Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. i The Application of Nanobubbles in Agriculture This thesis is submitted in accordance with the regulations, governing the award of the Degree of Master of Engineering with Honours in Chemical and Bio Process Engineering Department of Chemical and Bio Process Engineering Massey University, Manawatu, New Zealand By Ian Peter Gibbon June 2023 ii Abstract Nanobubbles have an extensive range of applications across engineering and industry. Our area of interest lies in agriculture and horticulture: Irrigation, using water enriched with oxygen nanobubbles, has been shown to greatly improve crop yields when compared to irrigation with untreated water. Currently, there are no manufacturers in New Zealand producing nanobubble generators to meet the requirements of large-scale irrigation. Field trials were undertaken, to quantify the increase in plant and crop growth through irrigation with oxygen nanobubbles. The results showed a marked increase in root mass and crop growth, supporting the viability and development of a nanobubble generator. The purpose to this thesis is to provide the research, testing and development of nanobubble tubes leading to a commercially viable, scalable nanobubble generator. Research was carried out to provide an understanding of the science behind nanobubbles. Applying this knowledge to the iterative process of design, print, test and evaluate, allowed comparisons to be made for various tube designs and allowed one design to be selected and taken forward for commercial development. Test results show that the optimum tube design, in terms of performance, is a compromise between the dissolved oxygen readings taken from the product water and the head loss across the nanobubble tube. The results also showed that smaller multiple tube arrangements out-performed large-scale single tubes. This led to a patent application for a multi-tube design. This thesis describes the 3-D printing of nanobubble tubes, nanobubble generators and current commercial installations under evaluation. The thesis concludes by discussing future development opportunities for the nanobubble generator. Key Words: nanobubble, generator, agriculture, horticulture, irrigation, crops, yield iii Acknowledgements I would like to take this opportunity to thank the following: Dr. Emilia Nowak, thesis supervisor, for her help, support and guidance. Leon Powers and Daneel Ovtcharenko, Nanobubble Agritech, for their help and assistance. iv Contents Page Chapter 1 - Nanobubbles 1.0 Introduction 1 1.1 Nanobubbles 3 1.1.1 Definitions for ‘fine bubbles’ 3 1.1.2 Physico-chemical characteristics of nanobubbles 3 1.1.3 Factors effecting nanobubbles 11 1.1.4 Free radical generation from bulk nanobubbles 12 1.2 Applications of nanobubbles 15 1.3 Nanobubbles in agriculture 17 1.3.1 Plant nutrients in soil 17 1.3.2 Mass transfer of plant nutrients in soil 17 1.3.3 Conditions affecting the transfer of nutrients 19 1.4 Nanobubble Generation 21 1.4.1 Electrolysis 22 1.4.2 Ultrasonication 23 1.4.3 Cavitation 24 Chapter 2 – Project Design for a Nanobubble Generator 2.0 Introduction 31 2.1 Design considerations 32 2.2 Method selected for the generation of nanobubbles 34 2.3 Design Assessment 34 2.3.1 Design intent of components 36 2.3.2 Identify opportunities to develop and optimise the design 37 2.3.3 Prioritise development 39 2.3.4 Preliminary Design 40 v Chapter 3 - Testing 3.0 Testing 42 3.1 Fabrication, Testing and Development Strategy 43 3.1.1 Fabricate a nanobubble generator which allows for the nanobubble tubes to be readily and easily changed out. 44 3.1.2 Determine the data required for the evaluation of performance and comparison between tube profiles. 46 3.1.3 Set up the test rig 50 3.1.4 Testing - Method 54 3.1.5 Install the commercially available tube, run performance test and record results 55 3.1.6 Replicate the commercially available tube 56 3.1.7 Install and perform test run 56 3.1.8 Trial different designs and record data 56 Chapter 4 – Nanobubble Tube Design and Development 4.0 Analysis of the commercially available tube 57 4.1 Tube Design 59 4.2 Tube Development 60 4.3 Profile variations 63 4.3.1 Number of helixes 63 4.3.2 Helix twist rate – number of full rotations v length 64 4.3.3 Core diameters 65 4.3.4 Venturi 66 4.3.5 Entry Profiles 67 4.3.6 Exit profiles 67 4.4 CAD – Computer Aided Designs 68 vi Chapter 5 – Results 5.0 Field trials 70 5.1 Nanobubble tube test results 77 5.2 Conclusions 82 Chapter 6 – Commercial Development 6.0 Evaluation 83 6.1 Kauri Park 84 6.2 NZ Cropping Farm 87 6.3 South Island Dairy Farm 88 6.4 Field Days and Agricultural Conferences 89 6.5 Patent 89 Chapter 7 – Future Development 7.0 Future Development 90 Appendix 8.0 Appendix 94 8.1 International Standards Organisation - ISO 20480-1:2017 : Standard definition for ‘fine bubbles’. 94 8.2 Project boundaries and parameters 96 8.3 Early Tube Designs – versions 8-24 97 8.4 Nanobubble tube test results 101 8.5 HAZOP - Hazard and Operability Study 102 8.6 HAZOP study – Kauri Park 106 8.7 Patent Application Summary 115 vii References 9.0 References 116 viii Figures Page Figure 1 International Standards Organisation, Bubble diameters 3 Figure 2 Graphic representation of bubble size comparison 4 Figure 3 Graphic representation of electrical charges at the gas bubble-water interface. 6 Figure 4 Fate of macro, micro, and nanobubbles in liquids with time. 9 Figure 5 The Bauer method 24 Figure 6 Example of multiple venturi NB systems in parallel, using ozone for a golf course turf application. 25 Figure 7 Example of mechanical cavitation NB machine 26 Figure 8 Pump impellor used in the EDUR multiphase pump 28 Figure 9 Graphical abstract - Generation of nanobubbles by ceramic membranes 30 Figure 10 Process Flow Diagram for a simplified Nanobubble Generator 36 Figure 11 Process Flow Diagram showing NBA’s preliminary design 40 Figure 12 The NBA Nanobubble Generator showing the commercially available NBT, (front view) 44 Figure 13 The NBA Nanobubble Generator showing the commercially available NBT, (rear view) 45 Figure 14 Extract taken from test data sheet 46 Figure 15 Simplified schematic diagram depicting the Nanobubble Generator test rig 50 Figure 16 Photo image showing nanobubble generator triple tube design set up for test run 51 Figure 17 ProSolo ODO Optical Dissolved Oxygen Meter 53 Figure 18 Cross sectional drawing depicting the commercially available nanobubble tube 57 Figure 19 Flow profile reproduced from, ‘Fluid Mechanics – Worked Examples for Engineers’, Carl Schaschke, 1998 59 Figure 20 Simultaneous printing of two nanobubble tubes 61 Figure 21 Photo image of 3-D Printer and PETG print reel cassettes 61 Figure 22 Photo image showing printing of a triple tube design 62 Figure 23 Photo image of a Nanobubble tube, showing 6-fins 63 ix Figure 24 Nanobubble tube showing each helix with 2 full rotations and an increasing twist rate 64 Figure 25 Photo image showing internal profile of figure 23 65 Figure 26 Diagram representing a Venturi profile 66 Figure 27 Image showing dripline irrigation trials 70 Figure 28 Image showing cropping trials, Massey University, Manawatu 71 Figure 29 Image showing soil samples after 4-months irrigation 72 Figure 30 Image shows a 3-span pivot irrigation scheme 83 Figure 31 Image showing a 10-span pivot irrigation scheme 84 Figure 32 Image showing the NBA generator prepared for shipment to Kauri Park 85 Figure 33 Nanobubble Agritech Generator fully installed at Kauri Park 86 Figure 34 Nanobubble Agritech mobile drip line installation, NZ Cropping Farm 87 Figure 35 Nanobubble Agritech Generator, South Island dairy farm 88 Figure 36 Nanobubble Agritech Drip-line installation, South Island dairy farm 88 Figure 37 Images showing a flow straightener and turbine meter. 90 x Tables Table Page 1 Applications of nanobubbles 15 2 2-D drawings generated by the CAD software package, showing tube designs. Versions 8 – 24 are shown in appendix 8.3, pages 97-100. 68 3 Summary of Nanobubble Tube Profile test results – 4 lps 77 4 Summary of Nanobubble Tube Profile test results – 10 lps 77 5 Summary of Nanobubble Tube Profile test results – 30 lps 78 6 Standard definition for fine bubbles, International Standards Organisation - ISO 20480-1:2017 94 7 Project boundaries and parameters 96 8 Nanobubble tube test results 101 xi Terms and Abbreviations Agritech Agricultural Technology Brownian motion ‘the continuous random movement of microscopic particles, approximate size 1µm, suspended in a fluid.’ ‘…the smaller the particles, the more extensive the movement’, (Oxford Science dictionary, 1997) BNB Bulk Nanobubbles CMP Centrifugal Multiphase Pump DO Dissolved Oxygen H+ Hydrogen ion - cationic form of atomic hydrogen H2O2 Hydrogen Peroxide – strong oxidising agent – mild antiseptic – bleaching agent IBC Integrated Bulk Container, typically having cubic dimensions holding 1 or 2m3 of water or aqueous solution lps Litres per second lpm Litres per minute mg/l milligrams per litre…may be expressed as mg.l-1 mV Millivolts NaCl Sodium Chloride – common salt NB Nanobubble NBA Nanobubble Agritech NBG Nanobubble Generator NBT Nanobubble Tube OH- Hydroxyl ion O2 Oxygen PETG Polyethylene Terephthalate Glycol PFD Process Flow Diagram pH Scale used to specify the acidity or basicity of an aqueous solution. PSI Pounds per Square Inch - pressure skid Frame-work upon which the nanobubble generator is mounted and secured. 1 Chapter 1 Nanobubbles 1.0 Introduction Nanobubble technology is relatively new with research and development very much in its early stages however, applications and uptake of this emerging technology are becoming more prevalent. There are few companies building nanobubble generators specifically for the agricultural industry. These generators produce both micro-bubbles and nanobubbles of oxygen in water. Irrigation, using water enriched with oxygen nanobubbles, has been shown to greatly improve crop yields. Nanobubble Agritech, (2021) verified this with data recorded from field trials, (chapter 5). This is also in accordance with published academic research. The potential economic benefits are significant in terms of increased yields. Furthermore, the benefit to the environment could be realised through reduced water usage leading to sustainable development and a reduced net energy consumption. Crops may not be viable in dry regions due to a lack of water. If water is used more efficiently and to better effect, then these areas where crops are considered non-viable may become viable, through the use of irrigation using water enriched with oxygen nanobubbles. At the time of writing, (2023) there are no known companies within New Zealand building nanobubble generators. Globally, there are few companies building nanobubble generators, specifically for irrigation applications. The agricultural sector would therefore be obliged to outsource, from the limited number of international manufacturers and retrofit or modify their products to meet the requirements for large-scale irrigation. These units currently represent a significant investment for crop growers and farmers. Current prices and options place this technology beyond the reach of many. The challenge is to produce affordable units in New Zealand that can be made available to the agricultural industry, benefiting the farmer, the environment and New Zealand enterprise. 2 The remit for this thesis and project is to research, investigate and develop low-cost units that are accessible to most agricultural businesses, providing them with significant commercial and environmental benefits. To improve on designs offered by the ‘market leaders’, it was necessary to procure a nanobubble tube and build a nanobubble generator for the purposes of testing and analysing the design. It became apparent, that to understand the design and seek out new ways to improve on each component, requires a detailed knowledge and understanding of engineering principles and fluid dynamics. The formation and generation of nanobubbles, and indeed the function of each component, must be fully understood to allow the design and its constituent components to be developed and improved upon. Chapter 1 provides the research and understanding necessary, laying the foundations on which to develop the nanobubble tubes and nanobubble generators. The subsequent chapters describe the testing and development of the nanobubble tubes and nanobubble generator. 3 1.1 Nanobubbles 1.1.1 Definitions The ISO, International Standards Organisation, defines ‘Ultra-fine’ bubbles, or nanobubbles as: Ultrafine bubbles with a volume equivalent diameter of less than 1 μm. The ISO specifies the terminology and definitions to be used in the area of fine bubble technology. The Standard definitions for fine bubbles can be found in Table 1 in the appendix, section 8.1. 1.1.2 Physico-chemical characteristics of nanobubbles i. Bubble diameter: Scale diagram showing bubble diameters Dimensions in μm, where 1 μm, (one micron) = 10-6 m Key: 1. bubble; 2. fine bubble ; 3. ultrafine bubble ; 4. microbubble Figure 1. Bubble diameters, International Standards Organisation - ISO 20480-1:2017 : Standard definition for ‘fine bubbles’. 4 Measured examples of ultrafine bubbles in water, mostly range between 100 nm and 200 nm, (Standard definition for ‘fine bubbles’, ISO 20480-1, 2017). Nanobubbles, or ultrafine bubbles, are defined as cavities of gases with a diameter < 200 nm in aqueous solutions, (Chaplin, 2017). 200 nm = 200 x 10-9 m Ushikubo et al. (2010), reported mean diameter of 137 nm for oxygen nanobubbles formed in de-ionised water. To put this in context; at 137 nm, the oxygen nanobubble is larger than a virus and smaller than a bacterium: Figure 2. Graphic representation of bubble size comparison, (Moleaer.com, 2022) ii. Surface area One of the key characteristics of nanobubbles, is that they provide a very high specific surface area for the mass transfer of gas, (shown mathematically, in Chapter 5.0). The figures used in the calculation, provide an indication of the order of magnitude for the 5 increase in surface area available, for the mass transfer of oxygen. The calculation showed that when the diameter of the bubbles decreased by a factor of 1000, the total surface area available for mass transfer increased by a factor of 1000. As bubble size decreases, total mass transfer area increases for a given total volume. Total mass transfer area is therefore inversely proportional to bubble size for a given volume of gas. iii. Buoyancy Due to their size and mass, nanobubbles are neutrally buoyant; they do not rise or sink. They take on a random movement, (Brownian motion) within the body of the water. Brown, (1827), showed this movement in pollen particles suspended in air. This random movement further reduces the likelihood of aggregation and coalescence. The zeta potential also plays a significant part in this effect, where negative charges at the gas bubble-water interfaces will cause bubbles to repel each other due to like-charges. This too, helps promote the stability of the nanobubble. “The stability of bubbles is increased by low rising velocity, which is negligible due to Brownian motion and low buoyancy forces.” (Chaplin, 2017). 6 iv. Zeta Potential Figure 3. Graphic representation of electrical charges at the gas bubble – water interface, (Acniti.com, 2022). Electrical double layer around an ultrafine bubble The net electrical charge around gas bubbles in liquid, are important in understanding the interaction of nanobubbles. When a bubble is suspended in liquid, it is surrounded by oppositely charged ions. The immediate area surrounding the nanobubble comprises two regions; an inner region called the Stern layer, where the ions are strongly bound, and an outer diffuse region where the ions are less firmly attached. The outer region is cloud-like and held together by electrostatic force. It consists of ions of opposite polarities. The system, as a whole, forms an electrical double layer. The development of a net charge at the 7 bubble- aqueous interface, affects the distribution of ions in the neighbouring interfacial region. This results in an increased concentration of oppositely charged ions close to the surface. When the nanobubble moves through the liquid, ions within the boundary move with the bubble. Any ions beyond the boundary do not move with the bubble. This boundary is referred to as the hydrodynamic shear or the slipping plane. The potential on this surface is called the zeta potential. High and Low Zeta potential The Zeta potential, measured in mV, gives an indication of the magnitude of the electrostatic charge, giving rise to repulsion or attraction between particles, bubbles or droplets in dispersed phase. Zeta potential is one of the fundamental parameters to affect dispersion and subsequent nanobubble stability. Zeta potential provides a detailed insight into the causes of dispersion, aggregation and coalescence. H+ and OH- ions Higher zeta potentials can be positive or negative, and give stability to nanobubbles due to repulsion between the bubbles. A lower zeta potential leads to coalescence and is less stable. A lower Zeta Potential means a value closer to zero. The charge of the solution depends on the agglomeration of positive ions, H+, or the agglomeration of negative ions, OH-. Studies have demonstrated that bubbles in distilled water are negatively charged. Meegoda et al (2018), reported a zeta potential of 22.4 mV, ± 3.4 mV for O2 in de-ionised water at 20 oC, pH7, 0.002M NaCl and that; when measuring different bubble sizes there is no relationship between the magnitude of the zeta potential and the bubble diameter. H+ ions, OH- ions and pH, are factors that determine Zeta potential. When the zeta potential is negative, the gas-water interface is negatively charged with OH- ions. Water has an excess of OH- ions at the interface, compared to H+ ions. The negative value of the zeta 8 potential under a wide range of pH suggests that OH- is more effectively adsorbed at the interface than H+. Zeta potential may be positive; this occurs under stronger acidic conditions, for example; when dissolved CO2 concentration is high. Researchers have identified that the adsorption of OH- onto the interface, by the difference of hydration energy between H+ and OH,- or by the orientation of water dipoles at the interface with hydrogen atoms pointing toward the water phase and oxygen atoms towards the gas phase, thus causing an attraction of anions to the interface. (for full review, see acniti.com, 2023) Meegoda et al., (2018), carried out a series of experiments to understand the behaviour of nanobubbles and showed that the size and zeta potential values of oxygen, nitrogen, air, and ozone nanobubbles were a function of the properties of the gas, specifically the gas solubility. Nitrogen, with the least solubility, had the smallest bubble diameter, while ozone with the highest gas solubility produced the largest diameter bubbles. The negative zeta potential value of nanobubbles is due to the number of OH- ions on the bubble surface. Since all the parameters are identical except the gas, it can be concluded that the zeta potential is a function of gas diffusion rates, solubility, and would contribute to the generation of OH- ions on the bubble surface. v. Coalescence The electrical charges at the liquid–gas interface creates repulsive forces that prevent bubble coalescence. This gives rise to highly dissolved gas concentrations in water. This in turn, results in smaller concentration gradients between the interface and the bulk liquid, (Ushikubo et al., 2010). 9 vi. Stability and Longevity Figure 4. Fate of macro, micro, and nanobubbles in liquids over time. * indicates that bubbles shrink & disappear, (Meegoda et al., 2018). Macrobubbles rise to the surface rapidly due to buoyancy forces, as shown in figure 4, and burst, while microbubbles rise at a slower rate. This increase in rise time allows for a greater transfer of gas from the bubble to the bulk liquid. This substantial loss of mass, causes the microbubble to shrink and disappear after a few hours. Microbubbles and nanobubbles have different swelling and shrinkage properties from macrobubbles. It is reported that the critical diameter separating bubble swelling and shrinkage is 50 to 65nm, (Li et al., 2013). Bubbles larger than this critical value will swell, while smaller bubbles will shrink. Microbubbles tend to gradually decrease in size and subsequently disappear due to long stagnation and dissolution of interior gases into the surrounding water, whereas nanobubbles can remain in the solution for weeks, under the right conditions, (Takahashi, 2005). Experimental data, from literature review, has shown that smaller bubbles with a high zeta potential, are more stable with time. Smaller bubbles tend to stay in solution longer because their motion is governed by both Brownian motion and buoyancy force. With this random motion, gas inside a bubble continuously diffuses and is supposed to decrease in size and eventually disappear. It is hypothesised that, with the loss of charges on the surface due to 10 diffusion, eventually, nanobubbles would shrink and disappear in a similar way to that of micro-bubbles. Figure 4 illustrates the fate of macro, micro, and nanobubbles over time. A theory is being developed to validate the above, based on diffused double layer theory and molecular dynamic simulations, Meegoda et.al., (2018). Nanobubbles are found to exist in solution for several weeks. Azevedo et al. (2016), reported that bubbles of radii 150–200 nm were identified in solution for 2 weeks after creation. With stable existence in liquids for over several weeks, nanobubbles have an extensive range of applications across many fields of science and engineering. For an effective and functional use of these bubbles, it is important to know the reason for their long-term stability. Meegoda et.al., (2018) carried out a comprehensive laboratory investigation to determine bubble size distributions and zeta potentials of nanobubbles, first with four different gases (test series I), then with different salt concentrations, pH levels, and temperatures of the solution (test series II). Experimental results from (test series I) showed that the average bubble size depended on the gas solubility in water, and zeta potential depended on the ability of the gas to generate OH- ions at the water/gas interface. Experimental results from test series II showed that high pH solutions produced smaller but stable nanobubbles. Bubble diameter increased slightly, with increasing salt concentration however, bubble size did not show considerable dependence on solution temperature. Long-term tests showed that with time zeta potential of bubbles decreased while the bubble size increased. Even though bubble sizes are expected to decrease with time due to gas diffusion, results indicate increased bubble sizes. This is because of a decrease in zeta potential and bubble movement, due to Brownian motion, which causes bubble coalescence over time, forming larger bubbles. (For full review see ‘Stability of Nanobubbles’, Meegoda et al., 2018). 11 1.1.3 Factors affecting nanobubbles i. pH Meegoda et.al., (2018) showed that negative zeta potential values increase when the pH value of the solution increases. This is due to the increase in OH- ions. It was also observed that smaller sized bubbles were generated, using hydrodynamic cavitation, under high solution pH values. Conversely, bubbles were larger and unstable in acidic solutions. These observations also supported the hypothesis that the amount of OH- ions on the surface of nanobubbles, governed stability. Higher pH levels with a high concentration of OH- ions generated smaller, stable nanobubbles with higher zeta potential values. Experimental results also showed that, with increased NaCl concentrations, zeta potential values decreased, while the bubble diameter increased. ii. Temperature Meegoda et al., (2018), showed by way of experimental results, that zeta potential decreased as temperature increased. There was no significant change in bubble size with increased solution temperatures. The change in zeta potential value may be due to the change in OH- ion concentration on the bubble surface, and with elevated temperatures and increased ion mobility it reduced the OH- concentration on the bubble surface. Jia et al., (2013), reported similar data showing a decrease in negative zeta potential values with increasing temperature. 12 1.1.4 Free Radical Generation from Bulk Nanobubbles Nanobubbles have been shown to have a mild disinfectant effect. They are frequently used in the treatment of wastewater and drinking water, recently developed, due to their ability to generate highly reactive free radicals, (Agarwal et al., 2011). This section provides the background to understanding the ‘mild disinfectant effect’ of water enriched with oxygen nanobubbles. Terms used include: i. Ion An atom, or group of atoms that has lost one or more electrons, making it positively charged (cation), or negatively charged (anion). ii. Radical A group of atoms either in a compound or existing alone. iii. Free Radical An atom, or group of atoms with an unpaired valence electron. Because of their unpaired valence electron, most free radicals are extremely reactive. (Definitions reproduced from Oxford Concise Science Dictionary, 1997). iv. Hydroxyl Ions, OH- and Hydroxyl Radicals, OH There is a significant difference between the hydroxyl ion, and the hydroxyl radical. The hydroxyl ion acts as an antioxidant in the body. It reduces harmful radicals that destroy tissues and DNA, protecting us from premature aging and the development of degenerative diseases. Reduction is one half of a chemical process referred to as Redox, (reduction /oxidation). It is a reaction that reduces a radical’s ability to oxidize other atoms and compounds. The other half of the redox reaction is oxidation. Oxidation is the loss of electrons and Reduction is the gain of electrons. 13 Hydroxyl radicals cause damage, by way of oxidation to tissues and DNA. The hydroxyl ion reduces the radicals’ ability to oxidize tissues and DNA. The difference between the Hydroxyl Ion and the Hydroxyl Radical is one electron. The hydroxyl radical is defined as OH (neutral) and the hydroxyl ion is defined as OH- (negative). The negative character represents the negative charge for the hydroxyl ion. An ion can be an atom or molecule that has a net charge. That charge can be positive or negative; it cannot be neutral. The hydroxyl ion has a negative charge, which means it has an excess of one electron that it can donate to a radical, or atom that lack electrons. A free radical is always looking for an electron, or electrons, to become paired again. Hydroxyl radicals are extremely damaging to living tissues. ACniti.com, (2022), suggested that when fine bubbles are compressed at high concentrations, zeta potential will increase as the ion concentration around the nanobubbles, increases. After several minutes of compression, an excess of OH- ions are formed, leading to the generation of OH radicals. It is the OH radicals that have an oxidising effect. While the explanation offered by ACniti may not be wholly accurate, or proven, the effect of free radicals is known, however ‘the how and why’ OH radicals are created, is not yet fully understood. Research has been carried out to gain an understanding of the generation of free radicals, and provide scientific reason for their creation. Takahashi et.al., (2021), attempted to provide such an explanation: “Microbubbles are very fine bubbles that shrink and collapse underwater within several minutes, leading to the generation of free radicals. The drastic environmental change, caused by the collapse of the microbubbles, may trigger the generation of free radicals, by dispersion of the elevated chemical potential accumulated at the gas-water interface.” This property gives rise to many practical applications, such as wastewater treatment and semiconductor cleaning. The mechanism for the generation of free radicals, proposed by Takahashi et.al., (2021), is related to the electrical charges present when microbubbles 14 collapse; Takahashi et.al., used electrophoresis studies to show that the gas-water interface is negatively charged over a wide pH range. Observations confirmed the generation of free radicals from bulk nanobubbles, through the collapsing process leading inducing cavitation. However, the existence of radical generation from bulk nanobubbles may indicate energy potential levels of bulk nanobubbles that could be useful in developing practical applications. The dispersion of physicochemical potentials may create some of these properties.” (For full review see, Takahashi et.al., 2021). While the research did not fully explain how and why, free radicals are generated, it did prove the presence and generation of free radicals from microbubbles and nanobubbles. The amount of hydroxyl radicals produced by NBs in solution is assumed to be very small. “The description used in the NB industry to portray the oxidation effect is that of a ‘mild’ disinfection or oxidation. For this purpose, it is used in algal mitigation of water bodies and in certain industries such as cleaning and food processing, to decrease pathogen counts,” (Meegoda, et.al.,2018). 15 1.2 Applications of nanobubbles (Table 1 – Applications of nanobubbles) Application Description Reference Drinking water Treatment of wastewater and drinking water: recently developed, due to their ability to generate highly reactive free radicals; oxygen nanobubbles have the effect of a mild disinfectant Agarwal et al., (2011) Nanobubbles are used in sparkling water and sports drinks. With the addition of nanobubbles, the water can potentially retain gases for a longer period Bauer Nanobubbles, (2017) Waste-water treatment Artificial flotation in water: This is accomplished by altering the ionic equilibria of dissolved ions in solutions and by changing the net charge on particle surfaces. Nanobubbles adhere to colloidal and emulsified materials promoting coagulation and facilitating separation through flotation and/or filtration. Moleaer, (2023) Decontamination of ground water Remediation of groundwater using ozone microbubbles and nanobubbles Hu and Xia, (2018) Bio-medical engineering Delivery of cancer drugs, where nanobubbles are placed in the body and given the ability to identify tumour cells. The bubbles are blown up when they approach tumour cells, thereby destroying the cancer NHI, (2017) Nanobubbles have been used in emergency procedures: O2 nanobubbles are injected directly into the bloodstream; This can give up to 15 mins when breathing is restricted allowing additional time to reach to hospital and a greater chance of survival Narayan, (2017) Agriculture Nanobubbles have shown the ability to create reactive O2 species which contribute to seed germination. This increase in reactive oxygen species has the same effect Liu et al., (2015) 16 as adding H2O2, (hydrogen peroxide), resulting in higher germination rates. O2 nanobubbles have been shown to have a significant effect on plants and crops, increasing growth, yield and improving soil quality and health. (Chapter5, Results, figure 27) Nanobubble Agritech, (2021) Fisheries O2 nanobubbles increase the DO levels in water and help reduce pathogens. This improves the health, growth rate and survival rates of farmed fish. Moleaer, (2017) Food CO2 Nanobubbles help regulate pH levels in liquids; they remain in suspension for prolonged periods helping to regulate solution pH. Moleaer, (2017) Poultry farms Ozone nanobubbles are used during the cleaning and disinfection cycles: O3 nanobubbles are pumped through the water lines to disinfect and clean the pipes and drinking vessels. O2 nanobubbles are injected into the drinking water to enhance poultry growth and health. Acniti, (2023) Paint In the presence of nanobubbles; paint drying times are reduced; paint shows a resistance to mould and shows an increase in brightness. Bauer, (2014) Mining The use of nanobubbles in the processing of tailings / fines, improves the separation efficiency allowing companies to comply with and meet strict environmental limits. Chipakwe, V., (2021) Separation Nanobubble technology has been adopted in the Oil and Gas industry to enhance phase and colloidal separation; also helping to reduce the volumes of chemicals used in chemical treatment, (surface and sub-surface) and stimulants, (sub-surface). Moleaer, (2023) 17 1.3 Nanobubbles in agriculture Nanobubbles, due to their size and numbers, provide huge surface areas for mass transfer of gas, for a given volume of gas, in solution. They have been shown to assist greatly, in the transportation and uptake of nutrients at the plant roots. A detailed knowledge of the effects of oxygenated water, as NBs, on plants and soil, provides a foundation on which to promote the NB generator. Plant life thrives on a healthy balance of water, oxygen and nutrients. Water provides the transport to deliver nutrients and oxygen to plant roots. 1.3.1 Plant Nutrients in Soil There are 17 essential elements that act as nutrients for growth: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Sulphur (S), Calcium (Ca), Magnesium (Mg), Boron (B), Chlorine (Cl), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Nickel (Ni), and Zinc (Zn). The three main nutrients are nitrogen (N), phosphorus (P) and potassium (K). Together they said to be, or known as NPK. The three macro-nutrients that plants take from water, air, or both, are; Carbon (C), Hydrogen (H) and Oxygen (O) while micro nutrients for plants include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. (For full review, see https://agritutorials.com/plant-nutrients-in-the-soil, 2022) 1.3.2 Mass Transfer of Plant Nutrients in Soil As the plant transpires, water, oxygen and nutrients are absorbed up into the plant. This causes a chemical imbalance between the root bowl and the surrounding soil. This, in turn, creates a concentration gradient. Water, oxygen and nutrients will migrate toward the 18 root bowl of the plant to reach chemical equilibrium. On reaching the plant roots, they are readily absorbed into the root structure, passing up through the plant as cells metabolise the nutrients and the plant transpires. This maintains a difference in concentration of water, oxygen and nutrients at the root bowl and the surrounding soil . This difference, or chemical imbalance, gives rise to a concentration gradient which provides the driving force, ensuring delivery and uptake of nutrients from the soil and oxygen from the water. The huge surface area and Brownian movement of the O2 nanobubbles in water, promote the mass transfer of gas from the water to the plant roots, further promoting the chemical imbalance. (For full review, see https://agritutorials.com/plant-nutrients-in-the-soil, 2022) i. Mass transfer Mass transfer is the movement of dissolved materials into a plant, as the plant absorbs water for transpiration. This process is responsible for the bulk transport of nitrates, sulphates, calcium and magnesium. Plant transpiration provides evaporative cooling, forming a major component of the leaf energy balance. Transpiration also provides the driving force for transport of water and nutrients from roots to shoots. ii. Diffusion Diffusion is the movement of nutrients, through water, to the plant roots, in response to a concentration gradient. When nutrients are found in higher concentrations in one area than another, there is a net movement to the low concentration area until chemical equilibrium is reached. Thus, a high concentration of nutrients, as chemical elements in the soil, will diffuse to an area of low concentration, specifically at the plant roots. This is particularly important for the uptake of potassium and phosphorous. iii. Root interception 19 Root interception occurs when the plant root comes into contact with colloids containing nutrients, within the soil. The plant root then absorbs the nutrients. This is an important transport mechanism for calcium and magnesium and provides a minor pathway for other nutrients. (For full review see: ‘Basic Concepts of Soil Fertility’, North East Region, Certified Crop Advisor, Study Resources, Cornell University, 2010.) 1.3.3 Conditions affecting the transfer of nutrients There are a number of direct and indirect factors which affect the transfer of water, oxygen and nutrients to plant roots. Weather conditions affect the rate at which water evaporates from soil to atmosphere. This has an indirect effect on the transfer of nutrients, reducing the water available for the transport of O2 and nutrients. Heavy clay-like soils have relatively poor porosity and permeability and directly effect the transfer of nutrients. i. Temperature As ambient temperature increases, soil will lose water through evaporation and without irrigation or adequate rainfall, the soil will dry out and crack to the point where the soil loses its porosity. If water cannot reach the plant root bowl, then nutrients are not transported and absorbed by the plant roots. If ambient temperature and soil temperature are too cold then there is minimal temperature differential and a significantly reduced driving force for plant respiration; this will slow or abate plant growth. Plant growth is very much dependent upon local soil conditions and species. Optimum temperatures will vary accordingly. ii. Pressure Atmospheric pressure is a contributing factor to the rate at which water is lost to atmosphere which in turn, will affect the transport of nutrients. 20 iii. Wind Wind can be considered as a consequence of temperature and pressure differentials. Wind also directly affects the rate at which water is lost to atmosphere. iv. Humidity Atmospheric humidity, (the degree of water saturation in air), has a direct effect on the rate and mass at which water is lost to atmosphere. v. Porosity Porosity in soil can be defined as the pore space, or ‘free space,’ between mineral particles and solid organic matter. These spaces are filled with air or water. The greater the porosity, then the greater the capacity to hold water, air and nutrients. In engineering terms, porosity can be calculated as Voidage where Voidage, ε , is defined as the fraction of the total volume which is free space available for the flow, or hold-up of fluids, and thus the fractional volume occupied by solid material is (1 − ε). vi. Permeability Permeability defines how easily a fluid flows through a porous material. Soil with a high permeability will allow easy unrestricted flow of water and nutrients, whereas soil with a low permeability will resist flow. vii. Concentration Irrigation, using water enriched with oxygen nanobubbles, has been shown to significantly improve plant growth, (Nanobubble Agritech, 2021). It is believed that the Brownian movement of nanobubbles helps to disperse the essential nutrients in water. Dispersing the 21 nutrients greatly improves absorption at the plant roots. This also maintains the chemical imbalance required for the concentration gradient and resultant driving force needed. Thus, concentration of nanobubbles and nutrients has a direct effect on mass transfer. 1.4 Nanobubble Generation There are a number of different ways in which nanobubbles can be produced. Some of these methods include electrolysis; sonication using ultrasonic sound waves; shearing by hydrodynamic cavitation and separation using membranes and diffusion. There is an energy cost and practical aspect to each method. Hydrodynamic cavitation has been shown to require less energy input compared to other methods and has proven to be comparatively simple and more practical; the generation of NBs for commercial applications, requires large scale production. Other methods are not conducive to large- scale production. To date, (2022) cavitation is the predominant method for producing NBs in commercial generators. Cavitation occurs when the fluid pressure, within a pipe, pump impellor, or system, falls below the gas vapour pressure. This results in gas breaking out of solution to form gas bubbles or gas filled cavities. As the localised pressure returns to its equilibrium state, the fluid pressure rises above the gas vapour pressure causing a rapid and violent collapse of said gas bubbles and cavities, giving rise to the generation of nano- and micro-bubbles. Cavitation can cause significant damage in process systems and mechanical systems. Early and experimental NBGs used ‘cavitation pumps’ designed purposefully, to induce cavitation. A limited number of manufacturers have developed the cavitation pump and use this method in their NB generator options. Cavitation can also be induced in centrifuges, although no commercially available generators, employing this method have been identified. 22 The use of modified or developed venturi tubes appears to be the most common method used in commercial NBGs. It is by far, the simplest and most economical method for producing nanobubbles. Essentially, it has no moving parts. Current commercial nanobubble tube designs resemble a modified venturi tube which combines the venturi and cavitation effects. Nanobubbles are produced by injecting gas into the nanobubble tube where shearing and cavitation takes effect. “Cavitation mechanisms can be classified into four different types, (Maoming et al., 2010; Agarwal et al., 2011; Padilla-Martinez et al., 2014). o Hydrodynamic - variation in the pressure of liquid flux due to system geometry (Maoming et al., 2010; Agarwal et al., 2011; Oliveira et al., 2018). o Acoustic - produced by applying ultrasound to liquids, (Ashokkumar and Mason, 2000; Ashokkumar, 2011). o Particle - passing high intensity light photons in liquids, (Poulain et al., 2015). o Optical - short-pulsed lasers focused into low absorption coefficient solutions, (Lauterborn, 1979; Martinez et al., 2014).” (For full review see ‘Stability of Nanobubbles’, Meegoda et al., 2018) 1.4.1 Electrolysis This method has typically been used in laboratory experiments only, with several research articles using this method of generation, such as Kikuchi et al., (2009). However, no known large-scale applications could be found, at this time, (2022) 23 1.4.2 Ultrasonication The principle of the acoustic cavitation method is based on subjecting the solution, or fluid, to periodic sound waves. These sound waves apply positive and negative pressure to the solution. Negative pressure gives rise to nucleation and expansion of bubbles. Positive pressure causes these bubbles to contract and implode when they reach their critical size. The implosion results in fragmented bubbles. Some of these bubbles continue to shrink over time, as the internal gas diffuses into the solution, and disappear, whereas others attain nanobubble size and stability. This method has only been found in lab-based research: It is believed that this is due primarily, to the relatively low nanobubble concentrations being produced. Bubble generation by acoustic cavitation was discovered in the mid-nineteenth century and has been studied by several researchers however, research on bubble generation using acoustic cavitation has been dominantly focused on micro/macro-sized bubbles, visible to the naked eye; furthermore, limited research has been conducted on the generation and growth of nano-sized bubbles using ultrasound. Moreover, no experimental report on the effect of dissolved gas concentration on NB generation using ultrasonication is available. (For full review see ‘Effect of Dissolved Gas Concentration on Bulk nanobubble Generation using Ultrasonication, Lee et al., 2020) 24 1.4.3 Cavitation i. The Bauer Method Figure 5. The Bauer method, (Nannobubbles.com., 2022) The Bauer Nanobubble Generator comprises a number of partial plates, housed in a pipe and installed inline to a pumped water system. The plates are placed in such a way that it forces the water to expand and contract as it flows over the plates. This rapid expansion and contraction creates cavitation. Bauer claim that their NBG produces a high concentration oxygen nanobubbles, with a mean diameter between 50 -100 nm. The Bauer method and Bauer NBG are proprietary and protected under patent hence, the information on the internal design and method of generation is limited. (For full review see nanobubbles.com.,2022) 25 ii. Hydrodynamic cavitation, (Venturi) Figure 6. Example of a multiple venturi NB system, using ozone for a golf course turf application, primarily for the elimination of fungal disease, (Nano Bubble Technologies, 2020). Hydrodynamic, (or Venturi) cavitation occurs through the depressurisation, of a gas- saturated water stream, through a flow constrictor. When the supersaturated water is forced through a needle valve, or constriction, the flow velocity rises, and the pressure falls below the vapour pressure of the liquid. Consequently, cavities (gas nuclei, or nanobubbles) form and grow into microbubbles by gas mass transfer from the liquid to the gas phase, (Azevedo et al., 2019). Advantages of this system include; simplicity; ability to process large volumes of water and a relatively high gas transfer rate. 26 iii. Mechanical cavitation High speed rotation of a pump impeller creates high pressure at the tip, or circumference of the impellor and low pressure at the eye of the impellor, (pump inlet). This results in a pressure gradient and the formation of gas cavities in the low-pressure zones. This system does not directly inject gas; NB’s are formed from gases already dissolved in water. However, this process results in lower NB concentrations per pass than other NB machines. Figure 7. Example of mechanical cavitation NB machine, (Nanobubbles, 2020) iv. Multiphase pumps Etchepare et al., (2017), were commissioned to develop and research the potential for large scale nanobubble generation, using multiphase pumps: The main objective of their work was to develop a new method for generating highly-loaded nanobubbles in aqueous solutions by hydrodynamic cavitation, using a centrifugal multiphase pump (CMP), and a needle valve. Nanobubbles at 150–200 nm diameter were formed at 22 °C, with a pump 27 and a recycle column, at various operating pressures. Nanobubbles were found to be resistant to shearing by pump impellers throughout several bubble generation cycles. Nanobubble size remained constant and their concentration increased as a function of these cycles, reaching equilibrium after 29 cycles; this was dependent on pump pressure and the surface tension of the solution. The highest concentration, 4 × 109 nanobubbles ml-1 was obtained at 5 bar and 49 mN m-1 surface tension. The mean diameter and concentration of these nanobubbles did not vary significantly over a period of two months, demonstrating the high stability of these concentrated nanobubbles. It is concluded that the procedure has great potential in future applications in ore flotation and wastewater treatment and reuse. Although residence times in multiphase pumps is generally lower than that using saturation vessels, the mechanism of hydrodynamic cavitation, operating in the various zones of the pump, allowed the formation of higher NB concentrations compared with the results obtained with a saturator vessel, (Azevedo et al., 2016). Advantages for sustainable bubble generation with CMP include: i. Higher volumetric efficiency, providing a large mass of air per unit volume of recirculation. ii. Elimination of saturation chambers. The results suggest that centrifugal multiphase pumps (CMP) have a great potential for bulk nanobubble generation at high rates, reaching a maximum concentration (4.1 × 109 nanobubbles ml-1, after approximately 29 operation cycles (residence time = 2.1 min). This optimum condition was obtained at 4–5 bar and low liquid/air surface tension (49 mN m−1). The bulk nanobubble dispersions were stable for over 60 days, with no decrease in nanobubbles concentration and mean size. For full review see, (Etchepare et al., 2017). 28 EDUR multiphase pumps Multiphase pumps are designed to supply integrated liquid-gas mixtures and to generate dispersions. They enable the supply of up to 30% gas proportions. EDUR-multiphase pumps are used in water and wastewater technology and varied industrial technologies. Applications include; flotation plants, ozonisation, water treatment, crude oil-water separation, fuel production and general process technology. Multiphase pumps can supply liquid-gas mixtures, enriching liquids with gases, such as air, oxygen or ozone. The multi- phase pump differs, in construction and mode of operation, from conventional centrifugal pumps, allowing throttling at the pump suction inlet without cavitation arising at this point. EDUR states: The features of their multi-phase pumps include; self-priming; up to 30% by volume, gas entrainment; dynamic mixing and very high gas saturation. Multiphase pumps are sometimes referred to as Flotation pumps, Dissolved Air Flotation pumps or gas- saturation pumps. Advantages: eliminates the need for compressor systems, pressure vessels, control units, valves and associated maintenance costs. Gaseous fluids can be fed directly into the pump. For full review see, (edur.com, 2023). Figure 8, showing the pump impellor used in the EDUR multiphase pump, (edur.com,2023) 29 v. Axial Flow - Shearing The water flow is rotated into several vortices, while moving along a special chamber and exiting from the discharge pipework. This rotational movement is maintained until the kinetic energy has depleted. This produces an intense shear, through respective centrifugal forces, triggering nucleation and gas bubble formation as a result of the pressure fluctuation, (Azevedo, 2019). This requires a pump for high water flows. vi. Diffusion through a porous membrane NBs are generated by flowing air, or gas through porous membranes, usually glass or ceramic, under high pressure. A key benefit of this type of machine is that a high concentration of NBs can be generated from a low energy requirement; only a low flow of water is required to pass through the NB generator, with gas pressure providing the key driving mechanism for NB production. This method is better suited to waste water treatment applications. (For full review see ‘Generation of nanobubbles by ceramic membrane filters’, Ahmed et.al., 2018) 30 Figure 9. Graphical abstract - Generation of nanobubbles by ceramic membrane filters, (Ahmed et.al., 2018). The porous membrane forms a tube through which, the gas diffuses into the bulk liquid. Disadvantages of this type of machine include: o In order to achieve efficient gas transfer relative to the power required, the gas must be pre-conditioned, (filtered and pre-heated) and supplied at relatively high pressure. o Porous membranes require a very high degree of filtration. o Pressurising gases, such as oxygen and ozone, has inherent safety implications which must be carefully engineered and managed. o Membranes are expensive. o Not suited high continuous water flows 31 Chapter 2 Project Design for a Nanobubble Generator 2.0 Introduction The boundaries and parameters have been set by those bodies that have contributed to the funding for project development and Nanobubble Agritech, the ‘developer’. (The project boundaries and parameters can be found in the appendix, section 8.2, table 2). The project has to be of environmental and commercial benefit to New Zealand. Nanobubble generators are available from international suppliers and manufacturers offering various designs however, at present, (2023) there are no local suppliers or NBGs built in New Zealand. The units available for import, are not specific to the Agricultural industry; they would require modification to meet the needs of large scale irrigation and are thus, not suitable. These units are also relatively costly. The capital outlay plus the additional cost of modification would place them beyond the reach of many farmers. The challenge for Nanobubble Agritech is to develop and manufacture low cost nanobubble generators, specifically for high-flow irrigation applications. Nanobubble Agritech’s vision was to develop a nanobubble generator that is ‘simple’, uses readily available low cost parts and nanobubble tubes that can be manufactured in-house. The tubes had to be designed for optimum efficiency in terms of energy losses and nanobubbles generated. The nanobubble tubes, designed and engineered by NBA, Nanobubble Agritech, are innovative in that the designs are unique. The environmental benefit to New Zealand will be realised through the combination of a reduced energy input, potential to reduce water usage and significant increases in plant growth and yield. 32 The commercial benefit will be realised in terms of increased crop production and the respective income and taxation streams. Increased production may also benefit tertiary and supporting industries. Employment opportunities may arise from the manufacture, supply and installation of NBGs. Employment opportunities may also arise to meet the labour demands of increased production, in terms of growth and yield. 2.1 Design considerations Nanobubble Agritech’s market is the agricultural industry. Most, if not all of NBA’s clients will come from the Agriculture sector where costs and capital investment present a significant out-lay and risk. With this in mind, NBA had to design and develop nanobubble generators that are affordable and attainable to agricultural businesses. This required a number of key factors to be considered. These include: Safety Safety is of paramount importance and was a prominent design consideration. The design team had to ensure that harm, injury or financial loss cannot occur as a result of human or mechanical failure, whilst operating a nanobubble generator. A full HAZOP, (Hazard and Operability study), was carried out to identify the risks and mitigations required. (The HAZOP process is explained in the appendix, section 8.4 and a full HAZOP study can be found in the appendix, section 8.5.) ‘Fit for purpose’ In essence, ‘fit for purpose’ refers to the optimum capital expenditure for a required component, piece of equipment or unit that will provide adequate performance. For example: “The requirement of the Agritech nanobubble generator is to reliably supply, irrigation water, enriched with oxygen nanobubbles”. The first component in the nanobubble generator is a water pump. Water pumps vary in cost and quality and the 33 selection criteria must be carefully considered: It is not good business practice to buy the most expensive pump available, on the assumption that it will be the most reliable, when cheaper pumps may perform adequately. Conversely, it is not necessarily good business practice to purchase the cheapest pump available; the initial cost may be cheaper however, it may prove to be more costly in terms of premature wear, breakdowns, maintenance call- outs to remote locations and the resulting inability to supply enriched irrigation water. Simplicity The design of the nanobubble generator has to be simple. Simple, in this context, means using the minimum number of parts which, are easily accessible and easy to maintain or replace. Reliability and durability The nanobubble generators will be installed in remote rural locations and subject to inclement conditions, therefore the NBGs must be robust, reliable and durable. This was an important consideration when selecting materials and equipment. Practicality Above all, the NBG had to be a practical solution to the design considerations. This included the feasibility and cost of manufacture. Energy costs The energy costs pertain to the electrical loading of the water pump, the oxygen generators’ compressor and the associated charges for the electricity used. The design had to be such that energy costs can be kept to a practical minimum. Total costs 34 The project team had to remain mindful of the total cost, in real terms, for the manufacture and supply of a nanobubble generator; the cost has to be viable. From the farmers’ perspective, this includes initial purchase cost, reliability and running costs. From the development team’s perspective, these factors had to be considered in parallel with the cost to Nanobubble Agritech in respect of manufacture, supply, installation, servicing and guarantees. 2.2 Method selected for the generation of nanobubbles The research and study carried out in Chapter 1, provided the necessary understanding and foundations on which, to design and build an adaptable nanobubble generator to meet the needs of large scale irrigation. Different methods of generating nanobubbles and current commercially available nanobubble generators were reviewed: Their principal of operation and component parts were studied, in conjunction with the different methods by which nanobubbles can be generated, in order to identify the most suitable and effective method for large scale irrigation. The shearing of O2 bubbles, as a result of pumping water through a nanobubble tube and injecting O2 at close proximity to the tube inlet, was identified as the only practical and feasible method that could be used in a commercial generator, specific to irrigation. Scientific, commercial and practical research, consolidated NBA’s decision to induce cavitation as a consequence axial flow shearing. 2.3 Design assessment To provide clarity: The nanobubble generator is the name given to the skid mounted unit, in its entirety. The nanobubble tube is the single item where the oxygen nanobubbles are produced. Under advice, the patent for a nanobubble tube has been lodged as a ‘nanobubble generator’ with no reference to the word ‘tube’, within the naming. This thesis, when making reference to the nanobubble generator, refers to the entire unit. 35 A nanobubble tube was purchased and a nanobubble generator was fabricated, to allow Nanobubble Agritech to complete performance trials. On completion of the trials, the generator was subsequently dismantled to facilitate a critical and analytical assessment of the design and it’s components. In essence, the design utilises a water pump, an oxygen generator, a nanobubble tube, associated pipework and an elementary control panel. The design also aligns with Nanobubble Agritech’s vision of an adaptable design suitable for low and high water flow rates. The drawing below, depicts a simplified NBG. Figure 10 Process Flow Diagram for a simplified Nanobubble Generator 36 2.3.1 Design intent of components The Nanobubble Generator comprises the following parts: Water Pump The design intent of the water pump is to supply water to the nanobubble tube at a specified flow rate and pressure. Nanobubble tube The intent of the NBT, is to generate nanobubbles, enriching the water with oxygen. Receiving vessel / Dispersion Tank The receiving vessel, or dispersion tank, provides a buffer between the nanobubble tube and the water outlet to the irrigation distribution pipe. Water enters the dispersion tank tangentially, and rotates around the circumference of the tank, where the water continues to decelerate as the momentum and kinetic energy deplete. Results from testing and evaluation, show that nanobubbles continue to be generated as the water continues to rotate. The design intent is facilitate this rotational motion and provide a degree of residence time. Oxygen Generator The intent of the Oxygen generator is to supply oxygen to the nanobubble tube at the required flow rate and pressure. The O2 generator may also be replaced with an O2 cylinder, fitted with a relief valve and a pressure regulator. Control Panel The control panel, or box, provides the stop / start and logic functions for the water pump and the O2 generator. 37 2.3.2 Identify opportunities to develop and optimise the design Each component was analysed to further understand its function and suitability with a view to NBA improving the design and adapting for high water flow rates, in rural locations. Water Pump The pump must be durable, capable of handling brackish water and not subject to rust. It must be low-cost and readily available. It should provide up to 10m lift, (lift, or negative head, are terms used to quantify the difference in elevation between the water, at the inlet to the pump suction hose, and the pump). Typically, the water supply will be drawn from an irrigation stream which is below the level of the nanobubble generator and the field on which it sits. Due to the remoteness of installations, the pumps must be ‘self-priming’. ‘Self-priming’, in this context, refers to the ability to evacuate all air from the pump and pump outlet. If there is air in the system then the pump may become air locked and unable to pump. NBA will fit an auto-priming device at the pump outlet. This is a considerably cheaper option to sourcing and buying self-priming pumps. Large irrigation schemes, found in New Zealand’s dairy farms, require flow rates ranging from 50 to 250 lps, (litres/second). The pump selected must be capable of delivering the required flow rate. The pressure losses across the entire irrigation pipework system must be taken into account and the total figure added to the required pressure at the outlet of the irrigation system. A 10% design margin is then applied. The result is the maximum pressure required at the pump outlet. This figure will be determined after the local pipework requirements have been assessed. For the purposes of a nanobubble generator, designed for agricultural applications, the pump selected will be rated for the specified flow rate and pressure requirements. It will be 38 a stainless-steel, single-stage centrifugal pump. These pumps are readily available from local farm supplies; should a pump need replacing, then one can be readily and locally sourced. NBA will source these pumps directly from the manufacturer when building nanobubble generators. There are no development or improvement opportunities identified with respect to the water pumps. A recommendation will be made to farmers, that a readily accessible and maintainable inline water filter is installed before the NBG, to reduce the risk of blockage at the inlet to the water pump. This is particularly pertinent when brackish water is supplied to the NBG. Nanobubble tube A commercially available nanobubble tube was purchased to allow performance testing and assessment. A second, identical tube was also purchased which was sectioned to understand the internal profile and the potential to further develop. After initial assessment, it was determined that NBA would design, develop, patent and manufacture their own NBTs. Receiving vessel / Dispersion tank Further development could be undertaken to enhance the internal geometry of the vessel and the inlet nozzles, in order to optimise fluid rotation and shear at the circumference of the vessel. The development and modifications to the vessel could be extended to allow for O2 recovery. Oxygen Generator Development opportunities were identified for the O2 generator primarily around safety, efficiencies and gas re-capture. Control Panel NBA will use a PLC, (Programmable Logic Controller), to control the start-up and shutdown sequencing. NBA will write specific logic and algorithms for each installation. 39 2.3.3 Prioritise development The nanobubble tube was identified as the primary and priority focus. NBA recognised the development potential and the opportunity to lodge a patent application for a unique and improved design. The receiving vessel / dispersion tank was also identified as having potential for design improvements. The vessel will be further developed when resources become available. There is scope to make improvements to the O2 generator and the supply of oxygen. The improvements focus primarily on efficiency and gas recapture. It is recognised that O2 generators represent the greatest cost in the manufacture and supply of NBGs however, the research and development required to improve the efficiency, through gas re-capture, represents a significant, stand-alone project requiring engineering and development. For this reason, a lower priority was placed on O2 recapture and recovery. NBA anticipates that this will be developed after initial sales and cash flow has been established. 40 2.3.4 Nanobubble Agritech - Preliminary design Figure 11 - Process Flow Diagram showing NBA’s preliminary design The preliminary design shown in figure 10, differs from the “Process Flow Diagram for a simplified Nanobubble Generator”, (shown in figure 9), by the addition of the following components: Auto-Prime The auto prime unit, fitted at close proximity to the pump discharge outlet, (described in section 2.3.2). Pressure Gauges Pressure gauges have been provided to allow; the O2 supply pressure to the nanobubble tube to be monitored and the water pressure, upstream and downstream of the nanobubble 41 tube, to be measured in order to determine the differential pressure, (or pressure drop), across the tube. Flow meter The flow meter allows the flow of oxygen to be monitored and regulated to the nanobubble tube. Non-return valve The non-return valve prevents the migration of water from the nanobubble tube, into the O2 supply line. If water migrates into the O2 generator then the membranes would have to be replaced. This would be costly, in terms of time and capital expenditure. 42 Chapter 3 Testing 3.0 Testing Purpose of testing The purpose of testing is to gather meaningful data for evaluation: It allows the performance of the 3-D printer to be evaluated and the performance of nanobubble tubes to be compared against each other. Desired / expected outcomes o Set up and calibrate the 3-D printer to reliably and consistently produce robust nanobubble tubes o Evaluate structural design and integrity of 3-D prints o Evaluate inlet and outlet manifolds for strength and integrity o Recommend a preferred tube design to take forward for up-scaling o Recommend a preferred tube design to take forward for patenting o Recommend tube profile design for commercial use A strategy was written such that the approach to testing and evaluation, followed a structured and sequential plan: 43 3.1 Fabrication, Testing and Development Strategy 1. Build a nanobubble generator which allows for the nanobubble tubes to be readily and easily changed out. 2. Determine the data required for performance evaluation and comparison between tube profiles. 3. Set up the test rig 4. Write the testing method 5. Install the commercially available nanobubble tube. This will be the ‘reference tube’ Run the NBG, in single pass mode and record data. 6. Using 3-D printer, replicate the commercially available nanobubble tube. Evaluate the tube for quality, structural strength and accuracy. Assess the performance of the 3-D printer. 7. Install the commercially available nanobubble tube replica, carry out testing, record data and compare against the recorded data for the commercially available nanobubble tube. 8. Use CAD imaging to develop a new profile based on test evaluation. 9. 3-D print new profile. 10. Performance test 1st tube profile and evaluate 11. Use CAD to improve / modify tube profile designs. 12. 3-D print tube profiles and test 13. Identify / select best profile to take forward 14. Scale-up - single tube 30 lps, print, test and evaluate 15. Multi-tube designs – triples; print, test and evaluate 16. Identify potential profile designs for patent. 17. Select design / designs for patenting. 18. Select a preferred tube profile design for commercial nanobubble generators. 44 3.1.1 Fabricate a nanobubble generator which allows for the nanobubble tubes to be readily and easily changed out. The nanobubble generator, shown below, was constructed, based on the preliminary design and the criteria set-out in chapter 2. Figures 12 and 13, are the physical representation of figure 11, page 40, (Process Flow Diagram showing NBA’s preliminary design). Figure 12 - The NBA Nanobubble Generator showing the commercially available NBT, (front view). The water inlet is on the right hand side and the outlet on the left 45 Figure 13 - The NBA Nanobubble Generator showing the commercially available NBT, (rear view). The water inlet is on the left hand side and the outlet on the right 46 3.1.2 Determine the data required for the evaluation of performance and comparison between tube profiles. Figure 14 - Extract taken from test data sheet. Figure 14 shows an extract taken from the test data sheet. (The data sheet can be found in Appendix 8.4, Nanobubble tube test results.) Consideration was given to the data required, that would allow the performance of different nanobubble tubes to be evaluated. The data and reasons are explained below: Dissolved O2 in IBC It is important to record the dissolved O2 in the water supply tank. (The supply tank is an IBC, integrated bulk container, providing 1 m3 of water). If we know the dissolved oxygen reading before and after the NBG, then this will allow the uptake of O2 injected and the efficiency to be determined. Feed water temp Temperature will affect the saturation level of dissolved O2 in water, under ambient conditions. The water supply temperature was initially recorded for this reason. O2 flow rate The O2 flow rate was recorded to quantify the amount of O2 injected. This information is a required factor in determining O2 consumption, efficiency and the size of the O2 generator required. 47 O2 pressure It is important to monitor the pressure of the O2 supplied to the nanobubble tube to ensure that a pressure differential is maintained between O2 supply and the water supplied to the nanobubble tube. The oxygen pressure must be greater than the water pressure to allow the O2 to flow into the nanobubble tube. It must also be greater to prevent back-flow, (reverse flow), of water into the O2 supply tubes. Time to fill 200 litres This information was recorded to allow the volume of oxygen injected to be determined. The total volume is the product of time and flow rate. The amount of O2 injected is required to calculate efficiency and up-take of O2. Drum capacity Fixed at 200 litres. This figure was a practical choice: The water discharged from the NBG is collected in a 200 litre plastic drum. Pump discharge pressure This gives an indication of pump discharge pressure and the back-pressure on the system, by inference; the greater the restriction to flow, within the system, then the greater the pressure observed at the pump discharge. The pump discharge pressure gauge also gives an indication of pump performance and partial restrictions or blockages at the pump suction strainer. Pump discharge flow rate The product of drum capacity and time, allows the water flow rate to be calculated. This allows comparisons to be made for the different nanobubble tube profiles and their respective performance. Dispersion tank pressure The Dispersion Tank is a practical place to mount the down-stream pressure gauge. The pressure taken from this point was subtracted from the pump discharge pressure gauge to 48 give the differential pressure across the nanobubble tube. The pressure drop across the tube gives a direct indication of the restriction to flow, due to the internal profile of the tube, and is termed ‘Head loss’. ‘Head loss’ was a criterion stipulated in the Boundaries and Parameters: “To improve and out-perform current market units, in respect of head losses across the Nanobubble Generator.” Head losses are measured as the pressure drop across the nanobubble tube and the pressure drop across the nanobubble generator from inlet to outlet. The greater the head loss then the greater the pumping requirements. This will increase energy consumption, which will increase the running costs. Increased energy consumption will have also have an environmental impact hence; head losses are a criterion. Product tank water temp When water is pumped, the water temperature will rise, fractionally, as a result of friction. At a practical level, the amount of heat added, due to friction is negligible however, if the water is recirculated and the heat added becomes greater than the heat losses within the system, then the water temperature will rise. This is a consideration when trialling multiple passes through the nanobubble tube and recycling the product, (enriched water), back to the water supply tank. Water temperature affects the saturation level of O2 in water, thus when trialling nanobubble tubes, the conditions under which the testing is carried out, must remain constant across trials. Dissolved O2 – mg/l This reading pertains to the dissolved O2 measured when lowering the DO meter probe into the 200 litre product drum. The product water discharges from the nanobubble generator dispersion tank, into the collection drum. After 2-minutes, the probe is lowered into the drum to a depth of 1/3 drum height, from the base of the drum. The DO reading is taken after a further 2 minutes has elapsed. This allows the fluid motion in the collection drum, and the DO meter readings to stabilise. These conditions were set to ensure consistency and expediency across trial runs. 49 By inference, the DO reading gives an indication of O2 as nanobubbles: If bubbles larger than nanobubbles are produced, then these bubbles will coalesce, rise to the surface and the oxygen will be lost to atmosphere: Dissolved oxygen readings taken will initially appear high however, these readings will fall markedly in a relatively short period of time, whereas the readings taken when the water is enriched or saturated with nanobubbles will remain high and stable for extended periods. A trial was carried to show the longevity of nanobubbles. Nanobubbles were generated using a commercially available nanobubble tube. Samples were taken and compared against control samples, from the same water supply. Dissolved oxygen readings were taken over a six week period. There was a marked difference observed between the control samples and the nanobubble-enriched water samples: The control samples varied between 9.0 and 9.8 mg.l-1 The initial readings taken after the water had passed through the nanobubble generator varied between 17.0 and 17.7 mg.l-1. The DO remained high and stable, showing depletion after 2-weeks. After six-weeks, the DO readings had fallen, showing values between 10 and 13 mg.l-1 The DO results, and the results from field trials and commercial installations, were sufficient evidence needed to support the commercial viability of the NBG. NBA does not have the facility to quantify NB size and distribution however, the generation of nanobubbles has been proven in two ways: by inference of high and stable DO readings taken after extended periods and the increase in plant growth, measured and recorded during crop trials at Massey University. 50 3.1.3 Test rig Figure 15 - Simplified schematic diagram depicting the Nanobubble Generator test rig 51 Figure 16 – Nanobubble generator triple tube design set up for test run 52 Figure 16 shows a triple tube design installed in the test rig and set up ready for testing. Water enters at the base of the image and exits at the top of the image. The rotameter, (variable are flow meter) seen at the base of the image allows the oxygen flow rate to be set and monitored. The blue hose supplies O2 to the flow meter. The clear 10 mm hose from the outlet of the flow meter supplies the nanobubble tubes, teeing off to supply each individual tube with O2. The two pressure gauges allow the upstream and downstream water pressure to be recorded and allow the differential pressure / head loss across the tube arrangement to be determined. Describe equipment used IBC The integrated bulk container, is a polythene container holding 1 m3 of water and supplies the nanobubble generator. The container has a water outlet valve at its base. Water is supplied to the nanobubble generator from this point, via a connecting hose. The IBC is filled by removing the screw down lid at the top of the container and running a water hose into the IBC. Nanobubble Generator The Agritech nanobubble generator is shown in figures 11 and 12, (p 44 and 45). It includes the dispersion tank, or vessel. RCD, (Residual Current Device), and electrical extension lead. The RCD is an electrical protection device designed to prevent electrocution and damage to equipment. This is particularly important when water is present. O2 cylinder For testing purposes, an O2 cylinder, fitted with a pressure regulating valve and a pressure relief valve, supplies oxygen to the nanobubble tube. An O2 generator will be supplied for commercial sales. 53 200 litre drum A 200 litre polypropylene drum receives water from the nanobubble generator. This is the collection vessel for the nanobubble enriched water, (referred to as product water). Dissolved oxygen meter Figure 17 - ProSolo ODO Optical Dissolved Oxygen Meter, used to measure the dissolved oxygen in the 200 litre product water collection drum 54 3.1.4 Testing - Method Pre-test run 1. Connect all pipework following the schematic diagram shown in figure 14. Note: The outlet from the nanobubble generator discharge hose should be placed on the ground to allow run-off to an open drain. The discharge hose will be placed in the 200 litre drum for the duration of a test run. 2. Confirm that the pressure regulator fitted to the O2 cylinder, is fully wound out, (anti-clock- wise). This prevents over-pressurisation of the test equipment. 3. Connect the O2 tubing from the O2 cylinder to the NBG oxygen regulating flow meter. 4. Connect the O2 tubing from the flow meter to the injection point at the nanobubble tube. 5. Confirm that the regulating valve at the O2 flow meter, (on the meter panel), is fully shut. 6. Slowly open the shut-off valve at the O2 cylinder to approximately ¼ open. 7. Slowly wind the pressure regulator, at the O2 cylinder, clockwise until a pressure of 50 psi is seen on the O2 cylinder regulator gauge. The O2 supply pressure is now set to a maximum of 50 psi. 8. Adjust the O2 flow to the nanobubble tube by slowly opening the valve at the base of the flow meter on the meter panel. Set the flow rate to the desired value for the test; 4 litres / min for the initial low-flow tests. Note: adjustments made at the O2 flow meter must be done very slowly so as not to damage the flow meter. Fine adjustment will be required when water flow is established. 9. Close the shut-off valve at the top of the O2 cylinder. 10. Confirm that the On/Off switch at the flow meter panel is in the off position. 11. Connect an extension lead to the power outlet. 12. Connect an RCD, (Residual Current Device), to the extension lead. The RCD is an electrical protection device designed to prevent electrocution. Note: This is particularly important where electrical equipment may come into contact with water. 13. Take the electrical cable from the water pump and plug this into the RCD 14. Take the pre-start readings, specifically the DO and temperature for the water in the IBC. Record time and date. The equipment is now set up and ready for the test run. 55 Testing 1. Open the water outlet valve at the base of the IBC to allow water to flow to the NBG. 2. Slowly loosen the vent plug at the top of the water pump casing and bleed air from the system until water shows. Close and hand tighten vent plug. This will purge air from the system and prime the pump. 3. Open the shut-off valve, at the top of the O2 cylinder, to the ¼ open position. 4. Confirm O2 flow rate is 4 l/min 5. Turn the On/Off switch, at the meter panel to the On position. Note: The pump will start at this point and the water level in the IBC will fall rapidly. Safety note: Do not allow the pump to run dry; this will damage the pump. 6. Allow the pump to run for a few seconds to purge any remaining air from the system and to allow the water flow rate to stabilise. 7. Check the O2 flow rate and confirm 4 litres/min. Make a fine adjustment if required. 8. Place NBG discharge hose into the 200 litre drum. As soon as water enters the drum, 2nd person starts the timer. 9. 2nd person records; pump discharge pressure, dispersion tank pressure, O2 pressure and monitors O2 flow rate. 10. When the water level reaches the 200 litre mark in the Product drum, stop the timer 11. Switch off the water pump at the NBG meter panel. 12. Turn off the O2 supply at the cylinder outlet valve 13. Close the outlet valve at the base of the IBC. 14. Record the time taken for the water to reach the 200 litre mark. 15. Confirm that all the required data has been validated. Note: The nanobubble tube may now be removed and the next tube installed in readiness for the next test. 3.1.5 Install the commercially available tube, run performance test and record results The commercially available tube was installed in the nanobubble generator and a number of test runs were performed. This allowed familiarity with the test rig and the equipment. This 56 also allowed the testing method to be revised. The commissioning of the test rig and the nanobubble generator was carried out at this point, prior to the collation of test data. Primarily, we were looking for consistency and repeatability with the data. The final test with the commercially available tube provided said data, against which the 3-D replica print would be measured. The commercially available tube was considered to be the reference tube. 3.1.6 Replicate the commercially available tube The commercially available tube was sectioned at the centre, along the vertical plane. This allowed the internal profile to be analytically and critically assessed. The tube could then be replicated using 3-D printing software, and the 3-D Printer. 3.1.7 Install and perform test run The replica tube was installed in the NBG and testing performed. Comparing the test data recorded from the commercially available tube with the data recorded from the replica tube allowed the performance of the 3-D printer to be assessed and adjustments to be made. The performance of the replica tube was compared against the original. The results are discussed in Chapter 5. 3.1.8 Trial different designs and record data Nanobubble tube design evolved as an iterative process by applying the knowledge gained in Chapter 1, and an understanding of fluid dynamics. This insight allowed the results from each design to be analytically and critically appraised. Each appraisal would influence or change the profile for the next iteration. The development of the nanobubble tubes and their respective profiles followed the strategy outlined in section 3.1. 57 Chapter 4 Nanobubble Tube Development 4.0 Analysis of a commercially available nanobubble tube Figure 18 – Cross sectional drawing depicting a commercially available nanobubble tube, (From prior art search, carried out on behalf of Nanobubble Agritech Ltd. Patent Assigned to Gaia USA Inc.) Principle of operation: Water enters the nanobubble tube at the left hand side of the image and exits from the right, enriched with oxygen nanobubbles. Items 1020 / 1002 form a solid cone. The tip of the cone faces into the upstream flow. As water hits this point, it diverts around the O2 injector 58 and out to the walls of the tube, (1012a, 1012b). O2 is injected into the flow-stream via an injector and an injection port, (910, 1016, 1006). Water is forced to flow on both sides of the first helix, making one full rotation. As the water makes this rotation it is subjected to centrifugal force. The water on either side of the helix can be considered as two streams. There is a mixing region before the second helix, (1050). This region effectively provides an increase in volume. The increase in volume reduces the water pressure to a point below the vapour pressure of the O2. This induces cavitation and promotes the generation of nanobubbles. The two streams come together in this ‘mixing’ region creating more turbulence. Turbulence gives rise to attrition between the O2 bubbles and the water. Water then passes to either side of the second helix, prior to exiting the tube. Nanobubbles are generated as a consequence to the effects of centrifugal force and turbulence. Centrifugal force and turbulence give rise to attrition at the molecular level. It is hypothesised that low pressures are created when attrition occurs. The reduction in localised pressure leads to cavitation which in turn, generates nanobubbles. Analysis When the water diverts around the O2 injection port, vortices will be shed giving rise to a slight increase in turbulence with additional head loss, albeit fractional. The design is such that, the O2 can be considered to be drawn into the water flow stream as opposed to being injected into the flow stream. Mixing and dispersion of the oxygen is a consequence of the turbulence created, as the water changes direction and diverts to either side of the first helix. The two streams from either side of the helix collide as they enter the mixing zone between the two helixes. This causes more turbulence and additional head loss. The water must change direction when it enters the mixing region and change direction again, as it flows to either side of the second helix, adding to the overall head loss. Head losses are an important design criterion and are stipulated in the parameters set out for the NBA generator. The NBA profiles must improve on existing designs and must not encroach or infringe on existing patents or copyrights. Remaining mindful of the above, the Nanobubble Agritech designs must be innovative and NBA must look for a new approach 59 whereby head losses can be kept to a minimum and DO readings match or better, the commercially available tube. 4.1 Tube Design In order to produce a preliminary tube design, the fundamental principles for the generation of nanobubbles had to be considered: Nanobubbles are generated through cavitation; a consequence of ‘Shear’, which can be achieved through attrition. Attrition can be facilitated through, friction against the internal profile of the tube, acceleration, turbulence and centrifugal forces. This was taken into consideration, leading to the conclusion that tube profiles would be designed to facilitate and maximise shear, whilst optimising acceptable head losses. ‘Shear’ has to be understood and indeed, how to maximise shear: Figure 19 – Development of laminar flow, ‘Fluid Mechanics – Worked Examples for Engineers’, (Schaschke, C., 1998) Figure 19 shows the flow profile for fluid flow within a pipe. The left hand side of the drawing shows the profile for turbulent flow with laminar flow shown on the right hand side. The fluid, (water), may be considered to flow in successive layers, in both the horizontal and vertical planes, where friction exists between each successive layer and each plane. The ‘layers’ become less defined as turbulence increases. Friction between successive layers leads to shear, promoting cavitation and the generation of nanobubbles 60 from the entrained, and/or, injected O2. Water has relatively low viscosity and is discharged from a pump at high velocity thus, the flow will be turbulent however, if we can reduce or minimise the degree of turbulence throughout the nanobubble tube then this will minimise head losses and minimise the disturbance between the successive layers in the flow profile. The intent of Nanobubble Agritech tube designs, is to maximise shear within the oxygen saturated, or enriched, water. This will be achieved by causing the water to rotate as it accelerates, in centrifugal and linear directions through the length of the tube. 4.2 Tube development Nanobubble tubes can potentially be manufactured from a number of materials including glass, fibre glass, aluminium, stainless steel and various grades and types of plastic. The material type will have an effect on performance, (in terms of pressure drop, or head losses); this is due to the surface roughness of the material used and the respective friction factors. Durability, robustness and reliability are also important considerations. These factors were taken into account, remaining mindful of cost, ease of manufacture and the criterion, ‘fit for purpose’. The material had to be a compromise between performance and cost. Polyethylene Terephthalate Glycol, PET or PETG, was selected: It is cheap, durable, light- weight, robust and ideal for 3-D printing. It is also commonly used in the manufacture of plastic drink bottles and containers. 61 Figure 20 - Simultaneous printing of two nanobubble tubes Figure 21 - 3-D Printer and PETG print reel cassettes 62 Figure 22 - Printing of a triple tube design Figure 20 shows a 3-D print in progress. Two nanobubble tubes are being printed simultaneously. The blue polyethylene base provides an anchor and the stability required as the prints increase in length and height. Figure 21 shows the 3-D printer and the PETG cassettes, (print reels). Figure 22 shows a triple tube design. The O2 injection ports and threads for the injectors are approximately 50% complete and can be seen at the surface of the print. The honeycomb or lattice structure of the cross-section will give strength and rigidity to the finished nanobubble tube arrangement, while saving printing time and PETG used for the print. 63 4.3 Profile variations There are a number of variations that can be made to the internal profile of the nanobubble tube and these include: 4.3.1 Number of helixes A helix can be defined as a profile having a 3-dimensional shape which rotates around a central axis. Corkscrews and spiral staircases provide examples of a helix. Fins, or flutes extend from the internal wall of the nanobubble tube toward the centre of the tube forming a helix profile. It is the geometry and dimensions of the helix which will determine the acceleration of the water, the centrifugal force imparted to the water and the pressure drop across the length of the tube, as the water passes through. Figure 23 - Nanobubble tube showing 6 fins 64 Figure 24 - Nanobubble tube showing each helix with 2 full rotations and an increasing twist rate 4.3.2 Helix twist rate – number of full rotations v length The ‘twist rate’ can be defined as the number of full rotations for a given length. Figure 23 shows a nanobubble tube design where each internal flute, forms a helix which makes two full rotations across the length of the tube. The entry point is at the top of the image and the exit point at the bottom. It can be seen from the profile that twist rate increases from the entry point to the exit. A combination of increasing the twist rate and varying the diameter of the internal hollow core, causes the water to accelerate and the velocity to increase, promoting shear. Water entering the tube will accelerate as it travels the length of the tube and will have rotated through 720 0 at the point of exit. 65 4.3.3 Core diameters The core diameter refers to the hollow region extending along the length of the tube, around the centre axis. (This region can be seen in figures 23 and 25). Figure 25 - Image showing internal profile of figure 23, (Nanobubble tube showing 6 fins). Fins and flutes do not extend into this region. A percentage of the water will pass straight through the nanobubble tube, along this axis. Determining the optimum core diameter for a tube design, is an iterative process of development and testing. The optimum core diameter is compromise between the pressure drop across the length of the tube and the effectiveness in the generation of nanobubbles: A larger core diameter results in a lower pressure drop. The advantage of having a lower pressure drop, or head loss, is that the overall energy requirement is less. This suggests that a smaller pump may be selected for the nanobubble generator and the energy costs may be potentially lower. However, the pressure difference between the water at the point of entry and the water at the point of exit, provides the driving force which causes the acceleration and velocity of the water thus, a smaller core diameter favours the generation of nanobubbles. 66 Summarising: A larger core diameter favours energy costs, but results in a lower performance in terms of the nanobubbles generated. A smaller core diameter favours the generation of nanobubbles but does not favour the energy required and the associated costs. Thus, the optimum diameter will be a compromise between acceptable head losses and the DO reading observed at the product tank. 4.3.4 Venturi The internal profile of the nanobubble tube can be designed as a venturi. The core diameter decreases up to the ‘throat region’. The diameter remains constant throughout the length of the throat and increases from the exit point of the throat and continues to increase up to the point of exit from the tube. Figure 26 – Diagram representing a Venturi profile The rationale behind this design is to create high and low pressure regions within the tube. The throat is a restriction to flow. As water approaches this restriction, the pressure increases creating a high pressure region. As water passes through the throat, and exits, a low pressure region occurs within the throat. This pressure reduction leads to cavitation within the water. Azevedo, (2019), showed this experimentally and provided an explanation for his findings: Hydrodynamic, (or Venturi) cavitation occurs as a consequence of 67 depressurisation of a gas-saturated water stream, flowing through a restriction. When the supersaturated water is forced through the restriction, the velocity rises, and the pressure falls below the vapour pressure of the liquid. Cavities (gas nuclei, nanobubbles and microbubbles) form as mass transfer of gas occurs from the water to the gas phase. 4.3.5 Entry Profiles The ‘lead-in’ or entry profile refers to the point of entry to the nanobubble tube, prior to the flow path being made to change direction or deviate. If the flow direction deviates immediately on entry to the nanobubble tube, then the lead-in, or entry profile can be considered upstream of this point. The design intent is to minimise the disturbance to flow at the point of entry. This will reduce head loss and provide the water with a clean entry to the tube. A clean entry allows the water to enter the tube with minimal disturbance caused by physical restriction or deviation to the flow path. Flow straighteners would provide a clean entry to the nanobubble