Browsing by Author "Zou, Qian"
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- ItemCFD modelling of air flow and heat transfer in a ventilated carton : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Agricultural Engineering at Massey University(Massey University, 1998) Zou, QianForced-air cooling is widely adopted for cooling fresh produce in ventilated packages. Air distribution inside the package is therefore an important factor for efficient cooling, since the heat transfer between product and air is largely affected by air motions. Computational fluid dynamics (CFD) provides a sophisticated but economic tool for modelling air flow. A CFD-based mathematical model was developed to simulate air flow patterns in a ventilated apple carton during precooling. The model took account of both laminar and turbulent situations. In the model, air mass, momentum and energy conservation, as well as energy conservation of apples and packaging materials were described by a set of partial differential equations (PDEs) plus boundary conditions. For the turbulent flow a Lam and Bremhorst Low-Reynolds-number k-ε model was introduced to calculate local turbulent eddy viscosity. Two modelling strategies were adopted. In the first approach, the air flow was assumed to be steady-state while the buoyancy force due to natural convention was neglected. Steady-state Navier-Stokes equations were solved first, and the outputs of fluid velocity were then used as input data to solve energy equations. For the second approach, all transport equations were solved simultaneously with consideration of the effect of natural convection on air flow patterns. All together, three air flow scenarios were considered: steady-state laminar flow, steady-state turbulent flow, and unsteady-state laminar flow. The CFD package PHOENICS (CHAM, UK Ltd) was used to solve the set of PDEs. The curvilinear Body-Fitted Coordinates (BFC) grid system was used for mesh generation. The entire grid system had 19964 cells. Five sets of PHOENICS codes were written for the three different flow situations. An additional PHOENICS programme was also used to calculate the heat transfer coefficients on the carton external surfaces. It took much longer time to reach convergence for unsteady-state laminar flow (91 hours) than for steady-state laminar flow (9-14 hours). The predicted flow patterns and temperature profiles were very similar for steady-state laminar and turbulent flows under 0.5 m/s inlet velocity. By comparing predictions for steady-state and unsteady-state laminar flows, effects of natural convection were considered negligible in unsteady-state laminar flow. Thus it was reasonable to adopt the programme for steady-state laminar flow instead of unsteady-state laminar flow because of much less computing time in solving steady-state flow. A trial of apple precooling was conducted in which temperature in the centres of apples in various positions were measured. Good agreement between model predictions and experimental data was obtained in most locations, but fairly large errors were found in the apples near carton inlets and outlets. Further work is required to refine the model and to validate air temperature and velocity predictions.
- ItemA CFD modelling system for air flow and heat transfer in ventilated packing systems during forced-air cooling of fresh produce : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Engineering at Massey University(Massey University, 2002) Zou, QianForced-air cooling is the common method for precooling horticultural produce. Ventilated packaging systems are often used to facilitate cooling efficiency. A computational fluid dynamics (CFD) modelling system was developed to simulate airflow and heat transfer processes in the layered and bulk packaging systems during the forced-air cooling of fresh produce. Airflow and heat transfer models were developed using a porous media approach. The areas inside the packaging systems were categorised as solid, plain air, and produce-air regions. The produce-air regions inside the bulk packages or between trays in the layered packages were treated as porous media, in which the volume-average transport equations were employed. This approach avoids dealing with the situation-specific and complex geometries inside the packaging systems, and therefore enables the development of a general modelling system suitable for a wide range of packaging designs and produce. The calculation domains were discretised with a block-structured mesh system that was referenced by global and local grid systems. The global grid system specifies the positions of individual packages in a stack, and the local grid system describes the structural details inside individual package. The solution methods for airflow and heat transfer models were based on SIMPLER (Semi-Implicit Method for Pressure-Linked equations Revised) method schemes, and the systems of linear algebraic equations were solved with GMRES (Generalised Minimum Residual) method. A prototype software package CoolSimu was developed to implement the solution methods. The software package hid the core components (airflow and heat solvers) from user, so that the users without any knowledge of CFD and heat transfer can utilise the software to study cooling operations and package designs. The user interaction components in CoolSimu enable users to specify packaging systems and cooling conditions, control the simulation processes, and visualise the predicted airflow patterns and temperature profiles. When the predicted and measured product centre temperatures were compared during the forced-air cooling of fresh fruit in several layered and bulk packaging systems, good agreements between the model predictions and experimental data were obtained. Overall, the developed CFD modelling system predicted airflow patterns and temperature profiles with satisfactory accuracy.