Design and performance assessment for a novel friction smoke generator : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Chemical and Bioprocessing at Massey University, Palmerston North, New Zealand
Friction is one of the methods used to generate smoke for food smoking applications. The
method involves pressing a plank of wood against a spinning wheel, roughened to provide
frictional heating. The heating raises the interface temperature above 240 °C, where
smouldering occurs. The primary objective of this project was to understand the dynamics
of a novel friction smoke generator, designed in a prior project, but optimised here. Subobjectives
included understanding the frictional system and its thermodynamic behaviour,
and preliminary attempts to define the composition of the smoke.
The novel aspect of the design is supplementary heating additional to the heat generated by
friction. This means the interface temperature is less dependent on frictional heating. A
system control strategy was developed to control temperature and force.
Twenty seven experiments were carried out. Nine of them investigated the smouldering
limits without supplementary heating for various pressing forces and sliding speeds. The
other twelve runs were conducted with supplementary heating for 100, 150 and 200 ˚C and
various forces at constant sliding speed. The last six experiments were selected runs from
the previous experiments where smoke was collected for composition analysis.
With no supplementary heating, pyrolysis takes place when the pressing force is ≥49.1 N
and the wheel speed is at ≥2500 rpm. These conditions generate interfacial temperatures
within the pyrolysis range. When the system was heated, the limit where smouldering
starts when 9.81 N and 200 ˚C were applied. Two significant results were obtained. First,
the progression of smouldering, resulted in a low and high wear rate of wood. The shift
between these is proposed to be an endothermic to exothermic transition. Second, the time
to reach this shift is a function of the pressing force and system temperature, becoming
instantaneous at 200°C for forces > 29.4 N. These allowed insight to be gained into the
dynamics of heat and mass transfer during friction smoking. The smoke composition
analysis indicates that controlling the volatiles formation is highly achievable by varying the
smoking conditions (i.e. auxiliary heat, pressing force).
The current design has some limitations, which include uncertainties in the conversion of
electrical to mechanical power, vibration of the wood plank, conduction along the motor
shaft and ingress of air. Recommendations are to address these by placing a thermal break
on the shaft, preventing ambient air ingress into the chamber and adding a torque
transducer. Further study is also recommended on the roughness and design of the friction
wheel, and on scale up.