10-08-2013, 12:45 PM
Study of Thermoacoustic Refrigerator
Introduction
From creating comfortable home environments to manufacturing fast and efficient electronic
devices, air conditioning and refrigeration remain expensive, yet essential, services for both
homes and industries. However, in an age of impending energy and environmental crises, current
cooling technologies continue to generate greenhouse gases with high energy
costs.
Thermoacoustic refrigeration is an innovative alternative for cooling that is both clean and
inexpensive. Refrigeration relies on two major thermodynamic principles. First, a fluid‟s
temperature rises when compressed and falls when expanded. Second, when two substances are
placed indirect contact, heat will flow from the hotter substance to the cooler one. While
conventional refrigerators use pumps to transfer heat on a macroscopic scale, thermoacoustic
refrigerators rely on sound to generate waves of pressure that alternately compress and relax the
gas particles within the tube.
Objectives of present study
The study of thermoacoustic refrigeration will lead to study the another unorthodox way of
producing low temperature refrigeration.
It discusses the various components used in
refrigeration and their detailed study. Also, the study allows us to get brief knowledge about the
design of various components of the system.
Acoustic driver
The driver consists of a commercial moving-coil loudspeaker . The choice of the driver is
based on requirements like compactness, lightweight, low losses, and high Bl-factor . A plastic
housing, which originally covered the back of the loudspeaker, was removed so that our
resonance back volume control system could be mounted. The original dome is not rigid enough
to generate the high dynamic pressures required for our experiments. The fabric dome was cut
off near the voice coil and replaced by an aluminium cone which is glued onto the voice coil.
Furthermore, the loudspeaker coil has a diameter of 54 mm, but a resonator with a diameter of 38
mm is used to meet the required cooling power. A rubber rolling diaphragm is used to seal the
driver housing from the resonator. This diaphragm consists of an annular rolled membrane,
which is fixed between the basis plate and the cone . The membrane is clamped between the
plate and the cone. The outer side of the membrane is clamped between two annular aluminium
plates who support the driver and contain the capillary .
Gas spring System
The electroacoustic efficiency of the loudspeaker, defined as the output acoustic power
divided by the input electric power, can be maximized by matching the mechanical resonance
frequency of the driver to the acoustical resonance frequency of the resonator . It was shown that
this shift can be realized by using the volume of gas at the back of the driver housing. This
volume can be varied from the outside of the housing by using a cylinder and a piston (Fig. 1.1).
In this way an extra spring can be added to the loudspeaker. By varying the spring constant, the
mechanical resonance frequency of the loudspeaker can be shifted to match the acoustical
resonance frequency of the resonator. The back volume can be varied by varying the height of
the piston. The gas–spring system consists of a brass cylinder which is mounted on the back of
the driver. By turning a crank, fixed to the lid of the housing, a screw system varies the height of
the piston. The cylinder has an inner diameter of 8 cm and a height of 15 cm and accepts a piston
system mounted on the lid of the driver housing. A second cylinder of diameter 4.5 cm and
height 15 cm can be used instead of the large diameter cylinder whenever a fine tuning is
necessary.
Stack holder
The design and material requirements for the stack, which forms the heart of the refrigerator,
have been discussed elsewhere. To guarantee low thermal conductivity, Mylar material is
chosen. Because of the difficulty of construction and the fragility of the parallel-plate stack, an
approximate geometry is usually used , which consists of winding a long sheet around a rod to
get a spiral stack. The spacing between the layers is realized by fishing line spacers glued on to
the surface of the sheet. The gaps between the layers are considered to approximate the parallel-
plate channels.
Design Strategy
We start by considering the design and optimization of the stack which forms the heart of the
cooler. The coefficient of performance of the stack, defined as the ratio of the heat pumped by
the stack to the acoustic power used by the stack, is to be maximized. The exact theoretical
expressions of the acoustic power and cooling power in the stack are complicated, so one can try
to use the simplified expressions deduced from the short stack, and boundary-layer
approximations . These expressions still look complicated and they contain a large number of
parameters of the working gas, material and geometrical parameters of the stack. It is difficult to
deal in engineering with so many parameters.
Design of Cold heat exchanger
The whole resonator part on the right of the stack, cools down so a cold heat exchanger is
necessary for a good thermal contact between the cold side of the stack and the small tube
resonator. An electrical heater is placed at the cold heat exchanger to measure cooling power.
The length of the heat exchanger is determined by the distance over which heat is transferred by
gas. The optimum length corresponds to the peak-to-peak displacement of the gas at the cold
heat exchanger location.
Conclusion
In this report the manufacturing procedure of a thermoacoustic refrigerator is discussed. The
construction of the different parts of the refrigerator is described in detail. Also how the different
parameters are affecting the design of each component of the system is also discussed in detail.
It also discuss the heart of refrigeration system resonator. Report gives us brief idea about the
performance of refrigeration system.