Mumbai University > Mechanical Engineering > Sem 8 > Refrigeration and air conditioning

Marks: 05M

Year: May 2016

Over the past two decades, physicists and engineers have been working on a class of heat engines and compression-driven refrigerators that use no oscillating pistons, oil seals or lubricants. Thermo acoustic devices take advantage of sound waves reverberating within them to convert a temperature differential into mechanical energy or mechanical energy into a temperature differential.

STEVEN L. GARRETT Leading Researcher United Technologies Corporation Professor of Acoustics The Pennsylvania State University.He invented the thermoacoustic refrigerator in the year 1992 and that TAR was used in the space shuttle Discovery(STS-42)

The principle can be imagined as a loud speaker creating high amplitude sound waves that can compress refrigerant allowing heat absorption.The researches have exploited the fact that sound waves travel by compressing and expanding the gas they are generated in.Suppose that the above said wave is traveling through a tube. Now, a temperature gradient can be generated by putting a stack of plates in the right place in the tube, in which sound waves are bouncing around.

Some plates in the stack will get hotter while the others get colder.All it takes to make a refrigerator out of this is to attach heat exchangers to the end of these stacks.

Acoustic or sound waves can be utilized to produce cooling. The pressure variations in the acoustic wave are accompanied by temperature variations due to compression's and expansions of the gas. For a single medium, the average temperature at a certain location does not change. When a second medium is present in the form of a solid wall, heat is exchanged with the wall. An expanded gas parcel will take heat from the wall, while a compressed parcel will reject heat to the wall.

Fig 1. Thermoacustic refrigeration principle.

As expansion and compression in an acoustic wave is inherently associated with a displacement, a net transport of heat results.To fix the direction of heat flow, a standing wave pattern is generated in an acoustic resonator.The reverse effect also exists: when a large enough temperature gradient is imposed to the wall, net heat is absorbed and an acoustic wave is generated, so that heat is converted to work.

Thermoacoustics combines the branches of acoustics and thermodynamics together to move heat by using sound. While acoustics is primarily concerned with the macroscopic effects of sound transfer like coupled pressure and motion oscillations, thermoacoustics focuses on the microscopic temperature oscillations that accompany these pressure changes. Thermoacoustics takes advantage of these pressure oscillations to move heat on a macroscopic level.This results in a large temperature difference between the hot and cold sides of the device and causes refrigeration.