REFRIGERATION AND AIRCONDITIONING

SELF STUDY

PREPARED BY BASAVARAJA METI DHEEMANTHA BHAT

THERMOACOUSTIC REFRIGERATION

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Introduction One ordinarily thinks of a sound wave as consisting

only of coupled pressure and position oscillations. In fact, temperature oscillations accompany the pressure oscillations.

In an extremely intense sound wave in a pressurized gas, these thermoacoustic effects can be harnessed.

Whereas typical engines and refrigerators rely on crankshaft-coupled pistons or rotating turbines, thermoacoustic engines and refrigerators have no moving parts.

This simplicity, coupled with reliability and relatively low cost, has the potential of thermoacoustic devices for practical use.

Principle Sound waves travel in a longitudinal fashion. They travel

with successive compression and rarefaction of the medium in which they travel.

This compression and expansion respectively lead to the heating and cooling of the gas.

This principle is employed to bring about the refrigeration effect in a thermoacoustic refrigerator.

When a sound wave is sent down a half-wavelength tube with a vibrating diaphragm or a loudspeaker, the pressure pulsations make the gas inside slosh back and forth. This forms regions where compression and heating take place, plus other areas characterized by gas expansion and cooling.

A thermoacoustic refrigerator is a resonator cavity that contains a stack of thermal storage elements (connected to hot and cold heat exchangers) positioned so the back-and-forth gas motion occurs within the stack.

The oscillating gas parcels pick up heat from the stack and deposit it to the stack at a different location. The device "acts like a bucket brigade" to remove heat from the cold heat exchanger and deposit it at the hot heat exchanger, thus forming the basis of a refrigeration unit.

Components of Thermoacoustic Refrigerator

Acoustic driver Stack Heat exchanger Resonator

LOUDSPEAKER

The loudspeaker, which acts as the driver, sustains acoustic standing waves in the gas at the fundamental resonance frequency of the resonator.

The acoustic standing wave displaces the gas in the channels of the stack while compressing and expanding respectively leading to heating and cooling of the gas.

HEAT EXCHANGER The heat exchangers employed in a

thermoacoustic refrigerator influence the acoustic field created in the resonator.

There are many design constraints such as porosity of the heat exchanger and high heat transfer coefficient for efficiency. Due to these constraints, special kind of heat exchangers are used.

One typical micro channel aluminum heat exchanger is shown below.

Microchannel Aluminium Heat Exchanger

STACK

It is also called as regenerator.

The most important piece of a thermoacoustic device is the stack.

The stack consists of a large number of closely spaced surfaces that are

aligned parallel to the to the resonator tube.

In a usual resonator tube, heat transfer occurs between the walls of cylinder

and the gas.

RESONATOR

This the part of refrigerator which is only there for maintaining the acoustic wave. Because it is a dead volume which causes heat loss and adds bulk, quarter wavelength resonators are preferred over half wavelength

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The figure traces the basic thermoacoustic cycle for a packet of gas, a collection of gas molecules that act and move together.

Starting from point 1, the packet of gas is compressed and moves to the left.

As the packet is compressed, the sound wave does work on the packet of gas, providing the power for the refrigerator.

THERMOACOUSTIC CYCLE

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When the gas packet is at maximum

compression, the gas rejects the heat

back into the stack since the

temperature of the gas is now higher

than the temperature of the stack.

As the packet is compressed, the sound wave does work on

the packet of gas, providing the power for the refrigerator.

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In the second phase of the cycle, the gas is returned to the initial

state. As the gas packet moves back towards the right, the sound

wave expands the gas.

Although some work is expended

to return the gas to the initial state,

the heat released on the top of the

stack is greater than the work

expended to return the gas to the

initial state.

This process results in a net

transfer of heat to the left side of

the stack.

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Finally, in step 4, the packets of

gas reabsorb heat from the cold

reservoir.

The heat transfer repeats and

hence the thermoacoustic

refrigeration cycle.

ADVANTAGES OF TAR

No moving parts for the process, so very reliable and a long life span.

Environmentally friendly working medium (air, noble gas).

Use of simple materials with no special requirements, which are

commercially available in large quantities and therefore relatively cheap.

Also sonic compression or ‘sound wave refrigeration’ uses sound to

compress refrigerants which replace the traditional compressor and need

for lubricants.

ADVANTAGES OF TAR

On the same technology base a large variety of

applications can be covered.

Thermoacoustic refrigeration works best with inert

gases such as helium and argon, which are

harmless, nonflammable, nontoxic, non-ozone

depleting or global warming and is judged

inexpensive to manufacture.

DISADVANTAGES OF TAR

Efficiency: Thermoacoustic refrigeration is currently less

efficient than the traditional refrigerators.

Lack of suppliers producing customized components.

Talent Bottleneck: There are not enough people who have

expertise on the combination of relevant disciplines such as

acoustic, heat exchanger design etc.

APPLICATIONS

Chip cooling

Electronic equipment cooling on navy ships:

this application, a speaker generates sound waves. Again a thermo acoustic pump

is used to provide the cooling.

Upgrading industrial waste heat:

Acoustic energy is created by means of industrial waste heat in a thermo acoustic

engine. In a thermo acoustic heat pump this acoustic energy is used to upgrade

the same waste heat to a useful temperature level.

FUTURE SCOPE

Experimenting with different frequencies and stack placements could yield greater efficiency

Improvements to the resonator tube would involve further research into effects that differently shaped tubes would affect on the thermoacoustic effect

Modeling the acoustic properties by computer simulation and predict efficient tube-frequency combinations .

CONCLUSION

The Thermoacoustic Refrigeration System consists of no moving parts. Hence the maintenance cost is also low. The system is not bulky. It doesn’t use any refrigerant and hence has no polluting effects

Thermo acoustic refrigerators were already being considered for specialized applications, where their simplicity, lack of lubrication and sliding seals.

REFERENCES

https://en.wikibooks.org/wiki/Engineering_Acoustics/Thermoacoustics#/media/File:Poese.jpg

International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 2, Volume 2 (February 2015) Page -160 A Study of Thermoacoustic Refrigeration System

Using porous material for heat transfer enhancement in heat exchanger: Review International Journal of Heat and Technology 31(2) · December 2012

Tabletop thermoacoustic refrigerator for demonstrations Daniel A. Russell and Pontus Weibulla http://www.slideshare.net/Nimalan_I/thermoacoustic-refrigeration

THANK YOU