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Thermoacoustictechnology forfutureadvancement inEngines, Air-conditioning &Cryogenics”

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A

PAPER PRESENTATION

ON

“Thermoacoustic technology for future

advancement in Engines, Air-

conditioning & Cryogenics”

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ABSTRACTAUTHORS

NILESH N. KUNKULOL UMESH H. MALI

B. E. Mech. B. E. Mech.

TITLE - “Thermoacoustic technology for future advancement in refrigeration,

air-conditioning & cryogenics”

As conventional energy sources are limited using them efficiently is

very much necessary. Heat, the most degradable form of energy, is generated

by various ways. Converting available heat efficiently is the main focus in

today’s era. The objective of this seminar is to demonstrate the thermoacoustic

phenomenon, which uses heat to initiate the oscillation of gas without moving

parts. The new breakthrough in this heat conversion field is thermoacoustic

heat engine. This engine can convert heat in to useful work with maximum

efficiency. These engines will serve as source of energy in the new

millennium.

This literature critically examines the above aspects of Thermoacoustic

engines.

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INTRODUCTIONThermoacoustics can be simply defined as the physics of the interaction of thermal

& acoustic fields, especially in the form in which one gives rise to a significant componentof the other. (1)

Thermoacoustics as the name suggests is a field, which involves the use ofknowledge in both acoustics & thermodynamics. Due to the theoretical complexity of eachof these fields on their own, there has been little progress in thermoacoustics, particularlyhere in India. The numerical complexities of thermoacoustic engines are out weighed by theadvantages of using the phenomenon. Thermoacoustic devices in operation are "low tech"devices, which have no moving parts & hence should require low maintenance.

This makes the potential for their application desirable in many fields, applicationswould include, aerospace, industrial & in the third world. Thermoacoustic devises arecurrently used by high budget industries but are still able to be constructed from smallerbudgets. They are silent in operation & will operate from any source of heat, includingchemical fuels, solar radiations, waste heat from industrial processes etc.

BASIC THERMOACOUSTICThermoacoustics is the study of the thermoacoustic effect & the attempt to harness

the effect as a useful heat engine. A thermoacoustic prime mover (engine) uses heat tocreate sound. (1)

Simply put, thermoacoustic effect is the conversion of heat energy to sound energyor vice versa. Utilizing the Thermoacoustic effect, engines can be developed that use heat asenergy source & have no moving parts To explain the thermoacoustic effect, consider a highamplitude sound wave in a tube. As the sound wave travels back & forth in the tube, the gascompresses & expands (that's what a sound wave is). When the gas compresses it heats up& when it expands it cools off. The gas also moves back & forth, stopping to reversedirection at the time when the gas is maximally compressed (hot) or expanded (cool). (1)

Now, put a plate of material in the tube at the same temperature as the gas before thesound wave is started. The sound wave compresses & heats the gas. As the gas slows to turnaround & expand, the gas close to the plate gives up heat to the plate. The gas cools slightly& the plate below the hot gas warms slightly. The gas then moves, expands, & cools off,becoming colder than the plate. As the gas slows to turn around & expand, the cool gastakes heat from the plate, heating slightly & leaving the plate below the gas cooler than itwas.

So, what has happened is one part of the plate gets cooler, & one part gets hotter. Ifwe stack up many plates atop each other (making sure to leave space for the sound to gothrough), place the plates of an optimal length in the optimal area of the tube & attach heatexchangers to get heat in & out of the ends of the plates.

Even more spectacular is the fact that it can work in reverse. If we have a stack ofplates & force one end to be hot & the other cold & put that in a tube, we can create a veryloud sound. Thus by using waste heat we could create sound in a tube & use that sound tocool off another part of the tube. A device that creates sound from heat is called athermoacoustic heat engine.

Thermoacoustics is a technology long in search of a non-niche application. The roarof a jet engine is a thermoacoustic phenomenon. While many thermoacoustic events are

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simply incidental to some other occurrence, there are applications of thermoacoustics thathave potential utility. For example, a tube closed at one end & dipped into liquid nitrogenwill make loud sound at the frequency corresponding to a wavelength equal to twice thelength of the tube. Conversely, if acoustic energy is used as the prime mover, the tube canbe made to cool & be used as a thermoacoustic refrigerator with NO moving parts. Bothstanding wave & traveling wave tubes are being studied. These devices all operate on theprinciple that the compression & rarefaction of gas (air & other gases & gas mixtures)causes heating & cooling of the gas as defined by the gas equation of state. This heating &cooling & the expansion & contraction that accompany it can be used to drive devices. (2)

This technology is the first new breakthrough in thermal energy conversion indecades. These engines convert thermal energy into electric current at high efficiency. Theycost less than one fourth that of photovoltaic cells per peak watt & have applications frompollution frees lawn & garden equipments to automobiles to stationery power generation.They are silent in operation & will operate from any source of heat, including chemicalfuels, solar radiations, waste heat from industrial processes etc.

THERMOACOUSTIC ENGINEOscillatory thermal expansion & contraction of a gas could create acoustic power "if

heat be given to the air at the moment of greatest condensation, or be taken from it at themoment of greatest rarefaction," & that the oscillatory thermal expansion & contractioncould themselves be caused by the acoustic wave under consideration, in a channel with atemperature gradient. (2)

The gas is being compressed by the passing pressure wave (compression).Successively the gas parcel is moved to a hotter part of the regenerator. Since thetemperature over there is higher than the gas parcel, the gas is heated (heating). Then thepressure wave that first compressed the gas parcel is now expanding it (expansion). Finally,the gas parcel is moved back to its original position. The parcel of gas is still hotter than thestructure (regenerator) resulting in heat transfer from the gas to the structure (Cooling).

Fig : In a Stirling engine (left), two pistons oscillating with the correct relative time phasing carry agas in two heat exchangers & a regenerator through a cycle of pressurization, motion from ambient to hot,depressurization, & motion from hot to ambient.

STANDING-WAVE ENGINE (2)Rayleigh's criterion for spontaneous thermoacoustic oscillation that heat should flow

into the gas while its density is high & out of the gas while its density is low—isaccomplished in the Sondhauss tube & in other standing-wave engines

As a typical parcel of the gas oscillates along the axis of the channel, it experiences

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changes in temperature, caused by adiabatic compression & expansion of the gas by thesound pressure & by heat exchange with the solid wall of the channel. A thermodynamiccycle, with the time phasing called for by Rayleigh, results from the coupled pressure,temperature, position, & heat oscillations.

The time phasing between gas motion & gas pressure is such that the gas moveshotward while the pressure is rising & coolward while the pressure is falling. Deliberatelyimperfect heat exchange between the gas & the solid wall of the channel is required in orderto introduce a significant time delay between gas motion & gas thermalexpansion/contraction, so that Rayleigh's criterion is met. The imperfect thermal contactresults when the characteristic lateral dimension of the channel is one or more thermalpenetration depth in the gas at the frequency of the oscillation. The time phasing describedabove is that of a standing acoustic wave.

In standing-wave engines, the process occurs in many channels in parallel, all ofwhich contribute to the acoustic power generation. Such a set of parallel channels, nowcalled a stack, was not added to a Sondhauss tube until the 1960s. This importantdevelopment allowed filling a large-diameter tube with small channels, creating a largevolume of strong thermoacoustic power production, while leaving the rest of the resonatoropen & relatively low in dissipation. Heat exchangers spanning the ends of the stack areneeded for efficient delivery & extraction of the large amounts of heat needed by a stack..Figure 3 shows a recent example of such an engine, which produced acoustic powers up to17 kW & operated at efficiency as high as 18%. (Here, efficiency is the ratio of acousticpower flow rightward out of the ambient heat exchanger to the heater power supplied to thehot heat exchanger by the combustion of natural gas.)

Fig : Powerful standing-wave thermoacoustic engine

Although Rayleigh gave the correct qualitative description of the oscillatingthermodynamics that is at the core of standing-wave engines, an accurate theory was notdeveloped derived the wave equation & energy equation for monofrequency soundpropagating along a temperature gradient in a channel.

TRAVELING-WAVE ENGINES (2)In Stirling engines & traveling-wave engines, the conversion of heat to acoustic

power occurs in the regenerator, which smoothly spans the temperature difference betweenthe hot heat exchanger & the ambient heat exchanger & contains small channels throughwhich the gas oscillates. The channels must be much smaller than those of the stacksdescribed above—small enough that the gas in them is in excellent local thermal contactwith their walls. A solid matrix such as a pile of fine-mesh metal screens is often used.Proper design causes the gas in the channels to move toward the hot heat exchanger whilethe pressure is high & toward the ambient heat exchanger while the pressure is low, as

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shown in Fig. 4 (cf. "while...rising" & "while...falling" in the standing- wave description forFig. 3). Hence, the oscillating thermal expansion & contraction of the gas in the regenerator,attending its oscillating motion along the temperature gradient in the pores, has the correcttime phasing with respect to the oscillating pressure to meet Rayleigh's requirement forpower production.

The time phasing described above is that of a traveling acoustic wave, which carriesacoustic power from ambient to hot. In contrast to standing-wave engines, acoustic powermust be injected into the ambient end of a regenerator in order to create more acousticpower; the regenerator is an amplifier of acoustic power. (This point is important forunderstanding the cascaded engines described below.) A simple, dead-ended resonatorcannot provide the ambient power injection, so an ambient piston or toroidal resonator (Fig.5) is necessary.

The conversion of heat to acoustic power occurs in the regenerator between two heatexchangers, which are structurally & functionally similar to those of a Stirling engine.Proper design of the acoustic network (including, principally, the feedback inertance &compliance) causes the gas in the channels of the regenerator to move toward the hot heatexchanger while the pressure is high & toward the main ambient heat exchanger while thepressure is low. Excellent thermal contact between the gas & the regenerator matrix ensuresthat Rayleigh's criterion is satisfied as in a Stirling engine, but without moving parts. With awire screen or parallel-plate regenerator, the engine of Fig. 5 has produced acoustic powerof 710 W or 1750 W, respectively, each with an efficiency of 30%.

Several mechanisms might convect heat from hot to ambient without creatingacoustic power, thereby reducing the engine's efficiency. A thermal buffer tube (Fig. 5) isneeded to thermally isolate the hot heat exchanger from ambient- temperature componentsbelow. Ideally, a slug of the gas in the axially central portion of a thermal buffer tubeexperiences adiabatic pressure oscillations & thermally stratified velocity/motionoscillations, so that this slug of gas behaves like an axially compressible, thermallyinsulating, oscillating piston.

Fig - Thermoacoustic-Stirling hybrid engine, producing 1 kW of power at an efficiency of 30%without moving parts. The E's show the circulation & flow of acoustic power.

CASCADED STANDING-WAVE AND TRAVELIN-WAVE ENGINES (2)None of the systems described thus far provides high efficiency & great reliability &

low fabrication costs. For example, the traditional Stirling engine has high efficiency, but itsmoving parts (requiring tight seals between the pistons & their surrounding cylinders)compromise reliability & are responsible for high fabrication costs. The thermoacoustic-

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Stirling hybrid engine has reasonably high efficiency & very high reliability, but thetoroidal topology needed is responsible for high fabrication costs, for two reasons: It isdifficult to provide flexibility in the toroidal pressure vessel to accommodate the thermalexpansion of the hot heat exchanger & surrounding hot parts. Finally, the stack-basedstanding-wave thermoacoustic engine is reliable & costs little to fabricate, but its efficiencyis only about 2/3 that of a regenerator based system.

Hoping to enjoy the best features of all these systems, we have begun to build acombination in which one standing-wave engine & two traveling-wave engines arecascaded in series, as shown in Fig. 6. All three engines will be within one pressuremaximum in the standing wave, with the stack at a location where z ~ 5 pa & theregenerators at locations of higher z. The two cascaded regenerator units will provide greatamplification of the small amount of acoustic power that will be created by the small stackunit. Only about 20% of the total acoustic power will be created in the stack, so the stack'scomparatively low efficiency will have a small impact on the entire system's efficiency.

Fig. : A cascade of one stack & two regenerators, with the necessary adjacent heat exchangers &intervening thermal buffer tubes, should provide high efficiency in a simple, reliable package.

The performance of our engine will be judged by its output efficiency. G. Swift hasmade several thermoacoustic devices & claims efficiencies in the order of 23% of theCarnot efficiency. Efficiencies of 23% of the Carnot are still poor, relative to currentmechanical technology. It is hoped that efficiencies of thermoacoustic devises can beimproved with further development. Still, thermoacoustic devices have real worldapplications due to their low maintenance & lack of environmentally harmful gases.

Product FeatureThermo acoustic engine has some specifications that meet most of these

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requirements & some others can easily be added to it.Competitively efficient: Although the efficiency is not the main criteria of theThermoacoustic Engine, a device that can compete with the existing products is possible.This is supported with the adjustable temperature control system.Adjustable temperature control: Enables the consumers to control the temperature at thenecessary level. The existing products run until some temperature level & then stop & thenstart again when the temperature gets too high. This decreases the efficiency of the engine.Temperature control with the thermo acoustic devices can be done by simply decreasing orincreasing the sound volume.Minimum moving parts: These engines have no sliding seals & can be built by using fewor no moving parts. Since there are no moving parts there is no need to use chemicals aslubricants. This will increase the life span of the product a decrease the maintenance cost.

Application of Thermoacoustic TechnologyIn principle there is a large variety of applications possible for Thermoacoustic

engines. Below, some concrete examples are given of possible applications:Ø Liquefaction of natural gas: TA-engine generates acoustic energy. This acoustic

energy is used in a TA-heat pump to liquefy natural gas.Ø Chip cooling: In this case a piezo-electric element generates the sound wave. A TA-

heat pump cools the chip.Ø Electricity from sunlight: Concentrated thermal solar energy generates an acoustic

wave in a heated TA-engine. A linear motor generates electricity from this.Ø Cogeneration (combined heat and power): A burner heats a TA-engine, therewith

generating acoustic energy. A linear motor converts this acoustic energy intoelectricity.

Ø Upgrading industrial waste heat: Acoustic energy is created by means ofindustrial waste heat in a TA-engine. In a TA-heat pump this acoustic energy is usedto upgrade the same waste heat to a useful temperature level.

Ø Thermoacoustic Refrigerator: Acoustic energy is created by heat supplied & usedfor refrigerating effect.

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CONCLUSION

Thermoacoustics has shown how promising a technology it can be. The future mayhold some very large uses for thermoacoustics, depending on how industry chooses torespond to the idea of integrating thermoacoustic devices into their products. The researchis, however, going to continue regardless of the way industry acts and further advances willbe made in the field of applicable thermoacoustics. The future may see thermoacousticrefrigerators dominating the market and thermoacoustic engines powering transportationvehicles. Although they are nothing more than air pressures, sound waves hold thetechnological power to provide a safe, efficient & clean method of heating, cooling, andrunning engines. It could be said, with much honesty, that these technological advances aretruly the "wave" of the future.

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REFERENCES

1. G.W. Swift: "Thermoacoustic engines. J. Acoust. Soc.Am."G.W. Swift: "Thermoacoustic engines and refrigerators" Los Alamos ScienceNumber 21 1993.

2. Scott Backhauss & G.W. Swift: "Thermoacoustic engines. J. Acoust. Soc.Am."G.W. Swift: "New varieties Of Thermoacoustic engines" LA-UR-02-2721

3. Owen Lucas and Karel Meeuwissen “Design And Construction Of AThermoacoustic Device”

4. M.E.H. Tijani, S. Spoelstra, P.W. Bach “Thermal-Relaxation Dissipation InThermoacoustic Systems” ECN-RX--03-054

5. Feng Wu, Chih Wu, Fangzhong Guo, Qing Li and Lingen Chen “Optimization of aThermoacoustic Engine with a Complex Heat Transfer Exponent” Entropy 2003, 5,444-451

6. Insu Paek, James E. Braun, and Luc Mongeau “Heat Transfer Coefficients of HeatExchangers in Thermoacoustic Coolers” ICR0568

7. Jay A. Adeff, Thomas J. Hofler “Design & Construction of a Solarpowered,Thermoacoustically Driven, Thermoacoustic Refrigerator” 43.10.Ln, 43.35.Ud[Heb]

8. "A Thermoacoustic Characterization of a Rijke-type Tube Combustor" By Dr.William R. Saunders.

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