Thermoacoustic Technology

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Thermoacoustic technology for future advancement in Engines, Airconditioning & Cryogenics” BNCE Pusad 4/4/2006

Brought to you byRitesh Bhusari

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A PAPER PRESENTATION ON

“Thermoacoustic technology for future advancement in Engines, Airconditioning & Cryogenics”

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ABSTRACT AUTHORS

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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INTRODUCTION Thermoacoustics 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 component of the other. (1) Thermoacoustics as the name suggests is a field, which involves the use of knowledge in both acoustics & thermodynamics. Due to the theoretical complexity of each of these fields on their own, there has been little progress in thermoacoustics, particularly here in India. The numerical complexities of thermoacoustic engines are out weighed by the advantages 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, applications would include, aerospace, industrial & in the third world. Thermoacoustic devises are currently used by high budget industries but are still able to be constructed from smaller budgets. They are silent in operation & will operate from any source of heat, including chemical fuels, solar radiations, waste heat from industrial processes etc.

BASIC THERMOACOUSTIC Thermoacoustics 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 to create sound. (1) Simply put, thermoacoustic effect is the conversion of heat energy to sound energy or vice versa. Utilizing the Thermoacoustic effect, engines can be developed that use heat as energy source & have no moving parts To explain the thermoacoustic effect, consider a high amplitude sound wave in a tube. As the sound wave travels back & forth in the tube, the gas compresses & 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 reverse direction 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 the sound wave is started. The sound wave compresses & heats the gas. As the gas slows to turn around & 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 gas takes heat from the plate, heating slightly & leaving the plate below the gas cooler than it was. So, what has happened is one part of the plate gets cooler, & one part gets hotter. If we stack up many plates atop each other (making sure to leave space for the sound to go through), place the plates of an optimal length in the optimal area of the tube & attach heat exchangers 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 of plates & force one end to be hot & the other cold & put that in a tube, we can create a very loud sound. Thus by using waste heat we could create sound in a tube & use that sound to cool off another part of the tube. A device that creates sound from heat is called a thermoacoustic heat engine. Thermoacoustics is a technology long in search of a non-niche application. The roar of 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 that have potential utility. For example, a tube closed at one end & dipped into liquid nitrogen will make loud sound at the frequency corresponding to a wavelength equal to twice the length of the tube. Conversely, if acoustic energy is used as the prime mover, the tube can be made to cool & be used as a thermoacoustic refrigerator with NO moving parts. Both standing wave & traveling wave tubes are being studied. These devices all operate on the principle 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 in decades. These engines convert thermal energy into electric current at high efficiency. They cost less than one fourth that of photovoltaic cells per peak watt & have applications from pollution frees lawn & garden equipments to automobiles to stationery power generation. They are silent in operation & will operate from any source of heat, including chemical fuels, solar radiations, waste heat from industrial processes etc.

THERMOACOUSTIC ENGINE Oscillatory 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 the moment of greatest rarefaction," & that the oscillatory thermal expansion & contraction could themselves be caused by the acoustic wave under consideration, in a channel with a temperature 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 the temperature over there is higher than the gas parcel, the gas is heated (heating). Then the pressure 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 the structure (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 a gas 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—is accomplished 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 5

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changes in temperature, caused by adiabatic compression & expansion of the gas by the sound pressure & by heat exchange with the solid wall of the channel. A thermodynamic cycle, 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 moves hotward while the pressure is rising & coolward while the pressure is falling. Deliberately imperfect heat exchange between the gas & the solid wall of the channel is required in order to introduce a significant time delay between gas motion & gas thermal expansion/contraction, so that Rayleigh's criterion is met. The imperfect thermal contact results when the characteristic lateral dimension of the channel is one or more thermal penetration depth in the gas at the frequency of the oscillation. The time phasing described above is that of a standing acoustic wave. In standing-wave engines, the process occurs in many channels in parallel, all of which contribute to the acoustic power generation. Such a set of parallel channels, now called a stack, was not added to a Sondhauss tube until the 1960s. This important development allowed filling a large-diameter tube with small channels, creating a large volume of strong thermoacoustic power production, while leaving the rest of the resonator open & relatively low in dissipation. Heat exchangers spanning the ends of the stack are needed 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 to 17 kW & operated at efficiency as high as 18%. (Here, efficiency is the ratio of acoustic power flow rightward out of the ambient heat exchanger to the heater power supplied to the hot heat exchanger by the combustion of natural gas.)

Fig : Powerful standing-wave thermoacoustic engine

Although Rayleigh gave the correct qualitative description of the oscillating thermodynamics that is at the core of standing-wave engines, an accurate theory was not developed derived the wave equation & energy equation for monofrequency sound propagating 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 between the hot heat exchanger & the ambient heat exchanger & contains small channels through which the gas oscillates. The channels must be much smaller than those of the stacks described above—small enough that the gas in them is in excellent local thermal contact with 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 while the 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 for Fig. 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 correct time phasing with respect to the oscillating pressure to meet Rayleigh's requirement for power production. The time phasing described above is that of a traveling acoustic wave, which carries acoustic power from ambient to hot. In contrast to standing-wave engines, acoustic power must be injected into the ambient end of a regenerator in order to create more acoustic power; the regenerator is an amplifier of acoustic power. (This point is important for understanding the cascaded engines described below.) A simple, dead-ended resonator cannot 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 heat exchangers, 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 heat exchanger while the pressure is high & toward the main ambient heat exchanger while the pressure is low. Excellent thermal contact between the gas & the regenerator matrix ensures that Rayleigh's criterion is satisfied as in a Stirling engine, but without moving parts. With a wire screen or parallel-plate regenerator, the engine of Fig. 5 has produced acoustic power of 710 W or 1750 W, respectively, each with an efficiency of 30%. Several mechanisms might convect heat from hot to ambient without creating acoustic power, thereby reducing the engine's efficiency. A thermal buffer tube (Fig. 5) is needed to thermally isolate the hot heat exchanger from ambient- temperature components below. Ideally, a slug of the gas in the axially central portion of a thermal buffer tube experiences adiabatic pressure oscillations & thermally stratified velocity/motion oscillations, so that this slug of gas behaves like an axially compressible, thermally insulating, 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 its moving parts (requiring tight seals between the pistons & their surrounding cylinders) compromise reliability & are responsible for high fabrication costs. The thermoacoustic7

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Stirling hybrid engine has reasonably high efficiency & very high reliability, but the toroidal topology needed is responsible for high fabrication costs, for two reasons: It is difficult to provide flexibility in the toroidal pressure vessel to accommodate the thermal expansion of the hot heat exchanger & surrounding hot parts. Finally, the stack-based standing-wave thermoacoustic engine is reliable & costs little to fabricate, but its efficiency is 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 a combination in which one standing-wave engine & two traveling-wave engines are cascaded in series, as shown in Fig. 6. All three engines will be within one pressure maximum in the standing wave, with the stack at a location where z ~ 5 pa & the regenerators at locations of higher z. The two cascaded regenerator units will provide great amplification of the small amount of acoustic power that will be created by the small stack unit. Only about 20% of the total acoustic power will be created in the stack, so the stack's comparatively 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 has made several thermoacoustic devices & claims efficiencies in the order of 23% of the Carnot efficiency. Efficiencies of 23% of the Carnot are still poor, relative to current mechanical technology. It is hoped that efficiencies of thermoacoustic devises can be improved with further development. Still, thermoacoustic devices have real world applications due to their low maintenance & lack of environmentally harmful gases.

Product Feature Thermo 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 the Thermoacoustic 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 the necessary level. The existing products run until some temperature level & then stop & then start 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 or increasing the sound volume. Minimum moving parts: These engines have no sliding seals & can be built by using few or no moving parts. Since there are no moving parts there is no need to use chemicals as lubricants. This will increase the life span of the product a decrease the maintenance cost.

Application of Thermoacoustic Technology In 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 TAheat 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 into electricity. Ø Upgrading industrial waste heat: Acoustic energy is created by means of industrial waste heat in a TA-engine. In a TA-heat pump this acoustic energy is used to upgrade the same waste heat to a useful temperature level. Ø Thermoacoustic Refrigerator: Acoustic energy is created by heat supplied & used for refrigerating effect.

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CONCLUSION Thermoacoustics has shown how promising a technology it can be. The future may hold some very large uses for thermoacoustics, depending on how industry chooses to respond to the idea of integrating thermoacoustic devices into their products. The research is, however, going to continue regardless of the way industry acts and further advances will be made in the field of applicable thermoacoustics. The future may see thermoacoustic refrigerators dominating the market and thermoacoustic engines powering transportation vehicles. Although they are nothing more than air pressures, sound waves hold the technological power to provide a safe, efficient & clean method of heating, cooling, and running engines. It could be said, with much honesty, that these technological advances are truly 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 Science Number 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 A Thermoacoustic Device”

4.

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

5.

Feng Wu, Chih Wu, Fangzhong Guo, Qing Li and Lingen Chen “Optimization of a Thermoacoustic 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 Heat Exchangers 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 William R. Saunders.

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