characteristics of blowout phenomena from a...

ISTP-16, 2005, PRAGUE 16TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA

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CHARACTERISTICS OF BLOWOUT PHENOMENA FROM A THERMOACOUSTIC REFRIGERATION TUBE

Toshitsugu Hara

Nippon Institute of Technology, JAPAN e-mail: [email protected] phone: +81-480-33-7719, fax: +81-480-33-7745

Keywords: thermoacoustics, refrigeration, air-conditioning, acoustic pressure

Abstract Examination of characteristics of blowout phenomena from thermoacoustic refrigeration tube was made experimentally. Air was found to blow out from the tube through a small hole successively even when the inside pressure changed periodically. 1 Introduction

Clorofluorocarbons (CFCs) were found to be destroying the Earth's protective ozone layer, and the production of CFCs has now been prohibited in the world. Alternative compounds such as HCFCs and HFCs have problems associated with their pollution potential, their toxicity and their incompatibility with lubricants. Thermo-acoustic refrigeration has been thought one of new systems which can operate with natural working fluid.

A schematic diagram of a simple, one-quarter wavelength λ/4, thermoacoustic

refrigerator is shown in Figure 1. The loudspeaker at the left sets up the standing wave within the gas-filled tube. Its frequency is chosen so that the loudspeaker excites the fundamental (λ /4) resonance of the tube. At the right-hand end is a stack of plates. The spacing of the stack is made close enough for heat to be easily transferred between the gas and the plate.

Thermoacoustic refrigeration has been investigated and examined from the early 1980s [1, 2]. Wheatley et al. [1] showed phenomena in thermoacoustics and acoustical heat engines. Swift [3] reviewed the fundamentals of thermoacoustic engines, by analysis, intuition and examples Garrett and Hofler [4] showed that thermoacoustics offered COPs competitive edge on conventional refrigerators by using resonant high amplitude sound to pump heat.

The first purpose of this paper is to show the phenomena of air blowing out successively from the thermoacoustic tube even when the

Figure 1 Schematic diagram of thermoacoustic refrigeration tube

gas flow

T"1

T'1

T'2

T"2

wall temperature

gas temperature

tem

pera

ture

X ' X "

stack

Figure 2 Principle of thermoacoustic refrigeration

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Toshitsugu Hara

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inside pressure changes periodically. The second purpose is to measure the air flow rate and temperature difference of blowout air from the tube. The third purpose is to discuss the phenomena air blows out and ambient air flows in the tube simultaneously through the same hole.

The mechanism is visualized in Figure 2. In the first part of this four step cycle, the working fluid (inert gas) is transported along the plate from position X' to position X", and is heated by adiabatic compression from a temperature of T1' to T1". Work in the form of sound was done on the gas parcel. Hence, at its present location (T1">T2"), it is now at a higher temperature than the plate. In the second step, the warmer gas parcel transfers an amount of heat to the plate, and its temperature decreases to that of the plate T2". In the third step, the working fluid is transported back along the plate to position X'', and is cooled by adiabatic expansion to a temperature T2'. This temperature is lower than the original temperature at location X'. In the fourth step, the gas parcel absorbs an amount of heat from the plate. This raises its temperature back to its original value T1'. The net effects of these processes are that the system has completed a cycle that returns it to its original state, and heat has been transported up to a determined temperature gradient by work done in the form of sound. Its performance can be

understood by plotting the cycle for one tiny mass on a T-S diagram, where its cycle can be compared with the Stirling and Carnot cycles. 2 Experimental Apparatus

The resonance tube for the measurement airflow rate and temperature difference was a square duct of 30 mm in diameter and 400 mm long and was placed horizontally, shown in Figure 3. The duct length was equivalent to /4 of the sound wave. It was closed at one end and attached to the acoustic driver at the other end. The acoustic driver was a full-range speaker. The stack was made of a bundle of glass tubes. The glass tubes were 1.2 mm in diameter and 50 mm in length. About 500 pieces of the glass tubes were bound for the stack. The outlet hole through which cooled air blows out was drilled near the cold end of the stack where the pressure amplitude was large. It is shown in Figure 4. For visualization, the duct was made by transparency acrylic plate to be taken pictures by video camera and digital still camera for observing airflow in the tube. 3 Experimental Results and Discussion Experimental result of longitudinal and axial temperature distributions are shown in Figure 5.

Figure 3 Experimental apparatus for measuring flow rate of blowout air

Figure 4 Configuration and location of the nozzle for blowout air


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Figure 5 shows the thermograph results under the electric power input of 20 Watt and frequency of 216 Hz to the audio speaker. In the figure, temperature distribution was indicated by several colors. At the left end of the duct (speaker side), temperature was higher than expected. The highest temperature was taken at the right hand side of the duct. It was just the position of the right side of the stack. The lowest temperature was taken near the left side of the stack. Temperature difference between the lowest temperature and ambient temperature was smaller than the difference of highest temperature and ambient temperature. Thermo-acoustic effect was thought to influence on heating larger than the effect on cooling. Figure 6 shows the photograph of the blowout cooled air from the thermoacoustic tube. Blowout phenomena was seen not periodically but successively although inside pressure changed periodically. The speaker was driven by 30W and 216Hz. The cooled air blew out from the small hole at the left side of the stack. The diameter of the hole was larger, the flow rate was larger and the temperature difference from the ambient temperature was smaller. Air velocity injected from the hole was calculated by air flow rate. Calculated velocity was 5 - 10 m/s. Figure 7 shows the thermograph of the cooled air injection from the thermoacoustic

tube. In order to visualize an injected air temperature, a thin thermally insulated paper was set vertically on the outlet hole. In the figure, cooled air injected upward from the outlet hole is shown in blue color. It means the temperature was lower than ambient temperature shown in green color. Temperature contour calculated from Figure 7 is shown in Figure 8. The lowest temperature was taken at the center of the jet and the temperature was –1.4 degree C from the ambient temperature. After the visualization, flow characteristics of blowout air flow was examined experimentally. Figure 9 shows the relation of the air flow rate and the nozzle(hole) position on the tube. As the nozzle position approached to the right end of the tube, the flow rate increased. However, it decreased at the adjacent to the tube end. Since the stack was located at the position, air had to flow through the stack (a bundle of the small tubes) and resisted. When the nozzle position moved to the speaker side, flow rate decreased and finally it became negative (evacuated). The maximum flow rate was gained at the left (cold) side of the stack.

Figure 5 Longitudinal temperature distribution along the tube (P=20W)

Toshitsugu Hara

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Figure 7 Thermograph of blowout air which was cooled in the tube

Figure 10 shows the relation of the outlet flow rate and acoustic input power when the nozzle(hole) diameter was changed. Figure 10 shows the flow rate of blowout air and electric input power under constant nozzle(hole) diameter. Air flow rate increased with the increase of input power and the diameter of the nozzle. From the figure, outlet flow rate increased with the increase of input power and the nozzle(hole) diameter. However, air flow rate did not linearly increased and finally they gradually saturated when the electric input power increased. Figure 11 shows the relation of the temperature outlet nozzle(hole) diameter changed. In the figure, temperature difference ∆T was defined as follows, ∆T = T - Ta (1) where T is the cooled air temperature adjacent to the cold side of the stack, and Ta is an ambient temperature (or room temperature). From the figure, it was found that the temperature difference decreased when the electric input power increased and nozzle diameter decreased. This means the temperature difference decreased inversely with the increase of the flow rate. Figure 12 shows the temperature difference and flow rate with the nozzle(hole) diameter. When the outlet nozzle diameter increased, the flow rate increased and temperature difference decreased.

Figure 6 Photograph of blowout air from a tube

Figure 8 Temperature contour of blowout air

-1.4 degree C

-0.7

-0.4

0.0

-0.9

tube wall

injedtion tube


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The optimum diameter for the flow rate of blowout air and temperature difference was found to exist. As the refrigeration capacity was define by the product of refrigeration capacity. Experimental apparatus was made tightly for air leak. Air was thought to flow successively in the tube through some opening of the experimental apparatus, since air blew out through the hole form the tube. We could not find any opening of the apparatus. Finally, we found the blow-in air after some experiments. Figure 13 shows the photograph of the particle tracer which was moved by blow-in air in the tube. In the figure, the hole was set at lower side of the tube. The tube inside was seen upside. The polystyrene particle of about 3mm diameter can be seen to be flown from the bottom side of the wall to the upper side. Can air go in and out the same hole simultaneously? Figure 14 shows photograph of the trace polystyrene particle which was inside the tube and vinyl strings which was set outside the tube. We can see the polystyrene particle flew upward and the vinyl strings swung down simultaneously. They did not move periodically but successively. Air flew in and out simultaneously through the same hole. It could be seen the phenomena that air flew in the tube through the center of the hole and inside air blew out through the outer side of the hole simultaneously. 4 Conclusion Visualization of temperature distribution in a thermoacoustic refrigeration tube and some measurement of flow rate of blow-out air were examined experimentally. The following results were obtained.

1) Air which was cooled inside of the thermoacoustic tube blew out automatically and successively through the small hole of the tube.

2) Ambient air flew in the tube through the same hole simultaneously although inside air blew out through the hole.

Figure 9 Flow rate of blow-out air vs. hole position

Figure 10 Flow rate of blow-out air vs. electric power

Figure 11 Temperature difference of blow-out air

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3) The flow rate of outlet air increased with the increase of the electric input power to the acoustic speaker and the increased of the hole diameter.

4) Temperature difference of the blowout air decreased with the increase of the flow rate of the blowout air.

5 Acknowledgments The author acknowledge the support of the students, M. Oyama, K. Kashiwase, H. TAkeda, Y. Nakanishi and T. Sano, their assistance in the experiments and measurements. 5 References [1] Wheatley, J., Hofler, T., Swift, G. W. and Migliori, A., An intrinsically irreversible thermoacoustic heat engine, Journal of Acoustic Society of America, Vol. 74, pp. 153-170, 1983. [2] Wheatly, J., Hofler, T., Swift, G. W. and Migliori, A., Understanding some simple phenomena in thermoacoustics with applications to acoustical heat engines, American Journal of Physics, Vol. 53, No. 2, pp. 147-162, 1985. [3] Swift, G. W., Thermoacoustic engines, Journal of Acoustic Society of America, Vol. 84, No. 4, pp. 1145-1180, 1988. [4] Garret, S. L., and Hofler, T. J. Thermoacoustic refrigeration, ASHRAE Journal, pp. 28-36, 1992. [5] Hara, T. et al., Temperature distribution in a thermoacoustic refrigerator, Proceeding of the Asia-Pacific Conference on the Built Environment, Singapore, Vol. 1, pp. 152-159, 1995. [6] Hara, T. et al., Experimental studies on thermoacoustic refrigeration, Proceeding of the Conference of Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Brussels, Vol. 4, pp. 2451-2457, 1997.

Figure 14 Air blew in and blew out, simultaneously

Figure 12 Air flow rate and temperature difference

Figure 13 Photograph of blow-in air


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[7] Hara, T. Experimental Investigation of Cooled Air Injection in Thermoacoustic Refrigerator, Proc. of the Int’l Conf. Refrigeration, Sydney , 1999.

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