With the increasing environmental challenges, the search for an environmentally benign cooling technology that has simple and robust architecture continues. Thermo-acoustic refrigeration seems to be a promising candidate to fulfil these requirements. In this study, a simple thermo-acoustic refrigeration system was fabricated and tested. The thermo-acoustic refrigerator consists of acoustic driver (loudspeaker), resonator, stack, vacuum system and testing system. The effect of wave patterns and frequency on thermo-acoustic cooling effect was studied. It was found that a square wave pattern would yield superior cooling effects compared to other wave patterns tested

CHEN BAIMAN

MUSHTAK AL-ATABI

YOUSIF A. ABAKR

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Journal of Engineering Science and Technology Vol. 6, No. 3 (2011) 392 - 396 © School of Engineering, Taylor’s University   392 THE INFLUENCE OF WAVE PATTERNS AND FREQUENCY     ON THERMO-ACOUSTIC COOLING EFFECT YOUSIF A. ABAKR1, MUSHTAK AL-ATABI2,*, CHEN BAIMAN1 1Department of Mechanical, Materials and Manufacturing Engineering,                          The University of Nottingham Malaysia Campus, Semenyih, Selangor, Malaysia  2School of Engineering, Taylor’s University, Taylor's Lakeside Campus,                          No. 1 Jalan Taylor's, 47500, Subang Jaya, Selangor DE, Malaysia *Corresponding Author: Mushtak.Talib@taylors.edu.my           Abstract With the increasing environmental challenges, the search for an environmentally benign  cooling  technology  that  has  simple  and  robust  architecture  continues. Thermo-acoustic refrigeration seems to be a promising candidate to fulfil these requirements. In this study, a simple thermo-acoustic refrigeration system was fabricated and tested. The thermo-acoustic refrigerator consists of acoustic driver (loudspeaker), resonator, stack, vacuum system and testing system. The effect of wave patterns and frequency on thermo-acoustic cooling effect was studied. It was found that a square wave pattern would yield superior cooling effects compared to other wave patterns tested. Keywords: Thermo-acoustic, Refrigeration, Frequency, Wave, Vacuum.   1.   Introduction There  is  a  growing  need  for  heat  removal  from  an  increasing  thermal  source density that is a result of the creation of comfortable living environment and the advancements  in  manufacturing  of  fast,  efficient  and  miniaturised  electronic devices.  However,  in  an  age  of  impending  energy  and  environmental  crises, current  cooling  technologies  continue  to  generate  greenhouse  gases  with  high energy costs [1].  One  of  the  promising  innovations  to  mitigate  the  negative  environmental impact  of  the  traditional  cooling  systems  is  the  use  of  the  thermo-acoustic refrigeration  systems  which  generate  cooling  effects  using  a  high-amplitude acoustic standing wave. This concept is the reverse of a phenomena observed for The influence of wave patterns and frequency on Thermo-Acoustic Cooling Effect     393    Journal of Engineering Science and Technology                 June 2011, Vol. 6(3)  centuries by glassblowers who occasionally reported hearing their heated vessels emitting pure tones [2]. The recognition that the thermo-acoustic heat pumping cycle  was  intrinsically  irreversible  made  this  discovery  even  more  significant.  Traditional  heat  engine  cycles,  such  as  the  Carnot  Cycle  typically  studied  in elementary thermodynamics courses, assume that the individual steps in the cycle are reversible. In thermo-acoustic engines, the irreversibility due to the imperfect (diffusive) thermal contact between the acoustically oscillating working fluid and a stationary second thermodynamic medium (the "stack") provides the required phasing.  This  "natural  phasing"  has  produced  heat  engines  which  require  no moving parts other than the self maintained oscillations of the working fluid [3]. The recent growing interest in this technology is attributed to its simple yet robust architecture and its use of environmentally safe gases. The use of inert gases  as  the  thermodynamic  working  fluid  in  thermo-acoustic  refrigerators provides  a  potentially  attractive  alternative  to  the  conventional  refrigeration technology that employs man-made chemicals. In 1979, Ceperley proposed a pistonless Stirling engine without any moving parts [4] using the propagation of acoustical waves through a differentially heated regenerator  which  resulted  in  gas  in  the  regenerator  undergoing  a  Stirling thermodynamic cycle. Yazaki et al. [5] demonstrated a prototype acoustic cooler that  uses  Stirling  cycles  executed  by  a  traveling  wave  with  high  acoustic impedance thermoacoustically induced in a looped tube. The tube has no moving parts,  only  a  pair  of  stacks  sandwiched  between  two  heat  exchangers:  one amplifies the acoustic power and the amplified wave supplies the driving energy to pump heat directly within the second stack. Russell and Weibull [6] built a low cost  tabletop  thermo-acoustic  refrigerator  for  demonstration  purposes.  This thermo-acoustic refrigerator was built from a boxed loudspeaker, acrylic tubing and  sheet,  a  roll  of  35  mm  film,  fishing  line,  an  aluminium  plug,  and  two thermocouples. Temperature differences of more than 15°C were achieved after running the cooler for several minutes. Thermo-acoustic cooling units have been demonstrated aboard the Space Shuttle in 1992 and aboard the USS Deyo (DD 989) in 1995 to cool radar electronics [7]. This  paper  is  aimed  at  exploring  the  basic  principles  of  thermo-acoustic refrigeration  with  special  attention  to  the  influence  of  wave  patterns  and frequency on the effectiveness of the thermo-acoustic cooling effect.  2.   Experimental Setup and Procedure  The  thermo-acoustic  refrigerator  used  in  this  study  is  based  on  the  design proposed by Russell and Weibull [6] and shown schematically in Fig. 1. To construct the set up, a 25 cm long acrylic tube with an inner diameter of 5 cm was glued into a hole at the centre of 15 cm ×15 cm × 1 cm acrylic cover sheet. This sheet was used to securely cover a box containing a 25 cm, 300W loudspeaker. An aluminium plug was milled to fit snugly into the end of the tube to create the closed end. Three thermocouples were used in this study. One was used to measure the room temperature and the other two were placed to the two ends of the stack. A computer was used to provide the output frequency to the loudspeaker through an 180W audio amplifier and display the temperatures history. Fig. 2(a) shows the physical set up while Fig. 2(b) depicts the stack which consists of a large number of 394       Y. A. Abakr et al.                     Journal of Engineering Science and Technology                 June 2011, Vol. 6(3)  closely spaced surfaces aligned parallel to the length of the resonator tube. The stack was made from a 35 mm photographic film role separated by fishing line to allow the air to move along the longitudinal axis of the acrylic tube.   Fig. 1.  (a) Schematic Diagram of the Thermo-acoustic Refrigerator,            (b) Cross Section of the Stack Showing How the Film Layers were Separated by Fishing Line [6].   (a)  (b) Fig. 2. (a) Experimental Setup, (b) The Stack. The experiments were conducted for a frequency range of 365-425 Hz for three  wave  patterns,  namely  sine  wave,  square  wave  and  triangle  wave.  Each experiment was conducted at the same power and frequency level for 300 seconds and the temperature difference across the stack was measured.   The influence of wave patterns and frequency on Thermo-Acoustic Cooling Effect     395    Journal of Engineering Science and Technology                 June 2011, Vol. 6(3)  3.   Results and Discussion Figure 3 shows the maximum temperature differences between the two ends of the stack resulted from the use of different wave patterns, covering a range of frequencies ranging from 365 Hz to 425 Hz.  Fig. 3. Maximum Temperature Differences vs. Frequencies                            for Different Wave Patterns.  The three wave patterns produce their maximum temperature differences at a frequency  of  405  Hz.  The  maximum  temperature  difference  of  28oC  of temperature difference is achieved with square wave, followed by sine wave and triangle wave which were 26oC and 21oC respectively. As a matter of fact, the square wave achieved superior results when compared to the other wave patterns throughout  the  frequency  range  tested.  This  could  be  attributed  to  the  higher power input of the square sound wave as Fig. 4 shows that the square wave has more area under the curve integrated over the same period.           Fig. 4. Square, Sine, and Triangle Waves. 396       Y. A. Abakr et al.                     Journal of Engineering Science and Technology                 June 2011, Vol. 6(3)  4.   Conclusion A  simple  thermo-acoustic  refrigeration  apparatus  was  constructed  and  tested using different wave patterns. A maximum temperature difference of 28oC was achieved  using  square  wave  at  frequency  of  405  Hz.  The  thermo-acoustic refrigeration seems to be a promising technology worth further investigation.  References 1.  Newman, J.; Cariste, B.; Queiruga, A.; Davis, I.; Plotnick, B.; Gordon, M.; and  San  Martín,  S.  (2006).  Thermoacoustic  refrigeration.  GSET  Research Journal, 1-9. 2.  Rayliegh, J.W.S.; and Lindsay, R.B. (1945). The theory of sound. (2nd Ed.), Volume Two, Dover Publication. 3.  Garrett,  S.L.;  Hofler,  T.J.;  and  Perkins,  D.K.  (1993).  Thermoacoustic refrigeration.  Alternative Fluorocarbons Environmental Acceptability Study, Refrigeration  and  Air  Conditioning  Technology  Workshop  Breckenridge Hilton, Breckenridge, CO. 4.  Ceperley, P.H. (1979). A pistonless Stirling engine -The traveling wave heat engine. The Journal of the Acoustical Society of America, 66(5), 1508-1513. 5.  Yazaki, T.; Biwa, T.; and Tominaga, A. (2002). A pistonless Stirling cooler. Applied Physics Letter, 80(1), 157-164. 6.  Russell, D.A.; and Weibull, P. (2002). Tabletop thermoacoustic refrigerator for demonstrations.  American Journal of Physics, 70(12), 1231-1233. 7.  Johnson,  R.A.;  Garrett,  S.L.;  and  Keolian,  R.M.  (2009).  Thermoacoustic cooling for surface combatants. Naval Engineers Journal, 112(4), 335- 345. 

THE INFLUENCE OF WAVE PATTERNS AND FREQUENCY ON THERMO-ACOUSTIC COOLING EFFECT

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