Paper_development And Evaluation Of Thermoacoustic Refrigeration System.docx

Development and Evaluation of Thermoacoustic Refrigeration System PAWAN KUMAR(1MV11ME068) SIDHARTH SAHAY(1MV11ME098) SYED SADIQ SUHEB(1MV11ME104)

ABSTRACT This paper deals with the construction and performance of a thermoacoustic refrigerator. The manufacturing of the different components of the apparatus will be explained along with the reasons for using specific materials. The temperature difference of 4.6K is recorded with the use of Kodak film roll material used for stack construction. The performance of refrigerator is checked with different stack material and different dimensions of stack and resonator tube.

INTRODUCTION Fig 1.(a) shows the schematic diagram of a thermoacoutic refrigerator. Thermoacoustic is a branch of science which deals with conversion of sound energy into heat energy and vice versa. Thermoacoustic refrigerator is the device which converts sound energy into heat energy. It is a robust design consisting of four important parts namely resonator tube, stack, heat exchangers and an acoustic driver. These are arranged in a systematic manner to get efficient performance. Various designs can be made with different materials according to the availability and feasibility.

Fig. 1.(a) Schematic diagram of thermoacoustic refrigeration Fig. 1. (b) Velocity amplitude and pressure amplitude Fig. 1. (c) Temperature variation

Fig. 2 shows the constructive and destructive sine waves. Thermoacoustic refrigeration relies on the principle that sound waves are pressure waves. These sound waves propagate through a medium (preferably inert gas such as helium) which in turn creates molecular collisions. The molecular collisions cause a disturbance in the medium which in turn creates constructive and destructive interference. The constructive interference makes the molecules compress and the destructive interference makes the molecules expand. Due to these two interferences, the molecules of the medium gets compressed and expanded in a cyclic manner as demonstrated in the figure.

Fig.2. Constructive and Destructive sine wave

LITERATURE REVIEW Modern research and development of thermoacoustic systems is largely based upon the work of Rott and later Steven Garrett, and Greg Swift, in which linear thermoacoustic models were developed to form a basic quantitative understanding, and numeric models for computation. Commercial interest has resulted in niche applications such as small to medium scale cryogenic applications. The history of thermoacoustic hot air engines started about 1887, when Lord Rayleigh discussed the possibility of pumping heat with sound. Little further research occurred until Rott's work in 1969. A very simple thermoacoustic hot air engine is the Rijke tube that converts heat into acoustic energy. This device however uses natural convection. Orest Symko at University of Utah began a research project in 2005 called Thermal Acoustic Piezo Energy Conversion (TAPEC). Cool Sound Industries, Inc. is developing an air-conditioning system that uses thermoacoustic technology, with a focus on HVAC applications. The system is claimed to have high efficiency and low costs compared to competing refrigeration technologies, and uses no HFC, no HCFC, and no mechanical compressor.

Q-Drive, Inc. is also engaged in developing thermoacoustic devices for refrigeration, with a focus on cryogenic applications. A radioisotope-heated thermo-acoustic system has been proposed and prototyped for deep space exploration missions by Airbus. The system has theoretical slight advantages over other generator systems like existing thermocouple based systems, or proposed Stirling engine used in ASRG prototype.

METHODOLOGY The following parts are used in the construction of a thermoacoustic refrigerator: 1. Resonator tube, 2. Stack, 3.Heat exchanger, 4. Acoustic driver housing

1. Resonator tube: Fig. 3 shows the types of acrylic tubes used as resonator tubes. Resonator is a hollow tube in which sound wave propagates. All the interference happens inside this tube. The stack and the heat exchangers are placed inside the resonator tube. Its design is important. The tube length is generally half of the wavelength of the sound wave used. In different applications even one-fourth of the wavelength of sound used is considered as length of the resonator tube. The length of resonator tube which is equal to one-fourth of the wavelength is preferable because the losses in this tube are less as compared to the resonator tube which is half the wavelength. The material used in our design is acrylic tube and medium inside the resonator tube is air at atmospheric pressure. Inert gas is generally used as a medium inside the resonator tube but because of project limitation air is used. Total length of resonator tube can be combined with tubes of different diameters. The ratio of diameters used should be 0.54.

Fig.3. Acrylic tubes

Due to time constraint we managed to get acrylic tubes of diameter 32mm and 16mm. We used a PVC reducer to join the two acrylic tubes. The length of resonator tube can be calculated using the formula given below: Cot (k l) = (D1/D2)2 tan(k ( L t – l ) ) Where, k= wave number l= length of the larger diameter tube D1= diameter of large tube D2= diameter of small tube Lt= total length of the resonator The length of large diameter tube is taken as 200mm and by using the above formula we found out that the total length of the resonator tube should be equal to 320 mm.

2. Stack: Stack is the main part of the thermoacoustic refrigerator. Its construction is difficult. Different materials can be used as stack in thermoacoustic refrigerator such as Mylar, corrugated paper (Fig.4.), foam (Fig.5.), ceramics, Kodak film roll (Fig.6.) etc. The stack material should have low thermal conductivity and the heat capacity should be larger than the gas medium which is used in refrigerator. There are many types of stack geometry such as parallel plate stack, circular stack, triangular stack etc. Due to fabrication limitation we used a circular stack whose diameter is equal to the resonator tube’s diameter.

Fig.4. Corrugated paper

Fig.5. Sponge

Fig.6. film roll

Mylar is the best known material that should be used for stack but unfortunately we couldn’t find Mylar sheet because of our project budget constrained so we opted for other materials. The stack is constructed by rolling a corrugated paper. The stack diameter should be equal to the diameter of the resonator tube (larger diameter tube, 32mm). Wheatley has stated that in order not to alter the acoustic field the spacing between the stack’s adjacent plates should be in between 2δk to 4δk. So we took spacing distance as 3δk. 2𝑘

δk=√𝜌𝑚.𝑐𝑝.𝜔 Where, δk= Thermal penetration depth k = Gas thermal conductivity ρm = Gas density Cp= Isobaric heat capacity ω = Angular frequency The spacing between the stack plates should be small enough to ensure that there is heat transfer between the gas and the stack layer and it should also be large enough to allow the acoustic field to pass through the stack.

The blockage ratio should be 0.75 as suggested by Tijani. Blockage ratio can be calculated as follows: B= y0/(y0+l) Where, y0 = Half of the stack plate thickness l = Half of the stack spacing between the adjacent plates

The stack is positioned near the driver housing and the length of the stack and distance of stack from the driver housing is calculated as follows: Lsn = k Ls Where, Lsn = Normalized stack length k= Wave number Ls = Stack length xn =k xs Where, xn= Normalized stack center k= Wave number xs= Stack center Wave number can be found out by using the below relation: k= ω / a Where, ω = Angular frequency a= Sound velocity xh = xs – Ls/2 Where, xh = Distance between acoustic housing and the hot end of stack. As starting with the iteration process we took the frequency as 400 Hz and did the calculations. The speed of sound in air is 344m/s and the both normalized stack center and normalized stack length is taken as 0.22 because the value greater than this is difficult to fabricate. By using the above relation we found out that the length of stack is 30mm and distance between acoustic driver and stack is 15mm.

Fig. 7. Stack details Fig.7. shows the stack details used in the construction of the stack. We constructed stack various stack according to the dimensions required. The corrugated paper stack was constructed by rolling the corrugated paper into a cylindrical shape. The sponge stack was constructed by cutting out sponge in the form of cylinder. The Kodak film roll stack was constructed as described below. We took the Kodak film roll and the thin wire (diameter less than 1mm) and the wire was cut into equal length of 25mm and these wires was pasted on the film roll and the spacing between the wires were kept around 30mm. After pasting the wires throughout the length of film roll, the roll was made in the form of cylinder and was fitted to the resonator tube.

3. Heat Exchanger: Two heat exchangers are used in the thermoacoustic refrigerator. These heat exchangers are placed on each side of the stack and are useful in pumping the heat in and out of the system. The heat exchanger geometry should be like that of stack as it allows the sound wave to propagate without acting as a blockage to the acoustic field. The length of the hot heat exchanger should be twice the length of the cold heat exchanger. The cold heat exchanger takes the heat from the space that has to be cooled down and the hot heat exchanger rejects the heat to the surrounding. We used copper mesh (Fig.8.) and copper wool (Fig.9.) as the heat exchanger as it has high thermal conductivity and it is easily available.

Fig. 8. Copper mesh

Fig. 9. Copper wool

4. Acoustic driver housing: Fig.10. shows the applications used to generate sine waves. The acoustic driver housing consists of a loudspeaker that is connected to amplifier. Sound wave at the required frequency is played with the loudspeaker. The resonator tube is connected to this housing. We used a PVC slip cap to make the speaker housing and Sony loudspeaker is used for providing the acoustic waves. The frequency is set by an android application, FG (frequency generator) and the output of this application is supplied to the loudspeaker by AUX cable via amplifier.

Fig. 10. Sine wave generator application snap-shot

CONCLUSION We were able to make a model of thermoacoustic refrigerator. The ΔT (temperature difference across stack) being little low (4.5K). There can be many modifications done to the set up. One of which being use of other better materials which are more insulating than one used for resonator tube. Another modification can be using a heat sink as the aluminium we used as the heat conductor to the surroundings. This wasn’t enough to dissipate heat as the temperature in the bottom reached room temperature fast. When too much heat is in the system the bottom temperature stays at the surrounding temperature, while area on the top of the tube becomes very hot. So better heat sinks can be used. Thermistors can be used to measure temperatures as they are more reliable. The system can be isolated and better working gas can be used for improving performances.

Future Scope of work :Our recommendations to future investigations into thermoacoustic refrigerators revolve around avoiding the mistakes and failures discovered by this project. Without further testing, there is no way for us to detail what exactly caused the failure to achieve an efficient performance of refrigerator; however, we can elaborate on key potential reasons which should be avoided by subsequent attempts to construct a refrigerator. The first and possibly most crucial problem may have been stack placement. According to the design specifications of Tijani, the resonance frequency used did not match our resonator dimensions, and so the stack heat exchangers were not on a pressure node and anti-node for either side. The copper wool placed at either end of the stack may not have done their job. The wool was intended to carry heat to and from the stack to the outside resonator tubes but we are skeptical that the wool has enough surface area to complete this task due to their small size. Either a new set of larger wools are needed or the entire heat exchanger part of the refrigerator must be redesigned.

BIBLIOGRAPHY      

Environmentally friendly refrigeration with thermoacoustic by Dr Normah Mohd Ghazali, Prof Dr Azhar Abd Aziz. Loudspeaker-driven thermo-acoustic refrigeration by Tijani. Influence of stack geometry and resonator length on the performance Of thermoacoustic engine by N.M. Hariharan, S. Kasthurirengan, P. Sivashanmugam. Construction and performance of a thermoacoustic refrigerator by M.E.H. Tijani. Experiments with a thermoacoustic refrigerator by Vineet Barot. Design and construction of a thermoacoustic refrigerator by Meghan Labounty.