Experimental investigations of the performance of a ... - Springer Link

Journal of Mechanical Science and Technology 32 (2) (2018) 929~935 www.springerlink.com/content/1738-494x(Print)/1976-3824(Online)

DOI 10.1007/s12206-018-0143-x

Experimental investigations of the performance of a thermoacoustic refrigerator based on the Taguchi method † K. Augustine Babu1,* and P. Sherjin2 1

Department of Mechanical Engineering, Sri Ramakrishna Institute of Technology, Coimbatore-641010, Tamil Nadu, India Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore-641114, Tamil Nadu, India

2

(Manuscript Received June 3, 2017; Revised November 9, 2017; Accepted November 18, 2017) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract Conventional refrigeration system poses a major threat to the environment due to emission of harmful gases (CFC, HCFC). Hence, there is a need for an alternative. Thermoacoustic refrigeration, an alternative to conventional refrigeration, offers a wide scope for further research. It functions by passing high intensity sound waves through a porous stack or regenerator placed inside a resonator tube. Due to the pressure pulsations and the oscillatory motion of the gas, a temperature gradient is created on either side of the stack. Heat exchangers utilize this cooling. This paper deals with the fabrication of the model of the system and analyzes the performance in terms of temperature difference, by varying the stack material, its position inside the resonator, type of input wave and the frequency of the wave. Optimio zation by design of experiments is also done. A maximum temperature difference of 5.42 C was obtained at the best combination of its parameters. Results obtained from the experiment are in agreement with the results obtained from Taguchi analysis. Keywords: Cooling; Design of experiments; Refrigeration; Stack; Taguchi analysis; Thermoacoustics ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Conventional method of refrigeration plays a major role in the modern world. The process occurs by means of vapour compression, which uses a specific refrigerant. However, the conventional system of refrigeration poses a major threat to the environment by the generation of ozone depleting greenhouse gases. Potential to cause global warming is also high. Thermoacoustic refrigeration is a good alternative as it offers the desired level of cooling without using any harmful substances. Mathewlal, Yadav, Mahamuni and Zink have stressed on the need for alternate refrigeration system and the benefits of using thermoacoustic refrigeration [1-4]. The use of harmless refrigerants like air, helium or any inert gas is environment friendly. Thermoacoustic refrigeration technique offers added advantages of less maintenance cost as there are no moving parts, and hence, there is no requirement of lubrication system. Thermoacoustics is the combination of thermodynamics and acoustics. It involves the study of transfer of heat by using sound waves. Mathewlal et al. designed and fabricated a thermoacoustic model and then tested it [1]. The resonator used was of quarter wavelength type that used air in the ambi*

Corresponding author. Tel.: +91 9500874847 E-mail address: [email protected] † Recommended by Associate Editor Jae Dong Chung © KSME & Springer 2018

ent conditions as the working fluid. While the model was tested, the temperature at the hot end of the stack increased at a faster rate than the temperature at the cold end of the stack. The reduction in the temperature at cold end was retarded because the heat from the hot end of the stack flowed back to the cold side and raised its temperature, instead of dissipating it to the surroundings. Owing to the benefits and ideal potential of thermoacoustic refrigeration system to replace the conventional refrigeration system, efforts are being made to improve the COP of the system. While the focus is on improving the performance, importance is to be given to the resonator geometry, stack, the acoustic driver, the frequency of sound wave and heat exchangers. Yadav and Solanki have stated that the researchers till now have focused on the length of the stack and its position [2]. However, there is a lack of practical systems. Due to the low COP for the systems to date, research is to be made on stacks of different materials and in different gases, in order to find the best combination that will yield a better COP. The length of the resonator and the stack spacing also affects the performance of the refrigerator. Hariharan et al. constructed a thermoacoustic engine, which produced high energy acoustic waves from heat [5, 6]. These sound waves were used to drive a thermoacoustic refrigerator or a cryocooler. They found that as the length of the resonator increases, the difference in the temperature across the stack increases and also that the stack with minimum spacing produces higher

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K. A. Babu and P. Sherjin / Journal of Mechanical Science and Technology 32 (2) (2018) 929~935

temperature difference. The stack is one of the most significant parts of a thermoacoustic system that affects its performance. Alcock et al. investigated the performance of the ceramic substrates used as a stack material in standing wave thermoacoustic refrigeration [7]. The geometric configuration of the stack influences the performance of the system. He stressed that further study on the interdependency between the frequencies and the geometrical parameters is required. An overview of the design of the stack is presented by Bhansali et al. [8]. Thermal properties such as thermal conductivity and the specific heat are vital for the design of stack. Optimal spacing of the stack plates based on the thermal penetration depth and the viscous penetration depth are discussed. A small scale thermoacoustic refrigeration system was fabricated and studied experimentally by Rao et al. [9]. Through his experiment, he found that the temperature variation inside the resonator was under 0.5 oC i.e., temperature remained almost constant, when no stack was placed inside the resonator. When the stack was used, the difference in temperature of 15 oC was obtained across the stack ends after 300 seconds. The maximum difference in temperature was found to occur when the stack is placed at the pressure antinodes. The insulation around the resonator was found to have very little effect on the obtained refrigeration. The methods to improve the performance and the temperature difference were detailed by George [10]. Increasing the length of the resonator caused an increase in the temperature at the hot end of the stack. Use of aluminium plug improved the performance when compared with plastic plug by creating more difference in temperature. The increased temperature difference can be achieved by the use of heat exchangers at the ends of the stack. A stack made of low conductivity material enabled to reduce the heat diffusion across the stack. Optimum spacing of the plates in the stack played a crucial role on the performance of the system. The influence of wave patterns and the frequency on the thermoacoustic cooling effect was investigated by Abakr et al. [11]. Simple thermoacoustic refrigerator system was designed and tested for the effects of wave patterns and frequency on cooling. It was observed that the square wave pattern yielded better results when compared with other wave patterns. Thermoacoustic system has the potential to develop renewable energy systems by the utilization of waste heat or solar energy as stated by Babaei et al. [12]. Mathewlal and Alcock have made it clear that the environmentally friendly attributes and features of the system, the benefit of no moving parts and lubrication have turned the attention of researches towards this field [1, 7]. Global warming potential and ozone depleting potential are also minimized by this method [13]. Though thermoacoustic refrigeration system has wide potential and offers more benefits when compared with the conventional vapour compression refrigeration system. One major factor that limits the implementation of this technique in various applications is the low efficiency and COP of the system.

Fig. 1. Layout of the experimental set-up.

Future developments in the domain may be directed towards improving the efficiency and the COP of the system by varying the resonator geometry, by changing the parameters of the acoustic driver, by using stacks of different materials and geometry, and by varying the working medium.

2. Experimentation The experimental arrangement is as shown in the layout in Fig. 1. The apparatus consists of a function generator which is used to set the resonant frequency. It also serves as a means to set the type of input wave. The function generator is connected with a Cathode ray oscilloscope (CRO) to view the input wave sent into the resonator. From intensity of the waves are increased by means of amplification by amplifier. From the amplifier the waves are sent into the resonator through the loudspeaker. The other end of resonator is closed by an aluminium end plug. The resonant frequency (f) of the input wave is calculated by the following formula. f =

v . 4L

(1)

However, the experiment is repeated with three different frequencies namely, the resonant frequency, one above the resonant frequency and one below the resonant frequency in order to determine the variations in the obtained temperature difference. Spiral stack of 0.7 mm plate spacing is placed inside the resonator. The temperature at the hot end and the cold end of the stack are measured either by using LM-35 (Temperature sensor) connected to an LCD display through an Arduino microprocessor or by means of a thermocouple connected to a temperature indicator. While preparing the stack, two parameters play a major role namely the thermal penetration depth and viscous penetration depth. Thermal penetration depth is the distance up to which the gas molecules involve in thermoacoustic effect. It is given by


Fig. 2. CAD model of the set-up.

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Fig. 4. P-V diagram of the process.

through a resonator at a particular frequency, the pressure pulsations form a standing wave. This causes the oscillatory motion of the gas in the resonator along the axial direction. The combination of the pressure pulsations and oscillatory motion of the gas inside the tube causes heat transfer, when there exists a thermal contact of the gas with a stationary surface i.e., the stack. 3.3 Thermodynamic process involved in thermoacoustic refrigeration Fig. 3. Actual arrangement of the set-up.

dk =

2k

w * r * Cp

.

Thermoacoustic refrigerator follows Brayton cycle. It consists of four steps as shown in Fig. 4. (2)

Viscous penetration depth deals with the losses at the surface of the stack and the walls, which is given by

dv =

2m . w*r

(3)

The CAD model of the set-up is shown in Fig. 2 and the actual arrangement of the prototype is shown in Fig. 3.

3. Methodology 3.1 Formation of standing waves The sound waves of the pre-set frequency are sent into the resonator. As the resonator is closed on the other side, the initial wave gets reflected back into the resonator. Because of this, interference between the original and the reflected wave takes place. This interference may be of two forms; either constructive interference or destructive interference. Constructive interference occurs with waves of same phase while destructive interference occurs with waves of opposite phase. When this process happens inside a closed medium, the waves created are called standing waves. 3.2 Thermoacoustic effect When the sound wave from the acoustic driver is passed

(1) Adiabatic compression When the gas parcel moves along the stack, it gets compressed as there is a reduction in volume when it passes through the stack. (2) Isobaric heat rejection When the gas parcel reaches the other end of the stack, it rejects heat to the surface because of increase in temperature due to compression, causing an increase in the temperature. (3) Adiabatic expansion of gas When gas parcel moves toward the other end of stack, gas has more volume i.e., volume of the resonator. Hence, it undergoes expansion. (4) Isobaric heat absorption Due to expansion of gas, the temperature is reduced and hence, heat is absorbed from stack walls resulting in cooling effect at one of the stack ends. 3.4 Taguchi analysis Optimization by design of experiments is done using Minitab software (2016). Taguchi is a statistical tool used in experimental designing to minimize the variation of noise factors and determine the optimal parameters using Signal to noise (SN) ratio graphs. The main objective is to determine the optimum parameters

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to get maximum temperature difference across the ends of the stack. In the analysis, three different types of quality characteristics can be used namely smaller is better, nominal is best and larger is better. Performance of thermoacoustic refrigerator is based on the temperature difference across the stack, where our objective is to get larger difference and so we selected larger the better quality characteristics for the Taguchi analysis.

4. Results and discussions The experimental set-up was tested at room temperature of 27 oC for different combinations of stack material (Photo film, aluminium foil and corrugated sheet), stack position (50 mm, 150 mm and 250 mm from the aluminium plug end), frequency of input wave (150 Hz, 175 Hz and 200 Hz) and the type of the input wave (Sine, square and triangular wave). These combinations provide information on how the temperature difference varies in each case and to determine the combination that creates the maximum temperature difference. The stack position is the distance measured from the aluminium plug end to the center of the stack. The results are presented in the form of graphs.

Fig. 7. Corrugated sheet stack-150 Hz.

4.2 Results for frequency of 175 Hz


Fig. 8. Photo film stack-175 Hz.

Fig. 5. Photo film stack-150 Hz. Fig. 9. Aluminium foil stack-175 Hz.

Fig. 6. Aluminium foil stack-150 Hz.


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Table 1. Parameters considered for the analysis. Parameters

Level 1

Level 2

Level 3

Stack position from the aluminium plug end (mm) (A)

50

150

250

Stack material (B)

Photo film Aluminium foil

Corrugated sheet

Frequency (Hz) (C)

150

175

200

Wave type (D)

Sine

Square

Triangular

Table 2. Inputs given for the analysis. Test Stack no. position

Stack material

Frequency

Wave type

Temperature difference

A

B

C

D

Response

1

1

1

1

1

3.54

2

1

2

2

2

3.26

3

1

3

3

3

0.47

4

2

1

2

3

4.55

5

2

2

3

1

1.64

6

2

3

1

2

1.21

7

3

1

3

2

2.98

8

3

2

1

3

2.58

9

3

3

2

1

0.89

Fig. 11. Photo film stack-200 Hz.

4.4 Taguchi analysis Fig. 12. Aluminium foil stack-200 Hz.


The difference in temperature between the hot and the cold end of the stack occurs after a certain duration of operation. The final temperature difference was measured after steady state has been attained. A maximum of approximately 5.42 oC was obtained. Better temperature difference occurs only for a particular combination of the parameters. This indicates that the parameters, namely, stack material, position of the stack inside the resonator, wave frequency and type, have an influence on each other and on the refrigeration effect.

The Taguchi analysis was also done. The results obtained are given as above. The Taguchi analysis was performed with the criterion larger is better, because larger temperature difference (Response) is our objective. Main effects plot shows the severity of different levels of the factors on the response (Temperature difference). The plots obtained from the analysis, as shown in Figs. 14 and 15, indicate that the stack position and the frequency have the most significant effect on the response. The maximum value of response in main effect plot for SN ratio helps to select the optimum combination, which is 150 mm stack position(A2), photo film stack (B1), 175 Hz frequency (C2) and square wave (D2). The predicted temperature difference at this condition is 4.5 oC. The interaction plot shown in Fig. 16 gives the relationship between one categorical factor and a continuous response that depends on the value of the second categorical factor. The lines are evaluated to understand how the interactions affect the relationship between the factors and the response. Parallel lines indicate that no relation is present and non-parallel lines indicate the occurrence of interaction. The more non-parallel the lines are, the more is the strength of the interaction. Contour plots are used to explore the relationship between three variables at a time. Generally, there are two predictors and one response variable. The contour plots shown in Figs. 17-19 indicate how each of the parameters when combined

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Fig. 14. Main effects plot for SN ratios.

Fig. 17. Contour plot-1.

Fig. 15. Main effects plot for means.


Fig. 16. Interaction plot.


together affects the temperature difference. The darker regions indicate that more temperature difference can be obtained at the corresponding combinations. The combination of the parameters that produce the maximum temperature difference as predicted by the analysis is found to be in correlation with the experimental results. Also, the predicted temperature difference was almost close to the experimental results in each case of combination of parameters. Thus, instead of conducting many experiments to obtain the best combination of parameters, optimization is possible by design of experiments using Minitab (2016) software that helps to identify the best combination by conducting few experimental runs.

The method of obtaining cooling by this method has many advantages as it is much simple in construction, easy to operate, no moving parts, no wear and tear, no requirement for lubrication, no green house and global warming potential.

5. Conclusion A prototype of the thermoacoustic refrigerator was successfully fabricated and investigated. The set up was tested by varying the main parameters namely, the stack material, the position of the stack inside the resonator, the frequency of the wave and the type of the wave used. From the experiment, it can be concluded that the maximum temperature difference of


5.42 oC occurs for a particular set of combination of these parameters. Optimization by design of experiments using Minitab (2016) software was done. Taguchi analysis gives the optimized combination of the parameters along with the interaction and contour plots. The predicted results obtained from the analysis were found to be nearly close to the actual experimental results.

Nomenclature-----------------------------------------------------------------------f v L δk δv k

⍵ ⍴ μ Cp

: Frequency of the input wave : Velocity of the wave in the medium : Length of the resonator : Thermal penetration depth : Viscous penetration depth : Thermal conductivity : Angular frequency : Density of gas : Gas viscosity : Isobaric molar heat capacity per unit mass

References [1] T. Mathewlal, G. Singh, C. Devadiga and N. Mendhe, Demonstration of thermo acoustic refrigeration by setting up an experimental model, IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) (2014) 29-33. [2] M. Yadav and K. I. Solanki, Effect of working gases and stack material on a thermoacoustic refrigerator – A review, International Journal for Innovative Research in Science & Technology (IJIRST), 1 (11) (2015) 142-144. [3] P. Mahamuni, P. Bhansali, N. Shah and Y. Parikh, A study of thermoacoustic refrigeration system, International Journal of Innovative Research in Advanced Engineering (IJIRAE), 2 (2) (2015) 160-164. [4] F. Zink, J. S. Vipperman and L. A. Schaefer, Environmental motivation to switch to thermoacoustic refrigeration, Applied Thermal Engineering, 30 (2010) 119-126. [5] N. M. Hariharan, P. Sivashanmugam and S. Kasthurirengan, Influence of stack geometry and resonator length on the performance of thermoacoustic engine, Applied Acoustics, 73 (2012) 1052-1058. [6] N. M. Hariharan, P. Sivashanmugam and S. Kasthurirengan, Experimental investigation of a thermoacoustic refrigerator driven by a standing wave twin thermoacoustic prime mover, International Journal of Refrigeration, 36 (2013) 2420-2425. [7] A. C. Alcock, L. K. Tartibu and T. C. Jen, Experimental investigation of ceramic substrates in standing wave thermoacoustic refrigerator, International Conference on Sus-

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tainable Materials Processing and Manufacturing (SMPM 2017), Procedia Manufacturing, 7 (2017) 79-85. [8] P. S. Bhansali, P. P. Patunkar, S. V. Gorade, S. S. Adhav and S. S. Botre, An overview of stack design for thermoacoustic refrigerator, International Journal of Research in Engineering and Technology (IJRET), 4 (6) (2015) 68-72. [9] B. A. Rao, M. P. Kumar and D. S. Rao, Design and experimental study of small-scale fabricated thermo-acoustic Refrigerator, International Journal of Engineering Trends and Technology (IJETT), 4 (9) (2013) 3830-3836. [10] J. George, Loud speaker driven thermo acoustic refrigeration, International Journal of Scientific & Engineering Research, 7 (4) (2016) 465-468. [11] Y. A. Abakr, M. Al-Atabi and C. Baiman, The influence of wave patterns and frequency on thermo-acoustic cooling effect, Journal of Engineering Science and Technology, 6 (3) (2011) 392-396. [12] H. Babaei and K. Siddiqui, Design and optimization of thermoacoustic devices, Energy Conversion and Management, 49 (2008) 3585-3598. [13] N. Yassen, Impact of temperature gradient on thermoacoustics refrigerator, International Conference on Technologies and Materials for Renewable Energy, Environment and Sustainability (TMREES15), Energy Procedia, 74 (2015) 11821191.

Augustine Babu completed his B.E. in Mechanical Engineering from Sri Ramakrishna Institute of Technology, Coimbatore, Tamil Nadu, India in the year 2011. He received his Master’s degree in Thermal Engineering from Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India in the year 2013. He is currently working as an Assistant Professor in Sri Ramakrishna Institute of Technology, Coimbatore, Tamil Nadu, India. His areas of interest include refrigeration and cooling systems, nanofluids and heat exchangers. P. Sherjin completed his B.E. in Mechanical Engineering from Sri Ramakrishna Institute of Technology, Coimbatore, Tamil Nadu, India in the year 2017. He is currently pursuing his Master’s in Thermal Engineering at Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India. His areas of interest include refrigeration and cooling systems, nanofluids and composite materials.