Environmental issues are on the rise and becoming more crucial every day. The global demand for energy is exponentially growing. This trend is mirrored in the increasing need for refrigeration, a vital component in various applications and aspects of modern life. Most of the current refrigeration systems use environmentally harmful gases, which emphasizes the importance of exploring environmentally friendly cooling methods. Thermoacoustic refrigeration, as the name implies, involves the conversion of acoustic energy into heat pumping effect. This method presents a significant potential as a green cooling method due to several key advantages. It employs environmentally friendly gases (e.g., air, helium, and argon). Moreover, the system is typically simple to construct, lacking moving components, which enhances its reliability. Additionally, it can harness waste or solar heat to generate the necessary acoustic energy through a thermoacoustic engine (TAE). This coupling eliminates the requirement for electricity and fossil fuels, thereby mitigating CO2 emissions. In this thesis, a standing wave thermoacoustic refrigerator (TAR) is studied in detail using both experiments and computational fluid dynamics (CFD) numerical simulations, where the effect of various parameters have been revealed and subsequently optimized. Firstly, based on several design considerations, an experimental setup was developed to examine a standing wave TAR at low drive ratios, in which the design of loudspeaker was not considered. The experimented stack of 6.9 cm length is 3D printed and has a low thermal conductivity was placed in an acrylic resonator tube that has a length of 55 cm. The performance of the TAR was evaluated based on the temperature difference it achieves. Throughout the experiments, the stack was positioned at five different locations within the resonator, with the frequency altered for each position. The findings indicated that the optimal stack position for achieving the highest temperature difference was closer to the pressure antinode rather than the pressure node. In particular, a normalized stack position ( ) of 0.33 and frequency of 121 Hz achieved the highest temperature difference of 11 K. Secondly, a high-drive ratio system was constructed, where the loudspeaker design was considered to achieve the highest electroacoustic efficiency of the loudspeaker when coupled with the TAR. It was found that the optimum operation can be achieved when matching the resonance frequencies of loudspeaker and resonator tube. However, other operating conditions with relatively high efficiency were possible. The setup achieved a maximum temperature difference of 67 K at drive ratio of 6.4 %, where the stack was positioned at =0.18 from the pressure antinode, which is closer to the pressure antinode than the optimum position of low drive ratio system. Thirdly, numerically, a framework of utilizing both global and local CFD analysis to achieve an accurate simulation of TARs was presented. The approach is to utilize a global computational domain that includes the whole system to identify flow characteristics near the stack. This aided in establishing accurate boundary conditions for the local domain, which focused on a smaller part of the system (i.e., one plate only). The model was validated against our experimental data, demonstrating a high accuracy with a maximum error of 2% at low drive ratios and 5% at high drive ratios. With this validated model, parametric analyses were conducted, exploring various parameters, including stack length, plates’ spacing, blockage ratio, working fluid, mean pressure, and drive ratio. Based on these analyses, the following conditions were identified as offering an efficient operation of the TAR: spacing of 3.3 , blockage ratio of 0.7, normalized stack length of 0.24, and helium as working fluid. The highest of 1.55 was attained at temperature difference of 5 K, where the mean pressure was 101.325 kPa and the drive ratio is 1.5 %. The results indicated that increasing the drive ratio or/and the mean pressure led to a decrease in the coefficient of performance ( ) at low temperature gradients, such as 5 K. However, high drive ratio configurations demonstrated better performance at higher temperature differences, which is suitable for most refrigeration applications. Finally, to showcase the sustainable operation of TARs, the coupling between TAE and TAR, or the so-called Thermoacoustically Driven Refrigerator (TADR) was investigated using CFD. Two TADR configurations were considered: a) Helium with stacks at xn = 0.43 (TAE) and xn = 0.77 (TAR), prioritizing cooling power, achieving 60.9 W at 5 K temperature difference; b) Argon with stacks at xn = 0.25 (TAE) and xn = 0.58 (TAR), favouring efficiency, achieving up to 22.82% overall efficiency at 5 K temperature difference, and a maximum temperature drop of 34 K below ambient (300 K).
| Date of Award | 18 Dec 2023 |
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| Original language | American English |
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| Supervisor | Isam Janajreh (Supervisor) |
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