Parametric study on flow streaming for thermoacoustic

Lok, Yong Kheng (2024) Parametric study on flow streaming for thermoacoustic. Project Report. Universiti Teknikal Malaysia Melaka, Melaka, Malaysia. (Submitted)

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Abstract

Thermoacoustics is a study that deals with the conversion of sound energy to heat energy and vice versa. Thermoacoustic systems can be divided into two main types of systems which are engines and refrigerators. Although conventional energy production methods are more efficient than thermoacoustic devices, they often result in pollution. Therefore, further parametric research is needed in thermoacoustics to optimize the system. In this study, a two-dimensional CFD model is employed to assess flow streaming, parameters that may influence system’s performance. Experimental work was also conducted in the laboratory, where anemometers and vibration sensors were used to measure air velocity and wall displacement, respectively. The experimental results were used to validate the CFD models with comparison to theoretical predictions. The CFD model operates at a frequency of 23.6Hz, with the drive ratio of flow and working fluid as the manipulated variable and was solved using the SST k-w turbulence model with and without the activation of the acoustic model as well as the vibrating wall condition. Flow streaming is evaluated based on velocity profiles and vortex shedding pattern. Different velocity profiles are observed at various locations as the fluid oscillates around the stack. Additionally, the study reveals that the effect of vortex shredding and the boundary layer of flow within parallel plates, particularly for helium, is more pronounced compared to air. A thicker boundary layer facilitates better heat penetration to the center of the channel. Moreover, it is observed that cases with higher drive ratios result in thicker vortex layers. The model is further improved with the application of dynamic mesh to the lateral walls to examine the impact of vibration that was measured in the experiment. It is found that in cases with moving walls, the secondary boundary layer of vortex shredding at the end of the plate extends slightly further compared to the normal case. Then, the velocity at location P2 was compared with experimental results, revealing that the case with moving walls showed the lowest percentage difference, indicating a closer match to experimental conditions. Further improvement of the CFD model is done via the inclusion of the Ffowcs-Williams & Hawkings (FW-H) acoustic model to study the Sound Pressure Level (SPL) at three receivers positioned at the inlet, middle, and outlet locations. The best sound prediction was observed at Receiver 3, using an acoustic source configured at the plate wall with velocity-inlet and wall type. Additional observation on variation of vortex shedding over time shows that moving walls created more complex and dynamic vortex patterns with significant fluctuations. Additionally, the vortex structure over 20 phases was analyzed, revealing that moving walls provide a more accurate representation of actual conditions and better insights into vortex shedding. For future research, it is recommended to use helium as a working medium to further understand the fluid dynamics and heat transfer in the thermoacoustic design. Additionally, enhancing sound prediction through different receiver selections, acoustic source configurations, and evaluating vortex shedding variations with helium as the working medium are also suggested.

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