Thermo-acoustic engine pressure Wave: Analysis of working fluid effect

Thermo-acoustic Engines (TAE) utilize the production of acoustic waves to generate mechanical power when a thermal gradient is applied to a stack placed in the resonator of TAE. Owing to non-existence of moving parts that a conventional engine has, TAEs are typically mechanically more efficient and reliable, hence are an important area of research. The thermos-acoustic phenomenon for TAEs is only driven by temperature gradient that induces fluid flow. However, in the previous works related to numerical study of standing wave TAEs, an initial disturbance in the form of pressure gradient has been imposed to generate fluid flow. In this paper, a 2D numerical analysis of a standing wave TAE is performed using computational fluid dynamics (CFD) modeling to capture the pressure fluctuations (without any initial disturbance) with time in the resonator channel in order to assess its thermo-acoustic performance. The results are obtained for pressure variation at specific points and the development of temperature profiles within the resonator. Using the pressure variations, FFT analysis was performed to identify sound pressure levels and resonant frequencies. Also, a sensitivity study has been carried out. The objective is to analyze the pressure wave development under different fluid properties. In this study, equivalent properties of a certain mixture of gases are prescribed to represent a composite working fluid. Two cases are considered i.e. mixture of air and helium and mixture of air and carbon dioxide. The compositions are varied in each case. It is noticed that in He mixtures the onset of pressure wave is quicker than in only air or CO2 mixtures, this due to the higher thermal conductivities. However, when only He is considered there is no pressure wave unlike only air or only CO2 cases due to low molecular weight. Frequency in He mixtures rises as He composition is increased, and the contrary is seen in CO2 mixtures. This is due to the collective consequence of the Cp, thermal conductivity and molecular weight. The study shows how important the thermal properties of the working fluid are for the pressure wave.