Effect of an alternate stack geometry on the performance of phase-change thermoacoustic standing wave engine

Abstract

Thermoacoustic engines convert thermal energy, conceivably at relatively low temperatures, to mechanical energy in the form of high-amplitude acoustic oscillations. The thermoacoustic engine, a potential green technology for power generation from low-grade heat, has a critical limitation of high onset temperature difference. Wet thermoacoustic engines (WTE) can solve this problem by utilizing phase change heat transfer and hence resulting in devices with low onset temperature difference and increased acoustic power density. However, the method for introducing condensable liquid into wet thermoacoustic engines has yet to be established. Therefore, the current study proposes a modified parallel stack design for wet thermoacoustic engines that incorporates mesh screen packing as a spacer between parallel plates as well as a liquid-retaining component of the device. A prototype of a quarter-wavelength wet thermoacoustic engine is designed, fabricated, and operated using air as the working fluid and water as the condensable liquid at atmospheric operating conditions. The thermoacoustic performance of the modified parallel stack (onset temperature difference, pressure amplitude, generated acoustic power, produced acoustic energy, harmonics ratio, and fundamental frequency) is evaluated for a range of geometrical parameter variations in order to optimize the proposed stack configuration within the parametric space (plate spacing, mesh packing density, mesh number, mesh packing length, and number of mesh layers). The impact of injected water mass in the stack on the thermoacoustic performance of the WTE is also studied. The engine’s performance is examined during complete runs, which include start-up, steady operation, and shutdown. The findings show that when the wet thermoacoustic engines start oscillating at a low temperature difference, they acquire more harmonics. It is found that by blocking the higher harmonics in the quarter wavelength engine, the redesigned stack can aid in the suppression of harmonic losses. Squegging, which is an oscillation that rises up and then dies down with a significantly longer time constant than the oscillation’s fundamental frequency, can occur before or after engine operation, depending on the modified parallel stack’s conversion capability. When mesh packing is added to the parallel stack, onset temperature difference decreases and produced acoustic energy increases dramatically compared to that when only the parallel plate stack is used. At a plate spacing of 2.63 rh/δk of the stack (where rh is the hydraulic radius and δk is the thermal penetration depth), there is a low onset temperature difference and a large acoustic power. The wet thermoacoustic engine produces a high amount of acoustic energy even at a low onset temperature difference of 13◦C at 70% mesh packing density inside the modified parallel stack due to higher condensable vapor and harmonic suppression.

Because of relatively high heat transfer area and low viscous and mass diffusion resistances, Mesh 100 performs best among mesh numbers in the range of 60-200. The acoustic power and the generated acoustic energy is reduced when the mesh packing length decreases, however this has little effect on other parameters. The onset temperature difference increases as the number of layers in the mesh packing is reduced, and the produced acoustic energy reduces, resulting in deterioration of thermoacoustic performance. The direct input of water mass into the stack clogs the channel and reduces performance. Within the parametric space (plate spacing, mesh packing density and mesh number) of the present study, the optimal performance of the wet thermoacoustic engine is found to be achieved a plate spacing of 2.63 rh/δk, 70% mesh packing density, and mesh number 100 of the modified parallel stack. At this configuration of stack, the onset temperature difference, generated acoustic power, and produced acoustic energy are determined to be 13◦C, 28.9 mW, and 9219 mJ, respectively.