Resonator Shape Effect on the Performance of a Standing-Wave Thermoacoustic Heat Engine

Abstract

This thesis demonstrates an attempt to make a design of an about 1-meter-long thermoacoustic heat engine that has an optimum efficiency. This will be done using DeltaEC, software which was developed especially for low amplitude thermoacoustic devices modeling. The optimization process includes geometrical parameters of the resonator tube and the stack, the working fluid, and the heat input to the engine. The present optimization process has shown that slab stacks made of Celcor (a Ceramic material) demonstrated much better performance than other stack shapes and materials. For a 1.1239-meter-long and 0.011 m2 square-shaped resonator tube, a 7.75 cm long slab stack made of Celcor having 0.304 mm-thickplates, spaced by 0.648 mm, giving a porosity ratio of 0.68067, will theoretically convert heat to acoustic power at an efficiency of 30.611% which is equivalent to 47.97% of Carnot’s efficiency. The thesis ends with a brief summary of conclusions. Thermoacoustics is a branch of science concerned mainly with the conversion of heat energy into sound energy and vice versa. The device that converts heat energy in sound or acoustic work is called thermoacoustic heat engine or prime mover and the device that transfers heat from a low temperature reservoir to a high temperature reservoir by utilizing sound or acoustic work is called thermoacoustic refrigerator. There are several advantages of heat engines based on thermoacoustic technology as compared to the conventional ones. These devices have fewer components with at most one moving component with no sliding seals and no harmful refrigerants or chemicals are required. Air or any inert gas can be used as working fluids which are environmentally friendly. Furthermore, the simple design of the devices reduces the fabrication and maintenance costs. However, significant efforts are needed to bring this technology to maturity and develop competitive thermoacoustic devices. The thermoacoustic (TA) procedure uses a sound wave to achieve local heat exchange between the gas in which it propagates and a solid medium. Heat transfer occurs simultaneously along the length of the solid walls of the structure in which the gas is held. A sound wave is the propagation of a disturbance, the passage of which induces a reversible variation in the local physical properties (temperature, pressure) of the medium in which it propagates. It transports energy, but not matter. The propagation medium undergoes macroscopic displacement in the same direction as the propagating wave, and is therefore a longitudinal wave. The pressure wave causes the volumes of gas to oscillate around a mean value. Thus, half-way through the cycle, the gas is on one side of this mean and is compressed and hot, whereas at the end of the cycle, it is on the other side of the mean and is expanded and cold. If a solid medium, such as a metal plate, is used, this solid medium is likely to accumulate heat or to slow heat transfer. During the phases of compression and expansion, heat is exchanged with the wall, generating a difference in temperature between the two ends. In this study, four different resonator shapes are investigated and compared for a thermoacoustic heat engine of 1.12 m length to select the resonator shape that gives the best efficiency of the device. The selected shape will then undergo some changes in the geometrical parameters in order to obtain further performance enhancement.