Investigation of travelling-wave thermoacoustic engines with different configurations

[摘要] Thermoacoustic systems can either generate acoustic work (i.e., p-v work) from thermalenergy, or consume acoustic work to transfer heat from low to high temperaturesources. They are the so-called thermoacoustic prime movers or heat pumps, essentiallyacting as the acoustical equivalents of Stirling engines or coolers. If a travelling soundwave propagates through a regenerator with a positive temperature gradient along thedirection of sound wave propagation, the gas parcels experience a Stirling-likethermodynamic cycle. As such, thermal energy can be converted to acoustic power.Similar to Stirling engines and thermo-fluidic oscillators, thermoacoustic engines can beexternally heated with various heat sources and are capable of utilising low-gradethermal energy such as industrial waste heat and solar thermal energy. Both thesimplicity, and even the absence of moving parts of thermoacoustic enginesdemonstrate that they have the potential for developing low-cost power generatorstherefore, they have attracted significant research effort for developing coolers orelectric generators.The target design principle of a thermoacoustic engine is to maximiseacoustic power production within the thermoacoustic core whilst minimising theacoustic losses in the resonator. One of the main issues with current thermoacousticsystems is low efficiency, which is largely attributed to acoustic losses in the resonatorand the regenerator. There would be a significant impact on the thermoacoustic field ifa suitable travelling wave resonator were developed with the least losses. Despite thedifferent engine configurations for developing these engines, they all work on the samethermodynamic principle, i.e., the Stirling cycle. In this study, the first issue is resolvedby employing a by-pass configuration, and the second is addressed by using a side-branchedvolume technique.The current study focuses on the investigation of looped-tube travelling-wavethermoacoustic engines with a by-pass pipe. The novelty of such a by-passconfiguration is that the by-pass and feedback pipes actually create a pure travelling wave resonator. The engine unit extracts a small amount of acoustic work from theresonator, amplifies it and sends it back to it. As the pure travelling waveresonator has very low losses, it requires very little acoustic power to sustain anacoustic resonance. This idea is analogous to children playing on swings, where a smallpush could sustain the swinging for a long time. The present research demonstrates thattravelling wave thermoacoustic engines with such a by-pass configuration can achievecomparable performances with other types of travelling wave thermoacousticengines which have been intensively researched.According to the results, this type of engine essentially operates on thesame thermodynamic principle as other travelling wave thermoacoustic engines,differing only in the design of the acoustic resonator. The looped-tube travelling-wavethermoacoustic engine with a by-pass pipe was then implemented in the design of anengine with a much longer regenerator and higher mean pressure to increase its powerdensity. A thermoacoustic cooler was also coupled to the engine to utilise its acousticpower, allowing evaluation of thermal efficiency. A linear alternator has also beencoupled to the tested engine to develop an electric generator.This research additionally addresses the effect of a side-branched Helmholtz resonatorto tune the phase in looped- tube travelling wave thermoacoustic engine. This action isperformed in order to obtain the correct time-phasing between the acoustic velocity andpressure oscillations within the regenerator, to force gas parcels to execute a Stirling-likethermodynamic cycle, so that thermal energy can be converted to mechanical work(i.e., high-intensity pressure waves). By changing its volume one can change theacoustic impedance at the opening of the Helmholtz resonator, and thus adjust theacoustic field within the loop-tubed engine. It can essentially shunt away part of thevolumetric velocity at the low impedance region of the engine, so that the acoustic losscan be reduced within the engine. Both the simulations and the experimental resultshave demonstrated that the proposed side-branched volume can effectively adjust theacoustic field within the looped-tube engine and affect its performance. There is an optimal acoustic compliance corresponding to the best performance in terms ofacoustic power output and energy efficiency when the heating power input is fixed.

[发布机构] University:University of Glasgow;Department:School of Engineering

[关键词] Thermoacoustic, travelling wave,by-pass configuration, generator, phase tuning, multi-stage, cooler