Thermoacoustic systems can either generate acoustic work (i.e., p-v work) from thermal energy, or consume acoustic work to transfer heat from low to high temperature sources. They are the so-called thermoacoustic prime movers or heat pumps, essentially acting as the acoustical equivalents of Stirling engines or coolers. If a travelling sound wave propagates through a regenerator with a positive temperature gradient along the direction of sound wave propagation, the gas parcels experience a Stirling-like thermodynamic cycle. As such, thermal energy can be converted to acoustic power. Similar to Stirling engines and thermo-fluidic oscillators, thermoacoustic engines can be externally heated with various heat sources and are capable of utilising low-grade thermal energy such as industrial waste heat and solar thermal energy. Both the simplicity, and even the absence of moving parts of thermoacoustic engines demonstrate that they have the potential for developing low-cost power generators therefore, they have attracted significant research effort for developing coolers or electric generators. The target design principle of a thermoacoustic engine is to maximise acoustic power production within the thermoacoustic core whilst minimising the acoustic losses in the resonator. One of the main issues with current thermoacoustic systems is low efficiency, which is largely attributed to acoustic losses in the resonator and the regenerator. There would be a significant impact on the thermoacoustic field if a suitable travelling wave resonator were developed with the least losses. Despite the different engine configurations for developing these engines, they all work on the same thermodynamic principle, i.e., the Stirling cycle. In this study, the first issue is resolved by employing a by-pass configuration, and the second is addressed by using a side-branched volume technique. The current study focuses on the investigation of looped-tube travelling-wave thermoacoustic engines with a by-pass pipe. The novelty of such a by-pass configuration 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 the resonator, amplifies it and sends it back to it. As the pure travelling wave resonator has very low losses, it requires very little acoustic power to sustain an acoustic resonance. This idea is analogous to children playing on swings, where a small push could sustain the swinging for a long time. The present research demonstrates that travelling wave thermoacoustic engines with such a by-pass configuration can achieve comparable performances with other types of travelling wave thermoacoustic engines which have been intensively researched. According to the results, this type of engine essentially operates on the same thermodynamic principle as other travelling wave thermoacoustic engines, differing only in the design of the acoustic resonator. The looped-tube travelling-wave thermoacoustic engine with a by-pass pipe was then implemented in the design of an engine with a much longer regenerator and higher mean pressure to increase its power density. A thermoacoustic cooler was also coupled to the engine to utilise its acoustic power, allowing evaluation of thermal efficiency. A linear alternator has also been coupled to the tested engine to develop an electric generator. This research additionally addresses the effect of a side-branched Helmholtz resonator to tune the phase in looped- tube travelling wave thermoacoustic engine. This action is performed in order to obtain the correct time-phasing between the acoustic velocity and pressure oscillations within the regenerator, to force gas parcels to execute a Stirling-like thermodynamic cycle, so that thermal energy can be converted to mechanical work (i.e., high-intensity pressure waves). By changing its volume one can change the acoustic impedance at the opening of the Helmholtz resonator, and thus adjust the acoustic field within the loop-tubed engine. It can essentially shunt away part of the volumetric velocity at the low impedance region of the engine, so that the acoustic loss can be reduced within the engine. Both the simulations and the experimental results have demonstrated that the proposed side-branched volume can effectively adjust the acoustic field within the looped-tube engine and affect its performance. There is an optimal acoustic compliance corresponding to the best performance in terms of acoustic power output and energy efficiency when the heating power input is fixed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:726726 |
| Date | January 2017 |
| Creators | Al-Kayiem, Ali Abbas Hameed |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/8565/ |
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