Investigation of travelling-wave thermoacoustic engines with different configurations

Al-Kayiem, Ali Abbas Hameed (2017) Investigation of travelling-wave thermoacoustic engines with different configurations. PhD thesis, University of Glasgow.

Abstract

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.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Thermoacoustic, travelling wave, by-pass configuration, generator, phase tuning, multi-stage, cooler
• Subjects: T Technology > TJ Mechanical engineering and machinery
• Colleges/Schools: College of Science and Engineering > School of Engineering > Systems Power and Energy
• Supervisor's Name: Yu, Dr. Zhibin
• Date of Award: 2017
• Depositing User: Mr Ali Abbas Hameed Al-Kayiem
• Unique ID: glathesis:2017-8565
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 03 Nov 2017 09:12
• Last Modified: 01 Dec 2017 12:20
• URI: https://theses.gla.ac.uk/id/eprint/8565