Experimental Investigations on a Standing-Wave Thermoacoustic Engine
Creators: name: Emam, Mahmoud
Publisher: name: Figshare
Year: 2017
Subjects: subject: Applied Physics
subject: Mechanics
subject: Thermodynamics
subject: Entropy
subject: Mechanical Engineering
subject: FOS: Mechanical engineering

description: <strong>Thermoacoustic engine</strong> is one of the emerging technologies in the field of energy conversion. Thermoacoustic engines convert thermal energy, possibly at relatively low temperatures to mechanical energy in the form of acoustic oscillations of high amplitudes that can drive a thermoacoustic refrigerator or can be converted to electrical energy using a linear alternator. In this work, a prototype of a thermoacoustic engine was built, operated and tested. Sustainable operation of the engine is possible due to the relatively efficient hot and cold heat exchangers that were able to sustain the temperature gradient across the stack.
Both the transient and steady-state performance of the thermoacoustic engine are investigated using variable gas mixtures and using square-cell stacks of variable porosities. The engine performance is studied during full runs consisting of start-up, steady operation, and shutting-down. The steady-state engine performance and wave characteristics are investigated at different heat input values, different gas mixtures, and different stack porosities. Inserts with different patterns and open areas are used inside the resonator to investigate their effect on suppressing the high harmonics of the produced acoustic wave. A numerical model of the current prototype is built using the free simulation software DELTAEC. The model is validated using the experimental data and is then used to obtain more information about the engine performance.
Results show that the used gas mixture, the pressure ratio, and the stack cell dimensions significantly affect the engine power output, the wave dynamic pressure amplitude, and the harmonic content of the wave. Experimental data agrees with DELTAEC calculations in trend, but the values of acoustic power and dynamic pressure are significantly lower than the numerical model calculations. The values of the frequency are within 3% of the measured values.
The transient analysis shows that the dynamic pressure amplitude of the gas reaches a peak after onset and then decreases to a steady value during the run. The onset temperature of the engine is always higher than the temperature at which the system stops producing oscillations while shut-down. The hysteresis characteristics is significantly affected by the thermal conductivity of the gas mixture, the stack cell dimensions, the mean temperature at starting, and the time of the cycle. The heat exchange process at the cold side is enhanced by the existence of the wave.
In this work, several inserts of different thicknesses, open area, positions, and hole patterns are inserted into the resonator to suppress the harmonics. All inserts caused lower harmonic content with respect to the case with no inserts. Inserts of low open area cause the performance to degrade and produce low acoustic powers. Inserts with higher thickness gave better performance. Amongst all inserts used, the highest acoustic power is obtained using an insert with an open area of 50% of the resonator area, a pattern of several distributed holes, a thickness of 3 cm, and at a position of 0.33 of the fundamental wavelength measured from the cold-side blind flange.