Theoretical And Experimental Investigation On Oscillation Mechanism Of A Standing Wave Thermoacoustic Engine

As a novel heat-work conversion device, the thermoacoustic engine has advantages of simple structure and environmental friendliness because it has no moving parts and uses inert working gas. The engine due to utilizing industrial waste heat and solar energe becomes more and more attractive to the field of natural gas liquefaction, electronic cooling, and gas separation etc. in recent years. The study on oscillation mechanism of a thermoacoustic engine is of importance for lowering the onset temperature and improving the thermal efficiency of the engine. Because of the complex oscillation flow and heat exchange, and the nonlinear acoustics involved in the thermoacoustic effects, the oscillation mechanism of thermoacoustic effects has not be fully understood. The following theoretical and experimental studies are carried out to reveal the oscillation mechanism of a standing wave thermoacoustic engine:1. Simulation of a standing wave thermoacoustic engine in frequency domain based on thermoacoustic network theory. The onset conditions of a standing wave thermoacoustic engine are studied in frequency domain based on thermoacoustic network theory for further understanding of the oscillation mechanism of thermoacoustic effects. Using the control differential equations of linear thermoacoustic theory, the transfer arrays of the stack with temperature gradient are derived. Based on thermoacoustic network theory, a standing wave thermoacoustic engine is simulated. The calculation results agree well with previous experiments by Arnott, which validate the model for calculating onset frequencies and temperatures. The maximum deviation for frequency is less than1.4%, while for onset temperature difference is about17.9%. The onset temperature differences and frequencies of a self-designed standing wave thermoacoustic engine are calculated for different resonator lengths and charge pressures. The calculated onset temperature differences agree well with the experimental results qualitatively. The deviations between the calculated onset frequencies and the experimental results are2.3-4.9%. For the self-designed standing wave thermoacoustic engine in this thesis, the stack spacing is about2.7times the thermal penetration depth to reach the lowest onset temperature difference. The simulation results are constructive for optimizing the stack spacing in the standing wave thermoacoustic engine.2. Simulation of the onset characteristics in a standing wave thermoacoustic engine in time domain based on thermodynamic analysis. In order to simulate the oscillation process of a standing wave thermoacoustic engine in time domain, a simplified physical model for the standing wave thermoacoustic engine is developed by means of converting the components analogously. The governing differential equations are deduced based on the thermodynamic and fluid dynamic analysis of the various components. The transient frictional coefficients and heat transfer coefficients, which are indispensable for the simulation in time domain, are insufficient in literatures, so most of the previous simulations have to use the formulas for steady flow. In this study, a new method based on linear thermoacoustic theory is proposed, which provides an alternative solution for calculating the pressure drop and heat transfer terms in oscillation flow. The effects of charge pressure on onset temperature with different resonator lengths are obtained. The calculated onset temperatures are in good agreement with the experiments in tendency. The results show that a longer resonator leads to smaller deviations of onset temperatures and frequencies because the effects of the compliance are less remarkable for the longer resonator. When the resonator length is3.1m, the average deviation between calculated onset temperature differences and experiments is4.5%, while the maximum deviation is15.2%. And the frequencies are about3.9%deviating from the experimental results in the same condition. The calculations indicated that the optimal stack spacing was about2.6-2.7times the thermal penetration depth, which is of significant importance for the design and optimization of the standing wave thermoacoustic engine.3. Two-dimensional Computational Fluid Dynamics (CFD) simulation of a standing wave thermoacoustic engine. A self-designed standing wave thermoacoustic engine is numerically simulated using the commercial CFD program FLUENT to understand the distributions and variations of each parameters during onset process. A two-dimension axial symmetric model is developed. Considering the dimension magnitude differences for different parts in the standing wave thermoacoustic engine, the non-conformal grid technology is introduced in the model to decrease the mesh number and improve the calculation efficiency. The pressure evolution processes under the conditions of constant heating temperature and constant heating power are obtained and analyzed, respectively. The effects of temperature distribution in the stack on the performance of the thermoacoustic engine are investigated by using of CFD program. Results show that a higher middle temperature in the stack leads to a lower onset temperature and a higher saturated pressure amplitude, which is of significance for lowering the onset temperature and improving the performance of the thermoacoustic engine.4. Experimental investigation on oscillation characteristics of a standing wave thermoacoustic engine. The onset temperature, the frequency transition characteristics, and the infrared observation on the onset and damping process are experimental investigated on the self-designed standing wave thermoacoustic engine to obtain further understanding on the oscillation characteristics. The onset temperatures and frequencies for different charge pressures and resonator lengths were measured and compared with the above simulation models based on the thermoacoustic network theory and thermodynamic analysis. The frequency transition characteristics were experimentally studied in the standing wave thermoacoustic engine. The magnitude of quality factor is first proposed to be the criterion for frequency transition. Calculations of quality factor agree well with the experiments, which indicate the feasibility of using the quality factor as the criterion of frequency transition in the engine. To study the energy flow in the stack, the stack of the self-designed standing wave thermoacoustic engine was observed using an infrared camera. The axial and radial temperature distributions during the onset and damping processes are quantitatively analyzed. The uneven temperature distribution in radial direction of the stack due to the natural convection is first observed after analyzing the infrared images for different installation angles of the stack. The results indicate that the temperature unevenness in axial direction mainly depends on the charge pressure, and the uneven temperature distribution due to natural convection will gradually disappear after the onset.