Numerical modeling of thermoacoustic systems

Abstract

The subject of this thesis is a relatively new class of heat engines and refrigerators, called thermoacoustic (TA) systems. TA systems have gained commercial interest due to their low number of moving parts and potentially high efficiency. In the case of a TA engine, heat is converted to acoustic power. This power can subsequently be converted to electricity using a “reversed” loudspeaker, called a linear alternator. In a TA refrigerator, a speaker or linear alternator is used to generate a strong acoustic wave, which is used to pump heat. To achieve competitive power densities, thermoacoustic systems are generally run at such high amplitudes, that performance deteriorating nonlinear effects can no longer be neglected. To accu- rately predict performance in the nonlinear regime, nonlinear models are required. This thesis describes two contributions to the field of thermoacoustic system modeling. Firstly, a one-dimensional heat transfer model has been developed. This model can be used to estimate the performance of often used parallel-plate heat exchangers for thermoacoustic systems. These heat exchangers are located close to the stack or regenerator of a TA system and are responsible for the heat in/output required to let the system execute its thermodynamic cycle. The results of the model show a good match with a different heat transfer model from the literature, and the model provides guidelines for future heat exchanger design. Secondly, a nonlinear frequency domain method is developed with which the initial transient start-up process can be skipped in the simulations. The method can be used to directly simulate a TA system in its periodic steady-state. This significantly reduces computational cost, since the initial transient regime often involves several hundred oscillation cycles. The method is applied to a one-dimensional nonlinear model of TA systems. The model is used to simulate an experimental standing wave thermoacoustic engine from the literature. The obtained results are in agreement with literature results.