AN ENGINEERING NON-LINEAR MODEL FOR THERMO-ACOUSTIC ENGINES

摘要

A thermoacoustic engine is a device converting thermal energy into high amplitude acoustic waves that can be harvested, for example, to electricity. For the practical application of this technique it is vital to identify optimum design parameters and operating conditions. There are numerous reports and tools based on the application of the well-established linear theory first derived by Rott. This is useful for determining the working frequency and yields a first indication of the amplification potential of a given design, but cannot predict the saturation amplitude that is limited by non-linear loss mechanisms. In this work an engineering approach for estimating the final output power of a device is discussed. It is assumed that the fundamental mode of the device is dominating, neglecting the loss of acoustic energy into the harmonics. The core of the engine (heat exchangers and stack/regenerator) is represented as an amplitude-dependent acoustic two-port in the frequency domain. To close the system the duct network and acoustic load are treated similarly as the core; all parts are then connected to form a low-order acoustic network. One major difficulty is to represent the non-linear losses in the duct network. Here they are lumped and matched to available data in the literature. Starting at a moderate amplitude, the model is then iterated until the amplification is balanced with the losses in the system. At this stage of balance, the saturation pressure is obtained and the final output acoustic power to the acoustic load is found. Subsequently, parameter studies such as frequency sweeps and altering of the phase of the incoming pressure waves are carried out, to note their effect on the system efficiency.