Study On The Onset Mechanism Of Thermoacoustic Systems Based On Thermodynamic Stability Analysis

For a thermoacoustic engine, the onset process is a key process deciding whether a thermoacoustic engine functions properly or not, while deducing the onset temperature is of great importance for utilization of low grade energy. So far, the reported research about the onset process focused mainly on the dynamic characteristics, and seldom on the thermodynamic behavior. It should be noted that the thermodynamic instability is the basic precondition of onset process. Thus, in this thesis, Glansdorff-Prigogine (G-P) criterion is introduced to the study on onset mechanism from the viewpoint of non-equilibrium state thermodynamics, i.e., the onset phenomenon is studied by means of analyzing the thermodynamic stability of the thermoacoustic engine system. The present work mainly includes:1. Qualitative analysis about the non-linear non-equilibrium thermodynamic behavior of onset phenomenon in a thermoacoustic engine is performed, which indicates that the evolution of the fluctuation or the perturbation in a steady state dominates the onset process. Thus, G-P criterion for the thermodynamic stability is introduced to the study on the onset process. Further discussions focus on the problems about the application of G-P criterion for the stability analysis, and then the steps of the method are presented. The formula of excess entropy for thermoacoustic engine system, a single component system with ideal-gas model and without chemical reaction, is deduced.2. With a standing-wave thermoacoustic engine as an example, computation module for the excess entropy is coded, and the time evolution of excess entropy and excess entropy production is computed by means of CFD method. The distinct evolution feathers of the stable and unstable cases are analyzed. The difficulty of numerical excess entropy production problem, deriving from the influence of numerical error on the steady state and its stability, is discussed. A technique has been proposed to deal with this difficulty. And based on the technique, we develop a method to analyze the thermodynamic stability by means of G-P criterion, and then the onset temperature is predicted.3. With the above mentioned method, the impact of mean operating pressure on the onset temperature is analyzed for a standing-wave thermoacoustic engine. The computed onset temperature is compared with the experimental data to validate the effectiveness of the developed method.