Theoretical And Experimental Study On The Onset Mechanism Of Standing-wave Thermoacoustic Systems

Thermoacoustic engine is a novel energy conversion device based on thermoacoustic effects. It has the advantages of simple configuration, high reliability, environmental friendliness due to no moving parts and usage of inert working gases. Therefore, it has been attracting more and more attentions with respect to the promising applications concerning driving pulse tube cryocooler, liquefaction of natural gas, cooling of electronic devices and so on. The study of onset mechanism, which is one of fundamental issues in thermoacoustics, is of significant value for deep understanding the onset process, lowering onset temperature and improving the energy conversion efficiency of thermoacoustic engines. However, up to now the onset mechanism has not been fully explored mainly due to complex nonlinear effects in the onset process and insufficient theoretical study of the practical criteria concerning the system stability. To further understand the onset mechanism of thermoacoustic oscillation, theoretical and experimental research has been performed as follows:1. Developments of a design method of standing-wave thermoacoustic engine based on DeltaE On the basis of studying widely-accepted DeltaE (Design Environment for Low Amplitude ThermoAcoustic Engines) software used to calculate the performance of thermoacoustic systems, a design method of standing-wave thermoacoustic engine is proposed. A double-parameter evaluation method is proposed to evaluate the engine efficiency, which is helpful for achieving higher conversion efficiency. This design method is instructive for thermoacoustic engine design. According to calculation results, a standing-wave thermoacoustic engine setup was designed and constructed. A novel analogous annular stack, which has the comparable performance as the parallel stack, is suggested and tested.2. A new criteria for onset in standing-wave thermoacoustic engine based on circuit network theory On the basis of the change analysis before and after the thermoacoustic engine onset, it is noted that there are two changes occurring when thermoacoustic engine starts to oscillate according to thermoacoustic network theory. One is from passive to active mode and the other is from stable to instable state. By analogy of the activity and stability criterion in the circuit network theory, a new thermoacoustic onset criteria is proposed. The experiment shows that the onset temperatures calculated using these criteria are in reasonable agreement with the experimental results.3. Dynamic equation for the oscillation of standing-wave thermoacoustic engine According to the working principle of standing-wave thermoacoustic engine, the gas in the resonance tube can be regarded as a "gas piston". By analyzing the force condition of this "piston", the dynamic complex equation, which can be used to calculate the onset characteristics of the system quantitatively, is derived under the condition of nonzero displacement of the "gas piston". This equation poses a clear physical meaning and simple expression. The theoretical results show good agreement with experimental ones.4. CFD modeling of the onset process in standing-wave thermoacoustic engine Based on the self-designed standing-wave thermoacoustic engine, a 2-D axially-symmetrical numerical model is developed using non-conformal grid. Two methods of imposing temperature gradient across the stack are studied, and the amplification and saturation process of pressure and velocity are obtained and compared. The flow field distribution near the two ends and the center of the stack are observed in one cycle. In addition, the evolution process of flow vortices is analyzed. This work is helpful for the visualization study of the complex flow distribution in thermoacoustic engines.5. Nonlinear characteristics of onset process in a standing-wave thermoacoustic engine Two typical nonlinear phenomena, i.e. double-threshold effect and second-order onset mode, have been studied experimentally. The results show that the influence of the charging pressure and heating power on the double-threshold onset process is significant. The double-threshold damping process is dependent on the charging pressure and pressure amplitude at the time of damping. According to the experimental results, there are two critical charging pressures separating the onset mode into three sub-modes, that is, one first-order onset mode, one second-order onset mode and one frequency hopping onset mode, respectively. This study will contribute to further understanding of the thermoacoustic oscillation.