Based on the results of fluid dynamics, heat transfer and acoustics, a Computational Fluid Dynamics （CFD） method was utilized to study the acoustic characteristics and self-excited pulsation mechanism inside a Rijke pipe. To avoid settling the irrational boundary conditions of the finite-amplitude standing wave in the Rijke thermo-acoustic system, the simulation modeling in the flow field, which coupled the inner of pipe with its outer space, was carried out to replace the traditional way in form of internal flow field numerical investigations. A hypothesis for heat source in energy equation including the relationship on unsteady heat of air around heat source, oscillation pressure and oscillation velocity was presented. To reflect the essence of Rijke pipe, simulation on self-excited oscillation was conducted by means of its own pulsation of pressure, velocity and temperature. This method can make the convergence process steady and effectively avoid divergence. The physical phenomenon of the self-excited Rijke pipe was analyzed. Moreover, the mechanisms on the Rijke pipe＇s self-excited oscillation were explained. Based on this method, comparative researches on the acoustic characteristic of the Rijke pipe with different size and different shape of nozzle were performed. The simulation results agreed with the experimental data satisfactorily. The results show that this numerical simulation can be used to study the sound pressure of nozzle for the engineering application of Rijke pipes.
ZHONG Yingjie DENG Kai ZHANG Guojun LIN Haihao LI Hua
Based on the energy conservation relationship, nonlinear thermo-acoustic effects of Rijke tube including instability range, saturation processes and higher harmonics modes were investigated. With coupling between the external flow and the inner space of a Rijke tube, the acoustic characteristics of self-excited oscillation were simulated. The experimental study was also carried out and the results were compared with those from simulation. The nonlinear factors which distort the acoustic waveform distortion were analyzed. From the results, it is seen that varying size of the nozzle outlet changes the acoustic impedance in the boundary, and leads to reduction of the nonlinear effects. The results show that the modes of self-excited oscillation could be influenced by the position of higher harmonics. In the large amplitude oscillation, the distortion of pressure wave within Rijke tube could be induced by the acoustic losses due to vortices on nozzle. It is found that the waveform distortion could be avoided by the shrinkage of nozzle.
