Study On Acoustic Characteristics Of Mean Flow Acoustic Engine With Cross-junction Configuration

A mean flow acoustic engine (MFAE) is a new type of energy conversion device based on the aerodynamic effects, which utilizes a mean flow (e.g., wind) to induce an acoustic field featured with high energy-flux density and steady monofrequency. The acoustic energy can be used to drive thermoacoustic refrigerators and transducers without any mechanical moving parts. Because of its wide application prospect, we carried out the numerical and experimental study on MFAE.Based on vortex-sound theory and computational fluid dynamics (CFD) method, large-eddy simulation (LES) of turbulence model is applied to simulate the flow field and the acoustic field inside the MFAE. The coupling relationship of vortex in the mouth of resonator and acoustic field was analyzed. The computational results show similar variation trends to the experimental results in the region where relatively large acoustic pressure amplitude occurs.With natural wind simulated by a centrifugal air fan, the MFAE with a cross-junction configuration was first developed in China. Stable standing wave acoustic fields were established in specific ranges of the mean flow velocity. Experimental and computational results reveal the acoustic field distribution in the engine and show the effects of mean flow velocity and Strouhal number on acoustic field characteristics. With the resonator tube length regulated steplessly, the effects of the resonator tube length on the MFAE were revealed. The power level of acoustic source induced by vortices is calculated based on energy balance method. It is shown that there exist stable oscillation regions and non-oscillation regions with each resonator tube length in the certain ranges of the mean flow velocity. When the one-sided resonator tube length is between150mm and230mm, the acoustic field is in the fundamental mode; more odd acoustic modes appear in turn with the increase of the resonator tube length. The acoustic source power is related with the hydrodynamic mode and generally decreases with the increase of the resonator tube length. Acoustic oscillation occurs in the certain region of Strouhal number which reflects the hydrodynamic mode of the flow field. Furthermore, the critical length occurring between the transition points of acoustic modes is determined experimentally. With the mean pressure, the mean flow velocity and the one-sided resonator tube length of106.36kPa,50.35m/s, and190mm, respectively, the MFAE obtains a pressure amplitude of15.54kPa that is14.61%of the mean pressure, with the acoustic power induced by the vortex reaching6.28W. The research demonstrates the application potential of MFAE in wind energy exploitation and has established a foundation for future research.