Oscillating Flow Fields in Thermoacoustic Energy Systems

A thermoacoustic Stirling engine can produce electric power by simultaneously utilizing the cold energy from liquefied natural gas re-gasification process and the waste heat from the exhaust of gas turbines using a pair of linear alternators. However, the actual efficiency of the device when tested was much lower than that predicted by the design software. This might be due to un-accounted non-linear effects like entrance effects, vortex shedding, acoustic streaming etc. which affect the energy transfer processes occurring in the vicinity of the thermoacoustic core or stack/regenerator. It is therefore necessary to further understand and investigate these non-linear effects in an oscillating flow in order to develop more accurate models to predict the performance of thermoacoustic systems and improve the efficiency. To study and observe these phenomenon, a standing wave thermoacoustic system was setup with a parallel plate stack made of acrylic for its simplicity. A loudspeaker is used to generate pressure oscillations inside the standing wave resonator and create a temperature gradient across the stack ends. The flow near the thermoacoustic stack is visualized using particle image velocimetry technique. We can observe vortex generation at the edges of the plates followed by elongation and shredding as the cycle progresses. These vortices interact with the stack and each other in reverse flow to form complex structures that might affect the heat transfer process and the efficiency of the system.