Experimental study of heat transfer from plain fins in oscillatory flow - A microscopic view of heat exchange processes in thermoacoustic devices

An experimental study of heat transfer processes taking place in a stack of plain fins placed in an acoustically induced oscillatory flow is reported. This is of critical importance for understanding the performance of stacks and heat exchangers in thermoacoustic systems. Here, a pair of heat exchangers, consisting of an electrically heated heat exchanger and a water-cooled one, both having controlled temperatures, is used. The simplified "stack-less" arrangement is adopted here to mimic the processes taking place at the interface between the stack and heat exchanger. Measurement techniques such as planar laser induced fluorescence (PLIF) thermometry and particle image velocimetry (PIV) are applied to obtain the temperature and velocity fields around the fins. By using a phase-locking mechanism, the instantaneous temperature or velocity fields around the fins, corresponding to same phases in acoustic oscillation cycles, are obtained. The statistics of the temperature and velocity data are calculated for further analysis. From the mean temperature field, the heat flux on the fin surface is calculated from the temperature gradient. Combined with the mean velocity field, the local and/or time-dependent heat fluxes are studied to reveal the mechanism of the heat transfer process between the fins and the flow. The deduced time- and spatial-averaged heat flux is used to find the heat transfer coefficient to evaluate the performance of the convective heat transfer between the fins and the oscillatory flow. Furthermore, the effects of the fluid displacement amplitude on the heat transfer process are studied. It is thought that this study will be also helpful to the understanding of the general heat transfer from a solid body in an oscillatory flow.