Experimental study of heat transfer in oscillatory gas flow inside a parallel-plate channel with imposed axial temperature gradient

Understanding of heat transfer processes between a solid boundary and an oscillatory gas flow is important for the design of internal components such as heat exchangers, regenerators and thermal buffer tubes in thermoacoustic and Stirling thermodynamic machines where the flow oscillations are part of the power production or transfer processes. The heat transfer between the flow and an arrangement of hot and cold plates forming a parallel-plate channel is studied experimentally and numerically. Here, the particular focus is on the processes occurring within the thermal and viscous boundary layers. The measured phase-dependent temperature fields combined with the velocity fields are studied in detail. Near wall temperature minima and maxima of the cross sectional temperature profiles are observed when the gas moves from cold to hot or from hot to cold parts of the channel. They are referred to as “temperature undershoot” and “overshoot”, respectively. The existence of these temperature minima and maxima causes large local temperature gradients in the direction normal to the wall, which result in large heat diffusion between gas layers in the direction normal to the wall. It is observed that the flow history has a strong impact on the temperature fields. This invalidates the so-called “Iguchi hypothesis”, based on which the heat transfer data for steady flows is being applied to oscillatory flow conditions. General guidelines for the design of heat exchangers used in thermoacoustic devices are discussed.