Development of experimental techniques to investigate the heat transfer processes in oscillatory flows

Heat exchangers are important components of thermoacoustic devices. In oscillatory flow conditions, the flow and temperature fields around the heat exchangers can be quite complex, and may significantly affect heat transfer behaviour. As a result, one cannot directly apply the heat transfer correlations for steady flows to the design of heat exchangers for oscillatory flows. The fundamental knowledge of heat transfer in oscillatory flows, however, is still not well-established. The aim of the current work is to investigate the influence of certain geometric parameters of heat exchangers, and of operating conditions in oscillatory flow on heat transfer performance. The heat transferred between two heat exchangers forming a couple was measured over a range of testing conditions. Three couples of finned-tube heat exchangers with different fin spacing were selected for the experiment. The main parameters considered were fin spacing, fin length, thermal penetration depth and gas displacement amplitude. Their effects on the heat exchanger performance were studied. The results are summarised and analysed in terms of heat transfer rate, Nusselt number and heat transfer effectiveness; the latter defined by the ratio of the actual heat transfer rate to the maximum possible heat transfer rate. The measurement results are compared with results from models widely used in the design of thermoacoustic heat exchangers: Time-Average Steady-Flow Equivalent (TASFE), Root Mean Square Reynolds Number (RMSRe), boundary layer conduction model and selected correlations developed by different authors. Based on the experimental data, a new correlation is established aimed at improving the reliability of oscillatory flow heat transfer predictions. The correlation is proposed for the relationship between heat transfer effectiveness, and the normalized displacement amplitude and the normalized fin spacing (the ratio of fin spacing to thermal penetration depth). The uncertainties associated with the measurement of heat transfer rate are also considered.