Comparative Performance of Thermoacoustic Heat Exchangers with Circulat Pores and Rectangular Pores in Oscillatory Flow: Development of experimental techniques

Heat exchangers constitute key components of thermoacoustic devices and play an important role in determining the overall engine performance. In oscillatory flow conditions the flow and temperature fields around the heat exchangers can be quite complex and the underlying physical mechanism subtending the heat transfer process is not fully understood. As a consequence, standard heat transfer correlations for steady flows are of little practical value and cannot be directly applied to thermoacoustic heat exchangers, for which reliable and univocal design criteria are still lacking. This work is concerned with the initial stage of a research aimed at studying the thermal performance of thermoacoustic HXs working under oscillatory flow conditions. The paper reports a detailed discussion of the design and fabrication of the experimental set-up, measurement methodology and test-HXs characterized by two different pore geometries, namely a circular pore geometry and a rectangular (i.e. straight fins) geometry. The test rig is constituted by a standing wave engine working with air at atmospheric pressure where the test heat exchangers play the role of ambient heat exchangers. The experiment is conceived to allow the variation of a range of testing conditions such as drive ratio, operation frequency, acoustic particle velocity, temperature differences, etc. The procedure for estimating the gas side heat transfer coefficient for the two involved geometries and its dependence on the acoustic Reynolds number is described. The present research could help in achieving a deeper understanding of the heat transfer processes affecting heat exchangers under oscillating flow regime and in developing optimization procedures of their performance in the design phase.