Heat transfer from a cylinder in an acoustic standing wave

Heat transfer was measured from wires of various diameters located at a velocity antinode in an acoustic standing wave. A transient method was used, in which the rate of heat transfer was deduced from the rate of change of temperature after heating was turned off. For fixed wire diameter and acoustic frequency, the dimensionless heat-transfer coefficient (Nusselt number) Nu shows a distinctive variation with acoustic amplitude. At high amplitude, Nu follows the well-known, steady-flow, forced-convection correlation, time averaged over an acoustic cycle, while at low amplitude, Nu has a constant value determined by natural convection. The transition between these regimes, which occurs rather abruptly when the streaming Reynolds number (based on wire diameter and acoustic velocity amplitude) equals 88, is discussed in terms of a heat-transfer bottleneck that is opened by acoustic streaming. An empirical correlation is presented. Applicability to heat exchangers for thermoacoustic heat engines is considered.