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 w as 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 coeff icient (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 an d acoustic velocity amplitude) equals 88, is discussed in terms of a h eat-transfer bottleneck that is opened by acoustic streaming. An empir ical correlation is presented. Applicability to heat exchangers for th ermoacoustic heat engines is considered. (C) 1995 Acoustical Society o f America.