The Local Heat-Transfer Coefficient Around a Heated Horizontal Cylinder in an Intense Sound Field

Abstract

In order to achieve a better understanding of the physical mechanism of interaction between free convection and sound, an experimental investigation of the local heat-transfer coefficient around the circumference of a heated horizontal cylinder, both in the presence and absence of a strong stationary sound field, has been carried out. The results show that superposition of intense sound upon the free-convection temperature-velocity field about a heated horizontal cylinder increases the heat-transfer coefficient both on the under and upper portions of the cylinder’s surface. In the presence of a sound field for which SPL = 146 db (re 0.0002 microbar) and f = 1500 cps, the maximum measured increases in the local heat-transfer coefficient on the under and upper portions of a 3/4-in-diam cylinder—relative to the free convection case at the same temperature potential—were found to be approximately 250 and 1200 per cent, respectively. A comparison of these results with earlier flow-visualization studies indicates that the relatively large percentage increase in the heat-transfer coefficient on the upper portion of the cylinder is caused by the oscillating vortex flow which is characteristic of thermoacoustic streaming. The reasons for the increase in the heat-transfer coefficient on the lower portion of the cylinder appear to be: (a) An increase in laminar boundary-layer velocities (steady components) in this region; and (b) modification of the boundary-layer temperature profile due to acoustically induced oscillations (unsteady components) within the laminar boundary layer. The experimental data presented can be used to check the validity of future analytical investigations of thermoacoustic phenomena.