FREE-STREAM TURBULENCE EFFECTS ON LOCAL HEAT TRANSFER FROM A SPHERE

Experiments involving both heat-transfer and turbulence- field measurements were performed to determine the influence of free- stream turbulence on the local heat transfer from a sphere situated in a forced-convection airflow. The research was facilitated by a minature heat-flux sensor which could be positioned at any circumferential location on the equation of the sphere. Turbulence grids were employed to generate freestream turbulence with intensities of up to 9. 4%. The Reynolds number range of the experiments was from 20,000 to 62,000. Theesults indicate that the local heat flux in the forward region of the sphere is uninfluenced by free-stream turbulence levels of up to about 5%. For higher turbulence levels, the heat-flux increases with the turbulence intensity, the greatest heat-flux increases with the turbulence intensity, the greatest heat-flux augmentation found here being about 15%. At the higher turbulence intensities, there appears to be a departure from the half-power Reynoldsnumber dependence of the stgnation point Nusselt number. Turbulent separation occurred at Reynolds numbers of 42,000 and 62,000 for a turbulence level of 9. 4%, these values being well below the transition Reynolds number of 2x10//5 for a sphere situated in a low-turbulence flow.