Free-Stream Turbulence Effects on Local Heat Transfer from a Sphere

Abstract

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 miniature heat-flux sensor which could be positioned at any circumferential location on the equator of the sphere. Turbulence grids were employed to generate free-stream turbulence with intensities of up to 9.4 percent. The Reynolds-number range of the experiments was from 20,000 to 62,000. The results 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 percent. For higher turbulence levels, the heat-flux increases with the turbulence intensity, the greatest heat-flux augmentation found here being about 15 percent. Furthermore, at the higher turbulence intensities, there appears to be a departure from the half-power Reynolds-number dependence of the stagnation-point Nusselt number. Turbulent separation occurred at Reynolds numbers of 42,000 and 62,000 for a turbulence level of 9.4 percent, these values being well below the transition Reynolds number of 2 × 105 for a sphere situated in a low-turbulence flow.

Tansley Review No. 59 Leaf boundary layers

“…between 1-5 and 1-79 (depending on Re) at TI = 40%, with significant increase only when the turbulent Reynolds number Re.j, (as defined above) exceeds 7000. The contrast between the small observed increase in heat transfer near the forward stagnation point of spheres (y5< M2forTI < 9-4% at 19000 < Re < 62000), and the pronounced effects {j3 between 1 34 and 1 82, depending on Re) at the 120° wake position (Newman, Sparrow & Eckert, 1972), suggests that effect of free stream turbulence may depend strongly on its interaction with the separated flow in the wake. Data obtained for plates (leaves) in parallel flow will differ, therefore, from those obtained in inclined flow (section V).…”

Measurement of air temperature in the presence of a large radiant flux: an assessment of assively ventilated thermometer screens

“…The extent to which turbulence intensity enhances heat exchange from the surface of the sensor. Several studies of flow over cylinders and spheres show that in fact free-stream turbulence may enhance convective heat transfer between the object and the fluid (Newman et al, 1972;Schlichting, 1979), but give varying estimates of the magnitude of this effect, from 5% to 50% or more. However, the effect is relatively small at low Reynolds numbers (Lowery and Vachon, 1975).…”

On the effects of the free-stream turbulence on the heat transfer from a sphere

“…They found the local Nusselt number near the front stagnation to decrease with the level of turbulence intensity up to a point and then to increase again as incoming turbulence increases. Newman et al [23], for larger Reynolds numbers in the range 2 ˆ10 4 -6.2 ˆ10 4 , found no effect whatsoever on the Nusselt number for turbulence intensities lower for level of intensity of 9.4%. In these studies, the scale of the turbulence was not reported.…”

“…Similar findings, but at larger Reynolds numbers, were obtained by Raithby & Eckert [22], who also found an increase in the Nusselt number with the decrease in the turbulence length scale. Newman et al [23] studied the effects of turbulence on heat transfer at Reynolds numbers in the range 2.0 ˆ10 4 -6.2 ˆ10 4 , and concluded that up to 5% no influence in the heat flux at the front stagnation point was observed. However, the overall heat transfer coefficient increased with the increase in the free-stream turbulence.…”