Influence of Turbulence Parameters, Reynolds Number, and Body Shape on Stagnation-Region Heat Transfer

Abstract

JTrade names or manufacturers' names are used in this report for identification only. This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. SummaryThe purpose of the present work was threefold: (1) to determine if a free-stream turbulence length scale existed that would cause the greatest augmentation in stagnation-region heat transfer over laminar levels; (2) to investigate the effect of velocity gradient on stagnation-region heat transfer augmentation by free-stream turbulence; and (3) to develop a prediction tool for stagnation heat transfer in the presence of free-stream turbulence. Heat transfer was measured in the stagnation region of four models with elliptical leading edges that had ratios of major to minor axes of 1: 1, 1.5: 1, 2.25: 1, and 3: 1. Five turbulence-generating grids were fabricated; four were square mesh, biplane grids made from square bars. The fifth grid was an array of fine parallel wires that were perpendicular to the model spanwise direction .Heat transfer data were taken at Reynolds numbers ranging from 37 000 to 228 000. Turbulence intensities were in the range of 1.1 to 15.9 percent while the ratio of integral length scale to leading-edge diameter ranged from 0.05 to 0.30. Stagnation-point velocity gradient was varied by nearly 50 percent.Stagnation-region heat transfer augmentation was found to increase with decreasing length scale but no optimum length scale was found. Heat transfer augmentation due to turbulence was found to be unaffected by the velocity gradient near the leading edge. A correlation was developed that fit heat transfer data for the square-bar grids to within ±4 percent.

Stagnation Point Flow and Heat Transfer Under Free-Stream Turbulence

“…Very good agreement between the numerical simulation and experimental measurement on the heat transfer enhancement (Van Fossen et al, 1995) is obtained.…”

“…To compare directly with experimental measurements, throughout this report the turbulence integral scale L is determined by a least-squares curvefitting to the autocorrelation data using an exponential function (Van Fossen et al, 1995) R{r) = e-^l^ (4.2) Data between 0.33 < R{r) < 1.0 were used for curve fitting. The exponential function does not have the correct limiting behavior for very small values of r, but the fit is satisfactory over the main portion and the fitted R{r) can be integrated from 0 to oo to give the turbulence integral length scale.…”

“…Further experiments have been reported recently (Ames et al, 2002) with a larger Reynolds number range and different turbulence conditions. Van Fossen et al (1995) made a systematic study on the influence of various flow parameters on stagnation point heat transfer, focusing on the relatively short turbulence length scale (^ = 0.05 -0.3). The turbulence intensity Tu varies from 1.1 to 15.9 percent, and Reynolds number Rep from 37000 to 228000.…”

Effects of High Intensity, Large-Scale Freestream Combustor Turbulence on Heat Transfer in Transonic Turbine Blades

“…The work was performed in the same facility as published work by Van Fossen, et al (1995), using the same facility, turbulence grids and heat transfer models.…”

Effects of Turbulence on Mass Transfer of Cylindrical Disks

“…Measurement of the sublimation depth would yield local mass transfer rates. The aforementioned advantages promoted research in different flow regimes (Mach numbers >1 or <1), various body shapes (airfoils , circular cylinders, disks, spheres, prolate spheroids, cones etc), [3], [4], [5], [6] and flow conditions (turbulent , non-turbulent flows), [7], [8]. There is still some controversy concerning the effects of free stream turbulence level and the length scale on the average Sherwood number on disk shapes.…”