Diffusion of heat from an instantaneous point source in a turbulent boundary layer

Abstract

Diffusion of heat from an instantaneous point source located in a thick turbulent boundary layer over a wind-tunnel floor was investigated. A technique was developed for production of heat spots and detection of temperature fluctuations downstream from the point of release. As a heat source a short length of platinum-iridium wire 0.0004 in. in diameter and approximately 1/10 in. long was used which was heated by a short pulse of electric current. A high response resistance thermometer was employed for detection of temperature fluctuations. The output of the resistance-thermometer bridge was amplified and applied to an oscilloscope with a "memory screen". The instantaneous temperature profiles of the convected heat spots were displayed on the screen and readings of the maximum temperatures were taken. From about 100-120 readings, mean maximum temperatures were calculated. Data were taken at different distances up to 5 in. from the source with the source placed at 2-1/2 in. and 5-7/8 in. from the wall. The velocity of air was kept at 15 ft./sec (R = 16 x 106). The obtained horizontal distributions of heat spots are very close to Gaussian curves. The vertical distributions show a skewness. The skewness is such that the greater spread occurs at the side of the greater value of the mean velocity. The skew distribution obtained was compared with the Hinze's skewed temperature distribution and the agreement is satisfactory.