Experimental study on turbulent thermal convection

Abstract

Experiments have been conducted to investigate mean thermal structure in unstable turbulent thermal convection between two horizontal flat plates. The upper plate was kept at a constant cold temperature and the bottom plate at a constant hot temperature. Both air and water were used as its working fluid. Chamber aspect ratios were 3.08 and 6.17, the mean temperature difference between two plates were 0.9-7.9℃, which gave us Rayleigh number in a range $6.13×10^5$-$1.25×10^8$. The heat transfer correlation found through the experiments are $Nu=0.139R_a^0.285$ for air and $Nu=0.087R_a^0.319$ for water. Mean temperature profiles and fluctuations were obtained by using a Pt-Pt Rh thermocouple of 0.025mm diameter. The measured profiles have been differentiated to get the mean temperature gradient profiles. The results clearly identified the -2 and -4/3 power law regions which have been proposed by Kraichnan (1962) and Chung & Adrian (1981). The buoyancy defect region proposed by Chung et. al. was not detected, and the defect region for water was detected possibly due to slight unsteadiness of the convective motion. It was found that a correlation formula for mean temperature of Chung et. al. is in good agreement with this experimental data.