Convective Heat Transfer from Exposed Flat Horizontal Surface in Outdoorconditions at Low Wind Speeds: An Application to Flat Plate Solar Collector

Abstract

Estimation of various heat losses in flat plate solar collectors is important for their thermal performance evaluation under different operating conditions. Upward heat losses have a major contribution in the total heat losses in flat plate collectors (FPC). Equations for estimation of top heat loss coefficient for single and double glazed FPC have already been proposed [1-2]. Top heat loss coefficient is a function of wind heat transfer coefficient. Different forced convection studies [3-6] conducted in indoor and outdoor conditions have been reported for estimation of wind heat transfer coefficient. At sites/locations with very low wind, use of different velocity based correlations [3-6] would give a wrong estimation of wind heat transfer coefficient. In present work wind heat transfer coefficient is estimated by employing an exposed blackened test plate (of size 925mm x 865mm x 2mm) in outdoor conditions. Experiments have been conducted on rooftop of a building in IIT Delhi. Delhi, in general, has low wind throughout the year [9]. From experimental data of test plate, mixed convection effects have been observed at very low wind speeds.