Experiments on mixed convection heat transfer for airflow in a horizontal and inclined channel

Abstract

Experiments have been performed to investigate mixed convection heat transfer in the thermal entry region of a parallel plate channel heated uniformly from below. The effect of surface heat flux and channel orientation on the local Nusselt number was studied for Pr = 0.7, 125 < Re < 500, 7 × 10 3 < Gr∗ < 1 × 10 6 , and 0 < 0 < 30°. Heat transfer was initially dominated by forced convection and showed a rapid decline in the Nusselt number. Following the onset of thermal instability, secondary flow development caused a sharp increase in the Nusselt number, which was followed by a maximum and subsequent oscillations. The Nusselt number oscillations eventually decayed, yielding a fully developed value which depended on the Grashof number. The onset of instability was delayed by decreasing the Grashof number and/or by increasing the Reynolds number and the inclination angle. For the inclined channel, significant heat transfer enhancement occurred prior to onset of the secondary flow.