Experimental investigation on the mixed convection in a vertical rectangular duct

The mixed convection heat transfer of the entrance region in a vertical rectangular duct with a uniformly heated plate was experimentally investigated when the forced air flowed upward and downward. The temperature distributions and pressure drops were measured when the Reynolds number ranged from 329 to 5565 and Grashof number ranged from 9.7 × 10⁵ to 7.7 × 10⁶. The heat conduction through the duct walls in the experiment effectively influenced the characteristic of the convection. The experimental results showed that the average Nusselt number increased with the increase of the Reynolds number. For Re < 1000, the average Nusselt number of downward flow was larger than that of upward flow. But when the Reynolds number increased up to 1800, the both Nusselt numbers were closely equal. The pressure drop was approximately in the power function of the Reynolds number and changed little with the heat flux.