Effect of thermal entrance region on turbulent forced-convective heat transfer for an asymmetrically heated rectangular duct with uniform heat flux

Abstract To calculate the heat transfer rates in the thermal entrance section of a solar heater, it is essential to be able to predict average Nusselt numbers for any length of duct in this developing flow region. Experimental work has been carried out to determine local Nusselt numbers in the thermally developing region of an asymmetrically heated rectangular duct with uniform heat flux using air as the working fluid. The range of Reynolds numbers tested was from 9500 to 22,000. In an attempt to estimate average Nusselt numbers, an expression in the form Nu av = Nu ∞ (1 + SD e L ) was used to correlate the test results obtained from the duct. The factor S was found to be adequately represented by S = 14·3 log ( L D e ) − 7·9 for 0 L D e ⩽ 60 . For all ( L D e ) > 60 , S is constant at the value of S for ( L D e ) = 60 .