Natural convection effects on graetz problem in horizontal rectangular channels with uniform wall temperature for large Pr

Abstract

The effect of buoyancy forces on laminar forced convective heat transfer in the thermal entrance region of horizontal rectangular channels with uniform wall temperature is studied by a numerical method for the case of large Prandtl number fluids. The numerical results are presented for the aspect ratios (width/height) 0·5, 1, 2 and Rayleigh numbers 0 ~ 5 × 10 5. The correlation equations for the prediction on the onset of significant free convection effect are developed. The asymptotic behavior of local Nusselt number is compared against the known asymptote for the uniform wall temperature boundary condition. The entrance region where the free convection effect is significant is clearly established and the classical Graetz problem is shown to be a limiting case and is applicable only when Ra ⩽ 10 3.