MIXED CONVECTION IN THE THERMAL ENTRANCE REGION OF SYMMETRICALLY AND ASYMMETRICALLY HEATED VERTICAL FLAT DUCT WITH UPWARD OR DOWNWARD AIR-FLOW

摘要

A numerical investigation has been conducted on the effect of body force on pure forced convection of the upward or downward air-flow in the thermal entrance region between vertical parallel plates with uniform wall temperature. The governing equations based on the usual Boussinesq approximation are solved for the symmetrically and asymmetrically heated parallel plates. Numerically predicted friction factors C{sub}f and local Nusselt numbers Nu{sub}x are compared with their counterparts, C{sub}f{sup}* and Nu{sub}x{sup}*, for pure forced convection. In order to quantitatively determine the effect of buoyancy, the correlation equations of C{sub}f/C{sub}f{sup}* and Nu{sub}x/Nu{sub}x{sup}* for 30≤Re≤300 and 0≤Ra≤5000 were deduced as follows: At 0≤ζ≤0.5, {(C{sub}f/C{sub}f{sup}*)-(C{sub}f/C{sub}f{sup}*){sub}0}/A{sub}u = {(Nu{sub}x/Nu{sub}x{sup}*)-(Nu{sub}x/Nu{sub}x{sup}*){sub}0}/A{sub}t = ζ(1-ζ)At 0.5≤ζ≤1, {(C{sub}f/C{sub}f{sup}*)-1}/α{sub}u = {(Nu{sub}x/Nu{sub}x{sup}*)-1}/α{sub}t = 4(4ζ{sup}3-9ζ{sup}2+6ζ-1) for upward flow, or = 1+aη+bη{sup}2+cη{sup}3+dη{sub}4+eη{sup}5 (η = ζ-0.5) for downward flow. These correlation equations are shown with the solid lines in Figures A-1 and A-2. Furthermore, the critical value of Ra or Re for the flow reversal can be predicted from the correlation equations of (C{sub}f/C{sub}f{sup}*){sub}0 and (C{sub}f/C{sub}f{sup}*){sub}(min).