Transient mixed convection heat transfer for opposing flow from two discrete flush-mounted heaters in a rectangular channel of finite length: Effect of buoyancy and inclination angle

An experimental investigation in a vertical rectangular channel using water as the working fluid is carried out to study the transient laminar opposing mixed convection heat transfer from two flushmounted, symmetric and discrete heat sources subjected to a constant wall heat flux boundary condition while the other bounding walls are insulated and adiabatic. The experiments are done under different values of buoyancy strength or modified Richardson number Ri∗ = Gr∗/Re², Reynolds number of 300 ≤ Re ≤ 900 and channel inclination of 0° ≤ γ ≤ 90°. From experimental measurements, surface temperature distributions and averaged Nusselt number for each heat source are obtained. In general, for a fixed value of the buoyancy parameter, the averaged Nusselt number increases for increasing values of the Reynolds number. In the vertical channel configuration, it is observed that for fixed values of Re and high Ri∗ number, because buoyancy acts directly against convective flow, higher heat transfer rates are achieved. As the duct approaches the horizontal configuration, buoyancy strength is reduced and the averaged Nusselt number decreases for decreasing values of the inclination angle with marked variations. Here, the effect on the heat transfer rates is more pronounced at γ = 60° for low Ri∗. For the horizontal configuration, because buoyancy only acts indirectly, higher threshold values of Ri∗ are required to induce instability. The results show that for relatively large values of buoyancy strength, the surface temperature presents strong spanwise and axial variations, and for all of the inclination angles considered in this study, the values of the surface temperatures achieve higher values at the middle spanwise positions of both heaters than those registered at other spanwise locations. This indicates that because of the secondary three-dimensional flow, heat transfer augmentation takes place close to the channel corners while the higher surface temperatures and hence, lower heat transfer rates are achieved at the centerline of the discrete heat sources.