Effects of Flow Reversal on Wall Heat Transfer in Three-Dimensional Mixed Convection at Various Inlet Fluid Temperatures
Koichi Ichimiya and Koji Toriyam
In the present study, the characteristics of a three-dimensional heat transfer and flow in a horizontal square duct with three heat transfer walls (two side walls and a bottom wall, Tw=30oC) were examined numerically and experimentally at various inlet fluid temperatures (T0=10, 20, 40 and 50oC). The flow reversal (i.e., buoyancy- driven separation) occurs at low flow rates in a horizontal duct, and its effects on the wall heat transfer were examined. In the numerical analysis, water was selected as the working fluid, and the temperature dependence of the kinetic viscosity and thermal conductivity of water was considered. Three-dimensional elliptic governing equations for the conservation of mass, momentum and energy were solved by a finite-difference method in an unsteady state. In the experiment, the flow was visualized using a dye injection technique.
The flow behavior depends on the relative temperature difference between the wall and inlet temperatures. Fluid near the wall flows backward to compensate for the buoyancy-induced flow. The size of the flow reversal increases with the product of the Richardson number Ri (=Gr/Re2) by the Prandtl number Pr (=(m /r)/(l/Cpr)). Heat transfer on the wall is enhanced or depressed corresponding to the flow reversal. The heat transfer behavior corresponding to the flow is discussed.