Effects of buoyancy and inclination for opposing mixed convection in a symmetrical heated duct with a plane symmetric sudden contraction–expansion

• Mixed convection in discretely heated rectangular ducts is studied experimentally.
• Buoyancy strength and duct orientation effects are presented.
• The effect of the partial blockage on the heat transfer characteristics is assessed.
• Local and overall heat transfer characteristics are reported.
• Steady and oscillatory 3D flow and thermal characteristics are reported.

In this work, transient laminar opposing mixed convection is studied experimentally in an open vertical rectangular channel configuration with plane symmetric forward–backward facing steps located at the middle of the test section with uniform heat flux imposed to the lateral walls of each step while the other bounding walls are treated as adiabatic surfaces. The effect of opposing buoyancy and the geometrical configuration of partial blockage on the heat transfer behavior for the double stepped wall is analyzed for a Reynolds number of 300 ⩽⩽ Re ⩽⩽ 900, channel inclination of 0° ⩽⩽ γγ ⩽⩽ 90°, and different values of buoyancy strength or modified Richardson number. From experimental measurements, space-averaged surface temperatures and overall Nusselt number of each simulated electronic chip are obtained for a wide range in the parametric space. Also, phase-space plots of the self-oscillatory system, characteristic times of temperature oscillations and spectral distribution of the fluctuating energy are presented. Results show that for relatively large values of buoyancy strength, strong three-dimensional secondary flow oscillations develop in the axial and spanwise directions. The temperature measurements show that for a fixed value of the modified Richardson number, there is not a linear dependence between the duct orientation and the heat transfer rates achieved. Also, when the duct is inclined with respect to the horizontal, the right (upper) and left (lower) oscillating vortical structures present large and small amplitude thermal fluctuations, respectively. In addition, it is pointed out that the highest flow reversal takes place at the channel corners of the upper heater block, and that higher surface temperatures are reached at the centerline of the latter. The analysis brings out the significance of the three-dimensional configuration of the vortical structure and how the buoyancy induced secondary flow is affected by the partial blockage.