Experimental investigation of unsteady laminar mixed convection from a horizontal heated cylinder in contra-flow: Buoyancy and confinement effects on the three-dimensional heat transfer response

Transient laminar opposing mixed convection heat transfer from a circular cylinder confined inside a vertical closed-loop downward rectangular water channel has been studied experimentally. The cylinder is subjected to a constant wall heat flux boundary condition while the other bounding walls are insulated and adiabatic. The cylinder is placed horizontally at the mid-plane with a 40% blockage ratio (BR, ratio between the cylinder diameter and the thickness of the rectangular section) and a cylinder aspect ratio (AR, ratio between the length and diameter of the cylinder) of 6. Experiments are performed for flow conditions with Reynolds number based on cylinder diameter in the range 170≤Re≤260, Prandtl number of Pr = 7, and for values of the modified Richardson number, Ri ^∗ =Gr ^∗ /Re ² , from 3.6 to 142.5. From experimental measurements, the variation in time-averaged local temperature distributions with angular position are obtained at nondimensional distances of 0.25, 0.5 and 0.75 of the cylinder span. Also, space-averaged surface temperatures and overall Nusselt number are obtained for a wide range in the parametric space. It is observed that for all values of Re and Ri ^∗ up to 40.1, a steep increase in the overall Nusselt number is observed with increasing buoyancy. However, because of the channel blockage, for values of Ri ^∗ > 40.1 and up to Ri ^∗ = 142.5, the overall Nusselt number reaches a plateau and its value remains practically fixed. Results indicate that for relatively large values of Ri ^∗ , because of the buoyancy-induced secondary flow resulting from the proximity of the cylinder to the channel walls, the three-dimensional (3D) configuration of the flow is no longer symmetric with respect to the channel centerline and the heat transfer rates vary along the span. The analysis brings out the significance of the end conditions and the buoyancy-induced secondary flow on the local and overall heat transfer characteristics of the bounded flow.