Mixed convective transport around tandem circular cylinders in an unconfined medium

We perform two-dimensional numerical investigation to analyze the mixed convective transport around tandem circular cylinders immersed in an unconfined medium at low Reynolds numbers (Re = 10–40) with air as the working fluid. The problem has significant importance as the results are immensely useful for the design of heat transfer equipment. The transient laminar flow simulations are carried out by invoking the finite volume based CFD package ANSYS Fluent. The effect of the gap spacing of the cylinders is analyzed by changing the dimensionless gaps (g*) in the range 0.2–3.0. For the above range of Re in pure forced convection, the flow is steady separated. With the introduction of the cross thermal buoyancy, the steady flow may be transformed into an unsteady pattern with triggering of vortex shedding. This flow transition occurs at a critical value of the buoyancy parameter (Ri_cr). This aptly brings the novelty of the work. Precisely, the quantitative estimation of Ri_cr for the transformation of the steady separated flow into an unsteady periodic flow for such flow and geometric configuration is the new contribution to the available body of literature. The results show that Ri_cr decreases as g* and the Re increase. As Re increases from 10 to 40 for g* = 3.0, 13 % reduction in Ri_cr can be observed. Whereas, at Re = 40, as g* increases from 0.2 to 3.0, Ri_cr decreases by 35 %. Furthermore, the drag on the cylinders decreases with Re and Richardson number (Ri). However, it increases with increasing g*. Overall, the upstream cylinder experiences more drag, lift (negative) and heat transfer than the downstream cylinder. The heat transfer is also found to increase with g* and Ri.