Flow and heat transfer from heated-unheated tandem cylinders: Effect of diameters, gap ratios and heating scenarios

Two-dimensional numerical simulations for flow and heat transfer around two heated and unheated tandem cylinders are done for the Reynolds number Re = 150, and the temperature ratio Ts* = 1-2.3. Ts* is defined as a ratio of cylinder surface temperature Ts and free stream flow temperature T infinity. The spacing ratio L/Dd is varied in the range of 2to8, and diameter ratios Du/Dd in between 0.5to2, where L is the center-to-center distance between the upstream cylinder having diameter Du and the downstream cylinder having diameter Dd. Four different heating scenarios were considered: (1) both unheated cylinders (reference), (2) heated upstream cylinder, (3) heated downstream cylinder, and (4) both heated cylinders. The mean drag coefficient CD, fluctuations in the amplitude of the drag coefficient CD, root mean square (RMS) of lift coefficient CL ', mean surface heat transfer coefficient h and Strouhal number St were obtained as a function of L/Dd and Du/Dd for both upstream and downstream cylinders. CD on the downstream cylinder reduces to a value lower than that of a single cylinder in all cases due to the presence of the upstream cylinder. In addition, CD on both cylinders generally increases with the increase in L/Dd for a constant Du/Dd. A major fall of 89.7% in CD is seen at L/Dd = 4 and Du/Dd = 1 for the downstream cylinder when only the downstream cylinder is heated compared with both unheated cylinder scenarios. Similarly, a fall of 117.5% is seen in CD on the downstream cylinder is observed for both the cylinders heated scenario at L/Dd = 8 and Du/Dd = 2. For upstream heated cylinder cases, around 96.4% rise in h is observed for the Du/Dd from 2 to 0.5 for all L/Dd, while for the downstream heated cylinder scenario, for L/Dd = 2 and 4, the rise in h is 46.2% and 71.5%, respectively, when reducing the Du/Dd from 2 to 0.5.