The heat transfer characteristics of a cylinder submerged in wake

Fluid flow and heat transfer of an isothermal cylinder in the wake of another of different diameters in tandem arrangement is numerically simulated at a Reynolds number Re = 200 and Prandtl number, Pr = 0.71. The diameter, d, of upstream cylinder is changed while diameter, D, of downstream cylinder is kept constant. Transient two-dimensional simulations are carried at various range of diameter ratios (d/D = 0.24, 0.4, 0.6, 0.8, 1.0) and spacing ratio (distance from center of upstream cylinder to forward stagnation point of downstream cylinder) L =5.5d. It is found that both cylinders shed vortices for 0.4 ≤ d/D ≤1.0, two prominent vortex shedding frequencies are detected in the wake of downstream cylinder, one represent the primary Kármán vortex street, and other with large vortical structures having low frequency evolved due to merging of vortices in the wake, nomenclature as secondary vortex street. With decrease in d/D, the frequency of secondary vortex street is increased, while vortices getting stronger. Novel tertiary frequencies are also found in the spectrum of Nusselt number, involved in the heat transfer enhancement with decrease in d/D. Based on the numerically simulated results heat transfer around isothermal cylinder is classified into three different regions, characterized by impingement of vortices from upstream cylinder, shear layer characteristics and correlation of vortex evolution in the near wake. Heat transfer is enhanced for d/D ≥ 0.4, both cylinders lies in the co-shedding regime, standing vortices in the reattachment regime have a curtailing tendency with respect to heat transfer at d/D =0.24. Heat is transported by vortical structures in the wake.