Flow and heat transfer analysis around tandem cylinders: critical gap ratio and thermal cross-buoyancy

Abstract

The focus of the current numerical investigation is to study the fluid flow and mixed convective (thermal cross-buoyancy) heat transfer of incompressible fluid across identical cylinders organized in a confined tandem configuration because of their enormous engineering and industrial applications such as heat exchange tubes, electronic cooling, twin chimney stacks, cooling tower, etc. The involved flow and energy equations are solved for different gap ratios (S/D = 2.5, 3, 3.5, 4, 5, and 5.5) with varying Richardson number (Ri = 0, 0.5, and 1) at Reynolds number Re = 100, Prandtl number Pr = 0.7 and wall confinement β = 25% by using a finite volume method-based commercial solver ANSYS Fluent. It is found that after a certain gap ratio there is a drastic change in the physical parameters and after that gap ratio the change is gradual; this gap ratio is termed as a critical gap ratio. The introduction of thermal cross-buoyancy (Ri > 0) plays a deep impact on the physical parameters, and it is to be noted that the critical spacing shifts towards a lower value with increasing Ri. The analysis of lift coefficients shows that the fluctuations in lift signal shift from zero average value at Ri = 0 towards the nonzero negative average value for the tandem cylinders at Ri > 0. The local Nusselt number shows the shift in the front stagnation point on both cylinders with increasing thermal cross-buoyancy. The drag coefficient and Nusselt number of the downstream cylinder are always less than the upstream cylinder, but the percentage increment in the physical parameters of the downstream cylinder after critical spacing is much more than its upstream counterpart.