Unsteady mixed convection heat transfer from two confined isothermal circular cylinders in tandem: Buoyancy and tube spacing effects

In this work, two-dimensional numerical simulations are carried out to investigate the unsteady mixed convection heat transfer in a laminar cross-flow from two equal-sized isothermal in-line cylinders confined inside a vertical channel. The governing equations are solved using the vorticity-stream function formulation of the incompressible Navier-Stokes and energy equations using the control-volume method on a non-uniform orthogonal Cartesian grid. The numerical scheme is validated for the standard case of a symmetrically confined isothermal circular cylinder in a plane channel. Calculations are performed for flow conditions with Reynolds number of Re_D = 200, a fixed value of the Prandtl number of Pr = 0.744, values of the buoyancy parameter (Richardson number) in the range -1≤Ri≤4, and a blockage ratio of BR=D/H=0.3. All possible flow regimes are considered by setting the pitch-to-diameter ratios (σ = L/D) to 2, 3 and 5. The interference effects and complex flow features are presented in the form of mean and instantaneous velocity, vorticity and temperature distributions. In addition, separation angles, time traces of velocity fluctuation, Strouhal number, characteristic times of flow oscillation, phase-space relation between the longitudinal and transverse velocity signals, wake structure, and recirculation length behind each cylinder have been determined. Local and space-averaged Nusselt numbers for the upstream and downstream cylinders have also been obtained. The results reported herein demonstrate how the flow and heat transfer characteristics are significantly modified by the wall confinement, tube spacing, and thermal effects for a wide range in the parametric space.