Numerical analysis of effects of fins and conductive walls on heat transfer in side heated cavities - Onset of three-dimensional phenomena in natural convection

In the present study, we analyse individual and combined effects of conductive horizontal walls and conductive fins on the natural convection of air in side heated cavities (SHC). The flow and heat transfer are studied for Rayleigh numbers in the range of 10(4) - 10(9): Direct Numerical Simulation (DNS) is conducted for the lower and Large Eddy Simulation (LES) for the higher Rayleigh numbers (> 10(8)). Thermally conductive walls destabilize the flow yielding an earlier transition to turbulence and expedite the decay in boundary layer thickness with increase in Rayleigh number. The preheating/precooling along the conductive walls reduces the actual heat transfer at the vertical walls. Above the fin, instabilities are only marginally enhanced for adiabatic horizontal walls, whereas for conductive horizontal walls, plumes erupt from the fin. This localized Rayleigh-Benard-like effect triggers 3D instabilities in the entire flow field and yields a steeper slope in Nusselt-Rayleigh diagram. The presence of a fin increases the integral heat transfer by 18% for adiabatic and 21% for conductive horizontal walls. We show that 2D and 3D simulations are similar for the smooth cases (i.e., without fin), but differ by 4% and 13% for the adiabatic and conductive fin cases respectively. The local heat transfer characteristics even deviates up to 50%, therefore a 2D simplification should be avoided. (C) 2021 The Authors. Published by Elsevier Ltd.

Automated summary

This study investigates the individual and combined effects of conductive horizontal walls and conductive fins on the natural convection of air in side heated cavities. The results show that thermally conductive walls accelerate the transition to turbulence and decrease the boundary layer thickness. Preheating/precooling along the conductive walls reduces the actual heat transfer at the vertical walls. Plumes erupt from the fin in the case of conductive horizontal walls, triggering 3D instabilities in the entire flow field. The presence of a fin increases the integral heat transfer.