Numerical Study of the Effects of a Hot Obstacle on Natural Convection Flow Regimes

Abstract

A numerical study was conducted for natural turbulent and laminar convection induced by a circular hot obstacle in two different positions, the heat obstacle is introduced at the inlet of the cylinder and inside the cylinder. To determine the effects of a circular hot obstacle on the regime of the thermosiphon flow generated by a heated cylindrical channel, we suggest a theoretical and numerical model using the Navier–Stokes equations and the finite volume method. An experimental apparatus was used for the validation of the numerical model. A comparison of the numerical and experimental results showed a good agreement in the global flow behavior. The temperature and vertical velocity profiles demonstrated the presence of a boundary-layer regime along the heated channel's wall in the absence of a hot obstacle and the presence of the hot obstacle at different positions. The introduction of the hot obstacle generates a significant modification of the evolution of the flow above the hot obstacle. The numerical simulation was conducted for a specific radius of the hot obstacle that ensures the existence in the laminar flow regime, and different Rayleigh numbers (Ra=105; 107; 108; 109; and1010). The flow rate, velocity, temperature and Nusselt number profiles were presented and discussed. The effects of the hot obstacle are presented in our results by two maxima of the vertical component of the velocity profile. The temperature profile at the entrance of the vertical cylinder is presented by the appearance of a maximum above the hot obstacle. As the cylinder height increases, the air temperature decreases. The vertical displacement of the obstacle upward induces a modification in the behavior of the dynamical flow. A CFD code was used to study the different natural convection regimes presented by three different form factors, including the critical regime with a form factor of A=0.1.