Numerical solution turbulent flow over backward facing step - the effects of step height, duct angle

Two-dimensional simulations are presented for compressible turbulent convection flow adjacent to backward-facing step in rectangular duct and the effects of step height, duct angle and Reynolds number on turbulent separated flow and heat transfer characteristics are investigated. The step geometry consists of an adiabatic backward-facing step, an upstream wall and a downstream wall. Both the upstream and downstream walls are heated to a uniform and constant temperature. The simulation was done for step heights of 0, 22, 33 and 44 mm, at a free stream air velocity, U , of .41, 1, 1.5 and 2m s, and duct angles of 90, 60, 45 and 30 degree, also a temperature difference, T,of 30 ◦C between the heated walls and the free stream a (corresponding to a local Grashof number xi Gr = 6.45 × 10 10 ). The first model which is used in simulation of this flow is k  model. Certainly this model consider of problem in some point such as recirculation point. In this paper the feasibility of using commercial LUENT software for k model is investigated. This model will be modified in next step and replace with LES model. It was found that both the eattachment length and the heat transfer rate from the downstream heated wall increase with increasing step height. The inclination angle, on the other hand, affects significantly the temperature and velocity distributions. As the inclination angle from the vertical increases, the local Nusselt number decreases, while the reattachment length and the location of the maximum Nusselt number behind the backward-facing step increase.

Keywords:

backward-facing step , turbulent flow , Separated flows , Nusselt number , CFD