Large eddy simulation of flow and heat transfer over forward-facing steps with upstream injection

Forward-facing step flows occur in many kinds of applications such as wind power generation at the top of abrupt changes in landscape, in electronics cooling for airflow over chips, and in gas turbine engines where individual components may be misaligned due to differential thermal expansion or assembly mismatch. Unlike the more commonly studied backward facing step flow, forward-facing step flows feature two recirculation zones, one at the base of the step, and one on top of the step. High amounts of turbulence and convective heat transfer occur in the recirculation regions. In some applications (particularly the hot components in a gas turbine engine), leakage flow is injected at the base of the forward step to provide cool air and prevent melting downstream of the step, but also can locally increase the heat transfer coefficient by the disturbance of the injection. In this study, the turbulent convective heat transfer over a forward-facing step with upstream injection flow was studied using scale-resolving computational simulations. The continuity, Navier-Stokes, and energy equations were solved numerically and large eddy simulation (LES) with the Wall-Adapting Local-Eddy Viscosity (WALE) subgrid scale model was used to resolve the turbulence. By adding upstream injection, the separation and reattachment lengths around the step increased by about 20% for an injection velocity ratio (injection to mainstream) of 0.25, and by 50% for an injection velocity ratio of 0.5. Injection increased the turbulence downstream of the forward-facing step, resulting in higher heat transfer coefficients downstream of the step. These effects further increased with increasing injection velocity ratio.