Effects of heat transfer on separated boundary layer behavior under adverse pressure gradients

Separated boundary layers in a converging-diverging channel under two different wall heat transfer conditions, including the adiabatic wall and the cooled isothermal wall, have been investigated by using a large eddy simulation (LES) code coupled with a wall heat transfer module. The wall temperature of the cooled isothermal wall is 0.8 times of the inflow temperature, which is a typical value in a low-pressure turbine. The pressure distribution of the converging-diverging channel bottom wall is similar to that of a typical low-pressure turbine blade suction surface. The flow conditions are set as the exit isentropic Reynolds number (Re = 60,154) and the exit isentropic Mach number (Ma = 0.402). The results show that the cooled isothermal wall condition can increase the static pressure rise coefficient significantly, accelerate the transition process of the separation boundary layer, and suppress the separation. Furthermore, the cooled isothermal wall condition can decrease the frequency of the large-scale spanwise vortex rolled-up, reducing the influence of the spanwise vortex on the separated shear layer and decreasing the scales of the downstream hairpin vortex. In addition, the instability of the separated shear layer can be regarded as the superposition result of both the Tollmien-Schlichting (T-S) instability mechanism and the Kelvin-Helmholtz (K-H) instability mechanism, while the K-H instability mechanism on the whole is slightly more dominant in the presence of separation and reattachment. This trend is more obvious under the cooled isothermal wall condition.