Roughness Effects on Dense-Gas Turbine Flow: Comparison of Experiments and Simulations

This article presents a combined numerical and experimental study of the high-subsonic organic vapor flow in a linear turbine cascade. The profile geometry is the well-documented LS59 highly loaded rotor blade and the working fluid is Novec649, a dense gas used in organic Rankine cycles. Large eddy simulations are carried out with and without the roughness introduced by the additive manufacturing process. The results for the rough blade are in fair agreement with experiments, while the smooth surface induces a change in the vortex shedding regime. A detached shedding, characterized by a long recirculation downstream of the trailing edge and a base pressure plateau, is obtained in the experiments and by discretizing the roughness in the simulation. By contrast, a transonic vortex shedding is established when the surface is smooth: intense vortices roll up immediately after the trailing edge, yielding a short bubble and a lattice of shock waves. A strong pressure drop is observed at the trailing edge, resulting in high profile losses. In both cases, the boundary layer is turbulent ahead of the separation, but its thickness is significantly greater in the rough configuration, which may be the reason for the change of regime.