Large eddy simulation of secondary flow structures in low-pressure turbine blade

Increasing the efficiency of a gas turbine engine and reducing fuel consumption depend strongly on the efficiency of gas turbine components. The low pressure turbine (LPT), which is one of the heaviest components in a gas turbine engine, is subjected to very high temperatures and complex flow fields. The efficiency of a turbine is crucial in an engine since the power is generated by the turbine. Turbine efficiency can be increased by reducing the aerodynamic losses. Aerodynamic losses in turbine stages can be classified as profile losses, tip leakage losses, and secondary flow losses. The secondary flow losses are responsible for a significant amount (30% - 50%) of losses in the turbine. Hence, in this study, secondary flow structures, their interactions with each other, reasons for the formation of these structures and effects of the inflow boundary condition on these structures are studied. Simulations are performed with pisoFoam (pressure implicit with splitting of operators) solver in OpenFOAM (Open Field Operation and Manipulation) software which is an open source computational fluid dynamics (CFD) solver. Large eddy simulation (LES) method is used to investigate the unsteady, turbulent flow in a turbine passage. The untwisted T106 turbine blade geometry is used in the analysis. Half span is used for all simulations thanks to symmetry plane boundary condition. The computational domain split into 29 blocks in order to generate structural hexahedral elements. Three different grids are generated in order to study the grid resolution effect on statistics and to identify most appropriate grid. The coarse grid has 3 million grid points, medium grid has 15 million grid points, and fine grid has 30 million grid points. First simulation is performed with the coarse grid in order to validate and investigate the capability of the LES solver in OpenFOAM software. This case has uniform inlet and an inflow angle of 45 degree. After the numeric validation, an inflow angle of 37.7 degree is used for the rest of the simulations presented in this study. Then, the coarse mesh with an endwall is used to investigate the effect of the subgrid model on flow field. In these simulations, the effects of Smagorinsky model and k-equation model on turbulence and secondary flow structures are compared. Results indicate that k-equation model gives more accurate representation of the flow field than the Smagorinsky model. As a result of that, rest of the simulations are performed with k-equation model. After that, the effect of grid resolution (medium and fine grids) on the flow field is investigated. In these simulations, analysis of the energy density spectrum and the instantaneous second invariant velocity gradient tensor show that medium grid is fine enough to capture the relevant secondary flow structures. Hence, other studies on the effects of the inflow boundary condition on secondary flow structures and losses are performed with the medium grid. In order to investigate effects of the inlet boundary conditions on the secondary flow structures and losses, four different inlet boundary conditions are applied. These are uniform inlet, laminar boundary layer (LBL) velocity profile inlet with a shape factor of 2.2, turbulent boundary layer (TBL) velocity profile inlet with a shape factor of 1.5, and unsteady wake with secondary flows (UWwSF) inlet boundary condition, which is obtained from the wake results of a simulation with TBL inlet. Results indicate that inlet boundary conditions cause significant changes on the horseshoe vortex system, the strength of secondary flow structures, and total pressure losses along the flow field.