Characteristic And Mechanism Of The Momentum And Scalar Transport In Gas Turbine Film Cooling Jet Flow

Gas turbine is a high-efficient machine for energy conversion.It is widely used in national defense and civil area,such as jet propulsion and electricity generation.Film cooling is one of the most e ffective cooling methods adopted in the vanes and blades of the gas turbine.In a fluid-dynamical point of view,film cooling is essentially a wall jet problem.Predicting the temperature precisely is the goal of film cooling studies,which boils down to t he study of scalar transport process in a wall jet flow field.In this paper,we synthetically combined the methods of heat transfer measurement and large eddy simulation(LES)to study the characteristic and mechanism of the momentum and scalar transport.Moreover,the scalar flux model is also investigated.We developed the recycling and rescaling method by imposing constraints to the mean and temporal field at the inlet of the simulation domain.With the modification we made,the recycling and rescaling method becomes more robust.The method is to generate the inlet wall turbulence upstream the wall jet,which is vital for the large eddy simulation.Further,we developed a multi-block mesh generation method,which is constructed upon blocks with curved-faces.With this method,the mesh points can automatically adjust position itself to be of higher quality.This feature is especially useful for meshing the geometry of the wall plane with an inclined cylindrical hole,which can be tough by hand.Moreover,we conducted a detailed investigation on the mesh scale required to resolve wall turbulence in LES.Besides,we also implemented a vortex identification technique,which can track the position of the vortex core and tell the strength of it.Based on all the methods we developed and implemented,we built a platform called ‘les Film Jet'.The platform aims to resolve the turbulence in film cooling jet flow with high quality and details.By utilizing the platform that we built,along with some measurement techniques,we investigated the flow field and scalar transport process in an inclined jet-in-crossflow.The result shows that the complex vortices can be categorized into four vortex systems,namely the in-hole vortex system,the Kelvin-Helmholtz shear layers,the horseshoe vortex system,and the counter-rotating vortex pair(CRVP).We found that the inner-rolling of the K-H shear layer transport the scalar upstream,which is to the opposite of its gradient direction.This inner-rolling process is the reason for the well-diffusion of scalar in the jet flow fields.The K-H shear layer contributes equally by its turbulent transport effects,compared with the mean field transport by the CRVP.We further reveal that the shear flow is the driving force of the turbulent scalar transport.It has two different transport patterns,one along the direction of the scalar gradient,another perpendicular to it.Moreover,we also investigated the effect of the boundary layer states on the flow and scalar field of the inclined jet-in-crossflow.It is found that the jet induces a forced transition when the upstream boundary layer is laminar.It is also noticed that a thicker turbulent boundary layer alters the characteristic of the jet penetration and separation,which results in a decrease of film cooling effectiveness and increase of lateral diffusion on the wall.We make use of the very same method to study the flow and scalar field of the wall jet with an inclined cylindrical hole,either with forward or backward injection.The results show that the backward-injected jet produces a more uniform coolant coverage.We found that this phenomenon is attributed to the separation zone downstream the jet exit when the coolant is injected backward.For the cases with a trench,we reveal th at the downstream trench wall blocks the coolant from separation and produced a lateral redistribution of coolant.The reason for the huge difference of effectiveness enhancement with a trench for the forward and backward injection is discussed.Considering the scalar flux models,we studied its performance in predicting the scalar field,based on the time-averaged LES data.The results demonstrate that the gradient diffusion hypothesis always produce a scalar field of insufficient scalar diffusion.We revealed that the logic of the high-order gradient diffusion models is to add a component perpendicular to the gradient direction.A better understanding of the mechanism in scalar transport of the film cooling jet flow,can be very helpful in improving the sc alar models and performance of scalar field prediction.