Large Eddy Simulation Of Plasma Aerodynamic Actuation For Improving The Flow Characteristics Of Film Cooling

Film cooling is one of the most important cooling methods in modern gas turbine engines.To improve their performance,gas turbines are required to operate at a continuously higher turbine inlet temperature which will exceed 2 300 K in the future.As a result,the traditional film cooling technology cannot meet the needs of advanced cooling configuration.So it is urgent to explore new and more efficient film cooling technologies.For the film cooling,how to suppress the cold jet penetrating into the hot gas and broaden the coverage of cooling film in the streamwise and spanwise directions are the key scientific problem to improve film cooling efficiency.As a new type active flow control technology,DBD(Dielectric barrier discharge)plasma flow control technology takes on a look of prosperity in recent years,providing new concepts and methods to further enhance the film cooling efficiency.Firstly,based on the discharge characteristics of DBD plasma actuators and experimental data,the plasma aerodynamic actuation phenomenological models were constructed and verified.Then large eddy simulation(LES)was performed to investigate the turbulent flow structures and vortex dynamics of flat plate film cooling with a standard plasma actuator installed downstream cylindrical hole.Subsequently,the effects of plasma aerodynamic actuation on the cooling jet exit velocity components,jet trajectory and turbulent flow field were analyzed considering different geometric parameters and aerodynamic parameters.Meanwhile,the evolution characteristics of film cooling flow filed before and after plasma aerodynamic actuation were also studied to reveal the mechanism of the standard plasma actuator for enhancing the film cooling efficiency.On this basis,the influences of the plasma aerodynamic actuation on the turbulent mixing process in the film cooling were investigated when the standard plasma actuator s were arranged symmetrically in spanwise direction.The results show that the downward force and streamwise momentum injection effects induced by plasma aerodynamic actuation are dominant when the standard plasma actuator is arranged in streamwise direction.Consequently,the cold fluid stays closer to the wall and flows faster,and the coherent structures in the film cooling reduce in size and break into smal l scale near-wall streaks,preventing the cold jet lift-off from the wall.Furthermore,the plasma aerodynamic actuation exerts a predominant spanwise momentum injection effect on the film cooling flow field as the standard plasma actuator is arranged in spanwise direction.This promotes the spanwise wall coverage of the cooling film and induces a new vortex pair which has an opposite rotating direction from that of counter rotating vortex pair(CRVP).On the basis of the above study,we proposed a saw-tooth plasma actuator which rendered the plasma aerodynamic actuation induced flow in both streamwise and spanwise directions.Firstly,the impacts of the saw-tooth plasma actuator arrangements on the vortex dynamics of flat plate film cooling with cylindrical hole were studied by LES.The results show that the plasma aerodynamic actuation induces a new vortex pair which rotating in opposite direction with the CRVP and produces a spanwise momentum injection effect which entrains the cooling fluid along the spanwise direction when the tip of the saw-tooth locates at the centerline of cooling hole.Thus the cooling film on the wall expands while propagati ng downstream.Whereas the root of the saw-tooth locates at the centerline of cooling hole,the new vortex pair induced by the plasma actuator has a same rotating direction from that of the CRVP,which entrains the hot gas underneath the cold jet.This results in that the cooling film on the wall shrinks while approaching downstream.Secondly,the effects of the saw-tooth plasma actuator on the turbulent flow structures and aerodynamic loss of flat plate film cooling with cylindrical and fan-shaped holes are thorough studied considering different blowing ratios.Then the impacts of the saw-tooth plasma actuator on the development process,spatial correlation and spectral analysis of coherent structures were investigated by direct comparison of before and after the plasma aerodynamic actuation.The loss separation method and entropy generation analysis method are adop ted to analyze the aerodynamic loss of the film cooling concurrently.All of these revealed the unique physical mechanisms of the interactions among the saw-tooth plasma aerodynamic actuation,cold jet and hot gas.The results show that the novel film cooling design of the fan-shaped hole with the saw-tooth plasma actuator can significantly improve the film cooling efficiency.Thanks to this novel film cooling design,the coherent structures in the film cooling flow reduce in size and break into near-wall streaks early.And the two order space correlation function and spectrum function also confirm that the saw-tooth plasma actuator aerodynamic actuation decreases the size and promotes the organizational of coherent structures from the physical space and wave-number space,respectively.Meanwhile,the new vortex pair induced by the saw-tooth plasma actuator enhances the turbulent transportation of cooling fluids in the spanwise direction.Furthermore,the saw-tooth plasma actuator aerodynamic actuation reduces the mixing loss downstream of cooling hole,especially the mixing loss distributions in the cross-sections downstream fan-shaped hole are developed from single peak to double peaks.And the irreversible entropy creation due to the film cooling is also diminished after the saw-tooth plasma actuator aerodynamic actuation.Finally,the saw-tooth plasma actuator aerodynamic actuation could increase the film cooling efficiency and decrease the aerodynamic loss of the film cooling differently for all LES cases.At last,in order to assess the performance of DBD plasma actuator on the turbine vane film cooling,LES had been conducted to investigate the film cooling flow structures on the pressure side of C3 X turbine vane with DBD plasma actuator.Then the space-time evolvement process of vortex structures and coherent structures in the film cooling flow fields were presented,and the physical mechanisms of the standard and saw-tooth plasma actuators for improving the film cooling efficiency on pressure side were made a thorough analysis.After that,the influences of the novel film cooling design on the flow structures of the pressure side film cooling were analyzed systematically.The results show that due to the high speed hot gas and favorable pressure gradient on the pressure side,the CRVP is stretched far downstream along the wall surface,and then its terminal initially breaks into discrete coherent structures.As the saw-tooth plasma actuator exerts a aerodynamic actuation on the film cooling of fan-shaped hole,the phenomenon of the cold jet detaching from the wall disappears and the coherent structures reduce in size and intermittency,thus promoting the continuity and evenness of the cooling film.Eventually the centerline film cooling efficiency and spanwise-averaged film cooling efficiency are enhanced.