Investigation On Flow Characteristics And Cooling Performance Of Turbine Trenched Film Cooling Hole

Film cooling is one of the important cooling technologies for the hot components of the gas turbine.With the higher demand for power output and efficiency,the cooling requirements in the hot components are increasing,and high-performance film cooling technologies are urgently needed.However,the turbine film cooling is limited by some factors such as manufacture,aerodynamic performance and complexity of auxiliary devices.Therefore,this dissertation starts with the cooling structure of feasible manufacture to explore the main flow characteristics and details that affect the cooling performance.Then,based on this,its cooling performance should be further tapped and optimized.Meanwhile,the influences of its complex factors should also be taken into full account.These can provide a reliable theoretical guarantee for its industrial application in the future.In this dissertation,the trenched film cooling hole is taken as the main research object.The large eddy simulation(LES)and Reynolds Averaged Navier-Stokes(RANS)models are used,considering their advantages.Starting from the time-averaged and unsteady flow mechanism of the trenched hole,the improved structures are proposed based on the flow characteristics.Then,the cooling improvement of trenched holes is also investigated under the stagnation flow characteristics of turbine leading edge.In the end,their cooling characteristics are further evaluated on the actual turbine vane.Firstly,the LES method is employed to systematically study the time-averaged and unsteady flow fields of the trenched film cooling hole on the flat plate model.The differences with the cylindrical hole are included,and the effects of different flow and geometrical parameters are also revealed.The results show that,for the 0° compound angle,the downstream instantaneous vortex structures mainly include Hairpin-like vortices,K-H vortices,stagnation vortices,and a series of unsteady vortex tubes.The Hairpin-like vortices at different positions provide vorticity for the counter-rotating vortex pair(CVP)with a larger spatial size and the anti-CVP in the time-averaged result.Their different vorticity and vortex distributions at different blowing ratios change the local film cooling effectiveness accordingly.For the 90° compound angle,there is an asymmetric vortex with a larger spatial size downstream of the trenched hole.What is different from the 0° compound angle lies in that the separation region in the windward side is another source of the asymmetric vortex.Besides,the trench has limited influence on downstream development of the flow in the hole.Therefore,the downstream cooling effectiveness is more affected by the different coolant inflow directions in the plenum.Secondly,based on the mechanism study of the trenched hole,the RANS model is used to investigate the enhancement of the improved downstream trench lip and the trenched cooling unit with a stronger anti-CVP.The applicable flow conditions are summarized,and then the LES method is applied to study the streamwise instantaneous vortex development under the condition of vortex interaction.The results show that the fillet trench lip is beneficial to the reduction of coolant normal velocity and eliminates the adverse effect of the downstream separation region.Compared with the baseline case,the fillet trench lip can be used at the blowing ratios ranging from 0.5 to 1.0.In the trenched cooling unit with a stronger anti-CVP,another coolant jet is introduced to provide the vorticity.Whether the coolant jet comes from a cylindrical or a trenched hole,an obvious enhancement of film cooling effectiveness can be obtained.The upstream trenched hole can significantly accelerate the breaking of the detrimental Hairpin-like vortices.However,a series of unsteady vortices intensifies the mixing between the coolant and mainstream,which weakens the streamwise extension of coolant.Thirdly,in the simplified turbine leading edge model,the differences of cooling characteristics between cylindrical and trenched holes are investigated by the RANS model.The LES method is used to summarize the differences of flow and cooling mechanism.Based on the flow features of leading-edge trenched holes,the stable cooling performance is further demonstrated under the condition of mainstream inlet swirl.The results show that the trenched hole on the stagnation line can only obtain a superior coolant coverage at the blowing ratios between 1.0 and 2.0.The unsteady vortex tube near the stagnation line is an important factor for the downstream transport of coolant.At blowing ratios ranging from 0.5 to 3.0,the the cooling performance can be enhanced by arranging the trenched hole at the position 25° away from the stagnation line.This arrangement can also narrow the region with high heat transfer.Besides,the trenched hole can also reduce the adverse effect of mainstream inlet swirl on the coolant distribution of the leading edge.The improvement becomes more obvious as the blowing ratio increases.Finally,in order to further evaluate the application of the above cooling structures and conclusions in the actual turbine,the flow fields and cooling performance on the turbine leading edge and suction surface are studied using the RANS model.The tendency of temperature distributions on the thermal barrier coating and the metal surface is summarized by aero-thermal coupling calculation,comparing with the adiabatic simulation.The results show that,when three rows of trenched holes with a 25° inclined angle are arranged on the leading edge surface,a significant cooling improvement can be brought.However,the applicable range of coolant mass flow is varied because of the different local flow conditions on the suction and pressure side.The cooling unit with the trenched hole can also improve cooling effectiveness under different blowing ratios and Reynolds numbers.In addition,the cooling structures with the trenched hole would not bring the aerodynamic losses.The results in the aero-thermal coupling simulation show that the advantages of the trenched structures are more obvious on the thermal barrier coating.By comparison,the internal metal surface is more affected by the coolant flow characteristics in the plenum.