Study On The Flow And Heat Transfer Characteristics Of High Efficiency Shaped Film Cooling Structures

Advanced aero-engines and gas turbines require an increasing cooling performance of turbine blades.External film cooling is a very important cooling technology for turbine blades.In order to obtain an efficient external cooling effect,three new types of film cooling structures are investigated in this paper,which are with high cooling efficiency,including the chevron hole,the sine-wave shaped trench hole and the mesh-fed slot.Experimental measurements and numerical simulations were used to study the film cooling characteristics,flow resistance and turbine loss,based on both plate and blade models,and to reveal their flow and heat transfer mechanism and to obtain empirical correlations that can be used for engineering design.At last,the comprehensive comparison among these three structures was conducted on the blade.For chevron hole,the main mechanism for improving the film cooling effectiveness are the span-wise guiding effect of the chevron grooves and the deceleration effect of the outlet expansion.At the outlet of the chevron hole,the normal velocity of the jet is significantly reduced.At the same time,the jet in the chevron grooves still has a certain spreading velocity,which is conducive to the spread of the jet.Under the influence of these two factors,the coupled vortex pair downstream of the chevron hole is flattened,so that the coverage span of the jet is equal to the outlet width,and the good attachment can be maintained.Also affected by the coupling vortex pairs downstream of the hole,the heat transfer coefficient is higher downstream of the chevron grooves.Chevron holes have less flow resistance than the cylindrical holes.Increasing the divergence angle of the chevron hole will further enhance the effect of lateral guiding,which is beneficial to the jet coverage.Increasing the main hole inclination will weaken the lateral guiding effects,which is not conducive to the jet coverage.The film flow state of the chevron hole on blade is different from the flat plate model.At the suction 1 position,due to the influence of the inverse normal pressure gradient,the film is better attached;at the pressure side,due to the positive normal pressure gradient,the edge of the coupled vortex pair is significantly lifted,and the film cooling effectiveness is reduced.The discharge of chevron hole has a higher effect on the loss of the cascade than the cylindrical hole.The sine-wave shaped trench hole is a new structure proposed in this paper on the basis of the transverse trench hole,and the film cooling effectiveness is higher than that of the transverse trench hole.The reason is that the diversion effect of the sine-wave shaped trench can enable the film jet to form a large span-wise velocity in the groove.This causes the gas film jet to be mixed with the main stream after jumping out of the trench.The coupled vortex pair formed is an anti-kidney vortex structure,which is more conducive to the spreading of the jet.The increase of wave peak and trench depth will strengthen the diversion effect of the sine-wave shaped trench,thereby further improving the film cooling effectiveness.However,when the diversion effect is too large,the collision loss of adjacent jets will be increased and the jets will be lifted,which is not conducive to film coverage.Under the effect of coupled vortex pairs,the heat transfer coefficient downstream of the region between the holes is higher.Because the sine-wave shaped trench has sufficient flow guidance to the jet,its flow resistance is smaller than that of the transverse trench.The film cooling characteristics of the sine-wave shaped trench hole on the blade model are similar to those of the flat plate model.Affected by different local normal pressure gradients,the film cooling effectiveness on the suction side is higher than that on the pressure side.The effect of film discharge from the shaped trench hole on the loss of the cascade is not much different from that of the transverse trench hole.The film cooling effectiveness distribution of mesh-fed slot is close to the ideal twodimensional slot,and the uniformity of the laterally distribution is much higher than that of the discrete holes.However,a certain degree of non-uniformity in the distribution of the film cooling effectiveness is still found.The reason is that a separated reflux vortex is formed at the backward side of the pin fin,and the vortex has a higher normal velocity than the jet between pin fins.This velocity gradient will be formed coupled vortex under the force of mainstream downstream the slot.More jets will be concentrated downstream the pin fin by the coupled vortex,which leads to a higher film cooling effectiveness.This phenomenon doesn't changed with different pin fin arrangement.Also affected by the coupled vortex,the heat transfer coefficient distribution still has a non-uniformity.In the suction side of the blade model,the distribution characteristics of the film jet of the mesh-fed slot is opposite to that of the flat plate model.The reason is that there is a normal reverse pressure gradient at the slot outlet,which causes the mainstream to be trapped in the low pressure area on the backward side of the pin fin.The diffusion of the jets to the pin fin back side is blocked so that the film cooling effectiveness downstream of the pin fin is lower.The effect of film discharge from the staggered structure on the loss of the cascade is greater than the in-line structure.The flow characteristics of the film cooling structures studied in this paper can be summarized as three types of export deceleration,outlet guiding,and two dimensional intake.The forced guiding at the outlet is the more effective method,and this is also reflected in the comparative analysis of various structures.The sine-wave shaped trench hole shows the best comprehensive performance and can be used in many positon on the blade.The chevron hole can be used at the leading edge area of suction side and the trailing edge area of pressure side.The mesh-fed slot can be used at the leading edge area and mid-span area of the pressure side.