Study On Superposition Prediction Of Film Cooling Effectiveness Of Double Rows Of Discrete Holes

For advanced turbine blade,film cooling is used as an external cooling method.Multi-row film holes are usually arranged on the leading edge,the suction side and the pressure side.It’s cooling performance can be predicted by the known single-row cooling efficiency data and the superposition prediction method in engineering application,while its accuracy is limited.Thus,studying on the interaction between the multi rows of film holes and finding the principle of the superposition prediction are beneficial to supply an accurate thermal boundary condition.This paper focuses on the interaction between jet flows.In order to study that,the experimental and numerical researches are made.Two-row cylinder holes and shaped holes are studied on flat plate,firstly.Also,the superposition is studied on blade.The experiment data are used to obtain the principle of superposition prediction,while the numerical simulations are used to find out the interaction between film holes.According to these,the main factors that affect superposition prediction is found.At last,a more accurate prediction method is carried out.For the research on plate,the influences of blowing ratio,film hole arrangement,film hole spacing and deflection are analyzed.For the two-dimensional slit film hole,it is with a weak interaction between the two layers of the cooling film.Under this condition,the prediction of film cooling efficiency is relatively more accurate.For cylindrical holes,the interaction between the two rows of film holes is not strong at a low blow ratio.As the blow ratio increases,the interaction between the front and rear cooling film is enhanced,which results in decreasing the accuracy of prediction.The arrangement of film holes can affect the accuracy of superposition prediction.The inline arrangement makes front and rear jets overlap and increases the thickness with which the superposition condition is layered.Also,the kidney vortex pair from the front row plays a "suction" effect on the rear film for which makes the film coverage worse.Because of these,the predictions of film cooling efficiency are higher than the measure ones.For the staggered structure,the front and rear jets are arranged in a staggered arrangement spanwisely,and the interaction of jets is lumpy superposition,which is weaker than that in inline arrangement.The front jet plays an oppressive effect on the rear jet and the film coverage is improved.The superposition calculation is more accurate for the staggered arrangement structure.The hole row distance is one of the factors that influence the accuracy of cooling efficiency superposition calculation.Under the condition of small hole row spacing,the interaction is strong and the accuracy of superposition calculation is poor.The influence of the front flow under the condition of large spacing is weaker,and the accuracy of the superposition calculation is better.When there is a deflection angle of the film hole,the accuracy of superposition prediction is improved.Because of the interaction of the jet between the front and back film being similar to the superposition of the two dimensional slit hole,the prediction is more accurate.Thus,the accuracy of the superposition prediction depends on films interaction.Therefore,the two parameters of blow ratio and distance from the film hole are introduced to the basic Sellers formula.As for the superposition of cylindrical film hole on blade,besides the parameters above,the influence of the blade curvature is studied.At a small blow ratio,the film is more easily to be attached to the surface under the influence of the curvature on suction side.With the increase of the blowing ratio,the film is more easily to escape from the wall surface.In addition,on the suction side,the principle is different from it on the pressure side.For the cylindrical hole on the suction surface,the accuracy of superposition calculation is better.On the pressure side,the jet flow is easily to escape from the wall and the interaction between two rows of film holes is relatively strong,which result in a lower superposition calculation accuracy.For the superposition of shaped hole in the cascade channel,the dustpan film hole and the converging-expanding hole are studied in this paper.Because of the expansion effect in the outlet for dustpan hole,it makes the jet flow uneasily to escape from the wall,the spanwise coverage larger and the coverage shorter in the streamline direction.The interaction on the suction side is weak and the accuracy of superposition calculation for dustpan hole is better.While on the pressure side,the accuracy is lower for the enhanced interaction between the cooling films.As for the converging-expanding hole on the suction side,the accuracy of superposition calculation is higher.On the pressure side,although the interaction between the film holes is stronger,the accuracy is ensured because of it being similar to the “layered” superposition effect.