The Combined Influence Of Hole-Blockage And Surface Deposition On The Performance Of Film Cooling

Film cooling has been widely used in modern gas turbine designs,and there are a lot of previous researchers,who paid a large number of attentions to film cooling techniques,and most of them are based on an ideal environment.During the past decades,a lot of experiments were conducted to study the effects of main parameters on film cooling performances,such as the angle and shape of film holes,blowing ratio,mainstream turbulence intensity,and so on.Actually,in real environments,gas turbines may operate in dust-laden environments,for example,taking off and landing on desert ground or flying through volcano dust cloud.The foreign particles in these environments may frequently deposit at the hot surfaces near film holes,and it could cause an increase in surface roughness.The foreign particles could also result in a partial blockage of film holes,which leads to a decrease in cooling effect,and even a catastrophic damage.In the real environments,particle deposition at hot surface and film hole blockage may form during thousands hours,so it is still difficult to study deposition at hot surface and hole blockage.This problem is not well solved up to now,because of the tremendous costs in time and money.There have been a lot of investigations on the influences of individual deposition and individual hole-blockage on film cooling performances.However,it is well known that surface-deposition and hole-blockage may exist simultaneously under the real operation conditions of gas turbine,but in the previous studies,the combined influences of surface-deposition and hole-blockage on film cooling performances were not deeply discussed and analyzed.Therefore,in this paper,the topics are not only in the individual deposition and hole-blockage,but also including the combined influences of surface-deposition and hole-blockage,which has never been studied before.In this experimental investigation,Planar Laser Induced Fluorescence(PLIF)technique is used to detect the effects of individual hole-blockage and surface-deposition on film cooling performances.The experiments are carried out in a low speed water tunnel.The mixture of Rhdamine B with purified water is utilized as coolant and injected from film holes,and then the concentration distributions of Rhdamine B in different mainstream planes are captured by a high speed camera.Wall temperature fields and film cooling effectiveness are obtained by the similarity principle of heat and mass transfer.Firstly,two surface-deposition models are studied,1)Particle deposition is limited to upstream of hole with a peak height of 1.5 diameter of holes(D 1.5-BO);2)Particle forming a trench downstream film hole with a peak height of 1.0 diameter(D1-B0).Secondly,two types of individual hole-blockage are tested:1)hole-blockage at the leading edge of film hole(LB);2)hole-blockage at the trailing edge(TB).Finally,two types of combined models are investigated:1)Surface deposition with leading edge hole-blockage(D1.5-LB);2)Surface deposition with trailing edge hole-blockage(D1-TB).This experimental work reveals some interesting phenomena,which were not reported in previous literatures.For the individual hole-blockage,the location of blockage has a significant influence on film cooling effectiveness,particle blockage at leading edge(LB)results in an increase in film coverage length and cooling effectiveness,and it can be found that film coverage length increases with blockage ratio and blowing ratio.At the same blowing ratio,blockage at leading edge with larger blockage ratio has a longer film coverage and higher cooling effectiveness.At the same blockage ratio,film coverage length and cooling effectiveness increase with blowing ratio.The reason is that the area of hole-exit is reduced due to hole-blockage,which causes an acceleration of coolant flow.If hole-blockage occurs in the trailing edge(TB),it is different,because the blockage(TB)induces a decrease in film coverage length and cooling effectiveness.From the concentration distribution,one can find that film coverage length decreases when blockage ratio increases.At the same blowing ratio,TB with a larger blockage ratio corresponds to a shorter film coverage length and a lower cooling effectiveness.The reason is that a trailing edge blockage causes a stronger penetration of coolant into mainstream.But at the same blockage ratio,film coverage length and larger cooling effectiveness increase with blowing ratio.The experiments of individual deposition model also exhibit some interesting phenomena.The surface-deposition model D1.5-B0 can improve film cooling effect.The reason is that the upstream surface-deposition can uplift mainstream from the wall,and cause an area with low pressure behind the deposition.The second surface-deposition model D1-B0 can enhance the lateral-averaged cooling effectiveness,but decrease the area of film coverage.For the most important part in the paper,the combined effects of models D1.5-LB and D1-TB reveal the following interesting phenomena.1)For individual blockage,the blockage at trailing edge TB results in a decrease of cooling effectiveness,but at leading edge LB results in an increase of film cooling effectiveness.2)For individual deposition model,the cooling effectiveness of individual deposition model D1.5-B0 is higher than that of perfect hole,DO-BO.Although individual deposition model D1-BO results in higher effectiveness,the cooling area is decreased.3)The cooling effectiveness of combined model D1.5-LB is higher than that of perfect model DO-BO(without deposition and blockage).4)The effect of blockage ratio on cooling performances of combined D1.5-LB is opposite to the individual DO-LB,i.e.in the case of combined model D1.5-LB,a higher blockage ratio corresponds to a lower cooling effectiveness;whereas,in the case of DO-LB,the cooling effectiveness increases with the blockage ratio.5)For different models,cooling effectiveness increases with blowing ratio.6)The combined effects of models Dl-TB also reveal some interesting phenomena,which has been never exhibited before.At lower blockage ratios from 0.1 to 0.3,the averaged cooling effectiveness of combined models,D1-TB0.1 and D1-TB0.3,is higher than that of perfect model DO-B0,because deposition effect is dominant.At a large blockage ratio of 0.5,and the averaged cooling effectiveness of the combined model D1-TB0.5 is lower than that of DO-B0,because the blockage effect is dominant.In the case of individual deposition model D1-B0,although the lateral-averaged film cooling effectiveness is augmented,the area of film cooling is reduced.