Numerical And Experimental Investigations Of Flow And Heat Transfer Characteristics On A Squealer Tip

To avoid the friction between the high pressure turbine blades and the casing,there is usually a small gap between them,which is called tip clearance gap.A part of the mainstream near the tip flows through the tip clearance gap under the pressure difference between the pressure side(PS)and the suction side(SS)of the blade,known as tip leakage flow.The leakage flow not only reduces the efficiency and output work of the turbine,but also makes the tip of blade expose to high heat load.So it is very important to design an improved tip shape with good cooling system.The squealer tip can not only allow smaller tip clearance gap,but also form the labyrinth seal,these all can reduce the the leakage flow,and then the leakage loss and the tip heat transfer rate are reduced.So the squealer tip is widely used in engineering.In this paper,the tip leakage flow and heat transfer characteristics of the squealer tip are investigated numerically and experimentally.The main research contents and conclusions are as follows:1.Numerical study was performed to study the formation mechanism of the tip leakage flow and heat transfer.The tip leakage flow field,the development of the leakage vortex,the distribution of pressure coefficient and heat transfer coefficient on the tip surface were showed.Moreover,the leakage flow filed was divided in detail.The effects of tip clearance height,the squealer height and casing relative motion on the leakage flow and heat transfer characteristics of squealer tip were analyzed.The results show that the the tip leakage flow rate increases with the increase of the tip clearance height,so the leakage loss and the heat transfer coefficient on the tip surface are increased.The blockage effect of the squealer to the tip leakage flow increases with the increase of the squealer height,so the leakage flow rate and leakage loss decrease,and the heat transfer coefficient on the tip surface firstly increases and then decreases.The relative movement of the casing leads to the formation of a scraping vortex within the cavity,which is under the casing and near the SS squealer.The scraping vortex has a blockage effect to the tip leakage flow,so the tip leakge flow rate,leakage loss and heat transfer coefficient on the tip surface are all decreased.2.According to the characteristics of the tip clearance leakage flow field,3 cutback structures on the squeaelr tip were proposed from the point of reducing the mixing loss between the mainstream and tip leakge flow.The tip leakage flow and heat transfer characteristics were numerically investigated.The results show that the pressure side trailing edge cutback will decrease the leakage flow resistance near the trailing edge,so the tip leakge flow rate,leakage loss and heat transfer coefficient on the tip surface are all increased.And the effect increases with the increase of the cutback length.On the one hand,the suction side trailing edge cutback can reduce the leakage flow resistance near the trailing edge to the leakage flow,so the total leakage flow rate increases.On the other hand,the suction side trailing edge cutback can change the leakage flow path near the cutback area,and suppress the mixing of the leakage flow and the mainstream effectively compared to the cavity tip.The tip leakge flow will be reduced when the suction side trailing edge is appropriate,which is SS-TEcut2 in the study range of this paper.The suction side leading edge cutback will decrease the leakage flow resistance near the leading edge,so the leakage flow rate increases.However,part of the leakage flow will flows towards the traiing edge along the suction side of the blade instead of mixing with the mainstream.So the tip leakage loss is reduced.In addition,the suction side leading edge cutback will increase the heat transfer coefficient at the leading edge of the blade tip near the suction side and decrease the heat transfer coefficient at the leading edge of the blade tip near the pressure side.3.The effects of gas film-hole location,film-hole distance and film-hole angle on tip leakage flow and cooling characteristics of squealer tip with tip-cooling were investigated numerically.The results show that the cooling jet can make the leakge flow rate decrease slightly,while the leakage loss increase slightly.The influence of film-hole location on the leakge flow rate and leakage loss of squealer tip with different film-hole arrangements is small.The flim coverage on the squealer blade tip surface varies with the film-hole location.The entire tip surface obtains high film-cooling effectiveness when the film holes are arranged as type-C.The film-cooling effectiveness on the squealer tip can be increased by improving the local film-hole distance and angle on the base of type-C.4.The influence of the pressure side and suction side trailing edge cutback on the leakage flow and cooling characteristics of the squealer tip with film cooling were numerically investigated.The results show that the effect of pressure side trailing edge cutback on the flow path of cooling air is exhibited in two aspects.On the one hand,most colling air will leave the caivity floor before the cutback,on the other hand,some colling air forms local film coverage on the caivity bottom near the cutback.Compared to the full squeaelr tip,the tip film-cooling effectiveness of type-A decreases as the cutback length is longer(TA-PScut1),while increases when the cutback length is longer(TA-PScut3).The tip film-cooling effectiveness of type-D decreases with all pressure side cutback lengthes.The suction side trailing edge will make some cooling air leave the cavity floor through the cutback,so the trailing edge of the caivyt floor is well protected by the film.The film coverage on the cavity bottom increases with the increase of the suction side trailing edge cutback length.5.The effect of relative casing motion on the leakage flow and cooling characteristics of the squealer tip with film cooling were numerically investigated.The results show that the tip leakage rate and loss decreases greatly when a speed is applied to the casing.The effect of it on the film-cooling effectiveness of squealer tip is closely related to the film-hole arrangement.The casing relative motion has the samallest influence on the film-cooling effectiveness of squealer tip with type-B and largest influence on that with type-D.The relative moment of casing will make the film coverage near the pressure side squealer at the middle chord region of squealer tip with film-hole arrangement of type-D decrease.The effect of relative casing motion on the squealer tip with suction side trailing edge cutback is the same with it on the full squealer tip,and its influence on the film-cooling effectiveness on the trailing edge of cavity floor is small.Additionally,this paper also presents a new cooling squealer tip with suction side trailing edge cutback,which can decrease the tip leakage loss and increase the tip film-cooling effectiveness.6.The effects of film-hole location,tip clearance height and blowing ratio on the tip clearance leakage flow and cooling characteristics of the squealer tip with film cooling were experimentally investigated and numerically analyzed.The results show that the numerical calculation and the experimental results are in good agreement.The film-cooling effectiveness on squeaeler tips varies with different film-hole location.The film-cooling effectiveness on the front tip is much better when the film holes are placed as type-EF compared to the other film-hole configurations.The film-cooling effectiveness on the squeaeler tip increases with the decrease of clearance height and the increase of blowing ratio.Under the same coolant usage,in general,type-EF produces the most favorable film cooling performance except for the trailing edge,and type-EC produces wider film coverage extending to trailing edge,while type-ED is demonstrated to be the worst film-hole configuration among the current six film-hole arrangements.