Study On Film Cooling Of Turbine Blades And Coolant Mixing Loss

With the constant demand on the high performance of gas turbines,the requirements on turbine cooling technology are also increasing.Film cooling technology is one of the key technologies for gas turbine design.The use of film cooling reduces the metal temperature of turbine vanes/blades.Meanwhile,the mixing loss between the coolant and the mainstream caused by film cooling reduces the turbine aerodynamic performance,which in turn affects the thermal efficiency of the gas turbine.In order to reduce the mixing loss due to film cooling,it is necessary to further study the mixing process and the mixing mechanism while study the film cooling effectiveness,and develop a proper correlation for the better prediction of the mixing loss.In this paper,DES and SAS methods were employed to simulate the film cooling on flat plate model.With the frequency domain analysis on the velocity field downstream of the film hole,it was revealed that the coolant breaks into small-scale vortices with higher characteristic frequencies at the exit of the film hole.These small vortices merged with each other after exiting the film hole to form slightly larger vortices,whose characteristic frequencies is lower.The large vortices then flow steadily downstream and gradually dissipate in the process of mixing with the main stream.According to the change of vortex structure,the mixing process of coolant and mainstream can be divided into three stages:"detach-merge-dissipate",while the film cooling field can be divided into four regions:the film hole,the detaching area,the merging area and the developing area.The merged vortices form the three-dimensional structure of the kidney-shaped vortex pair.The RANS method was used to simulate the film cooling on the pressure surface and suction surface of a cascade.The entropy generation analysis method was used to analyze the numerical results,verifying the feasibility of this method in predicting the mixing losses,and at the same time,demonstrating its limitations in dealing with changes in blade losses.Using the total pressure loss coefficient to analyze the results,it was found that the inner hole loss,mixing loss,and total loss were linearly related to the kinetic energy ratio of the coolant and the mainstream.Based on this finding,a correlation was preliminarily fitted to predict the total pressure loss coefficient of film cooling cascades.This correlation is the basic form of the mixing loss model.The five-hole probe measurement technique and PSP measurement technique were respectively employed to measure the aerodynamic losses and the film cooling characteristics of the cascade.In order to make the total pressure loss coefficient have the same physical definition for uncooled blades and film cooling blades,the calculation process of the traditional total pressure loss coefficient is analyzed,and an ideal virtual mixing process is proposed to find the equivalent inlet state to the inlet state of coolant and the mainstream.Then the total pressure loss coefficient can be calculated based on this state as a basis.This new method of calculating the total pressure loss coefficient takes the status difference between the coolant and the mainstream into account.Therefore,it has a clear physical meaning compared with the traditional method.The experimental results were processed and analyzed using this new method.It was found that the position of the film hole row and the shape of the hole had a significant effect on the mixing loss,and the influence of the two was in the same magnitude.Based on this,a non-dimensional mainstream velocity factor is added to the correlation of the total pressure loss coefficient to account for the effect of the hole row position.And the correlation is fitted with the experimental data as the mixing loss model.The position of the hole row also has a great influence on the film cooling effectiveness.By adding a factor of the main stream speed,an improved correlation for the cascade film cooling effectiveness is proposed.This correlation can be used to calculate the cooling effectiveness based on the cooling effectiveness of flat film cooling.The film cooling performance of the leading edge,trailing edge and end wall was measured and analyzed.The film holes arranged in parallel enhanced the lateral spread of coolant,improved the cooling effectiveness of the film-covered area,and also increased the mixing loss due to the mutual interference of the coolant between the nearby two rows.The influence of the staggered row arrangement is on the contrary.The mutual interference the coolant from different rows is weakened,and the spread of the coolant in the lateral direction is also weak.However,this arrangement improves the downstream film covering area,and the averaged film cooling effectiveness is improved.In addition,the mixing loss of the staggered row arrangement is also smaller.Increasing the width of the slot of the trailing edge will significantly increase the spread of the coolant and the cooling effectiveness,but at the same time it also makes the interaction between the coolant and the slot sidewall intensify,and increases the secondary flow loss.Different rows of holes in the end walls have different cascade flow environments,and the main influencing factors of the coolant are different.The outflow of coolant before the leading edge of the cascade is mainly affected by the stagnation of the leading edge of the blade;the pressure gradients near the front of the tunnel are larger,and the outflow of coolant of the hole row on this position is not uniform;the hole rows at the rear of the channel are in areas where channel vortices and wall secondary flow are fully developed,the outflow of coolant is limited to the corner areas of the suction surface and the end wall,which improves the film cooling effectiveness locally,and also increases the mixing loss drastically.