Investigation Of Passive Flow Control For A Highly Loaded Low-pressure Turbine At Low Reynolds Number Conditions

Reducing the number of low-pressure turbine blades through high loaded blade profile design is one of the important development directions for reducing the mass of low-pressure turbine components and improving the thrust-to-weight ratio of engines.However,the boundary layer on the suction side of the blade suffers greater adverse pressure gradient owing to the elevated blade loading whereby the laminar separation is more likely to occur under the working environment of low Reynolds number.Appearance of the separation area results in a sharp increase in losses,which seriously affects the working efficiency of the engine and the endurance time of the aircraft.Therefore,it has become one of the important technical directions to improve the performance of low-pressure turbines and develop a new generation of high-performance low-pressure turbines to research and develop effective flow control means to restrain the adverse effects of laminar flow separation at low Reynolds number caused by load increase.In this paper,the dimple passive flow control technology is studied.Firstly,the development law of suction side boundary layer of the prototype low-pressure turbine cascade is investigated by means of Reynolds average(RANS)method when the turbulence is 1% and Reynolds number is 25,000,45,000 and 100,000,respectively.On this basis,the key geometric parameters of the dimple,including the depth H,the span height D,the spacing S and the direction position of the center of the dimple on the control effect of the dimple flow,were studied under the Reynolds number of 45,000.It is found that the dimple structure can effectively inhibit laminar separation and reduce the loss by promoting boundary layer transition in advance.The optimal dimple scheme reduces the profile loss by 22.0%.Among the studied geometric parameters,dimple depth has the greatest influence on the control effect of dimple flow.When the depth of dimple increases to a certain value,dimple structure will promote boundary layer to complete transition in advance,greatly increasing the flow control effect.Further,in order to explore the difference in flow control effect between the dimple structure and some "quasi-three-dimensional" structures,the U-shaped groove structure and the dimple structure were compared and studied.Compared with u-shaped slot structure,at the same depth and the width,the dimple although the blade was the smaller area of the processing of the processing,but under the low Reynolds number flow field produce larger disturbance effect,boundary layer transition ahead of the inhibition of separation,flow control effect is better,at the same time under the high Reynolds number,the dimple in the boundary layer has a lower level of turbulent kinetic energy,Therefore,the flow control is also better.Considering the RANS method for the separation of boundary layer transition has carried on the modeling and processing,calculation precision is limited,therefore understanding the dimple,on the basis of passive control technology flow control law,and large eddy simulation(LES)method in this Reynolds number in 45000,was 1% under the condition of turbulence degrees,further explore the flow control mechanism of the dimple.By analyzing the displacement thickness,momentum thickness,shape factor and flow wall shear stress of the boundary layer in the calculated results,the development history of the boundary layer under each scheme is defined.By analyzing the evolution process of vortex structure in the flow field,the flow control mechanism of dimple structure was clarified.In addition,different flow structures generated by Ushaped groove structure and dimple structure in the flow field were compared and studied.It is found that the roll-up and breaking of the spanwise vortexes caused by K-H instability are the key factors affecting the transition process in the separated shear layer.The rollup and breaking of the spanwise vortexes are observed in all the calculated schemes,but the roll-up position and breaking process of the spanwise vortexes are affected by the socket structure and U-shaped groove structure.There is a vortex structure with periodic shedding at the trailing edge of the blob-and-cavity structure.In the process of developing to the downstream,this vortex structure promotes the rolling up of the spanwise vortex on the one hand,and the breakage of the spanwise vortex on the other hand,which effectively promotes the process of boundary layer transition.The U-shaped groove structure can promote the boundary layer to roll up the spanwise vortex earlier,and also promote the transition process of the boundary layer.However,compared with the dimple structure,the periodic shedding vortex structure generated by the dimple structure can more effectively promote the spanwise vortex breakage,so it has better flow control effect.