Investigation On The Heat Transfer/Flow Characteristics And Multidisciplinary Design Optimization Of The Turbine Casing With The Coupled Cooling Structure

The blade tip clearance between turbine rotor blade and stator casing is essential to avoid abrasion between the blade and casing during the operation of gas engine.However,a larger tip clearance will cause flow loss and then decrease the turbine efficiency.A lot of methods have been developed to control the blade tip clearance at a proper value so that both the space between blade and casing and the turbine efficiency can be maintained at the same time,for example,the passive(PCC)and active clearance control(ACC).In particular,an effective manner is the thermal ACC of the turbine casing,which controls the thermal displacement by the cooling air and the temperature distribution by the impingement jet cooling.However,the thermal ACC has a limited capability on the displacement control of high pressure turbine(HPT)thin-walled casing under the hot gas.For a further enhancement of the thermal discplacement control and the turbine efficiency,it is urgent to develop a more effective cooling manner to control temperature distribution,displacement,and deformation of the casing.In this thesis,we propose a coupled cooling method of the turbine casing.The heat transfer/flow characteristics and the thermal displacement have been investigated,and the multidisciplinary design optimization(MDO)of the internal cooling structures has also been conducted.The main work includes:(1)Considering the impingement cooling structure of the HPT casing,the influence of the impingement cooling hole parameters on the casing heat transfer and flow performance has been investigated,including the hole diameter(i.e.,1 mm,2 mm,3 mm),inclined angle(i.e.,90°,±60°,±45°),and arrangement(i.e.,the inline and staggered arrangement).The results have shown that local convection effect can be improved with the decrease of the impingement cooling hole diameter,while a small diameter causes nonuniformity of the overall cooling effect.A better overall cooling performance of the turbine casing is obtained when the hole diameter is 2 mm.Although the hole inclination can not improve the convective heat transfer in this impingement cooling system,it can be used to guide the cooling air on the thermal-load components to improve the cooling effect.Compared with the inline arrangement case,the cooling effect on the HPT casing is enhanced with the staggered arrangement of the impingement holes.However,the impingement jet has been deflected distinctly with the staggered arrangement,which has led to the heat transfer degradation on the target surface by 10%.(2)The roughness elements including the cambered rib,square,and round pin-fin have been designed on the target surface in the impingement cooling system,i.e.,a coupled cooling system with the impingement holes and the roughness elements.The influences of the roughness elements and the cambered rib parameters(height and width)on the heat transfer and flow characteristics of the HPT casing have been analyzed.It can be concluded that the presence of roughness elements disturbing the cooling flow has degraded the effect of crossflow and enhanced the convective heat transfer.Especially for the cambered rib structure,the average Nusselt number of the target surface is increased by up to 15%and the decrease of the average temperature is 28 K compared with the smooth case.The influence of cambered rib parameters on the heat transfer characteristics is distinct in the cooling passage.It can obtain better heat transfer characteristics on the target surface when the upstream width of the cambered ribs is 2.5 mm and the downstream width is 2 mm.(3)The fluid-thermal-structural coupling analysis has been conducted for the HPT casing with the impingement holes and the cambered ribs.The influences of cooling manners and cambered rib parameters on the von Mises stress and the radial displacement of the turbine casing have been investigated in detail.The results have shown that the maximum von Mises stress and the radial displacement of the turbine casing with the coupled cooling structures are decreased by 21%and 10%compared with those of the smooth case.These comparisons demonstrate that the cambered ribs can reduce the casing thermal stress and displacement.On the other hand,the change of the cambered rib height and width has a great influence on the casing displacement.When the cambered rib height is 1.5 mm and its width is 2 mm,the maximum radial displacement has been decreased by 11%compared with that of the smooth case.(4)The multidisciplinary design and optimization of the casing internal cooling structures,including the impingement hole diameter and the cambered ribs heights and widths,have been conducted based on the design of experiment and the surrogate model methods.The optimized result of the impingement hole diameter is 1.21 mm.The widths of cambered ribs change more significantly than the heights along the cooling passange,and the optimized values of the cambered rib height are located in a close range of 1.5 mm.Compared with the original structure of the coupled cooling system,the average temperature reduction of the optimized casing is 16 K and the maximum radial displacement has been decreased by 8%.(5)The influence of blade tip structures on the casing/tip heat transfer,flow characteristics,and the tip flow pattern has been studied based on the optimized casing structure.The tip configurations with four types of squealer tip(i.e.,the pressure side squealer,the suction side squealer,the cavity and the flat tip with streamwise ribs)and three tip injection locations(i.e.,near the pressure side,near the suction side,and at the camber line of the blade tip)have been investigated.The results have indicated that the presence of turbine blade has a significant influence on the heat transfer and flow characteristics of the casing interaction surface which has a direct contact with the hot gas,while it presents negligible influence on the casing internal heat transfer performance.For the flat tip with streamwise ribs case,the casing heat transfer performace and the tip temperature uniformity have been promoted,and the passage loss has also been reduced.The tip injection has separated the casing/tip surface from the hot gas.Especially for the tip injection location along the camber line,the tip average temperature reduction is 138 K compared with the non-jet tip.Furthermore,the tip injection has a better advantage on the decrease of the casing temperature gradient than the squealer tip case.The maximum von Mises stress and radial displacement have been decreased by 27%and 11%,which show that the tip injection can improve the overall turbine performance.