Research On Matching Design Of The Meridional Expansion Angle For The Last Stage In High Power Steam Turbine

The actual last-stage long blades of a steam turbine are taken as the research object,different combinations of expansion angle combinations are chosen to combine with four basic types of end-wall profiles: equal speed gradient,equal pressure gradient,Witozinsky curve and fifth degree polynomial curve.The upper end wall of the cascade is designed to examine the effect of the matching of different expansion angle combinations and end wall profile types on the operation of the last stage cascade of the turbine.The flow of wet steam in the cascade is numerically simulated in CFX,the boundary conditions of the numerical simulation are set according to the actual operating conditions.Since the actual droplet parameters are not the concern of research,the wet vapor equilibrium phase transition which sets the flow medium and treat the steam as a homogeneous mixture of liquid water and water vapor is selected.The three-dimensional N-S equations are solved.The numerical simulations of the last stage prototype cascades were performed to investigate the flow of wet steam in the last stage.The results were analyzed from the aspects of profile pressure distribution,humidity,Mach number,etc.The flow at the root and middle sections was obviously transonic while subsonic at the top section where the wet steam does a simple expansion flow.The lowest pressure point generally occurs after the axial chord length 0.7,which has a significant post-loading characteristic.There is a high humidity zone near the throat,corresponding to the lowest point of pressure,that is,the location of condensation.The shock wave structure can be observed in the root and middle section of the stator blade,and the cross section in the middle is most obvious.The flow in the moving blade cascade has obvious supersonic characteristics,and the Mach number at the top section even reaches 1.8,resulting in a large loss.Based on the analysis of the flow field structure of the prototype cascade,the four basic end-wall profiles such as equal speed gradient,equal pressure gradient,Witozinsky curve and fifth degree polynomial curve were selected.The end-wall profiles were designed to obtain 16 schemes,which were numerically simulated in CFX.The results were analyzed from many aspects.The results show that for the same expansion angle,the intensity of the condensation shock in the equal pressure gradient cascade is the smallest,and the strength in the equal speed gradient cascade is the largest.However,due to the mutual interference with the pneumatic shock waves,additional losses are produced near the exit in equal pressure gradient cascades.As the expansion angle increases,the condensation phenomenon is enhanced and the humidity in the grid is significantly increased.Due to the latent heat released by the condensation,the area of the local high temperature zone on the suction surface of the blade increases,and the uneven temperature distribution around the blade is enhanced.Large meridian expansion will cause serious separation of internal flow.When the expansion angle is small,equal pressure gradient endwalls can better suppress the separation at the upper end wall and reduce the export loss,but considering the spatial flow,the fifth degree polynomial curve endwalls can improve the flow near the lower end wall.When the degree of meridional expansion is small,the selection of equal pressure gradient type end wall profile can improve the aerodynamic performance of the cascade,obtaining higher stage efficiency and more reasonable distribution of radial reaction,and when the meridional expansion is large,the fifth degree polynomial curve endwall should be selected.In practical projects,meridian expansion is an unavoidable issue.Choosing the appropriate end wall according to the degree of expansion can minimize the losses caused by the expansion and ensure the best performance of the cascade as possible.Given the adequate designing conditions,the design of composite profiles can be considered to optimize specific local locations to achieve overall performance improvement.