| Internal flow in turbomachinery is a complex flow affected by many factors,and flow separation is the most common and complex flow phenomenon.In axial turbomachinery,the combined action of complex geometry and viscosity causes various forms of secondary flow,which is represented by vortices of different properties.The separation vortices have non-linear characteristics in three-dimensional space.The interaction between dynamic and static cascades forms a complex and changeable unsteady process.Stall and surge phenomena occurring under non-design conditions are closely related to flow separation.The internal flow field structure and the mechanism of loss can be obtained by analyzing the flow separation phenomena and the separation eddy structure model in the axial turbomachinery,which is the key to improving the mechanical performance of the axial turbomachinery.In this thesis,aiming at the internal flow separation mechanism and separation vortices structure of axial turbomachinery,the internal separation vortices structure of cascade is topologically generated using non-linear dynamic analysis,topological analysis and cross-section flow pattern method by numerical simulation combined with experimental verification,and the influence of external excitation conditions on flow separation morphology is studied in depth.Based on the flow separation mechanism of the stationary cascade,the formation mechanism of flow separation in the rotor and the vortex structure was studied,and the structure of leakage vortices in tip clearance and shock-induced flow separation vortices were analyzed.The steady-state flow separation was extended to non-steady-state flow separation,and the influence of rotor wake on the flow separation of stationary blades and the mechanism of flow separation formation were studied.Finally,the influence of roughness on cascade flow separation and its mechanism was explored.The main research contents and achievements of this thesis are as follows:(1)Summarize the research results of flow separation and vortices structure at home and abroad,and derive the characteristic value of flow separation in stationary cascade based on the conical separation theory;The characteristics of 3-D flow separation were deduced and the singular distribution characteristics of blade surface separation were given.The distribution regularity and shape of singular points along the separation line are obtained by combining the nonlinear dynamic analysis.Finally,the applicability of the cross-section flow method to study the mechanical vortex structure of the axial turbomachinery is verified.(2)The condition of two-dimensional flow separation is verified based on the 2-D cascade,and the relationship between the separation form,separation scale,attack angle and Reynolds number of the 2-D cascade is obtained.The singular point distribution of flow separation of the 2-D cascade under different conditions is analyzed by combining the topology.Thereafter,the 2-D cascade was extended to a 3-D cascade and the relationship between separation form,separation scale and Reynolds number at attack angle was mainly studied by experimental analysis.The topological structures of vortices were given by the cross-section flow pattern method.The results of the experiment and numerical simulation show that the flow separation of planar and three-dimensional cascades is affected by both Reynolds number and attack angle,in which the attack angle mainly affects the separation form and Reynolds number affects the separation scale,lays a foundation for the research on the vortex structure inside the rotor.(3)The rotor is divided into transonic rotor and low-speed rotor.First,the low-speed rotor is taken as the research object,its internal flow state is obtained by numerical simulation and the singular point analysis is carried out.The singular point distribution on hub and cascade is analyzed by combining the singular point coordination to get more accurate flow field information and to find the spiral joint hidden at the trailing edge of the blade accurately.For forward-swept blades,flow separation occurs mainly at the corner and the top of the blade,accompanied by obvious pressure fluctuations,and the resulting shedding vortices break up and diffuse into the mainstream.As the flow rate decreases,the number of odd points in the rotor channel increases and the position of the shedding vortex moves forward,and the annihilation speed of the vortex structure slows down gradually.Shock/boundary-layer interference separation is induced when inlet velocity exceeds sound velocity,shock wave is formed at the leading edge of the blade and refracted at the suction surface when rotating speed is high,and weak compression wave is formed at the leaf-like turning point on the suction surface when rotating speed is low.At higher airflow speeds,the resulting shock waves refract in the flow path and separate the pressure surface,while at lower speeds the compression waves do not intersect with adjacent blades.(4)When the turbomachinery operates under variable operating conditions,its downstream inlet flow parameters of the cascade will change.In steady-state,the decrease of speed and flow will result in the increase of inlet airflow angle and the decrease of inlet speed.Therefore,the flow separation of the cascade will occur when the rotor runs under variable operating conditions,which is mainly reflected by the change of the position of the shedding vortices formed in the top area and the corner vortices formed in the root area.In the unsteady state,the separated flow pattern on the cascade surface changes periodically due to the effect of the moving blade wake on the cascade.In addition,when the cascade is in critical condition,the surface topology of the suction surface is complex,and periodic changes of open and closed separation occur at the top of the cascade.(5)Taking the relationship between cascade flow separation morphology and cascade total pressure loss coefficient as the starting point,Reynolds number and equivalent sand roughness are unified into dimensionless roughness.The effects of different roughness and roughness location on cascade flow separation and its loss are analyzed by numerical simulation.The results show that there is an optimum relative roughness6)~+for the airfoil in this thesis to minimize the cascade loss.Roughness can change the flow separation form on the blade surface and effectively restrain the flow separation.The influence of open separation and separation bubbles on the flow state on the blade surface is suppressed or even eliminated with the increase of roughness.When there are separation bubbles in the cascade,increasing the roughness can accelerate the transition process of the boundary layer,reduce the thickness of the boundary layer and the width of the wake,smooth the outletairflow thee air flow,and improve the efficiency of the cascade.When the separation bubbles disappear,increasing the roughness will result in the accumulation of low energy fluid at the trailing edge of the cascade,increase the thickness of the boundary layer and the width of the wake,increase the outlet angle of the airflow and reduce the efficiency of the cascade.The effect of local roughness on the cascade performance shows that the roughness S1R distributed in the separation area has the best suppression effect on separation,but results in greater wake loss.The roughness LER of the leading edge position inhibits the flow separation with minimal wake loss and minimal cascade loss coefficient.This thesis contains 98 figures,11 tables and 155 references. |
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