Study On Flow And Heat Transfer Characteristics Of Large Meridional Expansion Turbine And Improvement Schemes

The design of high-efficiency,high-load blades for marine gas turbine turbines is one of the development directions.The high load leads to the large turning angle of the guide vane,and the design of the blade shape of the large turning angle will lead to high blade shape loss.By increasing the expansion angle of the turbine meridional channel,the turning angle of the turbine shape can be effectively reduced,which makes the marine gas turbine have a large expansion angle of the channel.The thickening of the boundary layer on the endwall of a large meridional expansion turbine is easy to separate,which results in strong secondary flow loss at the endwall and affects the non-uniformity of thermal load.The key to control and reduce the secondary flow loss at the end of a large meridional expansion turbine is to have a detailed understanding of the causes of secondary flow and its flow structure in the cascade passage of a large meridional expansion turbine.The large meridional expansion turbine is in a high temperature and high pressure environment,and the influence from the upstream leakage vortex causes serious secondary flow loss and more complex thermal load in the near endwall area of the large meridional expansion turbine.Therefore,the research focus of the large meridional expansion turbine is to explore the flow loss caused by the large meridional expansion of the endwall and propose effective flow control methods for the endwall loss.In this dissertation,endwall strong secondary flow characteristics and complex thermal load characteristics caused by large meridional expansion turbines are studied in detail,and experimental studies on high subsonic cascade of large meridional expansion turbines under multiple conditions are carried out to master the flow law under variable conditions.In this dissertation,the fillet technology of turbine blade with large meridional expansion,meridional modeling technology,non-axisymmetric endwall technology,and surface tripping technology are also proposed,and their effects on flow and heat transfer are studied.In this dissertation,the vortex system structure and wall thermal load characteristics of a large meridional expansion turbine are firstly analyzed,and the relationship and interaction between flow state and heat transfer characteristics are studied.The inlet flow angle and inlet turbulence intensity,which affect the flow of large meridional expansion turbine,are also studied.It is found that the maximum heat transfer coefficient area at the leading edge is the stagnation area formed by horseshoe vortex,and the change of angle of attack has obvious influence on the flow loss and heat load distribution of the large meridional expansion turbine.The increase of inlet turbulence intensity increased the profile loss.Two sets of fan cascades of a large meridional expansion low pressure turbine were tested respectively.The flow characteristics of turbine cascades at different Mach numbers and the variable angle of attack characteristics at the outlet Mach number are measured respectively.According to the test results,the characteristics of flow loss and secondary flow law of high subsonic cascade in large meridional turbine are analyzed in detail.Secondly,the unsteady flow and heat transfer characteristics of the upper endwall of a1.5-stage large meridional expansion turbine are studied by numerical simulation.The results show that the leakage vortexes,passage vortexes and wake at the exit of swimming blades in a large meridional expansion turbine are the main factors affecting the unsteady flow and heat transfer of stationary blades.The influence of tip clearance of rotor cotyledons on the downstream stator blades of a 1.5-stage large meridional expansion turbine is also numerically studied.The results show that the tip leakage vortex significantly increases the negative angle of attack at the downstream inlet,while the shrouded turbine rotor reduces the intensity of the high heat transfer zone on the suction surface of the downstream stator blade.Then,the curved-sweep blade technique of large meridional expansion stator blades in low pressure turbines is studied,and the effects of different shaped stacking lines such as sweep forward,sweep back,forward and reverse are simulated.The height and angle of sweep and curved are controlled by parameters to optimize the aerodynamic and heat transfer performance.It is found that positively curved blades can improve the static pressure and heat load distribution in the endwall region,effectively reduce the heat flux density and loss coefficient of the upper endwall.Five typical static blade fillets are also designed in this dissertation to study the different configuration of the second stage static blade of a 1.5 stage turbine.Reasonable fillet structure distribution can not only improve the flow state of the large meridional expansion turbine,but also reduce the heat load on the wall of the large meridional expansion turbine.The endwall profile on the meridional surface of a 1.5-stage large meridional expansion turbine was reconstructed to explore the influence of the meridional profile on the flow characteristics and heat transfer characteristics of the stator blade,and a scheme was expected to be found to reduce the flow loss and improve the heat transfer characteristics.Based on the idea of non-asymmetric endwall design,a non-axisymmetric endwall design method for large meridional expansion turbine is proposed,and the design scheme of non-axisymmetric upper endwall for large meridional expansion turbine is presented according to the simulation results.In this dissertation,a trip wire is also added to the turbine endwall to study the flow field variation and heat transfer characteristics of the end region by changing the geometric parameters of the trip wire.The results showed that the trip wire weakened the stagnation point of the blade leading edge and the separation point of the blade leading edge was advanced.The trip wire increases the strength of the leakage vortex from the rotor blade of the turbine,but weakens the upper passage vortex.The end wall trip wire reduces the total pressure loss.