Numerical And Experimental Study On The Turbine Tip Clearance Leakage Control By Using The Honeycomb Tip With Injection

With the continuous improvement of the aero engine economic indicators,the turbine components are already facing increasingly severe inflow temperature and single stage load.For turbine rotor blades,the tip clearance leakage flow reduces the work extracted from hot gas,and the leakage fluid would form the tip leakage vortex structure in the suction side passage,thereby exacerbating the secondary flow loss inside the rotor passage.The tip clearance leakage flow also worsens the tip heat transfer conditions and impairs the effective service life of the rotor blades.The tip clearance leakage control technologies include passive control technologies and active control technologies,such as the tip shroud,the squealer tip and the tip injection.Honeycomb tip is a tip structure based on the honeycomb seal,which could improve the tip clearance leakage flow and the relative losses.Such tip structure could protect the rotor blade body during friction and collision between the tip and the casing,and thus has application potential in the field of turbine tip clearance leakage control.The present work attempts to enhance the suppression on the tip clearance leakage flow of the honeycomb tip by means of the injection technology.The aerodynamic and performance of such tip structure is analyzed in a turbine cascade model by combining numerical calculations and wind tunnel experiments.Since the sealing mechanism of the above mentioned tip structures is unclear,a flat seal model is established with reference to the related studies in the field of seal to explore the honeycomb structure and the honeycomb structure with injection.Attempt is also made to apply the above mentioned tip structures in a real 1.5-stage turbine model,to evaluate the improvement of the tip structures on the real turbine performance.Besides,the optimization study of the composite honeycomb tip is also carried out.Firstly,the seal mechanism of the honeycomb structure and the honeycomb structure with injection is studied in the flat seal model.The velocity and static pressure distributions inside the gap are extracted to determine the effect of these two structures.The thickness of flow loss and the thickness of momentum loss are proposed to accurately describe the disturbance of the seal structures on the leakage flow field.The applicability of the seal structures under different Reynolds number or gap height conditions has been explored.The results show that both the honeycomb structure and the honeycomb structure with injection increase the friction factor of the sealing section,and reduce the leakage mass flow rate by about 13.3% and 19.7%,respectively.These two structures increase the thickness of flow loss and the thickness of momentum loss.That is,the effective flow area of the leakage fluid is reduced,and the resistance during the leakage flow is increased.Some leakage fluid is blocked by the honeycomb sidewalls and then enters the cavities,thereby maintaining the cavity vortices.Besides,the stagnant high pressure forms a local blocking effect inside the upper gap.Secondly,the structure optimization of the composite honeycomb tip is carried out in the turbine cascade model.The geometric parameters of the honeycomb tip and the tip clearance leakage mass flow rate are chosen as the input variables and the design goal,respectively.The genetic algorithm based on surrogate model is used to search and obtain the optimal composite tip structure.Then the calculation and wind tunnel experiment of the optimal tip structure are conducted.The results show that the numerical results of the optimal tip structure confirm the prediction and search accuracy of the above combined optimization strategy.The optimal tip structure reduces the tip clearance leakage mass flow rate by about 16.81%.The honeycomb depth plays an important role in suppressing the tip clearance leakage flow,while the effects of the height ratio and the length ratio are complicated.Except for the pressure side separation bubble,the leakage flow features inside the gap are found to be similar to the flat seal model.The tip structures also affect the positions and the loss feature of the secondary flow structures in the cascade passage,such as the tip leakage vortex and the passage vortex near the casing.At the same time,the performance of the honeycomb tip with injection is studied in the turbine cascade model.The effects of the honeycomb tip and the honeycomb tip with injection on the aerodynamic performance and the tip thermal conditions are analyzed.The effects of the injection orientation,the injection position,the honeycomb depth and the injection intensity on the turbine cascade performance are discussed.The aerodynamic performance of the tip structures under different relative casing speed or gap height conditions is explored.Besides,the turbine cascade wind tunnel experiment of the honeycomb tip with injection is conducted.The results show that the honeycomb tip with injection could reduce the tip clearance leakage mass flow rate and the average total pressure loss downstream of the cascade by about 27.6% and 2.6%,respectively.The injection accelerates the circulation of the leakage fluid inside the honeycomb cavities and enhances the suppression on the tip clearance leakage flow of the honeycomb tip.Besides,the cooling injection could reduce the overall heat transfer intensity of the honeycomb tip and improve local thermal conditions on the suction surface.Finally,the application study of the honeycomb tip with injection is carried out in the real 1.5-stage turbine model.The main performance parameters,such as the tip clearance leakage mass flow rate,the turbine isentropic efficiency and the average total pressure loss at the rotor exit are compared and analyzed.The flow structures inside the gap and the rotor passage are analyzed,and the effects of the honeycomb depth and the injection intensity on the turbine aerodynamic performance are discussed.The aerodynamic performance of the honeycomb tip with injection under different rotational speed or gap height conditions is also explored.The results show that the honeycomb tip and the honeycomb tip with injection could reduce the tip clearance leakage mass flow rate by about 8.74% and 63.95%,respectively,and further improve the turbine isentropic efficiency.In the rotor-relative frame,the flow structures inside the gap are found to be similar to those in the turbine cascade model.The momentum exchange between the injected fluid and the leakage fluid increases the resistance during the leakage flow.