Conjugate Heat Transfer Simulation And Experimental Validation On The Transient Thermal Behavior Of Ultra-supercritical Steam Turbine Control Valves During Cold-start Warming-up Process

The ultra-supercritical power generation is the most advanced technology in modern coal-fired power plant with better operation reliability,longer unit life and higher thermal efficiency.However,due to the increase of the unit parameters,the mainstream overheating parameters have reached26.25 MPa and above 600℃,posing higher requirements and challenges to the thermal design,structural manufacturing and parameter operation of the whole unit.As the primary flow control unit of coal-fired power plants,control valves are generally assembled between the boiler and steam turbine system.This allows them to operate flexibly in response to the required power output.In practice,control valves are exposed to the unsteady hightemperature steam flow issuing from an upstream boiler,subjecting their structural integrity to essential temperature variations,low-cycle fatigue and even structural damage.Especially in the cold start process of a steam turbine system,the significant temperature difference between the hot steam flow and cold valve bodies causes considerable temperature gradients and thermal stresses.Therefore,the heat damage has become one of the most serious challenges for the safety operation of steam turbine units.This study focuses on comprehensively analyzing the transient thermal behaviors of full-scale steam turbine control valves during the cold start warm-up process employing computational fluid dynamics methods,validated with high temperature experiment and field data measurements.Particularly,advanced conjugate heat transfer numerical simulation(CHT)was used to comprehensively investigate the transient heat transfer characteristics of ultra-supercritical steam turbine under cold start warm up process.A conjugate heat transfer simulation of transient turbulent flow in a scaled turbine valve was performed against experimental validation.A hightemperature(615 °C)experimental system with a scaled(1:3)turbine valve was set up at Pennsylvania State University.Eighty thermocouples were flush-mounted in streamwise and circumferential directions inside the valve body,and spatio-temporally varying temperature and temperature gradients were acquired as the mainstream temperature and pressure rapidly varied.A simulation using the shear stress transport model showed considerably better agreement with the measured temperature than the standard 6)6)- model and the realized 6)6)- model.The overall numerical errors of temperature and temperature gradient were below 8%.However,the largest errors located in the upper diffuser were confirmed to be associated with alternating oscillations of the annular attachment jet along the diffuser surfaces.Further investigations of transient thermal behaviors demonstrated that the instability of large-scale vortical structures inside the valve diffuser significantly enhanced heat transfer between the valve body and the air flow.In addition,upstream straighteners enabled the formation of separated secondary flow structures inside the diffuser,resulting in non-uniform heat transfer along the valve’s circumferential direction.To further reveal the transient thermal behaviors of ultra-supercritical steam turbine control valves during the cold start warm-up process,comprehensively studies on steam turbine systems was employed using conjugate heat transfer(CHT)simulation.The geometrical configurations and boundary conditions used in simulation were identical to the field setup in a thermal power plant.The simulated temperature variations were first validated using measurements by the flush-mounted thermocouples inside the solid valve bodies.The CHT simulation implementing the shear stress transport(SST)turbulence model demonstrated good agreement with the field data,and the overall numerical errors were below 10%;however,the numerical errors of the simulation,which used empirical heat transfer coefficients at the fluid-solid interfaces,reached 40%.The determined temperature differences between the cold valve bodies with the hot steam flow decreased significantly.Specifically,the temperature differences along the inner wall surfaces of the valve bodies decreased to less than 50℃.Further investigation of the transient heat flux distributions and Nusselt number distributions confirmed that the unsteady flow behaviors,such as the alternating oscillations of the annular wall-attached jet,the central reverse flow and the intermediate shear layer instabilities,enhanced the fluid-solid heat convection process and thus contributed to the warming up of the solid valve bodies.These findings indicate that the conjugate heat transfer simulation can determine fast spatio-temporal variation in the temperature distribution inside turbine valves,to serve as important inputs to low-cycle fatigue analysis and structural integrity evaluation.