Research Of Steam Wetness Measurement Uncertainty And Water Film Thickness Measurement Based On Resonant Cavity

Wet steam of two-phase flow seriously affects the safe and economic operation of steam turbine.Accurately measuring steam wetness is helpful to determine wet steam level,efficiency of low pressure cylinder, and provides design reference for dehumidification equipment, blade protection and so on.The wetness measurement method of microwave resonant cavity medium perturbation is a promising new method. The system of steam wetness measurement is convenient operation, high measurement precision, strong adaptability to environment, which can realize the online measurement.Paper analyzes some key problems of wetness measurement,based on resonant cavity medium perturbation theory.The theory feasibility of water film thickness measurement is analyzed,which has an important guiding significance to safe operation of steam turbine unit.1.The influence of thermal deformation of microwave cavity to steam wetness measurement is analyzed by fluid-structure interaction method.The temperature field and thermal deformation of resonant cavity is calculated, under the condition of different steam parameters and flow velocity.According to the average mesh node deformation of effective inner surface of cavity, the influence of cavity deformation to resonance frequency is analyzed, adopting resonance frequency calculation formula of cylindrical cavity TE011 mode.With the cavity deformation increase, the offset of resonant frequency increases.The cavity deformation will bring larger influence to the normal steam wetness measurement. It is suggested that improve the cavity structure design,measurement scheme to reduce the influence of fluid-solid coupling heat transfer to wetness measurement.2.The sensitivity coefficient of wetness measurement equation is deduced.The uncertainty of wetness measurement system is analyzed,taking different steam pressure and wetness conditions for example.The system measurement accuracy is higher and synthetic standard uncertainty is less than 0.004%.With wet steam wetness and pressure value increase, the synthetic standard uncertainty and expanded uncertainty of wetness measurement system increase, a linear growth.Considering thermal expansion of cavity, the wall sedimentary water film or salt scale of cavity, big drop across cavity, the sampling bias of cavity etc, The integrated uncertainty of wetness measurement system is less than 0.764%. The heat expansion of cavity and sampling bias affect the accurate wetness measurement of the steam heavily.3.The medium thickness measurement model is established and thickness measurement equation is deduced, based on equivalent circuit of low frequency coaxial resonant cavity.Thickness measurement sensor of water film is designed. The measurement range of water film is0-1 mm. The sensor structure is small, high precision, easy installation. The resonance frequency of resonant cavity reduces, the attenuation increases, resolution gradually declines, with water film thickness increase.Thickness measurement sensor of water film can be used to monitor steam turbine stator blade, cylinder walls, guide ring structure of the water film and so on. The water film thickness measurement technology of low frequency coaxial resonant cavity needs to further study.