| In China,the last stages of large steam turbines and all stages of nuclear steam turbines work in the wet steam zone,the two-phase flow of wet steam seriously affects the economic and safe operation of units.Accurate measurement of wet steam humidity is not only beneficial to determine the working efficiency of the whole unit,but also provides a powerful basis for the design of the dehumidification device,blade optimization and blade protection of the steam turbine.Microwave resonant cavity medium perturbation technique for measuring wet steam humidity does not need to operate excess steam sample,can be continuously measured and has high accuracy and great potential for development.Based on microwave cavity perturbation technique,the dielectric constant of the dielectric,the measurement system of resonant cavity and the temperature characteristic of resonant cavity are analyzed.1)In this paper,single and double Debye relaxation equations were compared to get the accurate saturated water complex permittivity equation,using double Debye relaxation equations can effectively reduce the error;The model of complex permittivity of wet steam with high accuracy is obtained by model comparison.The effective medium model is more suitable for the measurement of wet steam than the model of Lichtenecker.When the temperature and the frequency are certain,the complex dielectric constant of the wet steam does not change much with the humidity,which is suitable for measuring the humidity of the wet steam using the resonant cavity perturbation technique.2)The theoretical analysis shows that due to the thermal expansion and contraction of the resonant cavity,when the temperature changes,the resonance frequency shift of the resonant cavity is not only related to the change of dielectric constant in the resonant cavity,but also to the change of the resonant cavity itself size.HFSS electromagnetic simulation software simulation and experimental analysis verify that the change of medium and size change caused by the resonant frequency of the same order of magnitude.When measuring the humidity of the wet steam with a resonant cavity,it is necessary to eliminate the offset of the resonant frequency caused by the change of the cavity temperature.3)The resonant cavity measurement system is optimized,and two important defects of the measurement system are solved,two measuring systems are designed: using the ball valve control instead of the probe bar;using the shutter structure instead of the probe bar.The shell of the steam turbine also has relatively small vibration during the operation,which has little influence on the stability and safety of the cavity. |
