Design Of A Microwave Resonant Cavity Sensor For Measuring The Water Film Thickness On The Inner Wall Of A Cylindrical Cavity

Accurate measurement of the steam wetness is of great importance to the long-term stability and life of steam turbines.Microwave resonator perturbation technology uses a microwave cylindrical resonator as the wetness sensor,which is very suitable for measuring the steam wetness in the last stage of steam turbines.Since the wetness sensor works in the wet steam environment for a long time,a water film would deposit on the inner wall of the cylindrical cavity,which would aff ect the accuracy of the wetness measurement.Therefore,we design ed a hyperbolic slot-shaped resonant cavity for measuring the water film thickness on the inner wall of a cylindrical cavity,working in the TE₁₁₁ mode to measure the water film thickness.Then the measurement error caused by the water film is deducted through theoretical analysis to realize accurate measurement of the wetness.?1?The feasibility of measuring the water film thickness in the TE₁₁₁ mode is verified by theory and simulation.Based on the field equation of the TE₁₁₁ mode and the principle of medium perturbation,we established the relationship between the water film thickness and resonant frequency and obtained a linear relationship.The influence of temperature and steam wetness on measuring the water film thickness in the TE₁₁₁ mode was studied by HFSS simulation software.It is found that the wetness has no effect on the measurement of the water film thickness in a certain range.?2?We derived the mathematical expression of the current density distribution line in the TE₁₁₁ mode by the vector field equation method and designed a hyperbolic slot-shaped resonant cavity for measuring the water film thickness on the inner wall of a cylindrical cavity.The simulation results show that the el ectromagnetic,radiation and flow characteristics of the water film measuring cavity are suitable.The electromagnetic leakage rate could be as small as 0%and the sampling error is no more than-1.3%.The water film thickness has a linear relationship with the frequency offset and the correlation coefficient is 0.9978,which c ould measure the water film thickness accurately.?3?The water film measuring cavity is applied to measure the thickness of other dielectric films,such as the salt deposit on the inner wall of the wetness sensor,the lubricating oil mold and the dirt in the steam turbine,etc.The simulation results show that their corresponding resonant frequency offsets are linear with the water film thickness.The experimental results show that when the thicknesses of the dielectric films are within 100?m,the measuring error does not exceed 0.245.?4?Considering the influence of temperature and deposited water film on steam wetness measurement,a dual-mode dual-channel wetness measurement system was proposed based on the dual-cavity compensation system.This system introduces a reference cavity with the same structure as the measuring cavity to eliminate the influence of temperature on measuring the wetness and water film thickness.The water film measuring cavity working in the TE₁₁₁ mode to measure the water film thickness.Then the measurement error caused by the water film is deducted through theoretical analysis to realize accurate measurement of the wetness.