Theoretical and experimental research on microwave cavity dielectric perturbation technique to measure steam wetness

Abstract

The accurate measurement of steam wetness is important to the safe, economic operation and optimized design for turbine. The relationship between complex permittivity and static dielectric parameters, frequency of alternating electric field was substituted into the Maxwell-Wagner non-homogeneous dielectric theory. An equivalent complex permittivity model of wet steam was established. The complex permittivity distribution of dry saturated steam and wet steam with wavelength and temperature changing was obtained. According to the dielectric properties of wet steam mixture, using microwave cavity dielectric perturbation theory, a relationship of cylindrical cavity for wetness measurement in TE011 mode was established. Thermal properties of wet steam, temperature (pressure), resonance frequency and relative frequency deviation of the cavity would affect the wetness measurement. Wetness measurement increases with temperature (pressure) decreasing, cavity resonant frequency and relative frequency deviation increasing, vice versa. The accuracy of wetness measurement increases with resonance frequency decreasing and temperature increasing. This suggests that the appropriate cavity resonance frequency for steam wetness measurements should be between 5∼10 GHz. The experiment for measuring exhaust wetness in a 200MW steam turbine was carried out and the results agreed well with the theoretical calculation results.