Research On The Wetness Measurement System Based On A Microwave Cavity Resonator

A microwave cavity resonator based system for measuring the wetness of steamin a turbine has been developed in this thesis. To solve the problem caused by thedifficulties in the accurate calculation of permittivity of wet steam, a new method,named "water bead moving method", by which the perturbation analysis of theresonator can be made has been proposed based on the studies of the characters ofsteam in the turbine and the distribution of electric field in the resonator. Therelationship between wetness of the steam and resonant frequency excursion of theresonator and the relationship between wetness of the steam and quality factor of theresonator has been derived using the proposed method. An automatic frequency-tracking unitwas designed based on the response of the reflective resonator to frequency-modulated signal,and inserted in the wetness measurement system. The frequency-tracking unit can not onlylock the frequency of VCO to that of the resonator but also offset the frequency drift of VCO.The permittivities of saturated air at different temperatures were measured utilizing aself-made steam generator. The measurements were compared with the theoretical value, andthe reasons for deviations between them were analyzed.