Theoretical And Experimental Research On Resonant Cavity Dielectric Perturbation Technique To Measure Steam Humidity

Wet steam two-phase flow affects the safe and economic operation of steam turbine seriously. The accurate measurement of steam humidity is helpful to calculate the operating efficiency of the wet steam stages, as well as the low-pressure cylinder, and can provide guidance for turbine's operation. Besides, also it can provide references for the optimization design of moisture removal device, levels protection, wet steam stages and flow passage. Therefore, measurement technology of steam humidity in turbine has very important significance. Based on the microwave resonant cavity dielectric perturbation theory, the paper has earried out theoretical and experimental research on the measurement method of the steam humidity in turbine.1. 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 saturated water, dry saturated steam and wet steam with temperature (or pressure) and the microwave wavelength changing were obtained.2. According to the complex dielectric properties of wet steam, using microwave resonant cavity dielectric perturbation theory, a relationship of cylindrical cavity working in TE011mode for steam humidity measurement was established. Thermal properties of wet steam, temperature (or pressure), resonance frequency and relative frequency offset of the cavity affect the humidity measurement.3. The loop coupling cavity and the hole-coupling cavity which are suitable to measure the humidity of flowing wet steam were designed, as well as the slicer structure of the cavity. The important parameters of the effective size of the cavity, the length of the slicer, the size of the coupling loop, the rectangular waveguide, the radius of the coupling hole and so on, were also determined by the simulation. The measurement system used to measure steam humidity of flowing wet steam was designed.The microwave devices were chosen, and the operating circuit and control process of the measurement system were designed according to the design and measurement requirements. The measured data from the measurement system was sented to the computer to be analyzed. Besides, the methods of processing and analysis about the frequency sweep data and the pulse data were introduced.4. The influences which caused several factors on the humidity measurement were analyzed quantitatively. The factors include the expansion of the cavity when the temperature changed, the deposition water film or salt depositing on the cavity wall, large water droplets (water mass) passing through the cavity, the sampling bias of the cavity and heat transfer lag of the cavity when wet steam parameters abrupt changed and so on. A microwave cavity with automatic temperature compensation structure was designed. Therefore, the effects of the cavity thermal expansion and the heat transfer lag between the cavity and wet steam to humidity measurement were solved completely.5. The experimental program of wet steam sampling, measurement outside and the experimental program of measurement in exhaust cylinder were designsed, and the steam humidity measurement experiment was earried out. In the external cylinder measurement experiment, due to the droplets deposition and the steam condensation on the wall, large areas of water film formed in the wall and pipe, and it affected the humidity measurement seriously. In the interior cylinder measurement experiment, the vibration and thermal expansion of the cavity had a great effect on the measurement. The influence can be eliminated by using proper coupling structure and the temperature compensation method. The meaurement experiment shows that it is feasible to use the cavity medium perturbation techniques to measure the steam humidity.