Investigation Of Moisture Steam Characteristic Measurement Methods In Steam Turbine Based On Light Scattering

Considering the influence of the moisture steam on the security and economy of steam turbine operation, the methods for droplet size and humity distribution measurement in steam turbine were chosed to studied in this thesis under the synthetical analysis of the actuality and problems presented in previous study, and the formation mechanism of droplets and scattering characteristics were also investigated in detail. An experimental platform was set up to generate the wet steam, and the optical measurement experiments were conducted in this facility to validate the proposed measurement methods based on the Mie scattering theory, then the droplet sizes and humidity distribution were obtained by means of numerical calculation.The main research works are as follows:(1) The effects of steam humidity on the efficiency of steam turbine and distribution characteristic of moistness were studied according to thermodynamics principle. The variation of the efficiency of a certain 300MW steam turbine with the steam wetness was obtained by numerical calculation. The model of wet steam condensation was set up based on the theory of vapor-liquid two-phase flow and droplet nucleus formation, and the distribution characteristic of steam humidity in wet steam generator was obtained.(2) The Mie scattering properties of single water droplet and water droplets were investigated. According to the Mie scattering theory, the dependence of the water droplet scattered light intensity on the angle of scattered light and the laser's wavelength was found through the numerical calculation and analysis of the scattering energy distribution function and phase function for single droplet. Then the water droplets'scattering phase function and energy distribution function characteristics were further studied. In the forward small angle range, the empirical formula for the droplet mass middle radius and curve fitting parameters was obtained through the Gaussian function fitting method, which established a fundamental basis for parameter measurement such as steam humidity.(3) Two approaches, the single wavelength/milt-angles method based on CCD (Charge-coupled Device) imaging and the double wavelengths/double angles method, were proposed for droplet size and humidity measurement. Based on the theory of light scattering, the transporting and measuring model of forward small angle scattering light was set up. Besides this, optical measurement methods of the single wavelength /multi-angles, the single-angle/multi-wavelengths and the multi-angles/ multi-wavelengths were studied, and the computational model of steam humidity was derived. Scattering light measuring method of single wavelength/multi-angles using CCD imaging technique was proposed, and the principle of steam humidity measurement method and the moisture parameter solving processes were studied systematically.(4) The experimental rig of wet steam generator was developed. According to the requirement of measurement process, the experimental rig of wet steam generator was designed, which can simulate the wet steam environment under arbitrary running conditions. By theoretical analysis and experiments, it is found that the rig can meet the experimental requirement completely.(5) Steam humidity and the droplet mass middle radius in wet steam generator were inspected by using CCD imaging method and dual wavelengths/ double angles method. Scattered light distribution of wet steam under steady-state condition and varying duty was measured by using CCD, while the steam humidity and particle size distribution of droplet were obtained by means of parameter's optimization fitting. Under steady-state condition, it can be found that the droplet mass middle radius is 0.97μm, the density of droplet and medial humidity are 1.43×1012m-3 and 1.11% in respective. Under varying work conditions, it is found that the droplet mass middle radius is 1.12μm, the density of droplet is 2.4×1012 m-3 and the range of humidity is 0.71%~2.26%. Comparatively, the measuring experiment was carried out using the 650 nm and 532 nm laser, and it is calculated that the range of the droplet mass middle radius is 0.7μm~1.3μm though empirical formula of the Gaussian fitting parameters. To explain the fluctuation characteristics of scattering light intensity, real-time estimation method of scattering light mean intensity was proposed based on the ARMA(Autoregressive moving average model) model parameter recognition and the Kalman filtering, and the random variation factors affecting measurment results were analyzed in detail here.It is concluded that the proposed methods for measuring the characteritics of the moisture steam are feasible, especially, the CCD measuring method will have wide application prospect in measurement of steam humidity and real time monitoring for steam turbine through further studying and improvements.