Estimating The Thermal Boundary Conditions Of The Membrane Water Wall

The membrane water wall is one of the main heating surfaces of power plant boilers, the working condition and performance of which directly affects the safe and economic operation of the boilers. The thermal boundary conditions of membrane water wall are mainly comprised of the radiation heat flux at the fireside, the temperature of the working steam-water-mixtures and the convective heat transfer coefficient in the water wall tubes. Obtaining the above thermal boundary conditions is the basic premise to master the information such as the metal surface temperature distribution of the membrane water wall and the positions and temperatures of the dangerous points, also is the necessary requirement and content of the operation state analysis and monitoring of power plant boilers. Because of the difficulty of direct measurement, it is an effective approach to study the thermal boundary conditions of membrane water wall by using the method of inverse heat transfer problem, which bases on the local measurable temperatures on the membrane water wall.The inverse problem of estimating the thermal boundary conditions of membrane water wall was studied by using Conjugate Gradient Method (CGM) in this paper, aiming at the status and the main problems in the investigations of that. The main tasks of this paper are as follows.1) The mathematic model of the heat conduction process in the membrane water wall of power plant boiler was established. The differential equation of heat conduction was dispersed by Finite Volume Method, and the discrete algebraic equations were solved iteratively. Then the applicability was validated. The effects of different thermal boundary conditions on the temperature distribution, position of the highest temperature and temperature level of membrane water wall were analyzed through numerical experiments.2) The basic concept of conjugate gradient method was introduced. Combining the heat conduction process (forward problem) of membrane water wall established in Chapter 2, the inverse model of estimating the thermal boundary conditions of membrane water wall was set up by Conjugate Gradient Method (CGM), which took the radiation heat flux at the fireside of membrane water wall q0, the temperature of the working steam-water-mixtures tf and the convective heat transfer coefficient h in the water wall tubes as the inversed parameters. 3) The estimation of the thermal boundary conditions of membrane water wall was conducted by numerical experiments, and the influences of the parameter numbers, the initial guesses of inversed parameters, the number and the position of measured points and the measurement errors on the accuracy of inverse solutions and the temperature distribution at the fireside of membrane water wall were discussed. The results of numerical experiments showed that, the appropriate increase of measured points was benefit to improve the inverse accuracy, while the solution with three measured points can meet the accuracy requierment of engineering generally. Though the inversed results of the thermal boundary conditions depended on the initial values definitely, preferable results could be achieved by the inverse approach in this paper. When the measured temperatures contained some errors, satisfactory results can also be obtained by Conjugate Gradient Method.