Reconstruction Of Boiler Drum Temperature Field Based On Inverse Heat Transfer Problem

The steam drum is an important part of the natural circulation boiler.In the process of long-term operation of the thermal power unit,because of the complex working conditions and the difference of heat transfer strength between the wall inside the drum and the working fluids including steam and water,the temperature difference between the inner and outer walls of the drum is in flux.Thereby allowing the drum to withstand alternating thermal stresses and produce low cycle fatigue losses,resulting in creep of metal materials and performance degradation.Therefore,the monitoring and control of the temperature difference between the inner and outer walls of the drum are important means to ensure the safety of operation.The inner working condition of drum is high temperature and high pressure,for the actual operation of the thermal power unit,it is not convenient that installing the thermocouples in the inner wall to monitor the temperature.When using the method of heat transfer inverse problem,the temperature field of the whole drum wall can be reconstructed through the temperature measurement information of the measuring points at the outer wall of the drum,which has strong practical value.In view of the domestic and foreign existing research results and the main problems on calculating the temperature field of the drum's wall,in this paper,the dynamic matrix control algorithm(DMC)is used to study the inversion of the internal heat transfer coefficient and the reconstruction of the temperature field.The main research work includes:(1)The physical model and the mathematical model of the heat transfer process of the drum are established based on the reasonable assumptions and simplification.And then discretize the differential equations of heat conduction through heat balance method after dividing the spatial grid of drum model.We solve the discretized algebraic equations by the Gauss-Seidel iteration method.Through the numerical simulation,the temperature field distribution of the drum metal is obtained,the temperature field and the internal and external temperature difference of the top,middle and bottom of the steam drum is analyzed in the earlier period of cold start.(2)The basic principle of DMC algorithm is introduced,and then take the heat transfer coefficient between the wall inside the drum and the working fluids including steam and water as the parameter to be inverted.The heat transfer coefficient inversion model based on DMC is established by the mathematical model of steam drum heat transfer.In this method,the dynamic step response coefficient matrix at the measurement point is obtained by the step change of the parameters that need be inverted,and the temperature prediction model at the measuring point is established.And then through the temperature measurement information and temperature prediction information of k,k+1,,k+r-1 time,the inversion result of the heat transfer coefficient at k time is obtained.The DMC inversion algorithm only takes the inversion result of heat transfer coefficient at the current time,and then proceeds to the next moment to achieve the "rolling optimization" by the same optimization method.(3)The simulation of the heat transfer coefficient inside the drum is carried out,and then the temperature field of drum wall is reconstructed based on this.The validity of the DMC inversion algorithm is verified,and the influence of the measuring point arrangement scheme,the measurement error and the future time step on the inversion accuracy and the temperature field reconstruction of the drum wall are discussed.The numerical results show that the inversion accuracy and the temperature field reconstruction effect of the drum wall which meets the engineering requirements can be obtained when arranging the single measuring point on the upper and lower sides of the water level of the outer wall of the drum,which greatly simplifies the work of measuring points in engineering practice.It is found that the inversion model established in this paper can still obtain satisfactory inversion accuracy of heat transfer coefficient and temperature field reconstruction results within a certain range of measurement error.In addition,as the future time step increases,the accuracy of inversion increases gradually,but the rate of increase will be gradually reduced.When the smaller future time step is selected,the inversion of the internal heat transfer coefficient and the effect of temperature field reconstruction are also ideal,which indicates that the established inversion model reduces the dependence on future time steps.