Experimental Investigation On The Estimation Of Temperature Distributions And Thermal Radiative Properties In Furnaces

Combustion in industrial furnaces is one of the most complex physical and chemical processes occurring in large-scale spaces. The measurement of temperature distributions is essential for the development of combustion theory and technology, as well as the increase of security, economic operation, and low-pollutant emission of boilers. Thermal radiative parameters in furnaces, such as radiative properties of particulate medium (absorption coefficient and scatter coefficient) and emissivity of wall surface, are very important to the processes of radiative heat transfer, and also the necessary input parameters for numerical simulation of combustion processes.The main research of this paper is experimental investigation on the simultaneous reconstruction of temperature distributions and thermal radiative parameters through the flame image processing and thermal radiation analysis, on the basis of the flame radiative imaging. Main work is as follows.Color flame image processing method, radiative imaging model and the corresponding reconstruction algorithm used in the experimental investigations were described in this dissertation. Then numerical simulation based on the radiative imaging model was conducted under the two special cases, cavity radiation and isothermal radiation system. Simulation results of the boundary temperature and emissivity distributions agreed well with the theoretical results, which proved the accuracy of radiative imaging model. At the same time, simulation of the simultaneous reconstruction of temperature distributions, absorption coefficients of combustion medium and emissivity of wall surface was conducted. The results show that temperature distributions and thermal radiative parameters can still be reconstructed well when the measurement error is 5%.The corresponding experimental investigations were carried out on a coal-fired boiler furnace and an oil-fired furnace, respectively. Firstly, in a four-cornered, tangentially firing boiler coal-fired furnace of a 200 MW power generation unit, flame images were captured by a portable imaging processing system. Flame radiative temperatures and emissivity were calculated and compared with the results measured by infrared pyrometer. Average relative error is less than 4%. The reconstruction results of cross section temperature distributions and radiative properties of granular medium show that, because of the way of four-cornered tangentially firing, temperature distributions appear to be a single-peaked shape with temperatures higher in the center and lower near the wall surfaces, and flame temperatures increase with the load. Scattering coefficients of coal combustion medium are greater than absorption coefficients. Furthermore, experimental investigations were conducted in an oil-fired tunnel furnace. Temperatures of wall surface were calculated from the radiative image of refractory wall and compared with the measured temperature of a thermocouple. Relative error between two methods was only about 2%. Then temperature distributions, absorption coefficients of combustion medium, and emissivity of refractory wall within the 12 cross sections were reconstructed from oil-fired flame images, the results show that: temperature distributions in furnace can reflect the distributions of combustion flame, and material used in furnace wall has a strong reflectivity in the visible region.