Dynamical Compensation Of Total Heat Exchange Factor Based On Non-gray Radiation Properties Of Gas In Reheating Furnace

Optimization of online control of reheating furnace will bring significant economic and energy saving effects. Total heat exchange factor method simplifies heat flux to slab surface dependant on total heat exchange factor, slab surface temperature and furnace temperature, which has been widely used in online control mathematical models in reheating furnace for its simple algorithm and small computational cost. However, studies showed that the distribution of total heat exchange factor along length of reheating furnace was not a group of constant. Total heat exchange factor which is the key parameter in this method is related to combustion, flowing and heat transfer, and it is dependant on not only structural parameters but also operating parameters. Heat transfer is the most important process, and radiation is the dominant mode in reheating furnace which takes over 90% of the total heat. To date, most reheating furnaces burn mainly gas fuels concluding carbon and hydrogen elements. H2O and CO2 as the main products have strong non-gray radiation properties. In this dissertation, dynamical compensation of total heat exchange factor was carried on considering non-gray radiation properties of gas, aiming at improving the precision of online control mathematical models in reheating furnace. The main work includes the following five aspects of investigations.(1) Non-gray radiation properties of gas were studied. Emissivities and absorptivities were adopted to describe radiation properties of gas, and emissivities of gas to surfaces and absorptivies of gas to components of effective radiations of surfaces were distinguished. In distinguishing mean beam lengths of gas to parts of its boundary, inverse radiation problems were referred. An inverse method was presented to calculate the mean beam length from absorbing gas to parts of its boundary and it was validated. Edwards exponential wide band model and Leckner series were applied to calculate emissivities of H2O, CO2 as well as combustion products of typical gases. The results show that the maximal error of Leckner series is 16% based on results of Edwards exponential wide band model, but the computational speed of the former is almost 10 times of the latter.(2) Total radiative exchange areas were studied in non-gray participating media. Considering non-gray radiation properties of gas, two methods called ray tracing method and radiation net-chart method were given to calculate total radiative exchange areas. Expressions of total radiative exchange areas in ternary system in reheating furnace were derived by applying the two methods, and they agreed well with each other. Total radiative exchange areas of a walking beam reheating furnace were calculated and integrity of total radiative exchange areas was validated. The results show that reciprocity of total radiative exchange areas was not existed, especially in low temperature zones. With the increase of wall emissivities, the total radiative exchange areas between slab surface and wall increase, while total radiative exchange areas of slab surface to ifself and to gas decrease.(3) Advanced mathematical models were developed by introducing imaginary plane into ternary system, including steady and unsteady states, conventional and regenerative reheating furnaces, which were validated by experimental data in the literature. A conventional and a regenerative reheating furnaces were taken as research objects, and total radiative exchange areas, heat flux to slab surface, temperature field, total heat exchange factor along length of reheating furnace coupled with thermal efficiency were simulated, with which results of gray gas were compared. The errors were also given. Effects of operating etc. parameters on heat transfer process were also studied, including the effects of wall emissivities both in steady state and typical unsteady state called delay rolling. The results show that thermal efficiency will decrease when wall emissivity increases in steady state, but it may be different in delay rolling.(4) Conventional zone method mathematical models were developed in reheating furnace. The conventional and regenerative reheating furnaces were taken as research objects, and the results of ternary model, ternary model by introducing imaginary plane and zone method were compared. Furthermore, effects of flame distribution on heat transfer process were studied.(5) Three methods for calculation of total heat exchange factor were briefly introduced. An analytical solution of unsteady one-dimensional heat conduction equation with variable/time-dependant heat flux boundary conditions and distributed initial condition was derived, and expressions of cross-section temperature difference of slab were given. By analyzing typical analytical solutions, one can get that the method called cross-section temperature difference will bring errors at any place along length of reheating furnace, and the errors depend on heat flux distribution.(6) Studies on dynamical compensation of total heat exchange factor were carried on. For the above conventional and regenerative reheating furnaces, effects of operating etc. parameters on total heat exchange factor were investigated based on mathematical models considering non-gray properties of gas, which can be stored and used as experts’ knowledge database for compensation of total heat exchange factor in online models.