【作者】 崔苗；

【导师】 陈海耿；

【作者基本信息】 东北大学 ， 热能工程， 2009， 博士

【摘要】 实现加热炉在线最优控制可以取得明显的经济效益和节能效果。总括热吸收率法将钢坯表面热流简化为炉膛总括热吸收率、钢坯表面温度和炉温的函数,因其算法简单、计算量小,在加热炉在线控制数学模型中已得到广泛应用。然而,研究表明,该方法的关键参数——炉膛总括热吸收率,沿炉长方向的分布并非一组常数,与加热炉内的燃烧、流动和传热密切相关,不仅受炉子等结构参数的影响,也受热工操作等参数的影响。传热是加热炉内发生的最重要过程,加热炉炉膛内的传热以热辐射为主,通常占总热交换量的90%以上。目前,大多数加热炉主要燃烧碳氢气体燃料,作为主要燃烧产物的H2O和CO2气体,具有强烈的非灰辐射特性。因此,本文从加热炉炉膛内的传热机理和炉气的非灰辐射特性出发,对炉膛总括热吸收率进行动态补偿研究,为提高加热炉在线控制数学模型的精度提供理论依据,主要研究内容包括以下六个方面：(1)对炉气的非灰辐射特性进行研究。采用发射率和吸收率来描述气体的辐射特性,区别了炉气对不同表面的发射率和炉气对不同表面有效辐射中各分量的吸收率。给出吸收性气体对其部分边界平均射线行程的一种反演方法,通过模型检验证实了该方法的可靠性与可行性。分别采用Edwards指数宽带模型和Leckner级数式,计算了水蒸气和二氧化碳发射率,进而计算了典型煤气燃烧产物的发射率。结果表明,在加热炉的温度和压力行程范围内,以Edwards指数宽带模型计算结果为基准,Leckner级数式的最大误差为16%,而Leckner级数式的计算速度约为Edwards指数宽带模型的10倍。(2)对非灰气体参与的辐射全交换面积进行研究。给出考虑气体非灰特性的辐射全交换面积的两种求解方法：射线跟踪方法和辐射网络图方法。以炉子三元辐射体系为例,应用上述两种方法分别推导了各辐射全交换面积表达式,取得一致的结果,实现了相互印证。在此基础上,计算了连续加热炉各模型段三元辐射体系内的辐射全交换面积,并通过了完整性校验。结果表明：考虑炉气非灰特性的辐射全交换面积已不存在互换性,低温段中尤为明显；随着炉围发射率的增大,钢坯表面段与炉围表面段间的辐射全交换面积增大,而钢坯表面段对自身和对气体段的辐射全交换面积减小。(3)在上述工作的基础上,建立了考虑炉气非灰辐射特性与引入假想面的加热炉三元数学模型,并采用文献中的实验数据对模型进行验证。分别以实际的换热式和蓄热式加热炉为例,对各辐射全交换面积、钢坯表面热流、炉内温度、炉膛总括热吸收率和炉子热效率等进行数值模拟,并计算了将炉气简化为灰体时的上述变量及其误差。研究了热工操作等参数对炉内热过程的影响,其中有炉围发射率的影响,包括稳态工况和典型动态工况——待轧。本文的稳态研究表明,提高炉围发射率,炉子热效率略有下降,但对待轧的模拟可能出现相反的情况。(4)建立了区域法数学模型。以实际的换热式和蓄热式加热炉为例,比较了三元模型、引入假想面的三元模型和区域法的计算结果,研究了火焰分布对炉内热过程的影响。(5)简述炉膛总括热吸收率的辨识方法,为了对炉膛总括热吸收率辨识方法中的断面温差法进行分析,推导了一维、非稳态热传导方程在分布的初始条件和变热流边界条件下的解析解,并导出了钢坯表面热流与钢坯断面温差间的关系式。通过对典型热流分布的分析可知,应用断面温差法对炉膛总括热吸收率进行辨识时,沿炉长方向的任何一点都存在误差,误差的大小取决于钢坯表面热流分布函数。(6)对炉膛总括热吸收率进行动态补偿研究。应用考虑炉气非灰辐射特性的加热炉数学模型,分别以实际的换热式和蓄热式加热炉为例,研究了热工操作等参数对炉膛总括热吸收率的影响,形成数据库,这样的数据库相当于专家系统的知识库,以备炉膛总括热吸收率动态补偿之用。更多还原

【Abstract】 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.更多还原