Performance Evaluation of Two Heat Transfer Models of a Walking Beam Type Reheat Furnace

Two different heat transfer models for predicting the transient heat transfer characteristics of the slabs in a walking beam type reheat furnace are compared in this work. The prediction of heat flux on the slab surface and the temperature distribution inside the slab have been determined by considering thermal radiation in the furnace chamber and transient heat conduction in the slab. Both models have been compared for their accuracy and computational time. The furnace is modeled as an enclosure with a radiatively participating medium. In the first model, the three-dimensional (3D) transient heat conduction equation with a radiative heat flux boundary condition is solved using an in-house code. The radiative heat flux incident on the slab surface required in the boundary condition of the conduction code is calculated using the commercial software FLUENT. The second model uses entirely FLUENT along with a user-defined function, which has been developed to account for the movement of slabs. The results obtained from both models have a maximum temperature difference of 2.25%, whereas the computational time for the first model is 3 h and that for the second model is approximately 100 h.     

Transient radiative heating characteristics of slabs in a walking beam type reheating furnace

Transient radiative heating characteristics of slabs in a walking beam type reheating furnace is predicted by the finite-volume method (FVM) for radiation. The FVM can calculate the radiative intensity absorbed and emitted by hot gas as well as emitted by the wall with curvilinear geometry. The non-gray weighted sum of gray gas model (WSGGM) which is more realistic than the gray gas model is used for better accurate prediction of gas radiation. The block-off procedure is applied to the treatment of the slabs inside which intensity has no meaning. Entire domain is divided into eight sub-zones to specify temperature distribution, and each sub-zone has different temperatures and the same species composition. Temperature field of a slab is acquired by solving the transient 3D heat conduction equation. Incident radiation flux into a slab is used for the boundary condition of the heat conduction equation governing the slab temperature. The movement of the slabs is taken into account and calculation is performed during the residence time of a slab in the furnace. The slab heating characteristics is also investigated for the various slab residence times. Main interest of this study is the transient variation of the average temperature and temperature non-uniformity of the slabs.     

Comparisons of different heat transfer models of a walking beam type reheat furnace

Four different heat transfer models (Model-1 to -4) for the prediction of temperature of the slabs of a walking beam type reheat furnace have been compared. The models are classified based on the solution methodology and simplifications. In the first three models (Model-1 to -3), the furnace is modelled as radiating medium with spatially varying known temperature. Model-1 solves the 3D transient conduction in the slab and radiation in the furnace separately and is coupled via the boundary condition. In the second model, both radiation in the furnace and conduction in the slab are solved simultaneously. A user defined function (UDF) programme has been developed to process the movement of the slabs. Model-3 is similar to Model-2 but it includes additionally the skid support systems for the slabs. In the Model-4, convection in the furnace has been included in addition to all the features considered in Model-3. The convection has been modelled with the consideration of flow of hot gas through the inlet of the burners. All the models have been compared for their performance and computational time. Model-1 has been found to be quite economical and accurate. The inclusion of skid supporting system has little effect in the temperature distribution in the slab.     

A heat transfer model for the analysis of transient heating of the slab in a direct-fired walking beam type reheating furnace

A mathematical heat transfer model for the prediction of heat flux on the slab surface and temperature distribution in the slab has been developed by considering the thermal radiation in the furnace chamber and transient heat conduction governing equations in the slab, respectively. The furnace is modeled as radiating medium with spatially varying temperature and constant absorption coefficient. The steel slabs are moved on the next fixed beam by the walking beam after being heated up through the non-firing, charging, preheating, heating, and soaking zones in the furnace. Radiative heat flux calculated from the radiative heat exchange within the furnace modeled using the FVM by considering the effect of furnace wall, slab, and combustion gases is introduced as the boundary condition of the transient conduction equation of the slab. Heat transfer characteristics and temperature behavior of the slab is investigated by changing such parameters as absorption coefficient and emissivity of the slab. Comparison with the experimental work show that the present heat transfer model works well for the prediction of thermal behavior of the slab in the reheating furnace.     

Efficiency analysis of radiative slab heating in a walking-beam-type reheating furnace

The thermal efficiency of a reheating furnace was predicted by considering radiative heat transfer to the slabs and the furnace wall. The entire furnace was divided into fourteen sub-zones, and each sub-zone was assumed to be homogeneous in temperature distribution with one medium temperature and wall temperature, which were computed on the basis of the overall heat balance for all of the sub-zones. The thermal energy inflow, thermal energy outflow, heat generation by fuel combustion, heat loss by the skid system, and heat loss by radiation through the boundary of each sub-zone were considered to give the two temperatures of each sub-zone. The radiative heat transfer was solved by the FVM radiation method, and a blocked-off procedure was applied to the treatment of the slabs. The temperature field of a slab was calculated by solving the transient heat conduction equation with the boundary condition of impinging radiation heat flux from the hot combustion gas and furnace wall. Additionally, the slab heating characteristics and thermal behavior of the furnace were analyzed for various fuel feed conditions.     

THREE-DIMENSIONAL ANALYSIS OF THE WALKING-BEAM-TYPE SLAB REHEATING FURNACE IN HOT STRIP MILLS

Three-dimensional analysis is performed for the turbulent reactive flow and radiative heat transfer in the walking-beam-type slab reheating furnace by FLUENT. A simplified burner is validated against the results of the actual burner with the detailed grid resolution to avoid an excessive number of grids. The predicted temperature distribution in the furnace and global energy flow fractions are in reasonable agreement with available data. Distribution of the heat flux to the slabs, velocity vectors, and all major scalar variables in the furnace also are predicted. This study shows that three-dimensional analysis may be a useful tool to understand quantitatively the complicated combustion and heat transfer characteristics in the furnace.     

Optimum residence time analysis for a walking beam type reheating furnace

A 3D unsteady numerical simulation of a reheating furnace was performed to obtain the optimal slab residence time. Too long residence time decrease the efficiency of the reheating furnace, whereas too short residence time cannot satisfy the required heating quality of a slab. The total five cases of residence times – 6032 s, 6496 s, 6960 s, 7424 s and 7888 s – were investigated for the optimum residence time analysis with the two slab requirements, those of emission temperature and uniformity. In this study, the slab emission temperature should be in the range between 1373 K and 1573 K. The skid mark severity of an emitted slab should be lower than 50 K to satisfy the uniformity requirement. The numerical analysis was done for the identical geometry and operating condition of the reheating furnace using FLUENT. Slabs were assumed to move very quickly that it took no time for them to move next positions. The quick movements of slabs were processed with the own developed User-Defined Function program. Among the five cases of residence times, the residence time of 7427 s turned out to be most efficient.     

节能经济型加热炉的研究与应用

对已淘汰的轧钢厂进行异地改造,经过合理计算与设备选型,采用炉体结构、错位梁技术、高效换热器等多项节能减排技术的步进梁式加热炉,既能满足轧钢工艺要求又节能环保。     

板坯加热炉内加热特性的数值分析

以某公司热轧厂常规与双蓄热烧嘴组合供热的板坯加热炉为研究对象,建立该加热炉炉内流动、传热、燃烧和板坯运动吸热过程的三维物理数学模型,运用CFD仿真技术对其进行详细的数值计算,得到炉内稳态的速度场和温度场分布规律、板坯的升温曲线以及板坯温度分布均匀性,计算结果与"黑匣子"实验测量数据吻合良好。本文给出的板坯加热特性计算方法为研究加热炉新工艺、优化板坯加热温度制度提供了科学依据。     

混合煤气成分变化对加热炉内温度场影响的数值模拟研究

以燃用高炉、焦炉混合煤气的实验加热炉为研究对象,建立炉内二维稳态传热、流动及燃烧的数学模型,研究混合煤气成分变化对加热炉内温度场的影响,计算结果显示高炉煤气含量在一定范围内增加时,炉内温度水平和钢坯加热区温度均匀性逐渐降低.这些与华凌涟钢集团轧钢加热炉的实际情况基本一致.     

连续式轧钢加热炉在线控制数学模型与应用

以理论计算和实测数据为依据,开发了连续式加热炉最佳炉温设定和在线控制数学模型,并应用于安钢中型厂5点供热推钢式加热炉。结果表明：在轧机产量波动时,不但加热炉供热合理,而且钢温波动小、加热质量好、节能效果显著。     

辊底式连续热处理炉钢坯二维传热过程数学模型的研究

以某公司拟建的辊底式连续热处理炉为研究对象,在详细分析其传热机理的基础上,针对其钢坯厚度变化较大的特点,建立了钢坯在炉内连续加热和摆动加热过程数学模型,采用数值计算方法对其进行了仿真计算,并利用设计大纲提供的数据间接验证了所建模型的正确可靠性。所开发的辊底式热处理炉计算机数值仿真系统可以动态模拟不同燃料、不同规格的钢坯在炉内的运行状况及其钢坯各典型点的温度变化规律;可以确定在不同的热处理工艺制度下、不同规格的钢坯所需要的最佳运行方式和最佳工艺制度。     

Research on a 3D Temperature Field Model of a Slab with Oxide Layer Growth

Slab heating plays an important role in the production of iron and steel materials. However, it is a very complex process involving physical and chemical change. In this study, we built a numerical heat transfer model to predict the three‐dimensional transient temperature field of a slab based on the implicit finite difference method. The model takes the growth of the oxide layer into account, as well as the impact on heat transfer. Slab temperature and oxide layer thickness were calculated in each step. The model considers three kinds of boundary conditions. It displays the temperature variation of each part of the slab in the furnace at all time, the heating curve, and the growth in the thickness curve of the oxide layer. This model can be used to control heating time, optimize the heating curve, and improve production efficiency, thereby reducing cost. The model is also useful for calculation of rolling force, as well as the control of carbon isolation and product microstructure.     

Investigation of the slab heating characteristics in a reheating furnace with the formation and growth of scale on the slab surface

In this work, the development of a mathematical heat transfer model for a walking-beam type reheating furnace is described and preliminary model predictions are presented. The model can predict the heat flux distribution within the furnace and the temperature distribution in the slab throughout the reheating furnace process by considering the heat exchange between the slab and its surroundings, including the radiant heat transfer among the slabs, the skids, the hot combustion gases and the furnace wall as well as the gas convection heat transfer in the furnace. In addition, present model is designed to be able to predict the formation and growth of the scale layer on the slab in order to investigate its effect on the slab heating. A comparison is made between the predictions of the present model and the data from an in situ measurement in the furnace, and a reasonable agreement is found. The results of the present simulation show that the effect of the scale layer on the slab heating is considerable.     

A Coupled Numerical Study of Slab Temperature and Gas Temperature in the Walking-Beam-Type Slab Reheating Furnace

In the present study, a three-dimensional simulation is performed for the turbulent reactive flow and radiactive heat transfer in the walking-beam-type slab reheating furnace using STAR-CD software. The geometric model takes care of all components of the furnace. To obtain a steady solution, the walking beams are assumed fixed in the furnace and the slab is modeled as a laminar flow having a very high viscosity and thus moving at a nearly constant speed. The temperature distributions of the slab and the gas mixture are obtained through a coupled calculation. The simulation results successfully predict the temperature distribution inside the slab and the heat flux on the slab surfaces, providing an opportunity for a full exploration of the influence of the walking beam system on the skid marks. The simulation results show that the radiative shielding by the static beams is the main cause of the skid marks. The heat loss through the skid button to the cooling system worsens the skid marks.     

Time-varying heat transfer from adjacent slab-on-grade floors

A steady-periodic solution for the heat conduction problem under a slab-on-grade floor adjacent to another slab is presented. The two slabs can be uninsulated or uniformally insulated. A water table is assumed to exist at finite depth from the soil surface. The interzone temperature profile estimation technique is used to develop a semi-analytical solution for the heat conduction equation in the ground medium. The soil temperature field, and the total slab heat loss are obtained and analysed in detail. The effect of the distance that separates the adjacent slabs on the mean and amplitude of total slab heat loss is discussed. © 1998 John Wiley & Sons, Ltd.     

Fracture prediction of electromagnetic silicon steel in reheating furnace

This article proposes the use of strain energy density (SED) for the prediction of fracturing in silicon steel slabs undergoing reheating in a furnace. Reheating is commonly used to soften steel before hot rolling into ultra-thin silicon steel sheets referred to as electromagnetic steel. High heating rates are required to reduce the time spent in the reheating furnace to increase the efficiency of producing ultra-thin electromagnetic steel sheets and decrease fuel consumption. However, an excessive increase in heating rates may induce fracturing due to the comparatively brittle nature of the silicon used in electromagnetic steel slabs to enhance the electromagnetic properties. In this study, the authors used finite element numerical calculation to elucidate the fracture mechanism of electromagnetic steel slabs undergoing heating. The authors then used SED as a criterion by which to optimize the heating rates without inducing fracturing in electromagnetic steel. The proposed model could be used as a guide to shorten the time required for heating various types of steel in reheating furnaces.     

耐火陶瓷纤维在连续退火生产线退火炉高温段上的应用

较为系统地介绍了攀钢冷轧厂连续退火生产线退火炉改造前后的设备组成,分析了改造前退火炉高温段砌筑材料的不足,制定了采用陶瓷纤维进行炉衬改造的技术方案,通过耐火陶瓷纤维应用前后炉衬材料性能与使用效果的对比分析,取得了退火吨钢能耗下降标煤10．14kg、炉顶及炉墙的外表温度明显降低以及明显改善带钢表面质量的优良效果,对同类型机组的设备改造提供了可靠的参考依据。     

等离子体技术在多晶硅还原炉的应用

针对内蒙多晶硅项目,在等离子体发生器对还原炉内部进行加热的条件下,对炉内温度场进行了试验测试,同时采用FLUENT软件对其进行了数值模拟。理论和计算的综合结果表明,等离子体加热技术在多晶硅还原炉中有着良好的加热应用效果。与同类型加热设备卤素灯相比,等离子发生器不仅加热效率高且运行成本相对较低,具有良好的应用前景。     

Estimation of slab surface radiative emissivities by solving an inverse coupled conduction,convection, and radiation problem

Slab surface radiative emissivities severely affect the radiative heat transfer in a reheating furnace, as well as the slabs’ coupled conduction, convection, and radiation. Accurate evaluation of these parameters is of significance to ensure the high accuracy of the mathematical model for a reheating furnace, which is beneficial to the energy saving. However, it is difficult to directly and accurately measure these parameters. In this article, slab surface radiative emissivities in a reheating furnace are estimated by solving a nonlinear inverse problem, which is an inverse coupled conduction, convection, and radiation problem. An efficient and accurate gradient method, i.e., Levenberg–Marquardt algorithm, is applied to obtain the solution of the inverse problem. First, a finite difference method and the complex-variable-differentiation method are used for sensitivity analysis, and the inversion accuracy coupled with the efficiency is demonstrated. Then, effects of initial guesses, measurement errors, and measurement locations on estimated slab surface radiative emissivities are investigated in detail. Finally, conclusions are drawn based on the results and analysis.