Contribution to the metal-mold interfacial heat transfer

Formerly with the SVUSS Bechovice, Prague 9     

Measurement and modeling of heat transfer across interfacial mold flux layers

Surface quality problems in continuous cast steel are greatly affected by heat transfer across the interfacial layers in the gap between the solidifying steel shell and the mold. An experimental apparatus has been constructed to measure temperatures in the steel, mold flux layers, and copper under conditions approximating those in continuous casting. The flux solidified in multiple layers similar to those observed from continuous casting molds and contained many gas bubbles. Flux conductivities average about 1.0 W/m·K and appear to evolve with time. Contact resistances at both interfaces are significant and average about 0.0015 m²·K/W. Flux crystallization appears to be the only significant effect of flux composition. The one glassy flux tested had much greater thermal conductivities, presumably due to radiation transport. Temperature and gap thickness had a negligible effect on the properties. These properties depend on the model used to extract them. They are being implemented into a mathematical model to simulate heat transfer in the mold, interface, and solidifying shell of a continuous slab-casting machine.     

Determination of the interfacial heat transfer coefficient in a metal-metal system solving the inverse heat conduction problem

Simulation of several industrial processes involving solidification of metals requires characterization of heat transfer coefficient at the solidifying metal/metal-substrate interface. In the present investigation an attempt has been made to estimate this heat transfer coefficient, h_c, using simulated experiments in which the heat transfer from a heated stainless steel block (simulating solidifying metal) to a water cooled copper block (simulating metal-substrate) is monitored by continuously recording temperatures at a few internal locations both within the metal block and the substrate block. The problem of determining the interfacial heat transfer coefficient is recognized to be an inverse heat conduction problem (IHCP). A numerical method is employed to solve IHCP and to determine the h_c from the transient history of temperatures at a few locations. The effect of the physical nature of the interface, as well as the cooling conditions prevailing at the outer surface of the substrate on h_c is examined and discussed. While the physical nature of the interface, i.e. roughness on the metal as well as the substrate surfaces, has a significant effect on h_c, the cooling conditions have only a marginal effect. The h_c in the present investigation remains more or less time invariant.     

Biological verification of fluid-to-particle interfacial heat transfer coefficients in a pilot scale holding tube simulator

Mesothelioma is a malignant pleural or intraperitoneal tumor attributable to asbestos exposure in more than 80% of the cases. Manganese superoxide dismutase (MnSOD), a mitochondrial superoxide radical scavenging enzyme, is low in most tumors but is known to be induced by asbestos fibers and certain cytokines. Induction of MnSOD may be associated in asbestos-related pulmonary diseases in vivo. We investigated here MnSOD specific activity and MnSOD mRNA level using healthy human lung tissue, SV40-transformed human pleural mesothelial cells (Met5A), and six human malignant mesothelioma cell line cells. Total SOD (CuZnSOD + MnSOD) and MnSOD activities were 20.0 +/- 4.8 U/mg protein and 3.2 +/- 1.2 U/mg protein in healthy human lung tissue, and 25.6 +/- 10.7 U/mg and 3.8 +/- 1.0 U/mg in Met5A cells, respectively. In four mesothelioma cell lines MnSOD activity was significantly elevated, the highest activity (30.1 +/- 8.2 U/mg) was almost 10-fold compared to the activity in Met5A cells. The steady state mRNA level of MnSOD was low in Met5A cells and markedly higher in all mesothelioma cell lines roughly in proportion with enzyme activities. Cytotoxicity experiments, which were conducted in four cell lines, indicated that cells containing high MnSOD mRNA level and activity were resistant to the mitochondrial superoxide-producing agent menadione. In conclusion, our results suggest that human mesothelioma may express high levels of MnSOD, which is associated with high oxidant resistance of these cells.     

Research methods and influencing factors of interfacial heat transfer during sub-rapid solidification process of strip casting

Interfacial heat transfer behavior between the molten steel and twin-rolls is a key issue in the strip casting process,and it has already attracted wide attenti...     

A correlation to describe interfacial heat transfer during solidification simulation and its use in the optimal feeding design of castings

It is known from experimental data that for pure aluminum castings manufactured via the gravity die casting process, the interfacial heat-transfer coefficient can vary in the range 500 to 16,000 W/m² K. These coefficients are of significant importance for the numerical simulation of the solidification process. The experimentally determined variation of interfacial heat-transfer coefficients with respect to time has been recalculated to highlight the variation with respect to casting temperature at the interface. This variation was observed to be of an exponential nature. Also, the pattern of variation was found to be similar in all the experimental results. It has been found that all these patterns of interfacial heat-transfer coefficient variation can be matched by a unique equation that has been proposed as a correlation to model the metal-mold interfacial heat transfer. The benefit of this correlation is in its ability to approximate the combined effects of geometry variation, insulation, chills, die coatings, air gap formation, etc. during the numerical simulation and its use in the optimal design of heat transfer at the metal-mold interface.     

旋流叶片传热管内湍流强化传热数值模拟

基于管内核心流强化传热原理,在管内核心流布置旋流器以实现强化传热.建立了强化管内流体流动与传热的数学模型,结果表明:在强化传热管内布置旋流器可以显著强化湍流换热,而且其流动阻力增加幅度不大.当旋流器距圆管人口200mm,Re数为6000时,强化传热管的PEC值可达1.7.     

Regenerative heat transfer in rotary kilns

A mathematical model has been developed to determine the temperature distribution in the wall of a rotary kiln. The model, which incorporates a detailed formulation of the radiative and convective heat-transfer coefficients in a kiln, has been employed to examine the effect of different kiln variables on both the regenerative and the overall heat transfer to the solids. The variables include rotational speed, pct loading, temperature of gas and solids, emissivity of wall and solids, convective heattransfer coefficients at the exposed and covered wall, and thermal diffusivity of the wall. The model shows that the regenerative heat flow is most important in the cold end of a rotary kiln, but that generally the temperature distribution and heat flows are largely independent of these variables. Owing to this insensitivity it has been possible to simplify the model with the aid of a resistive analog. Calculations are presented indicating that both the shell loss and total heat flow to the bed may be estimated to within 5 pct using this simplified model.     

Determination of interfacial heat transfer coefficients in casting and quenching using a solution technique for inverse problems based on the boundary element method

Both casting and quenching are processes during which several physical phenomena like heat transfer, fluid flow, phase transformation,etc. interact in a complex manner. To obtain a nu-merical model which is capable of accurately simulating the actual process, one has to be able to quantify all the parameters affecting the process. One parameter which substantially influ-ences heat transfer in these processes is the heat transfer coefficient at the interface between the mold and the metal in casting and that between the metal and the quenchant in quenching. The heat transfer coefficient could vary on the surface of a casting or a quench metal both spatially and with time. Its accurate determination is imperative for a realistic simulation of these processes. In this work, an algorithm based on the boundary element technique is proposed to solve for the interface heat transfer coefficient. The problem is cast as one of inverse heat conduction in two dimensions where some of the boundary conditions, namely, the previously mentioned heat transfer coefficients, are unknowns. Since it is the boundary properties that are being determined, the boundary element method (BEM) is the most suitable technique to use. The algorithm uses experimentally measured temperature data inside the domain to determine the interface heat transfer coefficient. The technique is outlined in detail and some casting and quenching examples are presented to demonstrate its capability.     

Radiative heat transfer in rotary kilns

Radiative heat transfer between a nongray freeboard gas and the interior surfaces of a rotary kiln has been studied by evaluating the fundamental radiative exchange integrals using numerical methods. Direct gas-to-surface exchange, reflection of the gas radiation by the kiln wall, and kiln wall-to-solids exchange have been considered. Graphical representations of the results have been developed which facilitate the determination of the gas mean beamlength and the total heat flux to the wall and to the solids. These charts can be used to account for both kiln size and solids fill ratio as well as composition and temperature of the gas. Calculations using these charts and an equimolar CO₂−H₂O mixture at 1110 K indicate that gas-to-surface exchange is a very localized phenomenon. Radiation to a surface element from gas more than half a kiln diameter away is quite small and, as a result, even large axial gas temperature gradients have a negligible effect on total heat flux. Results are also presented which show that the radiant energy either reflected or emitted by a surface element is limited to regions less than 0.75 kiln diameters away. The radiative exchange integrals have been used, together with a modified reflection method, to develop a model for the net heat flux to the solids and to the kiln wall from a nongray gas. This model is compared to a simple resistive network/gray-gas model and it is shown that substantial errors may be incurred by the use of the simple models.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 1 Mould heat transfer

Abstract

With the drive to cast higher quality, many minimills are adopting mould powder as a lubricant for the continous casting of steel billets. Over the past three decades considerable experience has been accumulated on the relationship between mould behaviour and billet quality for oil lubrication, but comparatively few studies have been conducted for mould powder lubrication. This study, conducted at a Canadian minimill, involved instrumenting four faces of a copper mould with thermocouples and monitoring mould temperatures during casting of 208 × 208 mm billets with mould flux lubrication. Billet samples were also taken to coincide with periods of measurements. Mould temperatures were monitored for two different mould powder compositions, for different mould oscillation frequencies, two mould cooling water velocities, and a range of steel compositions. An inverse heat conduction model was developed to calculate mould heat transfer from the measured temperatures. In this paper, which is the first part of a two part series, details of the inverse heat conduction model and mould heat transfer data are presented. The results obtained for mould flux lubrication have been compared with those for mould heat transfer for oil lubrication. For peritectic steels, with carbon content in the range 0·12–0·14%, it was found that lubricant type has little influence on the measured mould heat flux distribution at the centreline of a face. The peak mould heat flux was found to be approximately 2500 kW m^-2 . In contrast, for medium carbon steels, mould heat transfer with mould powder was significantly lower than when oil was employed as a lubricant. For instance, at the meniscus, the peak heat flux with mould powder was approximately 2500 kW m^-2 , which was half that recorded with oil as a lubricant. The influence of oscillation frequency, mould cooling water velocity, and mould powder type on mould heat flux has also been presented.     

插入件换热器传热系数的数值分析

对插入件换热器的传热面积、传热系数及其结构尺寸进行了数值计算,分析了各初始条件对传热系数的影响并绘制了曲线.模拟结果表明,在其他初始条件不变时,传热系数与各初始条件呈线性,指数或幂函数关系.     

Fundamental phenomena governing heat transfer during rolling

To quantify the effect of roll chilling on the thermal history of a slab during hot rolling, tests were conducted at the Canada Center for Mineral and Energy Technology (CANMET) and at the University of British Columbia (UBC). In these tests, the surface and the interior temperatures of specimens were recorded during rolling using a data acquisition system. The corresponding heat-transfer coefficients in the roll bite were back-calculated by a trial-and-error method using a heat-transfer model. The heat-transfer coefficient was found to increase along the arc of contact and reach a maximum, followed by a decrease, until the exit of the roll bite. Its value was influenced by rolling parameters, such as percent reduction, rolling speed, rolling temperature, material type,etc. It was shown that the heat-transfer coefficient in the roll gap was strongly dependent on the roll pressure, and the effect of different variables on the interfacial heat-transfer coefficient can be related to their influence on pressure. At low mean roll pressure, such as in the case of rolling plain carbon steels at elevated temperature, the maximum heat-transfer coefficient in the roll bite was in the 25 to 35 kW/m² °C range. As the roll pressure increased with lower rolling temperature and higher deformation resistance of stainless steel and microalloyed grades, the maximum heat-transfer coefficient reached a value of 620 kW/m² °C. Obviously, the high pressure improved the contact between the roll and the slab surface, thereby reducing the resistance to heat flow. The mean roll-gap heat-transfer coefficient at the interface was shown to be linearly related to mean roll pressure. This finding is important because it permitted a determination of heat-transfer coefficients applicable to industrial rolling from pilot mill data. Thus, the thermal history of a slab during rough rolling was computed using a model in which the mean heat-transfer coefficient between the roll and the slab was determined from an estimate of the rolling load. It was found that the heat loss of a slab to the roll was 33 pet of the total, which emphasizes the importance of accurately characterizing the heat-transfer coefficient in the roll bite during hot rolling.     

Steady heat transfer in melt-irrigated blast-furnace zone

The steady heat transfer in the irrigated zone of the blast furnace is considered, taking account of the filtration of hot metal and slag through the coke bed. In this zone, the fluxes of coke, hot metal, and slag are heated simultaneously and exchange heat with one another, by both convection and radiation. Convection ensures heat treatment between the gas and all the batch materials and also between the slag and coke, since the slag running through the coke partially covers its surface. Radiant heat transfer develops among the coke, hot metal, and slag. It is more intense at higher temperatures. The heat transfer among the coke, hot metal, and slag in the irrigated zone of the blast furnace has a considerable influence on the temperature field in this zone. To calculate the heat transfer at the shoulder and hearth, information is required regarding the distribution of the thermal effects of direct reduction of iron, silicon, and manganese over the height of the irrigated zone and the proportion of the coke surface covered by slag.     

换热器强化传热技术的研究进展

介绍了被动式强化传热技术的研究进展,简述了典型和新型传热元件的开发和应用,针对换热器传热管表面处理技术,管的内插件和管束支撑结构的发展状况展开分析和论述;探讨了强化传热技术的发展方向,数值模拟和场协同原理技术的应用使换热器结构趋于最优化,强化传热技术由单一型向复合型方向发展,逐渐形成第三代传热技术.     

基于热量平衡的大方坯连铸传热数学模型研究

基于热量平衡原理建立了大方坯连铸过程的传热数学模型,通过对二冷区的分段和联立方程组求解,可以快速获得二冷区指定位置铸坯的坯壳厚度、表面温度、凝固潜热、散热等热状态参数。以某钢厂的大方坯连铸机生产45号钢为研究背景的数值模拟和现场测温表明,铸坯断面尺寸为260mm×340mm,铸机拉速为0．5m／min、0．6m／min时,模型的计算结果符合凝固规律,计算温度与实测温度的误差小于2％,计算结果可为大方坯动态轻压下工艺的优化提供可靠的依据。     

烧结余热回收装置强化传热研究进展

分析了钢铁企业烧结工序余热资源量及余热回收利用现状,探讨了烧结余热回收装置的传热特性和强化方向.分析表明基于多孔介质理论与场协同原理的第三代强化换热技术将使冷却机和余热锅炉的结构及回收效率趋于最优化.