Evaluation of temperature field and heat flux by inverse analysis during steel strip rolling

Knowledge of the temperature field in the roll is a critical factor of modern, high-speed rolling mills. In this paper, an inverse analytical method is developed to determine the temperature field and especially the temperature (and heat flux) at the surface of the roll by measuring the temperature with a thermocouple (fully embedded) at only one point inside the roll. Iterative methods are not studied because short computation times are desired. Some assumptions are done to resolve analytically the unsteady heat equation, taking into account the restrictions of the measurement system (e.g., measurement according to successive times). The solution is validated by comparing the outputs of the method and prescribed analytical temperature fields. Good agreement is obtained. Noise sensitivity is estimated by adding artificial random numbers to the inputs. Good accuracy is observed. A 10% error of the temperature sensor depth is also considered and does not compromise the method. On the other hand, the computation time (around 0.05 s by cycle) is studied to rapidly optimise the industrial parameters during the rolling process.     

Crack initiation and crack growth during thermal shock tests in the reactor pressure vessel of the HDR under corrosive medium conditions

This paper gives a survey of the status of comprehensive thermal shock investigations performed at a reactor pressure vessel (RPV). In a separate group of experiments it was first studied which impacts are exerted on the undamaged RPV wall by fluid transients resembling emergency cooling conditions. Furthermore, crack formation and crack growth were enforced in selected wall zones by providing appropriate boundary conditions. It is proposed to load in a final step the wall zones heavily damaged by quasi-reality crack fields under conditions similar to emergency cooling.

The investigations are performed to verify the capability of non-destructive testing methods and of computations (of temperatures, stresses, crack behaviour) to provide relevant information. The computations have yielded clear overestimations as well as underestimations. For instance, the investigations performed at the plant proper have produced the result that the thermal shocks acting on the RPV wall surface are substantially reduced by thermal mixing processes in water. In the subsequent computations of the RPV wall structure, rotation symmetric computer codes are bound to fail completely under conditions of intensive local cooling. Finally—although the crack growth computations for the special cyclic thermal shocks under corrosive medium conditions were extremely conservative—it has been demonstrated, by use of acoustic emission analysis and other techniques that the present crack growth computations do not adequately take into account crucial parameters influencing the material behaviour. This means that the results obtained with laboratory-scale specimens cannot be safely transferred to the components under the testing conditions prevailing for the time being.     

Thermal Stress Evolution of the Roll During Rolling and Idling in Hot Strip Rolling Process

This study examined a critical idling time for the roll to attain a target temperature prior to rolling the next strip in the hot strip rolling process. Thermoelastic finite element simulations were performed to compute the temperature evolution and resulting thermal stress variations of the roll during rolling and idling. The results showed that a circumferential compressive stress was generated in the roll during rolling. However, circumferential tensile stress on the roll surface was produced very shortly (about 0.76 s) and reached about 68 MPa around 28 s after the beginning of idling. The temperature distribution of the roll's 1.5-mm-thick outer layer was homogenized and reached a steady state approximately 28 s after the onset of idling. Hence, we may cut down the long idling time (90–120 s), generally adopted in a hot strip rolling process, to decrease (by 68.9–76.7%) the roll temperature elevated during rolling to a target temperature.     

THERMOELASTO-PLASTIC STRESSES AND THERMAL DISTORTIONS IN A BRAKE DRUM

Most analyses of thermal stresses generated in a brake drum are studies within either an elastic or a plastic region with constant yield strength. As a matter of fact, however, a severe brake application will create a thermal environment on the friction surface with an excessively high surface temperature. At a high temperature, the degradation of yield strength of a drum material may become significant and an important factor to be taken into account in the analysis. This article deals with the thermoelasto-plastic stresses and the thermal distortions produced in a brake drum during a severe braking condition, taking account of the temperature dependence of the yield strength of a drum material. It is found that high compressive thermoelastoplastic stresses are created during a brake application but eventually residual tensile stresses are developed in the vicinity of the rubbing surface in the course of cooling. These residual tensile stresses are strongly presumed to be one of the significant factors causing the heat crack in a brake drum. In addition, the thermal distortions of the drum suggest a loss of the contact area between the drum fictional surface and the brake shoe linings during braking.     

Initiation of Bulges in a Coke Drum Subjected to Cyclic Heating and Cooling,also Cyclic Mechanical Loads

Coke drums are equipments of an oil refinery system used to separate petroleum coke from lighter oils. During operations, a coke drum is subjected to cyclic heating and cooling also cyclic mechanical loads. Thus, the useful life of a coke drum is much shorter than the other equipments in the refinery. Bulges are commonly problems found in a coke drum. The initiation mechanisms of the bulges are not clear yet. However, there are two postulates have been proposed. First is that bulges are caused by contact stresses due to differential expansion between solid coke and steel. Second is that they are caused by thermal stresses due to presence of hot and cold spots in the coke drum wall. The present paper tends to agree with the second one. The main objective is to demonstrate that thermal stresses are sufficient to initiate the bulges. A coke drum with overall length, diameter, and thickness of 25.46 m, 6.4 m, and 42 mm, respectively has been taken into analysis. In order to provide actual temperature boundaries, operational temperatures of the coke drum have been measured and collected while it is operating. A cycle which shows the most severe operational temperature has been selected to be analyzed. Two-dimensional axisymmetric model was developed and stresses analysis upon the model was carried out by using ANSYS FEM commercial code. The equivalent stresses and the yield strength as a function of time are plotted. The results show that the maximum equivalent stress can reach the yield strength of the coke drum material. This concludes that the bulges are mainly initiated by thermal stresses.     

Cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film

The cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film were experimentally investigated in order to estimate the cooling performance of a rotating roll in a hot mill system. The following four conclusions were reached in the study. (1) In the case of a single spray jet, the local heat transfer coefficient at the center position depends on the forced convective heat transfer by the impinging jet. However, the average heat transfer coefficient is proportional to the flow rate density of the cooling water, and it does not depend on the distance between the nozzle and heated surface. (2) In the case of a double spray jet, liquid film interference occurs. The local heat transfer coefficient at the center position is greater, and the cooling performance increases with the increasing flow rate density of the cooling water. (3) The cooling performance of a multispray jet is proportional to the flow rate density of the cooling water. It does not depend on the nozzle construction, distance, or specifications. Also, there is no relation to the liquid film interference. (4) When the optimum specifications of the spray nozzle are used, thermal analysis of a rotating roll shows that the temperature at a depth of 1.3 mm from the surface is below 130 °C. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 280–299, 2000     

Comparative Analysis of Microchannel Heat Sink Configurations Subject to a Pressure Constraint

Leakage losses and ever-increasing power dissipation in the microprocessor are causing significant thermal, mechanical, and reliability problems. Conventional cooling methods are reaching their practical limits, and new methods of lowering the operating temperature of microprocessors are being explored. Microfluidics-based cooling schemes are one approach being considered. The implementation of microchannels for forced convection at the chip level shows much promise, as the effective heat transfer surface area and attainable heat flux are very favorable. A major design limitation to such an implementation is the pressure developed within such micro-flows and the stresses that could result. In this study, multiple discrete microchannel heat sink configurations are analyzed computationally and compared in a cooling capability sense, while total pressure drop across the flows is carefully considered. A single cooling channel over an energy source is split into two smaller channels, and so on, while total pressure drop is maintained constant, and specified such that all flows remain in the laminar regime. It is shown that for the configurations analyzed, there exists multiple-dependence optimum cooling configurations. In addition, it is shown that a slimmer design may be implemented with a relatively small effect on cooling capability. Furthermore, cooling capability dependence on total pressure drop of the flows is shown to be minimal for high-performing microchannel configurations.     

Comparative evaluation of preventive measures against primary side stress corrosion cracking of mill annealed inconel 600 steam generator tubes

Significant amounts of primary side cracking have been reported in the mechanically expanded area of the tubes of PWR steam generators in Europe, in Japan and to a lesser extent in the USA. The Belgian utilities are faced with the same problem. At Doel 2, where the tubes are rolled for only a part of the tubesheet, primary side cracking appeared in the roll transition. The Doel 3 and Tihange 2 steam generators, whose tubes are expanded for the full depth of the tube sheet, have experienced cracking after about 10 000 h of operation not only in the roll transition but also at roll overlaps. While some leaks and eddy current indications are associated with tubesheet or rolling anomalies, many of them are found on normal tubes.

A programme was launched by the Belgian utilities and was further co-sponsored by the Electric Power Research Institute (EPRI) to develop preventive actions applicable not only to hot steam generators but also to cold steam generators already installed on site. These preventive measures include stress relaxation and metallurgical improvement of the material by an in situ heat treatment of the whole tube sheet (a steam generator model was used to evaluate the feasibility of this treatment), and the introduction of residual compressive stresses on ID by rotopeening or shotpeening without inducing unacceptable tensile stresses on OD. A comparative evaluation of these measures was established on the basis of tests performed on representative mock-ups and specimens.     

The effect of limescale on heat transfer in injection molding

With the development of rapid prototyping technologies, injection mold inserts with conformal cooling systems can be manufactured from metal powder by direct metal laser sintering (DMLS). The conformal cooling channels are placed along the geometry of the injection molded product, thus they can extract more heat, and heat removal is more uniform than in the case of conventional cooling systems. But even the most efficient cooling circuits start to wear out, corrosion and limescale depositions precipitate on the wall of the cooling channel, which impede heat transfer from the mold to the coolant. The effect of the depositions cannot be neglected and the modeling of the impact on heat transfer is difficult. We developed a model to investigate the effect of limescale that formed on the wall of the cooling circuit. The thermal properties of the limescale are required for the simulation, therefore they were measured. We concluded that 2 mm thick limescale impedes heat removal so much that the more efficient conformal cooling system can only extract as much heat as the less efficient conventional system.     

发动机曲轴的激光冲击复合处理工艺研究

介绍了激光冲击强化和修复曲轴的新方法,并与其它曲轴处理方法进行了比较。实践表明选择合适的工艺参数,可在零件表面得到搭接均匀,表面硬度大于600 HV和厚度达到1．0 mm的硬化层,冲击区表层残余应力达到-495 MPa,而绝对磨损体积下降了133％以上。对磨损曲轴,可以先采用激光熔覆处理,然后对修补区进行激光冲击处理。激光冲击强化复合处理具有表面硬度高、零件变形小和不影响曲轴轴心部性能等优点,可以使性能超过无损伤曲轴,能实现曲轴的再循环利用,大幅度降低了生产成本。     

Design criteria for ribbed channels: Experimental investigation and theoretical analysis

The present study investigates heat transfer and pressure drop in flows through ribbed channel for application to turbine blade cooling. The experiments are conducted for different cross-sections, for Reynolds number from 20 to 60 × 10³. Local heat transfer coefficients are obtained using a transient thermochromic liquid crystal (TLC) technique. Detailed knowledge of the local heat transfer coefficient is essential to analyze thermal stresses in turbine components, while the combined effect of heat transfer and pressure drop should be taken into account for a proper cooling system design. As a compromise has always to be found, a new design criteria to choose the most appropriate solution for typical turbomachinery parameters is inferred and shown. Entrance effects for ribbed channels are also studied, as the common hypothesis of fully developed flow is rarely satisfied in real engine geometries; relevant results are revealed.     

双辊薄带连铸自动控制系统铸速控制建模优化

为了对双辊薄带连铸自动控制系统铸速进行检测和控制,保持工艺稳定,提高薄带成品质量,建立了凝固终点位置与铸速的数学模型,对其相互关系进行了研究.研究结果表明铸速与凝固终点位置近似线性关系：铸速越大,凝固终点越靠近铸口且易断带或漏钢;铸速越小,凝固终点离铸口越远且易裂纹或轧卡.采用先进的热像仪对熔池与侧封板的接触面温度分布进行实时检测,确定薄带质量最优时的温度分布和铸速.在实际生产过程中,通过调整铸速即可影响熔池与侧封板接触面的温度分布,进而稳定凝固终点最优位置.试验证明了此模型的准确性.     

Effect of cooling system on the polymer temperature and solidification during injection molding

Cooling system design is of great importance for plastic products industry by injection molding because it is crucial not only to reduce molding cycle time but also it significantly affects the productivity and quality of the final product. A numerical modeling for a T-mold plastic part having four cooling channels is performed. A cyclic transient cooling analysis using a finite volume approach is carried out. The objective of the mold cooling study is to determine the temperature profile along the cavity wall to improve the cooling system design. The effect of cooling channels form and the effect their location on the temperature distribution of the mold and the solidification degree of polymer are studied. To improve the productivity of the process, the cooling time should be minimized and at the same time a homogeneous cooling should be necessary for the quality of the product. The results indicate that the cooling system which leads to minimum cooling time is not achieving uniform cooling throughout the mould.     

铁道机车车辆轮轨的摩擦磨损与节能降耗

阐述了铁道机车车辆轮轨摩擦磨损的现状;研究了内燃机车车轮、闸瓦和钢轨的消耗数量及相应的维修费用;指出了采用适当的新技术之后,在节能降耗方面会产生显著的经济效益。     

轧钢加热炉技术在首钢的应用与发展

通过分析首钢加热炉近年发展状况,阐明轧钢加热炉向大型、高效、低污染和自动化等方向发展,表明首钢新建轧钢加热炉集成步进梁、蓄热式燃烧、汽化冷却和热装热送等先进节能技术,体现了首钢加热炉技术的升级换代。在首钢产品结构调整的过程,由于品种加热质量的提高,加热炉节能应注重品种钢的加热节能、钢坯加热均匀性、加热制度和氧化烧损等方面。上述研究为国内外加热炉技术节能提供参考。     

Cooling of spherical products: Part II—heat transfer parameters

This paper presents a study on the determination of the heat transfer parameters, namely surface heat transfer coefficients, thermal conductivities, thermal diffusivities, specific heats and Biot numbers, for the individual product being cooled with water and with air. An analytical model was developed to determine the surface heat transfer coefficients of the products depending on the thermal properties and cooling process parameters. The results of the present study indicate that surface heat transfer coefficients decrease with increasing batch weight in water cooling and increase with increasing air flow velocity in air cooling. The proposed model can be used to determine easily and accurately surface heat transfer coefficients of different spherically shaped objects subjected to cooling.     

“Model-based” design of thermal management system of a fuel cell “air-independent” propulsion system for underwater shipboard

The thermal management of the fuel cell stack is critical for high-performance long-term operation. A fuel cell system of underwater vessel has a liquid-to-liquid cooling circuits which is composed of two liquid cooling pumps, two three-way valves, and a shell and tube heat exchanger. The liquid-to-liquid cooling technique makes it easier to reject the reaction heat into the surrounding but it is required refined cooling structure and control algorithm. In this study, a model-based design is applied to evaluate the control algorithm. A fuel cell system simulation model is developed and polarization curves and transient response are validated. A proportional integral controller and control strategy is developed and a nominal state feedback controller is also developed. Those control algorithms are evaluated via the dynamic response under step increases of load, and the control performance of the nominal state feedback controller is compared with the conventional controller (PI). The results show that the presented the nominal state feedback control of performs better than the PI control method with less wear and less control effort on the two circuit cooling modules. The proposed the nominal state feedback control can increase the system energy by 23.9%.     

合金灰铸铁激光熔融处理后的耐磨性研究

应用对滚圆盘磨损试验机研究了合金铸铁经激光熔融处理后的耐磨性,试验结果指出,激光处理试样的磨损失重大为减少。同时着重应用扫描电子显微镜研究了磨损试样的表面形貌并指明了磨损产生的机理。     

Unsteady natural convection induced by diurnal temperature changes in a reservoir with slowly varying bottom topography

The present numerical investigation is concerned with the transient flow response in a reservoir model to periodic heating and cooling at the water surface. The numerical modelling reveals a stable stratification of the water body during the heating phase and an unsteady mixing flow in the reservoir during the cooling phase. It is shown that thermal instabilities play an important role in breaking up the residual circulation and initiating a reverse flow circulation in deep waters when the thermal conditions switch from heating to cooling. Further, if cooling is sufficiently strong, a clear undercurrent is formed, bringing cold water to the deep region of the reservoir. Moreover, heating from the water surface results in a stable large-scale convective roll which is clearly observed in the simulations. Understanding of the flow mechanisms pertinent to this problem is important for predicting the transport of nutrients and pollutants across reservoirs.     

汽油载重车排放净化研究

采用试验方法，研究载重车汽油机在使用过程中，由于气缸磨损和水套冷却状况的变化造成尾气排放中的ＨＣ和ＣＯ排放量增加，采用引入旁通空气和控制各气缸混合气空燃比的措施后，致使运行的载重其汽油机的ＨＣ和ＣＯ排放得到很好的控制。