Heat transfer with film quenching of vapourizing liquids

The heat transfer for film quenching was investigated theoretically and experimentally. With the aid of a mathematical model analytical equations were deduced, with which the locally transferred heat-flux density and the local thickness of the vapour film can be calculated during stable film boiling. The equations were confirmed by experimental results. Accordingly, the heat transfer can be technically influenced only by the velocity of the water leaving the nozzle. With increasing jet velocity of the water not only the transferred heat flux during stable film boiling becomes higher but also the thickness of the vapour film becomes smaller, so that the film collapses earlier, causing the Leidenfrost temperature to shift to higher values.     

Influence of heat transfer on the development of residual stresses in quenched steel cylinders

In this paper results of systematic FE-calculations about the influence of characteristic points of the temperature dependent heat transfer coefficient, especially the Leidenfrost point and the point of maximum heat transfer coefficient on the development of residual stresses are discussed. The numerical investigations were carried out for SAE 1045 and 4140 steel cylinders with 10 and 20 mm 0 quenched in water and oil, respectively. In this work experimentally determined h, T-curves are linearly approximated in the successive stages of heat transfer. Changes of the Leidenfrost-temperature do not influence the middle plane residual stresses of the cylinders investigated. Increasing maximum heat transfer coefficients and low temperatures of maximum heat transfer coefficient, respectively, cause higher magnitudes of residual stress. The development of residual stresses is determined by the temperature dependent gradient of the heat flux density δq/δT in the temperature range of martensitic transformation. Increasing Leidenfrost-temperatures cause more homogeneous stress and residual stress states at the surface of quenched cylinders due to the symmetrical cooling of the sample in axial as well as in radial direction. In particular, it was shown that during immersion cooling of cylindrical parts the heat transfer is locally dependent. Simulating immersion cooling this dependence has to be considered using effective local heat transfer coefficients.     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 1: heat transfer through hot-oxidized steel surfaces cooled by spray-water

For the cooling of steels in the continuous casting process it is necessary to know the heat transfer from the solidifying strand to the cooling water to enable calculation of the secondary cooling zone. Previous investigations have only determined this variable for non-oxidizing metallic surfaces. For many steels cast in practice, however, the formation of oxide layers prevents a direct transfer of the previous results. In the present research the influence of the oxide layers on the heat transfer has been investigated for spay-water cooling. Results have shown that heat transfer in the range of stable film boiling is determined for a constant spray-water temperature in the same way as for non-oxidizing metals, i.e. using the water mass flux density ·_s only. The changed surface qualities resulting from the oxide formation cause the Leidenfrost temperature, however, to shift considerably to higher values.     

An experimental study on heat transfer process of multiple mist impinging jets

Mist jet impingement cooling is an enhanced heat transfer method widely used after the continuous galvanizing process.The key of a successful design and operation of the mist jet impingement cooling system lies in mastering heat transfer coefficients.The heat transfer coefficients of high temperature steel plates cooled with multiple mist impinging jets were experimentally investigated,and the effects of gas and water flow rates on heat transfer coefficients were studied.The test results illustrate that the gas flow rate has little effect on the mist heat transfer rate.It is also found that the water flow rate has a great impact on the heat transfer coefficient.When the water flow rate ranges from 0.96m3/h to 1.59 m3/h,an increase in the rate will produce a higher heat transfer coefficient with a maximum of 5650 W/(m2·K).Compared with the conventional gas jet cooling,the heat transfer coefficient of the mist jet cooling will be much higher,which can effectively strengthen the after-pot cooling.     

高炉炉缸冷却水的探讨

根据炉缸的传热特点,推导了炉缸传热体系的计算公式,利用公式计算结果,分析了炉缸冷却水对延长高炉寿命的作用,重点是冷却水量、冷却水温对炉缸传热的影响规律。洒水冷却的炉壳温度比自然冷却的炉壳温度有显著降低,说明冷却水对维护炉缸安全生产具有重要的作用;在炉缸传热体系中,当水速大于2m/s时,增大冷却水量对炉内传出热量的影响是...     

Numerical determination of heat transfer coefficient for boiling phenomenon at runout table of hot strip mill

Abstract

The heat transfer coefficient during film boiling at the runout table of the hot strip mill is usually determined by experimental methods. Described in the present paper is a finite difference based model for analysis of the thermal behaviour of the strip during cooling at the runout table of the hot strip mill at Tata Steel, India. The model, developed for the prediction of strip temperature, is used to determine the heat transfer coefficient at the water/strip interface while water cooling occurs. A simple form of polynomial as a function of the strip surface temperature is proposed to describe the heat transfer coefficient at the water/strip interface. Good correlation has been found between model predicted temperatures considering the polynomial type heat transfer coefficient and the actual coiling temperature.     

基于热损失率目标的板坯连铸凝固传热过程多目标优化

基于构形理论,以板坯热损失率、表面温度梯度、矫直点表面温度、表面极限温度和液芯长度组成的惩罚函数为优化目标,在二冷总供水流量一定的条件下对板坯连铸凝固传热过程进行优化,得到二冷区各区段最佳水量分配比,分析水量分配比、二冷总供水流量和拉坯速度对复合函数(板坯热损失率和表面温度梯度的线性加权函数)、热损失率目标、表面温度梯度目标和板坯表面温度的影响。结果表明:与初始水量分配比结果相比,水量分配比优化后的复合函数、热损失率目标和表面温度梯度目标分别降低35.04%,2.14%和59.48%;优化后的二冷区出口温度和空冷区末端温度分别提高6.79%和1.59%。最佳水量分配比方案在提高板坯蓄能的同时也提高了板坯的质量。     

二冷配水对304不锈钢连铸板坯质量的影响

根据304不锈钢凝固时的结晶特性，模拟计算了连铸拉速为0．7m/min时，不同二冷配水下铸坯温度场的变化，分析了二冷配水对304不锈钢凝固时传热特性的影响，并提出提高铸坯质量的二冷配水原则。     

Removal of TDS from Cooling Tower Water by Using EDTA-Modified Bagasse Fibers

Modified by ethylenediaminetetraacetic acid (EDTA) salts and unmodified bagasse fibers were tested for the removal of total dissolved solids (TDSs) from cooling tower water. Parameters such as hydrogen ion concentration (pH), particle size of bagasse fibers, and the concentrations of adsorbent and adsorbate were studied to optimize the conditions to be applied on a commercial scale for the decontamination of effluents of cooling tower water. The optimum pH for TDS removal was between 6 and 6.5. The efficiency of TDS removal increased when the size of fiber particles decreased (100?μm) and when the concentration of EDTA salt increased to reach 78 mg/g of modified bagasse fibers. The adsorption parameters were determined using both Langmuir and Freundlich isotherms. The preferential mechanisms for the retention of TDSs are a complexation process between the TDSs and chemical functions present on the surface of fibers, and the chelation process with the EDTA attached to the fibers. The results obtained could be valuable for application to cooling tower water treatment and for the softening of hard drinking water.     

高碳钢线材轧后水冷换热过程的有限差分模拟

根据82B和72A钢Φ5.5～12.5mm高速线材轧后水冷传热特点,将水冷过程分成3个区域,并在水冷箱区域中根据环形喷嘴的开关状态,分别采用了水对流换热和水蒸气对流换热两种形式传热模型,建立了水冷过程按喷嘴个数分段处理的边界条件,通过有限差分模拟得出轧后水冷系统中线材断面的温度分布。通过实测温度值校正高碳钢线材水冷过程中水的对流换热系数,模拟结果表明,精轧机入口和吐丝机的预报和实测线材表面温度均值的绝对误差在±20℃以内。     

37Mn5钢 Φ210 mm圆坯连铸凝固影响因素的数学模拟分析

通过ANSYS软件建立了37Mn5钢Φ210 mm圆坯连铸的传热模型,研究了在铸坯传热过程中铸机拉速1.3~1.5 m/min,钢水过热度15°~60°,二冷比水量0.58~0.78 L/kg对铸坯表面温度、凝固坯壳厚度和凝固终点位置的影响。结果表明,控制稳定的较低拉速、低过热度、较弱二冷比水量可有效地避免37Mn5钢Φ210mm铸坯裂纹的形成,提高铸坯的冶金质量。     

冷却工艺对高速连铸结晶器传热的影响

为了分析冷却水的供水工艺对结晶器铜壁和冷却水温度场的影响,基于结晶器铜壁热电偶实测温度,构建铸坯/铜壁传热反问题和铜壁/冷却水正问题数学模型,采用ANSYS建立铸坯/铜壁/冷却水数值分析模型,对薄板坯结晶器温度场进行耦合传热分析,解析不同冷却工艺对高速薄板坯连铸结晶器内传热行为的影响.结果表明,水缝内冷却水流动方向对铜...     

喷淋结晶器传热特性的实验研究

喷淋结晶器是一种新型结晶器，它是用特殊的喷嘴将水喷淋到结晶器的铜板表面上进行铸坯冷却的，这种冷却方式具有较高的传热速度，便于调节结晶器的热流密度，可以提高拉坯速度和铸坯质量，喷淋结晶器的传热现象是很复杂的，本文通过建立传热实验台，对喷淋冷却过程进行较细致的实验研究，找出喷水量和换热系数的关系等，为设计喷淋冷却结晶器提供必要的参数。     

小方坯结晶器水温水速对传热过程影响

通过建立结晶器内钢液和水的二维对流-传热耦合模型过程,研究了小方坯结晶器冷却水入口温度和流速对铜管温度和结晶器内平均热流的影响.该模型使用Fluent进行求解,模拟了钢液和冷却水的流动和传热,凝固坯壳的生长,以及热量以辐射和导热两种通过保护渣和气隙.通过将坯壳厚度和铜管温度与其他研究的结果进行对比来验证模型准确性.研究结果表明,结晶器冷却水的温度显著影响铜管的冷面温度,水温超过313 K会导致铜管冷面最高温度超过水的沸点.水流速升高0.49 m·s-1能够消除水温升高4 K带来的不利影响.      

Calculation of temperature and heat flux in quenched cylinders for different wetting processes

The cooling of pieces being quenched in evapourable fluids is mainly determined by the process of wetting, which is defined as the sequence of the three known cooling stages film boiling, nucleate boiling and convective heat transfer. All these determine the properties of the finished product. The effect of different wetting processes and heat transfer coefficients on the time-dependent distribution of temperature and heat flux in quenched cylinders was examined using numerical methods. Only transformation-free quenching was considered. When the three cooling stages simultaneouly take place on the sample's surface the radial heat fluxes are superimposed by high axial heat fluxes, which could not be pointed out experimentally before. The consequences may be great axial differences of structure and hardness, especially for steels with low hardenability, as well as residual stresses. The axial temperature gradients can be reduced by producing high wetting velocities. An optimum wetting process involves a very fast breakdown of the vapour envelope on the whole surface of the quenched piece.     

On the development of a three-dimensional transient thermal model to predict ingot cooling behavior during the start-up phase of the direct chill-casting process for an AA5182 aluminum alloy ingot

The control of the heat transfer during the start-up phase of the direct-chill (DC) casting process for aluminum sheet ingots is critical from the standpoint of defect formation. Process control is difficult because of the various inter-related phenomena occurring during the cast start-up. First, the transport of heat to the mold is altered as the ingot base deforms and the sides are pulled inward during the start-up phase. Second, the range of temperatures and water flow conditions occurring on the ingot surface as it emerges from the mold results in the full range of boiling-water heat-transfer conditions—e.g., film boiling, transition boiling, nucleate boiling, and convection—making the rate of transport highly variable. For example, points on the ingot surface below the point of water impingement can experience film boiling, resulting in the water being ejected from the surface, causing a dramatic decrease in heat transfer below the point of ejection. Finally, the water flowing down the ingot sides may enter the gap formed between the ingot base and the bottom block due to butt curl. This process alters the heat transfer from the base of the ingot and, in turn, affects the surface temperature on the ingot faces, due to the transport of heat within the ingot in the vertical direction. A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the DC casting process. The model, based on the commercial finite-element package ABAQUS, includes primary cooling via the mold, secondary cooling via the chill water, and ingot-base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of ingot-surface temperature, water flow rate, impingement-point temperature, and position relative to the point of water impingement. In addition, a secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat-extraction rates). The algorithm used to describe ingot-base cooling includes both the drop in contact heat transfer due to gap formation between the ingot base and bottom block (arising from butt curl) as well as the increase in heat transfer due to water incursion within the gap. The model has been validated against temperature measurements obtained from two 711×1680 mm AA5182 ingots, cast under different start-up conditions (nontypical “cold” practice and nontypical “hot” practice). Temperature measurements were taken at various locations on the ingot rolling and narrow faces, ingot base, and top surface of the bottom block. Ingot-based deflection data were also obtained for the two test conditions. Comparison of the model predictions with the data collected from the cast/embedded thermocouples indicates that the model accounts for the processes of water ejection and water incursion and is capable of describing the flow of heat in the early stages of the casting process satisfactorily.     

热烧结矿竖式冷却过程传热特性

 与传统环冷相比,烧结竖冷拥有漏风率低、换热效率高的优势。为此利用竖冷试验装置,在某烧结厂取热风与热烧结矿研究了竖式冷却过程气固对流换热特性。研究表明,烧结矿竖式冷却过程中影响传热特性的主要因素为冷却风的流量及温度。烧结矿和冷却风对流换热系数随着烧结矿温度的升高而增大,且随着冷却风流量的增大以及冷却风温度的降低,对流换热系数也随之增大。基于白金汉定理,结合试验数据拟合得出了描述烧结矿与冷却风传热特性的准数关联式,其模拟性较好且平均相对误差为7.25%。     

水流量对热轧钢板层流冷却过程对流换热系数的影响

提高带钢层流冷却控制模型的精度,关键是建立精确的对流换热系数与冷却工艺之间的关系.采用有限差分法和反向热传导法,获得了实验条件下钢板表面的对流换热系数及表面温度.研究了不同水流量(0.9~2.1 m3·h-1)对换热系数与表面温度变化规律的影响.在层流冷却过程中,对流换热系数与表面温度呈非线性关系;在距离驻点70 mm内,水流量对换热系数随表面温度变化规律没影响;远离驻点70 mm外,对流换热系数比随远离冲击区驻点距离的增加而减小.采用所确定的换热系数计算得到的温降曲线与实测曲线吻合较好.      

80CrNiMo轧辊气雾冷却试验研究

通过热模拟机测量不同冷却方式试样温降曲线及相关试验,重点进行了80CrNiMo轧辊水冷与气雾冷却热传导、冷却强度对比分析及达到同等冷却强度时水量的消耗比较.试验结果表明,达到相同冷却效果,气雾冷却方式需要水量相当于水冷方式水量的1/15,气雾冷却水蒸发量大,水滴小,单位面积带走热量大,传热系数提高,冷却更均匀,冷却效果更好,应用到实际工业生产中,效益潜力巨大.     

Water spray cooling of stainless and C-Mn steel

This paper describes laboratory cooling experiments of C-Mn and stainless steel with a water/air nozzle. Experiments included use of both the steady state and the transient experimental method. The heat transfer coefficient was calculated both analytically and by a numerical method. The heat transfer coefficient was evaluated as a function of steel surface temperature, water flux, material and water/air flow. It was found that by increasing the water flux the heat transfer coefficient also increased. The Leidenfrost point was shifted to higher temperatures. Increasing air flow was also found to increase the heat transfer coefficient. Maximum heat transfer coefficient was twice as large for the C-Mn steel as for the stainless steel. Comparison of the steady state and transient experimental method showed good agreement in the temperature range 200 – 500°C, and with a water flux of 2I/m²s. Above 500°C it was difficult to obtain stable conditions for the steady state experiments.