Steady melting of rectangular thermoplastic bars induced by hot contacting surfaces

The steady melting of rectangular thermoplastic bars in contact with hot surfaces is analyzed by solving a simplified set of the momentum and energy balance equations, assuming a temperature and shear-rate dependent melt viscosity. A numerical model is developed for predicting the flow field and the temperature distribution in the solid and molten regions of the bar and the location of the solid/melt interface. Computer simulations show that the steady melting rate of the thermoplastic solid is mainly affected by the temperature sensitivity of the melt viscosity, by the pressure applied on the end of the bar, and by a balance between heat conduction and the convection of colder material into the molten region. For the amorphous and semicrystalline polymers considered, heat convection in the outflow direction of the molten material, viscous dissipation, and shear-thinning of the melt viscosity have a much smaller effect on the melting process. These results provide an insight into conduction-induced melting with forced melt removal caused by pressure-induced flow; they also provide a basis for developing a transient model for the hot-tool welding process.     

散热参数对冷心放肩微量提拉法生长蓝宝石晶体影响的数值模拟

利用数值模拟分析的方法,研究了冷心放肩微量提拉法生长蓝宝石晶体过程中热交换器散热参数对晶体质量的影响趋势;分析了热交换器的散热参数变化对晶体生长的固液界面凸出率和晶体内温度分布、温度梯度的影响.结果表明,热交换器的对流换热系数和工作流体的温度变化对晶体生长的固液界面突出率和温度梯度具有相似的影响效果;且在晶体长到一定尺寸后,只靠加大热交换器的散热热能力,不足使晶体继续生长.     

有限元方法求解化学气相淀积反应器模型(Ⅰ)——冷壁反应器模型及有限元解

建立了冷壁化学气相淀积反应器的数学模型,用Galerkin有限元方法对模型予以求解.计算中考虑了温度对物性参数的影响及自然对流因素,计算值和实验结果基本上一致.     

有限元方法求解化学气相淀积反应器模型(Ⅰ)——冷壁反应器模型及有限元解

建立了冷壁化学气相淀积反应器的数学模型,用Galerkin有限元方法对模型予以求解.计算中考虑了温度对物性参数的影响及自然对流因素,计算值和实验结果基本上一致.     

COMPUTER SIMULATION OF HEAT TRANSFER DURING DRYING AND PREHEATING OF WET IRON ORE IN A ROTARY KILN

In the present study, an improved numerical heat transfer model has been developed for a rotary kiln used for drying and preheating of wet iron ore. The present model includes radiation exchange among hot gas, refractory wall and the solid surface, transient conduction in the refractory wall, and mass and energy balances of the hot gas and the solids. The contribution of gas convection has also been taken into account in terms of a fraction of the radiative heat transfer to the inner refractory wall and the solid surface. The computer results show that the present model can predict the length of the kiln as well as axial solid and gas temperature distributions with reasonably good accuracy. A detailed parametric study reveals that a good design of a rotary kiln requires medium gas flow rate, small angle of inclination and low rotational speed of the kiln.     

回转窑筒体散热计算方法探讨

通过回转窑内气体辐射换热和耐火砖、窑皮的导热及窑筒体与环境之间的对流换热的研究,给出了回转窑筒体散热计算模型,基于该模型研究了窑筒体温度与耐火砖、窑皮厚度的关系。按此计算模型,估算5500t/d熟料线回转窑的筒体散热量与实际标定结果相近。     

Studies in liquid-liquid direct-contact heat transfer

Liquid-liquid direct-contact heat transfer in a spray column was studied numerically for a low hold-up. The solutions of the conservation equations of mass, momentum and energy for both phases agree well with earlier experimental results, if one assumes a rigid liquid-liquid interface. Heat transfer inside is dominated by transient conduction and natural convection. It was found that the Nusselt number for free convection depends essentially only on the Rayleigh number.     

真空玻璃传导和对流传热机理研究

利用传导和对流传热原理,建立了真空玻璃热量的流动和传递数学模型,推导了真空玻璃自由分子状态的导热量、支撑柱及边料的固体导热及真空玻璃的外表对流换热方程.分析了自由对流传热、纯气体、稀薄气体、支撑柱及边料的固体导热机理,探讨了真空玻璃的外表对流换热问题.研究结果表明,在低真空状态下的真空玻璃夹层厚度越小,越有利于由自由对流向纯气体导热方向发展,越有利于真空玻璃传热系数的降低.真空玻璃结构只要保持夹层真空度小于0.313 Pa就使得夹层内成为自由分子导热状态.     

MIXED CONVECTION BOUNDARY LAYER FLOW OF A MICROPOLAR FLUID ON A HORIZONTAL PLATE

An analysis is performed to study the heat transfer characteristics of laminar mixed convection boundary layer flow of a micropolar fluid past a semi-infinite horizontal fiat plate with variable surface heat flux. A nonsimilar mixed convection parameter £ and a pseudosimilarity variable v are introduced to cast the governing boundary layer equations into a system of dimensionless equations, which are solved numerically using the finite difference method. A single mixed convection parameter is used to cover the entire regime of mixed convection from the pure forced convection limit to the pure free convection limit. The effect of material parameters, the power-law variation of surface heat flux, nonsimilar mixed convection parameter and Prandtl number are considered. The micropolar fluids are observed to display drag reduction and reduced surface heat transfer rate when compared to Newtonian fluids. The effect of the buoyancy force results in the enhancements of friction factor, heat transfer rate and wall couple stress.     

Corrosion of refractory materials in iron-containing melts

The corrosion resistance of various refractory materials in an iron-containing glass melt is investigated. It is established that an increase in the melt temperature and iron concentration insubstantially affects the corrosion rate in KhATs-30 refractory but noticeably influences the rate of destruction of BK-33 refractory. The KhaTs-30 and Kor-95 refractories are shown to possess much higher corrosion resistance than the other investigated refractories.     

Free convective heat and mass transfer in a doubly stratified non-Darcy micropolar fluid

The flow, heat and mass transfer characteristics of the free convection on a vertical plate with uniform and constant heat and mass fluxes in a doubly stratified micropolar fluid saturated non-Darcy porous medium are studied. The nonlinear governing equations and their associated boundary conditions are initially cast into dimensionless forms by pseudo-similarity variables. The resulting system of equations is then solved numerically using the Keller-box method. The numerical results are compared and found to be in good agreement with previously published results as special cases of the present investigation. The effects of the micropolar, Darcy, non-Darcy and stratification parameters on the dimensionless velocity, microrotation, wall temperature, wall concentration, local skin-friction coefficient and wall couple stress coefficient are presented graphically.     

MELTING OF ICE AROUND A HORIZONTAL ISOTHERMAL CYLINDRICAL HEAT SOURCE

Processes during melting from a horizontal cylindrical heat source of uniform surface temperature embedded in ice have been studied experimentally. The volume of the melt and its shape were photographed at different times for various constant temperatures of the heat source. At early times and under all conditions, the melt occupied a cylindrical annulus. At later times free convective motion caused pear-shaped melt contours which pointed downward when the temperatures of the heat source were below 7°C and upward when the temperatures were above 8°C. Instabilities in cellular natural convection motion resulted in waviness of the interface. The location and magnitude of these ripples were found to depend on the temperature of the heat source and the melt layer thickness. Shadowgraph techniques were used to determine local heat transfer coefficients at the heat source surface.     

Free convection heat transfer over a horizontal plate at low prandtl number

Boundary layer equations for free convection heat transfer along a semi-infinite horizontal plate are derived by giving more importance to the energy equation. The equations are obtained for low Prandtl number and two separate polynomials are used to approximate the temperature and velocity profiles in these regions. The rate of heat transfer is compared with the available analytical and numerical results based on conventional boundary layer equations.     

MELTING OF ICE AROUND A HORIZONTAL ISOTHERMAL CYLINDRICAL HEAT SOURCE

Processes during melting from a horizontal cylindrical heat source of uniform surface temperature embedded in ice have been studied experimentally. The volume of the melt and its shape were photographed at different times for various constant temperatures of the heat source. At early times and under all conditions, the melt occupied a cylindrical annulus. At later times free convective motion caused pear-shaped melt contours which pointed downward when the temperatures of the heat source were below 7°C and upward when the temperatures were above 8°C. Instabilities in cellular natural convection motion resulted in waviness of the interface. The location and magnitude of these ripples were found to depend on the temperature of the heat source and the melt layer thickness. Shadowgraph techniques were used to determine local heat transfer coefficients at the heat source surface.     

Experimental investigation on evaporation interface temperature and evaporation rate of water in its own vapor at low pressures

Evaporative phase transitions are widely present in industrial production and daily life such as thin film processes and crystal growth. The evaporation of the liquid layer and the thermocapillary convection affect each other and restrict each other, making the energy transfer mechanism of the evaporation interface very complicated. To understand the evaporation characteristics of water in its low-pressure pure vapor environment, a series of experimental studies were carried out on the temperature distributions and evaporating rate of water evaporation in the annular pool. The cylinder temperature of the annular liquid pool is controlled between 3℃ and 15℃, and the evaporation environment pressure ranges from 394 Pa to 1467 Pa, when the temperature measurement starts, the depth of water is 10 mm. The results show that the temperature of the vapor side on the liquid-vapor interface is higher than that of the liquid side and there is an obvious temperature jump across the vapor-liquid interface. With the decrease of the pressure ratio, the evaporation rate increases, and the interface temperature jump is enlarged. Meanwhile, with the increase of the distance from the cylinder, the local evaporation rate decreases, thus, the temperature jump decreases. At the same pressure ratio, as the cylinder temperature increases, the heat flux from vapor side decreases, the temperature jump decreases at all measurement points. Within the experimental controlled parameters, the maximum temperature jump obtained in the measurements is 2.56℃. Due to the coupling effect of evaporation cooling and thermocapillary convection, there is a uniform temperature layer with a thickness of about 2 mm under the evaporation interface. The thickness of the uniform temperature layer near the cylinder is always larger than that in the middle of the evaporation interface. In the uniform temperature layer, the thermocapillary convection induced by radial temperature gradient transfers heat from the cylinder to the liquid-vapor interface to compensate for the latent heat of evaporation. Below the uniform temperature layer, the temperature rises rapidly due to heat conduction and buoyancy convection.     

低压蒸汽环境中水蒸发界面温度和蒸发速率的实验研究

蒸发相变广泛存在于薄膜过程及晶体生长等工业生产和日常生活中，液层表面蒸发和热毛细对流相互影响、相互制约，使得蒸发界面能量传递机制变得非常复杂。为了深入了解水在低压纯蒸汽环境中的蒸发特性，对环形液池内水蒸发时的温度分布和蒸发速率进行了一系列实验研究。环形液池壁温控制在3~15℃之间，蒸发环境压力在394~1467 Pa之间变化，开始测量时液层深度为10 mm。结果表明，蒸发界面气相侧温度总是高于液相侧，气液界面存在明显的温度跳跃。随着压比减小，蒸发速率增加，界面温度跳跃随之增大；随着距壁面距离增加，局部蒸发速率降低，温度跳跃值减小；相同压比下，随着壁面温度的升高，气相侧热通量减小，蒸发界面温度跳跃值整体降低；在实验范围内测得的最大温度跳跃值为2.56℃。由于蒸发冷却效应和热毛细对流的耦合作用，蒸发界面下液相侧存在一个厚度为2 mm左右的温度均匀层，且壁面附近温度均匀层厚度大于中间区域厚度。在温度均匀层内，径向温度梯度诱导的热毛细对流将热量从壁面传输至气液界面以补偿蒸发所需汽化潜热；在温度均匀层以下，浮力对流和导热共同作用使得液相温度迅速升高。     

Isothermal corrosion testing of frit furnace refractories

In this paper, the corrosion behaviour of aluminosilicate type refractories in frit melts is studied in an isothermal corrosion test setup. A refractory brick of largely andalusite and sillimanite composition was compared to another refractory brick of mullite and sillimanite composition, both of which were made by different manufacturers for use in different frit furnaces. The industrial frit used for corrosion tests was a commercial product used in a wall tile glaze formulation. Corrosion tests conducted under isothermal conditions provide quantitative and reproducible data about the corrosion performance of refractories. In this study, tests were performed by partially immersing a 15 mm × 15 mm × 115 mm refractory specimen into a frit melt at temperatures between 1404 and 1504 °C. The effects of temperature, duration of exposure and the refractory brick type were investigated using a statistically designed set of experiments. The ANOVA (analysis of variance) table indicated that temperature and test duration were the most important factor effects, as expected. Increasing both temperature and exposure duration led to an increased amount of corrosion as measured by the cross-sectional area loss of the corroded refractory specimen. Postmortem microstructural analysis was also done on the specimens, with extensive amount of ZnO·Al₂O₃ precipitation observed along the frit–refractory interface, where crystals of mullite and alumina were also found to precipitate. Increasing the amount of exposure time and temperature produced more ZnO·Al₂O₃ precipitation. As identified by SEM-EDS analysis, mullite crystals were in the needle-like morphology, while alumina crystals were generally cubic. Additional experiments were conducted by rotating the specimens in the melt at 50 rpm of rotational speed. Due to the reduction of boundary layer thickness, more dissolution was observed from the rotated specimens. In all specimens, corrosion was more pronounced in the bond phase than through the large filler grains of mullite and andalusite.     

Analysis of combined forced and free convection across a horizontal cylinder

A new analysis is performed to study the flow and heat transfer characteristics of laminar combined forced and free convection about a horizontal cylinder. The transformed conservation equations of the nonsimilar boundary layers are solved by a finite difference method. Numerical results for gases having a Prandtl number of 0.7 are presented for buoyancy parameters which cover the entire domain of the mixed convection, ranging from pure forced convection to pure free convection. It is found that both the local wall shear and the local heat transfer rate (1) increase with increasing buoyancy force for assisting flow, (2) decrease with increasing buoyancy force for opposing flow, and (3) depend very strongly on the variation of the local free stream velocity, particularly for small to moderate buoyancy forces. With regard to the heat transfer results, the buoyancy force effects on forced convection are found to be important for Gr/Re² ≥ 1.93 and ≤ –1.60, respectively for assisting and opposing flows, whereas the inertia force effects on free convection become significant for Re²/Gr ≥ 0.012.     

A numerical study on unsteady natural convection of air with variable viscosity over an isothermal vertical cylinder

The present study deals with the boundary layer flow and heat transfer of unsteady laminar free convection flow past a semi-infinite isothermal vertical cylinder immersed in air. The fluid viscosity is assumed to vary with the temperature. An implicit finite-difference method has been employed to solve the governing non-dimensional boundary layer equations. A parametric study is performed to illustrate the influence of variable viscosity on the velocity and temperature profiles. The numerical results reveal that the viscosity has significant influences on the transient velocity and temperature profiles, average skin-friction coefficient and the average heat transfer rate. The results indicate that as the viscosity parameter increases, the temperature and the skin-friction coefficient increase, while the velocity near the wall and the Nusselt number decrease.     

Natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium

This article deals with natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous medium and moisture content transfer in hygroscopic porous medium were solved by the finite element method. The radiative heat transfer is calculated by making use of the radiosity of the surfaces that are assumed to be grey. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation and Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and porous medium. The mean temperature at the interface decreases, the temperature and moisture content gradients are created on the upper two corners of the porous medium region, and the moisture content in the porous medium decreases in the porous medium as Ra increases.