An experimental and theoretical study of solidification in a free-convection flow inside a vertical annular enclosure

The natural convection and solidification in an annular enclosure has been studied experimentally and theoretically. Here the inner cylinder of the annular enclosure was cooled below the solidification temperature of the water, while the outer cylinder was kept on a uniform temperature well above 0 °C. The problem of the unsteady growth of the ice-layer on the inner cold cylindrical surface is studied theoretically and an approximate solution has been obtained for the quasi-steady development of the ice-layer thickness. In addition, it has been found that the influence of the contact layer between the frozen layer and the cold surface is of significant importance for the solidification process. Results are presented and compared between the experimental and the analytical investigation.     

Numerical study of the solidification of successive thick metal layers

Impact and solidification of droplets onto a dry solid substrate generates thin or thick substrates depending on size, velocity and physical properties of the fluid particles (during spray or deposition, for example). Numerous and complex phenomena occur in such processes. Difficulties are due to a lack of control of the resulting film, since fluid dynamics (due to droplets impact, for example) heat transfer and solidification interact and influence the final shape and properties of the film.This paper proposes the numerical study of the influence of some material properties upon the manufacturing of a thick coating. For this purpose, a simplified 1D model is used to simulate “deposition” and solidification of successive metal layers upon an initially cold substrate, assuming that there is no fluid flow. Then, the influence of several parameters is studied: layers thickness, initial substrate and layers temperatures, thermal contact resistance between two elements (substrate or layer) and impact frequency.     

Solidification and melting behaviors and characteristics of molten salt in cold filling pipe

The solidification and melting phenomena and performances of molten salt during cold filling process in a straight pipe are numerically investigated using volume of fluid model. As the molten salt is filled into a cold pipe, the molten salt adjacent to the cold wall is rapidly cooled, and the solidification phenomena appears. After the whole pipe is filled, the solidification layer begins to melt by high temperature fluid heating. Because of the solidification layer, the flow section obviously shrinks, and the pressure loss remarkably increases. During the solidification and melting processes, the fluid temperature in the region with phase change only varies near the freezing point, and it quickly rises after the melting process. Because of the absorption or release of latent heat, the boundary heat flux of molten salt is increased in the solidification region, while it will be decreased in the melting region. As the inlet temperature rises, the pressure loss apparently decreases with the thickness of solidification layer decreasing. However, when the inlet flow velocity increases, the thickness of solidification layer decreases, but the flow resistance without phase change increases, so the pressure loss has a maximum at moderate flow velocity.     

渠道混凝土防渗体下土体特性变化试验研究

渠道衬砌结构的冻胀破坏在寒冷地区普遍存在,对其使用效果和寿命造成了很大影响,为了解季节性冻土区渠道防渗体冻胀发生的原因,以梯形渠道弧形混凝土防渗体为例,从渠道衬砌结构的防渗体下土体温度变化及防渗体冻胀位移量变化特点入手,通过在灌区选取典型渠道进行原型原位观测试验,监测不同时间点不同影响因素下渠道不同位置土壤深度处地温及冻胀位移量变化,分析土体各特性的变化机理。结果表明,不同土层深度处土壤含水率的大小为未冻层>临界层>冻土层;不同土壤深度处的温度变化基本相同;10cm厚的保温板的保温效果优于20cm厚保温板的保温效果。研究成果为解决灌区渠道衬砌的冻害及渗漏问题以及未来大中型灌区渠道施工过程中采取有效的防冻胀措施提供了参考依据。     

相变蓄冷装置内传热及相变特性研究

建立了三种不同蓄冷球球径堆叠方式的相变蓄冷装置模型,对其进行了数值模拟以研究其内部传热及相变特性。结果表明：随着流速的提高、蓄冷球直径的减小,蓄冷结束后三种方案中装置内蓄冷球的凝固率提升显著;双球径方案与单球径方案装置内蓄冷球凝固率随时间变化的规律在蓄冷过程初段相类似,但两方案中相同球径部分蓄冷球相变结束,双球径方案中发生相变的蓄冷球由大直径转变为小直径时,其凝固率随时间变化的速度逐渐超过单球径方案。该研究可为相变蓄冷装置的实际设计及性能优化提供参考     

Dynamical cooling of semiconductor by contact layer

The subject of this paper is the presentation of a simple theoretical model of cooling of a semiconductor. The problem of the unsteady behaviour both of the semiconductor and of the cold plate that are in touch by the contact layer was studied analytically. The influence of the parameters of the materials and the cooling conditions was investigated. The results in the form of an analytical formula and graphs are presented and the heat stream is compared with the experimental results.     

Effect of thermal boundary layer on radiative heat transfer from combustion plasma

The effect of thermal boundary layer on radiative heat transfer considering nongray nonisothermal plasma has been calculated for potassium seeded watergas combustion plasma. The effect of combustion species concentration and seed concentration on radiative flux under the equilibrium flow and frozen flow condition has been studied. It has been estimated that reduction in radiative flux due to cold boundary layer may be upto 25%.     

Laminar film condensation on a finite-size horizontal wavy plate

A theoretical analysis of two-dimensional, steady laminar film condensation on a finite-size horizontal wavy plate is studied for the case in which a cold plate faces upwards. The present study considers that the wavy characteristic length, amplitude and the depth of boundary layer at the plate edge is equal to a minimum depth. The dimensionless heat transfer coefficients and the dimensionless film thickness along the surface are found to be functions of five parameters: dimensionless wave length and amplitude, Pr, Ja and Ra.     

Study of thawing behavior of liquid metal used as computer chip coolant

Liquid metal has recently been identified as an excellent coolant for the thermal management of computer chip. However, in an extremely low temperature environment, the liquid metals may subject to solidification due to local freezing, which may possibly cause failure of their flowing role in transferring heat away. To resolve this potential difficulty, we proposed here to quickly melt the frozen coolant by implanting in advance a wire heater into the liquid metal. A theoretical model was set up to characterize such phase change heat transfer problem through introduction of moving heat source principle. And a closed form analytical solution was obtained using Green's function under several typical boundary conditions. Meanwhile, proof-of-concept experiments were also performed in a refrigerator closet to validate the theoretical prediction. Both theoretical results and experimental measurements reveal that, the frozen metal can be successfully thawed within several dozens of seconds. This would guarantee a quick start and highly safe running of liquid metal for computer chip cooling.     

Numerical solutions for mixed controlled solidification of phase change materials

Kinetics of solidification of phase change materials (PCM) was analyzed for combined heat conduction in the PCM and container wall, and convection in the cold fluid. Stable and convergent numerical methods were derived after transformation to normalized size scale, and corresponding immobilization of the moving boundary. The accuracy was confirmed by comparing numerical solutions and corresponding analytical solutions for control by heat conduction in solidifying layers. The proposed method was used to assess when solidification is controlled solely by conduction in solidified layer, and to analyze relative roles of conduction through the container and convection in the cold fluid.     

Heat transfer characterization of the solidification process resulting from a spray forming process

Spray forming is achieved by atomizing a liquid metal sheet with an inert gas to form molten droplets, which are then subsequently deposited onto a moving cold substrate. During spray deposition processes, the developing pre-form loses thermal energy through a combination of heat transfer processes. To investigate such issues, a heat transfer model was developed to simulate the pre-form growth. This investigation involves the simultaneous heat transfer in the growing solid and mushy/melt region. The deposition rate is assumed to be continuous rather than discrete. Thus, the heat transfer process describing the growth of the deposit layer is mathematically formulated employing a continuous flow assumption. The influence of the system's controllable parameters such as substrate temperature and velocity, mass deposition rate, the superheat of the impinging metal droplets and environment conditions in the spray chamber on the final deposit and solidification are presented. All the parameters are found to have significant impact on the deposit development.     

Simulation of the microstructure of a thin metal layer quenched from a liquid state

In the paper, the solidification of a thin metal layer on quenching from a liquid state is considered. The conditions of the glass transition of the melt are considered. Based on these conditions, the thickness of the metal glass sublayer adjoint to the backing is determined. The proposed model of spontaneous crystallization of the melt allows calculation of the grain size distribution in the cross-section of the layer. It also predicts the variation in the size of the grains in the layer quenched on different backings.     

电站锅炉SCR系统流场的冷态试验与数值模拟的研究

采用冷态试验和数值模拟相结合对某电厂660 MW燃煤锅炉SCR脱硝系统中整流格栅、多孔板、导流板的设计方案进行了速度场和浓度场分布及压降的研究。结果表明数值模拟结果和冷态试验结果较为吻合。其中AIG上游速度分布CV值为15.4%,催化剂入口速度分布CV值为8.1%,浓度场CV值为5.7%,满足系统设计要求。催化剂入口处安装整流格栅可以优化进入催化剂层的气流方向,使烟气垂直进入催化剂层。     

On the prediction of frozen skin ratio in injection molding process by using boundary-tracking fdm

This research was aimed to realize the growth of frozen skin ratio in the multi-gapsize flow field during injection molding. By theoretical and physical approach, the relationship between the gapwise thickness of products and the rate of solidification had been investigated. Through the usage of the boundary-tracking skill based on the finite difference method (FDM), the location of solidified layer in injection molding process described by the compressible HeleShaw model for generalized Newtonian fluid was predicted precisely . It was found that the rate of solidification will achieve a stable value when the holding pressure was fully relaxed, and the flow aisle will be narrowed significantly due to the growth of frozen skin ratio, which of course will affectthe fluidity of the melt.     

Transient heat characteristics of water‐saturated porous media with freezing

Analytical and experimental investigations were performed to examine the transient heat characteristics of water‐saturated porous media with freezing. As a physical model, a two‐dimensional vertical cavity was considered. One vertical wall was abruptly cooled below the fusion temperature, the other three walls were thermally insulated. Three different sizes of glass, and iron, alumina, and copper beads were used as the porous media. The cold energy stored in the porous media and the average thickness of the frozen layer were measured in the experiments. Comparisons of the analytical results with the experimental ones were made, and the effects of Darcy number, Stefan number, and modified Prandtl number on the transient heat characteristics were discussed. The dimensionless equations for predicting the averaged frozen layer thickness and the stored cold energy were obtained as a function of various dimensionless parameters. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 300–316, 2000     

一种燃机叶片试板定向凝固过程的研究

明确铸件定向凝固过程中的温度变化规律．可以避免重型燃气轮机定向叶片铸造中出现的一些缺陷。文章选取了与某重型燃机第1级涡轮动叶尺寸相近的试板．采用定向凝固高温合金DZ445．研究了该试板在实际工业生产用定向结晶炉中的定向凝固过程,结果表明：在距离水冷铜盘位置〉50mm后固液界面形态将发生大的转变,温度梯度逐渐变小,〉100mm后温度梯度基本保持不变．这为后续制定叶片抽拉工艺给出了参考。该实验还通过热电偶获得的定向凝固过程中温度一时间曲线与ProCAST模拟结果基本一致．验证了模拟边界条件和设置参数的准确性．后续可以用于模拟结构复杂的叶片定向凝固过程．指导实际生产工艺.     

The effect of baking conditions on the effective contact areas of screen-printed silver layer on silicon substrate

In this paper, Ag-based paste was screen-printed on the polished as well as on the textured p-type (100) single crystalline silicon wafers. Three types of baking processes were studied: the tube furnace, the belt furnace and the hot plate baking. The effective contact areas of Ag/Si system were measured with a novel method, namely metal insulator semiconductor structure measurement. The results show that after baking on the hot plate at 400°C for 5 min, the size and number of pores in the Ag film layer as well as at the interface between silver layer and silicon decreases significantly, the effective contact area also increases about 20%, particularly on the textured silicon substrate.     

Visualization of Effects of Ultrasound on Liquid Droplet Solidification and Frost Formation on Cold Flat Surface

An experimental study was conducted to study the effect of ultrasonic vibrations on frost suppression on a cold flat surface in atmospheric air flow. A microscopic visualization on the liquid droplet solidification and frost formation process with and without the effect of 20-kHz ultrasound was made. The water droplets solidification and frost crystal growth on the solidified water droplet surface were comparatively observed. Meanwhile, the frost thickness variations were comparatively analyzed. It was found that almost no difference can be seen between the water droplet solidification onset time with the effect of ultrasound and that without the effect of ultrasound. However, after the water droplet solidifies, almost no growth of the frost crystal can be observed on the solidified water droplets surface with the effect of ultrasound. In addition, during the whole frost formation progress, very small growth of the frost layer can be seen on the flat surface with the effect of ultrasound. The experimental results showed that the frost growth process on the cold flat surface can be significantly restrained due to the effect of ultrasound.     

600 MW回转式空气预热器传热模型的解析-数值法

传统的数值迭代法只给出了两分仓回转式空气预热器稳态时端面的二维温度变化曲线,并不能直观地反映空气预热器从启停到正常运行时真实的温度分布.对回转式空气预热器的换热机理进行详细分析,提出采用解析-数值混合计算方法求解换热模型的数学方程,并用该方法模拟600 MW回转式空气预热器金属板和介质温度分布场,结果表明,解析-数值混合计算法,不仅克服了传统迭代法的精度不高、不易收敛的缺点,而且它可以真实地反映出空气预热器整个运行工况金属板和介质的温度分布情况,容易发现引起低温腐蚀的金属最低温度位置,因此对回转式空气预热器安全经济运行提供了更加科学的理论依据.     

Performance study and parametric analysis of a novel heat pipe PV/T system

A novel heat pipe photovoltaic/thermal (PV/T) system that could simultaneously supply electrical and thermal energy was proposed. Compared with a traditional water-type PV/T system, the heat pipe PV/T system can be used in cold regions without becoming frozen. A dynamic model of the heat pipe PV/T system was presented, and a test rig was constructed. Experiments were conducted to validate the results of the simulation. Based on the validated model, the performances of the heat pipe PV/T system were studied under different parametric conditions, such as water flow rates, PV cell covering factor of the collector, tube space of heat pipes, and kinds of solar absorptive coatings of the absorber plate.