Direct contact melting with asymmetric load

In this study, direct contact melting with an asymmetric load was investigated both experimentally and by numerical analysis. A rectangular parallelepiped solid on a heating surface was melted under various asymmetric loads, while total load acting on the solid and brine temperature were kept constant. In the numerical analysis, the melting process keeping the force balance between the pressure in the liquid film and the loads at all times was calculated. It was found that the average heat flux into the solid was independent of the moment acting on the solid. Analytical results of the time dependencies of the amount of melting and the inclination of the solid agreed with experimental ones for each condition.     

菱形腔内相变材料接触熔化分析

对菱形腔内固体相变材料的接触熔化进行了理论分析.建立了菱形腔内接触熔化分析模型,应用Nusselt 液体边界层理论求解接触熔化区熔化液体的流动控制方程,得到了液膜厚度、熔化速度和熔化体积的无量纲解析表达式.对熔化过程中腔体夹角以及熔化温差Ste的影响规律进行了分析,得到了决定固体完全熔化时间的特征参数,为工程应用提供了一定理论参考.     

An analytical solution of the heat transfer process during contact melting of phase change material inside a horizontal elliptical tube

Close-contact melting processes of phase change material (PCM) inside a horizontal elliptical tube are studied. The theoretical formulas of the melting rate, thickness of liquid layer, elapsed time of solid PCM and Nusselt number during the heat melting process are obtained by analyzing. The results include those of contact melting inside a horizontal cylinder. Finally, the influences of elliptical compression coefficient and temperature difference in melting are discussed, and useful conclusions are drawn. © 1998 by John Wiley & Sons, Ltd.     

Heat pipe-assisted melting of a phase change material

Heat pipe-assisted melting of a phase change material (PCM) housed within a vertical cylindrical enclosure is simulated and is compared to melting induced by heating from an isothermal surface, or with a solid rod or a hollow tube. A parametric study reveals that the heat pipe-assisted melting rates are significantly higher than those associated with the rod or tube, and approach the maximum attainable rates associated with the isothermal surface. Melting rates are enhanced as either the condenser length or the diameter of the heat pipe is increased. The heat pipe is particularly effective in augmenting melting in configurations involving PCM heating from above.     

Effects of convection and inertia on close contact melting

An analytical study was conducted in order to examine the role of convection and inertia on close contact melting of a phase change material (PCM) resting on a sliding heated plate. Results indicate that for high Prandtl substances, inertia has no effect on contact melting regardless of the magnitude of the melt layer Reynolds number Re. Convection however, enhances contact melting and its effect is increasingly perceptible for Re>10². Viscous dissipation may be ignored as long as Re<10⁴ and the Stefan number Ste0.001. On the other hand, for low Prandtl substances, both convection and inertia influence contact melting. Convection enhances melting while inertia hinders it. The effect of inertia is further accentuated as the Prandtl number becomes smaller. Viscous dissipation remains negligible as long as Re<10⁶ and Ste0.001.     

Analysis of contact problems of porous thermoelastic solids

In this article, we study a contact problem between a one-dimensional porous thermoelastic layer and a rigid obstacle. The mechanical problem consists of a coupled system of two hyperbolic partial differential equations and a parabolic one. By defining penalized problems, an energy decay property is obtained. Then, fully discrete algorithms are introduced to approximate both penalized and Signorini problems using the finite element method and the implicit Euler scheme. Stability properties are shown for both problems and a priori error estimates are proved for the penalized problem, from which the linear convergence of the algorithm is derived. Finally, some numerical simulations are performed to demonstrate the accuracy of the approximation and the behavior of the solution.     

Study of contact melting inside isothermally heated vertical cylindrical capsules

Close-contact melting processes of phase change material (PCM) inside vertical cylindrical capsule are studied. PCM are heated by the capsule isothermally at the bottom and side. The theoretical formulas of the melting rate and thickness of liquid layer during the heat transfer process are obtained by analysis, which are convenient for engineering predictions. Finally, the factors that affect melting are discussed, and conclusions are drawn.     

Direct contact condensation in packed beds

A diffusion driven desalination process was recently described where a very effective direct contact condenser with a packed bed is used to condense water vapor out of an air/vapor mixture. A laboratory scale direct contact condenser has been fabricated as a twin tower structure with two stages, co-current and countercurrent. Experiments have been operated in each stage with respective saturated air inlet temperatures of 36, 40 and 43 °C. The temperature and humidity data have been collected at the inlet and exit of the packed bed for different water to air mass flow ratios that vary between 0 and 2.5. A one-dimensional model based on conservation principles has been developed, which predicts the variation of temperature, humidity, and condensation rate through the condenser stages. Agreement between the model and experiments is very good. It is observed that the countercurrent flow stage condensation effectiveness is significantly higher than that for the co-current stage. The condensation heat and mass transfer rates were found to decrease when water blockages occur within the packed bed. Using high-speed digital cinematography, it was observed that this problem can occur at any operating condition, and is dependent on the packing surface wetting characteristics. This observation is used to explain the requirement for two different empirical constants, depending on packing diameter, suggested by Onda for the air side mass transfer coefficient correlation.     

围绕椭圆柱热源的定热流接触熔化研究

对第二类热边界条件下的接触熔化问题进行研究,针对水平椭圆柱热源,建立了定热流接触熔化模型。运用Nusselt液膜理论建立熔化控制方程,通过理论分析,得到了无量纲熔化方程组。对接触熔化速度,熔化液膜厚度和压力分布等参数的变化规律进行讨论,并与温差熔化结果进行比较,研究第二类热边界条件下的接触熔化规律。结果表明:接触熔化速度随热源施加的热流密度呈类线性变化;熔化速度随热源表面压缩系数J的增加而增大,当J=1时椭圆柱热源的熔化结果适用于圆柱热源,当J=0.1时,椭圆柱热源可按平板热源近似。     

Study of direct contact melting with hydrocarbon mixtures as the PCM

In this study, direct contact melting of hydrocarbon mixtures as phase change materials was investigated experimentally. Tetradecane and hexadecane binary mixtures, and pentadecane and octadecane binary mixtures were used as phase change materials, with various mixing ratios. It was found that the heat flux decreased significantly in the low surface temperature range. In addition, a rough and uneven melting surface was observed in the low surface temperature range. The convex portions at the melting surface were collected, and the ingredients of this region were measured by using differential scanning calorimetry (DSC). It was found that the DSC curve shifted to a higher temperature region, and the concentration of higher melting point material increased at the convex portions. Therefore, it seems that the melting point around the convex portions increased due to the higher concentration of higher melting point material, and this resulted in the decrease in the heat flux.     

Numerical simulation of contact melting using the cell-splitting modified enthalpy method

The present work deals with the numerical simulation of contact melting using a cell-splitting enthalpy method, which is an improvement over the conventional enthalpy-porosity method. It is demonstrated that such a method is far superior to the enthalpy-porosity method which not only is unable to capture the interface precisely but is also unable to capture the melt rates, unless the coefficients are back-fitted with the experimental data. In contact melting, the contact layer is thin and hence to resolve the flow, fine grids have to be used. A novel integral model is proposed where a single control volume is used in the contact layer. A parametric study is performed for contact melting in a square geometry and a correlation is evolved for the melt rates. The shapes of the solid during contact and noncontact melting are discussed and the physical mechanisms that decide the evolution are articulated.     

Direct contact PCM–water cold storage

Comfort cooling demand continues to increase throughout the world. Conventional cooling production results in high demand for electrical power during peak hours, leading to high emissions for producing cooling, and potential power shortages in electric grids. With a cold storage, the peak power demand is effectively managed and enables free-cooling. This paper examines one concept using phase change materials (PCM) for storing of cold, where the cold carrier (water) is in direct contact with the PCM. This is in order to enable high power for charging and discharging while providing a high storage capacity. A theoretical model highlights important design parameters for reaching large storage and power capacity. The capacity increases with the Packing Factor and temperature difference across the storage. For high power, the flow rate, temperature difference, and drop size is important parameters which is also verified in an experimental evaluation. The obtainable power is between 30 and 80 kW/m³ storage. Practical limitations of this concept are shown to be PCM–water bed expansion and non-uniform channeling due to asymmetric and unstable PCM shells.     

Direct contact condensation of superheated steam on water

Referring to the interaction of superheated steam (practically stagnant) with subcooled water (slowly moving) the results of an experimental research are presented and discussed. As foreseen in a theoretical investigation, the total thermal power (and consequently the total heat transfer coefficient) does not show an appreciable dependence on superheated steam temperature, so it can be practically evaluated by means of available correlations for saturated steam conditions. The direct contact condensation heat transfer coefficient is linked to the overall one and is slightly dependent on the degree of steam superheating, as experimentally confirmed.     

Analysis of contact melting driven by surface heat flux around a cylinder

The contact melting of phase change material around a moving horizontal cylindrical heat source, which descended under its own weight, is investigated in this article. A melting model under constant surface heat flux is established. The analytical results for thickness and pressure distributions inside melt layer and steady melting velocity are obtained by using contact melting theory. The melting law is discussed, and compared with that of contact melting driven by temperature difference. It is found that quasi-steady melting velocity is determined by heat flux of heat source, and the variation of heat source density has less effect on melting velocity.     

Fabrication of buried contact silicon solar cells using porous silicon

We report the fabrication of buried contact solar cells using porous silicon as sacrificial layer to create well-defined channels (for buried contacts) in silicon. In this paper, the salient features of the technology have been presented. No detrimental effect was found in the performance of buried contact solar cell with partially filled contact area compared to the solar cells having conventional planar contacts. However, a marked difference in the short circuit current density was seen when channel was fully filled with metal by screen printing, without degradation in the open-circuit voltage. It is expected that improved processing in combination with optimized buried metallic contact parameters may yield higher efficiencies that may result in substantial decrease in solar cell cost.     

Analysis of contact melting of phase change materials inside a heated rectangular capsule

Close-contact melting processes of phase change material (PCM) inside a horizontal rectangular capsule are studied. The PCM is heated by the capsule at constant heat flux at the top and isothermally at the bottom, and the sides are adiabatic. The theoretical formulas of the dimensionless melting rate and the thickness of the liquid layer during the heat transfer process are obtained by analysing, which is convenient for engineering predictions. Finally, the influences on the melting process are discussed, and conclusions are drawn.     

Study of close contact melting of ice from a sliding heated flat plate

A study is conducted to examine the effect of the relative motion between a PCM block and a heated flat plate in the process of close contact melting of a high Prandtl number phase change material. An analytical model is proposed and experimental results are reported. Results indicate that the relative velocity between the PCM block and the plate starts to play an important role in the close contact melting process when Re > 10⁴. Three distinct melting regimes are identified: for Re < 3 × 10⁵, close contact melting is the dominant mode of heat transfer in the melt layer. The relative motion may reduce the melting time by up to 66% compare to the melting time observed from a heated surface at rest. For Re > 5 × 10⁵, the thickness of the liquid melt layer is so small (δ¹ < 8 × 10⁻⁴) that the melting process is hindered and abrasion is observed. Finally, for 3 × 10⁵ < Re < 5 × 10⁵, a transition regime bridges the contact melting regime to the abrasion regime.