Three-dimensional thermocapillary-buoyancy flow in a shallow molten silicon pool with Cz configuration

In order to understand the characteristics of surface patterns on silicon melt in Czochralski furnaces, we conducted a series of unsteady three-dimensional numerical simulations of thermocapillary-buoyancy flow of a shallow molten silicon pool with Czochralski configuration (depth d = 3 mm). The crucible sidewall is maintained at constant temperature. Bottom and free surfaces are adiabatic or allow heat transfer in the vertical direction. The simulation results indicate that two flow transitions occur with increasing the radial temperature difference along the free surface. At first, the steady two-dimensional flow becomes steady three-dimensional flow and then oscillatory three-dimensional flow. The critical conditions for the onset of the instability were determined. Characteristics of the steady and the oscillatory three-dimensional flows were discussed.     

循环流化床锅炉几个问题的探讨

本文在收集了大量研究资料的基础上 ,结合实例针对当前国内投运的循环流化床锅炉运行状况和存在的因煤种变化、循环物料量变化、磨损和结焦等问题对锅炉运行的影响作了分析 ,并提出一些完善化措施     

流化床直接氮化硅粉技术的研究现状及发展方向

以硅氮反应机理为出发点,从流化床结构设计、原料选取、反应气氛和反应温度控制等四个方面介绍了现有流化床直接氮化硅粉技术的研究现状;同时,基于对现有方法的分析,提出了该技术目前存在的主要问题,并预测了今后的发展方向。     

Evaporative heat and mass transfer from the free surface of a liquid wicked into a bed of spheres

Evaporation of ethanol from square packed arrays of 3.95 mm diameter copper spheres in a transparent, enclosed chamber is investigated. The enclosure ensures that relatively saturated vapor conditions exist near the free surface. The desired heat flux is imposed on the copper substrate upon which the copper spheres are mounted, and the liquid level in the bed is maintained by wicking from a continuous supply of liquid provided by a syringe pump. Transparent windows in the enclosure allow for visualization of the evaporating liquid meniscus shape, which is recorded for different liquid feeding rates and heat fluxes. Experimentally measured meniscus profiles are compared to analytical results based on surface-energy minimization. A meniscus microregion is defined from the contact line to the length where the liquid thickness reaches 10 μm. An approximate kinetic theory-based analysis estimates that up to ～55% of the total meniscus mass transfer occurs in this microregion.     

Estimation of bed expansions in a freely-bubbling three-dimensional gas-fluidized bed

Knowledge of bed expansions is important in the design and operation of gas–solid fluidized beds. This paper presents a study on the estimation of expanded bed height in a large three-dimensional gas-fluidized bed with a square section of 0·61×0·61 m². All experiments were performed at the freely bubbling mode and the bed expansions were recorded by a video camera. Bed materials were used 593 μm raw perlite and 1233 μm sand falling within the categories of Geldart's Groups B and D, respectively. The bed height at minimum fluidization ranged from 0·0398 to 0·3176 m, while the excess air velocity from 0·034 m s⁻¹to 0·7453 m s⁻¹. Equations related to the bed expansion were given using a modified form of two-phase theory of fluidization. A correlation for the average bed expansion (void fraction) was also presented that has been derived from the principal form found successful in gas–liquid systems as follows: R=0·5482 d^−0·129_p(U_o−U_mf)^0·111 with an average deviation of less than 1%. The experimental findings were compared with previously reported results and were discussed in the light of available correlations. © 1998 John Wiley & Sons, Ltd.     

LCA of a molten carbonate fuel cell system

Fuel cells are recognized by all the scientific community to be ultra low emission energy conversion systems, because the pollutants associated with their operation are very low in concentration, compared to traditional energy systems. On the other hand, fuel cells are mainly fed with hydrogen, a chemical component that is not available as a pure component, but it must be extracted from other compounds. This practice involves energy consumption and emissions related to extraction of fuel, hydrogen conversion, transportation and clean up.In order to evaluate the environmental impact related to the energy production by the use of a fuel cell it is imperative to consider all the processes related to the fuel cell operation, and not only the FC operation itself.Life-cycle assessment (LCA) is a unique approach for evaluating the environmental impact related to the whole life of the system, i.e. considering all the processes associated to the system itself, including construction and decommissioning.In the present study a molten carbonate fuel cell (MCFC) system for electric energy production is considered and the related life-cycle environmental impact is considered. Finally a comparison between traditional energy conversion systems and the MCFC systems is conducted, in order to evaluate which are the advantages and the disadvantages that each supposed scenario can lead to.     

大盘进入调整 多晶硅行业配套看点更多

<正>自11月12日以来,沪深两市大幅调整,沪综指从3200点跌至将近2800点,4个交易日跌去12.5%;深成指从14000点跌至12000点,跌幅14.28%,让人不寒而栗。许多死守的投资者十月以来的获利吐出近一半。太阳能板块也未能幸免于难,相关指数从3700点跌至3100点,跌幅16%。这一轮下跌,促使部分市场人士思考,牛市行情是否结束?煤炭、有色在下一波反弹中,还会保持强势吗?下一轮热点是否会转向小盘、医药?     

The improvement of hog fuel by removing fines,using a trommel screen

The study tested the use of a trommel screen originally designed for compost materials to reject oversize particles from hog fuel, processed from several sources and by two different comminution devices. The experiment consisted in screening material previously comminuted by a convertible crusher, designed to use both hammers and knives. Three different feedstock types were used, and namely: discarded pallets, logs and branches from park maintenance. Each feedstock type came in two different qualities, depending on the tool used for comminution, i.e. hammers or knives. Trommel screen productivity varied between 4.2 t h⁻¹, and 5.2 t h⁻¹ of oven dry material. Screening hog fuel derived from pallets was 30% and 40% less productive than screening fuel derived from logs and branches, respectively. Screening cost varied from 16.2 € t⁻¹ dry material in the case of branches, to 19.9 € t⁻¹ oven dry material for pallets. Screening allowed an increase of fuel quality only when applied to pallet-derived hog fuel.     

工频有心感应炉中锌液对熔沟炉衬的渗透作用

从耐火材料组成和结构入手，运用有关理论分析了锌液的渗透作用使熔沟内衬产生的网状裂纹导致感应器破损的原因。     

循环流化床锅炉添加石灰石脱硫系统的探讨

本文首先论述炉内添加石灰石脱硫机理 ,并针对燃用福建无烟煤的循环流化床锅炉 ,进行适用性论证 ,提出相应的石灰石脱硫系统设置方案     

Full process for integrating silicon nanowire arrays into solar cells

A novel process was developed for integrating silicon nanowire arrays into solar cells. n-Type silicon nanowires were grown by chemical-vapour deposition via the gold-catalysed vapour-liquid-solid method, on a p-type silicon substrate. After the growth, the nanowire array was planarized, by embedding the nanowires in a spin-on glass matrix and subsequent chemical-mechanical polishing of the front surface. This planarization step allows to deposit a continuous and uniform conductive film on top of the nanowire array, and thus to form a high-quality front electrical contact. For an illumination intensity of 100 mW/cm², our devices exhibit an energy conversion efficiency of 1.9%. The main performance limiting factor is a high pn junction reverse current, due to contamination by the growth catalyst or to a lack of passivation of surface electronic defects.     

Thermal properties of a low-melting-point nitrate molten salt system

利用硝酸盐熔盐熔点低、比热容高、热分解温度高的特性,制备一种新型硝酸盐熔盐[在Ca（NO₃）₂：KNO₃为0.47：0.53的熔盐体系中添加0.1%~15%的新型添加剂],并对该熔盐的热力学特性及使用成本进行了测试分析。结果表明,新型熔盐的熔点为120.1℃,熔化潜热为76.37 J/g,分解温度为588℃,平均比热容为1.598 J/（g·K）,相比较于传统的Solar salt和Hitec熔盐热力学性能具有较大提升。采用测量范围内比热容的积分平均值来代替整个温度范围内熔盐的比热容,通过计算得出该体系熔盐的显热蓄热成本为108元/（kW·h）。此外,对新型熔盐的热扩散系数以及导热系数进行的测试分析,进一步证明了该熔盐在太阳能光热发电中作为蓄热传热介质具有巨大的潜在应用价值。     

Experimental investigation of a molten salt thermocline storage tank

Thermal energy storage is considered as an important subsystem for solar thermal power stations. Investigations into thermocline storage tanks have mainly focused on numerical simulations because conducting high-temperature experiments is difficult. In this paper, an experimental study of the heat transfer characteristics of a molten salt thermocline storage tank was conducted by using high-temperature molten salt as the heat transfer fluid and ceramic particle as the filler material. This experimental study can verify the effectiveness of numerical simulation results and provide reference for engineering design. Temperature distribution and thermal storage capacity during the charging process were obtained. A temperature gradient was observed during the charging process. The temperature change tendency showed that thermocline thickness increased continuously with charging time. The slope of the thermal storage capacity decreased gradually with the increase in time. The low-cost filler material can replace the expensive molten salt to achieve thermal storage purposes and help to maintain the ideal gravity flow or piston flow of molten salt fluid.     

Dynamic numerical simulation of a molten carbonate fuel cell

The paper presents a set of two-dimensional molten carbonate fuel cell mathematical models, considering the electrical, mass and heat transfer characteristics of a molten carbonate fuel cell. A dynamic simulation model was built under the VC++ environment. The steady state and dynamic simulation results for a cross-flow molten carbonate fuel cell were obtained using the simulation model. The results are appropriate for the design and operation of molten carbonate fuel cells.     

An experimental and theoretical investigation into the heat transfer of a finned water wall tube in a circulating fluidized bed boiler

Heat transfer improvement in a water wall tube with fins was investigated in a circulating fluidized bed (CFB) boiler. Experiments were first conducted in a 6 MWth CFB boiler then a model was developed to analyse and interpolate the results. Temperatures at some discrete points within the wall cross‐section of the tube were measured by burying 0.8 mm thermocouples within a tube. Experimental data showed an increase in heat absorption up to 45 per cent. A good agreement between measured and predicted values was noted. The distribution of temperature in the metal wall and of heat flux around the outer wall of a tube with longitudinal and lateral fins was analysed by numerical solution of a two‐dimensional heat conduction equation. Effects of bed‐to‐wall heat transfer coefficient, water‐to‐tube inside heat transfer coefficient, bed temperature, water temperature and thermal conductivity of the tube material on the heat flux around the water tube are discussed. The present work also examines the influence of the length of the longitudinal fin and the water tube thickness. Heat flux was highest at the tip of the longitudinal fin. It dropped, but increased again near the root of the lateral fin. Copyright © 2000 John Wiley & Sons, Ltd.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Nonlinear predictive control of a molten carbonate fuel cell stack

Operating temperature of a molten carbonate fuel cell stack should be controlled within a special range in order to improve the performance of fuel cell. In this paper, a nonlinear predictive control algorithm based on the Takagi–Sugeno fuzzy model is developed for the temperature of a molten carbonate fuel cell stack. Through predicting the outputs on a Takagi–Sugeno fuzzy model, a discrete optimization of the control action is adopted according to the principle of branch-and-bound method. The simulation results show the potential to introduce the predictive control based on Takagi–Sugeno fuzzy model for the development of fuel cells.     

Splashing of molten tin droplets on a rough steel surface

We photographed the impact of molten metal droplets on a flat plate. From these images we measured droplet dimensions during spreading and counted the number of fingers around a splashing drop. Experiments were done using stainless steel substrates with average roughness of 0.06, 0.07, 0.56, and 3.45 μm respectively. The temperature of the substrate was kept at either 25 or 240 °C. Droplet diameter (2.2 mm) and impact velocity (4 m/s) were kept constant, giving a Reynolds number (Re) of 31 135 and Weber number (We) of 463.Raising substrate roughness from 0.06 to 0.56 μm enhanced the tendency of droplet to splash, whereas increasing roughness even further to 3.45 μm suppressed splashing. This behaviour was attributed to changes in droplet solidification rate with surface roughness. A simple model of droplet spreading was used to estimate thermal contact resistance between the droplet and surface. Increasing surface roughness was found to raise thermal contact resistance and reduce heat transfer from the droplet to the substrate, delaying the onset of solidification and reducing splashing. The number of fingers formed around a droplet splashing on a smooth surface could be predicted reasonably well by a model based on Rayleigh-Taylor instability theory. Increasing surface roughness reduced the number of fingers while enlarging their size.     

Silicon tubes by a closed molten zone: a characterisation study

We report on the characterisation of silicon tubes recrystallised by closed molten zone, a technique developed as a step to a possible process for thin silicon sheet production. The tube faces are quite flat and have a smooth surface. For the electrical characterisation, samples were cut from the tube faces and simple photovoltaic solar cells were formed. The average diffusion length of minority carriers was found, from spectral response, to be around 100 μm. Low-resolution LBIC measurements showed lower diffusion lengths (around 40 μm) in the regions close to the tube edges. This behaviour was correlated to measurements of residual thermal stresses using infrared photoelasticity. Measurements of the changes of spectral response with increasing bias light intensity reveal an increase in the effective diffusion length, a known effect that is interpreted in terms of a density of trapping states.