THEORY AND COMPUTATION OF THE CHARACTERISTICS OF THE THERMAL ELECTRIC PLASMA ARC FOR CHEMICAL ENGINEERING APPLICATIONS

A generalized mathematical model has been developed for calculation of the characteristics of a thermal plasma arc under convective conditions. The geometry of the system analyzed is based on a novel technique which has demonstrated the capability of having a stream of gas injected into the core of the arc column around a cathode electrode with a high degree of penetration without affecting the arc stability allowing high throughputs of material. The general problem of the simultaneous solution of the conservation equations of mass, momentum, energy, and charge for a continuous plasma had been shown using incremental iterative techniques. High throughput convective arcs will now make practical many chemical and physical transformation processes. The inclusion of a more comprehensive current density makes the thermal arc more characterized than heretofore.     

裂解煤制乙炔的热等离子体发生器概述

热等离子体具有高温、高焓值和高反应活性等优点,可以用于裂解煤制取乙炔的工艺中。目前已运行的一些等离子体裂解煤制乙炔装置已经达到了中试规模。在此工艺流程中等离子体发生器是核心。介绍了一些裂解煤制乙炔工艺常见的热等离子体发生器,主要是电弧等离子体发生器,其中着重介绍各种类型的直流电弧等离子体发生器,包括直线式、同轴式和V型等离子体发生器。简述了多孔阳极等离子体发生器以及煤制乙炔等离子体发生器所面临的一些问题。     

热等离子体转化煤的研究进展

介绍了热等离子体裂解煤制乙炔、富勒烯、碳纳米管和炭黑,以及热等离子体煤气化制合成气的原理、研究现状、存在的主要问题及发展前景。指出热等离子煤转化是一种高效、洁净的技术,在煤化工等领域具有巨大的应用和发展空间,尤其是热等离子体裂解煤制乙炔联产氢与纳米碳材料工艺,值得重视和开发。     

DRYING OF WATER GELS: DETERMINATION OF THE CHARACTERISTIC CURVE OF AGAR-AGAR

Agar-agar is a polysaccharide extracted as a hydrocolloid from red seaweed, whose gels are homogeneous, stable and transparent. The characterization of ternary equilibrium and mass transfer kinetics in the agar-water-air system is essential for designing operations in the extractive process as well as for ascertaining the behaviour of these gels and sols during evaporation. humectation and swelling.

In this work, the convective thermal drying of agar gels using a laboratory scale discontinuous drier has been studied, the raw data being interfaced to an ancillary microprocessor and generalized according to the characteristic curve formalism useful in the design of full scale driers.     

H2/Ar等离子体射流反应器的模拟

建立了直流电弧等离子体射流反应器的计算流体动力学模型 ,在模型中同时考虑流动、对流换热和共轭热传导等过程 ,消除了以往对反应器内壁温度估计造成的误差 .对湍流的模拟采用了常见的标准k -ε双方程湍流模型 .用SIMPLEST算法模拟与计算了反应器内等离子体的温度场和速度场     

H2/Ar等离子体射流反应器的模拟

建立了直流电弧等离子体射流反应器的计算流体动力学模型 ,在模型中同时考虑流动、对流换热和共轭热传导等过程 ,消除了以往对反应器内壁温度估计造成的误差 .对湍流的模拟采用了常见的标准k -ε双方程湍流模型 .用SIMPLEST算法模拟与计算了反应器内等离子体的温度场和速度场     

New Synthetic Method of Forming Aluminum Oxynitride by Plasma Arc Melting

A new synthetic method of forming the gamma-phase of aluminum oxynitride (alon) has been proposed. alon has been successfully synthesized by the DC nitrogen plasma arc using alpha-Al₂O₃ and AlN as starting materials. Alon rapidly forms in a liquid state under thermal plasma. The obtained lattice parameter of alon has been determined as a function of the concentration of Al₂O₃. Evaporation takes place during arc melting. The condensed alon from the vapor consists of nanoscale-sized spherical particles, and these particles are in single crystals. The evaporation mechanism of alon during arc melting is discussed. Thus, arc plasma processing is a promising method for synthesizing alon and producing the ultrafine powder.     

陶瓷涂层三明治板的抗热震性

采用解析法研究了第3类边界条件下双面陶瓷涂层三明治板的瞬态温度场及瞬态热应力场.对不同Biot模数的热冲击过程中,Al2O3涂层/硬质合金(WC-8%Co,质量分数)基体/Al2O3涂层三明治板的瞬态热应力进行了数值计算.分析了涂层/基体厚度比、涂层与基体热-物理性能匹配对陶瓷涂层三明治板表面热应力峰值的影响.结果表明:陶瓷涂层三明治板的基体的热导率、线膨胀系数和弹性模量应高于涂层,这样可以降低其表面热应力,获得高抗热震性陶瓷涂层三明治板.此外,涂层厚度应尽可能小,以利于改善涂层的抗热震性.     

EVALUATION OF OVERALL HEAT TRANSFER RATES BETWEEN BUBBLING FLUIDIZED BEDS AND IMMERSED SURFACES

A mechanistic model to evaluate heat transfer rates between the dense phase of gas fluidized beds and immersed surfaces has been recently presented by the authors. This model, denoted Generalized Heterogeneous Model (GHM), is formulated in terms of effective thermal properties for particles and interstitial gas. It has been conceived with the purpose of achieving a generalized formulation accounting simultaneously for conductive, gas convective and radiant effects.

The model was previously tested as regards its capability to predict radiative heat transfer rates in beds at high temperature and gas convective contribution in beds of large particles and high operating pressures.

It is the principal object of this contribution to evaluate the performance of the GHM for a wide range of particle sizes, covering from The purely conductive regime to the gas convection dominant regime.

Also, the main assumptions incorporated in the model are revised and some modifications are introduced, mainly on the basis of the results obtained by Mazza et al. (1997b).     

热等离子体重整二氧化碳和甲烷制合成气的研究进展

合成气是一种非常重要的化工原料,近年来利用甲烷和二氧化碳重整制合成气成为了一个研究热点。本文讨论了等离子体重整的机理,重点介绍了不同重整方式,包括射流法、电弧法以及两者结合的方法在二氧化碳和甲烷重整中的应用。相对于冷等离子体,热等离子体重整更适合应用于工业放大生产,有较好的应用前景。     

基于超高温光谱的C/C材料烧蚀产物研究

用“星形”等离子发生器及国产的等离子体火炬进行了炭基复合材料烧蚀实验，采用实验中光谱检测对烧蚀产物进行了分析。经分析发现：在“星形”等离子发生器产生的3500K以下烧蚀环境的光谱中，产物以CO为主，而在国产的等离子体火炬产生的5500K以上烧蚀环境的光谱中，产物以CN、C2、C1和CO为主。光谱检测结果与热化学烧蚀计算的产物吻合得很好，验证了热化学计算的正确性。     

THE EFFECT OF CONVECTION ON DENDRITIC GROWTH UNDER MICROGRAVITY CONDITIONS

The Isothermal Dendritic Growth Experiment (IDGE) is an orbital space flight experiment, launched by NASA, in March, 1994, as part of the United States Microgravity Payload (USMP-2). The IDGE provided accurately measured dendritic growth rates, tip radii of curvature, and morphological observations of ultra-pure succinontrile obtained at supercoolings in the range 0.05-2.0 K. Data were received in the form of pairs of digitized binary images telemetered to the ground from orbit in near-real-time, and as 35mm photographic film received 3 months after the flight. The IDGE flight data has now been analyzed, permitting a comprehensive comparison between dendritic growth under terrestrial and microgravity conditions. The measured growth kinetics, in the form of velocity versus supercooling, is markedly different from those observed in terrestrial experiments. Above 0.4 K supercooling in microgravity, the process of dendritic growth is diffusion controlled, i.e., thermal conduction is the rate limiting process. Under terrestrial conditions, dendritic growth of SCN remains dominated by convective transport of heat until a supercooling of ca. 1.7 K is exceeded. Beyond a supercooling of 1.7 K, there is excellent agreement between terrestrial dendritic growth measurements, and a theory with one adjustable parameter determined form the microgravity measurements. Surprisingly, however, even under microgravity conditions, dendritic growth of SCN becomes dominated by convective transport at supercoolings of ca. 0.4 K and below. The observations confirm that convection, which depends as a sublinear power of the supercooling, will always dominate at low supercoolings, whereas diffusion, which depends on the superlinear power of the supercooling, will always dominate at high supercoolings.     

Oxidation behaviour of high quality freestanding diamond films by high power arcjet operating at gas recycling mode

Oxidation may cause degradation of mechanical, thermal and optical properties of freestanding CVD diamond films at elevated temperatures, and thus may impose severe technical limitations for applications of freestanding diamond films. In the present investigation oxidation behaviour of high quality freestanding diamond films prepared by high power d.c. arc plasma jet operating at gas recycling mode has been studied. It was found by thermogravimetry that the diamond films started to oxidize at approximately 650 °C, whilst the rate of oxidation increased substantially with increasing temperatures. Experimental observations confirmed that grain boundaries were the most preferred site for oxidation damage. Detailed studies were made on the influence of high temperature oxidation on the optical, thermal, and mechanical properties of high quality freestanding diamond films. Our results demonstrated that the high quality freestanding films prepared by high power d.c. arcjet can be safely used below 800 °C for a short time period of 180 s, which is more than enough for certain important IR applications.     

AN APPLICATION OF THE FINITE ELEMENT COLLOCATION METHOD IN SOLVING SELECTED PROBLEMS OF SUBMICRON AEROSOL FILTRATION. THE CASE OF THE SLIP FLOW ON A FIBRE

The convective diffusion equation of aerosols in the case of gas stream slip on the fibre cylindrical surface has been solved by means of the finite element collocation method. Half-analytical form of the solution makes it possible to examine directly changes the slip causes in the distribution of solids around the fibre surface. Concentration profiles and deposition efficiency computed by this method differ from the experiment results less than those obtained by the common method of finite differences. Additionally the finite element collocation has proved to converge more rapidly than that of finite differences (at optimal conditions of calculation for both methods).¹⁰     

THE THERMAL CONDUCTIVITY OF SOME REFRACTORIES1

The problem of measuring the thermal conductivity of materials up to high temperatures has been studied, especially in regard to the sources of error. This investigation has shown that many of the previous determinations of thermal conductivity may have had little precision due to a lack of appreciation of the errors involved. The values of thermal conductivity for a number of refractories are given, as obtained by a new type of apparatus designed to eliminate to a considerable extent the errors of measurement. However, it is believed that these values may have an error as high as ± 25 % for the better heat conductors; so there is still much work to be done in developing a method for measuring thermal conductivity with the precision usual in other physical measurements.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.     

莫来石抑制钛酸铝材料热分解的机理研究

运用扫描电镜（SEM）、X-射线衍射仪（XRD）、能谱仪（EDS）等现代测试手段,从材质的显微结构、晶界晶相组成分析入手,对莫来石抑制钛酸铝热分解的作用机理作了较为详细的分析研究,探明莫来石抑制钛酸铝热分解的作用机理是：（1）在高温下Si^4 进入钛酸铝晶格或间隙中,形成固溶化的钛酸铝晶体,稳定了晶格;（2）因莫来石与钛酸铝间的热膨胀差异,造成较大的压应力（37.65MPa）,致使固溶化的钛酸铝晶体更趋稳定。     

RELATIVE IMPORTANCE OF VAPOUR DIFFUSION AND CONVECTTVE FLOW IN MODELLING OF SOFTWOOD DRYING

The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves.     

Processing and properties of plasma sprayed silicon nitride-glass composite coatings

Abstract

Abstract

Silicon nitride decomposes before it can melt, and so thermal spraying of pure silicon nitride powder is impracticable. To address this difficulty, feedstock powder for plasma spray deposition has been developed in which each particle is a composite of silicon nitride in a low temperature borosilicate glass matrix. The research showed that the silicon nitride did not decompose in the plasma because the low thermal conductivity of the glass matrix ensured a low heat transfer rate and the particle temperature remaining below the decomposition temperature. The coating density initially increased with plasma arc power because of increasing splat flow but then declined at high power levels owing to decomposition of the glass matrix. The silicon nitride dispersion substantially reduced the splat flow, particularly near the maximum packing fraction, but also had the beneficial effect of restricting crack propagation, resulting in an optimum content for wear resistance of 30?vol.-% silicon nitride.