基于分子运动对流换热和热辐射下煤粉颗粒群粒子的加热分析

本文建立了在烟气对流加热和辐射状态下的煤粉颗粒群加热模型,通过数值模拟,模拟研究了不同锅炉炉膛尺寸下不同煤粉粒子粒径的煤粉群粒子加热时间以及粒子温升的关系,对对流换热和辐射换热在着火热源中所占比重进行了分析,模型很好的模拟了粒子的升温,能够较好的反映出煤粉粒子加热升温机理,为煤粉射流微元加热及着火提供了计算方法。     

微波辐射炼焦煤中噻吩硫结构的非热效应研究

采用XANES和XPS解析山西炼焦煤中有机硫的赋存特征,选择与煤中结构匹配的噻吩硫模型化合物进行微波辐射和水浴加热,通过Raman光谱比较研究两者对模型化合物中含硫结构的作用机制,利用Materials Studio构建、优化模型化合物结构,用密度泛函理论计算模拟微波场中模型化合物分子构型参数,解析含硫结构对微波的响应机理。结果表明：噻吩硫是炼焦煤中有机硫最主要的赋存形式。微波辐射后,模型化合物碳硫键和硫硫键的Raman谱吸收峰发生红移,温升速度快的模型化合物红移较小;相同温升的水浴加热后,几乎没有红移现象。微波能量不足以使模型化合物中碳硫键和硫硫键断裂,但能够改变分子构型,模型化合物含硫键在微波场中可能存在某种过渡态。微波作用对煤中噻吩硫结构存在非热效应。     

Modeling of the submicron particles formation and initial layer ash deposition during high temperature oxy-coal combustion

The future use of coal as a fuel for power generation in the US depends on the availability of financially viable technologies for capture and storage of CO₂ emissions from power plants. Key second-generation candidates for CO₂ capture include high temperature and pressurized oxy-firing of coal, which has the potential to increase efficiency, lower capital costs, avoid air ingress and reduce oxygen requirements. However, unquantified challenges, such as flame behavior, heat transfer, ash transformation, ash deposition and char oxidation, still exist for those technologies. This study specifically focuses on the formation of submicron particles and initial layer ash deposition during high temperature oxy-coal combustion. Previous work has shown that the initial layer deposits are mainly formed of submicron size ash aerosols transported by thermophoresis. Unfortunately, the importance of submicron particle deposition has not received much attention, probably due to the insignificant deposit mass and difficulty in prediction of the submicron particles formation. In this work, models including mineral matter vaporization model, scavenging model and deposition model are developed and applied into a three-dimensional CFD framework to predict the submicron particles formation and subsequent initial layer deposits formation. The model results are comparable to experimental data. The merits of this work are that it has led to the development of a novel approach to predict both submicron particle formation and initial layer ash deposition during oxy-coal combustion.     

Phenomenological models of microwave heating of a flat coal mass with release of absorbed heat by the convection law

Phenomenological models of electrodynamics and heat transfer in application to microwave heating are constructed. Analytically rigorous solutions to problems of heating a flat coal mass under microwave radiation are obtained. The boundary conditions correspond to convection mechanism of absorbed heat release to the ambient medium. Mathematical models of dielectric heating for homogeneous boundary conditions are solved by a method of dual integral Laplace and Fourier transform. In the presence of inhomogeneities in the boundary conditions, a quite universal method of Green functions is used. The obtained formulas have a constraint associated with constancy of electro- and thermophysical characteristics of coal fuel, or when their piecewise constant approximation is admissible. The obtained dependences form the basis for scientific support of the microwave heating technology.     

微波射频场在微波管中产生爆炸电子发射特性初步分析

 对微波射频场在微波管内引起的场致发射和爆炸电子发射及等离子体的产生进行了分析,推导了等离子体产生强度与微波振幅、材料的电阻率、热传导系数、质量密度和比热容之间的关系,得到了晶须温度分布的表达式,通过数值解析的方式总结出在远大于微波周期的时间尺度上晶须温度提高随时间线性上升。在模型所述材料特性下,温度的上升率达到了3.22×10¹⁰ ℃/s,在100 ns量级就可以使晶须发生气化形成等离子体。     

Prediction of soot formation characteristics in a pulverized-coal combustion field by large eddy simulations with the TDP model

In this study, the soot formation characteristics in a pulverized-coal combustion field formed by a 4 kW Central Research Institute of Electric Power Industry (CRIEPI) jet burner were predicted by large eddy simulation (LES) employing a tabulated-devolatilization-process model (TDP model) [N. Hashimoto et al., Combust. Flame 159 (2012) 353–366]. This model enables to take into account the effect of coal particle heating rate on coal pyrolysis. The coal-derived soot formation model proposed by Brown and Fletcher [A. L. Brown and T. H. Fletcher, Energy Fuels 12 (1998) 745–757] was employed in the LES. A comparison between the data predicted by LES and the soot volume fraction distribution data measured by laser induced incandescence confirmed that the soot formation characteristics in the coal combustion field of the CRIEPI burner can be accurately predicted by LES. A detailed analysis of the data predicted by LES showed that the soot particle distribution in this burner is narrow because the net soot formation rate is negative on both sides of the base of the soot volume fraction. At these positions, soot particles diffused from the peak position of soot volume fraction are oxidized due to a relatively high oxygen concentration. Finally, the effect of soot radiation on the predicted gas temperature distribution was examined by comparing the simulation results obtained with and without soot radiation. This comparison showed that the maximum gas temperature predicted by the simulation performed with soot radiation was over 100 K lower than that predicted by the simulation performed without soot radiation. From result strongly suggests the importance of considering a soot formation model for performing numerical simulations of a pulverized-coal combustion filed.     

Entropy generation applications in flow of viscoelastic nanofluid past a lubricated disk in presence of nonlinear thermal radiation and Joule heating

Entropy generation is the loss of energy in thermodynamical systems due to resistive forces,diffusion processes, radiation effects and chemical reactions. The main aim of this research is to address entropy generation due to magnetic field, nonlinear thermal radiation, viscous dissipation, thermal diffusion and nonlinear chemical reaction in the transport of viscoelastic fluid in the vicinity of a stagnation point over a lubricated disk. The conservation laws of mass and momentum along with the first law of thermodynamics and Fick's law are used to discuss the flow, heat and mass transfer, while the second law of thermodynamics is used to analyze the entropy and irreversibility. The numbers of independent variables in the modeled set of nonlinear partial differential equations are reduced using similarity variables and the resulting system is numerically approximated using the Keller box method. The effects of thermophoresis,Brownian motion and the magnetic parameter on temperature are presented for lubricated and rough disks. The local Nusselt and Sherwood numbers are documented for both linear and nonlinear thermal radiation and lubricated and rough disks. Graphical representations of the entropy generation number and Bejan number for various parameters are also shown for lubricated and rough disks. The concentration of nanoparticles at the lubricated surface reduces with the magnetic parameter and Brownian motion. The entropy generation declines for thermophoresis diffusion and Brownian motion when lubrication effects are dominant. It is concluded that both entropy generation and the magnitude of the Bejan number increase in the presence of slip. The current results present many applications in the lubrication phenomenon,heating processes, cooling of devices, thermal engineering, energy production, extrusion processes etc.     

Realization of oxyfuel combustion for near zero emission power generation

Oxyfuel combustion is one of the promising carbon capture and storage (CCS) technologies for coal-fired boilers. In oxyfuel combustion, combustion gas is oxygen and recirculating flue gas (FGR) and main component of combustion gas is O₂, CO₂ and H₂O rather than O₂, N₂ in air combustion. Fundamental researches showed that flame temperature and flame propagation velocity of pulverized cloud in oxyfuel combustion are lower than that in air with the same O₂ concentration due to higher heat capacity of CO₂. IHI pilot combustion test showed that stable burner combustion was obtained over 30% O₂ in secondary combustion gas and the same furnace heat transfer as that of air firing at 27% O₂ in overall combustion gas. Compared to emissions in air combustion, NOx emission per unit combustion energy decreased to 1/3 due to reducing NOx in the FGR, and SO_x emission was 30% lower. However SO_x concentration in the furnace for the oxyfuel mode was three to four times greater than for the air mode due to lower flow rate of exhaust gas. The higher SO₃ concentration results that the sulphuric acid dew point increases 15–20 °C compared to the air combustion. These results confirmed the oxyfuel pulverized coal combustion is reliable and promising technology for coal firing power plant for CCS.In 2008, based on R&D and a feasibility study of commercial plants, the Callide Oxyfuel Project was started in order to demonstrate entire oxyfuel CCS power plant system for the first time in the world. The general scope and progress of the project are introduced here. Finally, challenges for present and next generation oxyfuel combustion power plant technologies are addressed.     

强声波作用下烟气中滑移单颗粒煤粉传热传质特性

根据二维轴对称、非稳态,层流的质量、动量和能量守恒方程,研究强声波作用下烟气中滑移单颗粒煤粉的传热传质特性,颗粒与烟气之间有稳定的滑移速度.分析声压级范围为150 dB~170 dB,可听声频率范围内,及在声质点速度与滑移速度的不同速率比情况下,颗粒局部努赛尔数、表面平均努赛尔数以及时间-空间平均努赛尔数的分布规律.在可听声频率范围内,存在极值频率,此时煤颗粒表面的传热传质效果最佳.为强声波应用于电站锅炉中、强化煤颗粒燃烧提供理论基础.     

Diffusion Processes in Semiconductor Structures During Microwave Annealing

We present the results of experimental studies of the influence of microwave radiation on the diffusion processes in two different semiconductor materials, namely, InGaAs heterostructures with quantum wells and boron-ion implanted silicon. The experiments were performed in a 30 GHz gyrotron device for microwave processing of materials. We compare the results of heterostructure annealing under microwave and thermal heating. It is found that under microwave heating, the coefficient of In–Ga interdiffusion in InGaAs heterostructures is about an order of magnitude smaller than that under thermal heating at the same temperatures, while the activation energy of diffusion is approximately the same for both heating regimes. In boron-ion implanted silicon, the sheet resistance after annealing at a higher microwave power turns out to be lower than that after annealing at the same temperature and a lower power. This indicates that the microwave field exerts a nonthermal effect on electric activation of dopant atoms. The results show the possibility of optimization of the microwave-annealing regimes to obtain the maximum electric activation at a limited diffusion spreading of the density profile of implanted atoms.     

随机相位和随机频率微波加热效应的数值模拟

 利用时域有限差分法（FDTD）并结合蛙跳技术，通过联合求解Maxwell方程组和热传导方程，模拟了水的微波加热过程，计算了烧杯中的水的温度分布；研究了随机相位和随机频率微波功率源合成时水的加热情况，对比了随机相位和随机频率非相干微波功率源与相干微波功率源作用下水的吸热和温升。计算结果表明，随机相位功率源进行合成时，烧杯中的水温分布更均匀，水所吸收的热量也较相干功率源合成加热时有较大增加；而随机频率功率源进行合成时，加热效果没有明显的变化。     

A numerical simulation of transient ignition and ignition limit of a composite solid by a localised radiant source

An unsteady three-dimensional numerical model has been formulated, coded, and solved to study ignition and flame development over a composite solid fuel sample upon heating by a localised radiant beam in a buoyant atmosphere. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase formulation consists of full Navier-Stokes equations for the conservation of mass, momentum, energy, and species. A one-step, second-order overall Arrhenius reaction is adopted. Gas radiation is included by solving the radiation transfer equation. For the solid phase formulation, the energy (heat conduction) equation is employed to solve the transient solid temperature. A first-order in-depth solid pyrolysis relation between the solid fuel density and the local solid temperature is assumed. Numerical simulations provide time-and-space resolved details of the ignition transient and flame development and the existence of two types of ignition modes: one with reaction kernel initiated on the surface and the other with ignition kernel initiated in the gas phase. Other primary outputs of the computation are the minimum ignition energy (Joule) for the solid as a function of the external heating rate (Watt). Both the critical heat input for ignition and the optimal ignition energy are identified. Other parameters that were varied over the simulations include: sample thickness, ignition heat source spatial shape factor, and gravity level.     

Susceptor-Assisted Enhanced Microwave Processing of Ceramics - A Review

Susceptor-assisted microwave processing is a rapidly growing technology due to its superiority over the conventional processing. In contrast to the conventional heating from the surface, the microwave heating occurs volumetrically via direct interaction with the material. Correspondingly, the microwave heating rates are in general much faster than the heating rates in the conventional furnaces, where heat has to be transferred from the heat sources to the material via conduction, convection and radiation. The need for the susceptor stems from the fact that the majority of the ceramics are low lossy materials and they cannot couple well with the microwave at room temperatures. The susceptor provides an easy and non-invasive technique to exploit the rapid microwave processing even for the highly microwave transparent ceramics, such as alumina, silicon nitride, quartz, etc. This article critically evaluates the susceptor-assisted microwave sintering and solid state synthesis of ceramics which have been reported over the last two decades. A wide range of ceramics has been considered and each case has been analyzed in terms of the enhancement of the processing rates and product qualities (grain structure, material properties, etc.) compared to the conventional processing. It has been shown that the susceptor-assisted microwave processing can greatly reduce the processing time while providing an easy pathway to achieve the desired product qualities. The use of the appropriate susceptor is the key to achieve the fast, smooth, and reliable microwave processing of ceramics and this article provides the required database for the appropriate design of the susceptor based on the process requirement.     

HL-2A装置的约束改善和边界净输入功率分析

在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、H_α辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

微波加热技术的应用与研究进展

文章简述了微波加热的发展概况，阐述了微波加热的介电损耗机理和微波加热的特性．从微波加热与解冻、微波干燥、微波改性、微波烧结、微波杀菌等方面，介绍了微波加热技术在国内的研究与应用情况，指出微波加热技术具有广阔的发展前景，今后应重点加强微波与物料问相互作用理论、微波场中物料的传热和传质机制、微波加热工艺与设备、微波加热技术和其他技术的有机结合等方面的研究．     

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在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、Hα辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

Effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure

This study aims to clarify the effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure under various turbulence intensities. High-fuel-ratio coals are not usually used in coal-fired thermal power plants because of their low flame stability. The expectation is that ammonia as a hydrogen-energy carrier would improve the ignition capability of coal particles in co-combustion. Experiments on spherical turbulent flame propagation of co-combustion were conducted for various coal types under various turbulence intensities, using the unique experimental apparatus developed for the co-combustion. Experimental results show that the flame speed of co-combustion with a low equivalence ratio of ammonia/oxidizer mixture for bituminous coal case was found to be three times faster than that of pure coal combustion and two times faster than that of pure ammonia combustion. On the other hand, the flame speed of co-combustion for the highest-fuel-ratio coal case is lower than that of the pure ammonia combustion case, although the flame propagation can be sustained due to the ammonia mixing. To explain the difference of tendencies depending on the fuel ratio of coal, a flame propagation mechanism of ammonia/coal particle cloud co-combustion was proposed. Two positive effects are the increases of local equivalence ratio and the increases of radiation heat flux, which increases the flame speed. In opposite, a negative effect is the heat sink effect that decreases the flame speed. The two positive effects on the flame speed of co-combustion overwhelm a negative effect for bituminous coal case, while the negative effect overcomes both positive effects for the highest-fuel-ratio coal case. The findings of the study can contribute to the reduction of solid fuel costs when the ammonia is introduced as CO₂ free energy carrier and can improve the energy security through the utilization of high-fuel-ratio coals.     

Thermal regime of slotted channel with moving incompressible liquid under microwave conditions

The article studies mathematical simulation of microwave heating of flow in a slotted channel. The internal heat sources, which are proportional to the absorbed microwave energy, obey the Bouguer law. The stationary temperature distributions in the liquid, wall inner surface andmiddle of the wall along the channel have been found from the balance of heat supply and heat release. The maximum temperature values over channel cross sections have also been determined. The heat transfer to the flow was realized in the nonboiling convective regime. The microwave power was selected such that themaximum temperatures and heat fluxes did not exceed themaximumallowable values for the materials.     

惯性约束聚变中内爆混合的模型构建

从热斑质量方程和能量守恒方程入手,重新计算考虑混合后聚变燃料的比内能和比热容等热力学参数,分析混合效应在轫致辐射损失等能量输运方面的作用,构建有杂质混合情况的热斑燃烧动力学模型.根据静态模型中的热斑燃烧的功率平衡条件,研究烧蚀层杂质混合比例与点火阈值和热斑自持燃烧的关系.理论分析和数值计算表明,混合效应导致热斑中的轫致辐射增强是点火失败的重要因素之一.通过调整不同掺杂材料、混合浓度及混合方式,得到壳层混合与热斑面密度、热斑离子温度的演化之间的关系.最后,基于模拟结果给出两种降低混合影响的方法.     

Effect of radiative transfer of heat released from combustion reaction on temperature distribution: A numerical study for a 2-D system

Both light and heat are produced during a chemical reaction in a combustion process, but traditionally all the energy released is taken as to be transformed into the internal energy of the combustion medium. So the temperature of the medium increases, and then the thermal radiation emitted from it increases too. Chemiluminescence is generated during a chemical reaction and independent of the temperature, and has been used widely for combustion diagnostics. It was assumed in this paper that the total energy released in a combustion reaction is divided into two parts, one part is a self-absorbed heat, and the other is a directly emitted heat. The former is absorbed immediately by the products, becomes the internal energy and then increases the temperature of the products as treated in the traditional way. The latter is emitted directly as radiation into the combustion domain and should be included in the radiation transfer equation (RTE) as a part of radiation source. For a simple, 2-D, gray, emitting-absorbing, rectangular system, the numerical study showed that the temperatures in reaction zones depended on the fraction of the directly emitted energy, and the smaller the gas absorption coefficient was, the more strong the dependence appeared. Because the effect of the fraction of the directly emitted heat on the temperature distribution in the reacting zones for gas combustion is significant, it is required to conduct experimental measurements to determine the fraction of self-absorbed heat for different combustion processes.