变热流对可燃物热解着火特性影响的实验研究

固体可燃物热解过程中,热流是一个重要的影响因素。本文设计了线性及平方上升两种热流,实验研究了无点火源条件下榆木及PMMA的热解着火特性,测量了着火时间、表面温度及质量损失的变化规律。结果表明,随着热流上升系数增加,着火时间均减小,榆木着火时刻的表面温度增加,PMMA基本不变,质量损失速率均增大。热流上升系数和着火时间符合较好的幂函数关系。相同热流下,着火时刻PMMA的质量损失远小于榆木的质量损失。     

不同重力环境下辐射加热材料表面着火特性分析

研究外界辐射加热下，不同重力环境中热薄燃料的着火特性．探讨了重力、环境氧浓度、环境压力及外界辐射强度对着火的影响．结果表明，随着重力的变化，存在不同的着火机制．在微重力和在高的环境氧浓度中，材料的着火延迟时间变短．压力减小，着火延迟时间增大．随着辐射强度的增大，着火延迟时间变小．     

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。     

基于有限立体角测量的谱色测温法

辐射测温是通过测量物体表面发射的辐射来反演温度. 本文结合线性发射率模型从辐射测温方程封闭求解的角度, 解释了谱色测温通常需采用微元立体角测量或针对漫发射体的有限立体角辐射测量的原因, 并推导出了有限立体角辐射测量条件下, 具有非漫发射性质物体表面温度测量的辐射测温方程, 该方程具有测量普适性. 以此方程为基础, 推导了具有测量普适性的谱色测温方程组, 发现不同的辐射测量条件下, 发射率标尺的取值范围相同, 但物理意义发生了明显变化.     

水–酒精混合工质Marangoni凝结液珠表面温度随液珠尺寸变化研究

基于红外测温技术,开展了水平表面水–酒精混合工质Marangoni凝结实验,研究了液珠表面温度随液珠尺寸的变化规律,分析了射流水温、酒精蒸气质量浓度及蒸气压力等因素的影响。研究发现:液珠表面平均温度随液珠半径的增大而增大,并且同一工况下具有良好的相关性。当射流水温升高时,液珠表面温度分布曲线朝着更小半径和更高温度方向移动。当酒精蒸气质量浓度从0.5%增大到10%时,液珠表面平均温度整体下降约4.5℃。随着蒸气压力增大,表面温度分布向着更高温度及更大半径方向发展。     

三维柱腔内辐射输运的一维模拟

在惯性约束聚变实验中,使用一维辐射流体力学程序模拟辐射在管壁约束的介质中的传输,因为无法考虑管壁辐射漏失等二维或三维边缘效应而引入了较大的误差.通过解析的视角因子公式和反照率定标公式来模拟X射线在管壁中的损失以及腔壁开口的漏失等边缘效应,对一维辐射流体力学程序MULTI的辐射输运计算进行了修改,成功模拟了X射线辐照下金等离子体的界面运动轨迹,比不考虑边缘漏失更接近实验测量结果,证明了该方法的有效性.     

空间目标的可见光散射与红外辐射

利用Lowtran7大气传输模型计算0.4～0.8μm可见光波段的太阳辐射、大气自身的热辐射以及天地背景辐射.依据粗糙面光散射理论与双向反射分布函数计算空中目标表面对太阳辐射和云层对阳光反射的散射.利用传热学和背景辐射理论,根据能量守恒定律建立空间目标表面温度的热平衡方程.以气球为例,计算不同表面涂层材料的气球,在不同地理位置、不同高度和不同时间条件下,其温度及辐射功率的变化.分析空间目标红外辐射特性的一般规律和特征.     

原子在压缩光场中的共振辐射

本文应用随机统计理论方法获得了一二能级原子系综在多模压缩光驱动下的共振辐射光谱。对应于光场起伏的不同压缩程度,辐射光谱的中峰出现亚自然线宽与超自然线宽现象。它的边峰在任意情况下都被展宽,而且当光场起伏的压缩方向与光场振幅的相干激发方向不平行或不垂直时,辐射光谱均呈现不对称分布。 关键词：     

粲夸克辐射的死区效应

基于轻夸克辐射能量谱, 考虑夸克质量对部分子辐射能量损失的影响, 利用死区因子把轻部分子辐射能量损失谱推广到重部分子辐射能量损失的情形. 研究结果表明, 利用与夸克质量相关的死区因子推广轻夸克辐射能量损失谱得到的重夸克辐射能量损失谱, 与直接利用微扰QCD计算所得到的Djordjevic-Gyulassy(D-G)谱的结果一致, 且极大简化了重夸克辐射能量损失的计算.     

空间目标的可见光散射与红外辐射

利用Lowtran7大气传输模型计算了0．4～0．8μm可见光波段的太阳辐射、大气自身的热辐射以及天地背景辐射。利用粗糙面光散射理论与双向反射分布函数计算了空中目标表面对太阳辐射、云层对阳光反射的散射。并利用传热学和背景辐射理论，根据能量守恒定律建立了空间目标表面温度的热平衡方程。以气球为例，计算了不同表面涂层材料的气球在不同地理位置、不同高度、不同时间、温度及辐射功率的变化。分析了空间目标红外辐射特性的一般规律和特征。     

Theoretical Calculation on the Piloted Ignition of PMMA

This study presents the comparison of the experimental results and theoretical predictions of the piloted ignition of black PMMA. The model for theoretical calculations included heat, momentum, mass transfer equations and reaction kinetics both in the gas phase and the solid phase, to comprehensively describe the piloted ignition. The experimental samples were thick black PMMA pieces, with the ignition time and the critical surface temperatures at ignition measured using a cone heater under different external radiation heat fluxes. The predictions from the calculations showed good agreement with the experiment at high heat flux, but the deviation was distinct at low heat fluxes, especially for the critical surface temperatures. The fail of the prediction at low heat fluxes was regarded, by analysis, as the result of the neglecting of the decomposition energy term of PMMA in the energy balance equation.     

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.     

Piloted ignition delay of PMMA in space exploration atmospheres

In order to reduce the risk of decompression sickness associated with extra-vehicular activity (EVA), NASA is designing the next generation of exploration vehicles and habitats with a different cabin environment than used previously. The proposed environment uses a total cabin pressure of 52.7–58.6 kPa with an oxygen concentration of 30–34% by volume and was chosen with material flammability in mind. Because materials may burn differently under these conditions and there is little information on how this new environment affects the flammability of the materials onboard, it is important to conduct material flammability experiments at the intended exploration atmosphere. One method to evaluate material flammability is by its ease of ignition. To this end, piloted ignition delay tests were conducted in the Forced Ignition and Spread Test (FIST) apparatus subject to this new environment. In these tests, polymethylmethacylate (PMMA) was exposed to a range of oxidizer flow velocities and externally applied heat fluxes. Tests were conducted for a baseline case of normal pressure and oxygen concentration, low pressure (58.6 kPa) with normal oxygen (21%), and low pressure with 32% oxygen concentration conditions to determine the individual effect of pressure and the combined effect of pressure and oxygen concentration on the ignition delay. It was found that reducing the pressure while keeping the oxygen concentration at 21% reduced the ignition time by 17% on average. Increasing the oxygen concentration at low pressures reduced the ignition time by an additional 10%. It was also noted that the critical heat flux for ignition decreases at exploration atmospheres. These results show that tests conducted in standard atmospheric conditions will underpredict the ignition of materials intended for use on spacecraft and that, at these conditions, materials are more susceptible to ignition than at current spacecraft atmospheres.     

In depth fusion flame spreading with a deuterium-tritium plane fuel density profile for plasma block ignition

<正>Solid-state fuel ignition was given by Chu and Bobin according to the hydrodynamic theory at x = 0 qualitatively. A high threshold energy flux density,i.e.,E^* = 4.3×10¹² J/m²,has been reached.Recently,fast ignition by employing clean petawatt-picosecond laser pulses was performed.The anomalous phenomena were observed to be based on suppression of prepulses.The accelerated plasma block was used to ignite deuterium-tritium fuel at solid-state density. The detailed analysis of the thermonuclear wave propagation was investigated.Also the fusion conditions at x≠0 layers were clarified by exactly solving hydrodynamic equations for plasma block ignition.In this paper,the applied physical mechanisms are determined for nonlinear force laser driven plasma blocks,thermonuclear reaction,heat transfer, electron-ion equilibration,stopping power of alpha particles,bremsstrahlung,expansion,density dependence,and fluid dynamics.New ignition conditions may be obtained by using temperature equations,including the density profile that is obtained by the continuity equation and expansion velocity.The density is only a function of x and independent of time.The ignition energy flux density,E_t^*,for the x≠0 layers is 1.95×10¹² J/m².Thus threshold ignition energy in comparison with that at x = 0 layers would be reduced to less than 50 percent.     

激光点火煤粒着火的模型分析

本文对颗粒煤在激光加热条件下的着火和燃烧进行了数值模拟。采用的是一个简单的煤粒着火与燃烧的一维模型。该模型采用了热解和双平行反应模型,考虑了煤粒表面的多相反应和气相的基元反应以及气相中的传热与传质。从获得的煤粒表面和气相空间的温度随时间的变化规律,可以判断不同煤种的着火方式。     

表面催化反应对空间预混气体着火影响的研究

催化燃烧中表面反应和空间反应的相互影响是一个非常重要的问题,本文计算了表面有催化剂的高温小球利用催化反应放热点燃空间预混气体的温度和浓度分布情况,从而得到当空间预混气体着火时表面催化小球的温度低于无催化时空间预混气体着火温度,同时得到在利用催化反应放热点燃空间燃烧时,空间预混气体着火的温度却远远高于无催化下的空间预混气体着火温度,该结果与试验相符。     

甲烷-空气最小点火能量预测理论模型

 最小点火能是可燃气体危险性辨识的重要参数之一。为从理论上得到混合气体的最小点火能,建立了可燃气体火花点火的物理模型,给出了通过数值模拟得到的可燃气体最小点火能量的预测方法,采用该方法得到了甲烷-空气混合气火花点火的临界温度及最小点火能量。结果表明：甲烷-空气混合气的最小点火能量与浓度呈U型关系,浓度为8.5%的预混气的最小点火能量计算值为0.39 mJ,与实验值0.4 mJ吻合较好。     

Investigation of the third heat transfer crisis on a vertical surface

The process of development of the third heat transfer crisis for vertical orientation of the heating surface was studied experimentally. Experiments were carried out with acetone under the conditions of saturation for the pressures in the working volume from 20 to 28 kPa. In all experiments, the third heat transfer crisis was preceded by propagation of evaporation front along the heating surface. The threshold values of heat flux densities, above which a stable vapor film is formed on the whole heating surface, are lower for vertical orientation of this heating surface than for the horizontal one. Data on the threshold heat flux densities and overheating before boiling-up were obtained. Above these values, formation of evaporation fronts was observed. The range of operation parameters corresponding to formation of the sites of unstable film boiling on the heating surface after boiling-up was determined.     

Theoretical analysis of the evolution from ignition kernel to flame ball and planar flame

Dynamics of flame kernel evolution with and without external energy addition has been investigated analytically and numerically. Emphasis is placed on the effects of radiation heat loss, ignition power and Lewis number on the correlation and transition between the initial flame kernel, the self-extinguishing flame, the flame ball, the outwardly propagating spherical flame and the propagating planar flame. The present study extends previous results by bridging the theories of the non-adiabatic stationary flame balls and travelling flames and allowing rigorous consideration of radiation heat losses. The results show that the effects of radiation heat loss play an important role in flame regimes and flame transition and result in a new isolated self-extinguishing flame. Furthermore, it is found that radiation heat losses significantly increase the critical ignition radius and result in three different dependences of the minimum ignition power on the Lewis number. Comparisons between the results from the transient numerical simulation and those from the quasi-steady state analysis show a good agreement. The results suggest that prediction of flame initiation without appropriate consideration of radiation is not acceptable.     

An Earth-based equivalent low stretch apparatus for material flammability assessment in microgravity and extraterrestrial environments

The standard oxygen consumption (cone) calorimeter (described in ASTM E 1354 and NASA STD 6001 Test 2) is modified to provide a bench-scale test environment that simulates the low velocity buoyant or ventilation flow generated by or around a burning surface in a spacecraft or extraterrestrial gravity level. The equivalent low stretch apparatus (ELSA) uses an inverted cone geometry with the sample burning in a ceiling fire (stagnation flow) configuration. For a fixed radiant flux, ignition delay times for characterization material PMMA are shown to decrease by a factor of 3 at low stretch, demonstrating that ignition delay times determined from normal cone tests significantly underestimate the risk in microgravity. The critical heat flux for ignition is found to be lowered at low stretch as the convective cooling is reduced. At the limit of no stretch, any heat flux that exceeds the surface radiative loss at the surface ignition temperature is sufficient for ignition. Regression rates for PMMA increase with heat flux and stretch rate, but regression rates are much more sensitive to heat flux at the low stretch rates, where a modest increase in heat flux of 25 kW/m² increases the burning rates by an order of magnitude. The global equivalence ratio of these flames is very fuel rich, and the quantity of CO produced in this configuration is significantly higher than standard cone tests. These results demonstrate that the ELSA apparatus allows us to conduct normal gravity experiments that accurately and quantifiably evaluate a material’s flammability characteristics in the real-use environment of spacecraft or extraterrestrial gravitational acceleration. These results also demonstrate that current NASA STD 6001 Test 2 (standard cone) is not conservative since it evaluates a material’s flammability with a much higher inherent buoyant convective flow.