差分干涉法测量微重力环境蜡烛火焰的温度

从原理上介绍了用于测定微重力环境中蜡烛火焰温度分布的Ｗｏｌｌａｓｔｏｎ棱镜差分干涉仪,并给出了从差分干涉图计算轴对称蜡烛火焰温度分布的数学过程。测量与计算结果表明,微重力环境下的蜡烛火焰温度低于烟粒子的最小生成温度。由于自然对流消失,化学反应放热效率受到组分扩散速率的控制,辐射热损失的冷却作用相对增强,这是引起微重力环境中蜡烛火焰温度降低的主要原因。结论部分地证实微重力环境中蜡烛火焰出现暗蓝色无烟现象是由于火焰温度较低的推论。     

超细水雾与甲烷二维对冲火焰作用的模拟研究

基于Tsuji燃烧器模拟研究了常重力与微重力下超细水雾与二维对冲火焰作用的异同点,验证常重力下的试验结果对于微重力环境下的适用性.结果表明:无细水雾作用时,低应变率下火焰温度和位置对于重力较为敏感;细水雾作用后,高应变率下火焰温度对重力较为敏感.随着细水雾粒径的增大,重力和浮力对于粒子运动的影响逐渐增大.在常重力、低风速下使用较高浓度的小粒径水雾进行试验的结果对微重力下的情况有一定参考价值.     

低重力下典型固体燃料的火焰特征及燃烧速率

利用实验时间为2.2,s,低重力水平为10-2,g0简易实验系统,对易升华典型固体燃料试样从常重力到低重力环境过渡的瞬态燃烧过程进行了实验研究,考察了火焰形态、火焰亮度和温度以及燃料试样燃烧速率受重力水平影响的变化趋势.结果表明,随着重力水平的变化,火焰从常重力环境中的湍流扩散火焰,过渡为低重力环境中的层流火焰,火焰高度、亮度和温度以及燃烧速率都随重力水平的下降而降低.由于进入低重力状态后,常重力燃烧诱导的空气流动并没有完全消失,观察到了逆风侧火焰温度、火焰平均亮度,以及燃烧速率因气流作用而回升的现象,这种现象会使低重力环境中的火灾复杂性和危害性增强.     

有辐射热损射流扩散火焰条件矩模型数值模拟

假设火焰辐射区域为光学薄，将辐射计算的代数模型嵌入到条件矩模型中（CMC（rad）），对甲烷－空气射流扩散值班火焰进行了模拟。并将此模型的模拟结果分别与实验和未考虑辐射热损的条件矩模型（CMC（ad））的结果进行了比较。结果表明采用代数模型计算辐射热损失是合适的，且考虑辐射的条件矩模型对温度场和NO的浓度的模拟结果相比未考虑辐射热损条件矩模型的模拟结果，与实验结果符合得要好。     

浮力对皱折锋面预混V形火焰的影响

利用落塔微策略实验装置和OH平面激光诱导荧光（OH-PLIF）方法，在正常重力和微重力环境下观测了一系列具有皱折锋面的甲烷-空气预混V形火焰，以研究浮力对火焰的影响。借助于高速CCD摄像装置，在实验中记录了每一个火焰在正常重力和微重力下的大量火焰锋面瞬态图像。根据皱折锋面预混火焰的特点和现有的理论，作者开发了图像处理方法用于处理实验结果，利用统计计算得出火焰结构，从而评价浮力的影响。研究表明，浮力一方面影响火焰所在流场的平均流动，另一方面也影响湍流火焰的传播，浮力对火焰的影响在一定程度上呈现出与来流湍流度和层流火焰传播速度的相关性。     

炭黑非催化燃烧特性的固定床实验研究

含碳燃料在还原气氛下燃烧会生成炭黑，在动力设备的燃烧装置中，炭黑的后期氧化对污染控制是非常重要的。利用石英管固定床反应器对天然气扩散火焰中生成的炭黑在不同氧浓度下（20％、15％、10％和5％）的燃烧特性进行了研究，并选用了蜡烛炭黑、丁烷炭黑和煤焦作为对比。根据实验中得出的燃烧特性，与煤焦相比，炭黑的着火温度较低，但是炭黑的燃烧活化能相对更高。氧浓度对各试样着火温度影响不大，而却影响各试样燃烧过程。还进行了水蒸汽对天然气炭黑燃烧的影响研究，水蒸汽能引起炭黑燃烧速率的显著增大。图9参12     

热辐射对富氧扩散燃烧火焰结构和氮氧化物生成的影响

揭示了富氧燃烧过程中的火焰结构和氮氧化物生成机理,针对富氧火焰特性探讨NOx的抑制机理。本文以对向流扩散火焰为对象,利用基于详细的基元反应动力学模型的燃烧数值解析方法研究了热辐射对富氧空气(氧浓度为60％)／甲烷扩散火焰中火焰结构和氮氧化物生成的影响。结果表明,在速度梯度较大时,辐射对燃烧特性的影响可以忽视,当速度梯度K减小到约20s^-1以下,辐射的影响逐渐明显,需要考虑辐射项;同时发现随着速度梯度的减少,总的NO质量生成速率随着速度梯度的下降逐渐增大,在K≈33．3s^-1时达到峰值后又开始下降,直至熄火。     

二甲基醚/甲醇双燃料均质压燃低温氧化反应机理数值模拟

应用零维详细化学反应动力学模型，研究了二甲基醚(DME)／甲醇双燃料均质压燃低温氧化反应机理，考察了初始温度、甲醇浓度和二甲基醚浓度对低温氧化反应的影响．结果表明，甲醇改变了二甲基醚低温反应途径，二甲基醚的低温和二次加氧过程受到抑制，CH3OCH2直接裂解(β-scission)起主导作用，二甲基醚与甲醇高温反应几乎同时进行．温度升高，高温脱氢反应和β-scission增强；低温脱氢反应速率增大，反应时刻提前，高温脱氢反应速率先增大后减小，加氧反应速率随着DME浓度增大而增大，β-scission反应速率先增大后减小；甲醇浓度增大，DME低温脱氢反应速率降低，高温脱氢反应速率先增大后降低，β-scission反应速率随甲醇浓度增大而减小，加氧反应速率则随甲醇浓度增加而升高．     

耦合详细／半详细反应动力学机理的碳氢燃料预混气着火过程及火焰结构数值预测

理论分析碳氢类燃料，空气预混气热着火和流动燃烧过程数值计算方法，并对IPIC－CFDⅡ软件进行修改，使之适合燃料零维着火与火焰结构计算，程序采用了美国SANDIA国家实验室，NASA和BERKELEY大学热力不数据库中的相关参数以及大型化学反应动力学软件包CHEMKIN中相关的模型和子程序；火焰结构计算模块引入美国SANDIA国家实验室开发的PREMIX程序，运用开发的源码，以庚烷／空气预混气和碳氢类燃料中具有代表性的柴油为例；采用最新的化学反应动力学机理（其中庚烷氧化机理包含290个基元反应，涉及57种组分；柴油动力学机理包含327个基元反应，涉及71种组分），计算了C7H16／O2／N2，预混气在不同点火温度，不同当量比和不同压力下的着火延迟时间，预测了火争中反应物，主产物，自由基浓度以及温度变化的时间进程，同时模拟了柴油在不同工况下，其预混火焰中温度，反应物，主产物和自由基浓度随火争高度的变化关系。以上研究为反应设计提供指导。     

强弱射流型MILD富氧燃烧器燃烧特性的数值分析

采用计算流体力学(CFD)数值模拟方法对一种强弱射流型MILD富氧燃烧器的流动和燃烧特性进行了分析.采用有限速率/涡耗散(FR/EDM)模型预测了丙烷MILD富氧燃烧过程中烟气速度场、温度场、组分体积分数分布和烟气内循环流量比等宏观特征,并与已有实验数据进行比较,验证了模型的准确性.在此基础上深入分析了MILD富氧燃烧的化学反应区结构、湍流和化学反应时间尺度等微观特征.结果表明:强烈的烟气内循环充分地稀释并预热主反应区内的反应物,减缓了化学反应速率,从而降低了火焰峰值温度,揭示了强弱射流型MILD富氧燃烧的低氧温和燃烧特征.     

Radiative extinction of gaseous spherical diffusion flames in microgravity

Radiative extinction of spherical diffusion flames was investigated experimentally and numerically. The experiments involved microgravity spherical diffusion flames burning ethylene and propane at 0.98 bar. Both normal (fuel flowing into oxidizer) and inverse (oxidizer flowing into fuel) flames were studied, with nitrogen supplied to either the fuel or the oxygen. Flame conditions were chosen to ensure that the flames extinguished within the 2.2 s of available test time; thus extinction occurred during unsteady flame conditions. Diagnostics included color video and thin-filament pyrometry. The computations, which simulated flow from a porous sphere into a quiescent environment, included detailed chemistry, transport, and radiation and yielded transient results. Radiative extinction was observed experimentally and simulated numerically. Extinction time, peak temperature, and radiative loss fraction were found to be independent of flow rate except at very low flow rates. Radiative heat loss was dominated by the combustion products downstream of the flame and was found to scale with flame surface area, not volume. For large transient flames the heat release rate also scaled with surface area and thus the radiative loss fraction was largely independent of flow rate. Peak temperatures at extinction onset were about 1100 K, which is significantly lower than for kinetic extinction. An important observation of this work is that while radiative heat losses can drive transient extinction, this is not only because radiative losses are increasing with time but also because the heat release rate is falling off as the flame expands away from the burner and the reactant supply to the flame decreases.     

On the extinction characteristics of alcohol droplet combustion under microgravity conditions – A numerical study

Quasi-steady burning and extinction of droplets are of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in reduced gravity environments. Influences of diluent in the atmosphere on isolated droplet combustion characteristics including extinction provide insights to fire extinguishment phenomena and the effectiveness of various diluents as fire suppressants. Extinction of pure methanol and methanol–water droplets ranging from 1.5 to 7 mm size, for varying levels of ambient carbon-dioxide, helium and oxygen concentration – burning in a quiescent microgravity environment were studied numerically to compare the effectiveness of fire suppressant diluent selection and determining the limiting oxygen index. The results show distinct regimes of diffusive and radiative extinction. The transition from diffusive to radiative extinction is strongly influenced by the ambient diluent selection, especially by carbon dioxide concentration. Results for helium as the diluent showed increased burning rate and extinction due to diffusive heat loss. An “extinction characteristic” correlation is proposed that depends on burning rate, ambient diffusivity and flame standoff ratio. Recent methanol droplet experiments conducted over a wide range of operating conditions onboard the International Space Station were found to yield results that agree well with the proposed “extinction characteristic” correlation.     

Numerical modeling for structure and NOx formation characteristics of oxygen-enriched syngas turbulent non-premixed jet flames

The present study numerically investigated the effect of oxygen enrichment on the precise structure and NO_x formation characteristics of turbulent syngas non-premixed flames. The turbulence-chemistry interactions were represented by a Lagrangian flamelet model. In context with the Lagrangian flamelet model, the NO concentration was obtained directly from the flamelet calculation based on full NO_x chemistry, with radiative heat loss being accounted for through the flamelet energy equation. Computations were performed for three different syngas compositions with a designated nitrogen dilution level. Numerical results indicated that, for the CO-rich composition with the lowest LHV yielding the highest scalar dissipation rate and shortest flight time, the flame structure was dominantly influenced by turbulence-chemistry interactions. On the other hand, with regard to the H₂-rich composition with the highest LHV yielding the lowest injection velocity and longest flight time, the flame structure was strongly influenced by radiative cooling. The peak NO level was remarkably elevated by increased oxygen level due to the elevated temperature of the oxygen-enriched flame. In the enhanced oxygen level (30%), the H₂-rich case produced the highest NO level due to a higher temperature and longer residence time within the hot flame zone, while the CO-rich case yielded the lowest NO level due to a lower temperature and shorter residence time. It was also found that, by enhancing the oxygen level, contributions of NNH and N₂O to total NO emission rapidly decreased while the contributions of the thermal NO path were progressively dominant for all cases.     

Inverse influence of initial diameter on droplet burning rate in cold and hot ambiences: a thermal action of flame in balance with heat loss

Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K.     

Kinetic and radiative extinctions of spherical burner-stabilized diffusion flames

Extinction of steady, spherical diffusion flames stabilized by a spherical porous burner was investigated by activation energy asymptotics. An optically-thin radiation model was employed to study the effect of radiation on flame extinction. Four model flames with the same adiabatic flame temperature and fuel consumption rate but different stoichiometric mixture fraction and flow direction, namely the flames with fuel issuing into air, diluted fuel issuing into oxygen, air issuing into fuel, and oxygen issuing into diluted fuel, were adopted to understand the relative importance of residence time and radiation intensity. Results show that for a specified flow rate emerging from the burner, only the kinetic extinction limit at low Damköhler numbers (low residence times) exists. In the presence of radiative heat loss, extinction is promoted so that it occurs at a larger Damköhler number. By keeping the radiation intensity constant while varying the flow rate, both the kinetic and radiative extinction limits, representing the smallest and largest flow rates, between which steady burning is possible, are exhibited. For flames with low radiation intensity, extinction is primarily dominated by residence time such that the high-flow rate flames are easier to be extinguished. The opposite is found for flames suffering strong radiative heat loss. The kinetic extinction limit might occur at mass flow rates lower than what is needed to keep the flame outside of the burner and not observable. An extinction state on the radiative extinction branch can be either kinetic or radiative depending on the process.     

A computational study of spherical diffusion flames in microgravity with gas radiation. Part II: Parametric studies of the diluent effects on flame extinction

Spherical diffusion flame in microgravity was investigated computationally, considering gas radiation with statistical narrow band model (SNB) and discrete ordinate method (DOM). The parametric studies explored the relative effectiveness of fuel- versus oxidizer-side dilution on the flame radius and temperature behavior, and it was discovered that the oxidizer-side dilution has a stronger effect on flame transient behavior than the fuel-side dilution, thereby suggesting a more effective means to induce flame extinction by dilution. Study on different oxidizer-side dilution cases shows that CO₂ has a larger suppression effect than helium and nitrogen with the same dilution level. CO₂ dilution has multiple effects on flame behavior including radiation, thermodynamic, diffusion, and chemical effects. Quantitative analysis shows that the radiation effect is the primary factor accounting for flame temperature drop by approximately 60%, as compared to the thermal/diffusion (30%), and chemical effect (10%). Computational results over a wide range indicated a critical flame temperature of 1130 K at extinction, which appears to be a valid unified extinction criterion for the flame under study. Therefore, it is concluded that extinction of spherical diffusion flame is primarily dictated by the local condition in the flame zone rather than by the volumetric radiative heat transfer in the surrounding gases. Investigation on steady flame solution within different domain sizes shows that, a steady state spherical diffusion flame does not exist in microgravity because the flame keeps growing with the non-zero gradient on the flame outer edge, additionally flame temperature constantly decreases with gas radiation which eventually extinguishes the flame when the flame temperature drops down to a critical value.     

Imaging and radiation of laminar jet diffusion flames with low coflow air velocity in microgravity

We present imaging results and radiation measurements from laminar jet diffusion flames burning in coflowing air conditions. Color and pseudocolor flames are obtained and used to analyze flame brightness and shape, which show that flames under normal gravity are brighter than in microgravity. The longer residence times for microgravity flames result in increased radiative loss, which leads to local extinction and low temperature at the flame tip. Flame radiation fractions for microgravity flames are larger than those in normal gravity for C₂H ₄ and CH ₄. The velocity of coflowing air has a much more pronounced effect on radiation from microgravity flames compared to those in normal gravity. The radiation fractions from ethylene‐fueled flames in microgravity are large, leading to local extinction at the flame tip. We also analyzed the flame radiation fraction.     

发射CT法同时测量含烟黑火焰的温度与烟黑浓度分布的实验研究

基于烟黑辐射特性,利用烟黑单色辐射强度图像信息,采用CT算法同时重建含烟黑火焰温度与烟黑浓度分布,对蜡烛火焰与煤油火焰的温度与烟黑体积分数进行了测量．测量结果表明在两种火焰中,较大烟黑浓度都位于较高火焰温度之内,即在火焰外环的反应区内．另外,由于煤油火焰的燃料量大,因而会增大火焰中的烟黑浓度,辐射损失增大,降低火焰温度．这与有关实验结论是一致的．     

Methanol droplet extinction in carbon-dioxide-enriched environments in microgravity

Diffusive extinction of methanol droplets with initial diameters between 1.25 mm and 1.72 mm, burning in a quiescent microgravity environment at one atmosphere pressure, was obtained experimentally for varying levels of ambient carbon-dioxide concentrations with a fixed oxygen concentration of 21% and a balance of nitrogen. These experiments serve as precursors to those which are beginning to be performed on the International Space Station and are motivated by the need to understand the effectiveness of carbon-dioxide as a fire suppressant in low-gravity environments. In these experiments, the flame standoff distance, droplet diameter, and flame radiation are measured as functions of time. The results show that the droplet extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the initial droplet diameter leads to an increased extinction diameter, while increasing the carbon-dioxide concentration leads to a slight decrease in the extinction diameter. These results are interpreted using a critical Damköhler number for extinction as predicted by an earlier theory, which is extended here to be applicable in the presence of effects of heat conduction along the droplet support fibers and of the volume occupied by the support beads.     

碳氢火焰辐射特性快速判定方法研究

针对比色法计算碳氢火焰温度时要判定其是否为灰体，提出了一种基于图像处理的辐射特性判定方法。该方法用黑体炉标定CCD相机，拟合得到三基色值与单色辐射强度的函数关系，再根据普朗克定律和比色法测温原理计算蜡烛火焰的温度以及三基色值下的辐射率，计算三个辐射率的均方差并进行辐射特性判定。实验结果表明，三个快门速度下，计算温度的相对误差均小于0.500%，计算辐射率的相对误差均小于5.000%，说明标定实验的有效性；以蜡烛火焰为研究对象，其辐射率的均方差均大于0.024，可判定蜡烛火焰为非灰体。