窄通道内热薄燃料表面火焰传播特性研究

利用实验和数值模拟对微重力和常重力条件下高度为14mm和10mm的窄通道内热薄纸张表面火焰传播特性进行了研究。不同重力条件下窄通道内火焰传播速度随气流速度变化的规律符合得较好,说明地面窄通道实验能够模拟微重力条件下材料表面火焰传播的主要特征。地面窄通道中浮力流动速度的最大值约为5cm／s,与常规实验通道（高度较大）相比...     

微重力和常重力环境中柱状固体材料表面火焰传播实验研究

在微重力和常重力环境中,对不同氧气浓度下柱状聚甲基丙烯酸甲酯(PMMA)表面火焰传播现象进行了实验研究。微重力实验观测了低速强迫对流中的火焰传播,地面实验研究了浮力对流影响下火焰向下传播的规律,分析了氧气浓度与流动对火焰传播的影响。微重力和常重力下的火焰在形态和传播速度上具有显著区别。结合微重力和常重力的实验结果,将火焰传播速度随气流速度的变化关系分为三个区:辐射控制区,传热控制区和化学反应控制区。     

窄通道内热厚材料表面火焰传播的实验研究

利用高度为14 mm的水平窄通道对微重力条件下聚甲基丙烯酸甲酯(PMMA)和聚乙烯(PE)塑料材料表面的火焰传播进行了地面实验模拟研究。在环境气体氧气浓度为30%和50%、低速气流速度小于15 cm/s的实验条件下,实验测量了窄通道内材料表面火焰传播速度随气流速度的变化,它们与微重力下热厚材料火焰传播速度的理论预测结果符合得相当好。分析认为,窄通道能够有效地限制浮力对流,提高燃料表面固相热辐射在火焰传播中的相对作用,从而提供模拟微重力下热厚材料表面火焰传播特性的实验环境。     

近贫燃极限甲烷/空气火焰的层流燃烧速度研究

利用微重力条件下向外传播的球形火焰,对贫燃极限附近甲烷/空气预混火焰的层流燃烧速度进行了测量,得到当量比从0.512(本文微重力实验中测定的可燃极限)到0.601范围内的零拉伸层流燃烧速度,并与前人实验数据和使用3种化学反应动力学模型的计算结果进行了比较.本文实验结果与已有的微重力实验数据非常接近,而其他研究者在常重力...     

富燃料丙烷/空气预混火焰特性的微重力实验研究

地面常重力(1g)条件下,丙烷/空气预混火焰向上传播的富燃极限为9.2%C_3H_8,而向下传播时的富燃极限仅为6.3%C_3H_8,二者之间存在明显差距。利用微重力条件下的实验,对燃料浓度从6.5%到8.6%(微重力实验中测定的可燃极限)范围内的丙烷/空气预混火焰特性进行了研究。实验发现,重力对近极限丙烷/空气火焰的传播有显著影响,影响程度随着当量比的增加而增大。微重力下丙烷/空气的富燃极限为8.6%C_3H_8(φ=2.24),明显高于1g条件下向下传播火焰的可燃极限,略低于向上传播火焰的可燃极限。随着当量比的增大,根据压力变化曲线计算的火焰层流燃烧速度从8.5cm/s逐渐减小到2.7 cm/s,可燃极限处的层流燃烧速度与前人实验数据一致。     

用高空气球搭载微重力实验研究浮力对预混V形火焰的影响

在地面实验中观测到的燃烧现象,包含了浮力的影响。利用微重力实验在浮力消失后研究火焰,有助于深入理解燃烧过程。本文介绍了利用高空气球搭载微重力实验对甲烷－空气预混Ｖ形火焰的研究。实验提供了长时间微重力环境下火焰的动态图像。利用计算机图像处理方法对火焰图像的分析表明,在本实验的工况下,微重力下预混Ｖ形火焰锋面的张角比正常重力下变大,皱折和摆动加剧。这说明浮力确实影响预混燃烧过程。     

加压条件下甲烷/空气预混火焰可燃极限的微重力实验研究

本文成功搭建了适用于中国科学院力学研究所国家微重力实验室(NMLC)落塔的高压对冲火焰实验系统,并首次开展了微重力条件下加压对冲火焰实验,测定了一定张力条件下甲烷/空气层流预混火焰的熄灭极限。实验结果表明,随着压力的增高,甲烷/空气混合气体的可燃极限呈先增后降的非单调变化趋势,峰值发生在0.4 MPa左右。浮力对加压下微弱火焰熄灭极限的影响明显,在常重力条件下,相同张力下的熄灭极限较微重力条件下的偏大,峰值出现的压力略低。微重力条件下的实验结果与使用CHEMKIN的数值模拟的结果相当一致。     

微重力环境中空气流动与辐射热损失对火焰传播的影响

本文建立了包含辐射热损失的火焰沿热薄燃料表面传播的数学模型。燃毁点的密度作为待求参数出现在模型中。数值计算结果表明,在微重力环境中,火焰传播速度随空气流动速度的变化出现峰值。对比无辐射热损失模型和有辐射热损失模型的计算结果发现,辐射热损失是形成上述微重力燃烧特征的原因。在静止的微重力环境中或弱空气流动速度下,辐射热损失使燃毁点处有大量的残碳生成,但随着空气流动速度的增大,残碳生成量迅速减小。     

微重力池沸腾现象中的临界热流密度

本文利用我国第22颗返回式卫星实验研究了微重力条件下池沸腾临界热流现象,发现基于流体动力学不稳定性机制的LD-Zuber模型可以很好地预测不同重力条件下的池沸腾临界热流变化趋势,尽管热丝无量纲半径比该模型的适用范围扩大了3～4个数量级.这和地面常重力环境中关于临界热流尺度效应的研究结果有很大差异,表明在Bond数很小时,热丝无量纲半径已不再是描述临界热流尺度效应的唯一参数.此外,本文提出了"极限核化尺寸"的概念,来解释了不同条件下临界热流的变化特征.     

降压地面模拟技术中的临界压力研究

利用降压地面模拟技术,可以抑制自然对流的影响,从而通过地面试验结果还原空间密封舱内的流动换热情况.降压法的关键在于确定临界压力,使得自然对流对流动换热的影响刚好得到忽略.本文使用数值模拟方法确定临界压力比,以方腔密封舱内的流动换热情况为例,计算了不同 Gr/Re2 数和压力比下密封舱内的流动换热情况.得到了舱内气体温度分布和对流换热系数.通过比较空间情况和地面情况的计算结果,分析了自然对流给流动换热带来的影响,给出了判断临界压力的准则式,并给出了临界 Gr/Re2 数.     

微重力环境中的蜡烛火焰

对蜡烛火焰动态特征的分析表明,从正常重力状态过渡到微重力状态,火焰的空气动力学特征比质量和能量的传输特征的变化快。通过一台差分干涉仪首次测量得到了微重力环境中蜡烛火焰的温度。结果表明,微重力蜡烛火焰的温度小于正常重力蜡烛火焰的温度。微重力蜡烛火焰之所以呈蓝色是因为其温度小于烟黑生成的阈值温度1300K。但当环境氧浓度足够高时,火焰温度大于烟黑生成的阈值温度,火焰中明显有烟黑生成,颜色为亮黄色。     

On the role of radiation and dimensionality in predicting flow opposed flame spread over thin fuels

In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radiation model badly over-predicts the zero gravity flame spread rate due to under estimation of gas radiation loss to the ambient surrounding. The two-dimensional model was also found to be inadequate for predicting the zero gravity flame attributes, like the flame length and the flame width, correctly. The need for a three-dimensional model was found to be indispensable for consistently describing the zero gravity flame-spread experiments [1] (including flame spread rate and flame size) especially at high oxygen levels (>30%). On the other hand it was observed that for the normal gravity downward flame spread for oxygen levels up to 60%, the two-dimensional model was sufficient to predict flame spread rate and flame size reasonably well. Gas radiation is seen to increase the three-dimensional effect especially at elevated oxygen levels (>30% for zero gravity and >60% for normal gravity flames).     

Numerical study of the effects of gravity on soot formation in laminar coflow methane/air diffusion flames under different air stream velocities

Numerical simulations of laminar coflow methane/air diffusion flames at atmospheric pressure and different gravity levels were conducted to gain a better understanding of the effects of gravity on soot formation by using relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model. Thermal radiation was calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO₂, H₂O, and soot. Calculations were conducted for three coflow air velocities of 77.6, 30, and 5 cm/s to investigate how the coflowing air velocity affects the flame structure and soot formation at different levels of gravity. The coflow air velocity has a rather significant effect on the streamwise velocity and the fluid parcel residence time, especially at reduced gravity levels. The flame height and the visible flame height in general increase with decreasing the gravity level. The peak flame temperature decreases with decreasing either the coflow air stream velocity or the gravity level. The peak soot volume fraction of the flame at microgravity can either be greater or less than that of its normal gravity counterpart, depending on the coflow air velocity. At sufficiently high coflow air velocity, the peak soot volume fraction increases with decreasing the gravity level. When the coflow air velocity is low enough, soot formation is greatly suppressed at microgravity and extinguishment occurs in the upper portion of the flame with soot emission from the tip of the flame owing to incomplete oxidation. The numerical results provide further insights into the intimate coupling between flame size, residence time, thermal radiation, and soot formation at reduced gravity level. The importance of thermal radiation heat transfer and coflow air velocity to the flame structure and soot formation at microgravity is demonstrated for the first time.     

Flame spread over solids in buoyant and forced concurrent flows: Model computations and comparison with experiments

A detailed three-dimensional concurrent flame spread model is tested and compared with three sets of experiments. The parameters varied include: gravity, flow velocity, pressure, oxygen mole fraction, and sample width. In buoyant flows (normal and partial gravities), the computed steady spread rate and flame profiles agree favorably with experiment. The predicted extinction limits are lower but can be improved. Comparison in forced concurrent flow in microgravity shows correct trends. The predicted steady spread rates are lower than the experimental ones if the flames are short but higher than the experimental ones if the flames are long. It is believed that the experimental flames may not have fully reached steady state at the end of the 5-s microgravity drops. Longer duration microgravity experiments in future will be needed to substantiate this belief.     

Analytical predictions of shapes of laminar diffusion flames in microgravity and earth gravity

Flame shape is an important observed characteristic of flames that can be used to scale flame properties such as heat release rates and radiation. Flame shape is affected by fuel type, oxygen levels in the oxidiser, inverse burning and gravity. The objective of this study is to understand the effect of high oxygen concentrations, inverse burning, and gravity on the predictions of flame shapes. Flame shapes are obtained from recent analytical models and compared with experimental data for a number of inverse and normal ethane flame configurations with varying oxygen concentrations in the oxidiser and under earth gravity and microgravity conditions. The Roper flame shape model was extended to predict the complete flame shapes of laminar gas jet normal and inverse diffusion flames on round burners. The Spalding model was extended to inverse diffusion flames. The results show that the extended Roper model results in reasonable predictions for all microgravity and earth gravity flames except for enhanced oxygen normal diffusion flames under earth gravity conditions. The results also show trends towards cooler flames in microgravity that are in line with past experimental observations. Some key characteristics of the predicted flame shapes and parameters needed to describe the flame shape using the extended Roper model are discussed.     

湍流预混火焰中的浮力效应

本文利用数值模拟研究了浮力对湍流预混V形火焰平均速度场的影响,发现浮力效应主要体现在远场区域,而在火焰刷附近非常有限;利用落塔和 OH-PLIF 方法在正常重力和微重力下观测了火焰皱褶,发现浮力压制火焰皱褶的程度与湍流强度密切相关。分析表明斜压机理是浮力影响火焰皱褶的重要原因。     

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity.     

On the existence of steady-state gaseous microgravity spherical diffusion flames in the presence of radiation heat loss

Whether steady-state gaseous microgravity spherical diffusion exist in the presence of radiation heat loss is an important fundamental question and has important implications for spacecraft fire safety. In this work, experiments aboard the International Space Station and a transient numerical model are used to investigate the existence of steady-state microgravity spherical diffusion flames. Gaseous spherical diffusion flames stabilized on a porous spherical burner are employed in normal (i.e., fuel flowing into an ambient oxidizer) and inverse (i.e., oxidizer flowing into an ambient fuel) flame configurations. The fuel is ethylene and the oxidizer oxygen, both diluted with nitrogen. The flow rate of the reactant gas from the burner is held constant. It is found that steady-state gaseous microgravity spherical diffusion flames can exist in the presence of radiation heat loss, provided that the steady-state flame size is less than the flame size for radiative extinction, and the flame develops fast enough that radiation heat loss does not drop the flame temperature below the critical temperature for radiative extinction (1130 K). A simple model is provided that allows for the identification of initial conditions that can lead to steady-state spherical diffusion flames. In the spherical, infinite domain configuration, the characteristic time for the diffusion-controlled system to effectively reach steady-state is found to be on the order of 100,000 s. Despite a narrow range of attainable conditions, flames that exhibit steady-state behavior are observed aboard the ISS for up to 870 s, even with the constraint of a finite boundary. Steady-state flames are simulated using the numerical model for over 100,000 s.     

微重力池沸腾现象中的汽泡脱落直径

本文报道了空间微重力池沸腾过程中的汽泡脱落现象,观测到微重力条件下小汽泡行为与常重力时相似,但在中等尺寸范围内,汽泡往往粘附在加热丝上做横向振动,并不断合并所碰到的小汽泡,直到超过临界尺寸后脱落.本文在Lee模型(1992)的基础上引入热毛细作用力,成功地解释了实验观测到的独特的汽泡动力学行为特征.