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热厚固体材料表面近极限火焰传播机理研究
Alternative TitleMechanisms of Near-Limit Flame Spread over Thick Solid Fuels
朱凤
Thesis Advisor王双峰
2019-05-28
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline流体力学
Keyword火焰传播,热厚固体材料,熄灭极限,微重力,低速流动,环境压力,环境气氛
Abstract

固体材料的燃烧特性研究对于发展燃烧理论具有重要意义,相关知识也是防火安全的重要基础。本文围绕该问题,特别是熄灭极限附近热厚材料表面的火焰传播特性和机理开展了深入研究,将空间微重力实验结果、地面实验与数值模拟有机结合,获得了对低速流动中火焰传播和熄灭、环境压力影响下材料可燃极限和火焰特性以及不同环境平衡气体中火焰传播和熄灭等科学问题的系统认识。主要内容和研究成果如下:

通过分析空间实验结果,并开展地面模拟实验对低速流动中热厚聚甲基丙烯酸甲酯(polymethylmethacrylatePMMA表面火焰传播进行研究,揭示了低速流动中火焰传播模式的完整图谱、火焰对气流变化的动态响应特性和火焰传播与熄灭的控制机理,获得了热厚材料的可燃边界即火焰冷熄极限。在冷熄极限附近,有两种火焰传播模式:连续火焰和小火焰,在固定的氧气浓度下,气流速度大于临界值时火焰以连续火焰形态传播,小于临界值时以小火焰形态传播。火焰传播模式转变时,由于气相和固相特征热扩散时间尺度的不同引起火焰振荡现象。在低速流动区,无量纲火焰传播速度偏离热区理论预测,并且随着Damköhler数的增加而减小。随着火焰传播速度的降低,火焰前锋的驻离距离增加,而火焰倾斜角度减小。对火焰前锋附近固体表面的能量平衡分析表明,当火焰传播速度逐渐靠近熄灭条件时,总的热量损失(表面辐射热损失加上固体材料向内部传导的热量)在火焰传导热量输入中的比例迅速增加,当热损失率达到临界值时火焰稳定性发生转变,当热损失率达到极限值时火焰发生熄灭。在材料可燃性图谱上,冷熄极限边界和稳定性边界分别对应了极限热损失和临界热损失,或者说分别对应了极限火焰传播速度和临界火焰传播速度。地面窄通道模拟实验表明,低速流动中,对流传热的效率相对降低,火焰同向传播速度小于逆向传播速度,同向火焰熄灭的基本氧气浓度极限更低,对应的材料可燃范围更大。

利用变压力实验系统,研究了环境压力和氧气浓度耦合作用下PMMA材料的燃烧特性和近极限条件下的火焰特征,获得了压力对火焰传播影响的规律性认识。关于火焰传播,实验和理论分析结合研究了火焰前锋的驻离距离和火焰传播速度与压力的关系,建立了驻离距离和火焰传播速度依赖环境压力的关系式。驻离距离随着环境压力的增加呈幂次关系减小。火焰传播速度受到环境总压力和氧分压的影响,当环境总压力不变时,火焰传播速度随着氧分压的增加而呈幂次关系增加;当氧分压不变时,火焰传播速度随着环境总压力的增加而减小。在远离熄灭极限时火焰传热特征长度随着压力的减小而增加,而在靠近熄灭极限压力时,由于受到有限快化学反应和过量热损失的影响火焰传热特征长度减小。考虑辐射损失的影响,建立了火焰熄灭的临界压力与氧气浓度之间关系的预测关系式,分析辐射热损失的作用并与实验结果比较,发现仅从辐射损失的角度出发预测火焰熄灭极限会使材料的可燃范围偏大,压力对化学反应的影响不能忽略。对于同向传播火焰,实验发现存在与氧气浓度有关的临界环境压力,将火焰传播划分成两种模式:火焰根部退后传播模式和燃料退化燃烧模式,两种传播模式的转变压力随着氧气浓度的增加而降低。

利用窄通道实验系统获得低速流动条件并对环境压力进行控制,研究了火焰传播行为与压力和气流速度的关系。当压力和气流速度接近熄灭边界时,连续火焰不能维持,火焰以小火焰的形态稳定向前传播,压力越低,小火焰出现时对应的极限气流速度越大,压力对扩散-热不稳定性的影响是出现这种变化的主要原因。火焰的驻离距离随着压力和气流速度的增加而减小;火焰传播速度随着压力的增加而增加,低压下辐射热损失的影响变大,这使得在气流速度降低时火焰传播速度随压力增加的趋势变缓。

通过实验和数值模拟研究了低速流动中环境平衡气体(N2ArHeCO2)对火焰传播和熄灭的影响。通过改变环境气体热物性和辐射特性,不同的平衡气体对火焰形态、火焰传播速度和基本氧气浓度极限产生影响。数值计算的结果表明,在固体表面温度最高的位置,火焰辐射热流量最大,固体表面辐射热损失也最多,固体吸收的净辐射热流量出现最小值,而固体表面的净辐射热流量在火焰前锋的上游出现最大值。在低速流动中,当环境气体具有辐射特性时,火焰传热特征长度增加,但是由于火焰依然具有光学“薄”的特性,环境气体的辐射再吸收特性对火焰传播的影响较小,火焰向固体表面的热传导对火焰传播起主导作用。

Other Abstract

Combustion characteristics of solid materials are of great significance for the development of combustion theory, and relevant knowledge is an important basis for fire safety. The research interests in flame spread over thermally-thick solid fuel especially flame spread near the extinction limit. The objective of the present study is to gain a comprehensive understanding of flame spreading against a forced oxidizer flow over thick solid fuels in the low-velocity regime, flame extinction limit and spread characteristics under low-pressure ambient environment, as well as flame characteristics of spread and extinction against oxidizer with different balance gases by combining the microgravity results, earth-based experimental results and numerical simulation. The main contents and conclusions are as follows:

Flame spread and extinction phenomenon over a thermally-thick polymethylmethacrylate (PMMA) plate in low-velocity opposing flow have been investigated by analyzing microgravity combustion experiments conducted aboard the SJ-10 satellite of China, and carrying out simulation experiments in normal gravity environment. A flammability map, dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flow, as well as the controlling mechanism for flame spread and extinction are revealed. The flame quenching limit is obtained. Two distinct flame spread modes are identified near the quenching limit, namely the continuous flame mode for gas flow velocities greater than an oxygen-concentration dependent critical value, and the flamelet mode for subscritical gas flow velocities. The transition process between these two spread modes due to a step change in the gas flow velocity is usually accompanied by flame oscillations, and diffusive-thermal instability of the leading flame front is identified as the mechanism controlling such transition. A correlation of the flame spread rate data among different oxygen concentrations indicates that, in the presently considered radiation-controlled regime the normalized flame spread rate deviates from the predictions of the thermal theory and decreases monotonocally with the increase in the flame Damköhler number. Meanwhile, with the decrease in the flame spread rate, the standoff distance and the inclination angle at the flame leading edge show an increasing and decreasing trend, respectively. An energy balance analysis across the fuel surface beneath the flame leading edge indicates that with the approach of the vanishing spread rate limit, the proportion of the overall heat losses (i.e., the surface radiative heat losses plus the conductive losses to the fuel bed) among the total heat conducted from the flame undergoes a rapid growth. When the heat loss reaches a critical value, the stability changes, and the final extinction of entire flame occurs at a certain quenching limit. Moreover, the energy balance analysis suggests that the quenching boundary and the marginal stability boundary identified on the flammability map are, respectively, intrinsically associated with a certain specific ratio of the overall heat losses to the total heat conducted from the flame, or equivalently, associated with a certain specific value of the flame spread rate. The normal gravity narrow channel simulation experimental results show that in the low-velocity flow, the relative effectiveness of the heat transfer is reduced, the concurrent flame spread is slower than opposed spread, and the fundamental low limit oxygen concentration is lower allowing concurrent flame spread than opposed case, making concurrent spread has a wider flammable range than opposed case.

Combustion characteristics and flame spread behaviors near the extinction limit under the coupling effect of ambient pressure and oxygen concentration have been studied using an experimental system which can be used to regulate the pressure inside the chamber. The influence of pressure on on flame spread is obtained. For flame spread, the dependent relationship between the standoff distance at the leading edge and flame spread rate on ambient pressure is analysized theoretically and experimentally, and a relational formula which is used to predict the dependent relationship of standoff distance and flame spread rate on ambient pressures is built. Flame standoff distance decreases exponentially with the increase of environmental pressure. Flame spread rate depends on both total ambient pressure and oxygen partial pressure. When the total ambient pressure is a constant, flame spread rate increases exponentially with the increase of oxygen partial pressure, while flame spread rate decreases with the total ambient pressure at a constant oxygen partial pressure. When the ambient pressure is much higher than the quenching limit, flame characteristic thermal scale increases with the decrease of ambient pressure. As the ambient pressure is near the quenching limit, flame characteristic thermal scale decreases with the decrease of ambient pressure due to the effect of infinite kinetics and excessive heat loss. A prediction model used to correlate critical pressure at flame extinction and oxygen concentration is established base on the radiative heat loss. By comparing the theoretical and experimental results, it is found that the predicted extinction limit is wider than the experimental results, implying that the effect of low ambient pressure on the finite kinetics should not be ignored. For upward spread flame, two flame spread modes are identified with respect to a critical ambient pressure, which depends on oxygen concentration. When the ambient pressure is higher than the critical pressure, the flame spreads forward in the retreating flame base mode, whereas for lower pressures the flame spread takes the fuel regression mode. The critical pressure of flame spread mode transition decreases with the increase of ambient oxygen concentration.

Flame spread behaviors under low-pressure environment with low-velocity flow have been studied by using the narrow channel apparatus experimental system and the sealed chamber system. The relationships between flame spread rate and flow velocity as well the ambient pressure have been analyzed. It is found that near the quenching limit, the uniform flame can not be sustained, and flamelet is formed and spreads forward steadily. The flamelet is formed at higher flow velocity with lower ambient pressure. The diffusion-thermal instability is the main controlling mechanism for the formation of flamelet. Flame standoff distance decreases with the increased ambient pressure and flow velocity. Flame spread rate increases with ambient pressure. Under low ambient pressure environment, the effect of radiative heat loss increases. Flame spread rate increases with ambient pressure while the increasing trend becomes slower.

Effects of balance gases (N2, Ar, He, CO2) on flame spread and extinction behavior were studied by experiments and numerical simulation. In the environment with different balance gases, the thermal-physical and raditive properties of the oxidizer gases change, thus, flame appearance, flame spread rate and the fundamental low oxygen limit are different. The simulation results show that in the position where the solid temperature is highest, the incoming radiative heat flux is highest, while the radiative heat loss on the solid surface is the most. As a result, flame heat flux is the least. The maximum net radiative heat flux locates upwards flame leading edge. In low-velocity flow environment, if the balance gases have the ability to emit or absorb thermal radiation, the characteristic heat transfer length increases. However, the flame still can be treated as optically “thin”, thus the reabsorption of emitted radiation has small effects on flame spread rate, while the heat transfer from flame to the solid phase is the main controlling mechanism for flame spread.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79109
Collection国家微重力实验室
Recommended Citation
GB/T 7714
朱凤. 热厚固体材料表面近极限火焰传播机理研究[D]. 北京. 中国科学院大学,2019.
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