|Alternative Title||Mechanisms of Near-Limit Flame Spread over Thick Solid Fuels|
|Place of Conferral||北京|
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.
|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.|
|Files in This Item:|
|热厚固体材料表面近极限火焰传播机理研究.（5838KB）||学位论文||开放获取||CC BY-NC-SA||Application Full Text|
|Recommend this item|
|Export to Endnote|
|Similar articles in Google Scholar|
|Similar articles in Baidu academic|
|Similar articles in Bing Scholar|
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.