Free-convective regime of flame spread over a fuel film on a substrate

Flame spread over a liquid fuel film on a thin metallic substrate under free convection was studied experimentally. Instantaneous flame velocities correlate with the flame length. The average flame velocity increases from 2 to 30–40 cm/sec with the slope angle of the substrate to the horizon varying in the range of 0–90°. For a substrate of specified width, the flame velocity is inversely proportional to the heat capacity of the unit area of the substrate-fuel system and to the differences between the temperature corresponding to the formation of a stoichiometric mixture of the saturated fuel vapor and air and the ambient temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 21–30, September–October, 2007.     

Effect of modulations of opposed gas flow velocity on flame spread rate over a liquid surface

The effect of modulations of the velocity of the gas flow incident on the flame on the average flame velocity over a shallow liquid is studied. It is shown that the average flame velocity depends on the modulation frequency. If the modulation frequency is higher than the flame oscillation eigenfrequency, then, upon the imposition of the modulation, the flame velocity first increases and then gradually returns to the initial value. At frequencies close to the flame oscillation eigenfrequency, the average flame velocity is constant but is higher than the initial value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 11–17, January–February, 2009.     

倾斜角对液体燃料润湿条件下砂床表面火蔓延的影响

利用纹影系统、CCD相机以及K型热电偶对正丁醇润湿条件下不同角度的砂床表面火蔓延特性进行了研究,分析了不同砂床表面倾角对火蔓延的影响机理。研究结果表明:正丁醇润湿条件下变角度砂床表面火焰是稳定匀速蔓延的,向上蔓延时,火焰的高度、黄色发光区以及蔓延速度随着倾角的增加而增大;向下蔓延时,火焰的高度、黄色发光区以及蔓延速度随着倾角的增加而减小;蔓延火焰前方存在预热区,且预热区随表面倾角的增大而增大:砂层内部存在热边界层,且砂床热边界层厚度随表面倾角的增加而减小。得到的火蔓延特性及燃烧机理为此类液体燃料的储存、使用以及环境保护提供了一定的科学依据。     

Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10¹¹/cm² on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.     

阻燃硬质聚氨酯泡沫塑料垂直火蔓延特性研究

通过一步合成法制备了阻燃硬质聚氨酯泡沫,自主搭建保温材料火蔓延实验台,采用中小尺寸实验对比研究了阻燃及非阻燃硬质聚氨酯的垂直火蔓延特性,分析了火焰结构特性、火蔓延速度、火焰温度、质量损失速率等参数的变化规律。结果表明,火蔓延过程中,材料表面均出现了炭化现象,垂直双面燃烧过程中聚氨酯纯样RPUF燃烧最剧烈,阻燃剂膨胀石墨(EG)、次磷酸铝(AHP)和二乙基次膦酸铝(ADP)的加入,抑制了材料的燃烧和蔓延,使材料燃烧的火蔓延速度、质量损失速率及温度等参数都相应降低。RPUF/AHP5垂直双面火蔓延过程中,火焰稳定性差,在20 s后出现熄灭现象,原因是阻燃剂次磷酸铝(RPUF/AHP5)受热挥发出难燃气体。AHP降解后形成的含磷化合物可促进聚氨酯分子链成炭,导致产生熄灭现象。而RPUF/ADP5火蔓延过程中,同样出现了熄灭现象,其熄灭的程度低于阻燃剂次磷酸铝(RPUF/AHP5)试样。RPUF/EG5火蔓延过程中试样表面温度存在两个峰值,由于RPUF/EG5燃烧生成的炭层不稳定所致。当温度高于400℃时炭层被迅速氧化,热量穿透炭层使内部未燃样品热解,生成温度的第二个峰值。     

Experimental study on flame spread along fuel cylinders in high pressures

Flame spread over solid fuels in high‐pressure situations, such as nuclear containment shells during a pressurized period, has potential to result in catastrophic disaster, thus requiring further knowledge. This paper experimentally reveals the flame spread behaviors over fuel cylinders in high pressures. Polyethylene and polymethyl‐methacrylate cylinders with the diameter of 4.0 mm are used in this study. Ambient gas is air, and total pressures are varied from naturally normal pressure (100 kPa) to elevated pressure (500 kPa). Flame characteristics including flame appearance and flame size and burning rate and flame spread rate are investigated. Results show that in high pressure, the flame appearance is significantly affected. As the pressure increases, the blue flame disappeared, and the color of flame tip changes from luminous yellow to orange as well the orange part extends down towards the base of flame. The dimensionless flame height increases with pressure for pressure below 150 kPa and then decreases with pressure above that level. The burning rates show increasing trend with pressure and are proportional to P^0.6 and P^0.79 for polymethyl‐methacrylate and polyethylene, respectively. Besides, flame spread rates for polymethyl‐methacrylate and polyethylene both were found to be proportional to P^0.5.     

Flame spread over liquid fuel films on metallic substrates

New experimental studies of parametric dependences of the flame spread velocity and limits for liquid fuel films on metallic substrates confirmed the main features of the physical model proposed previously. For thermally thin layered systems “fuel-substrate,” a steady-state regime of flame spread is possible. It is shown that the flame velocity depends on the effective thermal diffusivity of the layer system, and its value is determined mainly by the volumetric heat capacities of the components of the system and, to a lesser degree, by their thermal conductivities. The mechanism of flame spread includes a series of interrelated elementary processes: heat conduction over the substrate from the combustion zone to the preflame zone, heating and evaporation of the fuel by the substrate, formation of a combustible mixture, and heating of the metallic substrate by the combustion products. The flame edge is located at the liquid surface, where the temperature corresponds to the formation of a stoichiometric mixture under equilibrium conditions. The liquid fuel is completely evaporated from the substrate at temperatures below the boiling point. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 3, pp. 25–30, May–June, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-03-32308) and the INTAS Fund (Grant No. 96-1173).     

Assessment of the flame velocity as a function of pressure and temperature

With the state in the chemical spike being taken as the initial state for deflagration burning behind the shock wave front, a dependence of the flame velocity on the initial parameters of the mixture can be derived. It is found that the most pronounced effect on the flame velocity is exerted by the initial temperature, while the dependence on pressure is rather weak. Results calculated for several mixtures in the pressure range of 0.001 to 100 atm and temperature range of 200 to 1800 K are presented.     

Dynamics of concurrent flame spread over a thin charring solid in microgravity

The dynamics of microgravity concurrent flame spread over thin cellulosic sheets are theoretically investigated. The mathematical model is based on the laminar, reactive Navier–Stokes equations coupled to solid-phase enthalpy and mass conservation equations. Simulations have been made for forced flow velocities in the range 0.25–15 cm/s, by decreasing the oxygen mass fraction of the concurrent flow below the ambient value and by increasing the solid charring rate (fire-retarded cellulose). For air, non-retarded cellulose and flow velocites larger than 5 cm/s, the dynamics of concurrent flame spread are qualitatively similar to those of normal gravity. As the concurrent flow is decreased below 5 cm/s, after short transients, a transition from fast flame spread to slow solid burning and then to flame quenching is predicted. Flame quenching is also observed, for relatively high flow velocities, in vitiated air or for fire-retarded cellulose. Finally, blow-off at the highest velocity considered (15 cm/s) is predicted only for sufficiently low oxygen concentrations. © 1998 John Wiley & Sons, Ltd.     

Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d ₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^?3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d ₁₀ = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d ₁₀ = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.     

点火位置对乳胶泡沫水平火蔓延规律的影响

为研究点火位置对乳胶泡沫材料水平方向火蔓延规律的影响。搭建小尺寸实验平台,在距离材料中心点0（x1）、3.54 cm（x2）、7.08 cm（x3）、10.62 cm（x4）、14.16 cm（x5）、17.70 cm（x6）位置处点火,研究了试样表面温度、质量损失、火焰高度、火蔓延速度等特性参数的变化规律。结果表明,随着点火位置由材料中心点向边缘点移动,平均火蔓延速度分别为0.24、0.23、0.19、0.31、0.42、0.51 cm·s^-1,呈现先减小后增大的规律;x3点火位置时的平均火焰高度较低,燃烧时间较长,平均质量损失速率较低,主要与火蔓延过程中的热量传递方式有关。研究结果显示了乳胶泡沫的火蔓延过程,得到了点火位置对火蔓延的影响规律。