环境风作用下聚氨酯泡沫水平火蔓延行为实验研究

为探究环境风速对聚氨酯泡沫水平顺流火蔓延的影响,开展多组工况对比实验,分析不同侧向风速对火蔓延典型特征参数（火焰形态、质量损失和近域场温度及辐射）影响。实验结果表明：侧向风速会缩短热解前锋预热角形成时间,且预热角度大小与风速呈负相关,预热角减小会增大火焰前锋对预热区热反馈面积,使火蔓延速度增加;火焰前锋受侧向风速拉伸效应主导下的空气卷吸作用,火蔓延过程中熔融滴落频率增大,加大次生火灾危险性;同时火蔓延过程中材料的质量损失率随风速增加而增加;风速的冷却效应在火蔓延前期占主导地位,但仍会促进火蔓延速度,且水平板材上下两侧辐射峰值差逐渐增大。     

Fire spread and flame shape

In laboratory tests, the size of a flame front can significantly affect the rate of fire spread. The configuration factor of a given flame shape provides a method for relating flame fronts of various widths. The author discusses an analog system for determining the configuration factor of an irregularly shaped flame and the effects of flame width and height on the rate of spread.     

Burning behavior of sedan passenger cars

Four full-scale fire experiments using 4-door sedan passenger cars were carried out. The cars were ignited either at the splashguard of the right rear wheel or at the left front seat in the passenger compartment with a gasoline spill. The temperature inside the burning car and the mass loss rate were measured. The burning of the 4-door sedan was composed of three compartmental fires: the engine compartment, the passenger compartment, and the rear part inclusive of the fuel. In the experiments where ignition was initiated at the splashguard, the flame spread in the following order: to the rear part of the car, to the passenger compartment, and to the engine compartment. Breakage of the window glass markedly affected the spread of fire into the passenger compartment. The quantity of gasoline in the fuel tank also affected the speed of spread of the fire, because the gasoline ignited at an early stage of the fire. In the experiment where ignition was initiated in the passenger compartment, the fire gained force after the windshield was broken entirely. The flame spread in the following order: to the passenger compartment, to the engine compartment, and to the rear part of the car. The temperature within the passenger compartment peaked at 1000 °C. The heat release rate (HRR) curves showed several peaks depending on the burning of the three compartments. The HRR increased markedly when the fire spread to several different parts of the car at the same time. The HHR peaked at 3 MW when the passenger compartment and fuel (gasoline) burned simultaneously. The measured HRR curves were characterized by superposition of a Boltzmann curve and a Gaussian curve in order to obtain a model, which allowed us to make a more precise prediction of the fire spread probability from a burning car to nearby structures. The HRRs of burning cars were described by the sum of HRR from each compartment.     

Experimental Studies on the Effects of Spacing on Upward Flame Spread over Thin PMMA

Experiments were carried out to study the effect of spacing between wall and thin fuels on upward flame spread. The front flame height, back flame height, pyrolysis height, burnout length, and pyrolysis spread rate were measured by video image analysis with spacings of 2 mm, 7 mm, 13 mm, 19 mm, and 25 mm. Experiments were performed on uniform PMMA (polymethyl-methacrylate) samples with 200 mm height, 50 mm width, and 1 mm thickness. The results are as follows: (1) As the spacing increased, the front flame height, back flame height, pyrolysis height, and burnout length showed the same trajectory, first increased and then decreased. The maximum trajectory was observed at a spacing of 6.5% of the wall height. (2) At an infinite length of PMMA, the pyrolysis zone and pyrolysis spread rate would reach an asymptotic steady state, and the pyrolysis and burnout spread rates will be asymptotically equal. (3) Of particular interest is the maximum mass-loss rate for a wall spacing/sample height ratio (0.065) due to enhanced the radiation fluxes. In this study, the effects of spacing between wall and fuels on upward flame spread was investigated for the first time using 1 mm thick PMMA sheets, including two-face burning case.     

可燃液体自由表面燃烧特性的数值模拟

依据小尺度汽油燃烧实验模拟法确定边界条件,采用数值模拟方法进行汽油池火燃烧模拟。数值模拟结果和实验结果具有一致性:用非稳态数值模拟的火焰热辐射随燃烧的进行逐步增加;热辐射分布以火焰的高温区分布较强;火焰中各组分如氧气消耗量、C(s)等质量浓度与燃烧时间的无量纲关系曲线与燃烧过程的温度分布一致,且遵循双曲线变化规律。采用稳态模拟方法得到不同风速条件下火焰倾角与风速之间的变化曲线,有风时火焰温度较无风时增加,且随着风速的增加而增加(实际上存在某个临界风速);有风时火焰热辐射分布随火焰的倾斜而改变,下风向热辐射明显,上风向则较低,且热辐射波及范围较无风时增大。     

Upward Flame Spread Over an Array of Discrete Thermally-Thin PMMA Plates

Experiments and theoretical analysis were conducted to investigate the upward flame spread over a homogenous PMMA plate and an array of discrete thermally thin PMMA elements. In the experiment, a digital video camera was used to record the flame spread process. An electronic balance and thermocouples were adopted to monitor the mass loss and pyrolysis front position, respectively, as a function of time. In the theoretical analysis, the mass loss rate of PMMA was correlated to the heat transfer during flame spread. The experimental results show that the flame spread rate peaks in the case of discrete PMMA elements with a fuel coverage around 80% rather than 100% (the homogenous case) because the gap with an appropriate spacing between neighboring elements accelerates the flame spread. However, the flame cannot spread over an array of discrete fuels at a coverage of 50% or smaller where the gap is too large to allow effective heat transfer required for flame spread. A smaller coverage of PMMA results in a larger mass loss rate per area since the gaps between elements can entrain more air to promote the burning. A logarithmic relation, that can well describe the mass loss rate as a function of PMMA coverage, was proposed based on the theoretical analysis and the fitting of experimental measurements.

     

卷积神经网络在建筑消防信息化的应用——以城市综合管廊火灾监控为例

随着科学技术的进步,越来越多的行业和领域在朝着信息化方向发展。综合管廊已成为城市能源输送的重要保障,但是在大型市政基础设施集中化发展背景下,其火灾隐患问题逐渐凸显。利用YOLO V5建立的卷积神经网络能够对火焰进行高精度识别,进而实现从识别结果中提取实时火焰蔓延位置、蔓延速度和火焰宽度等重要火灾发展关键参数。通过设计12组发展速度不同的综合管廊电缆火灾试验,对卷积神经网络进行训练并验证其信息提取的准确性。结果表明：卷积神经网络提取的火焰前锋蔓延位置平均相对误差为5%～15%、火焰蔓延速度平均相对误差为6%～20%、火焰宽度平均相对误差为10%～27%,进而证实该方法能够保证良好的提取精度。对建筑消防信息化监控来说,该方法能为火灾现场制定灭火救援战术提供关键依据,并让实时研判火灾发展趋势、评估事故严重性和估计事故损失成为可能。     

Correlation of Burning Rate with Spread Rate for Downward Flame Spread Over PMMA

Burning rate of solid fuel and laminar flame spread rate are both well studied topics for flame spread in downward configuration. Yet, despite well-developed theories, not much experimental data is available to correlate the two. In this work, experiments are performed under ambient conditions in downward spread configuration for a wide range of thicknesses (2 mm to 24 mm) for flat samples of Poly-Methyl Methacrylate (PMMA). The samples are held by two ceramic plates in order to obtain a two-dimensional propagation that is independent on the sample width. By analyzing videos of the experiments, the instantaneous spread rate is obtained using a recently developed MATLAB based tool. The shape of the pyrolyzing fuel is carefully measured after extinguishing the flame during a steady propagation. The spread rate and the burn angle, which is defined as the angle subtended by the pyrolyzing surface with respect of the fuel surface, are correlated, producing an expression for the burning rate in terms of the burn angle and flame spread rate. As the fuel thickness is increased, the burn angle and burning rate decrease and reach asymptotic limits for thermally thick fuels, in analogy with the spread rate limit. The comparison with data from literature suggests that in the thick limit the value of mass flux for PMMA (about 10 g/m² s) tends to the one of non-spreading flames. The presented geometrical approach to study the downward spread problem avoids the use of the B number and local gradients in order to calculate the mass burning rate of the fuel.     

A Theoretical Model of Major Fire Spread in a Tunnel

A model of major fire spread in a tunnel is described. It employs the concepts of non-linear dynamical systems theory and identifies the onset of instability with major fire spread in a tunnel. In particular, the model associates the existence of a fold bifurcation with dramatic fire spread from an initial fire to a ‘target object’. The purpose is to identify the thermo-physical and geometrical conditions which lead to instability and sudden fire spread. Flame impingement on the target object is assumed not to exist; fire spread is assumed to be by spontaneous ignition only. The case considered assumes the existence of a longitudinal forced ventilation and predicts the critical heat release rate needed for a fire to spread from an initial fire to an item with a given assumed shape. The target object may be taken to approximate a vehicle. The illustrative case approximating fire spread from an initial fire to a heavy goods vehicle (HGV) within the Channel Tunnel is presented; it is not restricted to this case, however. The model is identified with the name FIRE-SPRINT A3, which is an acronym of Fire Spread in Tunnels, Model A, Version 3. It is a development of an earlier model, FIRE-SPRINT A2. The current model takes greater account of thermal radiation than was done in the earlier work and also assumes a more extensive flame volume for a downstream flame section.     

The computational modelling of flame spread along a conveyor belt

This paper reports the results of an experimental and computational study conducted to characterize the initiation and spread of fire along the upper and lower surfaces of a conveyor belt mounted within a ventilated full-scale experimental fire test gallery. Experimental data were obtained from the instrumented fire test gallery that recorded the temperature gradients and airflow profiles produced within the gallery due to the spread of the flame front under various ventilation flow rates. The computational models were constructed using the three-dimensional commercial CFD software code, Fluent™. A novel modelling method is proposed to represent the observed flame spread along the conveyor belt surfaces. It is concluded that this model was able to qualitatively replicate the flame spread observed on the belt surfaces within the test gallery.     

Effect of oxygen on flame heat flux in horizontal and vertical orientations

A systematic empirical and analytical study was conducted to directly quantify the effect of enhanced ambient oxygen concentration on flame heat flux at bench scale and its ability to represent large-scale flame heat flux of well-ventilated fires. The Advanced Flammability Measurements Apparatus was used to conduct bench scale horizontal and single wall vertical orientation experiments for black polymethylmethacrylate, propylene gas and black polyoxymethylene. The key aspect of this study was direct experimental measurements of flame heat flux back to the burning surface for 20.9–40% ambient oxygen concentrations over a range of applied heat flux. The total flame heat flux, as well as the radiative and convective components, was experimentally measured with various gages. To gain more insight into the effects of oxygen, the flame emissivity, flame height and flame temperature were measured and used to calculate the radiative and convective components of the flame heat flux. Gas burner experiments were conducted to decouple the solid and gas phase effects of the ambient oxygen. Large scale tests of black polymethylmethacrylate were conducted in a horizontal orientation and literature data was used for single wall vertical orientations for comparison to the bench scale, enhanced oxygen results. The main conclusion is that the flame heat flux in enhanced ambient oxygen bench scale does not simulate large-scale flame heat flux in horizontal orientations but simulates a more severe large-scale geometry (parallel wall) in vertical orientations and is useful for evaluation of materials’ vertical flame spread potential.     

Laboratory study on the open channel flow reaeration: a dimensional approach

Results of laboratory experiments on open channel flow reaeration are presented and commented on. The tests were designed on the basis of the classical dimensional analysis. They were carried out using three 15 m long channels with different cross-sections: 1) 0.5 m wide semi-circular cross-section; 2) 0.4 m wide rectangular cross section; 3) 0.2 m wide rectangular cross-section. The longitudinal bottom slope, the roughness and the flow discharge were varied independently. The disturbed equilibrium approach was adopted within an innovative experimental procedure, i.e. comparing the dissolved oxygen measures acquired in tests without de-oxygenation agent (hereinafter ‘white tests’) with those performed in runs with de-oxygenation agent (‘reaeration tests’). A new relationship between the reaeration coefficient and the hydrodynamic characteristics of an open channel is proposed. The relationship is applicable to a wide range of values of hydraulic characteristics not previously analysed in the literature and typical of small rivers.     

Upward flame spread on composite materials

Three existing models of upward flame spread were tested against intermediate-scale experiments on a vinyl ester/glass composite. Characterization of rate of heat release per unit area, needed as input to the models, was obtained at external radiant fluxes below the minimum for ignition by adaptation of a method due to Kulkarni. There are several limitations on the accuracy of the material characterization when applied to composites. Each of the flame spread models has definite limitations as well. Nevertheless, all three models produced predictions of spread behavior in sufficiently quantitative agreement with the experiments that they should prove useful for engineering analyses of flame spread potential.     

Measurement of instantaneous flame spread rate over solid fuels using image analysis

Spread rate is an overall property of flame propagation that characterizes the condition of a flame better than any other property. As a result, prediction and measurement of spread rate is central to flame spread studies over solid fuels. Significant amount of data have been collected over last four decades of research on flame spread over various fuels under different conditions. In most of these studies, however, only average spread rate is reported which is adequate for steady phenomena. Given that a flame may not face the same conditions during the spread, it is possible for the spread rate to change during the duration of the spread continually. In this work a methodology for image analysis is presented with the goal of evaluating instantaneous spread rate to study time-dependent phenomena. The parameters that control the error and time resolution of the flame spread history are identified, and a sensitivity study is carried out to validate the results of a scale analysis. A MATLAB-based Flame Image Analyzer (FIA) package is developed and applied to flame spread videos recorded in several experiments in different regimes of opposed-flow flame spread. An expression for the error in spread rate for a given time resolution is expressed in terms of the imaging parameters. The two parameters that are found most important are the pixel resolution and the frame rate. A non-dimensional imaging parameter is identified that is shown to govern the quality of imaging for spread rate measurement. Theoretical prediction from the error analysis is confirmed by doing various case studies using the Analyzer.     

Smoke concentrations inside and outside of a corridor-like enclosure

This work presents smoke measurements and correlations inside and outside of a corridor-like enclosure fires in order to determine the effects of burning on smoke concentrations inside and outside the enclosure. Thirty eight experiments were performed in a three metre long corridor-like enclosure having a cross section 0.5 m×0.5 m, door like openings in the front panel and a gaseous burner located near the closed end. Smoke concentrations were measured at two locations inside the enclosure and also in the exhaust duct of a hood collecting the fire gases from the enclosure. It was found that smoke concentration in the exhaust duct decreased whereas smoke concentration inside the enclosure increased after the flames started moving towards the opening and external burning occurred. This increased smoke concentration inside the enclosure was caused by reversion of the flow pattern inside the enclosure after the flames moved past a point towards the opening. Namely, the flow pattern changed direction behind the flame front in the sense that hot gases in the upper layer were travelling backwards towards the closed end of the corridor thus contributing to smoke increase inside the enclosure. This change of flow pattern was confirmed in all experiments by bidirectional probe velocity measurements in the upper and lower layer as well as by oxygen concentrations and temperature measurements inside the enclosure. These results are useful for CFD validation and specifically applicable for assessing smoke hazards in corridor fires in buildings where smoke concentrations can be much larger than anticipated owing to leakage to adjacent rooms behind a moving flame front.     

燃料间距对PMMA竖向火蔓延行为的影响

以PMMA（聚甲基丙烯酸甲酯）板（8 cm×10 cm×1 cm）为实验材料,开展不同燃料间距（0~10 cm）的竖向火蔓延实验,分析火焰高度、燃烧速率、热解前锋位置及点燃滞后时间的变化规律。结果发现,随着燃料间距增加,固体表面的净火焰高度与燃烧速率均呈现先增大后减小的趋势。此外,火焰在相邻固体燃料表面的蔓延过程中出现“点燃滞后”现象,点燃滞后时间随着燃料间距的增加呈指数级增长。     

Metrological Tool for the Characterization of Flame Fronts Based on the Coupling of a Heat Flux Approach with Image Processing Data

The aim of this paper is to present some improvements in the metrology of forest fire flames by coupling image processing and radiative heat flux measurement. A new metrological tool using a visual video camera and a specific multiple thermal sensors is proposed. By means of an appropriate segmentation algorithm and the Direct Linear Transformation, the image processing methodology gives the forward or the backward fire front positions as input data to a radiative heat flux approach. Using a simplified flame model, this technique provides fire front positions versus time, and average values for the flame length, flame depth, flame tilt angle, apparent flame temperature and flame emissivity. The rates of spread, obtained by a linear regression of the determined fire front positions, as well as the thermal and radiative properties of the flame, are compared favorably to those given in the literature.     

An experimental study on the intermittent extension of flames in wind-driven fires

Experiments were conducted to study the intermittent extension of flames from wind-driven line fires using stationary burners. These fires are thought to share similar features with propagating wildland fires, where forward pulsations of flame have been observed to quickly ignite material far ahead of the mean flame front. However, stationary burners offer the ability to study the movement of the flame and its heating processes in greater detail than a spreading fire. In these stationary experiments, propane gas was used as a fuel with different burner sizes, 25–30 cm wide and 5–25 cm long in the direction of the flow. A specially-built wind tunnel was used to provide a well-characterized laminar flow for the experimental area. The free-stream flow velocity, measured by a hot-wire anemometer, ranged in the experiments from 0.2 to 2.7 m/s. The shape of the flame was measured using a high-speed video camera mounted perpendicular to the apparatus. A method was developed to track the extension of the flame close to the surface, simulating flame contact with unburnt fuel downstream of the fire. This extension length was then measured frame by frame and frequencies of flame presence/absence determined as a function of downstream distance. The location of maximum pulsation frequency, x_max, for each burner/wind configuration, was obtained using a level-crossing approach (essentially the variable-interval time-average (VITA) method). Further study indicates that x_max can be well estimated using mean flame properties. Probability distributions describing the location of the flame over time also showed that, the probability the flame extends far beyond the mean flame front is sensitive to increasing ambient winds and fire size.     

Gravimetric measurement of solid and liquid fuel burning rate near and at the low oxygen extinction limit

A precision mass balance coupled with a variable oxygen flow tunnel/nozzle enables us to measure the burning rate of solid and liquid fuels as a function of ambient oxygen percentage all the way to the extinction limit. Two sample configurations have been studied. The first is a liquid fueled wick flame (ethanol tea lamp). The total burning rate (mass/time) is measured as a function of wick length and oxygen percentage. Near the low oxygen limit, limit-cycle flame oscillation has been found that can last for many minutes to hours. The averaged mass burning rate of the oscillatory flame is about one-half that of the steady flame occurring at slightly higher oxygen. In the second configuration, local burning rate (mass/area/time) of poly(methyl methacrylate) spherical shell samples have been measured in the flame stabilization zone. Each sample has a different amount of heat loss and a different oxygen limit. The critical burning rate at their respective oxygen limits are different. This implies that critical burning rate is not a property of the material alone and it should not be used as the only criterion to judge the extinction or the ignition of materials.     

Fire spread experiment across Mediterranean shrub: Influence of wind on flame front properties

A fire spread experiment was conducted in the field under wind-blown conditions. The fuel consists of tall and dense Mediterranean shrub vegetation. The plot area was about 30 m wide and 80 m long. This experiment was conducted not only in order to increase the knowledge and understanding of the fire behaviour in the field but to provide data for the validation of physics based models of fire spread. In particular, the effects of wind on the geometric and thermal properties of the flame front in the field were investigated. The flame temperature along the vertical direction and the radiation emitted ahead of the flame front, were measured. The methodology employed in this experiment and some quantitative measurements of wind velocity and direction, flame geometric properties, are also presented and discussed. The measurements and observations exhibit that the behaviour of the fire and the flame structure character are very different from the one encountered at laboratory scale. These preliminary results show that large scale turbulence influence fire spread and affects the flame shape, temperature and radiation emission.