Study of vertical upward flame spread on charring materials—Part II: Numerical simulations

Simulation results, obtained by means of application of an enthalpy‐based pyrolysis model, are presented. The ultimate focus concerns the potential of the model to be used in flame spread simulations. As an example we discuss vertically upward flame spread over a charring material in a parallel plate configuration. First, the quality of the pyrolysis model is illustrated by means of cone calorimeter results for square (9.8 cm × 9.8 cm exposed area), 1.65 cm thick, horizontally mounted MDF samples. Temperatures are compared at the front surface and inside the material, for different externally imposed heat fluxes (20, 30 and 50 kW/m²), for dry and wet samples. The mass loss rate is also considered. Afterwards, vertically upward flame spread results are reported for large particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high), vertically mounted face‐to‐face, for different horizontal spacings between the two plates. The simulation results are compared to experimental data, indicating that, provided that a correct flame height and corresponding heat flux are applied as boundary conditions, flame spread can be predicted accordingly, using the present pyrolysis model. Copyright © 2010 John Wiley & Sons, Ltd.     

Correlations between pyrolysis combustion flow calorimetry and conventional flammability tests with halogen‐free flame retardant polyolefin compounds

Seven halogen‐free flame retardant (FR) compounds were evaluated using pyrolysis combustion flow calorimetry (PCFC) and cone calorimetry. Performance of wires coated with the compounds was evaluated using industry standard flame tests. The results suggest that time to peak heat release rate (PHRR) and total heat released (THR) in cone calorimetry (and THR and temperature at PHRR in PCFC) be given more attention in FR compound evaluation. Results were analyzed using flame spread theory. As predicted, the lateral flame spread velocity was independent of PHRR and heat release capacity. However, no angular dependence of flame spread velocity was observed. Thus, the thermal theory of ignition and flame spread, which assumes that ignition at the flame front occurs at a particular flame and ignition temperature, provides little insight into the performance of the compounds. However, results are consistent with a heat release rate greater than about 66kW/m² during flame propagation for sustained ignition of insulated wires containing mineral fillers, in agreement with a critical heat release rate criterion for burning. Mineral fillers can reduce heat release rate below the threshold value by lowering the flaming combustion efficiency and fuel content. A rapid screening procedure using PCFC is suggested by logistic regression of the binary (burn/no‐burn) results. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of shielding rates on upward flame spread over extruded polystyrene foam in vertical channel and heat transfer mechanism

Fire hazard of extruded polystyrene (XPS) thermal insulation materials has aroused public concern. In order to develop flame spread theory and the guideline for fire risk assessment of XPS, an experimental study on upward flame spread behavior and heat transfer mechanism of XPS in a vertical channel with different frontal shielding rates was conducted. Maximum temperature at the place 2 cm from XPS surface and at the center of channel first increase and then decrease as the shielding rate rises. The former is higher than the latter. Experimental value of average flame height rises as the shielding rate increases. A model for predicting the flame height is built, and the predicted results are consistent with the experimental results. Moreover, the relation between flame height and pyrolysis height under different shielding rates is obtained. The flame spread rate rises as the shielding rate increases. A prediction model of flame spread rate is established, and its prediction results are more accurate compared with those from previous models. The model also predicts that radiative heat transfer is the dominant heat transfer mode, accounting for 93% of the total heat transfer. This work is beneficial for fire risk assessment and fire safety design of building façade.     

稀薄燃气多孔介质燃烧二维火焰数值模拟

对稀薄低热值气体在多孔介质燃烧中的二维火焰温度分布进行了数值模拟。在一定工况参数下,考察了多孔介质二维火焰峰面和温度分布特性,以及火焰峰面传播过程。详细分析了当量比、燃气流速、及壁面热损失等工况参数对燃烧火焰和高温峰值的影响。结果表明,在火焰传播过程中,火焰峰面形状逐渐变化,导致高温区域分布逐渐改变;随着当量比增加,近壁面处的火焰峰面倾斜程度逐渐减少,火焰峰面逐渐向上游传播,火焰峰面形状由梯形分布逐渐转变成两侧向下倾斜的直线分布;燃气流速对火焰峰面形状和位置的影响与当量比相反;壁面热损失引起火焰峰面倾斜,对火焰位置影响较小。     

Combustion Process of Mg/TF Pyrotechnics

The combustion process of pyrotechnics was studied in order to obtain informations of the rate control parameters of burning rate. The pyrotechnics tested was made of Mg (magnesium) and TF (polyfluoroethylene). The burning rate measurements revealed that the burning rate of the Mg/TF propellants (pellet in shape) increases with increasing the weight fraction (ξ) of Mg in the range of ξ > 0.33. Though the adiabatic flame temperature is the maximum at ξ = 0.33, the burning rate increases with decreasing the flame temperature. The total burning surface area of the Mg particles mixed with in the unit mass of propellant plays an important role on the oxidation process in the gas phase just above the propellant burning surface. The heat flux feedback from the gas phase to the propellant burning surface increases with increasing ξ. Therefore, the burning rate increases as ξ increases.     

Numerical modeling of straw combustion in a fixed bed

Straw is being used as main renewable energy source in grate boilers in Denmark. For optimizing operating conditions and design parameters, a one-dimensional unsteady heterogeneous mathematical model has been developed and experiments have been carried out for straw combustion in a fixed bed. The straw combustion processes include moisture evaporation, straw pyrolysis, gas combustion, and char combustion. The model provides detailed information of the structure of the ignition flame front. Simulated gas species concentrations at the bed surface, ignition flame front rate, and bed temperature are in good agreement with measurements at different operating conditions such as primary air-flow rate, pre-heating of the primary air, oxygen concentration, moisture content in straw, and bulk density of the straw in the fixed bed. A parametric study indicates that the effective heat conductivity, straw packing condition, and heat capacity of the straw have considerable effects on the model predictions of straw combustion in the fixed bed.     

Effect of orientation on burning rate of solid combustible

The effect of orientation on the combustion phenomena of a PMMA plate was studied experimentally. A PMMA plate was held by a sample holder made of insulation boards with only one face exposed to the air. Observation of burning inclined plates revealed that flame shape varied with inclination angle. The burning behavior of the upper surface of an inclined plate appeared to differ from that of the lower surface. However, the surface regression rate was found to depend on the inclination angle from the vertical plane. This rate for the upper surface was identical to that of the lower one if the absolute value of the inclination angle was identical, and was found to depend on empirical relationships involving natural convective heat transfer around a heated plate.     

A theoretical basis for flammability properties

Theoretical formulations are presented for the fire growth processes under external radiant heating. They included ignition, burning and energy release rate, and flame spread. The behaviour of these processes with external heating is described along with the critical conditions that limit them. These include the critical heat fluxes for ignition, flame spread and burning rate. It is shown how these processes and their critical conditions depend on a limited number of properties measurable by a number of standard test methods. The properties include heat of combustion, the heat of gasification, ignition temperature and the thermal properties of the material. Alternatively, the properties could be related to parameters easily found from data; namely: (1) the critical heat flux (CHF) for ignition; (2) the slope of the energy release rate with externally imposed flux, defined as heat release parameter (HRP); and (3) the ignition parameter, defined as thermal response parameter (TRP). It is further shown that the flame heat flux differences between small laminar flame ignition sources and larger turbulent flames can affect flame spread due to heat flux and ignition length factors. Finally, it is found that the critical energy release rates theoretically needed for ignition, sustained burning, and turbulent upward flame spread are roughly 13, 52, and 100 kW/m², respectively, and independent of material properties. Copyright © 2005 John Wiley & Sons, Ltd.     

Study of pyrolysis and upward flame spread on charring materials—Part I: Experimental study

In this paper, we describe the results from an experimental campaign, focused on vertical upward flame spread over a charring material. First, for validation purposes of simulation tools, we report on cone calorimeter results for square (9.8cm×9.8cm), 1.65 cm thick, medium density fibre samples, mounted horizontally. Temperature is shown at the surface and at different depths. The mass of the sample is continuously measured. From the raw data, we derive the temporal evolution of the mass loss rate due to pyrolysis. Different externally imposed heat fluxes are investigated (20, 30 and 50kW/m²), onto dry and wet material. Afterwards, for the configuration of two particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high), vertically mounted face to face is considered. Two different horizontal spacing distances between the two plates are studied (30.5 and 10.5 cm). The purpose of this set‐up is to investigate the vertical upward flame spread with strong radiative heat feedback. To that purpose, the temporal evolution of surface temperature is measured over the height of the plates. The measurement data are used to test a pyrolysis model in numerical simulations. Copyright © 2010 John Wiley & Sons, Ltd.     

Flame lengths under ceilings

The evaluation of hazards from developing room fires often requires a knowledge of flame lengths developed by burning objects. Procedures for estimating flame lengths have been available only for vertical plume fires, where there is no flame impingement on the room ceiling. Calculational procedures are developed for approximate calculation of flame lengths when part of the flame flow is along the ceiling. Four common geometries are treated: unbounded ceiling, plume near corner, plume in corner and one-directional corridor spread. Ceiling flame lengths are calculated by use of the assumption that the total air entrained up to the flame tip is the same for ceiling flow as for the free fire. Comparison with limited experimental data suggests potential for prediction in full-scale room fires.     

粒度对煤粒燃烧和热解影响的理论分析

在化学热力学和动力学理论中引入表面项,并由此来分析和讨论粒度对煤颗粒燃烧和热解反应的影响规律.研究结果表明,煤颗粒的粒度对其燃烧和热解反应的热力学性质和动力学参数有明显的影响,粒度越小,影响越大;减小煤颗粒的粒径,化学反应的吉布斯函数差减小,煤颗粒燃烧和热解的趋势增大,使着火温度和热解温度降低,自燃容易发生;并且减小煤颗粒的粒径,其摩尔表面能增大,导致其燃烧和热解的表观活化能降低和速率常数增大,使煤颗粒的燃烧和热解速率加快,使转化率、燃尽度和热解度增加.     

Experimental studies on characteristics of fire whirl in a vertical shaft

An internal fire whirl can be generated readily in a tall shaft model with appropriate gap width at one corner. Experimental study was carried out to investigate the relationship between the characteristics of an IFW and the corner gap width in a 9‐m‐tall vertical shaft model. The vertical shaft had a 2.1 m by 2.1 m square section with gasoline pool fire of different diameters burning inside. The gap width was varied to investigate its impact on fire whirl characteristics, such as flame development, swirling intensity, flame height, flame temperature, and heat release rate of the gasoline pool fire. Vigorous flame swirling motions were generated when the ratio of the gap width to the shaft section perimeter was within the range 0.16 to 0.21. From the flame streamline angle, it was observed that the swirling component was much stronger than buoyancy component near the bottom of burning region. The swirling component decreased and became roughly the same as buoyancy near the middle. Finally, it diminished to being much weaker than buoyancy near the top of the fire. These observations suggest that the Froude number Fr decreased from a large number to 1, and then continued to decrease to 0.     

凹型结构下保温材料EPS典型垂直火蔓延特性对比研究

本工作通过自主搭建小尺寸火蔓延的实验平台,研究了凹型结构中保温材料EPS垂直向上和向下火蔓延特性。对比分析了火焰结构特性、火蔓延速度、质量损失速率、火焰温度等参数的变化规律。结果表明,在垂直向上蔓延过程中,EPS出现短暂的停滞现象。一方面由于烟囱效应易形成一个大的浮力压力差,导致火焰不稳定性;另一方面由于凹型结构易于烟气的聚集,导致燃烧不充分。而在垂直向下蔓延过程中,由于逆向烟囱效应的影响,火蔓延速度明显加速。火蔓延过程中质量损失速率,很大程度上受其火焰高度的影响,呈现上下振荡的特性。未燃区域部分,火焰温度经历两个温度峰值,向上蔓延过程中第一个温度峰值大于第二个;而向下蔓延过程中第一个温度峰值小于第二个。这主要是由于火焰结构形态及烟囱效应的方向特点,导致产生两个不同大小的峰值。本研究结果可为实际凹型结构下火蔓延特性研究提供前期理论和参考价值。     

聚磷酸蜜胺系膨胀型阻燃剂阻燃PA6的燃烧行为及热裂解研究

采用以聚磷酸蜜胺(MPP)为基的三元膨胀型阻燃剂阻燃聚酰胺6(PA6)，测定了阻燃PA6的氧指数(LOI)、UL94V阻燃性及热稳定性，以傅立叶变换红外光谱(FT—IR)分析了阻燃PA6的热分解残余物，以锥形量热仪(CONE)测定了阻燃PA6的诸多与火灾有关的阻燃参数(包括释热速度、质量损失速度、有效燃烧热、比消光面积等)，并以光电子能谱(XPS)测定了阻燃PA6残炭表面的元素组成及XPS曲线拟合数据。     

Combustion Wave Structure of AP Composite Propellants

The combustion mechanism of ammonium perchlorate (AP) composite propellants were studied. The oxidizer-rich propellants tested were made with excess concentrations of AP particles. The pressure deflagration limit of propellant decreases with increasing the concentration of binder. The combustion wave consists of two reaction regions I and II: the region I is the zone of AP monopropellant flame and the region II is the zone of diffusion flame. The heat flux feedback from the gas phase to the burning surface increases as pressure increases, and the heat flux is responsible for the burning rate characteristics.     

Impact of air velocity on ignition in the Intermediate Scale Calorimeter (ICAL)

Higher surface temperatures were required for ignition in the Intermediate Scale Calorimeter than in the Cone Calorimeter. Air velocity measurements were made in front of calcium silicate and particleboard specimens in the Intermediate Scale Calorimeter and temperature measurements were carried out on their exposed surfaces. The higher ignition temperature in the Intermediate Scale Calorimeter was the result of the high induced upward air velocity due to the large specimen size (1.0 m by 1.0 m). This caused the flame to blow off. A higher surface temperature was needed to produce a downward flame spread rate sufficient to overcome the upward air velocity. To counteract this effect, a horizontal projection plate extending 100 mm out from the bottom edge of the specimen was installed. This created an eddy, causing a downward air velocity near the surface at the bottom of the specimen. The igniter was a hot wire across the width of the specimen at a height of 100 mm. The flame quickly spread down from the igniter to the bottom edge and attached there. Under these conditions the temperature required for ignition of particleboard in the Intermediate Scale Calorimeter was less than it was in the Cone Calorimeter. © 1997 John Wiley & Sons, Ltd.     

亚大气压下AP/HTPB微尺度稳态燃烧的数值模拟

为研究亚大气压下高氯酸铵/端羟基聚丁二烯(AP/HTPB)的燃烧特性,采用三步反应动力学机理,建立二维三明治模型,耦合气固两相;对20~80kPa下AP/HTPB的微尺度燃烧进行模拟,并与高压下(4MPa)AP/HTPB燃烧特性差异进行对比。结果表明,亚大气压下BDP模型中第一步反应靠近燃面,放热量较大,在AP/HTPB推进剂燃烧过程中占主导地位;燃烧环境压强不同,导致火焰的特性不同,亚大气压下火焰中扩散与混合过程共存,高压下为扩散火焰;相比于高压,亚大气压火焰离燃面远,面积大;由于高低压下放热区域及放热率差异导致气固相温度分布不同,从而影响燃面形状,亚大气压下AP与HTPB交界处相对于整个燃面突出,而高压下交界处相对于整个燃面凹陷。     

Burning Phenomena of the Gun Propellant JA2

The combustion phenomena of the standard gun propellant JA2 are investigated in experiments and analyzed by a simplified theoretical model. Hereby energy transfer from the gas phase governs ignition and combustion of solid rocket and gun propellants. In addition to the heat conduction and convection, the radiation of the flame contributes to the heat feedback which controls the burning rate in dependence on pressure. The dependence on the initial temperature is given by physical parameters of the conversion from the solid to the gaseous state. Burning rates are measured in dependence on pressure and initial temperature confirming a simplified law for the burning rate. The evaluation yields that the pressure exponent can be directly assigned to the heat feedback and that the temperature of the conversion from the condensed to the gas phase lies at about 675 K. The experiments also comprise spectroscopic measurements at low pressures in the wavelength ranges from 300 nm to 14000 nm which are resolved spatially along the vertical flame profile. The analysis of the spectra delivers the profiles of species in the flames including di‐atomic radicals and tri‐atomic molecules of the final combustion products. In addition, gas phase temperatures are derived by application of the Single‐Line‐Group model which gives approximately 2800 K closely below the adiabatic flame temperature of 2900 K at low pressures. They are compared to temperatures assigned to soot particle emission. In summary, these data enable an estimation of the heat feedback from the flame to the burning surface.     

Measurement and Modelling of Pyrotechnic Time Delay Burning Rates: Application and Prediction of a Fast Burning Delay Composition

A predictive numerical model was implemented for a time delay based on the Si+Pb₃O₄ system. The reaction kinetic parameters were estimated by comparing predicted surface temperature profiles with experimental data acquired with an infrared camera. Fair agreement between the modelled and measured burning rates was achieved. The burning rate is predicted to increase by 9.4 % for every 50 °C increase in ambient temperature. The core diameter was found to have a slightly larger impact on the burning rate than the wall thickness. The effect of using different wall thickness materials was evaluated and indicated that the burning rate is significantly influenced by the wall material when the thermal conductivity is increased and the volumetric heat capacity is reduced. The shape of the combustion front was found to widen with a long tail for materials with a low thermal conductivity and a narrower combustion front with a short tail for materials with high thermal conductivity. Preheating occurred for pyrolytic graphite‐ and diamond‐based elements but no radial combustion was observed. The external heat transfer parameters (convection and radiation) did not affect the burning rate of the fast delay composition. It is concluded that the ambient temperature, volume fraction solids, molar heat of reaction, core and outer diameter are the factors that most significantly influence the burning rate of the Si+Pb₃O₄ composition in long cylindrical elements.