Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that, in most practical situations, the spread of fire in opposed gas flows occurs at near extinction or non-propagating conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemcial kinetics is unimportant in the flame spreading process, it is important in the establishment and extension of the diffusion ...

Controlling Mechanisms of Flame Spread

A review of physics and correlations of pool fire behaviour in wind and future challenges

CFD fire modeling tools are continuously developed and improved to increase their predictive capability of phenomena observed in practical applications. Such models require that “effective” material properties be provided so that the pyrolysis codes used in the models can properly estimate the thermal degradation of solid fuels involved in a fire situation. This paper presents analyses aimed at evaluating the plausibility of obtaining material properties numerically from pyrolysis data collected in a Fire Propagation Apparatus (FPA). A theoretical pyrolysis model is used to simulate the experimental data and the input parameters (i.e. the material properties) are adjusted to provide the best possible agreement between simulations and experiments. This is done via the application of evolutionary optimization methodologies. First, available optimization techniques are evaluated using synthetic data and it is shown that the Shuffled Complex Evolution approach ( [17] ) can recover the input parameters with high accuracy, efficiency, and robustness. Second, the algorithm is applied to experimental FPA pyrolysis data of practical materials: polymethyl methacrylate (PMMA), single-wall corrugated board, and chlorinated polyvinyl chloride (CPVC).

Evaluation of optimization schemes and determination of solid fuel properties for CFD fire models using bench-scale pyrolysis tests

New concepts are addressed for predicting the flame spread on materials from laboratory measurements. It focuses on heat transfer which precipitates and precedes upward flame spread on a vertical surface. Six materials have been featured in this study as well as in past related studies. Their flame spread properties are presented. In this particular study heat transfer and flame height results are presented for wall samples burned at varying levels of external irradiance. Also complementary results are presented for methane line burner wall fires. An approximate theoretical analysis is included to serve as a guide to identifying the important variables and their relationship for correlation purposes. Experimental results yield flame height proportional to energy release rate to the 2/3 power, and wall heat flux distributions are roughly correlated in terms of distance divided by flame height. These correlations appear to at least hold for the scale of these experiments: flame heights of 0.3 to 1....

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Wall Flames and Implications for Upward Flame Spread

Flame Heights in Turbulent Wall Fires with Significant Flame Radiation

Two-dimensional upward flame spread and subsequent steady turbulent burning of a thermally thick vertical fuel surface is examined theoretically and experimentally. The upward spread rate for vertical PMM slabs is observed to increase exponentially with time. This result is predicted in terms of measured fuel thermophysical properties, flame heights and heat feedback to the fuel surface. The local steady burning rates established after completion of upward spread exhibit a minimum at a height of 18 cm from the bottom edge and increase continuously beyond this height, becoming 70% larger at a height of 140 cm. This increase is shown to be entirely attributable to increasing flame radiation. Individual measurements of the various energy transfer components during steady burning of the PMM slabs are obtained from radiant intensity measurements of (1) the surface alone and (2) flame plus surface. Above 76 cm flame radiation ranges from 75 to 80% of the total (radiation plus convection) heat transfer from the flames to the fuel surface. Surface heat transfer by convection decreases slightly with height.

Upward turbulent fire spread and burning of fuel surface

The effect of equilibrium moisture content (ranging from 4 to 12.5% moisture, corresponding to 20 to 90% relative humidity) on piloted ignition times of single wall and tri-wall corrugated paperboard samples has been measured over a range of external radiant heat flux (10 to 60 kW/m 2 ) in the Fire Propagation Apparatus (ASTM E-2058). It has been shown for heat fluxes up to 50 kW/m 2 corrugated paperboard samples behave as thermally thin solids and satisfy a simple fundamental equation based on energy required for ignition: ) 1 ( ) ( w ig cr e m t q q γ α χ

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Effect of Moisture on Ignition Time of Cellulosic Materials

Flame heights and flame heat-flux distributions are measured for a wide range of fuels burning between two parallel panels. The flame heat flux levels are very sensitive to fuel sootiness. The heat flux distributions are obtained from the transient temperature rise of thermocouples peened into the steel parallel panel sidewalls. The measured flame heights imply an actual heat release rate per unit flame volume, 1110 = ′ ′ ′ q� kW/m 3 , consistent with literature values. This heat release rate per unit volume is independent of fuel type and fire scale. The heat flux distributions are integrated to obtain the net total heat transfer () 0 p Qq ′′ � � to the panels above an arbitrarily specified panel heat loss rate, 0 q� ′ ′ . The integration is performed only over areas for which 0 0 ≥ ′ ′ − ′ ′ q q f � � to obtain the net heat transfer, needed by fire growth models. The results are described by a simple theoretical model that assumes heat transfer occurs only by radiation. The model gives the net heat transfer p

Flame Heat Transfer Between Parallel Panels

It has been established that the release of both thermal radiation and products of incomplete combustion from well-ventilated buoyant turbulent diffusion-flames are well-correlated by the fuel's laminar flame smoke-point value. Thus the smoke-point of a material provides an important measure of its flammability. Standard methods are available for measuring the smoke-points of gaseous and liquid fuels, but not for solid fuels. The apparatus developed for the present study can be used for measuring the smoke-points of charring and noncharring solid fuels. A horizontal fuel sample is continuously fed into a downward pointing C02 laser beam which pyrolyzes a small area of the sample. The heat release rate (or flame height) of the steady laminar flame produced by the pyrolysis gases is controlled by the laser beam power andfor the sample feed rate. The flame height is measured by a video camera while the release of smoke is measured by its attenuation of an electronically chopped infrared beam. The smoke-point is defined here by the critical flame height (or heat release rate) at which significant smoke is released from the flame tip. This smoke-point criterion occurs at considerably greater flame heights than the "equal-wings" condition, which previously has been used by some investigators to define the smoke-point.

The Of Role Of Smoke-point In Material Flammability Testing

The study establishes a similarity relationship controlling the buoyant turbulent boundary layer combustion of wall fires. Measurements of soot deposition onto glass rods allow one to infer the characteristic thickness, S δ , of propylene flames at different heights, z , and mass transfer rates, m ′ ′ & . The normalized flame thickness, z S δ , is successfully correlated by the overall fuel to air mass ratio of the flames, Ψ . Similarly, Ψ , correlates the measured boundary layer temperature profiles of the flames normal to the wall. It correlates measurements of the outward directed radiance, r N from the same propylene wall fires with a uniform effective flame radiation temperature, f T , and uniform soot absorption/emission coefficient being independent of both height, z , and fuel mass transfer rate, m ′ ′ & . Finally the same fuel to air ratio, Ψ , correlates, here for the first time, the LDV velocity measurements of Most et al for turbulent ethane wall flames performed at Factory Mutual Research in 1982. The data presented here are suitable for development of analytical and CFD models of turbulent wall fires.

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J.L. De Ris

Papers

Upward turbulent fire spread and burning of fuel surface

Effect of Moisture on Ignition Time of Cellulosic Materials

Flame Heat Transfer Between Parallel Panels

The Of Role Of Smoke-point In Material Flammability Testing

Similarity Of Turbulent Wall Fires