Flame spread over solid fuel beds

INTRODUCTION

Flame spread over thin solid fuels is a useful paradigm for studying the behavior of more complex flame spread problems, e.g., in building fires. Our experiments on flame spread in non-standard atmospheres have shown that the unequal rates of diffusion of thermal energy and oxidizer (the "Lewis number" effect) are important influences which have not received much attention in prior studies. We have also discovered a new type of flame spread process in which the flame front is cellular and explained this in terms of Lewis number effects in conjunction with reactant interdiffusion near extinction. Our theoretical modeling has confirmed the importance of Lewis number effects and shown that such effects can be described via extensions of classical theory.

Current efforts focus on the effects of buoyancy-induced convection and radiative heat transport on flame spread over both thin and thick fuel bed materials. Experiments have been conducted at earth gravity and at microgravity (µg) in the drop towers at the NASA-Glenn Research Center in Cleveland, OH. Most experiments were conducted in a 20 liter combustion chamber (Fig. 1). Exactly the same apparatus was used for 1g and µg tests. In most thin-fuel experiments 5 cm wide Kimwipe samples 15 cm long were used and were held by aluminum quenching plates. Thick-fuel samples studied have included polystyrene, polyurethane, polyphenolic and carbon foams. The samples were ignited by an electrically-heated Kanthal wire. The flame spread process was imaged via three video cameras and a laser shearing interferometer.

The objectives of these experiments is to study the effect of diluent type, and particularly the radiative properties of the diluent gas, on flame spread rates and extinction conditions.  Our current state of understanding of these topics is discussed below.

EFFECT OF DILUENT TYPE

The effect of inert gases He, Ar, N2, CO2 and SF6 on flame spread over thermally-thin fuels were tested since these inerts provide a variety of radiative properties and oxygen Lewis numbers. For He, N2 and Ar diluents, the flame spread rates (Sf) at µg were always lower than the 1g values and the minimum oxygen concentrations that would support flame spread were lower at 1g than µg. These findings are consistent with prior studies in O2-N2 atmospheres and results from the greater radiative heat losses at µg due to the increased flame thickness (d) at µg. In contrast, for CO2 diluent, Sf was slightly higher at g and the minimum O2 concentration was lower (Fig. 2). For SF6 diluent, Sf was substantially higher at µg for all oxygen concentrations and the minimum O2 concentration was significantly lower (Fig. 2). These effects was attributed to changes in the character of radiative transfer for CO2 and SF6. He, Ar and N2 diluents do not emit thermal radiation and thus only the H2O and CO2 combustion products radiate significantly. For our test conditions tested the Planck mean absorption length (LP) of the combustion products in these diluents is typically 1 m, which is much larger than d, consequently, radiative transport is optically thin. However, for CO2 and especially SF6 diluents, LP is comparable to d or smaller, thus reabsorption effects cannot be neglected. With reabsorption, radiation emitted near the flame is not lost to the surroundings and instead augments conventional thermal conduction to increase Sf above radiation-free values. Interferometer images (not shown) showed that d increased at µg for all diluents but most dramatically for radiatively-active diluents, most likely due to these reabsorption effects.

Experiments on flame spread over thermally-thick fuels have been conducted using foam fuels to obtain low density and thermal conductivity, and thus large spread rate (Sf) over thermally-thick fuels compared to dense fuels such as PMMA.  This scheme enabled meaningful results to be obtained even in 2.2 second drop tower experiments.  It was found that, in contrast conventional understanding; steady spread can occur over thick fuels in quiescent microgravity environments, especially when a radiatively active diluent gas such as CO2 is employed (Figs. 3, 4).  This is proposed to be due to radiative transfer from the flame to the fuel surface that can lead to steady spread even when conductive heat transfer from the flame to the fuel bed is negligible.  Radiative effects are more significant at microgravity conditions because the flame thickness is larger and thus the volume of radiating combustion products is larger at microgravity.  The effects of oxygen concentration and pressure are shown and the transition from thermally-thick to thermally-thin behavior with decreasing bed thickness is demonstrated.  Radiative flux measurements confirm the proposed effects of diluent type and gravity level.  These results are particularly noteworthy considering that the International Space Station employs CO2 fire extinguishers; our results suggest that helium may be a better inerting agent on both mass and mole bases at microgravity even though CO2 is much better on a mole basis at earth gravity.

Download video of flame spread at 1g and g over a polyphenolic fuel sample in a 35% O2 - 65% CO2 mixture at a pressure of 4 atm (603 KB file).

Figure 1. Schematic of drop frame and interferometer apparatus. The fuel bed is mounted inside the chamber parallel to the plane of the page.

Figure 2. Flame spread rates vs. O2 mole fraction at 1 atm in helium, carbon dioxide and sulfur hexafluoride diluents.

Figure 3.  Effect of oxygen concentration on spread rates over thick solid fuel beds at µg and earth gravity.

Figure 4.  Effect of pressure on spread rate over thick solid fuel beds at µg and earth gravity (polyphenolic foam, density 0.0267 g/cm3, atmosphere 40% O2 - 60% CO2.).

FLAME SPREAD IN PARTIALLY-PREMIXED ATMOSPHERES

Fires in enclosures usually burn in under-ventilated conditions, leading to atmospheres containing unburned fuel or intermediates such as CO. With this motivation, thin-fuel flame spread experiments were conducted at 1g and g in atmospheres containing sub-flammability-limit concentrations of gaseous fuels in O2-N2 atmospheres. CO and CH4 were used for the gaseous fuels in partially-premixed atmosphere tests, plus H2, C3H8 and NH3 for 1g tests only. 1g tests showed that that for some fuels such as CO and H2, there is a substantial effect of gaseous fuel on Sf, whereas for other fuels such as NH3, there was practically no effect. Remarkably, for CO fuel (Fig. 5), a very important case for practical applications, Sf was higher and the minimum O2 concentration was lower when a given number of oxygen atoms in the ambient atmosphere was present in the form of CO rather than O2. Moreover, these data do not even account for the fact that in practical fires the vitiated air will be hotter than ambient due to the heat release associated with the partial oxidation. For all gaseous fuels enhancement of Sf correlated well with the characteristic chemical reaction rate of the premixed fuel and no correlation with the heating value or diffusive properties of the premixed fuel was observed. In the current flight definition study, this work will be extended to thick fuels using longer-duration g experiments.

It was found that the effect of adding gaseous fuel to the ambient atmosphere was qualitatively similar at 1g and g but the effect is stronger at g than 1g, and in fact Sf is actually higher at g than 1g at high premixed fuel concentrations (Fig. 6). Also, the effect of added gaseous fuel was found to be more substantial at higher oxygen concentration and with CO fuel. All of these results are consistent with the simple theoretical model proposed by the PI which shows the effect of the premixed fuel is to cause a partially-premixed flame sheet to occur upstream of the conventional non-premixed flame. This additional flame increases the total heat flux to the fuel bed and thus Sf. Finite-rate chemistry of the premixed flame was found to affect the additional heat flux, even when the nonpremixed flame is at the high Damkohler number (mixing-limited) condition.

Figure 5. Effect of O2 mole fraction and gaseous CO addition on Sf. Solid lines and data points: experiment; dashed lines: theoretical predictions. f  is the equivalence ratio of the premixed atmosphere.

Figure 6. Measured and predicted flame spread rates vs. f. Atmosphere: 18% O2 in N2 at 1 atm with CO as added gaseous fuel.

CONCURRENT-FLOW FLAME SPREAD

A study of upward flame spread over solid fuels was conducted to clarify the mechanisms of spread rates for concurrent-flow flame spread and in particular, buoyancy effects on this process. It was proposed that, contrary to many prior theoretical predictions, upward flame spread could be steady because convective losses to the sides of the fuel samples and/or surface radiative losses prevent the flame length and thus spread rate from growing indefinitely. These losses were argued to be unavoidable because the flame length will grow until these losses balance the heat generation rate. Scaling relations for the spread rates in the presence of convective and radiative losses, laminar and turbulent flow, buoyant and forced convection, and thin and thick fuels were derived. Tests of some of these relations were conducted for upward-propagating flames over tall, thermally-thin fuel samples, subject to buoyant convection only, for a range of pressures, oxygen mole fractions, diluents and fuel bed thicknesses. In this manner a seven-decade range of Grashof number, defined as gW3/n2, where g is the gravitational acceleration, W the fuel bed width and n the kinematic viscosity, was studied. Only conditions away from quenching were studied to minimize chemical influences. Flames were found to achieve steady values of both Sf and flame length when the sample was sufficient tall. Measured values of Sf, normalized by the opposed-flow (downward) spread rate with the same atmosphere and fuel bed (Sf,opp), are shown in Fig. 7. At low GrW, Sf/Sf,opp ~ GrW1 with the value of the proportionality constant being slightly different for different atmospheres. At higher W, Sf is independent of GrW, indicating a transition to radiatively-stabilized flame spread. At intermediate GrW, there is some indication of a region where Sf/Sf,opp ~ GrW4/7 as would be characteristic of turbulent buoyant regime. The data deviate from Sf/Sf,opp ~ GrW1 behavior towards Sf/Sf,opp ~ GrW4/7 near GrW = 20,000, which is close to where a predicted transition from laminar to turbulent behavior occurs. Furthermore, visually the flames were observed to change to a turbulent structure near this value of GrW.

Figure 7. Effect of Grashof number (GrW) on upward flame spread rate, normalized by downward spread rate, over thin fuel samples.

PUBLICATIONS

Son, Y., Ronney, P. D., "Radiation-Driven Flame Spread Over Thermally-Thick Fuels in Quiescent Microgravity Environments," Proceedings of the Combustion Institute, Vol. 29, pp. 2587-2594 (2002).

Honda, L. K. and Ronney, P. D., “Mechanisms of concurrent-flow flame spread over solid fuel beds,” Proceedings of the Combustion Institute, Vol. 28, pp. 2793-2801 (2000).

Honda, L. and Ronney, P. D., "Effects of Ambient Atmosphere on Flame Spread at Microgravity," Combustion Science and Technology, Vol. 133, pp. 267-291 (1998).

Ronney, P. D., Greenberg, J. B., Zhang, Y., Roegner, E. V., "Flame Spread Over Thin Solid Fuels in Partially Premixed Atmospheres," Combustion and Flame, Vol. 100, pp. 474-484 (1995).

Greenberg, J. B., Ronney, P. D., "Analysis of Lewis Number Effects in Flame Spread," International Journal of Heat and Mass Transfer, Vol. 36, pp. 315-323 (1993).

Zhang, Y., Ronney, P. D., Roegner, E., Greenberg, J. B., "Lewis Number Effects on Flame Spreading Over Thin Solid Fuels," Combustion and Flame, Vol. 90, pp. 71-83 (1992).