A.C. Fernandez-Pello

Controlling Mechanisms of Flame Spread

Abstract 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 ...

Flame spread in an opposed forced flow: the effect of ambient oxygen concentration

The velocity of flame propagation over the surface of thick PMMA and thin paper sheets has been measured as a function of the velocity and oxygen concentration of a forced gas flow opposing the direction of flame propagation. It is shown that although for thin fuels the flame spread rate always decreases as the opposed flow velocity increases, for thick fuels the dependence of the spread rate on the gas velocity is also a function of the ambient oxygen concentration. For low oxygen concentrations the flame spread rate decreases as the velocity of the gas flow increases. For high oxygen concentrations, however, the spread rate increases with the flow velocity, reaches a maximum and then decreases as the gas velocity increases. The velocity of the opposed flow at which the maximum occurs is a function of the oxygen concentration, decreasing as the concentration decreases. Following phenomenological considerations and simplified descriptions of the primary mechanisms occurring during the flame spread process, the experimental results are correlated by two non-dimensional parameters, one describing the gas phase kinetic effects and the other describing the process of heat transfer from the flame to the fuel. Such a correlation provides a powerful means of predicting the flame spread prcess as well as physical insight into the mechanisms controlling the propagation of the flame.

Forced forward smolder combustion

Abstract We consider porous cylindrical samples closed to the surrounding environment except at the ends, with gas forced into the sample through one of the ends. A smolder wave is initiated at that end and propagates in the same direction as the flow of the gas. We employ asymptotic methods to find smolder wave solutions with two different structures. Each structure has two interior layers, i.e., regions of relatively rapid variation in temperature separated by longer regions in which the temperature is essentially constant. One layer is that of the combustion reaction, while the other is due to heat transfer between the solid and the gas. The layers propagate with constant, though not necessarily the same, velocity, and are separated by a region of constant high temperature. A so-called reaction leading wave structure occurs when the velocity of the combustion layer exceeds that of the heat transfer layer, while a so-called reaction trailing wave structure is obtained when the combustion layer is slower than the heat transfer layer. The former (latter) occurs when the incoming oxygen concentration is sufficiently high (low). Reaction trailing structures allow for the possibility of quenching if the gas mass influx is large enough; that is, incomplete conversion can occur due to cooling of the reaction by the incoming gas. For each wave structure there exist stoichiometric, and kinetically controlled solutions in which the smolder velocity is determined, respectively, by the rate of oxygen supply to the reaction site and by the rate of consumption in the reaction, i.e., by the kinetic rate. Stoichiometric (kinetically controlled) solutions occur when the incoming gas flux is sufficiently low (high). For each of the four solution types, we determine analytical expressions for the propagation velocities of the two layers, the burning temperature, and the final degree of solid conversion. We also determine analytical expressions for the spatial profiles of temperature, gas flux, and oxygen concentration. Gravitational forces are considered and are shown to have a minimal effect provided the ambient pressure is large compared to the hydrostatic pressure drop. The solutions obtained provide qualitative theoretical descriptions of various experimental observations of forward smolder. In particular, the reaction trailing stoichiometric solution corresponds to the experimental observations of Ohlemiller and Lucca, while the reaction leading stoichiometric solution corresponds to the experimental observations of Torero et al.

Initial Observations on the Free Droplet Combustion Characteristics of Water-In-Fuel Emulsions†

A new mechanical technique for the production of well characterized small diameter isolated free droplets of multi-component and emulsified liquids is described. The technique is employed in development of an experimental facility to generate, inject, and combust free droplets of liquid fuels in a well defined, hot convective atmosphere. Initial observations of the burning characteristics of isolated free droplets of water in pure n-paraffin emulsions are reported. The existence of free droplet secondary atomization and the importance of fuel physical characteristics, water content, and internal phase structure to optimizing this phenomenon are confirmed. Observations are found to be in agreement with earlier predictions of the required internal phase super heat (Avedisian and Andres, 1978) and results suggest previously published suspended droplet combustion data on emulsified fuels to be of limited quantitative value. Observations are compared with the prediction of and physical models used in recent an...

Experimental Observations on the Disruptive Combustion of Free Droplets of Multicomponent Fuels

Abstract The disruptive burning characteristics of isolated free droplets of binary n-paraffin mixtures have been observed experimentally. For disruptions to occur a minimum difference in the normal boiling points of the components as well as a certain initial concentration of the more volatile component must exist. The initial concentration of the volatile component must be within a limited range defined by the relation of the homogeneous superheat limit of the mixture to the normal boiling point of the less volatile material. Disruptive burning is a result of homogeneous nucleation of the mixture somewhere within the interior of the droplet. Mass diffusion is the limiting liquid transport process which results in superheating of the droplet interior. Comparison with previous studies of micro-explosive atomization of water-in-fuel emulsions shows that the disruptive burning of a binary fuel mixture is a slower and less violent process. It is also concluded that through relieving the requirement for homog...

A theory of laminar flame spread over flat surfaces of solid combustibles

Abstract A theory is developed for predicting the steady rate of spread of a flame over the surface of a solid in directions ranging from downward to horizontal. The model involves a diffusion flame in a boundary layer downstream from a point of flame inception, heat transfer by natural convection from this flame to the gasifying fuel which supports it, heat conduction through the solid to the cooler fuel ahead of the flame, generation of an upstream boundary layer due to entrainment into the flame plume, upstream gasification and diffusion of fuel into this boundary layer, and thermal runaway of an ignition for the resulting premixed gas in the upstream boundary layer. The calculations make use of asymptotic analysis of the limit of high activation energy for the gasification and ignition reactions. It is shown that the predictions of the model can be made to agree well with many experimental observations.

Forward smolder of polyurethane foam in a forced air flow

An experimental study is conducted of forward smolder of polyurethane foam. Air is used as oxidizer, and is forced in the direction of smolder propagation under conditions that produce approximately one-dimensional forward smolder propagation. The objective of the study is to provide further understanding of the mechanisms controlling forward smolder and verification of theoretical models of the problem. Upward and downward forward smolder are compared to also observe the effect of buoyancy on the process. Measurements of the temperature histories at several locations throughout the foam sample are used to infer the characteristics of the smolder process, and to calculate the smolder propagation velocity along the sample length as a function of the air flow velocity and gravitational orientation. It is found that as the flow velocity is increased, there is a transition in the smolder characteristics from a smolder process that is characterized by the propagation of a single exothermic oxidation (smolder) reaction to one characterized by the propagation of two reactions, an oxidative smolder reaction preceded by an endothermic pyrolysis reaction. Buoyancy is observed to affect this mode of smolder at very low air velocities, or when the smolder front approaches the sample end. The smolder velocity data are correlated well in terms of a nondimensional smolder velocity derived from previously developed theoretical models of forward smolder. The good agreement between theory and experiments verifies that the smolder controlling mechanisms and simplifying assumptions implicit in the models are appropriate at least for the present experimental conditions.

Opposed Forced Flow Smoldering of Polyurethane Foam

Abstract An experimental study is carried out of the effect on the propagation of a smolder reaction through the interior of a porous fuel of a forced flow of oxidizer opposing the direction of smolder propagation. The potential effect of buoyancy in the process is also analyzed by conducting the experiments in the upward and downward propagation, and comparing the respective results. The experiments are conducted with a high void fraction flexible polyurethane foam as fuel and air as oxidizer, in a geometry that approximately produces a one-dimensional smolder propagation. Measurements are performed of the smolder reaction propagation velocity and temperature as a function of the location in the sample interior, the foam and air initial temperature, the direction of propagation, and the air flow velocity. For both downward and upward smoldering three zones with distinct smolder characteristics are identified along the foam sample. An initial zone near the igniter were the smolder process is influenced by...

Propagation and extinction of forced opposed flow smolder waves

Abstract Smoldering is a slow combustion process in a porous medium in which heat is released by oxidation of the solid. If the material is sufficiently porous to allow the oxidizer to easily filter through the pores, a smolder wave can propagate through the interior of the solid. We consider samples closed to the surrounding environment except at the ends, with gas forced into the sample through one of the ends. A smolder wave is initiated at the other end and propagates in a direction opposite to the flow of the oxidizer. Previous experimental results show that for opposed flow smolder, decomposition of the solid fuel into char is the chemical process which drives the smolder process. We model this decomposition as a one step reaction. The model suggests that extinction occurs when decomposition is complete. We employ large activation energy asymptotic methods to find uniformly propagating, planar smolder wave solutions. We determine their propagation velocity, burning temperature, final degree of fuel decomposition, and extinction limits. We also determine spatial profiles of gas flux, oxidizer concentration, temperature, and degree of decomposition of the solid. Comparison is made with previous experimental results.

Laminar flame spread over PMMA surfaces

A study is made of the mechanisms by which laminar flames spread over flat surfaces of polymethylmethacrylate, in directions ranging from downward to horizontal. Measurements of spread rates, temperature fields and velocity fields are reported. Techniques employed include thermocouple probing, photography, interferometry, radiometer measurements, sampling followed by gas chromatography, and particle-track photography. A simplified theoretical model of the spread process is developed, involving forward heat conduction through the solid as the major mode of the energy transfer and thermal runaway of a gas-phase ignition reaction of methylmethacrylate vapor in a boundary layer just upstream from the point of flame attachment. The extent to which this physical model applies to other materials will depend on the thermal and chemical-kinetic properties of those materials.