Indrek S. Wichman

Theory of opposed-flow flame spread

Abstract A critical, historical review of the flame spread literature is given, beginning with the first systematic studies of opposed-flow flame spread. Important modeling effects are described, including qualitative, simplified, μg and comprehensive numerical modeling. A brief discussion of subjects with the potential for further development is also given. Although this review focuses on flame-spread theory the emphasis is on the logical development, not the detailed mathematics.

A Simplified Model for the Pyrolysis of Charring Materials

A simplified model of the pyrolysis of charring materials is analyzed. The effects of moisture are neglected, and the heat of pyrolysis is assumed equal to zero. Four stages of pyrolysis are obtained: (i) inert heating, (ii) initial pyrolysis, (iii) thin char, and (iv) thick char. Formulas for the volatile mass efflux, m, are obtained in stages (ii), (iii), and (iv); m = 0 in the first stage. The calculations indicate that the surface temperature controls the volatile production rate in the initial pyrolysis stages (the kinetically controlled regime), while the temperature gradient controls the volatile production rate in the thick char stage (the diffusion-controlled regime). Comparisons of the calculated results with numerical computations are made for the volatile mass efflux, the surface temperature, and the density.

A simplified model of flame spread in an opposed flow along a flat surface of a semi-infinite solid

Abstract A flame-spread model is analyzed in which heat release occurs at the planar interface of two media, each of which moves with a different but constant velocity. The steady-state, two-dimensional equations for conservation of energy in each medium are solved subject to a prescribed temperature distribution on the downstream half of the interface and continuity of the normal heat flux on the upstream half. Differing thermal conductivities in normal and streamwise directions are allowed in each medium. The approach involves introduction of Fourier transforms in the streamwise coordinate and use of the Wiener-Hopf technique. The model is shown to be equivalent to that of de Ris with radiant transfer neglected and also may be interpreted in terms of distributed electrical or radiant heating without combustion. Parametric results are obtained for various heat fluxes and for spread rates. The study helps to improve understanding of mechanisms of flame spread under conditions controlled by heat transfer.

Flame spread in an opposed flow with a linear velocity gradient

Abstract A theoretical model is developed for estimating the rate of flame spread under conditions of heat-transfer control with account taken of the fact that the gas velocity is not uniform. It is shown that the functional dependence of the spread rate on parameters such as the external gas velocity is modified from that obtained in the classical study of deRis. The modified formula is capable of lending interpretation to previously anomalous experimental observations.

Material flammability, combustion, toxicity and fire hazard in transportation

Abstract Many materials used in transportation vehicles are combustible, often dramatically so. These materials may include gases, liquids and solids. They are sometimes employed in order to increase fuel efficiency, reduce manufacturing cost or meet other market demands such as durability or appearance. Extensive research has been conducted on such materials both with and without the expressed intent of applying the results directly to transportation. Combustion and fire research, considered generally, are much broader than (and in fact encompass), the specific applications to transportation issues discussed here. Much if not most of fire and combustion research, therefore, has a direct bearing on transportation. This review ties together disparate subjects of fundamental fire and combustion research that have relevance to transportation fire. The goal is to provide a technical overview of the combustion literature related to, but not exclusively concerned with or focused on, transportation fire safety. Although they are not a large percentage of transportation-related fatalities, fires can be costly in human and economic terms. In the United States, post-collision fires occur in 1–5% of fatal motor vehicle accidents, while approximately 20% of air traffic fatalities are attributed to fire. This review addresses scientific and technical engineering issues in the fields of fire initiation; fire spread; products of combustion and their toxicity; and practical fire prevention in vehicles and other modes of transportation.

Effects of solid-phase properties on flames spreading over composite materials

The problem of downward flame spread over composite, thermally thick materials is theoretically investigated by means of a thermal, analytical model and a numerical model based on the reactive fully elliptic Navier-Stokes equations. The solid, made of a thermoplastic polymer and inert additives, undergoes in-depth endothermic pyrolysis, for the active part, with volatile monomer formation. The effective thermal conductivity of the composite material depends on the content of inert and the variable content of the active part, whose thermal conductivity varies between that of the polymer and that of the melted phase monomer. Even though surface regression is not taken into account, polymer consumption is modeled through a mass balance. The changes in the flame spread mechanisms with solid phase properties are investigated. The spread process is strongly affected by the solid perpendicular (to the spread direction) thermal conductivity which, when it decreases, causes a continuous increase in the spread rate. On the other hand, both numerical and analytical solutions give no dependence of the spread rate on the solid parallel (to the spread direction) thermal conductivity for a wide range of variation. In agreement with previous experimental results, at very large values of the latter, the finite-rate reaction model predicts a decrease in the spread rate. However, in contrast to certain experiments performed for non homogeneous composite samples, an increase of the spread rate with the effective solid parallel thermal conductivity or thermal capacity has not been found.

A model describing the steady-state gasification of bubble-forming thermoplastics in response to an incident heat flux

A theoretical model is developed to describe the in-depth effect of bubbles on the steady-state transport of volatile gases from the surface of a thermoplastic material subjected to an incident conductive heat flux. In this model the effect of the bubbles on the surrounding liquid is felt through the bubble number distribution function, n, which appears in the equations for conservation of mass, momentum, species, and energy in the melt. The equation describing the evolution of n includes the effects of bubble growth, convection, and nucleation. Formulas for the mass flux of volatiles and the bubble void fraction for gasifying PMMA are developed from the conservation equations, for the special case of a constant polymer mass fraction in the liquid phase.

Compact microwave re-entrant cavity applicator for plasma-assisted combustion

Advantages of combining an electrical discharge with combustion include a faster process, higher intensities, leaner combustion, pollutant reduction by altering by-products of combustion, improved fuel efficiency by achieving more complete combustion, more reliable ignition of combustion, and combustion across a wider range of pressures, temperatures and mixture stoichiometries. The benefits may also include the operation of combustion processes at extreme limits, such as aerospace applications at high speeds and altitudes.

A one-dimensional model of piloted ignition

Abstract In this article a model of piloted ignition is analyzed. The two-dimensional coupled solid and gas phase problem is simplified by assuming that the mass evolution rate from the combustible solid is a known function of time and by employing a plane rather than a point ignition source. Thus only a transient one-dimensional analysis of the gas phase is necessary. The equations are solved numerically using a fast scheme especially suitable for combustion problems. The pilot flame is modeled as a thin slab of gas that is periodically raised to the adiabatic flame temperature of the stoichiometric mixture. The effects of (1) ignition source location, (2) fuel mass evolution rate from the surface, and (3) surface temperature of the solid are investigated. An explanation is offered for the preignition flashes often observed experimentally. A rational criterion for positioning of the pilot flame is proposed. The minimum fuel flow rate, by itself, is found insufficient for predicting the onset of piloted ignition; heat losses to the surface play an important role. Also, the conditions at extinction of a steady diffusion flame are found to be very similar to those for piloted ignition.

Upward Turbulent Flame Spread on Wood Under External Radiation

Experiments were performed to obtain histories of surface temperatures and rates of upward flame spread for velocity oriented, thermally thick wood slabs exposed to surface fluxes of thermal radiation up to 2.6 W/cm{sup 2}. Above a critical irradiance sustained upward flame spread occurred for Douglas-fir particle board with pilot initiation at the base of the fuel face. Data obtained included temperatures, flame heights, pyrolysis-front heights, combustion duration, and char-layer thickness for various irradiances and preheat times. The measurements were compared with theory.