Combustion Science and Technology

FERNANDEZ-PELLO, A. C.; HIRANO, T.

doi: 10.1080/00102208308923650pmid: N/A

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 that generates the spread process. The current experimental observations, although still in need of further verification, provide insight toward the development of accurate descriptions of the flame spread process.

RAY, STEVEN R.; GLASSMAN, IRVIN

doi: 10.1080/00102208308923651pmid: N/A

Abstract The variation of the flame propagation rate across a thermally-thick fuel as a function of opposed flow velocity is described as consisting of three regimes. The first at low opposed flow velocities is dominated by the naturally induced flow and shows relatively little variation of the propagation rate. The second regime shows a near linear increase of propagation rate with opposed flow and is dominated by thermal processes alone. The last regime shows a decline in rate with opposed flow and is an indication of the dominance of chemical rate processes. Surface temperature and flow field measurements ahead of the flame indicate that the mechanism by which the flame propagates is not by thermal conduction through the solid ahead of the flame, but rather by fuel diffusing from behind the flame through the quench layer to create a lean flammable mixture ahead of the flame.

ALTENKIRCH, R. A.; EICHHORN, R.; RIZVI, A. R.

doi: 10.1080/00102208308923652pmid: N/A

Abstract Flame spread rates are presented for polymethylmethacrylate fuel beds as a function of gravitational acceleration and ambient pressure and oxygen concentration. The data are correlated by plotting a dimensionless spread rate that is a measure of the heat transferred forward of the flame, required to sustain the flame, compared to the maximum possible forward heat transfer against a Damköhler number. The latter parameter indicates the approach of the flame to its maximum temperature, where the maximum possible forward heat transfer occurs, such that the dimensionless spread rate approaches unity at large Damköhler number. Surface regression effects on the flame spread process are incorporated into the dimensionless spread rate, and the final correlation is independent of bed thickness.

QUINTIERE, J.; HARKLEROAD, M.; WALTON, D.

doi: 10.1080/00102208308923653pmid: N/A

Abstract A concept was examined for measuring flame spread parameters suitable for predicting the performance of a material in fires. The study examines a radiant panel test apparatus used to measure downward and lateral flame spread, and ignition. An analysis of data from tests of Douglas fir particle board is presented. A procedure has been identified for measuring specific parameters useful in the general prediction of ignition and flame spread for complex materials.

WICHMAN, I. S.; WILLIAMS, F. A.

doi: 10.1080/00102208308923654pmid: N/A

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.

BORGESON, ROBERT A.; T'IEN, JAMES S.

doi: 10.1080/00102208308923655pmid: N/A

Abstract A numerical model of flame spread on a thermally thin fuel in opposed flow (Frey and T'ien, 1979) has been used to study the effect of the initial fuel bulk temperature on the extinction limit. The distribution of temperature, fuel fraction, reactivity, and local equivalence ratio are presented and analyzed for the near-limit case. When the fuel temperature is decreased, the fuel pyrolysis rate near the pyrolysis front decreases, the flame spread rate drops, the pyrolysis length diminishes and the flame size is reduced. When the flame reaches a certain critical size, depending on a number of parameters, extinction occurs.

HIRANO, TOSHISUKE; SATO, KENJI; SATO, YOSHIKO; SATO, JUN'ICHI

doi: 10.1080/00102208308923656pmid: N/A

Abstract The development of a model for a consistent prediction of metal fire in high pressure oxygen has been attempted. The reaction schemes and reaction rates of metal oxidation are inferred, and the burning region spread rates are predicted. The burning region spread rate V over an infinite thickness piece is shown to depend on the normal burning rate and the heat flux from the periphery of the burning region. For burning of a piece of a finite dimension, a plate or a cylinder, V is shown to increase with the decrease of its thickness or diameter. The most acceptable rate-determining step is inferred to be the incorporation of oxygen into the oxide layer. The dependency of V on the thermal conductivity is suggested to change largely with the temperature at which an intense heat release due to oxidation starts. The results obtained in the present study are shown to agree fairly well with the experimental results found in other studies.

CARRIER, GEORGE; FENDELL, FRANCIS; FINK, STANTON

doi: 10.1080/00102208308923657pmid: N/A

Abstract The spread of fire across the ceiling of a large room (or long corridor) is modeled as wind-aided flame spread along a horizontal char-forming thick slab, in the presence of significant convective, diffusive, and radiative transport. The goal is to predict the rate of streamwise advance of the site on the solid-gas interface at which the pristine solid undergoes endothermic degradation to a combination of (I) a porous carbonaceous heat-retaining matrix, and (2) a mixture of (partially combustible) vapors that move through the matrix to the outer gas. This rate of advance of the thermal-degradation site is sought as a function of normally available data concerning the thermodynamic and physical properties of the solid; the thermodynamic and dynamic state of the hot vitiated bulk gas that abruptly starts, and then continues, to flow over the slab; and the initial thermodynamic state of the slab. Downwind of the interfacial degradation site, hot product gases flowing over the slab preheat it from its ambient to its outgassing state; upwind of the degradation site, formation of a thickening char layer between the deep pristine solid and the bulk gas occurs, though, far enough upstream, the char layer thins because the carbonaceous matrix is eroded under surface attack. The degradation site is thus a “separatrix” whose time-varying position, to be found in the course of solution, conveniently characterizes the extent of slab “involvement” this moving separatrix implies a boundary/initial-value problem of unconventional, but tractable, Stefan type. A nonlinear, unsteady, two-spatial-dimension treatment in the Shvab-Zeldovich approximation entails boundary-layer simplification in the manner of Prandtl, convective-transport simplification in the manner of Oseen, and thin-flame simplification in the manner of Burke and Schumann. The char layer plays an important role because of its thermal resistance and its heat retention; and radiative transfer, is also important, via bulk-gas cooling, solid-surface emission, and near-flame radiation. It is not surprising that an analysis of the time-independent counterpart of this phenomenon in the presence of this plethora of mechanisms† has not yet been carried out, and, accordingly, here in Part I, we formulate the time-dependent problem in anticipation of its treatment in Part II and we solve the steady-state problem to get the insight it provides, to establish the methodology, and to obtain information which is needed in the postulation of the boundary conditions of the time-dependent phenomenon.