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

Mechanisms involved in many types of fire spread are described in a manner that sacrifices accuracy for the purpose of emphasizing general aspects of the underlying heat-transfer, fluid-flow and chemical-kinetic phenomena. Consideration is given to conditions for transition from one mode of propagation to another. Research on fire spread has been pursued intensively in recent years, and in the present contribution at attempt is made to provide a framework within which various studies can be placed. Entries to current literature are provided. Areas of apparent importance that do not seem to have been emphasized are suggested.

Mechanisms of fire spread

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.

Laminar flame spread over PMMA surfaces

Processes of flames spreading over the surface of charring fuels: Effects of the solid thickness

A method of carbonizing cellulose-containing plants is disclosed. Wood is used as a precursor material which is carbonized under controlled temperature and atmosphere conditions to produce a porous carbon product having substantially the same cellular structure as the precursor wood. The porous carbonized wood may be used for various applications such as filters, or may be further processed to form carbon-polymer or carbon--carbon composites. The carbonized wood may also be converted to a ceramic such as silicon carbide. Additional processing may be used to form ceramic-metal or ceramic--ceramic composites.

Carbonized wood and materials formed therefrom

Flame spreading velocity measurements were made using PMMA cylinders as fuel rods. The diameter of the rods varied from 1½10; to 1½2 inch, and burning was observed at angles of orientation from θ = −90° (vertically downward burning) θ = +40°. The length of the pyrolysis zone was measured and used to calculate an average surface regression rate. For the axisymmetric case of vertically downward burning, surface temperatures were measured in the pyrolysis zone and in front of the flame; internal temperatures were measured in front or the flame. The measured temperatures were used to evaluate a number of energy balances. As a result, values were obtained for the energy feedback from the flame to the fuel by the various modes of heat transfer.

Flame Propagation Measurements and Energy Feedback Analysis for Burning Cylinders

Pyrolysis of Pittsburgh Seam coal at fire-level surface heat fluxes of 0.76 and 2.0 cal/cm2-sec was investigated by means of measurements of mass-loss rate, density, temperature and internal gas pressure of pyrolyzing coal and thermal properties of coal and coke. Local and overall heats of pyrolysis were determined from these measurements at 2.0 cal/cm2-sec. It was found that as coal pyrolyzes, a surface coke layer (580 °C <T <800°C) grows behind a low-density plastic zone (410°C <T<580°C) that propagates into the virgin coal. The plastic zone is characterized by initial endothermic decomposition of virgin coal into a metaplast of liquid and gas and subsequent exothermic polymerization of these products to form coke. Exothermic reactions also occur in the surface coke layer. The sequential pyrolysis reactions were identified with respect to various density zones in the reacting coal

Coal Pyrolysis at Fire-Level Heat Flux

A method of forming a prechar-layer on wood to cause it to become fire and rot retardant. In an air or a low oxygen concentration atmosphere the wood is uniformly charred at a depth of at least 3 millimeters by a high-intensity radiant heat panel. Normally the heat panel is spaced from and moved with respect to the wood. The relative velocity of the wood past the radiant panel and the panel's emitted heat flux are used to control the depth of charring. Preferably fire and/or rot retard chemicals and/or treatments are added to the formed charred layer to further increase the wood's fire and rot retardancy.

Calvin K. Lee

Papers

Flame Propagation Measurements and Energy Feedback Analysis for Burning Cylinders

Coal Pyrolysis at Fire-Level Heat Flux

Method for wood precharring