Diffusive Burning of Liquid Fuels

It should be realized that the use of L values from Table 9.1 must be taken as representative for a given material. The values can vary, not just due to the issues associated with the interpretation of experimental data, but also because the materials listed are commercial products and are subject to manufacturing and environmental factors such as purety, moisture, grain orientation, aging, etc. 

We shall formalize our use of L by considering analyses for the steady burning of liquid fuels where the heat of gasification is a true fuel property. 

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Burning Rate - Defined as the rate (in millimeters per minute) at which a pool of liquid decreases as the liquid bums. Details of measurements are provided by D. S. Burgess, A. Strasser, and J. Grumer, Diffusive Burning of Liquid Fuels in Open Trays, Fire Research Abstracts and Reviews, 3, 177 (1961). 

The theoretical solution to the diffusion flame problem is best approached in the overall sense of a steady flowing gaseous system in which both the diffusion and chemical processes play a role. Even in the burning of liquid droplets, a fuel flow due to evaporation exists. This approach is much the same as that presented in Chapter 4, Section C2, except that the fuel and oxidizer are diffusing in opposite directions and in stoichiometric proportions relative to each other. If one selects a differential element along the x-direction of diffusion, the conservation balances for heat and mass may be obtained for the fluxes, as shown in Fig. 6.8. 

A variety of phenomena are exhibited by the burning of a spherical fuel particle in an infinite oxidizing atmosphere. Here we shall consider one of the simplest situations, the quasisteady, spherically symmetrical burning of a liquid fuel that vaporizes and reacts in a gas-phase flame, producing gaseous products that flow and diffuse to infinity. However, at the outset it is of interest to indicate some of the complexities that may arise in other situations. Because of the diversity of high-temperature oxidation mec-anisms of solids [24], [46], these complexities often are associated with the burning of solid fuels. 

The most commonly considered case of droplet combustion involves the combustion of a liquid fuel burning in a surrounding oxidizing atmosphere, usually air. The droplet evaporates and acts as a source of vapor, and, since oxidant and fuel are initially separated, the fuel vapor and oxidant bum in a diffusion flame surrounding the droplet. 

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