Microgravity studies of methanol droplet combustion.

摘要

The objective of this work is to investigate the structure and flame extinction of methanol droplets burning in air. Asymptotic methods are employed to describe the flame structure and extinction of quasisteady, spherically symmetrical diffusion flames around methanol droplets. Starting with a short mechanism involving fourteen steps, sequentially reduced overall descriptions of four, three and two steps are obtained by investigating the character of the departure of the H atom and its associated radicals from steady states and by demonstrating that, contrary to hydrocarbon flames, water-gas equilibrium is an excellent approximation in the reaction zones of these methanol flames. It is shown that a useful two-step approximation is described qualitatively by ;Unlike alkane droplet combustion, methanol is completely miscible with water. Interactions between the vaporization and combustion processes of methanol involve fairly complicated transport and flow phenomena. A time-dependent model with water absorption and re-evaporation has been introduced and developed by employing quasisteady gas-phase analysis with non-unity Lewis number for water, unity Lewis numbers for other species. By coupling with asymptotic analysis, this method yields a formulation which eliminates the transport effects, that is, the dependence of extinction diameter on the initial diameter is a thermodynamic and chemical-kinetic property. The perfect mixing model predicted that for methanol droplets burning in air, the extinction diameter increases linearly with the initial diameter. This linear relationship was confirmed later by the USML-2 space experiment. Reasonable agreement with theoretically reported extinction diameters, obtained from detailed time-dependent computations and from USML-2 space mission Fiber Supported Droplet Combustion experiments, is thereby achieved by including about 20% initial water absorption before droplet ignition in the theoretical model. The results provide an analytical method for calculating methanol droplet combustion and extinction.