Flames

Water Absorption

Since methanol is completely miscible in water, before or during methanol droplet burning in air, methanol will absorb water. The effect of water absorption results in a non d-square combustion behavior and promotes flame extinction. If a Damkohler number, defined as the ratio of a diffusion time to a chemical time, becomes too small, then there is insufficient time in the flame surrounding the liquid sphere for the chemical heat release to occur, and the flame is extinguished. This flame extinction occurs when the liquid sphere reaches a critical minimum diameter.

In our study, two limiting approaches, perfect liquid-phase mixing and pure diffusion, are adopted to pursue the water-absorption analysis. The corresponding results are summarized here.

The square of the droplet diameter as a function of time for droplets initially 1 mm in diameter, burning in air at pressure p = 1 atm and temperature T2 = 300 K and in a helium-oxygen mixture with the oxygen mole fraction of 0.5 at p = 1atm and T2 = 300 K; and for a droplet initially 0.7 mm in diameter, burning in air at p = 0.25 atm and T2 = 300 K; solid lines are for perfect liquid-phase mixing, dashed line are for time-dependent liquid-phase diffusion, points are experimental data from literature:


The droplet diameter and the extinction diameter as functions of time with time-dependent water absorption for perfect liquid-phase mixing, for droplet initially 1 mm in diameter, burning in two different atmospheres at p = 1 atm and T2 = 300 K, illustrating how extinction conditions are determined:


The extinction diameter and the water mass fraction in the liquid at extinction, as functions of initial droplet diameter, for perfect liquid-phase mixing, for burning in three different atmospheres at T2 = 300 K:


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