Transport of water vapor from combustion gases into liquid-phase in unsteady methanol pool flames

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Abstract Small-scale laboratory pool flames of methanol were studied experimentally to determine the liquid fuel depletion rates and the water transport between the gas-phase and the liquid-phase. All experimental observations were made in the unsteady burning mode covering the period from ignition to complete burnout. The pool diameters were 46.5, 97 and 138.5 mm. Five flame conditions with different pool diameters and various pool depths were investigated. Experimental results showed that the amount of water in the liquid-phase increased significantly as the burning progressed. It was shown that this was due to the transport of water in the gas-phase to the relatively cooler liquid surface. The physical mechanism that would produce the observed results was argued to be the following: water vapor condenses at the gas–liquid interface and dissolves into the pool liquid. The condensation heat release increases the enthalpy of the liquid-phase and hence can augment pool liquid evaporation. However, as the water fraction in the liquid-phase increases, so does the equilibrium saturation temperature of the water–methanol mixture. This means that a portion of the enthalpy contributed by water vapor condensation in the liquid-phase is spent to compensate for the increased saturation temperature. The vapor–liquid equilibrium compositions of methanol–water mixtures exhibit a nonlinear behavior such that the water vapor mole fraction above the liquid surface is smaller than the water mole fraction in liquid-phase resulting in an accumulation of water in the pool as the burning progresses.