One-dimensional numerical model for evaporation and oxidation of hydrocarbon fuels

Abstract

In this work, a detailed chemistry, one-dimensional, reactive-diffusive model is implemented to study the basic aspects of evaporation and oxidation of a thin liquid fuel layer exposed to an incoming premixed flame. In particular, the model is applied to predict the total evaporation and ensuing oxidation of a liquid layer under repeated cycles. Methanol was used as a baseline fuel. Simplifications in the flow, geometry, and operating conditions are made to restrict the problem to its fundamental mechanisms. The solution method solves the appropriate governing equations in the liquid and gas phases, observing mass and species conservation with phase-equilibrium at the interface. The resulting eigenvalue problem is solved for pure liquid layers, but the extension of multi-component liquids is possible. Results show that increasing pressures lead to relatively lean regions near the interface due to the inverse dependence of phase-equilibrium concentrations on pressure. As a premixed flame arrives at the interface, large temperature gradients evaporate fuel from the layer as the remaining oxygen diffuses back into core gases. A short-lived diffusion flame results, which greatly enhances the rate of evaporation, serving as both a source of energy and a sink of fuel. Similar results are observed for pressure histories that resemble those of operating spark-ignition engines. Decreasing liquid layer thicknesses, increasing wall temperatures, and decreasing heats of vaporization are all observed to enhance the rate of evaporation mainly due to their impact on the heat transfer characteristics of the problem. Since the liquid layer surface is restricted to temperatures below or equal to the liquid boiling point, however, boundary layer temperatures for all cases are very similar, and thus total survival rate of evaporated fuel, repeatedly found to be roughly 2.9% for methanol, is quite insensitive to these parameters.