Revealing the role of limiting oxygen concentration test for a wick flame in characterizing the liquid fuel flammability

A two-dimensional numerical model was established to simulate the wick combustion system for determining the limiting oxygen concentration (LOC) of organic solvents, called the wick-LOC method. The ethanol wick flame under forced flow with decreased oxygen concentrations in normal gravity was studied numerically. Three simplified reaction mechanisms, 1-step, 2-step, and quasi-global mechanisms of ethanol were employed in the simulations for comparison with experimental results. The responses of flame height and edge flame standoff distance to the oxygen decrease were analyzed for validation. As the oxygen concentration was reduced to the stability limit of the full flame, the blow-off of the edge flame can be observed. By comparing the simulated LOCs and near-limit stabilized flame structures, the 2-step mechanism can reproduce the wick-LOC well, while the quasi-global mechanism can provide a more detailed flame structure. Further investigation into flame blow-off processes revealed that the reaction rate in fuel-lean conditions is more important for determining the wick-LOC, and the local flow velocities at the flame kernel are almost constant until blow-off occurs. By combining the flame stabilization theory with simulations, the physical meaning of the wick-LOC value can be explained as the oxygen concentration where the local flow velocity can be balanced with critical edge flame speed (more fundamentally, the critical laminar burning velocity) in a certain fuel-lean equivalence ratio. Further applications of the wick-LOC method are expected to examine the simplified reaction mechanisms of other liquid fuels.