Experimental Study and Kinetic Modeling for the Laminar Flame Speed of Methyl Octanoate and n-Nonane

Abstract

ABSTRACT Biodiesel is one of the most important alternative fuels due to limited conventional resources. Methyl octanoate with long alkyl chain and mono-alkyl ester is one of the surrogate fuels of actual biodiesel. In order to clarify distinctive combustion characteristics of methyl octanoate, comparison with alkane in traditional diesel is necessary. Therefore, this work chose n-nonane with same carbon number as methyl octanoate. The laminar flame speeds of two fuels were measured at different conditions (T = 423 K, P = 1 and 3 atm, and φ = 0.7–1.5) in a constant volume combustion bomb. Results show that laminar flame speed of n-nonane is higher than that of methyl octanoate in most conditions, but lower in fuel-rich conditions. The combined chemical kinetic model of methyl octanoate and n-nonane was developed through eliminating unnecessary reactions, which can well predict the experimental results of laminar flame speed and ignition delay time. The adiabatic flame temperature results from the updated kinetic model show that the temperatures of n-nonane are higher than methyl octanoate in whole conditions, which is unexpectedly opposite to their laminar flame speeds order in fuel-rich conditions. In overall conditions, the intermediates of methyl octanoate tend to produce CH3, which consumes H radicals, thereby reducing the laminar flame speed, which can explain the higher laminar flame speed of n-nonane in most conditions. Meanwhile, the oxidation reactions with oxygen molecules of intermediates of n-nonane are suppressed due to low combustion temperature at φ = 1.5 unlike at φ = 0.7 and 1.0. On the contrary, those of methyl octanoate have similar reactions between φ = 0.7, 1.0, and φ = 1.5. It means that the laminar flame speed of n-nonane decreases more significantly than methyl octanoate as the equivalence ratio increases from φ = 1.0 to φ = 1.5.