Sub-millimeter sized methyl butanoate droplet combustion: Microgravity experiments and detailed numerical modeling

Combustion characteristics of isolated sub-millimeter sized methyl butanoate (MB) droplets are studied at low gravity (10 4 m/s) in a 1.2 s drop tower. In the experiments, droplets were grown and deployed onto the intersection of two 14 lm silicon carbide fibers in a cross-string arrangement and exposed to symmetrically placed spark ignition sources. The initial droplet diameter was fixed at 0.54 ± 0.01 mm, and experiments were carried out in room temperature air at atmospheric pressure. Detailed measurements of the evolution of droplet diameter, flame standoff ratio and burning rate are reported. The experimental results are compared against predictions from a comprehensive time-dependent, sphero-symmetric droplet combustion simulation that includes detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer, multi-component diffusive transport, full thermal property variations and tether fiber perturbation effects. The predicted combustion characteristics of MB are also compared with n-heptane droplets of nearly identical sizes over a range of oxygen concentrations. The results show that predicted burning histories, burning rates and flame standoff ratios are in excellent agreement with the measurements. The average burning rates and flame temperatures for both fuels were found to be similar even though the heat of combustion of n-heptane is higher by a factor of 1.6. However, the average flame standoff ratio for MB was found to be significantly smaller than for n-heptane, due to the presence of additional oxygen atoms in the parent fuel. Important differences in the diffusion flame chemistries of the two fuels are also discussed. 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.