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Droplet size and equivalence ratio effects on spark ignition of monodisperse N-heptane and methanol sprays
Journal article   Peer reviewed

Droplet size and equivalence ratio effects on spark ignition of monodisperse N-heptane and methanol sprays

Allen M. Danis, Izak Namer and Nicholas P. Cernansky
Combustion and flame, v 74(3)
1988
DOI: https://doi.org/10.1016/0010-2180(88)90074-0

Abstract

The effects of the droplet size and the equivalence ratio on the ignition of monodisperse n-heptane and methanol sprays at atmospheric pressure were investigated using a capacitive discharge spark ignition system. The minimum ignition energy E min of the sprays was measured over a range of droplet diameters, D = 30–57 μm, and equivalence ratios, φ = 0.44 – 1.8. As expected, results showed that E min decreased with decreasing droplet size, with increasing equivalence ratio, and with increasing fuel volatility. While the same trends have been observed previously for polydisperse sprays, this is the first study to quantify these effects for monodisperse sprays in this size range. The minimum ignition energy was also measured for fully prevaporized n-heptane and methanol. An optimum gas phase equivalence ratio for prevaporized n-heptane ignition was found between 1.5 < φ < 2.0. No corresponding optimum was obtained for prevaporized methanol due to difficulties in generating fuel-rich methanol mixtures. comparison of the spray and prevaporized ignition results indicated the existence of an optimum droplet size, D < 30 μm, for the ignition of fuel-lean sprays for both fuels. Extension of the lean prevaporized ignition limit, φ = 0.55, was also observed for all sprays. Experimental ignition results were compared to the predictions of two existing ignition models for quiescent sprays: the characteristic time model for ignition of Peters and Mellor and the general ignition model of Ballal and Lefebvre. Both models, using a characteristic time approach, predicted the experimentally determined ignition energies accurately for most conditions. Model performance deteriorated, however, for leaner ration, φ < 0.7, and for smaller droplet size, D < 35 μm, with both models increasingly underpredicting E min.

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Web of Science research areas
Energy & Fuels
Engineering, Chemical
Engineering, Mechanical
Engineering, Multidisciplinary
Thermodynamics
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