Ignition of uniform droplet-laden weakly turbulent flows following a laser spark
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Authors
de Oliveira, PM
Allison, PM
Mastorakos, E
Publication Date
2019Journal Title
Combustion and Flame
ISSN
0010-2180
Publisher
Elsevier BV
Volume
199
Pages
387-400
Type
Article
Metadata
Show full item recordCitation
de Oliveira, P., Allison, P., & Mastorakos, E. (2019). Ignition of uniform droplet-laden weakly turbulent flows following a laser spark. Combustion and Flame, 199 387-400. https://doi.org/10.1016/j.combustflame.2018.10.009
Abstract
The forced ignition process has a stochastic nature, which can be intensi ed
due to turbulence and mixture
uctuations. Although fuel droplets represent
strong inhomogeneities which are generally detrimental to ignition, the
presence of small droplets has been found to enhance
ame speeds, decrease
minimum ignition energy, and improve the ignitability of overall lean mixtures.
In order to understand which factors are conducive to ignition of
sprays, a spherically expanding
ame is investigated, which is produced by a
laser spark in a uniform dispersion of ethanol droplets in turbulent air. The
ame is visualised by schlieren and OH*-chemiluminescence for overall equivalence
ratios of 0.8 to 2, Sauter mean diameter of approximately 25 m, and
u0=SL ranging from 0.9 to 1.3, where u0 and SL denote the rms axial velocity
and laminar burning velocity, respectively. The timescales of the spark's effects
on the
ame are measured, as well as quenching timescales and initial
kernel sizes conditional on ignition or failure. Small kernels quenched faster
than approximately 0.6 ms, that is, the duration of the
ame overdrive, and
a minimum kernel radius for ignition of 1mm was observed. The short-mode
of ignition failure was suppressed by increasing the laser energy and, consequently,
the initial kernel size. Nevertheless, the ignitability of lean mixtures
was only e ectively improved through high-energy sparks and partial prevaporisation
of the fuel. Virtually all kernels ignited once prevaporisation was
increased, and the gas-phase equivalence ratio was approximately 75% of the
lower
ammability limit, with ignition being limited only by laser breakdown.
Sponsorship
European Commission Clean Sky project AMEL (641453);
Brazilian Space Agency and Brazil's National Council for Scientific and Technological Development
Funder references
European Commission (641453)
Identifiers
External DOI: https://doi.org/10.1016/j.combustflame.2018.10.009
This record's URL: https://www.repository.cam.ac.uk/handle/1810/286312
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