Abstract
Three-dimensional Direct Numerical Simulations (DNS) in canonical configuration have been employed to study the combustion of mono-disperse droplet-mist under turbulent flow conditions. A parametric study has been performed for a range of values of droplet equivalence ratio ϕd, droplet diameter ad and root-mean-square value of turbulent velocity u′. The fuel is supplied entirely in liquid phase such that the evaporation of the droplets gives rise to gaseous fuel which then facilitates flame propagation into the droplet-mist. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ϕd>1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion: the premixed mode ocurring mainly under fuel-lean conditions and the non-premixed mode under stoichiometric or fuel-rich conditions. The prevalence of premixed combustion was seen to decrease with increasing droplet size. Furthermore, droplet-fuelled turbulent flames have been found to be thicker than the corresponding turbulent stoichiometric premixed flames and this thickening increases with increasing droplet diameter. The flame thickening in droplet cases has been explained in terms of normal strain rate induced by fluid motion and due to flame normal propagation arising from different components of displacement speed. The statistical behaviours of the effective normal strain rate and flame stretching have been analysed in detail and detailed physical explanations have been provided for the observed behaviour. It has been found that the droplet cases show higher probability of finding positive effective normal strain rate (i.e. combined contribution of fluid motion and flame propagation), and negative values of stretch rate than in the stoichiometric premixed flame under similar flow conditions, which are responsible for higher flame thickness and smaller flame area generation in droplet cases.
Highlights
Flame propagation into droplet-laden mixtures plays a pivotal role in several engineering applications ranging from Internal Combustion (IC) engines (e.g. Direct Injection and Compression Ignition engines) [1, 2] to aero gas turbines [2, 3], as well as in hazard prediction and control [4]
It has been found that the rms value of turbulent velocity u, initial droplet diameter ad and droplet equivalence ratio φd have significant influences on the nature of the mixture arising due to droplet evaporation and that this is highly dependent on the position within the flame: c < 0.9 or c > 0.9
It was shown that both premixed and non-premixed modes exist simultaneously in flames propagating into a droplet mist, but they reside predominantly in different locations in mixture fraction space: premixed mode in ξ
Summary
Flame propagation into droplet-laden mixtures plays a pivotal role in several engineering applications ranging from Internal Combustion (IC) engines (e.g. Direct Injection and Compression Ignition engines) [1, 2] to aero gas turbines [2, 3], as well as in hazard prediction and control [4]. Faeth and his co-workers [6, 7] indicated in a number of experimental investigations that evaporation characteristics of the droplets can contribute to the subsequent flame propagation behaviour It was experimentally demonstrated by Ballal and Lefebvre [8] that the flame speed for droplets small enough to evaporate completely before reaching the flame front was similar to that of a purely gaseous mixture and that the flame speed decreases with increasing initial droplet diameter. Ballal and Lefebvre [8] experimentally observed that there exists an optimal initial droplet diameter for a given fuel and overall equivalence ratio, φov = φg + φd (where φg is the contribution arising due to gaseous fuel and φd the contribution arising due to liquid fuel), for which the laminar flame propagation was enhanced compared to that of a purely gaseous fuel. The equivalence ratio of fuel in the gaseous mixture following the evaporation process was shown to have significant effects on flame propagation [13], but these effects weaken with increasing level of turbulent velocity fluctuation [13]
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