Propagation Velocity and Structure of Flames in Droplet-Vapor-Air Mixtures

Hayashi, Sachiko; Kumagai, S.; Sakai, Tadami

doi:10.1080/00102207708946782

Cited by 105 publications

(63 citation statements)

References 3 publications

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“…Ballal and Lefebvre [8] experimentally observed that there exists an optimal initial droplet diameter for a given fuel and overall equivalence ratio, φ ov = φ g + φ d (where φ g is the contribution arising due to gaseous fuel and φ d the contribution arising due to liquid fuel), for which the laminar flame propagation was enhanced compared to that of a purely gaseous fuel. This enhanced flame speed was observed experimentally for both lean (φ ov < 1.0) and rich (φ ov > 1.0) overall equivalence ratios [9]. For an overall rich flame, it was suggested that the flame speed enhancement was due to the incomplete evaporation of large droplets on the unburned gas side which led to a local gaseous equivalence ratio close to unity (i.e.…”

Section: Introductionmentioning

confidence: 77%

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Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Wacks

Chakraborty

2016

Flow Turbulence Combust

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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 φ d , droplet diameter a d 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 dropletmist. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for φ d > 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.

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Section: Introductionmentioning

confidence: 77%

“…17 it is possible to derive a transport equation of c based on the transport equations for the oxidizer mass fraction Y O and the mixture fraction ξ [9]:…”

Section: Mathematical Backgroundmentioning

confidence: 99%

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Wacks

Chakraborty

2016

Flow Turbulence Combust

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“…The same system was used in a previous study on gas phase laser ignition (Bradley et al, 2004) Explosion vessel and aerosol characteristics Aerosol mixtures were generated in a combustion bomb by the Wilson cloud chamber technique (Wilson, 1911). This technique has been used previously in combustion studies by Hayashi (1976) and Lawes et al (2006). Well defined, near mono-dispersed, droplet suspensions was generated with diameters that could be varied between 0 and 25 µm.…”

Section: Laser Ignition Unitmentioning

confidence: 99%

Laser Ignition of Iso-Octane Air Aerosols

Lawes

Lee

Mokhtar

et al. 2007

Combustion Science and Technology

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Iso-octane aerosols in air have been ignited with a focused Nd:YAG laser at pressures and temperatures of 100 kPa and 270 K and imaged using schlieren photography. The aerosol was generated using the Wilson cloud chamber technique. The droplet diameter, gas phase equivalence ratio and droplet number density were determined. The input laser energy and overall equivalence ratio were varied. For 270 mJ pulse energies initial breakdown occurred at a number of sites along the laser beam axis. From measurements of the shock wave velocity it was found that energy was not deposited into the sites evenly. At pulse energies of 32 mJ a single ignition site was observed. Overall fuel lean flames were observed to locally extinguish, however both stoichiometric and fuel rich flames were ignited. The minimum ignition energy was found to depend on the likelihood of a droplet existing at the focus of the laser beam.3

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“…For ethanol and isooctane sprays, Hayashi and Kumagai (1975) [6] and Hayashi et al (1976) [7] reported velocity enhancement for rich sprays, and for lean sprays with large droplets. But, Ballal and Lefebvre (1981) [9] for isooctane, and Myers and Lefebvre (1986) [3] with six different fuels, did not observe the enhancement effect for lean sprays.…”

Section: Introductionmentioning

confidence: 99%

“…Let us particularly quote the works by Cekalin (1961) [4] and by Mizutani and Nakajima (1973) [5-a, 5-b], who added kerosene droplets to a propane air mixture and saw an increase in propagation speed. We also have to mention the pioneering works of Hayashi and Kumagai (1975) [6] and Hayashi et al (1976) [7], who used a Wilson cloud chamber to produce a nearly monodisperse spray. For polydisperse kerosene sprays, Polymeropoulos and Das (1975) [8] observed that burning velocity reaches a maximum for a certain domain of droplet size.…”

Section: Introductionmentioning

confidence: 99%