Statistical Analysis of Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Study

Abstract

Turbulent combustion of mono-disperse droplet-mist has been analysed based on three-dimensional Direct Numerical Simulations (DNS) in canonical configuration under decaying turbulence for a range of different values of droplet equivalence ratio (φ d ), droplet diameter (a d ) and root-mean-square value of turbulent velocity (u ). The fuel is supplied in liquid phase and the evaporation of droplets gives rise to gaseous fuel for the flame propagation into the droplet-mist. It has been found that initial droplet diameter, turbulence intensity and droplet equivalence ratio can have significant influences on the volume-integrated burning rate, flame surface area and burning rate per unit area. The droplets are found to evaporate predominantly in the preheat zone, but some droplets penetrate the flame front, reaching the burned gas side where they evaporate and some of the resulting fuel vapour diffuses back towards the flame front. 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 and this predominantly fuel-lean mode of combustion exhibits the attributes of low Damköhler number combustion and gives rise to thickening of flame with increasing droplet diameter. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion and the relative contribution of non-premixed combustion to overall heat release increases with increasing droplet size. Combust (2016) 96:573-607 of the flame propagation and mode of combustion have been analysed in detail and detailed physical explanations have been provided for the observed behaviour.

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

“…This behaviour arises due to incomplete evaporation of the droplets, which leads to a gaseous equivalence ratio value much lower than the overall equivalence ratio. Recently, Wacks et al [19] extended the analysis by Neophytou and Mastorakos [14] for turbulent flames by carrying out three-dimensional compressible Direct Numerical Simulations (DNS) of freely propagating turbulent flame propagation into droplet-laden mixtures. Wacks et al [19] used the DNS data to analyse the effects of initial droplet diameter, a d , droplet equivalence ratio, φ d , and turbulence intensity, u , on the flame structure and the statistical behaviour of flame propagation into droplet-laden mixtures.…”

“…Recently, Wacks et al [19] extended the analysis by Neophytou and Mastorakos [14] for turbulent flames by carrying out three-dimensional compressible Direct Numerical Simulations (DNS) of freely propagating turbulent flame propagation into droplet-laden mixtures. Wacks et al [19] used the DNS data to analyse the effects of initial droplet diameter, a d , droplet equivalence ratio, φ d , and turbulence intensity, u , on the flame structure and the statistical behaviour of flame propagation into droplet-laden mixtures. It has been found based on numerical results that the resulting spray flame structure exhibits a considerable extent of premixed combustion in addition to an expected diffusion mode of burning [15][16][17][18][19].…”

“…Wacks et al [19] used the DNS data to analyse the effects of initial droplet diameter, a d , droplet equivalence ratio, φ d , and turbulence intensity, u , on the flame structure and the statistical behaviour of flame propagation into droplet-laden mixtures. It has been found based on numerical results that the resulting spray flame structure exhibits a considerable extent of premixed combustion in addition to an expected diffusion mode of burning [15][16][17][18][19]. The extent of premixed mode of combustion has been found to increase with decreasing droplet diameter [15][16][17][18][19].…”

“…It has been found based on numerical results that the resulting spray flame structure exhibits a considerable extent of premixed combustion in addition to an expected diffusion mode of burning [15][16][17][18][19]. The extent of premixed mode of combustion has been found to increase with decreasing droplet diameter [15][16][17][18][19]. Moreover, the burned gas temperature can be different from a gaseous diffusion flame due to the combined effects of latent heat of evaporation and premixed combustion [15][16][17][18][19].…”

“…The extent of premixed mode of combustion has been found to increase with decreasing droplet diameter [15][16][17][18][19]. Moreover, the burned gas temperature can be different from a gaseous diffusion flame due to the combined effects of latent heat of evaporation and premixed combustion [15][16][17][18][19]. Nakamura et al [15] also argued that the group combustion number, which is defined based on a homogenous field of unburned droplets and air mixture, is not sufficient to characterize the combustion process in spray flames due to the premixed-like combustion.…”