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 (ϕd), 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 ϕd > 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 Damkohler 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. The statistical behaviours of the flame propagation and mode of combustion have been analysed in detail and detailed physical explanations have been provided for the observed behaviour.
Highlights
Flame propagation into turbulent droplet-laden mixtures has a number of important engineering applications including Internal Combustion (IC) engines (e.g. Direct Injection (DI) and Compression Ignition (CI) engines) [1, 2], gas turbines [2, 3], and hazard prediction and control [4]
Lawes and Saat [13] showed that the equivalence ratio of the gaseous fuel-air mixture which develops due to the evaporation process can have significant effects on both early and late stages of flame propagation, but, once again, these effects are diminished as a result of increasing turbulent velocity fluctuations
These regions arise both due to the presence of a droplet in the immediate vicinity of the reaction front, in which case the locally high temperature leads to rapid evaporation and augmentation of the gaseous fuel mass fraction, and due to post-flame evaporation, in which case the local fuel mass fraction is augmented by the back-diffusion of the newly-evaporated gaseous fuel from the burned gas region
Summary
Flame propagation into turbulent droplet-laden mixtures has a number of important engineering applications including Internal Combustion (IC) engines (e.g. Direct Injection (DI) and Compression Ignition (CI) engines) [1, 2], gas turbines [2, 3], and hazard prediction and control [4]. The analysis of Neophytou and Mastorakos [27] indicated that the high values of burning velocity and chemical reaction rate are obtained for ‘small’ droplets with overall equivalence ratio φov either less than or equal to 1.0 (i.e. φov ≤ 1.0) and for ‘large’ droplets with overall equivalence ratio greater than one (i.e. φov > 1.0) It has been found by Neophytou and Mastorakos [27] that combustion can be sustained even for φov values which are greater than the rich-flammability limit for gaseous fuel-air mixture. The simulation results by Fujita et al [23] and Watanabe et al [30] demonstrated that radiative heat transfer can play an important role in spray combustion and flame propagation under some conditions
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