Fundamental Understanding of Turbulent Combustion in Droplet-Laden Mixtures Using Direct Numerical Simulations

Abstract

The flame propagation in droplet-laden mixtures is of considerable importance in automotive engines, gas turbines, and accidental explosions. Despite the practical importance of turbulent combustion of droplet-laden mixtures, it remains one of the most challenging topics in thermo-fluid mechanics due to the involvement of complex interactions of evaporation, heat and mass transfer, fluid dynamics, and combustion thermochemistry. Thorough knowledge of these interactions, which occur over a wide range of scales, is necessary for fundamental understanding and modelling of turbulent spray flames. In this thesis, three dimensional compressible Direct Numerical Simulations (DNS) of spherically expanding and V-shaped flames propagating in droplet mists are considered for a fundamental physical understanding of the flame structure and flame speed statistics in turbulent spray flames. Simulations with modified single-step Arrhenius type chemical mechanism have been conducted for a range of different droplet diameters, overall equivalence ratios, and turbulence intensities. The influence of liquid droplets has been investigated by comparing the statistics for spray flames to those for the corresponding gaseous premixed spherically expanding flames with statistically similar initial turbulent flow conditions. It has been found that flame-droplet interaction promotes dropletinduced flame wrinkling for laminar flame kernels, and this strengthens with increasing overall equivalence ratio and droplet diameter. However, the effects of droplet-induced flame wrinkling cannot be readily distinguished from flame wrinkling due to fluid motion for turbulent spherically expanding spray flames. The combustion has been found to take place predominantly under fuel-lean mode in comparison to the overall equivalence ratio for all droplet sizes and this tendency strengthens with increasing droplet diameter due to slow evaporation of large droplets. Furthermore, increasing turbulence intensities enhances the availability of fuel-lean mixture. The statistics of the Surface Density Function (SDF = magnitude of the reaction progress variable gradient) and the strain rates, which affect the behaviour of SDF have been analysed for spherically expanding spray flames. Flame thickening has been observed for large droplets and at high turbulence intensities due to the predominance of fuel-lean combustion. Droplet size and turbulence intensity significantly affect the behaviour of scalar gradients and the infinitesimal distance between non-material surfaces. The flame propagation behaviour in droplet-laden mixtures has been analysed in terms of the statistics of density-weighted dis- placement and consumption speeds. Flame topologies associated with flame self-interaction events have been discussed along with the small-scale scalar geometries of flame isosurfaces. The presence of droplets, turbulence intensity and droplet diameter have been found to considerably alter the distributions of flame topologies. Additionally, flame-droplet interactions have been investigated in detail based on the source terms associated with two-phase coupling arising from droplet evaporation in various gaseous carrier phase transport equations and the modelling implications of the statistical behaviour of flame-droplet interactions have been addressed. Furthermore, hypothetical inertialess droplet motion is considered to identify the influence of droplet inertia on the combustion characteristics and the evolution of the flame surface area. The number density of droplets within the flame is greater for the inertial droplet cases than the corresponding inertialess droplet cases and this leads to higher availability of obtaining stoichiometric mixture in the flame. Finally, a comparison between the spherically expanding and V-shaped spray flames reveals that flame curvature, density-weighted displacement speed, and consumption speed varies considerably with droplet diameter in the case of spherically expanding spray flame cases, whereas the effects of droplet diameter are relatively weaker in V-flames. Simulations of V-shaped flames propagating in droplet mists for different mean inflow velocities indicate that reacting mixture composition significantly varies with the mean inflow velocity which also plays an important role in determining the flame structure and burning rate statistics.