Effects of turbulence on dispersion and vaporization of droplets in spray combustion

Abstract

Interactions between liquid-fuel spray combustion and turbulent fluctuations were investigated in a grid-generated turbulent flow. Based on experimental results, a novel formation mechanism of the dense droplet groups was proposed. The model is valid for the sprays in which liquid droplets can follow turbulent flow fluctuations exactly. Adopting some simplifying assumptions, trajectories of droplets can be described by an equation with only one parameter, the centrifugal Stokes number, defined as the ratio of the centrifugal and the drag forces acting on the droplet. When the values above the parameters increase, droplets move away from the vortex center, and their trajectories approach each other. This phenomenon is believed to be an important part of the mechanism responsible for formation of the dense droplet groups and nonuniformity of droplet concentration observed in almost every practical spray. Modification of droplet spatial distribution due to variation of flow parameters strongly affects temperature and species concentration distributions. Observed changes of the droplet size distribution were modest compared with those of the droplet spatial distribution. This indicates that for dense sprays, the turbulence effect on non-uniformity of spatial distribution is of much greater importance than the effect of turbulence on the vaporization rates of individual droplets. Another interesting aspect of the study was the coexistence of two contrary effects caused by turbulence modification. The change of flow properties that resulted in faster vaporization of droplets made the mixing of fuel vapors with ambient air slower. Thus, a spray flame with reduced droplet penetration distance but longer overall length was established.