A flamelet LES of turbulent dense spray flame using a detailed high-resolution VOF simulation of liquid fuel atomization

Abstract

A numerical framework used to model dense spray flames is proposed. In this framework, the liquid fuel (acetone) atomization is solved by a detailed high-resolution VOF simulation, and the Eulerian components of liquid droplets are transformed into Lagrangian droplets, which are stored in a database at a certain downstream cross-section. Then, the combustion process is solved by a LES/FPV (flamelet progress variable) adopting the pre-stored database of Lagrangian droplets (i.e., the position, size, and velocity of each droplet) as the inlet boundary conditions. This framework is a one-way coupling between a VOF simulation and a combustion simulation. The validity of this approach is investigated by comparing the computations with the experiments of the Sydney Piloted Needle Spray Burner. The VOF simulation shows that the volume flux of the droplets at the nozzle exit fluctuates both temporally and spatially and the larger droplets tend to be located away from the center axis compared to the small droplets. The computed breakup length is in good agreement with the empirical correlation. In the database of the Lagrangian droplets for the LES/FPV of spray flames, the location of the sampling cross-section, the sampling time, and the threshold value for Eulerian–Lagrangian (E-L) transformation strongly affect the properties of the Lagrangian droplets, and are critical for the successful use of the LES/FPV. Two spray flames with different recess distances are computed using their optimal pre-stored droplets databases and both show generally good agreement with the experiments in terms of the gas temperature and droplet size distributions. The spray flame with a longer recess distance, which is more representative of a dilute spray, is considered to have a longer and wider premixed core than that with a shorter recess distance representing a dense spray. The discrepancy in the prediction of denser spray flames becomes more evident leading to over-predictions of gas temperature further downstream. Reasons for this behavior are discussed in the text.