Analysis of segregation and bifurcation in turbulent spray flames: A 3D counterflow configuration

Abstract

International audienceThe understanding of spray combustion processes is of primary importance, as it is encountered in a wide range of industrial applications. In the present work, mesoscale-resolved simulations of a 3D turbulent counterflow spray configuration are conducted. Primary focus is on examining the effect of the coupling between turbulence, evap- oration, mixing, and combustion. By considering different initial droplet diameters and through comparisons with turbulent and laminar configurations at the same operating condition, it is shown that preferential concentration can lead to conditions of locally high mixture-fraction composition. In addition, local variability in strain rate and droplet diameter introduces a bifurcation of the spray flame. This bifurcation consists of spray flame structures exhibiting single-reaction or double-reaction structures. It is shown that this bimodal behavior is linked to the existence of a hysteresis in the laminar spray flame structure for droplet diameter variations, as well as the occurrence of a bifur- cation for strain rate variations. These results have direct implications for flamelet-based tabulation methods, since identifying the appropriate flamelet structure in turbulent spray flames would require informations about boundary conditions and the flamelet history

On the generalisation of the mixture fraction to a monotonic mixing-describing variable for the flamelet formulation of spray flames

“…10(b) as long as the strain rate remains below 350 s −1 . This type of bifurcation was also observed in [23], where two branches were identified without the occurrence of an hysteresis 1 . It may also be noted that the temperature is overestimated for the highest values of the strain rate when solving the system in η-space.…”

“…For all three cases considered, the first temperature peak is located on the gas side at stoichiometric condition. However, with increasing initial droplet diameter, a second temperature peak is formed on the spray side, which identifies the transition from a single-reaction to a double-reaction flame structure for large droplets, as observed in [16,23]. Moreover, by comparing profiles of Z g and Z l with the analytic solution, the diffusive contribution on the spray side can be clearly recognized.…”

“…The behavior of the flame to a variation in the droplet diameter strongly depends on the evaporation model and the reaction chemistry. Vié et al [23] identified a hysteresis for droplet diameter variations, which was characterized by a doublebranch structure. Following this analysis, the mean flamelet temperature is used as a robust metric to distinguish between single-and double-reaction structures:…”

“…Experiments in counterflow flames have been performed to examine extinction behavior of mono-and polydisperse spray flames through strain variation and vortex interaction [21,22]. More recently, bistable flame structures of laminar flames were considered for examining the bifurcation in three-dimensional turbulent counterflow spray flames [23]. As such, these studies demonstrated that the structure of spray flames is of fundamental relevance for a wide range of operating regimes.…”

Numerical study of the multi-flame structure in spray combustion

“…Most of them are on a simplified counterflow configuration, including early studies by Continillo and Sirignano [4] , Gutheil and Sirignano [9] , and more recently by Olguin and Gutheil [10] . Vié et al [11] performed a DNS study of a 3D spray counterflow configuration. It was found that the structure is strongly related to droplet size and strain rate.…”