Large-Eddy Simulation of Interactions Between a Reacting Jet and Evaporating Droplets

Abstract

Large-eddy simulation of a turbulent reactive jet with and without evaporating droplets is performed to investigate the interactions among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian-Lagrangian approach is used for the gas-liquid flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions, dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale similarity model. Various representative cases with different initial droplet sizes (St 0 ) and mass loading ratios (MLR) have been simulated, along with a case without droplets. It is found that compared with the bigger, slow responding droplets (St 0 =16), smaller droplets (St 0 =1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak temperature and intensity of temperature fluctuations are found to be reduced in all the droplet cases, to a varying extent depending on the droplet properties. Detailed analysis on the contributions of respective terms in a transport equation for gridscale kinetic energy (GSKE) shows that the droplet evaporation effect on GSKE is small, while the droplet momentum effect depends on St 0 . When the MLR is sufficiently high, the bigger (St 0 =16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures. On the other hand, the turbulence level is found to be lower in the droplet cases than in the pure flame case, due to the dissipative droplet dynamic effect.

Water Spray Dynamics in Fire Flows

“…Hence, the current position is obtained by velocity integration over τ ,1 ! " # $ ; expression for the constant p 1 is obtained upon relevant non-dimensionalisation of the dimensional counterpart of (8).…”

“…The studies [5,7] present relevant, but not directly related to the topic of the present paper studies, namely on multiphase Radiative Transfer Equation [5] and on water film flows over solid surfaces [7]. In the light of what has been just said about the studies [4,6] the best progress, in the author opinion, has been made by Luo and co-workers, e.g [8]. The major advantage of their approach is high accuracy of numerical treatment of turbulence in multiphase systems with the LES methodology.…”

“…[2,3]. There are several exceptions, most notable being reported in the studies [4][5][6][7][8]. Nmira et al, [4] porpose Eulerian-Eulerian model for multiphase spray flows in fire applications.…”

“…Not surprisingly, to the best of our knowledge, models comparable in accuracy to [8] are not used in fire engineering practice. It is clear that further fundamental developments of CFD models for fire suppression engineering are very much hindered by aforementioned high costs and efforts required for a descent progress in subgrid-scale suppression model development.…”

Detailed characteristics of drop-laden mixing layers: Large eddy simulation predictions compared to direct numerical simulation

“…[6][7][8][9][10] LES, in which only the large scales must be resolved, is a promising methodology for situations not accessible in DNS because LES does not have Re value limitations in turbulent free shear flows. As such, LES has been recently used to study various aspects of evaporating and/or chemically reacting sprays; [11][12][13][14] in all LES involving combustion ͑e.g., Ref. 15͒, prediction of mixing is a prerequisite for that of accurate heat release.…”