Impact of Evaporation Models and Droplet Size on Auto-ignition and Lift-off Height in a Spray Jet Flame

ABSTRACT

In the current work, the jet spray flame reflecting an experimental configuration of a piloted spray burner with a vitiated hot co-flow is modeled using large eddy simulation (LES). We consider a diluted polydisperse spray, which evaporates and diffuses from a fuel-rich spray core, mixes with an oxidizer and eventually auto-ignites. The objective of the present study is to assess an influence of evaporation models and mean droplet diameter on the combustion process in a lifted flame, starting from an auto-ignition up to the flame stabilization. Three evaporation models are involved, i.e., (i) “D² law”, (ii) infinite conductivity model with Stefan flow effect, and (iii) infinite conductivity non-equilibrium evaporation model based on a Langmuir-Knudsen law. An in-house high-order numerical code was used to carry out the computations. The continuous gas phase is modeled in the Eulerian coordinate system while the spray droplets are tracked in space in the Lagrangian manner. The obtained results show that in a stable combustion regime a flame size and liftoff height only slightly depend on evaporation model. However, in general, it is observed that the spray size can affect the flame structure to a much larger extent. On the other hand, for bigger droplets differences in initial size distribution cause only small flame movement downstream or upstream toward the fuel nozzle. For auto-ignition and initial flame development phase it is observed that both the droplets size and evaporation model affect the obtained results significantly.

KEYWORDS:

• LES
• turbulent combustion
• spray
• evaporation models

Acknowledgments

This work was supported by grant 2015/17/B/ST8/03217 (National Science Centre, Poland) and statutory funds BS/PB-1-103-3010/11/P. PL-Grid infrastructure was used to carry out the computations.