Understanding Soot Emission in Bluff Body Ethylene Flame under the Same Configuration

Abstract

A bluff body ethylene flame is studied through computational simulation in order to understand the evolution of soot throughout the turbulent flow field. The flame consists of three regions: recirculation zone, neck, and the jet-like region. Each region has a different scalar dissipation rate and mixture fraction that influence soot formation and growth. Using the integrated Large Eddy Simulation (LES) model developed by Mueller et al. [2], the entirety of the flame is simulated. Although the recirculation zone is still in a transient phase, instantaneous images of soot volume fraction, number density, and particle diameter show soot evolution that supports past experimental observations and current understanding of soot. Upstream in the recirculation zone, there is a high concentration of large soot particles, but the number density is low. These characteristics indicate coagulation, condensation, and surface growth to be at work. Downstream at the neck, there are minimal changes in soot due to high dissipation rate. In the jet-like region, soot begins to form and grow given the long residence time and diminishing scalar dissipation rate. An increase is noted across the three soot parameters. The jet-like region behaves like the Delft III flame, but with an increase in turbulence that breaks off the flow into many small mixture fraction pockets. Preliminary results of the recirculation zone confirms previous findings: Source terms for acetylene-based growth are much higher than that for PAH-based growth, although the values are an order of magnitude smaller as the recirculation zone is not fully developed. In terms of intermittency, the neck and the jet-like region have high soot intermittency as predicted. The recirculation zone also appears to be highly intermittent in the time-average image but it is believed to be a consequence of the underrated soot source terms.