Soot Evolution in Turbulent Reacting Flows: Refining the Mesh for Bluff Body Ethylene Flames

Abstract

Soot particles are the carbon-based nanoparticles that result from fuel-rich combustion. These particles have negative health and environmental effects, and, consequently, their emissions are regulated. Following past trends, these regulations will likely become more strict and require companies to produce lower emitting engines. Designing these low-emission systems will depend on combustion modeling, but soot modeling is especially difficult due to soot’s complex formation and degeneration. In addition to accurate models, appropriate methods for mapping the solution space must also be used. This thesis compares the simulation results of both a fine and coarse mesh to see if altering the solution space has any appreciable effect on the results of the ethylene bluff body, non-premixed flames. The long term goal of this research is to develop a modeling approach leading to more efficient engines. In general, the modeling framework of Large Eddy Simulation (LES) is applied in conjunction with a soot model based on a Hybrid Method of Moments (HMOM), whose closure is achieved through a presumed subfilter PDF approach. The models used within this thesis for describing the soot particle formation and interaction come from Mueller’s work. In order to compare the simulation results of different meshes, several steps are taken. The first step is to create a flamelet table for each fuel that can be referenced when computing the evolution of the flame. This flamelet table is created using Flamemaster code. The fine and coarse mesh are defined using a polynomial expression to concentrate computational power on reactive zones such as those around the fuel jet. After running the simulation, using NGA, on the two different meshes long enough for the transient turbulent effect of ignition to run out of the solution space, statistics are collected and measured against the experimental results given by Mueller et al. This thesis concludes that increasing the discretization space of the area around the fuel jet leads to more accurate data trends when compared to current knowledge of sooting flame structure. The current simulations have not yet developed into a steady state so the soot volume fractions are not directly comparable to the experimental statistics. The hydrogen enriched ethylene fuel yields less polycyclic aromatic hydrocarbons (PAH) than pure ethylene for a given dissipation rate and mixture fraction, though flame temperature is unaffected, showing that PAH formation is of a chemical nature. This is important because it verifies the flamelet tables and also may effect future conclusions drawn between steady state comparisons of the soot volume fractions in each of these flames, ultimately helping the overall understanding.