Simulation of Soot Volume Fraction and Size in High-Pressure Lifted Flames Using Detailed Reaction Mechanisms

Abstract

A simulation study of high-pressure lifted flames in a constant-volume chamber has been conducted using detailed reaction mechanisms in CFD to investigate ignition times, flame lift-off lengths, soot production, and fuel effects. The fuels considered include n-heptane, a two-component surrogate fuel (SR), conventional U.S. No. 2 diesel (D2), and world-averaged jet fuel (Jet-A). Conditions for the flames are those of the experiments performed at Sandia National Laboratories; the n-heptane flame is labeled Spray H [Idicheria and Pickett, SAE 2005-01-3834], and the conditions for all other fuels studied are labeled Spray A [Kook and Pickett, SAE 2012-01-0678]. 3D CFD simulations have been performed using the FORTÉ CFD package. Complex fuels D2 and Jet-A have been modeled using multi-component surrogates. Detailed reaction mechanisms for fuel combustion and emissions formation have been used in the simulations. The size of the fuel mechanisms varied from 326 species to 1000 species for the different fuels. For soot predictions, two different models were used in the simulations: a detailed soot-surface mechanism and a seven-step phenomenological soot model. Both soot models were coupled with the fuel mechanism precursor predictions that included aromatics from benzene to pyrene. While using the detailed soot-surface mechanism, particulate (PM) size and number density were determined using the Method of Moments, which is implemented in the CFD software to calculate particle size distribution characteristics. Results show excellent prediction of flame location and ignition for all fuels. Location and magnitude of soot fractions in the various flames show good agreement with the published data. Both the phenomenological soot model and the detailed soot-surface mechanism estimated comparable soot fractions in all flames. In addition, PM size information was predicted using the detailed soot-surface mechanism. Impacts of fuel, temperature, pressure, and oxygen concentrations on combustion and soot fractions have been captured by the simulations.