Numerical simulation of axisymmetric laminar diffusion flames with soot

Detailed numerical modeling of combustion phenomena, soot formation, and radiation is an active area of research. In this work a general-purpose, pressure-based, finite volume code for modeling laminar diffusion flames has been incorporated into the CFD code OpenFOAM. The code uses a mixture-averaged model for the calculation of transport coefficients, and can be used to perform detailed modeling of multi-dimensional laminar flames using realistic molecular transport, and with detailed chemical mechanisms containing hundreds of chemical species and reactions. Two soot models have been incorporated into the code: a semi-empirical two-equation model, as well as a detailed Method of Moments with Interpolative Closure (MOMIC). An emission-only, optically-thin radiation model has also been included in the code to account for the radiative heat loss, and sophisticated radiation models with detailed calculations of spectral properties and radiative intensity have also been included. The flame code showed excellent scalability on massively distributed, high-performance computer systems. The code has been validated by modeling four axisymmetric, co-flowing laminar diffusion flames, and the results have been found to be mostly within experimental uncertainty, and comparable to results reported in the literature for the same and similar configurations. A number of parametric studies to study the effects of detailed gas-phase chemistry, soot models and radiation have also been performed on these flame configurations. It has been found that the flames considered in this work are all optically thin, and so the simple, emission-only, optically-thin radiation model can be used to model these flames with good accuracy and a reasonable computational effort. In particular, the detailed radiation models increase the computational cost by two orders of magnitude, and thus their applicability in a detailed calculation may be limited. It was found that the two-equation soot model used in conjunction with a gasphase mechanism that adequately describes the combustion of C2 hydrocarbons iii produces results in close agreement with experimental data for a 1-bar ethylene-air flame, a 10 bar methane-air flame, as well as an ethane-air flame at 10 bar. The detailed MOMIC soot model requires the use of a larger, more detailed gas-phase chemical mechanism containing polycyclic aromatic hydrocarbons (PAH) with four rings, and thus the computational cost associated with the MOMIC soot model is significantly higher. The detailed model was used to model the flames, and computed soot levels were within a factor of two of the experimental values, which is typically considered good agreement considering the complex physics involved. The last flame studied using both the soot models was a N2-diluted ethylene-air flame, in which the predicted values of major gas-phase species were seen to be close to the experimental values, but the soot levels were off by an order of magnitude. Notwithstanding the lack of agreement with measurements for this flame, the flame solver with the soot models was demonstrated to be a robust, scalable, and general code with potential applications to a variety of laminar flames in the non-premixed, partially premixed and premixed regimes.