Somesh Roy

A Systematic Comparison of Detailed Soot Models and Gas-Phase Chemical Mechanisms in Laminar Premixed Flames

ABSTRACT This research is part of an ongoing assessment to identify the most promising soot models for applications to laboratory turbulent flames, and ultimately to practical combustion systems. To that end, it is of interest to compare soot model simulation results with experiments over a broad range of conditions. Here five steady, one-dimensional, laminar, premixed ethylene flames have been targeted that cover a range of C/O ratios (0.68 to 0.88), pressures (1 atm to 10 bar), and peak soot levels (0.03 to 3.2 ppm). Seven different chemical mechanisms have been considered. For each mechanism, results from several soot model variants are compared. Two different approaches for soot aerosol dynamics are used: a discrete sectional method (DSM), and a method of moments with interpolative closure (MOMIC). DSM and MOMIC results are also compared with those from a widely used semi-empirical two-equation soot model. The main figure-of-merit is the ratio of computed-to-measured peak soot volume fraction. The computed soot volume fraction is most sensitive to variations in the surface chemistry scheme. Sensitivities to variations in model configurations are reduced at high pressure. The DSM-based models yield slightly lower soot volume fraction and slightly larger particle size compared to the corresponding MOMIC-based models. A modified semi-empirical two-equation model produced a good match to experiment for all five flames. Particle sizes and number densities from the models are discussed for a low-sooting flame where such data are available. The DSM-based models produce bimodal particle size distributions that are qualitatively consistent with those observed experimentally. The best results overall are obtained using an underlying chemical mechanism that incorporates recent understanding of polycyclic aromatic hydrocarbons kinetics. There is no clear advantage of DSM over MOMIC in predicting global quantities, such as soot volume fraction, particle number density, and average particle size, while the computational cost of DSM is significantly higher than that of MOMIC.

Development of High Fidelity Soot Aerosol Dynamics Models using Method of Moments with Interpolative Closure

The method of moments with interpolative closure (MOMIC) for soot formation and growth provides a detailed modeling framework maintaining a good balance in generality, accuracy, robustness, and computational efficiency. This study presents several computational issues in the development and implementation of the MOMIC-based soot modeling for direct numerical simulations (DNS). The issues of concern include a wide dynamic range of numbers, choice of normalization, high effective Schmidt number of soot particles, and realizability of the soot particle size distribution function (PSDF). These problems are not unique to DNS, but they are often exacerbated by the high-order numerical schemes used in DNS. Four specific issues are discussed in this article: the treatment of soot diffusion, choice of interpolation scheme for MOMIC, an approach to deal with strongly oxidizing environments, and realizability of the PSDF. General, robust, and stable approaches are sought to address these issues, minimizing the use of ad hoc treatments such as clipping. The solutions proposed and demonstrated here are being applied to generate new physical insight into complex turbulence-chemistry-soot-radiation interactions in turbulent reacting flows using DNS. Copyright 2014 American Association for Aerosol Research

Dynamics of flow–soot interaction in wrinkled non-premixed ethylene–air flames

A two-dimensional simulation of a non-premixed ethylene–air flame was conducted by employing a detailed gas-phase reaction mechanism considering polycyclic aromatic hydrocarbons, an aerosol-dynamics-based soot model using a method of moments with interpolative closure, and a grey gas and soot radiation model using the discrete transfer method. Interaction of the sooting flame with a prescribed decaying random velocity field was investigated, with a primary interest in the effects of velocity fluctuations on the flame structure and the associated soot formation process for a fuel-strip configuration and a composition with mature soot growth. The temporally evolving simulation revealed a multi-layered soot formation process within the flame, at a level of detail not properly described by previous studies based on simplified soot models utilizing acetylene or naphthalene precursors for initial soot inception. The overall effect of the flame topology on the soot formation was found to be consistent with previous experimental studies, while a unique behaviour of localised strong oxidation was also noted. The imposed velocity fluctuations led to an increase of the scalar dissipation rate in the sooting zone, causing a net suppression in the soot production rate. Considering the complex structure of the soot formation layer, the effects of the imposed fluctuations vary depending on the individual soot reactions. For the conditions under study, the soot oxidation reaction was identified as the most sensitive to the fluctuations and was mainly responsible for the local suppression of the net soot production.

Detailed computational modeling of laminar and turbulent sooting flames

This study reports development and validation of two parallel flame solvers with soot models based on the open-source computation fluid dynamics (CFD) toolbox code OpenFOAM. First, a laminar flame solver is developed and validated against experimental data. A semi-empirical two-equation soot model and a detailed soot model using a method of moments with interpolative closure (MOMIC) are implemented in the laminar flame solver. An optically thin radiation model including gray soot radiation is also implemented. Preliminary results using these models show good agreement with experimental data for the laminar axisymmetric diffusion flame studied. Second, a turbulent flame solver is developed using Reynolds-averaged equations and transported probability density function (tPDF) method. The MOMIC soot model is implemented on this turbulent solver. A sophisticated photon Monte-Carlo (PMC) model with line-by-line spectral radiation database for modeling is also implemented on the turbulent solver. The validation of the turbulent solver is under progress. Both the solvers show good scalability for a moderate-sized chemical mechanism, and can be expected to scale even more strongly when larger chemical mechanisms are used.