Iliyana Naydenova

DETAILED KINETIC MODELING OF SOOT FORMATION DURING SHOCK-TUBE PYROLYSIS OF C6H6: DIRECT COMPARISON WITH THE RESULTS OF TIME-RESOLVED LASER-INDUCED INCANDESCENCE (LII) AND CW-LASER EXTINCTION MEASUREMENTS

The results of calculations of the main parameters of the soot formation process (τ, k f, SY, and r m) carried out with the use of the detailed kinetic model of soot formation are compared with the experimental measurements of these parameters by the continuous-wave (CW)-laser extinction technique and by the time-resolved laser-induced incandescence (LII) method during C6H6 pyrolysis behind reflected shock waves. The detailed kinetic model of soot formation that is developed incorporates the gas-phase mechanisms of acetylene pyrolysis and the mechanisms of formation of polyaromatic hydrocarbons, polyyne molecules, and pure carbon clusters. It combines the H abstraction/C2H2 addition and polyyne pathways of the soot formation process. The formation, growth, and coagulation of soot precursors and soot particles are described within the framework of the discrete Galerkin technique based on an error-controlled expansion of the size distribution function of heterogeneous species into the orthogonal polynomials of a discrete variable (in particular, the number of monomers in the heterogeneous particle) that makes it possible to preserve a discrete character of any elementary transformations of heterogeneous particles and to describe them as elementary chemical reactions for the heterogeneous particles of all sizes. The comparison of the calculations with the experimental measurements of the induction time τ, observable rate of soot particle growth, k f, and soot yield SY by the CW-laser extinction method in the pyrolysis of benzene/argon mixtures in shock-tube experiments clearly demonstrates that the coincidence is quantitatively good for all the main parameters of soot formation. A particular difference between the values of the mean soot particle radius r m experimentally measured by the time-resolved LII technique and calculated with the help of the detailed kinetic model is observed at the low and high temperatures. The results presented demonstrate the current level of the predictive capabilities of the detailed kinetic model of soot formation and the reliability of the time-resolved LII technique for the quantitative determination of the soot particle sizes.

A Simplified Model for Soot Formation in Gas Turbine Combustion Chambers

Soot resulting from combustion processes is known to have a negative impact on health and environment. Soot also may lead to material damages especially in gas turbines. Therefore, a deeper insight in the processes leading to the formation and consumption of the soot precursors and soot particles is needed. For describing these processes a reaction kinetical model is needed representing the different steps. The processes happening in the gas phase are described by elementary chemical reactions, whereas for the particle phase a more complex formalism is needed due to the enormous number of different particles possible. In this work the processes in the particle phase are represented by the detailed soot model. A kinetical description of the processes is developed along with a mathematical representation of the model. This is done for shock tube conditions that can be assumed to be spatially homogeneous, allowing to focus on models of the chemical processes. In a next step this model is implemented for laminar flame conditions. Here additional transport phenomena are considered which helps improving gas phase and particle reaction models for representing soot formation in practical combustion devices. Besides giving a deeper insight into soot formation processes, the detailed soot model is needed for calibrating and adjusting a simplified soot model. This is used for simulating soot formation in complex technical systems like gas turbines.

Lean Premixed Flames: A Direct Numerical Simulation Study of the Effect of Lewis Number at Large Scale Turbulence

The paper presents results from three-dimensional Direct Numerical Simulations of turbulent lean premixed hydrogen, propane and methane flames. The three fuels studied have Lewis numbers which range from small values (hydrogen) through near-unity values (methane) up to values larger than unity (propane). The computations make use of reduced chemical mechanisms with species number ranging from 9 to 28. It has been found that for the present time-explicit numerical method, the CPU-time scales non-linearly – with the power of 1.6 – with the number of chemical species.A new method to produce turbulent vortexes directly in the computational domain is presented. The vortexes are produced through localized forcing in physical space. This allows the full control of the length-scale of the vortexes as well as of their spatial and time distribution. The parallelization of the method requires no programming efforts, i.e. the parallelization follows automatically. The set of vortexes created for this particular investigation is identical for all flames studied: two methane, one hydrogen and one propane flames. Despite the identical turbulent vortex set, the flames of the different fuels react quite differently to it, exhibiting qualitatively different combustion regimes as well as different growth of the flame front area and of the consumption flame speed with time.The computations are carried out on the CRAY XE6 HERMIT supercomputer at the High Performance Computing Center Stuttgart (HLRS). Numerical issues and performance results for the three chemical mechanisms, corresponding to the three fuels used, are presented and discussed.