Kinetic modeling of soot formation in premixed burner-stabilized stagnation ethylene flames at heavily sooting condition

A detailed kinetic mechanism of soot formation and oxidation is revised and extended to include temperature-dependent collision efficiencies. The collision efficiency for various particle size is studied and compared with experimental data and molecular dynamics simulations for the PAH dimerization where the experimental data are not available. This revised kinetic model is validated in comparison with the premixed burner-stabilized stagnation ethylene flames at heavily sooting conditions. The results showed that quasi-one-dimensional numerical simulations can capture the flame structure and predict soot formation quite satisfactorily. The predicted particle size distribution function (PSDF) is in reasonable agreement with experimental results, but the model only partially reproduces the distinct separation between nucleation and coagulation modes observed experimentally. This leads to some discrepancies in the prediction of soot number density, while the predicted soot volume fraction, which is dominated by the large particles of the PSDF, is in generally good agreement with the experimental data. There is an overestimation of the initial soot volume fraction in the flame region close to the burner, which is a consequence of the over-prediction of the amount of young particles. Therefore, the prediction of PAHs formation and their condensation on soot, which controls the nucleation rate, will require further attention. The comparison between the temperature-dependent model and the model neglecting the temperature dependency showed that the temperature-dependent model could improve the prediction of soot number density, which is controlled by small particles

Theoretical and kinetic modelling study of phenol and phenoxy radical decomposition to CO and C5H6/C5H5 in pyrolysis conditions

Bio-oils from biomass fast-pyrolysis are an economically viable solution to reduce carbon footprint [1]. Lignin-derived bio-oils are a complex mixture of oxygenated species, including phenolic compounds such as phenol, anisole, guaiacol, catechol and vanillin (20-30% in weight) [2]. Hence, an accurate characterization of the pyrolysis and combustion kinetics of phenolic species, starting from phenol, is essential to assess the technical viability of these biooils. Furthermore, phenol plays a key role in the mechanism of oxidation of benzene, a building block of PAHs chemistry, precursors of soot and PM [3]. Finally, substituted phenolic species have recently gained attention for their antiknock properties and are being considered as possible octane boosters [4]. Nevertheless, the kinetics of phenol has not been systematically addressed yet, and the available experimental data are limited. Therefore, a theoretical approach for the prediction of accurate kinetics provides a major contribution to improve the current knowledge. This work investigates with ab initio methods the two main decomposition pathways of phenol: 1) the molecular pathway forming C5H6+CO, and 2) the radical pathway forming C6H5O+H. This latter pathway justifies the additional investigation of the decomposition of phenoxy radical (C6H5O) to CO and cyclopentadienyl (C5H5). For a consistent investigation of phenol kinetics, also the H-abstraction reactions from cyclopentadiene are included. The kinetic constants thus obtained are included in the CRECK kinetic model and validated with experimental data

New Dynamic Scale Similarity Based Finite-Rate Combustion Models for LES and a priori DNS Assessment in Non-premixed Jet Flames with High Level of Local Extinction

In this work, the performances of two recently developed finite-rate dynamic scale similarity (SS) sub-grid scale (SGS) combustion models (named DB and DC) for non-premixed turbulent combustion are a priori assessed based on three Direct Numerical Simulation (DNS) databases. These numerical experiments feature temporally evolving syngas jet flames with different Reynolds (Re) numbers (2510, 4487 and 9079), experiencing a high level of local extinction. For comparison purposes, the predicting capability of these models is compared with three classical non-dynamic SS models, namely the scale similarity resolved reaction rate model (SSRRRM or A), the scale similarity filtered reaction rate model (SSFRRM or B), and a SS model derived by the "test filtering" approach (C), as well as an existing dynamic version of SSRRRM (DA). Improvements in the prediction of heat release rates using a new dynamic model DC are observed in high Re flame case. By decreasing Re, dynamic procedures produce results roughly similar to their non-dynamic counterparts. In the lowest Re, the dynamic methods lead to higher errors

A comprehensive CFD model for the biomass pyrolysis

The present work addresses the study of the pyrolysis of biomass particle, with the aim to improve the comprehensive mathematical model of the thermochemical processes involving solids decomposition. A new CFD model for the biomass pyrolysis was developed at the particle scale in order to properly describe the relative role of reaction kinetics and transport phenomena. The model is able to solve the Navier-Stokes equations for both the gas and solid porous phase. The code employs the open-source OpenFOAM® framework to effectively manage the computational meshes and the discretization of fundamental governing equations. The mathematical algorithm is based on the PIMPLE method for transient solver and exploit the operator-splitting technique that allows the separation of the transport and the reactive term in order to handle complex computational geometries minimizing the computational effort. The model was tested with experimental data for both reactive and non-reactive conditions. The code is able to provide correct information about temperature distribution within the particle, gas, tar and char formation rates

The role of chemistry in the oscillating combustion of hydrocarbons : an experimental and theoretical study

The stable operation of low-temperature combustion processes is an open challenge, due to the presence of undesired deviations from steady-state conditions: among them, oscillatory behaviors have been raising significant interest. In this work, the establishment of limit cycles during the combustion of hydrocarbons in a wellstirred reactor was analyzed to investigate the role of chemistry in such phenomena. An experimental investigation of methane oxidation in dilute conditions was carried out, thus creating quasi-isothermal conditions and decoupling kinetic effects from thermal ones. The transient evolution of the mole fractions of the major species was obtained for different dilution levels (0.0025 <= X-CH4 <= 0.025), inlet temperatures (1080K <= T <= 1190K) and equivalence ratios (0.75 <= Phi <= 1). Rate of production analysis and sensitivity analysis on a fundamental kinetic model allowed to identify the role of the dominating recombination reactions, first driving ignition, then causing extinction. A bifurcation analysis provided further insight in the major role of these reactions for the reactor stability. One-parameter continuation allowed to identify a temperature range where a single, unstable solution exists, and where oscillations were actually observed. Multiple unstable states were identified below the upper branch, where the stable (cold) solution is preferred. The role of recombination reactions in determining the width of the unstable region could be captured, and bifurcation analysis showed that, by decreasing their strength, the unstable range was progressively reduced, up to the full disappearance of oscillations. This affected also the oxidation of heavier hydrocarbons, like ethylene. Finally, less dilute conditions were analyzed using propane as fuel: the coupling with heat exchange resulted in multiple Hopf Bifurcations, with the consequent formation of intermediate, stable regions within the instability range in agreement with the experimental observations

Evaporation of multicomponent fuel droplets in buoyancy driven convection

In this work, the evaporation process of multicomponent fuel droplets is analyzed, both from an experimental and numerical point of view. The droplets are hanged on a thin thermocouple against gravity and evaporated in natural convection regime, following the process by means of high speed shadowgraphs. The experimental analyses were performed hierarchically, starting from pure components (n-dodecane and n-hexadecane), then moving to their mixtures. The numerical modeling is performed with the DropletSMOKE++ code, a comprehensive CFD framework for the simulation of 3D evaporating droplets under gravity conditions. The numerical results present a good agreement with the experimental data, especially if compared with the same cased modeled in microgravity conditions. The difference evaporation rate is analyzed as well as the different surface temperature, highlighting the important role of internal and external convection on the evaporation process

Coupling chemical lumping to data-driven optimization for the kinetic modeling of dimethoxymethane (DMM) combustion

The kinetic mechanisms describing the combustion of longer-chain fuels often have limited applicability due to the high number of species involved in their oxidation and decomposition paths. This work proposes a combined methodology for developing compact but accurate kinetic mechanisms of these fuels and applies it to dimethoxymethane (DMM), or oxymethylene ether 1 (OME1). An automatic chemical lumping procedure, performed by grouping structural isomers into pseudospecies, was proposed and applied to a detailed kinetic model of DMM pyrolysis and oxidation, built from state-of-the-art kinetic sub-models. Such a methodology proved particularly efficient in delivering a compact kinetic mechanism, requiring only 11 species instead of 35 to describe DMM sub-chemistry. The obtained lumped kinetic model was then improved through a data-driven optimization procedure, targeting data artificially generated by the reference detailed mechanism. The optimization was performed on the physically-constrained parameters of the modified-Arrhenius rate constants of the controlling reaction steps, identified via local sensitivity analyses. The dissimilarities between the predictions of the detailed and lumped models were minimized using a Curve Matching objective function for a comprehensive and quantitative characterization. Above all, the optimized mechanism was found to behave comparably to the starting detailed one, throughout most of the operating space and target properties (ignition delay times in shock tubes, laminar flame speeds, and speciations in stirred and flow reactors). The successful application of the proposed methodology to the DMM chemistry paves the way for its extensive use in the kinetic modeling of longer OMEs as well as heavier fuels, for which the computational advantages are expected to be even higher

An experimental and kinetic modeling study of NH3 oxidation in a Jet Stirred Reactor

The increasing interest towards renewable, and more sustainable energy sources imposes a widerange analysis of the underlying chemistry, in order to maximize the efficiency of combustion devices and reduce pollutant emissions. In this context, ammonia chemistry has recently gained major attention: it is present in biogas and bio-oil, in trace amounts. Investigating ammonia chemistry can benefit from several studies carried out in the past decades on its pyrolysis and oxidation behavior. However, scarce literature is available on the conditions of interest previously mentioned, since the presence of ammonia in trace amounts results in superoxidative conditions. The available kinetic models of ammonia have been built up by mostly relying on hightemperature data, obtained in ideal reactors. On the other side, few work has been carried out to investigate its oxidation at lower temperatures. In order to further investigate this topic, and to provide a stronger support for kinetic model validation, in this study the oxidation of ammonia in diluted conditions, at relatively low temperatures (T < 1200 K) and a pressure close to atmospheric, is investigated by using a Jet Stirred Reactor. In addition to ammonia conversion, the formation of Nitrogen Oxides (NOx) is also analyzed. At the same time, a detailed kinetic mechanism for ammonia oxidation is developed by leveraging the most recently available kinetic data on experimental and theoretical reaction rates, and is used to analyze the obtained data, after being validated against the literature data in similar conditions