R.P. Lindstedt

Detailed Chemical -Kinetic Model for Aviation Fuels

Theintroductionofdetailedchemicalreactionmechanismsforaviationfuelsintocomplexmultidimensionale uid dynamics problems is not practical at the present time. Simplie ed reaction mechanisms that have been thoroughly evaluated must be developed to address specie c issues arising in realistic combustor cone gurations. The latter should be e rmly based on detailed mechanisms carefully evaluated against a wide range of experimental data. A detailed kinetic mechanism for hydrocarbon combustion is formulated to address the gas-phase chemistry of endothermic and conventional aviation fuels. Reaction paths are analysed for n-decane and kerosene premixed e ames, and the ability ofthemechanism to predictvarious premixed e ame featuresisassessed by comparison with experimental species proe les. Finally, the level of success achieved by the present kinetic model in the context of practical problems is discussed.

Joint scalar probability density function modeling of pollutant formation in piloted turbulent jet diffusion flames with comprehensive chemistry

A transported joint probability density function (pdf) approach was applied to model three ( Re ≅8200, 22,400, and 44,800) piloted CH 4 /O 2 /N 2 turbulent jet diffusion flames investigated experimentally by Barlow and co-workers. The flames covered conditions from weakly turbulent to close to extinction. The chemistry is a systematically reduced variant of the 48-species and 300-reaction C/H/N/O mechanism of Lindstedt and co-workers. The applied form features 16 independent, 4 dependent, and 28 steady-state scalars. The mechanism takes full account of the C 2 chemistry via the inclusion of C 2 H 2 , C 2 H 4 , and C 2 H 6 as solved species. The formation of oxides of nitrogen is treated by a 5-independent-scalar (NO, NO 2 , NH 3 , N 2 O, and HCN) submechanism. The level of detail retained in the chemical mechanism is novel in the present context and permits an assessment of the accuracy of other closure approximations. The computational method for the solution of the joint-scalar pdf features moving particles in a Lagrangian framework. The second moment closure by Speziale et al. is used for the velocity field, and molecular mixing is modeled using the ubiquitous modified Curls model of Janicka et al. It is shown that excellent flow and scalar field predictions are possible across the range of Reynolds numbers and that conditional pdf values are reproduced with good accuracy even close to extinction. A sensitivity to the mechanical/scalar timescale ratio is, however, observed for flames under conditions of the latter type. Levels of NO become progressively overpredicted at downstream locations, though the magnitude of the discrepancy is arguably consistent with the increasing influence of radiative heat losses. The latter are not accounted for in the present work due to current uncertainties surrounding the accuracy of radiation submodels in the present flames.

Modeling of premixed turbulent flames with second moment methods

Turbulent premixed flames feature complex interactions between turbulent transport of scalars and chemical reaction as well as a strong coupling between velocity and scalar turbulence. The present work concerns the modeling of such flames at the second moment closure level and mainly focuses on the critical role of pressure redistribution/scrambling. It is shown that the common and exclusive practice of past work in this area, which involves a density weighted rewriting of models derived on a constant density basis, is insufficient. To resolve this issue a class of model extensions is proposed to account for variable density effects. The accuracy of the proposed approach is illustrated by consideration of incompressible premixed turbulent flames in the laminar flamelet regime of combustion. However, the proposed approach is general and does not preclude the consideration of other combustion regimes or the use of sophisticated closures for reaction related terms. The full closure is here applied to the simulation of transient one-dimensional flames and fully two-dimensional flames stabilized in counterflow geometries. The obtained level of agreement with experimental data proves most encouraging and constitutes a significant advance over past modeling efforts.

A dimensionally reduced reaction mechanism for methanol oxidation

Methanol has potential as an alternative fuel due to favorable combustion properties that include lower emissions of particulates and oxides of nitrogen. Disadvantages include high miscibility with water and potential difficulties with emissions of oxygenated species. Theoretical investigations of methanol oxidation in practical (e.g., turbulent and/or multidimensional) flow fields require chemistry descriptions that balance thermochemical fidelity, compactness, and mathematical properties (e.g., “stiffness”). Recent studies of methane combustion in turbulent flames have shown, through the application of augmented (large scalar space) reduced reaction mechanisms, that accurate chemistry is a prerequisite for quantitative predictions of kinetically influenced phenomena. The latter include extinction/reignition and pollutant emissions. The present work extends past work on augmented reduced mechanisms to include methanol oxidation. The detailed and systematically reduced mechanisms are comprehensively validated against shock tube, flow reactor, premixed, and partially premixed flame data. It is shown that the detailed starting mechanism featuring 52 species and 326 reactions can be reduced to a 14-step mechanism for the C/H/O system and a 5-step submechanism for nitrogen-containing species without appreciable lack of generality.

The formation and oxidation of aromatics in cyclopentene and methyl-cyclopentadiene mixtures

Aromatic, hydrocarbon formation and oxidation is discussed in the context of detailed chemical kinetic modeling of cyclopentene flames and methyl-cyclopentadiene pyrolysis in a shock tube. The main objective is to explore the relative roles of cyclopentadienyl-and phenyl radical-based sequences leading to the formation of naphthalene and indene in a system featuring a high cyclopentadienyl, to phenyl radical ratio. The work is also motivated by a desire to explore the importance of possible linkages between cyclopentadiene/benzene and indene/naphthalene through CH 3 or 1 CH 2 / 3 CH 2 addition reactions. Naphthalene formation is considered in the context of a number of tentative detailed sequences that include C 3 H 5 +C 2 H 2 , C 6 H 5 +C 4 H 4 , and C 7 H 7 +C 3 H 3 routes as well as initiation via C 5 H 5 +C 5 H 5 . It is shown that that a fast cyclopentadienyl-based recombination route is not consistent with experimentally determined naphthalene concentrations under the conditions considered. The study lends support to the suggestion that the addition of CH 3 to C 5 H 5 is a viable path for benzene formation. Attention is also placed on paths leading to different isomeric C 9 H 8 structures, and it is shown that steps such as C 6 H 5 +C 3 H 8 have the potential to explain indene formation with good quantitative accuracy. The study further suggests that an important route to naphthalene formation is initiated via acetylene addition to the ethenylphenyl radical.

Molecular growth and oxygenated species formation in laminar ethylene flames

Ethylene constitutes a key intermediate in the oxidation of practical hydrocarbon fuels. Past studies of ethylene oxidation in shock tubes, flow/stirred reactors, and flames have provided an understanding of several key aspects of the fuel oxidation chemistry. However, the current trend, toward leaner premixed combustion in practical devices results in significant changes in the combustion regime, and studies under fuel-lean flame conditions have become essential. The changes in the main reaction channels are significant, and the chemistry of oxygenated species comes to the fore. The present work compares the main reaction paths in fuel-rich and fuel-lean ethylene flames, with particular emphasis on the relative importance of addition, abstraction, and isomerization paths in the context of the formation of oxygenated and aromatic species. It is shown that in the fuel-lean flame, ethylene is mainly consumed by addition reactions, while in the rich flame environment, abstraction reactions leading to vinyl radical formation dominate. The balancing of the destruction chemistry of the latter and the importance of the rate and product distribution of the dolecular oxygen attack are discussed in detail. It is noted that the channel C 2 H 4 +O=CH 2 CHO +H is the dominant route to vinoxy under lean flame conditions, and the present work supports the acetyl channel (CH 2 CHO→CH 3 CO→CH 3 +CO) for vinoxy destruction at elevated temperatures. Potential benzene formation paths are assessed, and it is concluded that vinyl radical addition to vinylacetylene and propargyl radical recombination constitute the major benzene formation paths in rich ethylene flames. The present work thus further emphasises the need to consider multiple benzene formation channels.

Joint scalar transported probability density function modeling of turbulent methanol jet diffusion flames

A transported joint probability density function (JPDF) approach is applied to computer four (M1–M4) turbulent ( Re ≅39,200, 53,500, 62,100, and 69,600) CH 3 OH/air jet diffusion flames investigated experimentally by Masri and coworkers. The flames cover conditions from stable combustion to close to extinction. The present work provides an assessment of the ability of the transported PDF approach to model the detailed chemical structure of such flames and related phenomena including local extinction/reignition. The chemical closure for methanol is obtained through a systematically reduced variant of a comprehensively validated 32 species and 167 reaction C/H/O mechanism of Lindstedt and coworkers. The applied form features 14 independent, 4 dependent, and 14 steady-state scalars. The mechanism takes full account of the C 2 chemistry. The computational method for the solution of the joint scalar JPDF features moving particles in a Lagrangian framework and the second moment closure by Speziale et al. is used for the velocity field. Molecular mixing is modeled using the ubiquitous modified Curls model of Janicka et al. and it is shown that good scalar field predictions are possible across the range of Reynolds numbers. An interesting result is that the computations are, despite the simplicity of the molecular mixing model (e.g., mixing of particles separated in composition space), able to predict high levels of local extinction for flame M4 close to the burner nozzle ( x/D =10) and subsequent reignition further downstream. A parametric analysis applied to the the time-scale ratio of velocity and scalar turbulence confirms a greatly increased sensitivity as blow-off is approached.

Joint Scalar-velocity pdf Modelling of Finite Rate Chemistry in a Scalar Mixing Layer

Abstract The joint scalar-velocity probability density function (pdf) modelling approach is here applied to the problem of mixing and chemical reaction in a scalar mixing layer. The cases considered are of direct relevance to the modelling of turbulent combustion applications and the main objectives of the work are: (i) To assess the applicability of joint scalar-velocity pdf solution methods in the context of realistic low Damkohler number turbulent reacting flows. (ii) To investigate a multi-scalar extension of a modified binomial Langevin scalar mixing model of Valifio and Dopazo. The latter is here used in conjunction with the generalised Langevin model for velocity statistics of Haworth and Pope. The experimental data sets by Bilger et al., obtained in chemically reacting flows with Darnkohler numbers of the order unity have been used to validate the approach. The excellent agreement obtained for both conserved and reacting scalars indicates that the reaction -diffusion coupling is accurately represe...

Second moment modeling of premixed turbulent flames stabilized in impinging jet geometries

The present work comprises a computational study of premixed turbulent flames stabilized in a stagnation point flow geometry. Past work in this area has focused mainly on the closure of the mean reaction rate and exclusively featured gradient diffusion-type approximations for the turbulent transport of momentum and scalars. The shortcomings of gradient diffusion-based closures in such flows are well established, and turbulent transport has been shown to exert a significant influence on the evolution of global and local flame properties. Thus, the present study comprises a full second moment closure for both velocity and scalar turbulent transport and includes extended variable density forms for the pressure redistribution, pressure scrambling, and dissipation generation terms. The proposed extensions are general and independent of the combustion regime considered. The accuracy of the approach is illustrated by the application of the resulting equations to the simulation of a stoichiometric ethylene and air premixed turbulent flame stabilized in an impinging jet geometry. Comparisons with available experimental data are highly encouraging and reveal excellent agreement for predictions of both mean and turbulent quantities. Furthermore, the generality and superiority of second moment methods over gradient diffusion closures is clearly established. The present study has fundamental implications for the development of turbulent transport and reaction rate models.

The modelling of direct chemical kinetic effects in turbulent flames

Abstract Combustion chemistry-related effects have traditionally been of secondary importance in the design of gas turbine combustors. However, the need to deal with issues such as flame stability, relight and pollutant emissions has served to bring chemical kinetics and the coupling of finite rate chemistry with turbulent flow fields to the centre of combustor design. Indeed, improved cycle efficiency and more stringent environmental legislation, as defined by the ICAO, are current key motivators in combustor design. Furthermore, lean premixed prevaporized (LPP) combustion systems, increasingly used for power generation, often operate close to the lean blow-off limit and are prone to extinction/reignition type phenomena. Thus, current key design issues require that direct chemical kinetic effects be accounted for accurately in any simulation procedure. The transported probability density function (PDF) approach uniquely offers the potential of facilitating the accurate modelling of such effects. The present paper thus assesses the ability of this technique to model kinetically controlled phenomena, such as carbon monoxide emissions and flame blow-off, through the application of a transported PDF method closed at the joint scalar level. The closure for the velocity field is at the second moment level, and a key feature of the present work is the use of comprehensive chemical kinetic mechanisms. The latter are derived from recent work by Lindstedt and co-workers that has resulted in a compact 141 reactions and 28 species mechanism for LNG combustion. The systematically reduced form used here features 14 independent C/H/O scalars, with the remaining species incorporated via steady state approximations. Computations have been performed for hydrogen/carbon dioxide and methane flames. The former (high Reynolds number) flames permit an assessment of the modelling of flame blow-off, and the methane flame has been selected to obtain an indication of the influence of differential diffusion effects among gaseous species. The agreement with experimental data is excellent. The predicted blow-off, velocity is within 10 per cent of the experimental value and it is further shown that experimental levels of major and minor species are well reproduced. Interestingly, comparisons of experimental data with prediction indicate only a modest influence of differential diffusion effects on gaseous species. A comparison with previous modelling efforts, featuring smaller scalar spaces, permits the conclusion that accurate chemistry is a prerequisite for quantitative predications of finite rate chemical kinetic effects.