A computational assessment of flame speed correlation in an ultra-leanpre-chamber engine
International Journal of Engine Research
Predictive modeling of pre-chamber combustion engines relies primarily on the correct description... more Predictive modeling of pre-chamber combustion engines relies primarily on the correct description of laminar and turbulent flame speeds. For engineering applications, the correlations of the flame speeds with physical variables involve empirical constants that are valid for a limited range of operating conditions. The current work aims at assessing the significance of laminar flame speed prediction in the simulation of ultra-lean pre-chamber engine combustion operated with methane. Gülder’s empirical correlation for laminar flame speed was chosen as a reference and was further modified for equivalence ratio, pressure and temperature ranges beyond what it was originally derived for, in order to confirm the original hypothesis; the pressure and temperature dependence were adopted as a power-law correlation. Based on the computational results using the skeletal reaction mechanism, the correlation was modified better represent the flame speeds at ultra-lean engine conditions, using GRI ...
International Conference on Liquid Atomization and Spray Systems (ICLASS), 2021
Complexity behind physical phenomena of supercritical and transcritical jet flows, still leaves a... more Complexity behind physical phenomena of supercritical and transcritical jet flows, still leaves an ambiguous understanding of such widespread technology, with applications ranging from diesel and liquid rocket engines to gas turbines. In this present numerical study, a new open-source CFD model construction is presented and validated using a liquid-rocket benchmark comprised of liquid-oxygen (LOX) and gaseous-hydrogen (H2) streams. Mixing process of liquid oxygenhydrogen streams under liquid rocket engine (LRE) relevant conditions is scrutinized using the pressure-based solution framework implemented in the versatile computation platform Open-FOAM. The model accounts for real fluid thermodynamics and transport properties, making use of the cubic Peng-Robinson equation of state (PR-EOS) and the Chung transport model. The solver capability to capture the mixing layer between the two separated streams is discussed as well as its capability to predict with adequate accuracy the thermophysical quantities. Following the thorough validation, a comparison of the contribution of the accurate laminar transport properties vs. the large eddy simulation (LES) subgrid scale (sgs) turbulent values is conducted in order to assess the relative importance of the turbulent viscosity. By means of an assessment of the pressure-based numerical framework with available data in the literature, this work contributes to a better understanding of well resolved simulations. In addition, it enables the further development of a real fluid pressure-based multi-species solver as an open-source code.
Large eddy simulations (LES) of a turbulent premixed jet flame in a confined chamber are performe... more Large eddy simulations (LES) of a turbulent premixed jet flame in a confined chamber are performed using the flamelet-generated manifold technique for tabulation of chemical kinetics and the OpenFOAM framework for computational fluid dynamics. The configuration is characterized by an off-center nozzle having an inner diameter of 10 mm, feeding a lean methane-air mixture with an equivalence ratio of 0.71 and mean velocity of 90 m/s, at 573 K and atmospheric pressure. Conductive heat loss is accounted for in the manifold via burner-stabilized flamelets and the subgrid-scale (SGS) turbulence-chemistry interaction is modeled via presumed filtered density functions. The effects of heat loss inclusion as well as SGS modeling for both the SGS stresses and SGS variance of progress variable on the numerical predictions are all systematically investigated. Comparisons between numerical results and measured data show a considerable improvement in the prediction of temperature when heat losses are incorporated into the manifold, as compared to the adiabatic one. In addition, further improvements in the LES predictions are achieved by employing SGS models based on transport equations.
The LES filtered species transport equation in turbulent reacting flow simulations contains the u... more The LES filtered species transport equation in turbulent reacting flow simulations contains the unclosed turbulent scalar flux that needs to be modelled. It is well known that the statistical behavior of this term and its alignment characteristics with resolved scalar gradient depend on the relative importance of heat release and turbulent velocity fluctuations. Counter-gradient transport has been reported in several earlier studies where the flames under investigation were located either in the corrugated flamelets or thin reaction zones regime of premixed turbulent combustion. Therefore it is useful to understand the statistical behavior of turbulent scalar fluxes if the flame represents the broken reaction zones regime (BRZR). The present analysis aims to provide improved understanding on this subject through an a-priori analysis of a detailed chemistry database consisting of three freely-propagating statistically planar turbulent H2-air premixed flames representing three different regimes of combustion. Results indicate that heat release effects weaken with increasing Karlovitz number, but that counter-gradient transport can still occur for large LES filter size in the BRZR. Furthermore the behaviour of the flux and in particular its sign are different for reactant and product species.
Michigan-Ann Arbor-The present work investigates the auto-ignition characteristics of composition... more Michigan-Ann Arbor-The present work investigates the auto-ignition characteristics of compositionally homogeneous reactant mixtures in the presence of thermal non-uniformities and turbulent velocity fluctuations. An auto-ignition regime diagram is briefly discussed, that provides the framework for predicting the expected ignition behavior based on the thermo-chemical properties of the reactant mixture and flow/scalar field conditions. The regime diagram classifies the ignition regimes mainly into three categories: weak (deflagration dominant), reaction-controlled strong and mixing-controlled strong (volumetric ignition/spontaneous propagation dominant) regimes. Two-dimensional direct numerical simulations (DNS) of autoignition in a lean thermally-stratified syngas/air turbulent mixture at high-pressure, low-temperature conditions are performed to assess the validity of the regime diagram. Various parametric cases are considered corresponding to different locations on the regime diagram, by varying the characteristic turbulent Damköhler and Reynolds numbers. Detailed analysis of the reaction front propagation and heat release indicates that the observed ignition behaviors agree very well with the corresponding predictions by the regime diagram.
Two-dimensional direct numerical simulation (DNS) databases of bluffbody-stabilized lean hydrogen... more Two-dimensional direct numerical simulation (DNS) databases of bluffbody-stabilized lean hydrogen flames representative of complicated reactivediffusive system are analysed using the combined approach of computational singular perturbation (CSP) and tangential stretching rate (TSR) to investigate chemical characteristics in blow-off dynamics. To assess the diagnostic approaches in flame and blow-off dynamics, Damköhler number and TSR
An analysis of soot formation pathways in laminar coflow ethylene flame at higher pressures
AIAA Scitech 2020 Forum, 2020
The work reported in this paper was sponsored by the King Abdullah University of Science and Tech... more The work reported in this paper was sponsored by the King Abdullah University of Science and Technology (KAUST) and computational resources were provided by the KAUST Supercomputing Laboratory (KSL).
Computational characterization of hydrogen direct injection and nonpremixed combustion in a compression-ignition engine
International Journal of Hydrogen Energy, 2021
Abstract With the revived interest in hydrogen (H2) as a direct combustion fuel for engine applic... more Abstract With the revived interest in hydrogen (H2) as a direct combustion fuel for engine applications, a computational study is conducted to assess the characteristics of H2 direct-injection (DI) compression-ignition (CI) non-premixed combustion concept. Development of a CFD modeling using CONVERGE CFD solver focuses on hydrogen's unique characteristics by utilizing a suitable numerical method to reproduce the direct H2 injection phenomena. A grid sensitivity study is performed to ensure the fidelity of results with optimal cost, and the models are validated against constant-volume optical chamber and diesel engine experimental data. The present study aims to contribute to the future development of DICI H2 combustion engines, providing detailed characterization of the combustion cycle, and highlighting several distinct aspects of CI nonpremixed H2 versus diesel combustion. First, unlike the common description of diesel sprays, hydrogen jets do not exhibit significant flame lift-off and air entrainment near injector nozzle, and the fuel-air interface is drastically more stratified with no sign of premixing. It is also found that the DICI H2 combustion concept is governed first by a free turbulent jet mixing phase, then by an in-cylinder global mixing phase. The former is drastically more dominant with the DICI H2 engine compared to conventional diesel engines. The free-jet mixing is also found to be more effective that the global mixing, which indicates the need to completely rethink the optimization strategies for CI engines when using H2 as fuel.
The study is aimed at demonstrating a methodology for the timescale characterization of the chemi... more The study is aimed at demonstrating a methodology for the timescale characterization of the chemistry-wall-heat-transfer interaction. The driving chemical timescale is estimated by means of the tangential stretching rate, and a proper thermal timescale for the temperature-time variation due to wall heat flux is presented. A thermal Damköhler number, Da th , is proposed as the ratio of the two. The methodology is applied on a prototypical laminar methane-oxygen diffusion flame impinging on an isothermal cold wall. Non-adiabatic effects are described qualitatively and a CSP-TSR analysis is performed to obtain topological information and physical insights. The thermal Damköhler number field is computed and discussed to highlight the interplay between chemical and diffusive processes and to a-priori assess the accuracy of the steady laminar flamelet assumption under non-adiabatic conditions.
Understanding detonation development from a flame kernel initiated by a pre-ignition event is imp... more Understanding detonation development from a flame kernel initiated by a pre-ignition event is important for modern internal combustion (IC) engines operating at boosted conditions. To provide fundamental insights into the effects of bulk gas temperature stratification on the characteristics of detonation development, one-dimensional high fidelity simulations were conducted for a constant volume reactor filled with a thermally stratified reactive stoichiometric hydrogen/air mixture. A linear temperature variation in the upstream end-gas was introduced to represent the thermal stratification of the bulk mixture, and the evolution from the initial deflagration flame front to detonation development was examined. The results showed that the bulk-gas temperature gradient has a significant effect on the run-up time and intensity of the developing detonation. Detailed analyses further revealed that the mechanism of detonation development is qualitatively different for the positive and negative temperature gradient cases. In the former, the detonation development is initiated by the end-gas autoignition at the wall, while the latter exhibits detonation development following the process of the self-acceleration of the flame similar to the deflagration-to-detonation transition. This behavior is attributed to the longer residence time in the end-gas allowing the reinforcement by the interaction of incident and reflected pressure waves during the flame propagation, and results in the peak pressure even higher than the case with the same level of positive temperature gradient. Furthermore, yet another detonation development pattern was observed for the negative temperature gradient condition in the presence of a uniform temperature region just ahead of the flame. In this case, autoignition was found to start in the middle of the bulk end-gas, and subsequently leads to the transition to detonation. The results demonstrate the importance of the bulk gas conditions in predicting the detonation development, which corroborate the existing theoretical framework.
Ciottoli P, et al. (2019) Investigation of the turbulent flame structure and topology at differen... more Ciottoli P, et al. (2019) Investigation of the turbulent flame structure and topology at different Karlovitz numbers using the tangential stretching rate index. Combustion and Flame 200: 155-167.
Linear instability sheet atomization (LISA) breakup model has been widely used for modeling hollo... more Linear instability sheet atomization (LISA) breakup model has been widely used for modeling hollow-cone spray. However, the model was originally developed for inwardlyopening pressure-swirl injectors by assuming toroidal ligament breakups. Therefore, LISA model is not suitable for simulating outwardly opening injectors having string-like structures at wide spray angles. Furthermore, the varying area and shape of the annular nozzle exit makes the modeling difficult. In this study, a new spray modeling was proposed for outwardly opening hollow-cone injector. The injection velocities are computed from the given mas flow rate and injection pressure regardless of ambiguous nozzle exit geometries. The modified Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) breakup model is used with adjusted initial Sauter mean diameter (SMD) for modeling breakup of string-like liquid film spray. Liquid spray injection was modeled using Lagrangian discrete parcel method within the framework of commercial CFD software CONVERGE, and the detailed model was implemented by user defined functions. It was found that the new model predicted the liquid penetration length and local SMD accurately for various fuels and chamber conditions.
Interactions of turbulence, molecular and energy transport coupled with chemistry play a crucial ... more Interactions of turbulence, molecular and energy transport coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen-air premixed turbulent flames. Direct numerical simulations (DNS) are conducted with detailed reaction mechanism and transport properties for hydrogen-air flames, at different parametric conditions. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (S d) of a temperature isosurface is considered and the contributions of transport, chemistry and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Principal curvature evolution statistics obtained using FPT suggest that these islands are ellipsoidal on average, enclosing fresh reactants. Further examinations show that the increase in S d is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients, as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number was found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description.
On the effects of fuel properties and injection timing in partially premixed compression ignition... more On the effects of fuel properties and injection timing in partially premixed compression ignition of low octane fuels. Fuel 207: 373-388.
44th AIAA Aerospace Sciences Meeting and Exhibit, 2006
Motivated by the potential use of catalytic materials in micro-combustor application, the primary... more Motivated by the potential use of catalytic materials in micro-combustor application, the primary scope of this study is to investigate the lean extinction characteristics of premixed flames in the presence of catalytic reaction. In particular, the effects of mixture dilution on the lean flammability limit are examined by adopting a stagnation-point flow system with a methane/air mixture over a platinum surface. For the reference fuel/air conditions, it was found that the lean flammability limit of homogeneous reaction is actually lower than that of the heterogeneous reaction. However, when sufficient amount of nitrogen is added to the reactant mixture, the surface reaction shows the benefit of extending the lean extinction limit to a lower value. It is also found that the magnitude of flammability extension by surface reaction is more pronounced in the presence of surface heat loss. Unsteady extinction behavior in response to equivalence ratio fluctuations shows an overall trend consistent with the steady results. These phenomena are explained as result of different characteristic time scale of homogeneous and heterogeneous reaction. Analytical calculation proved that under same reactant and thermal boundary condition, surface reaction has smaller time scale which is rather insensitive to dilution while gas phase reaction has larger one which changes more with different dilution magnitude.
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