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... 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 ...
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Research Interests: Physics and Turbulence
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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).
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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... 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.
Research Interests: Engineering, Aerospace Engineering, Materials Science, Computational Fluid Dynamics, Combustion, and 12 moreInternal Combustion Engine, Hydrogen Energy, CHEMICAL SCIENCES, Diesel Fuel, Exhaust Gas Recirculation, Hydrogen Engine, Cfd, Direct Injection, Compression Ignition, Homogeneous Charge Compression Ignition (hcci), Hydrogen Combustion, and Ignition System
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The flamelet approach is considered a viable framework to the modeling of homogeneous charge compression ignition (HCCI) engines under stratified mixture conditions. However, there are several issues that need further improvement. In... more
The flamelet approach is considered a viable framework to the modeling of homogeneous charge compression ignition (HCCI) engines under stratified mixture conditions. However, there are several issues that need further improvement. In particular, accurate representation of the scalar dissipation rate, which is the key parameter to connect the physical mixing space to the reactive space, requires further investigation. This involves a number of aspects: (i) probability density functions, (ii) mean scalar dissipation rates, and (iii) conditional scalar dissipation rates, for mixture fraction (Z) and total enthalpy (H). The present study aims to assess the validity of existing models in HCCI environments both in the RANS and LES contexts, and thereby suggest alternative models to improve on the above three aspects. Nomenclature Z = mixture fraction H = total enthalpy Z = Z scalar dissipation rate H = H scalar dissipation rate Z = Mean Z scalar dissipation rate H = Mean H scalar dissipat...
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Research Interests: Engineering and Lean Burn
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A coupled Eulerian-Lagrangian approach was employed to Engine Combustion Network (ECN) Spray-G simulations. The Eulerian in-nozzle flow simulation was conducted with a small plenum attached to the nozzles, and the results were fed to the... more
A coupled Eulerian-Lagrangian approach was employed to Engine Combustion Network (ECN) Spray-G simulations. The Eulerian in-nozzle flow simulation was conducted with a small plenum attached to the nozzles, and the results were fed to the Lagrangian spray simulation. For Eulerian simulation, the homogeneous relaxation model (HRM) coupled with the volume of fluid (VOF) method was used. HRM proved to be good at predicting the phase change phenomena due to vaporization mechanisms, that is, both cavitation and flash boiling. As a one-way coupling, quantities such as rate of injection (ROI), mass injected through each hole, discharge coefficient, spray plume angle and half cone angle predicted from the Eulerian simulations were used as the initial and boundary conditions for the subsequent Lagrangian spray simulations using the blob injection model. Non-flashing (Spray-G1) and flashing (Spray-G2) spray was simulated, and the results were validated quantitatively against the published data...
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Lagrangian particle tracking and Large-Eddy simulation were used to assess the effect of different fuels on spray characteristics. In such a two-way coupled modeling scenario, spray momentum accelerates the gaseous phase into an intense,... more
Lagrangian particle tracking and Large-Eddy simulation were used to assess the effect of different fuels on spray characteristics. In such a two-way coupled modeling scenario, spray momentum accelerates the gaseous phase into an intense, multiphase jet near the nozzle. To assess fuel property effects on liquid spray formation, the non-reacting Engine Combustion Network Spray A baseline condition was chosen as the reference case. The validated Spray A case was modified by replacing n-dodecane with diesel, methanol, dimethyl ether, or propane assuming 150 MPa injection pressure. The model features and performance for various fuels in the under-resolved near-nozzle region are discussed. The main findings of the paper are as follows. (1) We show that, in addition to the well-known liquid penetration [Formula: see text], and vapor penetration [Formula: see text], for all the investigated fuels, the modeled multiphase jets exhibit also a third length scale [Formula: see text], with discus...
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A three-dimensional Direct Numerical Simulation (DNS) database of statistically planar $$H_{2} -$$ H 2 - air turbulent premixed flames with an equivalence ratio of 0.7 spanning a large range of Karlovitz number has been utilised to assess... more
A three-dimensional Direct Numerical Simulation (DNS) database of statistically planar $$H_{2} -$$ H 2 - air turbulent premixed flames with an equivalence ratio of 0.7 spanning a large range of Karlovitz number has been utilised to assess the performances of the extrapolation relations, which approximate the stretch rate and curvature dependences of density-weighted displacement speed $$S_{d}^{*}$$ S d ∗ . It has been found that the correlation between $$S_{d}^{*}$$ S d ∗ and curvature remains negative and a significantly non-linear interrelation between $$S_{d}^{*}$$ S d ∗ and stretch rate has been observed for all cases considered here. Thus, an extrapolation relation, which assumes a linear stretch rate dependence of density-weighted displacement speed has been found to be inadequate. However, an alternative extrapolation relation, which assumes a linear curvature dependence of $$S_{d}^{*}$$ S d ∗ but allows for a non-linear stretch rate dependence of $$S_{d}^{*}$$ S d ∗ , has be...
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Abstract The statistics of entrainment velocity, defined as the displacement speed of an enstrophy isosurface, which can be taken to be the interface between turbulent/non-turbulent regions, have been analysed using a Direct Numerical... more
Abstract The statistics of entrainment velocity, defined as the displacement speed of an enstrophy isosurface, which can be taken to be the interface between turbulent/non-turbulent regions, have been analysed using a Direct Numerical Simulation database of statistically planar H 2 -air flames with a range of different Karlovitz numbers. It has been found that the component of the entrainment velocity arising from molecular dissipation plays a leading order role for all values of Karlovitz number, whereas the relative importance of the baroclinic torque and dilatation rate weakens with increasing Karlovitz number. By contrast, the relative contribution of the entrainment velocity component arising from vortex-stretching strengthens with increasing Karlovitz number K a . The mean entrainment velocity remains positive for the case representing the corrugated flamelets regime (i.e. K a 1 ), whereas it assumes negative values in the cases with large values of Karlovitz number (i.e. K a ≫ 1 ). The magnitude of the ratio of the mean values of entrainment velocity to the mean values of flame displacement speed conditional upon non-dimensional temperature within the flame front remains of the order of unity irrespective of Karlovitz number. However, the probability density functions of entrainment velocity exhibit considerably higher probabilities of finding large magnitudes than in the case of flame displacement speed. The alignments between the normal vector on the enstrophy isosurface and local principal strain rates have been found to be qualitatively similar to the corresponding alignments between flame normal and local principal strain rates, and the same holds true for the distributions of curvature shape factor of reaction progress variable and enstrophy isosurfaces. These findings indicate that the isosurface topologies and the alignments of normal vectors with local principal strain rates for enstrophy and reaction progress variable exhibit qualitatively similar behaviours. Consequently, turbulence and combustion modelling strategies cannot be considered in isolation in premixed turbulent flames.
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Direct numerical simulations (DNS) of ethylene/air diffusion flames in decaying two-dimensional turbulence were conducted in order to investigate flame extinction characteristics at different pressures. The computational configuration is... more
Direct numerical simulations (DNS) of ethylene/air diffusion flames in decaying two-dimensional turbulence were conducted in order to investigate flame extinction characteristics at different pressures. The computational configuration is an ethylene fuel strip surrounded by ambient air on both sides, superimposed by a prescribed turbulent flow field as the initial condition. The effect of pressure on extinction is studied by considering three different pressures: 0.1, 1.0, and 10 atm. To isolate the pressure effects on the chemistry, the physical parameters were modi- fied by artificially changing the gas transport properties in relation with the pressure change. This methodology allows a consideration of identical turbulent flow fields at different pressure conditions. An extinction criterion based on the local Damk ̈ohler number is tested in predict- ing various flame extinction events encountered. Results show that, despite the simplicity of the AEA flame extinction criterion, it...