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Now showing items 1 - 16 of 76

  • A phenomenological HCCI combustion model in 0D and 3D-CFD

    Blomberg, Christopher Kim   Wright, Yuri Martin   Boulouchos, Konstantinos  

    A phenomenological combustion model for HCCI applications - the 3-Stage heat release model - which predicts low, intermediate and high temperature heat release (LTR, ITR and HTR) is developed and validated for six automotive surrogate fuels. The contribution of all three combustion phases to the overall heat release and their durations are modeled based on rapid compression and expansion (RCEM) single-zone adiabatic simulations with detailed chemical kinetic mechanisms for all six fuels. This allows "pure" chemical effects on combustion to be included in the proposed 3-Stage heat release model, avoiding any combustion chamber related uncertainties such as blow-by, heat losses or trapped mass in crevices. Agreement between the 3-Stage heat release model and virtual RCEM (VRCEM) reference data is good. In a second step, a zero-dimensional HCCI model (0D model) is presented. To predict the onset of combustion, the low temperature ignition term of a previously developed and validated 3-Arrhenius auto-ignition model is employed in conjunction with the ignition integral of Livengood and Wu. Thereafter the 3-Stage heat release model predicts chemical heat release. Results show that the 0D model gives excellent predictions of ignition delays and combustion progress compared to the single-zone RCEM simulations using detailed chemical reaction mechanisms. Furthermore, including ITR in the 0D model is shown to be of essential importance to correctly predict high temperature ignition delay. As a final step a 3D HCCI model (3D model) is presented. The onset of combustion is again predicted by the low temperature ignition term of the 3-Arrhenius auto-ignition model, implemented via the source term of the transport equation into the 3D-CFD code. To predict heat release, the source term of the enthalpy equation is used to incorporate the 3-Stage heat release model into the 3D-CFD code. A simplified RCEM geometry is used for the simulations. Validation of the 3D model is made by a comparison to a chemical kinetic mechanism that is directly integrated into the 3D-CFD code. Combustion progress, temperature stratification in the combustion chamber and ignition delay times agree well with the reference data. The combination of the 3-Arrhenius auto-ignition model and 3-stage heat release model, their integration into 0D or 3D-CFD as a fully predictive HCCI model and their very low computational cost suggest the proposed model constitutes a promising approach for the development of future HCCI combustion engines.
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    In order to extent the operation range of an internal combustion engine, i.e. the operation range of a HCCI engine, or to increase the compression ratio of a spark ignition engine (Otto engine) the invention proposes generating a dual swirl in the cylinder. According to the invention the strength of the dual swirl is variable and adjustable or feedback controllable. For the HCCI combustion mode the stratification of the temperature field is adjustable by the strength of the dual swirl flow motion which allows extending the operation towards higher engine loads. In addition, the dual swirl prevents the mixing of the gases in the two intake channels, which can be used to generate stratified mixtures without employing a direct injection system. This can be used for extending the HCCI operation range towards lower loads. In case of spark ignition engines the dual swirl can generate a more homogeneous temperature distribution allowing higher compression ratios and thus higher engine efficiencies.
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  • A LES-CMC formulation for premixed flames including differential diffusion

    Farrace, Daniele   Chung, Kyoungseoun   Bolla, Michele   Wright, Yuri M.   Boulouchos, Konstantinos   Mastorakos, Epaminondas  

    A finite volume large eddy simulation-conditional moment closure (LES-CMC) numerical framework for premixed combustion developed in a previous studyhas been extended to account for differential diffusion. The non-unity Lewis number CMC transport equation has an additional convective term in sample space proportional to the conditional diffusion of the progress variable, that in turn accounts for diffusion normal to the flame front and curvature-induced effects. Planar laminar simulations are first performed using a spatially homogeneous non-unity Lewis number CMC formulation and validated against physical-space fully resolved reference solutions. The same CMC formulation is subsequently used to numerically investigate the effects of curvature for laminar flames having different effective Lewis numbers: a lean methane-air flame with Le(eff) =3D 0.99 and a lean hydrogen-air flame with Le(eff) =3D 0.33. Results suggest that curvature does not affect the conditional heat release if the effective Lewis number tends to unity, so that curvature-induced transport may be neglected. Finally, the effect of turbulence on the flame structure is qualitatively analysed using LES-CMC simulations with and without differential diffusion for a turbulent premixed bluff body methane-air flame exhibiting local extinction behaviour. Overall, both the unity and the non-unity computations predict the characteristic M-shaped flame observed experimentally, although some minor differences are identified. The findings suggest that for the high Karlovitz number (from 1 to 10) flame considered, turbulent mixing within the flame weakens the differential transport contribution by reducing the conditional scalar dissipation rate and accordingly the conditional diffusion of the progress variable.
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  • Effect of methane on pilot-fuel auto-ignition in dual-fuel engines

    Srna, Ales   Bolla, Michele   Wright, Yuri M.   Herrmann, Kai   Bombach, Rolf   Pandurangi, Sushant S.   Boulouchos, Konstantinos   Bruneaux, Gilles  

    The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100 mu m injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH2O-PLIF (10 kHz) and Schlieren (80 kHz) imaging for detection of the first-stage ignition, and simultaneous high-speed OH* chemiluminescence (40 kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH2O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low T and high T chemistry and micro-mixing for dual-fuel auto-ignition processes for which the governing processes were identified. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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  • Spectral Quasi-Equilibrium Manifold for Chemical Kinetics

    Kooshkbaghi, Mandi   Frouzakis, Christos E.   Boulouchos, Konstantinos   Karlin, Iliya V.  

    The Spectral Quasi-Equilibrium Manifold (SQEM) method is a model reduction technique for chemical kinetics based on entropy maximization under constraints built by the slowest eigenvectors at equilibrium. The method is revisited here and discussed and validated through the Michaelis-Menten kinetic scheme, and the quality of the reduction is related to the temporal evolution and the gap between eigenvalues. SQEM is then applied to detailed reaction mechanisms for the homogeneous combustion of hydrogen, syngas, and methane mixtures with air in adiabatic constant pressure reactors. The system states computed using SQEM are compared with those obtained by direct integration of the detailed mechanism, and good agreement between the reduced and the detailed descriptions is demonstrated. The SQEM reduced model of hydrogen/air combustion is also compared with another similar technique, the Rate-Controlled Constrained-Equilibrium (RCCE). For the same number of representative variables, SQEM is found to provide a more accurate description.
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  • Multiple Cycle LES Simulations of a Direct Injection Natural Gas Engine

    Schmitt, Martin   Hu, Rennan   Wright, Yuri Martin   Soltic, Patrik   Boulouchos, Konstantinos  

    In this work the flow field evolution, mixture formation and combustion process in an engine with methane Direct Injection (DI) is investigated using Large Eddy Simulations. The supersonic methane injection is modeled according to Muller et al. (2013) and combustion by a level set approach. The flame propagation showed to be dependent on the grid resolution. Higher grid resolutions have two opposing effects: first the fraction of unresolved turbulence is reduced, which decrease the flame speed and second flame wrinkling is increased resulting in faster flame propagation. For the observed setup the wrinkling effect was stronger. The average in-cylinder pressure traces as well as the cyclic variability thereof were compared to experimental data and very good agreement was found. During the supersonic gaseous injection the turbulence level in the cylinder is significantly increased, which dissipates quickly and thus has only a minor effect on the flame propagation. The introduced momentum showed a larger impact, since it enhances the tumble motion resulting in increased turbulence levels as the tumble decays shortly before ignition. During DI the cyclic differences in the tumble motion are preserved, but the impact on the average tumble level results in changing relative differences of the cyclic turbulence levels at ignition timing. Thus an injection direction supporting the tumble flows is expected to reduce the Cycle-to-Cycle Variations (CCV), while a reduction of the tumble strength could increase the CCV level. Compared to the fluctuations in the turbulence levels, the cyclic variability of the equivalence ratio at the injection location with DI showed a minor effect on the simulated CCVs.
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  • Reduction of Cold-Start Emissions for a Micro Combined Heat and Power Plant

    Zobel, Tammo   Schuerch, Christian   Boulouchos, Konstantinos   Onder, Christopher  

    Decentralized power generation by combined heat and power plants becomes increasingly popular as a measure to advance the energy transition. In this context, a substantial advantage of small combined heat and power plants is based on the relatively low pollutant emissions. However, a large proportion of the pollutant emissions is produced during a cold-start. This fact is not reflected in governmental and institutional emission guidelines, as these strongly focus on the emission levels under steady-state conditions. This study analyzes the spark advance, the reference air/fuel ratio and an electrically heated catalyst in terms of their potential to reduce the cold-start emissions of a micro combined heat and power plant which uses a natural gas fueled reciprocating internal combustion engine as prime mover and a three-way catalytic converter as aftertreatment system. Based on these measures, control approaches were developed that account for the specific operating conditions of the class of small combined heat and power plants, e.g., full-load operation and flexible, demand-driven runtimes. The experimental data demonstrates that even solutions with marginal adaptation/integration effort can reduce cold-start emissions to a great extent.
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  • Direct numerical simulation of multiple cycles in a valve/piston assembly

    Schmitt, Martin   Frouzakis, Christos E.   Tomboulides, Ananias G.   Wright, Yuri M.   Boulouchos, Konstantinos  

    The dynamics and multiple-cycle evolution of the incompressible flow induced by a moving piston through the open valve of a motored piston-cylinder assembly was investigated using direct numerical simulation. A spectral element solver, adapted for moving geometries using an Arbitrary Lagrange/Eulerian formulation, was employed. Eight cycles were simulated and the ensemble-and azimuthally-averaged data were found to be in good agreement with experimentally determined means and fluctuations at all measured points and times. During the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed a large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a dominant effect on the jet breakup and the vortex ring dynamics below the valve and on the observed significant cyclic variations. Based on statistical averaging, the evolution of the turbulent flow field during the first half of the intake stroke is dominated by the jet breakup process leading to a strongly anisotropic behavior. In the second part of the intake stroke, the decrease of the incoming jet velocity results in a more isotropic behavior. (C) 2014 AIP Publishing LLC.
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  • Hybrid LES/RANS with wall treatment in tangential and impinging flow configurations

    Keskinen, Karri   Nuutinen, Mika   Kaario, Ossi   Vuorinen, Ville   Koch, Jann   Wright, Yuri M.   Larmi, Martti   Boulouchos, Konstantinos  

    Scale-resolving simulation of high Reynolds number flows is a considerable numerical challenge. One approach to alleviate the matter is to relax near-wall resolution requirements of large eddy simulation (LES) with wall models or hybrid LES/Reynolds-averaged (RANS) methods. In-cylinder engine flows present a particular complexity as the process is inherently governed by wall tangential, wall normal and free shear flows with substantial temporal variation in Reynolds number and boundary layer gradients. In such conditions, robustness regarding wall-normal spacing would also be beneficial. Motivated by these factors, this study investigates the functionality of seamless and zonal hybrid LES/RANS methods in incompressible channel (Re-tau =3D 590) and impinging jet (Re-D =3D 23, 000) flows using relatively coarse grids. Standard (Smagorinsky) and more recent (sigma) subgrid-scale (SGS) models are utilized. As a novel contribution, we incorporate a recently developed RANS-based wall model in the zonal hybrid LES/RANS context with considerably coarser near-wall grids in the wall-normal direction. Results show (i) isotropy differences between the SGS models in both LES and hybrid LES/RANS cases and that (ii) the hybrid models mostly improve on corresponding LES results in terms of low-order statistics and wall friction. In addition, (iii) different hybrid implementations enhance different aspects of the solution, especially in the impinging jet flow. Results with the zonal method indicate only marginal interference with the core LES. Finally, (iv) combining zonal hybrid LES/RANS with the presented wall treatment provides favourable indications particularly in tangentially-dominated flow regions, while the complex jet stagnation region benefits from a moderately refined grid. This approach appears promising for decreased near-wall grid sensitivity in scale-resolving simulations. (C) 2017 Elsevier Inc. All rights reserved.
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    The invention relates to a combustion method, in particular for a four-stroke reciprocating internal combustion engine, wherein a pilot amount of fuel (e) is introduced before a main amount of the fuel (x) is introduced, intermediate products of the pilot amount of the fuel (f) are formed, the main amount of the fuel (x) is introduced during the compression phase (b) in such a way that complete ignition of the mixture of fresh gas and the intermediate products is suppressed and further intermediate products are formed until a monitored ignition of the mixture and of the further intermediate products occurs (c) and wherein the introduction of the main amount of the fuel (x) into the combustion chamber begins at a gas temperature in the combustion chamber between 700 K and 1000 K and the introduction of the main amount of the fuel (x) into the combustion chamber ends before the ignition top dead center (ZOT). By means of the combustion method according to the invention, the conflict of the goals of fuel consumption and NOx emissions in the lean combustion method (Otto and Diesel) can be resolved and thus future emissions regulations can be met without further reductions in fuel consumption.
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    The invention relates to a lean combustion method for a four-stroke reciprocating internal combustion engine having a cylinder, in which a combustion chamber is delimited by a cylinder head and a piston that can be moved in a reciprocating manner, the piston being able to vary the volume of the combustion chamber and wherein a fuel can be introduced directly into the combustion chamber, wherein at least one gas exchange inlet valve and one gas exchange outlet valve are provided for a gas exchange, and wherein an intermediate compression can be set in the combustion chamber at a gas exchange top dead center (LWOT) and a main compression can be set at an ignition top dead center (ZOT).
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    The invention relates to a combustion method, in particular for a four-stroke reciprocating engine comprising a combustion chamber, the volume of which can be modified by a reciprocating piston and fuel can be directly injected into the combustion chamber, at least one gas exchange inlet valve and a gas exchange outlet valve for a charge cycle being provided and the combustion chamber having a minimum volume at an upper charge-cycle dead-centre position (LOT) and at an upper ignition dead-centre position (ZOT). Said method comprises the following steps: introduction of an inlet gas into the combustion chamber (a) in an intake phase; introduction of a primary quantity of fuel (x) into the combustion chamber during the intake phase (a) and/or a compression phase (b); compression of the inlet gas and the fuel in the compression phase (b); ignition of a mixture of inlet gas and fuel formed in the combustion chamber (c); expansion and discharge of an exhaust gas formed by the combustion in an expansion phase (d), wherein a pilot quantity of fuel (e) is introduced into the combustion chamber before the primary quantity of fuel (x) is introduced; intermediate products of the pilot quantity of fuel are formed (f) and the primary quantity of fuel (x) is introduced into the combustion chamber during the compression phase (b) in such a way that the complete ignition of the mixture consisting of inlet gas and the intermediate products is suppressed and other intermediate products are formed until a controlled ignition of the mixture and other intermediate products takes place (c). The claimed combustion method fulfils the conflict of objectives of fuel consumption versus NOx emissions during lean combustion (spark-ignition and diesel) and thus future emission regulations, without fuel consumption losses.
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  • Dynamics of premixed hydrogen/air flames in microchannels

    Pizza, Gianmarco   Frouzakis, Christos E.   Mantzaras, John   Tomboulides, Ananias G.   Boulouchos, Konstantinos  

    The stabilization and dynamics of lean (psi = 0.5) premixed hydrogen/air atmospheric-pressure flames in planar microchannels of prescribed wall temperature are investigated with respect to the inflow velocity and channel height (0.3 to 1.0 mm) using direct numerical simulation with detailed chemistry and transport. Rich dynamics starting from periodic ignition and extinction of the flame and further transitioning to symmetric V-shaped flames, asymmetric flames, oscillating and pulsating flames, and finally again to asymmetric flames are observed as the inlet velocity is increased. The richest behavior is observed for the 1.0-mm-height channel. For narrower channels, some of the dynamics are suppressed. The asymmetric flames, in particular, vanish for channel heights roughly less than twice the laminar flame thickness. Stability maps delineating the regions of the different flame types in the inlet velocity/channel height parameter space are constructed. (C) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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  • Neat polyoxymethylene dimethyl ether in a diesel engine; part 1:Detailed combustion analysis

    Barro, Christophe   Parravicini, Matteo   Boulouchos, Konstantinos  

    Oxygenated fuels have already shown potential to be used in Diesel engines without major modifications. Polyoxymethylene dimethyl ether (POMDME, short OME) offers particular characteristics (i.e. no C-C bonds, high cetane number, high potential to reduce greenhouse gases, when produced with renewable energy, etc.) which are favourable to be combusted in a standard Diesel engine and allow the potential to overcome the classical NOx-PM trade off. In order to capture the full potential of this fuel, its combustion characteristics under diesel engine conditions need to be investigated. In the present work the combustion behaviour of neat OME in a composition of approximately 80% OME3 and 20% OME4 has been investigated. For the measurement campaign, a heavy duty single cylinder research engine was used. For the operation with OME, the eight injector nozzle holes were increased from 0.24 mm to 0.29 mm. The operating conditions included variations in fuel, nozzle size, EGR, boost pressure and load. A significant number of operating conditions with OME have been investigated under stoichiometric global air/fuel ratio conditions. It was observed that the ignition delay of OME is shorter than the one of Diesel, due to the higher cetane number of the oxygenated fuel. Moreover OME showed a faster combustion. The reason for this fast combustion behaviour has not been found in the lower demand on air directly, but mainly in the higher turbulence level for mixing due to combustion closer to the injector nozzle tip as a result of the lower oxygen demand. A computationally fast description for steady state sprays has been used to express a characteristic time scale which is required for the fuel to mix with surrounding air up to stoichiometric conditions. The resulting characteristic number tau(spray) correlates well with the characteristic mixing rate tau(mix), calculated from the measurement data. This description offers the possibility to predict the diffusion combustion rate in a very wide range of engine operation parameter variations, including changes in nozzle size, fuel injection pressure or fuel composition with strongly air demand.
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  • Numerical assessment of wall modelling approaches in scale-resolving in-cylinder simulations

    Keskinen, Karri   Koch, Jann   Wright, Yuri M.   Schmitt, Martin   Nuutinen, Mika   Kaario, Ossi   Vuorinen, Ville   Larmi, Martti   Boulouchos, Konstantinos  

    Wall modelling in internal combustion engines (ICEs) is a challenging task due to highly specific boundary layers and a dynamically changing flow environment. Recent experimental (Jainski et al., 2013, Renaud et al., 2018) and direct numerical simulation (DNS, Schmitt et al., 2015a) studies demonstrate that scaled near-wall velocity and temperature profiles in ICEs deviate considerably from the law of the wall. Utilising the DNS data, the present paper focusses on benchmarking a scale-resolving approach with a 1-D non-equilibrium wall model (HLR-WT, Keskinen et al., 2017) in ICE-like flows. Specific emphasis is put on the compression stroke using different grids and two additional wall-modelled large eddy simulation (WMLES) reference approaches. The standard wall law based WMLES-1 produces highly grid-dependent under-prediction of wall fluxes, to which WMLES-2 (Plensgaard and Rutland, 2013) and HLR-WT, employing engine-targeted wall treatments, yield considerable improvement. Differences between the improved methods are noted in detailed metrics. Throughout the compression stroke, HLR-WT provides a good match to the DNS in scaled mean boundary layer profiles for both velocity and temperature. With relevance to local heat flux distribution, the characteristic impingement-ejection process observed in the DNS is qualitatively replicated with WMLES-2 and HLR-WT. The non-equilibrium formulation of the latter allows for slight improvements in terms of local heat transfer fluctuation predictions. In contrast, coarse near-wall grids appear to be detrimental for such predictions with all approaches. The study provides evidence on the potential of the HLR-WT and WMLES-2 approaches in ICE near-wall flow prediction, advocating further investigations in more realistic engine configurations.
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  • Hydrogen-enhanced gasoline stratified combustion in SI-DI engines

    Conte, Enrico   Boulouchos, Konstantinos  

    Experimental investigations were carried out to assess the use of hydrogen in a gasoline direct injection (GDI) engine. Injection of small amounts of hydrogen (up to 27% on energy basis) in the intake port creates a reactive homogeneous background for the direct injection of gasoline in the cylinder In this way, it is possible to operate the engine with high exhaust gas recirculation (EGR) rates and, in certain conditions, to delay the ignition timing as compared to standard GDI operation, in order to reduce NO(x) and HC emissions to very low levels and possibly soot emissions. The results confirmed that high EGR rates can be achieved and NO., and HC emissions reduced, showed significant advantage in terms of combustion efficiency and gave unexpected results relative to the delaying of ignition, which only partly confirmed the expected behavior A realistic application would make use of hydrogen-containing reformer gas produced on board the vehicle but safety restrictions did not allow using carbon monoxide in the test facility. Thus, cure hydrogen was used for a best-case investigation. The expected difference in the use of the two gases is briefly discussed.
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