1,720,962 research outputs found
CFD Modeling of Gas-Fuel Interaction and Mixture Formation in a Gasoline Direct-Injection Engine Coupled with the ECN Spray G Injector
No full textThe thorough understanding of the effects due to the fuel direct injection process in modern gasoline direct injection engines has become a mandatory task to meet the most demanding regulations in terms of pollutant emissions. Within this context, computational fluid dynamics proves to be a powerful tool to investigate how the in-cylinder spray evolution influences the mixture distribution, the soot formation and the wall impingement. In this work, the authors proposed a comprehensive methodology to simulate the air-fuel mixture formation into a gasoline direct injection engine under multiple operating conditions. At first, a suitable set of spray sub-models, implemented into an open-source code, was tested on the Engine Combustion Network Spray G injector operating into a static vessel chamber. Such configuration was chosen as it represents a typical gasoline multi-hole injector, extensively used in modern gasoline direct injection engines. Afterwards, the Spray G injector was coupled with the Darmstadt optical engine and full-cycle simulations were carried out for three operating points, combining two engine speeds, respectively equal to 800 rpm and 1500 rpm, and two different engine loads, with pressures of 0.95 bar and 0.4 bar in the intake manifold. The case at 800 rpm and 0.95 bar represented the reference condition. By switching to 1500 rpm and 0.95 bar the effect of the piston speed on the in-cylinder flow and spray evolution was analysed, while the reduction of the intake pressure down to 0.4 bar, coupled with the engine speed of 800 rpm, allowed to study the effects of the engine load on spray evolution and mixture fraction formation. Furthermore, comparisons between the engine cases at 0.95 bar and the simulations in vessel allowed to understand the effects exerted by the turbulence generation on the spray morphology. A detailed post-processing was proposed for each condition. For the vessel, axial vapor and liquid penetrations were assessed, along with spray morphology and liquid mass distribution inside the jet. In the engine, quantities such as in-cylinder gas velocities, mixture fraction distribution and charge homogeneity were investigated. The achieved results demonstrated the potential of the computational fluid dynamics as an effective tool for direct-injection, spark ignition engines optimization towards the goals of emissions reduction and increased efficiency
Numerical Investigation on GDI Spray under High Injection Pressure up to 100 MPa
No full textIn recent years, the increase of gasoline fuel injection pressure is a way to improve thermal efficiency and lower engine-out emissions in GDI homogenous combustion concept. The challenge of controlling particulate formation as well in mass and number concentrations imposed by emissions regulations can be pursued improving the mixture preparation process and avoiding mixture inhomogeneity with ultra-high injection pressure values up to 100 MPa. The increase of the fuel injection pressure in GDI homogeneous systems meets the demand for increased injector static flow, while simultaneously improves the spray atomization and mixing characteristics with consequent better combustion performance. Few studies quantify the effects of high injection pressure on transient gasoline spray evolution. The aim of this work was to simulate with OpenFOAM the spray morphology of a commercial gasoline injected in a constant volume vessel by a prototypal GDI injector. Different operating conditions were considered under very high injection pressure up to 100 MPa. The transient spray evolution in a constant volume vessel was analyzed from an experimental and numerical point of view in different ambient conditions. The resulting development of the jet plumes was assessed, along with the physical effects of injection pressure. A RANS Eulerian-Lagrangian approach was adopted to couple the gas phase with the liquid jet and a complete validation of atomization and secondary breakup models was performed. Furthermore, different values of ambient pressure were investigated to validate the robustness of the proposed numerical set-up in different ambient conditions. Experimentally, an optical technique characterized by a hybrid Mie-scattering /shadowgraph approach were adopted registering images on a high-speed C-Mos camera. The spatial distribution and the time-resolved evolution of the free sprays were derived under different ambient conditions along with their characteristics. Numerical simulations allowed a good reproduction of the fuel penetration and spread in the constant vessel under very high fuel injection pressure, depicting the strong sensitivity of the spray profiles against the ambient conditions and confirming fundamental information on the physics of fuel provided by the experiments. Under flash-boiling settings, the very high injection pressures induced a loss of the classic mushroom morphology, related to the spray-collapse, because the increased droplet velocities, along the axial direction, become a dominant effect
CFD Modeling of Impinging Sprays under Large Two-Stroke Marine Engine-Like Conditions
No full textTo improve the combustion and emission characteristics of the large-bore marine engines, the spray is usually designed as an inter-spray impingement to promote the fuel-air mixing process, which implies frequent droplet collisions. Properly describing the collision dynamics of liquid droplets has been of interest in the field of spray modeling for marine engine applications. In this context, this work attempts to develop an accurate and efficient methodology for modeling impinging sprays under engine-like conditions. Experimental validations in terms of spray penetration and morphology are initially carried out at different operating conditions considering the parametric variations of ambient temperature and pressure, where the measurements are performed on a large-scale constant volume chamber with two symmetrical injectors. The existing models, including O'Rourke and Nordin's collision models, are also applied and the obtained results are compared to further assess the accuracy and efficiency of the present model. The collision regimes are analyzed to explore the outcomes of the spray-spray impingement and to guide the future work of injection optimization for large two-stroke marine engine applications. The results suggested that the O'Rourke model overestimates the collision probability, while the Nordin and Nordin-MR approaches show similar accuracy since the outcomes of reflective separation and shattering are negligible in current impinging spray configuration and the lack of bouncing does not make significant difference in modeling the fuel evaporation and mixing processes at 800-K ambient. The similar computational cost were achieved by the Nordin and Nordin-MR models, suggesting that both of them are applicable to practical marine engine simulations. Besides, the spray impingement could strongly stimulate the droplet breakup at the impinging location, which leads to a subsequent enhanced mixing with the potential to mitigate the soot emission in engine applications
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Validation of a comprehensive computational fluid dynamics methodology to predict the direct injection process of gasoline sprays using Spray G experimental data
No full textA detailed prediction of injection and air–fuel mixing is fundamental in modern direct injection, spark-ignition engines to guarantee a stable and efficient combustion process and to minimize pollutant formation. Within this context, computational fluid dynamics simulations nowadays represent a powerful tool to understand the in-cylinder evolution of spray and air–fuel charge. To guarantee the accuracy of the adopted multidimensional spray sub-models, it is mandatory to validate the computed results against available experimental data under well-defined operating conditions. To this end, in this work, the authors proposed the calibration and validation of a comprehensive set of spray sub-models by means of the simulation of the Spray G experiment, available in the context of the engine combustion network. For a suitable validation of the proposed numerical setup in addition to the baseline condition, gasoline direct injection operating points typical of early injection with homogeneous operation, late injection with high ambient density and flash boiling with enhanced fuel evaporation were also simulated. Numerical computations were validated against a wide set of available experimental data by means of an accurate post-processing analysis taking into account axial liquid and vapor penetrations, gas-phase velocity between spray plumes, droplet size, plume liquid velocity, direction and mass distribution. Satisfactory results were achieved with the proposed setup, which is able to predict gasoline spray evolution under different operating conditions
CFD Modeling and Validation of the ECN Spray G Experiment under a Wide Range of Operating Conditions
No full textThe increasing diffusion of gasoline direct injection (GDI) engines requires a more detailed and reliable description of the phenomena occurring during the fuel injection process. As well known the thermal and fluid-dynamic conditions present in the combustion chamber greatly influence the air-fuel mixture process deriving from GDI injectors. GDI fuel sprays typically evolve in wide range of ambient pressure and temperatures depending on the engine load. In some particular injection conditions, when in-cylinder pressure is relatively low, flash evaporation might occur significantly affecting the fuel-air mixing process. In some other particular injection conditions spray impingement on the piston wall might occur, causing high unburned hydrocarbons and soot emissions, so currently representing one of the main drawbacks of GDI engines. Within this context, a deep understanding of the spray evolution by means of accurate experimental and numerical investigations is of great importance for reduction of pollutant emissions and fuel consumption. This work is focused on the CFD simulation of the 8-hole, ECN Spray G injector under constant volume conditions. Calculations were carried out with the LibICE code, which is based on the OpenFOAM technology. Different ambient pressure and temperature conditions were investigated for a comprehensive evaluation of the proposed numerical setup. The resulting developments of the jet plumes were assessed, along with the physical effects of injection pressure and sub-atmospheric ambient pressure conditions. The capability of the code in describing the spray evolution in terms of penetration and diffusion under flash-boiling conditions was here analyzed. A validation of atomization, secondary breakup and wall impingement models was performed through a comparison with experimental data obtained with optical techniques characterized by a hybrid Mie-scattering/schlieren approach. The spatial distribution and the time-resolved evolution of the free sprays were derived along with their post-impingement characteristics
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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