1,721,032 research outputs found
3D-CFD Methodologies for a Fast and Reliable Design of Ultra-Lean SI Engines
No full textThe continuous pursuit of higher combustion efficiencies, as well as the possible usage of synthetic fuels with different properties than fossil-ones, require reliable and low-cost numerical approaches to support and speed-up engines industrial design. In this context, SI engines operated with homogeneous ultra-lean mixtures both characterized by a classical ignition configuration or equipped with an active prechamber represent the most promising solutions. In this work, for the classical ignition arrangement, a 3DCFD strategy to model the impact of the ignition system type on the CCV is developed using the RANS approach for turbulence modelling. The spark-discharge is modelled through a set of Lagrangian particles, whose velocity is modified with a zero-divergence perturbation at each discharge event, then evolved according to the Simplified Langevin Model (SLM) to simulate stochastic interactions with the surrounding gas flow. For the active prechamber configuration, instead, a CFD methodology based on a 3DRANS approach is proposed to model the combustion process, in which the air-fuel mixture distribution inside the prechamber is properly considered without a 3D simulation of the main chamber intake process. This is carried out through a non-reacting simulation of the prechamber-only until IVC, then results are interpolated on a different 3D engine mesh for the further compression and combustion stages. The proposed methodologies were assessed and validated against experimental measurements at different operating conditions
A novel simulation methodology for orthogonal cryogenic machining with CFD spray cooling integration
Full text linkThe performance of cryogenic machining depends on the effectiveness of the heat transfer between the coolant jet and the chip in the cutting area because it affects the material temperature and the mechanical properties of the chip. This is a complex multi-physics problem because the solid deformation depends on the thermal and fluid–dynamic interaction with the cryogenic droplets generated by the atomization of the coolant jet. Within this context, this work applies an innovative methodology based on computational fluid dynamics to simulate the cutting process accounting for the interaction with the cryogenic jet. The proposed approach does not require empirical correlations since it integrates a predictive machining analytical model with Conjugate Heat Transfer CFD simulation and spray modelling to accurately estimate the heat transfer process accounting for the cooling effect of the impinging droplets. Complete Ti6Al4V dry and cryogenic cooled orthogonal cutting simulations were performed and results were compared with literature experimental data and state-of-the-art Finite Element Modelling simulations. The proposed methodology correctly estimates the cutting forces to vary cutting velocity and depth. Average errors in the resultant force estimation are 11.85% in dry and 14.4% in cryogenic cutting. Moreover, the experimental increase of the cutting force due to cooling is better estimated by the proposed approach with respect to FEM simulations. Thanks to the results accuracy and reduced computational costs, the proposed methodology could improve the understanding and the design of this innovative machining technology
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
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
A Finite-Element Based Mesh Motion Technique for Internal Combustion Engine Simulations
No full textCFD codes are now widely used to investigate the in-cylinder thermofluid-dynamic processes. Despite the availability of complex physical models, the results are strongly influenced by the mesh quality, which should be kept high for the whole simulation even in presence of extreme boundary deformations like valve penetration into the piston bowl or piston approaching the cylinder head at the top dead center. For this reason the whole simulation is generally covered by using a different number of meshes and each one of them is used for a certain crank angle interval. This provides good mesh quality and resolution when efficient algorithms for field mapping and grid point motion are available: interpolation-related errors should be avoided and the number of target meshes should be kept at minimum to reduce the user turnaround time required for mesh generation and motion.
In this context, the authors have implemented in the OpenFOAM code moving mesh algorithms for internal combustion engine simulations. The grid points motion is described by a vertex-based unstructured mesh motion solver: the Laplace equation of motion is solved with variable diffusion on the mesh vertices, using a Finite Element method with polyhedral cell support. This guarantees that an initial valid mesh remains valid for arbitrary boundary motion. Different expressions for the motion diffusion can be used to improve the quality of the mesh during motion. A cell based, distance weighted interpolation technique is used to interpolate the geometric fields from the source to the target mesh.
The proposed approach has been applied to simulate the in-cylinder flows in different engine geometries. Firstly a flat-head geometry with a centrally located valve was used to evaluate the efficiency of the field mapping and mesh motion algorithms. Then two real engine configurations were modeled: for both of them the simulation of the intake, compression and combustion phases was performed and a comparison with experimental data of in-cylinder pressure is provided
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
A Constant Equivalence Ratio Multi-Zone Approach for a Detailed and Fast Prediction of Performances and Emission in CI Engines
No full textThe paper illustrates and validates a novel predictive combustion model for the estimation of performances and pollutant production in CI engines. The numerical methodology was developed by the authors for near real-time applications, while aiming at an accurate description of the air mixing process by means of a multi-zone approach of the air-fuel mass. Charge stratification is estimated via a 2D representation of the fuel spray distribution that is numerically derived by an axial one-dimensional control-volume description of the direct injection. The radial coordinate of each control volume is reconstructed a posteriori by means of a local distribution function. Fuel mass clustered in each zone is further split in 'liquid', 'unburnt' and 'burnt' sub-zones, given the local properties of the fuel spray control volumes with respect to space-time location of modelled ignition delay, liquid length, and flame lift-off. Multiple injections are described on the same numerical grid to account for jet-to-jet axial interactions, whose effects reflect on improved ignition characteristics. The multi-zones are open systems which are discretized on the equivalence ratio; mass is allowed to travel from one to another, causing a 2D charge stratification. For each zone, local thermodynamic properties and NOx production are determined to estimate cylinder-average performances and emissions. The apparent heat release rate, in-cylinder pressure, BSFC and NOx emissions are validated against experimental data of full map of a light-duty engine. The computational effort of the model is relatively low, which makes the approach suitable for static optimization, to be used in 1-D simulation codes for transient's optimization and can be run in parallel with a real time 1-D gas model for the simulation of long driving cycles
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
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