1,721,004 research outputs found
Enhanced splash models for high pressure diesel spray
No full textMixture preparation is a crucial aspect for the correct operation of modern direct injection (DI) Diesel engines as it greatly influences and alters the combustion process and, therefore, the exhaust emissions. The complete comprehension of the spray impingement phenomenon is a quite complete task and a mixed numerical-experimental approach has to be considered. On the modeling side, several studies can be found in the scientific literature but only in the last years complete multidimensional modeling has been developed and applied to engine simulations. Among the models available in literature, in this paper the models by Bai and Gosman (Bai, C., and Gosman, A. D., 1995, SAE Technical Paper No. 950283) and by Lee et al. (Lee, S., and Ryou, H., 2000, Proceedings of the Eighth International Conference on Liquid Atomization and Spray Systems, Pasadena, CA, pp. 586-593; Lee, S., Ko, G. H., Ryas, H., and Hong, K. B., 2001, KSME Int. J., 15(7), pp. 951-961) have been selected and implemented in the KIVA-3V code. On the experimental side, the behavior of a Diesel impinging spray emerging from a common rail injection system (injection pressures of 80 and 120 MPa) has been analyzed. The impinging spray has been lightened by a pulsed laser sheet generated from the second harmonic of a Nd-yttrium-aluminum-garnet laser. The images have been acquired by a charge coupled device camera at different times from the start of injection. Digital image processing software has enabled to extract the characteristic parameters of the impinging spray with respect to different operating conditions. The comparison of numerical and experimental data shows that both models should be modified in order to allow a proper simulation of the splash phenomena in modern Diesel engines. Then the numerical data in terms of radial growth, height and shape of the splash cloud, as predicted by modified versions of the models are compared to the experimental ones. Differences among the models are highlighted and discussed. Copyright © 2007 by ASME
Evaluation of Spray Impingement Models in Multidimensional Simulation of High Speed Diesel Engines
No full textSpray impingement on walls is an important physical
process in modern DI Diesel engines as it greatly
influences mixture formation, combustion process and
exhaust emissions. The mixture preparation is, in fact, a
crucial aspect for the correct operation of the engine as
it significantly affects the combustion process.
In this paper three models, among the available in
literature, have been selected and implemented in the
KIVA-3V code. Namely, the models by O’Rourke and
Amsden (OA model) [1, 2], by Bai and Gosman (BG
model) [3] and by Lee et al. (LR model) [4, 5] are
compared in terms of performance and capability of
representing the splash phenomenon. The model
capabilities are firstly tested comparing the numerical
results with four sets of experimental literature data,
characterized by low injection pressures.
The high injection pressures of modern Diesel engines
result in droplets velocities emerging from the nozzle
greater than 300 m/s. To better understand the models
behavior with high injection pressures, an experimental
investigation of the splash behavior of a Diesel spray
emerging from a Common Rail (CR) injection apparatus,
with injection pressures up to 120 MPa, is carried out.
The impinging spray is lightened by a pulsed laser sheet
generated from the second harmonic of a Nd - YAG
laser. The images are acquired by a CCD camera at
different times from the start of injection (SOI). A digital
image processing software enables to extract the
characteristic parameters of the impinging spray versus
different operative conditions.
The numerical data of the splash phenomenon, as
predicted by modified versions of the proposed models
are compared to the experimental ones. All the three
models in their original formulation result to be
inadequate to reproduce the splash phenomena.
Nevertheless, after slight modifications and a proper
updating phase, the BG and the LR models prove to
show a good agreement with the experimental data
High Pressure Diesel-Like Injections for GDI Engine: Experimental and Numerical Approach
No full textMultihole injectors, working like diesel valve coverage orifice (VCO) or mini-sac nozzle, seem to be the future trend for GDI (Gasoline Direct Injection) applications. The GDI approach through this injector type is very similar to diesel one.
A diesel-like electronically controlled Common Rail injection apparatus has been used for pressure up to 100 MPa. An axially disposed single hole 0.18 mm in diameter and 1.0 mm in length injected a gasoline-like fluid in an optically accessible vessel filled with inert gas (N2) and controlled in pressure up to 1.2 MPa. The jets emerging from the injector hole have been lightened by a pulsed laser sheet at different instant from the start of injection. The images were collected by a CCD camera, synchronized with the light, and processed by professional software to extract the significant parameters of the evolving spray (penetration, cone angle, velocity).
Some results of a work in progress aiming to select and validate proper models for the various stages of spray development are also discussed. Four different models have been compared, to evaluate the one that better represents the characteristics of the generated spray. The final goal of the research activity is to set up the KIVA 3V code for its extensive use in the design and development of GDI engines
Schlieren and Mie scattering techniques for the ECN “spray G” characterization and 3D CFD model validation
No full textPurpose – This paper aims to study the heat transfer phenomenon occurring between heated walls and impinging fuel, showing the strict relationship between cooling effect after impingement and enhancing of wallfilm formation. The study focuses on a fundamental task in terms of pollutant emissions in internal combustion engines, aiming at giving a major contribution to the optimization of energy conversion systems in terms of environmental impact. Design/methodology/approach – The paper is based on experimental campaigns relevant at taking measurements of an impinging spray over a heated wall in a confined vessel. The results, in both qualitative and quantitative terms (measurements of liquid and vapour radial penetration and thickness), are numerically reproduced by a computational model based on a Reynolds Averaged Navier Stokes approach, properly validated through customized sub-models. Findings – The paper provides quantitative results about the agreement between radial penetration and vapour thickness between measurements and simulation, achieved by taking into account the cooling effect determined by the fuel impingement. This validation of the numerical model allows the author to give more considerations about the link between wall temperature and wallfilm formation. Originality/value – This paper presents an original approach for the simulation of wall heat transfer, by imposing a boundary condition at the wall that may consider the heat conduction and temperature cooling given by fuel impingement in both lateral and normal directions. The classical Dirichlet boundary condition, characterized by imposing a fixed temperature value, is, instead, replaced by an approach based on calculating the unsteady process that couples the heat fluxes between the fluid and the solid material and within the solid itself
Experimental and numerical analysis of high pressure diesel spray-wall interaction
No full textThe interaction between impacting and splashed droplets and air motion plays a fundamental role on the mixture formation process, which is a crucial aspect for the correct operation of modern DI Diesel engines as it greatly influences the combustion process and the exhaust emissions. A complete understanding of spray impingement is quite complex. A mixed numerical-experimental approach is proposed in this paper. The experimental tests are carried out with a high pressure (up to 120 MPa) diesel spray emerging from an axial disposed single-hole nozzle in an optically accessible vessel, pressurized up to 5.0 MPa at ambient temperature. The jet impacts on a flat stainless steel wall heated up to 500 degrees C by a 200 W temperature regulated electrical resistance wire. The experimental analysis is performed using a Bosh tube as the injection mass flow meter, a pulsed laser sheet generated on the second harmonic of a Nd-YAG laser and a synchronized CCD camera. Digital image post-processing allows extraction of the radial penetration and thickness growth of the impacted fuel versus injection pressure, vessel back-pressure and wall temperature. Moreover, a procedure to relate light intensity to average fuel density is proposed. The numerical analysis is carried out by means of a multi-dimensional numerical tool, based on the KIVA-3V code. The spray wall interaction is simulated through a phenomenological splash model available in literature and validated for low injection pressures (up to 300 bar) and ambient back-pressure. The comparison between experimental and numerical results demonstrates the inability of the model in predicting high pressure spray-wall interaction, especially under increasing back-pressures. Based on the experimental evidences, a modified version of the model is proposed and the new model is proven to be an adequate representation for different injection pressures and back-pressures. (c) 2007 Elsevier Ltd. All rights reserved
A 3D CFD Simulation of GDI Sprays Accounting for Heat Transfer Effects on Wallfilm Formation
No full textDuring gasoline direct injection (GDI) in spark ignition engines, droplets may hit piston or liner surfaces and be rebounded or deposit in the liquid phase as wallfilm. This may determine slower secondary atomization and local enrichments of the mixture, hence be the reason of increased unburned hydrocarbons and particulate matter emissions at the exhaust. Complex phenomena indeed characterize the in-cylinder turbulent multi-phase system, where heat transfer involves the gaseous mixture (made of air and gasoline vapor), the liquid phase (droplets not yet evaporated and wallfilm) and the solid walls. A reliable 3D CFD modelling of the in-cylinder processes, therefore, necessarily requires also the correct simulation of the cooling effect due to the subtraction of the latent heat of vaporization of gasoline needed for secondary evaporation in the zone where droplets hit the wall. The related conductive heat transfer within the solid is to be taken into account. In this work, a preliminarily validated spray model is specifically implemented by solving the strongly coupled heat and mass transfer problem describing the liquid and vapor phases thermo-fluidynamics after impact and the wall change of temperature. The discussion is made considering a different boundary condition with respect to standard simulations. Sprays are assumed from to different injectors in order to verify the wallfilm simulation model: the impact over heated walls of the ECN “Spray G” is first discussed, by comparing numerical results with experimental measurements deriving from a combined use of the schlieren and Mie-scattering techniques, then the footprint on the wall of the spray delivered from a 6-hole Bosch injector is related with infrared thermography and LIF measurements taken from the literature
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