1,721,022 research outputs found
Numerical Analysis of Injector Flow and Spray Characteristics from Diesel Injectors Using Fossil and Biodiesel Fuels
No full textThe aim of the paper is the comparison of the injection process with two fuels, a standard diesel fuel and a pure biodiesel, methyl ester of soybean oil. Multiphase cavitating flows inside injector nozzles are calculated by means of unsteady CFD simulations on moving grids from needle opening to closure, using an Eulerian-Eulerian two-fluid approach which takes into account bubble dynamics. Afterward, spray evolutions are also evaluated in a Lagrangian framework using results of the first computing step, mapped onto the hole exit area, for the initialization of the primary breakup model.
Two nozzles with cylindrical and conical holes are studied and their behaviors are discussed in relation to fuel properties.
Nozzle flow simulations highlighted that the extent of cavitation regions is not much affected by the fuel type, whereas it is strongly dependent on the nozzle shape. Biodiesel provides a slightly higher mass flow in highly cavitating nozzles. On the contrary using hole shaped nozzles (to reduce cavitation) diesel provides similar or slightly higher mass flow. Comparing the two fuels, the effects of different viscosities and densities play main role which explains these behaviors.
Simulations of the spray evolution are also discussed highlighting the differences between the use of fossil and biodiesel fuels in terms of spray penetration, atomization and cone-angle. Usage of diesel fuel in the conical convergent nozzle gives higher liquid penetration
Diesel engine NOx emissions control: an advanced method for the O2 evaluation in the intake flow.
No full textIn recent decades, the increasingly tight emissions regulations, along with the ever-increasing price of
fuels and the request for more power from the engines, has pushed the world car industry to improve
the performances of the applications of electronics, designed to control the internal combustion engines
(ICE) and the pollutant emissions systems. At present, one of the main problems, in the development of
diesel engines is represented by the achievement of an increasingly strict control on the systems used for
the pollutant emission reduction. In particular, as far as NOx gas is concerned, EGR systems are mature
and widely used, but increased efficiency in terms of emissions abatement, is necessary in order to determine
as best possible the actual oxygen content in the charge at the engine intake manifold. The present
work compares the ability of the ANN and Neuro-Fuzzy approach (ANFIS) to predict the volumetric oxygen
concentration at the intake, using experimental data acquired on a compression ignition engine in transient
operational conditions. In an off-line evaluation of results, both models show good predicting abilities;
in particular the ANFIS model presents an absolute error value for the training and test phases
respectively equal to 0.7 and 0.9 (as a percentage of 3.5% and 4.5%), while, the same evaluation obtained
using the ANN-BP model provides 0.92 and 0.9 (as a percentage of 4.6% and 4.5%). The comparison shows
that the ANFIS model produces more accurate solutions in less time, using linear rules that bind the input
variables with the output. The linearity of the rules is a key feature to decrease the convergence time
especially when the model is used for the on-board periodic training activity due to the aging of the
engine
CFD Analysis of Unthrottled Part Load in a SI Engine with Different Valve Lift Strategies
No full textInvestigation of Homogeneous Relaxation Model Parameters and their Implications for Gasoline Injectors
No full textFlash boiling is known to be a common phenomenon for gasoline direct injection (GDI) engine sprays. The Homogeneous Relaxation Model has been adopted in many recent numerical studies for predicting cavitation and flash boiling. The Homogeneous Relaxation Model is assessed in this study. Sensitivity analysis of the model parameters has been documented to infer the driving factors for the flash-boiling predictions. The model parameters have been varied over a range and the differences in predictions of the extent of flashing have been studied. Apart from flashing in the near-nozzle regions, mild cavitation is also predicted inside the gasoline injectors. The variation in the predicted time scales through the model parameters for predicting these two different thermodynamic phenomena (cavitation, flash) have been elaborated in this study. Turbulence model effects have also been investigated by comparing predictions from the standard and Re-Normalization Group (RNG) k−ε turbulence models
Experimental and Numerical Momentum Flux Evaluation of High Pressure Diesel Spray
No full textIn the present work, an experimental and numerical analysis of high pressure Diesel spray evolution is carried out in terms of spray momentum flux time history and instantaneous injection rate. The final goal of spray momentum and of injection rate analyses is the evaluation of the nozzle outlet flow characteristics and of the nozzle internal geometry possible influences on cavitation phenomena, which are of primary importance for the spray evolution. Further, the evaluation of the flow characteristics at the nozzle exit is fundamental in order to obtain reliable boundary conditions for injection process 3D simulation. In this paper, spray momentum data obtained in ambient temperature, high counter-pressure conditions at the Perugia University Spray Laboratory are presented and compared with the results of 3D simulations of the momentum rig itself. The experimental tests, performed using a commercial common-rail injector, include high speed imaging of the spray during momentum tests and the measurement of instantaneous injection rate for a more detailed comparison with the numerical analysis. Numerical 3D simulations allowed to evaluate the actual contribution to global spray momentum given by the liquid phase and by the gaseous phase, as a function of time, nozzle-target distance and target-diameter. This analysis allowed to explain the observed influences of the measurement procedure on the spray momentum flux experimental data. In particular, the present CFD analysis highlighted a significant contribution to the jet momentum flux provided by gaseous phase, which tends to become predominant with growing distances from the nozzle. The findings suggested that the target design and position can affect significantly the spray momentum flux measurement. In some operating conditions, unexpected trends of the spray momentum time integral as a function of the target distance and target-diameter were obtained, suggesting that main hypotheses on which spray momentum measurement devices are based – i.e. orthogonal flow deviation – may not hold true for some system configurations. A numerical analysis of the spray impact force was developed to evaluate the target intrusiveness, pointing out the irrelevance of the target when present into the simulated domain. A practical policy to define the optimal combination of target-diameter and nozzle-target distance, for each set of operating conditions, as a compromise between the extreme configurations of the involved parameters, is explained
Comparison of Mixture and Multifluid Models for In-Nozzle Cavitation Prediction
No full textFuel injectors often feature cavitation because of large pressure gradients, which in
some regions lead to extremely low pressures. The main objective of this work is to
compare the prediction capabilities of two multiphase flow approaches for modeling cavitation
in small nozzles, like those used in high-pressure diesel or gasoline fuel injectors.
Numerical results are assessed against quantitative high resolution experimental data
collected at Argonne National Laboratory using synchrotron X-ray radiography of a
model nozzle. One numerical approach uses a homogeneous mixture model with the volume
of fluid (VOF) method, in which phase change is modeled via the homogeneous
relaxation model (HRM). The second approach is based on the multifluid nonhomogeneous
model and uses the Rayleigh bubble-dynamics model to account for cavitation. Both
models include three components, i.e., liquid, vapor, and air, and the flow is compressible.
Quantitatively, the amount of void predicted by the multifluid model is in good agreement
with measurements, while the mixture model overpredicts the values. Qualitatively,
void regions look similar and compare well with the experimental measurements. Grid
converged results have been achieved for the prediction of mass flow rate while gridconvergence
for void fraction is still an open point. Simulation results indicate that most
of the vapor is produced at the nozzle entrance. In addition, downstream along the centerline,
void due to expansion of noncondensable gases has been identified. The paper
also includes a discussion about the effect of turbulent pressure fluctuations on cavitation
inception
Numerical Study of SI Engine Part Load Operating Conditions Using Different VVA Strategies
No full textIn SI engines, VVA (Variable Valve Actuation) technology is mainly used for the reduction of pumping losses at part load. This paper presents the results of fluid dynamic analyses on a 4V engine about the effects of different VVA strategies, by comparing and discussing the results in terms of organized charge motions, turbulence levels, flame developments, NO and CO emissions. CFD simulations cover five load control cases: comparison is among conventional throttling, EIVC (Early Intake Valve Closure) with symmetric and asymmetric intake lifts, LIVC (Late Intake Valve Closure) and symmetrical Multi-Lift strategies. 3D U-RANS simulations are performed, adopting the Extended Coherent Flamelet Model (ECFM) for the description of premixed SI combustion. The 3D model is also coupled to a 1D engine model which provides inlet/outlet boundary conditions.
Simulation results highlight the potential of asymmetric Early Intake Valve Closure (EIVC) strategy which allows reducing pumping losses and, at the same time, achieving good turbulence intensity and combustion speed, if compared to other load control strategies. Multi-Lift strategy resulted excellent in terms of burn duration, but pumping losses are practically the same as in the throttled engine.
CFD Modeling of the Nozzle Flow and Near-field Spray on ECN Spray B Injector
No full textIt is important to understand the fluid dynamics inside the diesel injector and flow development in the
nozzle due to their direct impact on the fuel-air mixing and combustion. This work uses a Eulerian approach
and performs fully coupled simulations of the injector flow and near-field spray. This approach
treats the liquid fuel and gas phases as a single mixture in the nozzle and near nozzle dense sprays. The
liquid fuel mass fraction is transported with a model for the turbulent liquid diffusion flux using a Volume-
of-Fluid method. The cavitation is modeled by the homogeneous relaxation model. A standard kepsilon
turbulence model is used with round-jet correction. The well-characterized ECN spray B injector
is simulated under the x-ray radiography measurement conditions. The transient flow development and
cavitation phenomena are analyzed in details. The effect of the needle motion on the flow and hole-tohole
variations is discussed. A comparison between the multi-hole injector and single-hole Spray A injector
is also conducted
Sperimentazione di un motore ad accensione comandata alimentato con miscele di idrogeno e metano
No full textLa presente memoria descrive un’attività sperimentale di ricerca volta a quantificare le prestazioni, in termini di consumi ed
emissioni, ottenibili da un motore ad accensione comandata trasformato per essere alimentato con miscele idrogeno e metano.
Le prove sono state condotte su un motore Iveco Daily di cilindrata 2800 cc aspirato con iniezione elettronica multi-point
originariamente funzionante a metano. L’analisi ha previsto l’impiego di più miscele metano-idrogeno caratterizzate da diverse
concentrazioni di idrogeno, con valore massimo pari al 35% in volume.
Le attività presentate sono le prime effettuate nell’ambito di una ricerca piu’ ampia, e tuttora in svolgimento, sviluppata
nell’ambito del Progetto Europeo LIFE+ denominato H2 POWER - Hydrogen in fuel gas, finalizzato alla verifica della
possibilità di realizzare un motore per mezzi di trasporto pubblico nei quali la percentuale di idrogeno sia variabile in tempo
reale in funzione del carico richiesto su strada.
Nei test sperimentali è stato fatto funzionare il motore sia a pieno carico, per valutarne le prestazioni massime in tutto il
range ammissibile di giri motore, sia a carico parziale in un determinato punto motore per verificare gli andamenti di consumi,
rendimenti ed emissioni, dipendenti dalle miscele impiegate e dalle strategie di iniezione e controllo motore.
In questo lavoro è risultata molto evidente la drastica riduzione delle emissioni di NOx all’aumentare dello smagrimento
della miscela combustibile. In particolare con idro-metano al 35% si ottiene una notevole riduzione delle emissioni grazie
all’elevata infiammabilità dell’idrogeno che consente una corretta combustione anche con valori di miscela magra altrimenti
non raggiungibili. In prospettiva, questo potrebbe consentire l’eliminazione della funzione riducente del catalizzatore. Anche i
consumi specifici risultano nettamente inferiori, soprattutto in condizioni di alimentazione magra
Effect of Off-Axis Needle Motion on Internal Nozzle and Near Exit Flow in a Multi-Hole Diesel Injector
No full textThe internal structure of Diesel fuel injectors is known to have a significant impact on the nozzle flow and the resulting
spray emerging from each hole. In this paper the three-dimensional transient flow structures inside a Diesel injector is
studied under nominal (in-axis) and realistic (including off-axis lateral motion) operating conditions of the needle.
Numerical simulations are performed in the commercial CFD code CONVERGE, using a two-phase flow representation
based on a mixture model with Volume of Fluid (VOF) method. Moving boundaries are easily handled in the code, which
uses a cut-cell Cartesian method for grid generation at run time. First, a grid sensitivity study has been performed and
mesh requirements are discussed. Then the results of moving needle calculations are discussed. Realistic radial
perturbations (wobbles) of the needle motion have been applied to analyze their impact on the nozzle flow characteristics.
Needle radial motions are based on high-speed X-ray phase-contrast imaging collected at Argonne National Laboratory.
Different types of wobbles are presented and the results are compared to the nominal in-axis motion behavior. Hole-tohole
differences are discussed and quantified. Complex flow structures are observed in the sac region of the nozzle and
an explanation for the different flow structures emerging from each hole is provided. From this work it has been observed
that in presence of needle wobble, at low and medium lifts hole-to-hole differences are observed. Also, some holes
exhibits swirling flow which affects the near nozzle jet structure, the liquid mass distribution and the mass flow rate,
resulting in potential spray modifications
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