1,721,013 research outputs found
A Numerical Characterization of New High-Pressure Multi-Hole GDI Injector
No full textThe paper reports a numerical activity aiming at investigating the spray structure originated by a new-generation GDI injector. The spray is analyzed under quiescent conditions, injecting the fuel in a test vessel at non-evaporative ambient conditions. Results from 3D-CFD simulations are compared to experimental measurements available in literature: commer-cial gasoline at two different injection pressures (10 and 20 MPa) was injected and the spray evolution was ana-lyzed throughout the injection duration.The spray was investigated along the jet axis by the phase Doppler anemometry in order to provide droplet size and velocity, in terms of both axial and radial components. Data were analyzed using the ensemble averaging technique in order to provide mean values.Experimental measurements briefly described above are used to test and validate some lagrangian spray numerical sub-models and numerical parameters such as grid density, numerical setup, primary and secondary fuel breakup and droplet to droplet interaction. Particular care is devoted to the accurate representation of the spray primary breakup, in view of the lack of ad-hoc developed models available in literature. A wide CFD activity is then performed in order to investigate grid effects on the prediction of liquid spray penetration and droplet velocity.Results from the CFD analyses show a relevant dependency of the spray structure on both the computational cell size and the adopted CFD model ensemble
CFD Investigation of Fuel Film Formation within a GDI Engine under Cold Start Cranking Operation
No full textnumerical study on the investigation of spray evolution and liquid film formation within the combustion chamber of a current production automotive Gasoline Direct Injected (GDI) engine characterised by a swirl-type side mounted injector is presented.
Particularly, the paper focuses on low-temperature cranking operation of the engine, when, in view of the high injected fuel amount and the strongly reduced fuel vaporisation, wall wetting becomes a critical issue and plays a fundamental role on the early combustion stages. In fact, under such conditions, fuel deposits around the spark plug region can affect the ignition process, and even prevent engine start-up. In order to properly investigate and understand the many involved phenomena, experimental visualisation of the full injection process by means of an optically accessible engine would be a very useful tool. Nevertheless, the application of such technique, far from being feasible from an industrial point of view, appears to be very difficult even in research laboratories, due to the relevant wall wetting at cranking conditions. A numerical program was therefore carried out in order to analyze in depth and investigate the wall/spray interaction and the subsequent fuel deposit distribution on the combustion chamber walls. The CFD model describing the spray conditions at the injector nozzle was previously implemented and validated against experimental evidence. Many different injection strategies were tested and results compared in terms of both fuel film characteristics and fuel/air mixture distribution within the combustion chamber. Low-temperature cranking conditions proved to be an open challenge for the in-cylinder numerical simulations, due to the simultaneous presence of many physical sub-models (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) and the very low motored engine speed. Nevertheless, the use of a properly customized and validated numerical setup led to a good understanding of the overall injection process as well as of the effects of both injection strategy and spray orientation modifications on both the air/fuel and fuel/wall interaction
Detailed Experimental and Numerical Investigation of the Spray Structure in a GDI High-Pressure Swirling Injector
No full textThe paper reports a detailed experimental and numerical investigation of the spray evolution, in terms of flow pattern and droplets size, of a gasoline hollow-cone spray generated by a high-pressure swirl injector for Gasoline Direct Injected (GDI) engine applications. Experiments were carried out, injecting the fuel in a chamber at ambient temperature and atmospheric pressure, in the range of injection pressures between 6 and 10 MPa by means of a common rail injection system, a commercial swirled type injector with a nozzle diameter of 0.50 mm and a cone angle of 67°. A 2-D imaging technique was used to follow the global evolution of the spray as function of the injection time in order to estimate the jet development, the morphology of the spray, and the instantaneous velocity field of fuel droplets by Particle Image Velocimetry (PIV). Images of the spray and PIV shots were captured, firstly, aligning the light sheet to the vertical axis of the spray; then, experiments were also taken with the light sheet placed through the cross section of the spray in order to explore the structure and velocity field at different distances from the nozzle.A PDA system was used to acquire, simultaneously, the droplets velocity as well the droplets size (D10). The system, equipped with an argon-ion laser, was set in forward scattering mode at an off-axis of 30°. Measurements of the axial velocity component and size of droplets were performed, at the same operative conditions as for the PIV ones, close to the nozzle exit (distances of Z=7.5 and 10 mm) and at different radii over 100 injection cycles. As a result, for each measurement, a data set with a minimum of about 40,000 valid data were collected and analyzed, off-line, using the ensemble averaging technique.CFD computations are simultaneously carried out by means of the STAR-CD software. Fuel jet atomization and break-up are evaluated by means of a user-implemented set of models. A preliminary evaluation of the fuel droplet velocity at the injector exit is performed in order to define an instantaneous mass flow rate, aiming at capturing the spray temporal evolution throughout the injection process
Modelling of primary breakup process of a multi-hole spray for Gasoline Direct Engine applications
No full textThe paper proposes a numerical methodology for the simulation of a gasoline spray generated by a multi-hole injector of a current production wall-guided Gasoline Direct Injection engine. Particular care is dedicated to the accurate representation of the spray primary breakup by means of an atomization model. The model is purposely implemented to take into account cavitation phenomena and turbulent effects induced by the nozzle geometry through a simplified approach. Since a high primary breakup rate is expected, an initial distribution of atomized droplets is predicted at the nozzle hole exit by the numerical approach. The spray is at first experimentally investigated in a test vessel at non-evaporative ambient conditions and under quiescent conditions, injecting commercial gasoline at two different injection pressures (10.0 and 20.0 MPa). The spray is characterised in terms of both instantaneous mass flow rate and morphology. Numerical simulations are performed and then compared against experiments in order to evaluate their capability to correctly predict liquid spray penetration, droplet size distribution and spray morphology. The new approach is a fairly simple yet reliable solution able to predict the influence of the nozzle hole (in terms of discharge coefficient, diameter and length) and neglecting geometrical details usually far from being easily accessed by engine developers
Analisi Termofluidodinamica Multidimensionale del Ciclo di un Innovativo Motore Diesel, 2 Tempi Veloce
No full textL’analisi termofluidodinamica multidimensionale del ciclo di un motore a combustione interna presenta notevoli difficoltà, sia concettuali che pratiche. Da un punto di vista teorico occorre infatti definire una metodologia in grado di simulare in maniera sufficientemente accurata tutti i complessi fenomeni che avvengono all’interno del cilindro e dei condotti (in particolare la miscelazione aria-combustibile, la combustione, l’efflusso attraverso le valvole e/o le luci). Tutt’altro che banale risulta anche la generazione della griglia di calcolo e la gestione del movimento del pistone e degli eventi di apertura e chiusura delle valvole. L’affinamento dei modelli e l’esigenza di reiterare i calcoli per più cicli consecutivi del motore si scontra poi sempre con la limitatezza delle risorse di calcolo, per quanto la potenza dei sistemi di elaborazione sia in costante espansione.Il presente articolo descrive un’analisi multidimensionale e multiciclo applicata ad un innovativo motore Diesel 2 Tempi veloce, in corso di sviluppo presso il Dipartimento di Ingegneria Meccanica e Civile dell’Università di Modena e Reggio Emilia. In particolare, sono state considerate quattro condizioni operative, due a pieno carico e due a carico parziale.Lo strumento di simulazione CFD è un software commerciale, in cui i modelli di spray e combustione sono stati calibrati sulla base di dati sperimentali, ottenuti su di un motore Diesel veloce a 4 Tempi, avente medesimo alesaggio.La metodologia utilizzata si è dimostrata in grado di fornire risultati stabili mediamente dopo tre cicli, ciascuno dei quali ha richiesto circa 200 ore di calcolo su di una macchina quadri-processore. Molto significativa è risultata essere la variazione ciclica per quello che riguarda le emissioni inquinanti, a causa delle differenze di composizione iniziale della carica.E’ stata infine valutata l’influenza delle leggi di rilascio del calore, ottenute mediante queste simulazioni multidimensionali e multiciclo, sui parametri prestazionali del motore a due tempi, calcolati con l’ausilio di un codice gasdinamico mono-dimensionale
Wall Impingement Process of a Multi-Hole GDI Spray: Experimental and Numerical Investigation
No full textThe Direct Injection (DI) of gasoline in Spark Ignition (SI) engines is very attractive for fuel economy and performance improvements in spark ignition engines. Gasoline direct injection (GDI) offers the possibility of multi-mode operation, homogeneous and stratified charge, with benefits respect to conventional SI engines as higher compression ratio, zero pumping losses, control of the ignition process at very lean air-fuel mixture and good cold starting.The impingement of liquid fuel on the combustion chamber wall is generally one of the major drawbacks of GDI engines because its increasing of HC emissions and effects on the combustion process; in the wall guided engines an increasing attention is focusing on the fuel film deposits evolution and their role in the soot formation. Hence, the necessity of a detailed understanding of the spray-wall impingement process and its effects on the fuel distribution. The experimental results provide a fundamental data base for CFD predictions.In this paper investigations have been performed using a 7-hole injector, 0.179 mm in hole diameter, spraying in a constant volume vessel with optical accesses. To examine the effects of various factors on development of the spray impinging on the wall, experiments have been conducted at different injection pressures, diverse wall inclination angles and at atmospheric pressure. The acquired images have been processed for extracting the characteristic parameters of the impinging fuel at the different operative conditions.The multi-hole spray has been simulated by Star-CD code taking into account the commercial gasoline properties and the real mass flow rate derived from experimental measurements. In order to correctly reproduce spray impingement and fuel film evolution, a numerical methodology has been defined. Lagrangian sub-models and numerical parameters have been validated against experimental results
CFD Methodology Assessment for the Investigation of Convective Heat Transfer Properties of Engine Coolants under Boling Conditions
No full textThe paper presents a combined experimental and numerical program directed at defining a cost/effective methodology for conjugate heat transfer CFD simulations of engine water cooling jackets. As a first step in the process, deficiencies in current numerical strategies for the analysis of conjugate heat transfer problems under typical engine operating conditions are exposed and commented. Results are shown form a wide validation program based on the comparison between experimental measurements from a test facility at Villanova University and CFD predictions at the University of Modena. On the experimental side, the test apparatus consists of a test section, pump, accumulator tank, rejection heat exchanger and required pumping. The test section is provided with a constant volumetric flow rate, and consists of a cylindrical aluminum body with a drilled horizontal flow channel. The
section is heated by ten cartridge heaters located at a constant radial distance from the cylinder axis. The test section is connected to the flow loop by means of two calming sections, respectively at the cylinder inlet and exit.
Twenty thermocouples are used to measure the test section local temperature along a radial plane cutting the cylinder. Water / ethylene-glycol binary mixture and pure water are tested and compared during the experimental program, in order to reproduce a set of thermal situations as close as possible to actual engine cooling system operation. On the CFD side, an extensive program reproducing the experiments is carried out in order to assess the predictive capabilities of some of the most commonly used eddy viscosity models available in literature. Both non-evaporating and evaporating conditions are tested, showing severe limitations to the use of simplified boiling models to correctly capture the complex interaction between turbulent boundary layer and vapor bubble dynamics. In order to overcome the above stated deficiencies under boiling conditions, a methodology is then proposed to both improve the accuracy of the CFD forecasts and reduce the computational costs of the simulations. A few preliminary results from the validation process are shown and briefly discussed at the end of the paper
Numerical Analysis of GDI Engine Cold-Start at Low Ambient Temperatures
No full textThe paper investigates the low-temperature cranking operation of a current production automotive Gasoline Direct Injected (GDI) by means of 3D-CFD simulations. Particular care is devoted to the analysis of the hollow cone spray evolution within the combustion chamber and to the formation of fuel film deposits on the combustion chamber walls. Due to the high injected fuel amount and the strongly reduced fuel vaporization, wall wetting is a critical issue and plays a fundamental role on both the early combustion stages and the amount of unburnt hydrocarbons formation. In fact, it is commonly recognized that most of the unburnt hydrocarbon emissions from 4-stroke gasoline engines occur during cold start operations, when fuel film in the cylinder vaporize slowly and fuel can persist until the exhaust stroke.In view of the non-conventional engine operating conditions (in terms of injected fuel amount, engine speed, ambient and wall temperature and almost null fuel atomization and breakup), an understanding of the many involved phenomena by means of an optically accessible engine would be of crucial importance. Nevertheless, the application of such technique appears to be almost unfeasible even in research laboratories, mainly because of the relevant wall wetting.CFD analyses prove then to be a very useful tool to gain a full insight of the overall process as well as to correlate fuel deposits to both the combustion chamber design and the injection strategy. In order to better understand where, and how thick, these wall films are formed during the intake and compression, a detailed description of the spray interaction with both the piston wall and the intake valves was performed by the authors in a previous paper [ 1 ]. Subsequently, a wide set of injection strategies was simulated in order to better understand the physics of spray/wall interaction and to minimize the formation of deposits in the combustion chamber most critical locations [ 2 ].In order to limit the overall number of modeling uncertainties (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) the spray model was at first validated against experimental data under low injection pressure, and results from the comparison were reported in [ 1 ].In the present paper, cold start operations at decreasing ambient temperatures are modeled and results are analyzed in terms of both fuel film distribution on the combustion chamber walls and resulting fuel/air mixture distribution within the combustion chamber. The use of CFD simulations prove to be useful to investigate and understand the influence of both combustion chamber design and injection profile on the amount and distribution of fuel deposits, showing a high potential to address future engine optimizatio
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