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Venerdì, 18 Luglio 2014 14:59

Large Eddy Simulation (LES) of fuel spray transients: start and end of injection phenomena

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Background –  The shape of fuel injection ramps affects spray penetration and mixing process, especially with short multiple injection strategies [1] as generally used in direct ingection engines. It has been shown that after an injection pulse fuel dribbles can be produced, and a significant amount of ambient gas is also ingested in the nozzle sac depending on factors like ambient back pressure, hole sizes, etc. [2,3]. The start of a new injection event is consequently affected by the presence of residual gas in the sac. During the first 100 µs after Start-of-Injection (aSOI) liquid penetration is shorter and evaporation rate is altered [1]. This work reports investigations on diesel spray transients, accounting for internal nozzle flow and needle motion, carried out at Argonne National Laboratory (ANL) [8]. LES of end-of-injection and start-of-injection processes have been carried out on a single hole injector, trying to link the phenomena and provide insights in to the physics.
Battistoni-Xue-Som - les4ice abstract spray-transients v42-

 

Methodology – LES are conducted at the Laboratory Computing Resource Center at ANL. The multi-phase system is modeled using an Eulerian single-fluid approach [4]. A pseudo-density concept is used for the homogeneous mixture with Volume-of-Fluid (VOF) method. The multi-phase system is a three-component two-phase mixture comprising liquid fuel, vapor and air (the latter as non-condensable gas in the liquid and as ambient gas filling the chamber). Mass transfer due to cavitation is also accounted for. The model has been recently implemented in the commercial code CONVERGE [5]. Spatial discretization is second-order accurate, while temporal discretization is based on the implicit Euler scheme. Sub-grid-scale turbulence is modeled using the dynamic structure model [6].

Discussion – End-of-injection - Simulation results (Fig. 1) show that during the last part of the needle closure ramp, liquid jet decelerates dramatically. A rapid decrease of the pressure in the sac is observed, which eventually (just after the valve closure) drops down to a very low level, generating intense void fraction formation and cavitation in the sac region. The liquid column rupture eventually occurs forming dribbles. Ambient gas is then drawn in to the sac through the open orifice, rapidly re-establishing the ambient pressure. Vapor condenses promptly, and ambient gas refills the sac, where some liquid also remains. The LES provides unique insights into the physics during the end-of-injection.
Start-of-injection - Results are shown in Fig. 2.a and 2.b. Simulation is started assuming the sac is initially filled with gas. Needle motion (accounted for from 4 µm minimum lift) controls the sac filling process. The start of injection timing is predicted with a reasonable accuracy, compared to experimental near nozzle x-ray radiography data [7]. The predicted spray development can be analyzed in three steps. 1) Before the SOI, liquid fuel fills the nozzle sac, pushing previously ingested gas out of the orifice (cf. LVF, density and velocity fields at -25 µs aSOI). The initial plume is only a gas jet with fine turbulent structures clearly visible at its tip. 2) At 5 µs aSOI liquid penetration is around 1.5 mm and previously expelled gas tip is at 3 mm. 3) At 10 µs liquid tip reaches the gas tip. Also a shock wave is visible attached to the liquid tip, as it penetrates in to the chamber. It is expected that the simultaneous mixing process occurring with expulsed gases changes the mixing with fresh air and vaporization rate of the first part of the injection.

References
[1] Pickett, L., Manin, J., Payri, R., Bardi, M. et al., "Transient Rate of Injection Effects on Spray Development," SAE Technical Paper 2013-24-0001, 2013, doi:10.4271/2013-24-0001 (2013).
[2] Kastengren A., Powell C.F., Tilocco F.Z., Liu Z. Moon S., Zhang X., Gao J., “End-of-Injection Behavior of Diesel Sprays Measured With X-Ray Radiography”, J. Eng. Gas Turbines Power, 134(9), 094501 (2012).
[3] Battistoni M., Kastengren A., Powell C.F., Som S., “Fluid Dynamics Modeling of End-of-Injection Process”, ILASS Americas (2014).
[4] Xue, Q., Battistoni, M., Som, S., Quan, S. et al., "Eulerian CFD Modeling of Coupled Nozzle Flow and Spray with Validation Against X-Ray Radiography Data," SAE Int. J. Engines 7(2):2014, doi:10.4271/2014-01-1425 (2014).
[5] Richards et al., “CONVERGE (Version 2.1) Theory Manual”, (2013).
[6] Pomraning E. and Rutland C. J.,  "Dynamic One-Equation Nonviscosity Large-Eddy Simulation Model", AIAA Journal, Vol. 40, No. 4, pp. 689-701 (2002).
[7] http://www.sandia.gov/ecn/
[8] http://www.anl.gov

 

Background – This work reports investigations on diesel spray transients, accounting for internal nozzle flow and needle motion, carried out at Argonne National Laboratory (ANL). The shape of injection ramps affects spray penetration and mixing process, especially with short multiple injection strategies [1]. It has been shown that after an injection pulse fuel dribbles can be produced, and a significant amount of ambient gas is also ingested in the nozzle sac depending on factors like ambient back pressure, hole sizes, etc. [2,3]. The start of a new injection event is consequently affected by the presence of residual gas in the sac. During the first 100 µs after Start-of-Injection (aSOI) liquid penetration is shorter and evaporation rate is altered [1].  LES of end-of-injection and start-of-injection processes have been carried out on a single hole injector, trying to link the phenomena and provide insights in to the physics.

 

Methodology – LES are conducted at the Laboratory Computing Resource Center at ANL. The multi-phase system is modeled using an Eulerian single-fluid approach [4]. A pseudo-density concept is used for the homogeneous mixture with Volume-of-Fluid (VOF) method. The multi-phase system is a three-component two-phase mixture comprising liquid fuel, vapor and air (the latter as non-condensable gas in the liquid and as ambient gas filling the chamber). Mass transfer due to cavitation is also accounted for. The model has been recently implemented in the commercial code CONVERGE [5]. Spatial discretization is second-order accurate, while temporal discretization is based on the implicit Euler scheme. Sub-grid-scale turbulence is modeled using the dynamic structure model [6].

 

Discussion – End-of-injection - Simulation results (Fig. 1) show that during the last part of the needle closure ramp, liquid jet decelerates dramatically. A rapid decrease of the pressure in the sac is observed, which eventually (just after the valve closure) drops down to a very low level, generating intense void fraction formation and cavitation in the sac region. The liquid column rupture eventually occurs forming dribbles. Ambient gas is then drawn in to the sac through the open orifice, rapidly re-establishing the ambient pressure. Vapor condenses promptly, and ambient gas refills the sac, where some liquid also remains. The LES provides unique insights into the physics during the end-of-injection.

Start-of-injection - Results are shown in Fig. 2.a and 2.b. Simulation is started assuming the sac is initially filled with gas. Needle motion (accounted for from 4 µm minimum lift) controls the sac filling process. The start of injection timing is predicted with a reasonable accuracy, compared to experimental near nozzle x-ray radiography data [7]. The predicted spray development can be analyzed in three steps. 1) Before the SOI, liquid fuel fills the nozzle sac, pushing previously ingested gas out of the orifice (cf. LVF, density and velocity fields at -25 µs aSOI). The initial plume is only a gas jet with fine turbulent structures clearly visible at its tip. 2) At 5 µs aSOI liquid penetration is around 1.5 mm and previously expelled gas tip is at 3 mm. 3) At 10 µs liquid tip reaches the gas tip. Also a shock wave is visible attached to the liquid tip, as it penetrates in to the chamber. It is expected that the simultaneous mixing process occurring with expulsed gases changes the mixing with fresh air and vaporization rate of the first part of the injection.

Letto 60977 volte Ultima modifica il Venerdì, 18 Luglio 2014 16:33
Battistoni Michele

  • Michele Battistoni, Ph.D., is Assistant Professor at the Department of Engineering at the University of Perugia, where he became faculty in November 2006.
  • He has been visiting researcher at the Argonne National Laboratory's Center for Transportation Research (CTR), in Chicago (IL), for two years from 2012 to 2014, and for four months in 2016.
  • His research and teaching interests lie in the areas of energy-thermal-fluid sciences, thermo-fluid dynamics, multiphase flow, CFD modeling, and energy conversion systems, with main applications to propulsion systems, advanced fuels and injection systems, bio-derived and alternative fuel sprays and combustion.
  • He serves as session organizer worldwide for SAE International conferences in the area of fuel injection systems and sprays. http://www.sae.org/
  • He is leader of the "primary atomization" area within the Engine Combustion Network (ECN). https://ecn.sandia.gov/
  • He has been invited to give presentations at several prestigious Universities and Research Institutes, like Argonne (Chicago, IL, USA), Sandia (Livermore, CA, USA), IFP-EN (Paris, France), KAUST (Thuwal, Saudi Arabia) among others.
  • He is lecturer at the Dept. of Engineering where he teaches at undergraduate and graduate levels.
  • Prior to joining the University of Perugia, Dr. Michele Battistoni worked 2 years in Fiat Powertrain, in Turin, as a Mechanical Engineer in a team of Engine Design.
  • Dr. Michele Battistoni received his Laurea Degree (M.S.) in Mechanical Engineering, summa cum laude, in 1999, and his Ph.D. in Industrial Engineering in 2004, from University of Perugia.
  • He is a member of the Society of Automotive Engineers (SAE), American Society of Mechanical Engineers (ASME), American Physical Society - Division of Fluid Dynamics (APS-DFD) and IInstitute for Liquid Atomization and Spray Systems ILASS.
  • He has published over 70 scientific articles, 24 of which in international journals with referees, 25 in international congresses proceedings with referees, and the remaining presented at international or national conferences or symposia. He has given over 40 scientific presentations in Europe, USA and Japan. H-index: 16 in Google Scholar and 14 in Scopus (as of August 2017). 

Altre informazioni

  • Telefono
    +39 075 585 3749
  • Ruolo
    Ricercatore - Research fellow
  • Area
    Macchine e sistemi per l'energia e l'ambiente - Environmentally sustainable energy conversion systems

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