VOF-LES Simulations of Primary Atomization and Near-Nozzle Spray Structure

Transcription

1 VOF-LES Simulations of Primary Atomization and Near-Nozzle Spray Structure Michele Battistoni, Francesco Risi - University of Perugia Zongyu Yue, Sibendu Som - Argonne National Laboratory 2018 CONVERGE User Conference Europe - March 20, CONVERGE User Conference - Europe - Bologna, March 20 th, 2018

2 Motivation for near-nozzle studies Direct injection compression ignition engines have become preferred architectures for extending Diesel legacy (higher efficiencies, lower emissions): PPCI, GCI, etc Advancing the timing of start of injection (SOI) to control fuel-air mixing and auto-ignition processes, the link between mixture formation and combustion becomes more delicate. Direct injection spark ignition engines are also heavily relying on spray formation and mixing, in order to operate in the efficient lean area, close to unstable limit conditions. Primary atomization is still the critical link, in the modeling area, between the injector and the combustion at the device scale. The near-nozzle flow is characterized by: high liquid mass loadings, large momentum exchanges, complex evolution of the liquid-gas interface, which require specialized numerical techniques. This presentation aims to show progresses on highly-resolved two-phase flow simulations for diesel and gasoline sprays, leveraging new quantitative data made available from the Engine Combustion Network, which provides a unique testing ground for CFD multi-phase models for fuel sprays CONVERGE User Conference - Europe - Bologna, March 20 th,

3 Breakup Regimes and Modeling Approach Approach used Eulerian Volume of Fluid (VOF) Separated flows, with sharp interface Direct transport of void fraction Phase interface tracking is possible for moderate interfacial area Taylor et al., Exp. Fluids, 1983 Crua et al. Fuel, 2015 Huge literature: Zaleski, Herrmann, Pitsch, Desjardin, Umemura, Sussman, 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

4 Eulerian VOF model in CONVERGE v = 0 ρ v + ρ (vv) = p + T + f + S t α t + αv = 0 Setup: PLIC for interface geometric reconstruction Grid resolution = 2.5 μm LES (Dynamic Structure model) Wall functions PISO coupling CFL_u = 0.25 Single-fluid incompressible continuum Fluid properties: ρ = ρ 1 α α 1 ρ 2 μ = μ 1 α α 1 μ 2 Interface property: surface tension γ S() t S n x x ' ds ni x i surface tension effects via Continuum Surface Force model n i x j i x i 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

5 Diesel ECN Injectors Micron-scale high resolution real geometries Synchrotron X-ray scans µm resolution Gasoline spray A spray D spray G spray C spray G (hole closeup) spray B 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

6 Spray D: towards direct SMD predictions Fuel n-dodecane Orifice Diameter (nominal) 180 µm K-factor (nominal) 1.5 Injection Pressure 150 MPa Fuel Temperature 298 K Ambient Temperature 298 K Ambient Density 22.8 kg/m 3 ~50 M cells about two weeks on 256 cores to simulate ~10 μs at high resolution 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

7 Spray D: domain and grid 14.4 mm AMR active on velocity and alpha gradients 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

8 Spray D results (single snapshot) Void fraction, cut plane with grid Void fraction, cut plane Projected mass 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

9 projected mass (μg/mm2) projected mass (μg/mm2) projected mass (μg/mm2) Spray D results: mass distribution validation ANL - xray radiography - exp 0.1 mm VOF-LES sim 0.1 mm ANL - xray radiography - exp 2 mm VOF-LES sim 2.0 mm Y distance (mm) Y distance (mm) ANL - xray radiography - exp 4 mm VOF-LES sim 4.0 mm Simulation results averaged over 3 snapshots Y distance (mm) 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

10 Simulation results Spray D results: morphology 0.1 mm 0.6 mm 1 mm 2 mm 3 mm 6 mm ANL x-ray reconstructed density (μg/mm 3 ) at 0.1 mm from the nozzle exit video snapshot The jet cross section has already lost circular symmetry at ~0.5 D from the nozzle exit 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

11 Spray D phase interface area y Experiments: USAXS (Ultra Small Angle Xray Scattering) Axial sweep y Simulation postprocessing: mimicking USAXS method x y Transv. sweep x z Bin size in the USAXS experiments: 50 µm 500 µm x Calculation of the total area of phase interface, A, and the total volume, V. USAXS-like method: V/V box & A/V box Line-of-sight box, moving along X and Y, to collect info on each structure, including core On each box, collection of: - liquid volume V - interface area A 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

12 VOF post processing: volume and surface area calculations 1. Resampling on uniform grid, at min. resolution (no interpolation is involved) 2. Definition of sampling box (line of sight, annular, box, etc ) connectivity analysis is applied to identify each blob. Volume is retrieved 4. Patches to identify surfaces Surface area is retrieved SMD = 6 V/V box A/V box 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

13 Projected Surface Area [mm 2 /mm 2 ] Spray D phase interface area: results USAXS data simulations x = 1 mm x = 2 mm x = 3 mm x = 4 mm y [mm] Simulations explain the double peak area distribution: it is due to the intact core. It is therefore possible to identify the liquid core length. In this case about 5-6 mm 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

14 SMD [μm] Spray D Sauter Mean Diameter: results Battistoni, Magnotti, et al., SAE Model captures the SMD trend VOF-LES - line-of-sight centerline peak VOF-LES - detached structures average USAXS - line-of-sight centerline peak USAXS - line-of-sight average axial distance [mm] Actual droplet size (detached structures) is insensitive to the axial distance Room for improvement: more resolution still required to be fully predictive at diesel spray conditions Note: USAXS data include the liquid core 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

15 Spray D summary New USAXS data provides a unique dataset for understanding the characteristics of diesel primary breakup. Model should use these data properly and with caution for apple-to-apple comparison of quantities. Highly resolved VOF simulations with phase interface tracking are able to predict mean droplet sizes, provided resolution is sufficiently fine. We think this is the first time such analysis, in combination with experiments, has been done, unraveling the inner structure of diesel primary atomization. Detached droplet diameters appear reasonable, and the size insensitive to the axial distance, suggesting that the fluid-mechanics of droplet breakup do not change over these distances CONVERGE User Conference - Europe - Bologna, March 20 th,

16 Diesel ECN Injectors Micron-scale high resolution real geometries Synchrotron X-ray scans µm resolution Gasoline spray A spray D spray G spray C spray G (hole closeup) spray B 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

17 Spray G: simulation setups Fuel iso-octane Fuel Temperature 298 K Injection Pressure 19 MPa Ambient Temperature 298 K Ambient Density 3.5 kg/m 3 R1 Nominal Real Orifice Diameter, D1 165 µm 175 µm Orifice Length, L1 170 µm 150 µm L1/D Hole inlet corner radius, R1 0 µm 4.93 µm Counterbore diameter, D2 388 µm 394 µm Counterbore length, L2 470 µm 402 µm Matusik et al., ILASS-Europe, hole sector 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

18 Spray G: simulation setups Initial needle lift: 5 μm ~10 days on 300 cores, for 100 µs simulation Initialization of computational domain Pressure Species 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

19 Spray G: nominal vs. real surface Nominal Real Liquid core spreads into a sheet, rather than cylindrical column 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

20 Spray G: inside and near nozzle Nominal Real Nominal Real mm 1 mm downstream mean density field (time-averaged from 50 to 100 µs) Flow separation inside the nozzle due to sharp corner at hole inlet; Significant difference on spray breakup at near nozzle region (L/D ratio, surface variations). mm 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

21 droplet count Spray G: d v vs. d s d s = A π d v = 6V π 1Τ3 Real Real Nominal µs ASI Liquid structures with d v < 20 µm remain spherical; Liquid structures with 20 < d v < 200 µm are mainly ligaments before breaking into smaller droplets; Liquid structures with d v > 200 µm are considered as intact liquid core, representing more than 90% mass of the spray Distance to orifice exit [mm] 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

22 Equivalent SMD [μm] SMD [μm] Equivalent SMD [μm] Spray G: SMD SMD = 6 σ V i σ A i Overall SMD Real Nominal SMD data at z = 15 mm (GM experiments at ECN3) time [μs ASI] Intact liquid dv > 200 µm Detached droplet dv < 200 µm time [μs ASI] time [μs ASI] The use of real geometry predicts 11% lower overall SMD, attributed to the difference in the intact liquid morphology Droplet SMD in good agreement with experimental measurements available downstream 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,

23 Spray G summary Injector geometry scanned with micron resolution is used in comparison with a nominal geometry, to study the impact of manufacturing tolerance and deficiency of injector on the near nozzle flow and spray development. Compared to the nominal geometry, the use of the high resolution real geometry predicts about 11% smaller overall SMD, and also accelerates the droplet detachment in the upstream of spray, starting inside of the counter-bore during the start-of-injection. Droplet SMD is in good agreement with experimental values Future work Perform simulations using other real geometries to study the correlations between geometric features and spray characteristics. Decouple the effects of roughness and non-nominal orifice dimensions CONVERGE User Conference - Europe - Bologna, March 20 th,

24 Conclusions Numerical studies have been carried out to investigate the primary breakup characteristics for ECN diesel and gasoline sprays. 26 connectivity analysis is applied to extract information such as droplet volume and surface. Detailed and accurate description of the internal and external two-phase flow is feasible (seamlessly), capturing needle transients, real geometry features, and their impact on spray atomization. Necessity of highly resolved simulations (~2.5 μm grid size is tested). Droplet sizes are in very good agreement with measurements for the GDI injector, and are able to predict the correct trends for the high injection pressure Diesel case. The methodology has high relevance in the area of nozzle design and spray calculations, with impact on new/advanced combustion systems design CONVERGE User Conference - Europe - Bologna, March 20 th,

25 University of Perugia Michele Battistoni, Francesco Risi, Jacopo Zembi, Carlo N. Grimaldi, Francesco Mariani, Gabriele Discepoli, Valentino Cruccolini Argonne National Laboratory Sibendu Som, Zongyu Yue, Gina Magnotti, Riccardo Scarcelli, Kastengren, Powell, Matusik, and HPC clusters Blues/Bebop Convergent Science Group and Acknowledgments Kelly Senecal, Priyesh Srivastava, support team Cineca SCAI HPC center Partners & sponsors Magneti Marelli, Fondazione CR Perugia 2018 CONVERGE User Conference - Europe - Bologna, March 20 th,