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Large Eddy Simulations of Turbulent Spray Combustion in Internal Combustion Engines

Large Eddy Simulations of Turbulent Spray Combustion in Internal Combustion Engines. F arhad Jaberi Department of Mechanical Engineering Michigan State University East Lansing, Michigan. Background.

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Large Eddy Simulations of Turbulent Spray Combustion in Internal Combustion Engines

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  1. Large Eddy Simulations of Turbulent Spray Combustionin Internal Combustion Engines Farhad Jaberi Department of Mechanical Engineering Michigan State University East Lansing, Michigan

  2. Background • In-Cylinder Flow:Combination of highly unsteady turbulent flow, separated boundary and shear layers, pressure waves, spray, mixing and combustion in complex geometrical configurations with moving pistons and valves. • CFD & IC Engines:The solver should be able to handle complex geometries with dynamic mesh. LES needs high order numerical method and accurate subgrid turbulence models. For spray, advanced primary and secondary break-up models and fully coupled gas-droplet flow solvers with multi-component droplet evaporation models are needed. Turbulent combustion models with appropriate chemical kinetics mechanisms are also needed. • Previous Works: Mostly based on RANS or low-order LES. • Our Model: LES/FMDF, based on a new Lagrangian-Eulerian-Lagrangian mathematical/numerical methodology.

  3. LES/FMDF of Single-Phase Turbulent Reacting Flows Scalar FMDF - A Hybrid Eulerian-Lagrangian Methodology LES/FMDF of a Dump Combustor Eulerian Grid LagrangianMonte Carlo Particles • Eulerian: Conventional LES equations for velocity, pressure, density and temperature fields • - Deterministic simulations • Lagrangian: Transport equation for FMDF (PDF of SGS temperature and species mass fractions • - Monte Carlo simulations • Coupling of Eulerian and Lagrangian fields: A certain degree of “redundancy” (e.g. for filtered temperature)

  4. LES of Two-Phase Turbulent Reacting Flows A New Lagrangian-Eulerian-Lagrangian Methodology Filtered continuity and momentum equations via a generalized multi-block high-order finite difference Eulerian scheme for high Reynolds number turbulent flows in complex geometries Various closures for subgrid stresses Gasdynamics Field Scalar Field (mass fractionsand temperature) Filtered Mass Density Function (FMDF) equation via Lagrangian Monte Carlo method - Ito Eq. for convection, diffusion & reaction Lagrangian model for droplet equations with full mass, momentum and energy couplings between phases and a stochastic sub grid velocity model Droplet Field (spray) Kinetics: (I) global or reduced kinetics models with direct ODE or ISAT solvers, and (II) flamelet library with detailed mechanisms or complex reduced mechanisms Fuels considered so far: methane, propane, heptane, octane, decane, kerosene, gasoline, JP-10 and ethanol Chemistry

  5. LES of Two-Phase Turbulent Reacting FlowsA New Lagrangian-Eulerian-Lagrangian Methodology Spray-Controlled Dump Combustor Wall Fuel Injector Wall -Eulerian Grid • Monte Carlo Particles • Liquid Fuel Droplets Mass,Momentum,Scalar Terms from Droplets LES Solver Monte Carlo Particles Eulerian Finite Difference Grid Interpolation / Favre Filter

  6. KH RT Filtered Equations - Eulerian Droplet Equations Lagrangian KH/RT Break-up Droplet terms Reaction terms FMDF Equation Lagrangian Two-phase subgrid scalar FMDF: Reaction term Droplet terms

  7. Main Features of LES/FMDF • Large scale, unsteady, non-universal, geometry- depended quantities are explicitly computed in LES/FMDF • FMDF accounts for the effects of chemical reactions in an exact manner and may be used for various types of chemical reactions (premixed, nonpremixed, slow, • fast, endothermic, exothermic, etc.). • LES/FMDF can be implemented via complex chemical kinetics models and is applicable to 3D simulations of hydrocarbon flames in complex geometries. • FMDF contains high order information on sub-grid or small scale fluctuations. • The Lagrangian Monte Carlo solution of the FMDF is free of artificial (diffusion) numerical errors. This is very important in IC engine simulations as overprediction of temperature could cause numerical ignition!

  8. Application of LES/FMDF to Various Flows Axisymmetric Dump Combustor Double Swirl Spray Burner IC Engines with Moving Valves/Piston complex cylinder head/piston, spray and combustion Spray Controlled Lean Premixed Square Dump Combustor 10 degree After TDC Pressure Iso-Levels Temperature Contours Fuel Injector 24 Block grid for a 4-valve Diesel Engine Wall

  9. y x z y LES of Cold Flow Around a Poppet Valve Reynolds No = 30,000 Mass rate = 0.015 kg/s Dimensions in mm 5-block LES grid Graftieux et al. 2001 Axial Velocity Contours 20mm 70mm RMS of axial velocity Mean axial velocity Exp. Data Dyn. Smag-filtered Dyn. Smag-Averaged Smag Cd=0.01

  10. LES of Flow in a Piston-Cylinder Assembly 4-block moving structured grid for LES Grid compression or expansion Piston Morse et al. (1978) Comp. ratio 3:1 , RPM=200 , Re=2000 Crank angle=36o Crank angle=144o 5th cycle instantaneous axial velocity contours m/s

  11. LES of Flow in a Piston-Cylinder Assembly Mean values computed by doing both azimuthal and ensemble averaging over cycles Exp. Data Dynamic Smag Smag, Cd=0.01 CA=36o Mean Velocity RMS of Velocity CA=144o

  12. Rapid Compression Machine – LES/FMDF Predictions Simple Piston Groove In-Cylinder Piston Non-Reacting RCM Simulations Temperature piston piston Pressure Temperature Contours

  13. Rapid Compression Machine - LES/FMDF Predictions Reacting Simulations - Consistency between Finite-Difference (FD) and Monte Carlo (MC) values of Temperature and Fuel Mass Fraction FD MC Temperature Contours FD MC Fuel Mass Fraction Contours

  14. Rapid Compression Machine - LES/FMDF Predictions Non-Reacting Flows Temperature Contours Flat Piston Piston Non-Reacting Flows Temperature Contours Creviced Piston Piston Temperature Ethanol CO2 Piston Reacting Flows without Spray Creviced Piston at 5msec Reacting Flows with Ethanol Spray

  15. 3D Shock Tube Problem – LES/FMDF Predictions 3D Shock Tube p2 p1 Two-Block Grid p2/p1=15 • Compressibility effect is included in FMDF-MC . Without Compressible term FMDF-MC results are very erroneous. • Number of MC particles per cell is varied but particle number density does not affect the temperature. • By increasing the particle number per cell MC density • becomes smoother but temperature is the same for all cases. 5 MC per cell 20 MC per cell 50 MC per cell

  16. Modeling of Engine Configuration MSU 3-Valve Direct-Injection Spark-Ignition Single-Cylinder Engine Bore 90 mm Stroke 104 mm Compression Ratio 9.8/11 Engine Speed 2500 rpm Intake valves 2 tilted with 5.1o D = 33 mm Exhaust valve 1 tilted with 5.8o D = 37 mm Injector Spark Plug Exhaust Port Piston Cylinder fuel spray

  17. Direct-Injection Spark-Ignition Engine – LES Predictions MSU 3-Valve DISI Engine: Bore=90mm Stroke=106mm 18-block Grid Axial Velocity 2D Cross Section of 18-block LES Grid Valve lift= 11mm Piston velocity=13m/s Crank angle=100o Valve lift= 5mm Piston velocity=1.5m/s Crank angle=175o Pressure contours piston

  18. Direct-Injection Spark-Ignition Engine – LES Predictions CA=220o CA=340o CA=100o piston piston piston Contours of Evaporated Fuel Mass Fraction CA=90 CA=140 CA=270

  19. LES/FMDF of3-Valve DISI Engine with Spray and Combustion Consistency between Finite Difference (FD) and Monte Carlo (MC) parts of the hybrid LES/FMDF numerical solver Crank angle of 350 5 mm from TDC Instantaneous Values In-Cylinder Temperature Volume Averaged

  20. LES/FMDF Predictions of MSU’s 4-Valve Diesel Engine 24 Block grid for a 4-valve Diesel Engine Beginning of Compression CA=190 Pressure Iso-Levels Pressure Contours Temperature Contours

  21. LES/FMDF of MSU’s 4-Valve Diesel Engine 6o Before TDC 14o Before TDC 6o After TDC Temperature Contours Contours of Evaporated Fuel Mass Fraction and Fuel Droplets

  22. LES/FMDF of MSU’s 4-Valve Diesel Engine 10 degree After TDC Temperature Contours

  23. Numerical Simulations of 3-Valve DISI Engine Overall Validation of the model Without Spray air mass via cell volume = air mass via ideal gas Variations of mean Temperature With Spray – Valves Closed mass of liquid fuel+evaporated fuel = injected liquid fuel

  24. Simulations of 3-Valve Engine – Spray In-cylinder Spray Modeling: • Initial droplet size, position and velocity distribution • Droplet breakup and collision models • Multi-component non-equilibrium evaporation models • Wall collision and film models Primary Break-up Model: Parent droplets injected with specific velocities and diameters (bold model) Secondary Break-up Models: 1) Taylor Analogy Break-up (TAB) -Spring, mass and damper 2) Rayleigh-Taylor Break-up (RTB) - RT instable waves 3) Kelvin-Helmond Break-up (KHB) - KH invisid instable waves 4) KH/RT Break-up model • Stroke: 105.8 mm • Compression Ratio: 11:1 • Eight nozzles with cone angle of 8 degree each. Initial SMD: 30 m • Injection Velocity: 50 m/s

  25. Simulations of 3-Valve Engine – Chemistry • Ethanol • Detailed Kinetics: e.g. 372 elementary reactions and 57 species for ethanol • Multi-Step Reactions • Global Mechanisms • Ignition delays calculated from detailed Mechanism using CHEMKIN for homogeneous 0-D reactor based on equivalence ratio and temperature conditions prevalent in the cell • By addition of ignition delay, the unphysical phenomenon of autoignition in numerical simulation of SI engines do not occur.

  26. Simulations of 3-Valve DISI Engine – Effects of Fuel Vaporization No significant evaporation for ethanol Combustion No combustion for ethanol fuel • Operating conditions are the same for both fuels Mixtures are stoichiometric when all fuel is evaporated and mixed

  27. Summary and Conclusions • A robust and affordable LES model is developed for detailed simulations of various realistic single-cylinder engines: (i) A multi-block compressible LES solver in generalized coordinate system, (ii) Combustion and spray simulations are via a new Lagrangian-Eulerian-Lagrangian LES/FMDF methodology • Several test cases are simulated with the newly developed models: (i) flow around a poppet valve, (ii) flow in a piston-cylinder assembly, (iii) flow in a single-cylinder three-valve direct-injection spark engine, (iv) flow in a single-cylinder four-valve diesel engine • LES with high-order numerical methods, dynamic SGS models and two-phase FMDF can predict the complex in-cylinder turbulent flows with spray and combustion in realistic engines • Detailed experimental data, under controlled and well defined flow conditions are needed for complete validation of LES/FMDF • LES/FMDF is used for studying effects of (i) chemistry model, (ii) spray model and (iii) various parameters on turbulence, mixing and combustion,

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