LES Combustion Modeling for Diesel Engine Simulations

Bing Hu Professor Christopher J. Rutland

Sponsors: DOE, Caterpillar

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Background Motivation

Better predictive power: LES is potentially capable of capturing highly transient effects and more flow structures

New analysis capability: LES is more sensitive to initial and boundary conditions than RANS such that it is better suitable for studying cyclic variations and sensitivity to design parameters.

Primary components Turbulence model: a one-equation non-viscosity model called

dynamic structure model for subgrid scale stresses Scalar mixing models: a dynamic structure model for subgrid

scale scalar flux and a zero-equation model for scalar dissipation

Combustion model: a flamelet time scale model

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Large Eddy Simulations

u

x

Actual u

LES averaged u

RANS averaged u

Spatial filtering

Filtering of non-linear terms in Navier-Stokes equations results in subgrid scale terms needed to be modeling

i i iu u u

ij i j i ju u u u

Smagorinsky model use eddy viscosity

Dynamical structure modelone equation modelk: sub-grid turbulent kinetic energyCij :dynamically determined tensor coefficient

ij t ijS ij ijc k

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Flamelet Time Scale Combustion Model

Overview Flamelet mixture fraction approach: each species is a function of

mixture fraction and stretch rate , this functional dependence is solved using a 1-D flamelet code prior to the CFD computation

Use probability density function (PDF) to obtain mean values

Modification for slow chemistry using a time scale

Additional features PDF of mixture fraction is constructed from its first and second

moment which are solved from LES transport equations LES sub-grid model for scalar dissipation helps to construct PDF of

stretch rate

*i i iY YY

t

1

*

0 0

( , ) ( , )q

i iY Y P d d

( , )i iY Y

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Jet Flame Tests (Sandia Jet Flames)• Sandia piloted flames are simulated to validate models• A coarse grid is used: 15cm x 15cm x 60cm, about 230,000 cells• Instantaneous temperature fields are presented below• Black curves represent stoichiometric mixture fraction• Reynolds number at fuel jet for flame D = 22,400• Reynolds number at fuel jet for flame E = 33,600

flame D A Relatively stable flame

flame E Significant local extinctions result in lower temperature

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Engine Test Case (Caterpillar Diesel Engine)

Cylinder bore X stroke (mm) 137.6 X 165.1Displacement volume (L) 2.44Compression ratio 15.1Engine speed (rpm) 1600% Load 75START OF INJECTION -9 ATDCDuration of injection (degree) 21

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-10 -5 0 5 10 15 20 25 30 35 40

CA [oATDC]

Pre

ssu

re [

MP

a]

Experiment

Simulation

-30

70

170

270

370

470

-10 -5 0 5 10 15 20 25 30 35 40CA [

oATDC]

Hea

t R

elea

se [

J/D

egre

e]

Experiment

Simulation

Mixture fraction Mixture fraction variance

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Summary and Future Work A flamelet time scale combustion model was

integrated with LES dynamical structure turbulence and scalar mixing models

Model results agreed well with experiments of jet flames and a diesel engine

More accurate spray models are to be integrated with LES turbulence and scalar mixing models

More precise initial and inflow conditions are to be generated for LES simulations