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14/09/2015

CFD-Simulation of Ignition and

Combustion in Gas Engines

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Verbrennungsforschung in der Schweiz

9. September 2015 in Zürich

G. Xu†,*, Ch. Hanauer†, Y.M. Wright* and K. Boulouchos*

†Liebherr Machines Bulle SA, Bulle

*LAV, ETH Zürich

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List of contents

2

Introduction and motivation

Ignition modeling and validation

Ignition modeling

Ignition modeling validation

Ignition and combustion simulation in Liebherr gas engine

Grid independence analysis

Turbulent flame speed parametric sensitivity study

Simulation of various engine operating conditions

Conclusions and future work

Guoqing Xu &

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Introduction and motivation

3 Guoqing Xu &

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4 Guoqing Xu

Introduction and background

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Combustion chamber

Premixed lean combustion simulation involves different physics, the

understanding of fundamental processes and their interaction is essential

for a proper modeling work. Turbulence modeling

Flame theory Ignition description

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Motivation

Guoqing Xu

Find out appropriate predictive modeling approach for the

Liebherr gas engine and assist engine development.

Obtain knowledge and create solid basis for engine pre-

chamber combustion simulation.

Single cylinder

engine

Multi-cylinder

engine

Endurance/In-

field testing

En

gin

e d

ev

elo

pm

en

t

Engine

combustion

simulation

Concept

Serial

production

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Ignition modeling and validation

6 Guoqing Xu &

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7 Guoqing Xu

Ignition modeling

Ignition models (Herweg & Maly 1992)

𝑺𝑻𝑺𝑳

= 𝟏 + 𝑮𝑨 × (𝒖′

𝑺𝑳)𝟓/𝟔

𝑮𝑨 = [𝟏 − 𝒆𝒙𝒑(−𝒓𝒌𝑳)]𝟎.𝟓× [𝟏 − 𝒆𝒙𝒑(−

𝒖′ + 𝑺𝑳𝑳

𝒕)]𝟎.𝟓

𝒅𝒓𝒌𝒅𝒕

= 𝝆𝒖𝝆𝒃

× (𝑺𝑻 + 𝑺𝒑𝒍𝒂𝒔𝒎𝒂)

𝑺𝒑𝒍𝒂𝒔𝒎𝒂 = 𝑸𝒕𝒐𝒕𝒂𝒍η𝒆𝒇𝒇

𝟒𝝅𝒓𝒌𝟐 × [𝝆𝒖∆𝒉 + 𝑷𝒂𝒃𝒔

𝝆𝒖𝝆𝒃]

rk

𝒅𝒓𝒌𝒅𝒕

electrode

This 1D ignition model is capable of accounting local

turbulence and spark induced thermal expansion effect. Turbulent flame

speed

Spark induced

expansion

speed

This model is coupled with G-equation using user code usrtfs.f

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8 Guoqing Xu

Phi Pressure Temperature Turbulent

kinetic energy

Integral

length scale

LB1 1.0 5 bar 360 K 6 m2/s2 20mm

LB2 0.6 5 bar 360 K 6 m2/s2 20mm

Ignition modeling validation

Leeds MkII combustion bomb

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Ref: Lawes M. et al, Combustion Science and Technology, 177:7, 1273-1289, DOI: 10.1080/00102200590950467

Bradley D., et al, Combustion and Flame, volume 133 , page 415-430, 2003

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9 Guoqing Xu

Spark affect region Spark affect region

Switch to

combustion model

Switch to

combustion model

Under stoichiometric (λ=1) condition:

The ignition model can properly capture the flame kernel growing curve as well as

the flame speed evolution profile.

The discontinuity is observed when switch from ignition to combustion.

Switch radius is

10mm (lt= 20 mm)

Case: Stoichiometric condition (ϕ=1.0)

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Ignition modeling validation

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10 Guoqing Xu

Case: Lean condition (ϕ=0.6)

Spark affect region

Spark affect region

Switch to

combustion model

Under lean (λ=1.667) condition:

The ignition model can also properly capture the flame kernel growing curve as

well as flame speed evolution profile.

The spark induced expansion is represented well by the plasma speed.

The discontinuity is observed when switch from ignition to combustion.

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Ignition modeling validation

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Engine combustion simulation

11 Guoqing Xu &

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Engine Simulation --- Experimental setup

Liebherr 6-cylinder gas engine

Displacement volume 1.99 L/cylinder

Bore 130 mm

Stroke 150 mm

Compression ratio 13.5

Speed 1500 rpm

Norminal Power 246 kW

Liebherr gas engine test

bench

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13 Guoqing Xu

Average cell

size

Cell number at TDC

(including ports)

Cell number at BDC

(including ports)

CPU hours per

cycle G-equation

Coarse 2.25 mm 0.13 million 0.35 million 1176

Medium 1.5 mm 0.58 million 1.05 million 3600

Fine 1.0 mm 1.39 million 2.32 million 8000

Engine simulation --- Grid independence analysis

Computation domain

Exhaust Intake

Fine

Coarse

Medium

&

Asymmetric geometry

=> Full cylinder simulation

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14 Guoqing Xu

Considering accuracy and computational effort:

=> medium grid is appropriate for this application

Reference case with different grids

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Full load, 1500 rpm, λ=1.67,

ST=20.5 °BTDC

Engine simulation --- Grid independence analysis

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𝑺𝒕𝑺𝒍

= 𝟏 + 𝑨 × (𝒖′

𝑺𝒍)𝟓/𝟔

a proper value of A for the reference case is 3.2.

Guoqing Xu

Engine simulation ---- tunable parameter sensitivity analysis

Damköhler turbulent flame speed closure constant A

Full load, 1500 rpm,

λ=1.67, ST=20.5 °BTDC

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Various engine operating conditions

Variations Range Fixed parameter for

each variation

Spark timing (ST) 17.5°, 22.4° BTDC λ=1.67

Air-fuel ratio λ=1.65, 1.68, 1.72 ST= 18.5 ° BTDC

Engine speed n= 1200, 1500, 1800 rpm ST= 20.5 °; λ=1.68

Engine load BMEP= 16(100% load), 12

(75 % load), 8 (50%

load)bars

ST=20.5 °BTDC ; λ=1.67;

speed = 1500 rpm

Guoqing Xu &

Table of engine operating condition sweeps

Reference case

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17 Guoqing Xu

Ignition timing variation

ST= 17.5 °BTDC

ST= 22.4°BTDC

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The pressure is normalized with respect to the motored cycle peak pressure

The HRR is normalized with respect to total amount of heat release

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18 Guoqing Xu

Lambda variation

λ=1.65

λ=1.68

λ=1.72

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19 Guoqing Xu

Engine speed variation

n=1500 rpm

n=1800 rpm

n=1200 rpm

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20 Guoqing Xu

Engine load variation

Bmep =12 bar (Load 75%)

Bmep = 8 bar (Load 50%)

Bmep =16 bar (Load 100%)

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Conclusions and next steps

21 Guoqing Xu &

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Conclusions

From ignition modeling:

Ignition model based on Herweg & Maly can properly capture the flame

kernel size and kernel expansion speed at both lean and stoichiometric

conditions for the Leeds bomb measurements.

From the engine combustion simulation

Considering the result accuracy and computational effort and no further grid

refinement is necessary for an averaged cell size of 1.5 mm for Liebherr gas

engine application.

A proper model value of Damköhler turbulent flame speed closure is 3.2 for

Liebherr gas engine lean combustion application.

Good predictions in terms of cylinder pressure and chemical heat release

rate, at various engine operating conditions, were obtained.

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23

Next Steps

Inclusion of knocking and NOx emission models towards

higher compression ratio operating concept.

Apply the models on engine combustion coupled with

pre-chamber ignition system.

Guoqing Xu &