Inhomogeneous Mixing Behavior of
Recirculated Exhaust Gas in a Lean Premixed Flame
2nd Japan-China Joint Seminar
July 11, 2016, Gifu University, Japan

Masaharu Komiyama

Department of Mechanical Engineering
Gifu University, Japan

Contents
1. Background
Combustion technology

2. Objective
3. Experimental Methods(a) Rayleigh scattering(b) Laser-induced fluorescence

4. Experimental Apparatus
Premixer
Optical apparatus

5. Experimental Results
6. Summary

Gifu University
Department of Mechanical Engineering

1. BackgroundPremixed Combustion

reduce environmental loads

Fuel and air are mixed before they burn Homogeneous flame is formed

Flame Reduce NOx emissions

EGR (Exhaust Gas Recirculation)

Premixed combustion

Recycle a part of the exhaust gas tothe compressor
 Low oxygen concentration combustion
 Reduce the amount of oxygen combinedwith nitrogen Reduce NOx emissions

However …
The inhomogeneous oxygen distributions will be formed.

2. Objectives
To clarify the flame characteristics under theinhomogeneous oxygen concentrations
Approach Instantaneous measurements of 2-D flame temperature,
OH-LIF and oxygen concentration distributions
 Using N2 as a recirculated exhaust gas in order to reduce O2
concentration
Methods
Flame temperature

Rayleigh scattering method

OH-LIF and O2

Laser-Induced Fluorescence(LIF) method

NOx emissions

Gas analyzer

(a) Rayleigh Scattering Method
Rayleigh scattering is the elastic scattering of light The wavelength of the scattering light = that of theincident light The wavelength of the scattering light doesn’t change
depending on gas species
 The intensity of the scattering light The number density of the gas molecules,
The effective Rayleigh scattering cross section
Intensity of Rayleigh scattering light
The effective Rayleigh scattering cross section
Number density of gas molecules
The ideal gas equation
Temperature (is inversely proportional to scattering intensity)

Raleigh Scattering Intensity IRI R  PL N s Re ff L
I R : Rayleigh scattering intensity

 : Detection efficiency
PL : Laser intensity
N s : Number density of molecules N s  T 1

 Re ff : Effective Rayleigh scattering cross section
L : Length of measuring volume

Effective Rayleigh scattering cross section : Re ffRe ff： mole fraction of species i    iiii

 ： Rayleigh scattering cross section of species ii

The composition of species changes in combustion process changes in combustion process
Re ff

This technique is limited to the fuel whose  Re ff varies only slightly

Rayleigh scattering cross section ratio
σReff/σRair

Effective Rayleigh scattering cross section : Re ff

1.2
Unburned gas
Adiabatic-Equilibrium gas

1.1

1
1

2

3

Air ratio λ Relationship between air ratio  and Rayleigh scattering cross section, Reff
normalized by that of air, Rair in the CH4-Air flame.
• CH4 90 % + N2 10 % /Air premixed flame• Change of effective Rayleigh scattering cross section of the unburned mixture gasand burned gas within 3 %.
• Change of effective Rayleigh scattering cross section of the mixture gas in thereaction zone and burned gas within 4 %.

Optical Apparatus for Rayleigh Scattering•Temperature measurement by Rayleigh scattering:
Nd:YAG laser (λ=532 nm, 300 mJ)
CCD camera with 512×512 square pixels with single I.I. and a normallens (f 1.2)

Instantaneous Temperature Profiles in a Turbulent FlameH2 30%+N2 70% Diffusion Flame(a) Re=1000, (b) Re=4000,(c) Re=8000, (d) Re=12000,(e) Re=17000

(b) Laser-Induced Fluorescence (LIF)
Laser–induced fluorescence (LIF) is a spectroscopic method used for
detective of selective species and flow visualization and measurements
The excited species will de-excite and emit light at a wavelength
larger than the excitation wavelength in few ns to s.

＜Advantages＞
 It is possible to get two- and three-dimensional
images.
 The signal-to-noise ratio of the fluorescencesignal is relatively high, providing a goodsensitivity to the process.
 It is possible to distinguish between morespecies .

Two level transition equation of molecules for LIF

dn1
 n2 ( B21U L  Q21  A21 )  n1 B12U L
dt
dn2
 n1 B12U L  n2 ( B21U L  Q21  P21  A21 )dt
n0  n1  n2

LIF intensity of OH molecules: IFA2 (v' 1)  X2(v" 0) P2 (7) : 285.43nm
A21B12U L h 21VIF f B (T ) N
B12U L  B21U L  Q21  A21
n1 : number density of OH molecules in the ground staten2 : number density of OH molecules in the excithed state
no : total number density of OH molecules
B12 : Einstein coeffiient of stimulated absorption
B21 : Einstein coefficient of stimulated emission
A21 :Spontaneous emission rate
Q21 : electronic quenching rate
U L : Energy density of laser light
P21 : Predissociation rate

LIF intensity of OH A-X (1,0) System•

P2(7) A-X (1,0) has relatively weaker temperature dependency
compered by other absorption lines.

LIF signal intensity of OH AX (1,0) system

Relationships between OH existence
probability on energy level andtemperature

OH Concentration (N/N 0)

Relationship between OH-LIF and OH Concentration

21.00%
17.85%
15.75%

10.5001234OH-LIF Intensity (I F/IF0)•••

OH-LIF and OH concentration for air ratios of 2.0 with various oxygen
concentrations.
Calculated for the case of 1D PREMIX and GRI-Mech 3.0.
Relationship between OH-LIF and OH Concentration keeps a linear one.

Optical apparatus for OH LIF•OH-LIF:
Nd:YAG pumped dye laser (λ= 285.43 nm, 10 mJ) to excite the P2(7) lineof the (1-0) band of A2Σ+－X2Π .
CCD camera with 512×512 square pixels with single I.I. and a UV camera lens (f3.5)

OH Concentration Profiles in a Turbulent Flame

X=30 mm

X=60 mm

Acetone LIF for N2 Concentration Measurement

Acetone mole fraction is
proportional to its LIF
intensity in an unburned
region in this experimental
acetone mole fraction range.

•Nitrogen gas saturated with 1 % acetone vapor was used as a recirculated
exhaust gas.
•Fluorescence intensity modified by the homogeneous acetone-LIF image of air.

Schematic view of premixer for inhomogeneous mixing

CH4 Air
N2Air N2

Case A : HomogeneousFuelCondition

AB

Oxidizer
O2
Temperature
concentration

CH4AirN2

NL/minNL/minNL/min

%

K

5
5

80.7
71.2

14.3
23.7

17.9
15.8

570
570

4Air NCH
Air
2

Case B: InhomogeneousExperimental Apparatus for inhomogeneous mixing
••

•
•
•

Rayleigh: Nd:YAG 532nm, 400mJ
Acetone-LIF: Nd:YAG 266nm, 100mJ
OH-LIF: Nd:YAG pumped Dye laser 285.43 nm, 10mJ

Air supplied from acompressor after
cleaning moistureand dust.
Nitrogen added toreduce oxygen
concentration of anoxidizer.
Methane as a fuel is
supplied to fuelnozzle.
A Nd:YAG laser
CCD cameras forRayleigh scatteringand LIF are locatedat facing of the test
section.

Inhomogeneity ofoxygen concentration
To evaluate inhomogeneity of oxygen concentration quantitatively,
we used the Unmixedness : G given by the following equation.

̅ ∶ the average of LIF signals: the variance of LIF signals

Spatial Scale ofConcentration Fluctuation Using 2D FFT
 2DFFT→Relationships between Amplitude of Concentration
fluctuation and Wavenumber
 Spatial scale λ← Wavenumber based on Measuring Area

10050

00102030
r [Hz]

4050

(a)
(b)
(c)
(d)
(e)

10050

00102030
r [Hz]

4050

150
Amplitude [-]Amplitude [-]

(a)
(b)
(c)
(d)
(e)

Amplitude [-]

150

150

(a)
(b)
(c)
(d)
(e)

100

50

00102030
r [Hz]

4050

N2 profiles under unburned conditions
in x-z plane (Condition A)(a)

5.19 10
λ 0.900 mm
No N2 Nozzle

6.57 101.945 mm
N2 : h = 90 mm

1.15 102.476 mm
N2 : h = 70 mm(a) N2 concentration mixed with air from upstream of the premixer homogenously
Lager G and λ in (b) and (c) rather than in (a).

N2 profiles under unburned conditions
in x-y plane (Condition A)

Instantaneous Temperature Profiles(Condition B)
6.18

100.95 mm

4.35102.08 mm1.03102.60 mm

Instantaneous OH-LIF Intensity (Condition B)
6.18

100.95 mm

4.35102.08 mm

1.03102.60 mm

0.26
0.24

•

OH-maxNOx1.4

0.22

1.3

0.2

1.2

0.180•

1.50.002 0.004 0.006 0.008
G0.01

NOx(O2=15%) [ppm]OH-LIF max Intensity [-]Relationship between Max OH-LIF, NOx andUnmixedness G

1.1

When G increases, OH-LIF intensity and NOx concentration decreases in thebeginning. When G increases more, OH-LIF intensity and NOx concentration
increases conversely.
Moderate reaction in lower oxygen concentration regions is predominant when
the unmixedness increases, active reaction in higher fuel concentration regions
might become predominant when the unmixedness increases more.

0.26

•

OH-max
NOx

0.24

1.4
1.3

0.22
1.2
0.2
0.18

•

1.5

1.1
1

1.5

2
λ[mm]

2.5

NOx(O2=15%) [ppm]OH-LIF max Intensity [-]Relationship between Max OH-LIF, NOx andSpatial Scale

1

When spatial scale increases, OH-LIF intensity and NOx concentration decreases
in the beginning. When spatial scale increases more, OH-LIF intensity and NOx
concentration increases conversely.
Moderate reaction in lower oxygen concentration regions is predominant when thespatial scale increases, active reaction in higher fuel concentration regions might
be predominant when the spatial scale increases more.

Summary

Gifu University
Department of Mechanical Engineering

We investigated the flame characteristics under thehomogeneous and inhomogeneous oxygen concentrations.
 The inhomogeneous degree of oxygen concentration profiles wereevaluated by unmixedness G and spatial scale of the fluctuation.
 The shape of flame was complicated in the inhomogeneous oxygenconcentration profiles. It was caused by the locally differences ofburning velocity.
 OH-LIF and NOx concentration decreases, when inhomogeneousdegrees of oxygen concentration profiles increase, OH-LIF andNOx concentration increases conversely, when the inhomogeneousdegrees increase more.
 Moderate reaction in lower oxygen concentration regions ispredominant when the inhomogeneous degree increases, but theactive reaction in higher fuel concentration regions might bepredominant when the inhomogeneous degree increases more.