Experimental analysis on the effect of turbulence
on the structure and dynamics of a bluff-body
stabilized conical lean premixed flame
Bikram Roy Chowdhury and Baki M.Cetegen
Combustion and Gas Dynamics Laboratory
Mechanical Engineering Department
University of Connecticut
Outline
Motivation
Background
Experimental Setup
Results
Summary
Motivation
Physics behind bluff body flame
stabilization
Recirculation Zone
Flame Sheets
High level of turbulence intensities (~50%)
are encountered in practical combustors.
Bluff body stabilized flames find
widespread application in propulsion and
land based power generation systems.
Background
Most of the turbulent flame imaging experiments done prior to 2009 were restricted
to turbulent Reynolds numbers less than or around 2000.
Significant flame extinction was observed in highly strained piloted Bunsen flames
in the works of Mansour et al (1992), Chen et al (1996) and Chen and Mansour
(1997).
Premixed flames subjected to intense turbulence levels (Ret ~ 100,000) have been
studied recently by Temme et al. (2014, 2015) and Skiba et al (2015, 2016) in the
Michigan Hi-Pilot burner.
Experiments involving V-shaped flames have been studied by Namazian et al.
(1986), Shepherd et al. (1996), Kheirkhah et al.(2014, 2015) and others.
In a number of recent works, simultaneous PLIF imaging of OH and CH2O has been
performed for estimation of heat release.
Experimental Setup
Fuel-Air Mixing
Chamber
Air from
compressor
Mass Flow
Controller
Flow Controller
Module
Brass Burner
Perforated Plate - I
Bluff Body
Turbulence
Generator
Hot Wire
Anemometer ICCD # 1
Flow Analyzer
ICCD # 2
S.P/ P.P
Mesh
Jet
Bluff Body
PLIF
Field of View
Turbulence Generating
Arrangement
Slotted Plate used for T.I : 24%
(Dimensions are in mm)
Axisymmetric Bluff Body
Characterization of the turbulent flow field
0
0.1
0.2
0.3
0.4
0.5
0.6
0
2
4
6
8
10
12
0 5 10 15 20
Norm
ali
zed
Tu
rbu
len
t
Inte
nsi
ty
Mea
n V
elo
city
(in
m/s
)
Distance from the burner inner wall (in mm)
Bluff-Body
stem
Burner
inner wall
Axial velocity profile for the mean velocity of 10 m/s and
turbulence Intensity of 30%. Power spectrum based on measurements (black
line) with the correlative fit (red line) to
determine the integral length scale.
𝑈𝐸{𝑓}
𝑙𝑢′2= 4 1 +
8𝜋𝑓𝑙
3𝑈
2 −5/6
u’/Um lo (mm)
(H.W.A)
lo (mm)
(PIV) Ret
0.04 2.8 2.93 87
0.14 7.8 7.60 844
0.24 12.8 12.3 2176
0.30 13.6 13.1 2890
Properties of the turbulent flow field in
the experiments
Simultaneous PLIF imaging of OH and CH2O
Sheet Optics
355 nm
Intensified Camera #1
Sirah Dye
Laser
Continuum
Nd:YAG
532 nm
Quantum Pro
Nd:YAG
Dichroic
Mirror
Intensified Camera #2
Burner
Beam Dump
Schematics of the Laser system layout
ICCD # 2
(Exposure Time)
Laser pulse for OH
PLIF (282.67 nm)
Laser Pulse for CH2O
PLIF (355 nm)
dt = 300 ns
ICCD # 1
(Exposure Time)
Timing of the two beams
Sample PLIF image showing the OH
region (left) and CH2O region (right).
The border of the overlapping region is
shown by thick blue lines on the OH
PLIF image.
Results
Flame Front Topology
Low Turbulence Intensity ( ~ 04 %)
- Weakly wrinkled and moderately
symmetric flame front.
-Continuous heat release regions
Moderate Turbulence Intensity (~14 %)
- Increase in wrinkling of the flame front
- Formation of cusps and unburned
mixture fingers
Instantaneous PLIF image corresponding to the low
turbulence intensity condition
Instantaneous PLIF image corresponding to the moderately
turbulent condition
Results (contd.)
Intense Turbulence Intensity ( ~ 30 %)
- The flame front topology is highly
distorted with significant change in
the structure, both spatially and
temporally.
- Thickening of the pre-heat region
- Localized extinctions and flame
fragmentation
- Formation of pockets
- Intermittent change in the shape of the
stably burning flame from varicose to
sinuous mode
Instantaneous PLIF image corresponding to the intense
turbulent condition
Results (contd.)
Intermittent shift of the flame shape from
varicose to sinuous mode
- Localized extinctions and flame fragmentations
result in heat loss and ultimately, reduces the
density ratio.
fusionViscousDif
oductionVorticityBaroclinic
ExpansionGasStretchingVortex
SpVV
Dt
D
.).().(
2
Pr
Computed vorticity contours and
instantaneous flame positions at various
dilatation ratios. From Erickson et al. (2006)
Results (contd.)
Average PLIF of OH, CH2O and heat release for the low (4%) and
intense (30%) turbulence conditions agrees with the observations from
the instantaneous flame front topology images.
Mean PLIF images
T.I : 04%
T.I : 30%
Flame Front Statistics
Curvature Analysis
- Estimated along the leading edge of
the flame front by parameterizing
the instantaneous Cartesian
coordinates of the front by the
length parameter (s).
𝐶 = 𝑥 𝑦 − 𝑦 𝑥 /(𝑥 2 + 𝑦 2)3/2
- As expected, the curvature
distribution broadens with
increasing turbulence intensity.
Results (contd.)
Probability distribution functions of curvature for
the different conditions
Flame Surface Density
- Estimated by Shepherd’s method
using the mean map of progress
variable obtained by averaging the
binarized OH PLIF images.
Σ 𝑐 = 𝐿 𝑐 /𝐴 𝑐 𝑛𝑓
Flame Brush Thickness
- The thickness of the flame brush has
been defined as the full width at half-
maximum (FWHM) of the profiles
of 𝑐𝑟𝑚𝑠′ .
Results (contd.)
Measurement of FSD evaluated for different conditions Variation of flame brush thickness for different
conditions
Flame topology is strongly affected by turbulence.
Formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4 to 14 % but, the heat release region remained continuous.
For higher turbulence intensity condition, flame fragmentation, localized extinctions and islands of reactants in the product region with heat release along their boundaries were observed. Intermittent modification of the flame shape from varicose to sinuous mode was observed.
Broadening of the curvature pdfs occurs with increase in turbulence levels showing greater extent of wrinkling.
The brush thickness increased with increasing turbulence intensity but saturated beyond 24 % while the flame surface density decreased.
Summary
This research is supported by GE Graduate Fellowship for Innovation and UTC Chair Professorship Endowment fund
With greatest sincerity and gratefulness, I acknowledge the continuous motivation and mentorship of my advisor Prof. Baki Cetegen.
Acknowledgement