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