flame flashback in swirl burners: previously hidden ... · flame flashback in swirl burners: how...

WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN

Flame flashback in swirl burners: How high-speed laser diagnostics reveal previously hidden information on this transient combustion phenomenon

Dominik Ebi :: Research Scientist :: Laboratory for Thermal Processes and Combustion

Tagung Verbrennungsforschung in der Schweiz

07.09.2017

[email protected] 07.09.2017 2

1. Gas turbines: Strengths and challenges in future energy supply.

2. Flame flashback: New insights on the physics.

3. High-speed laser diagnostics:

Access to transient combustion phenomena.

Outline

[email protected] 07.09.2017

Fuel flexible including more reactive fuels

Low emissions even at part load

Reliability despite many load changes

High efficiency (combined cycle, CHP) even at part load (higher firing temperatures)

Fast ramp up/down and still stable combustion

Motivation

3

Strengths of and challenges for gas turbines as part of the future energy supply infrastructure.


Fuel flexible including more reactive fuels

Low emissions even at part load

Reliability despite many load changes

High efficiency (combined cycle, CHP) even at part load (higher firing temperatures)

Fast ramp up/down and still stable combustion

Motivation

4

Strengths of and challenges for gas turbines as part of the future energy supply infrastructure.

Increase in flashback propensity


Geometry of a Gas Turbine Combustor

5

http://efficiency.gepower.com/media.html

Meher-Homji et al., Proc. 39th Turbomachinery Symp., 2010.

Swirling flow for flame stabilization and enhanced mixing.

flame

prem

ix

sect

ion

com

bust

ion

cham

ber

swirler

mixing tube

air in

center body


What is Flame Flashback?

6

Danger for burner hardware: −sustained upstream flame

propagation −flame holding

Undesired upstream propagation of the flame from the combustion chamber into the premix section.

prem

ix

sect

ion

com

bust

ion

cham

ber

flame

Meher-Homji et al., Proc. 39th Turbomachinery Symp., 2010.


A single-burner model combustor

7

Experimental investigation of flashback in a swirling flow.

• CH4-H2 air flames • atmospheric

pressure • non-preheated • moderate Re • fully premixed

∅ 𝟓𝟓𝟓𝟓 𝒎𝒎𝒎𝒎

∅ 𝟏𝟏𝟏𝟏𝟏𝟏 𝒎𝒎𝒎𝒎


Time Scales of Flashback

8

High-speed diagnostics needed to resolve this transient (~ 100 𝑚𝑚𝑚𝑚) combustion phenomenon.

Real-time Slow motion • Flashback event

recorded with high-speed intensified camera at 4 kHz

• Artificial color map applied to greyscale images

• Flashback event recorded with regular camera (30 fps)

• CH4-air flame • Flashback

triggered by increase in fuel flow rate


Flashback: Classical View

Flashback results from an imbalance between (unperturbed) flow velocity and flame speed. Assumption: Isothermal flow.

𝒗𝒗𝒖𝒖: velocity unburnt gas

inflow (reactants)

outlet (products)

flashback

𝒗𝒗𝒇𝒇: lab-frame flame propagation velocity 𝑚𝑚𝑢𝑢: flame speed

flame channel wall


Flashback: Recent Findings

Flashback is associated with a strong coupling between propagating flame front and velocity field.

Heeger, C., Gordon, M., Tummers, M.J., Sattelmayer, T., Dreizler, A., Exp. in Fluids, 49, 2010

• The flow-flame interaction facilitating flashback is not yet well-understood, especially in swirling flows.

• So far no detailed fundamental investigations at gas turbine relevant conditions.


Measuring technique: Stereoscopic particle image velocimetry (PIV) • Flow seeded with oil droplets or

ceramic particles • Particles illuminated with a pair of

laser pulses • Images divided into smaller

interrogation windows • Particle displacements in each

window obtained from cross-correlation

• Displacement + time delay of laser pulses provides velocity vectors

• Lasers and cameras operated at 4 kHz to get time-resolved data

High-speed Laser Diagnostics

laser sheet

Accurate velocity field measurements with high spatial resolution inside the mixing tube at kilohertz-rate.


• Oil droplets − evaporate as they approach flame front − no droplets in burnt gas region − interface typically marks preheat zone of

premixed flames • Ceramic particles − strong gradient in particle number density

across the flame due to decrease in gas density

− location of steepest gradient good marker for reaction layer in premixed flames1

• These methods work well in flamelet regime for non-preheated reacting flows in areas without strong recirculation of combustion products

High-speed Laser Diagnostics

Flame front tracking based on PIV images.

Pfadler et al., Optics Express, 15 (23), 2007

Olive oil droplets:

Aluminum oxide particles:


Simultaneous PIV and CL imaging

Ebi, D., Clemens, N.T., 7th Int. Symp. on Appl. of Laser Techniques to Fluid Mechanics, Lisbon, 2014

flame front

reversed axial flow

Simultaneous application of different diagnostic techniques is often crucial for a correct interpretation of the results.


Simultaneous PIV and CL imaging

flame front

reversed axial flow

CL

Top view on burner:

center body

mixing tube

laser sheet

Simultaneous application of different diagnostic techniques is often crucial for a correct interpretation of the results.

PIV

Ebi, D., Clemens, N.T., 7th Int. Symp. on Appl. of Laser Techniques to Fluid Mechanics, Lisbon, 2014


Flow-flame Interaction during Flashback

z

θ

z r

field of view for PIV

Simultaneous stereoscopic PIV and CL imaging at 4 kHz reveal a blockage effect induced by the flame on the approach flow.

Ebi, D., Clemens, N.T., TSFP-9, Melbourne, 2015




Ebi, D., Clemens, N.T., TSFP-9, Melbourne, 2015




Ebi, D., Clemens, N.T., Measurement Science and Technology, 27, 2016

Ebi, D., Clemens, N.T., Combustion and Flame, 168, 2016

Volumetric measurements: Tomographic PIV and flame surface reconstruction

flame surface


Conclusions

• Desire to use alternative fuels and operate GTs with higher firing temperatures.

• Dealing with increase in flashback propensity a challenge.

• Need for improved fundamental understanding, especially for swirling flows at GT-relevant conditions.

• High-speed diagnostics provide us with valuable insight on this transient combustion phenomenon.

• Simultaneous application of multiple diagnostic techniques important for an unambiguous interpretation of the results.


Wir schaffen Wissen – heute für morgen

Acknowledgements • Flow Field Imaging Lab (Prof. Noel Clemens),

The University of Texas at Austin • US DOE NETL

flame flashback in swirl burners: previously hidden ... · flame flashback in swirl burners: how...

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