Flame flashback in swirl burners: previously hidden ... · Flame flashback in swirl burners: How...
Transcript of 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.
[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
4
Strengths of and challenges for gas turbines as part of the future energy supply infrastructure.
Increase in flashback propensity
[email protected] 07.09.2017
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
[email protected] 07.09.2017
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.
[email protected] 07.09.2017
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
[email protected] 07.09.2017
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
[email protected] 07.09.2017 Page 9
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
[email protected] 07.09.2017 Page 10
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.
[email protected] 07.09.2017
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
Page 11
laser sheet
Accurate velocity field measurements with high spatial resolution inside the mixing tube at kilohertz-rate.
[email protected] 07.09.2017
• 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
Page 12
Flame front tracking based on PIV images.
Pfadler et al., Optics Express, 15 (23), 2007
Olive oil droplets:
Aluminum oxide particles:
[email protected] 07.09.2017
Simultaneous PIV and CL imaging
Page 13
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.
[email protected] 07.09.2017
Simultaneous PIV and CL imaging
Page 14
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
[email protected] 07.09.2017
Flow-flame Interaction during Flashback
Page 15
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
[email protected] 07.09.2017
Flow-flame Interaction during Flashback
Page 16
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
[email protected] 07.09.2017
Flow-flame Interaction during Flashback
Page 17
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., 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
[email protected] 07.09.2017 Page 18
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.
[email protected] 07.09.2017 Page 19
Wir schaffen Wissen – heute für morgen
Acknowledgements • Flow Field Imaging Lab (Prof. Noel Clemens),
The University of Texas at Austin • US DOE NETL