Analysis of elevated flare ignition and resulting pressure wave generated as a function of ignition delay and flow rate.

1. www.inl.gov
Prediction and Measurement of
Flare Ignition Using the LES
based C3d
J. D. Smith, Ph.D., Idaho National Laboratory
A. Suo-Anttila, Ph.D., Systems Analyses and Solutions
S. Smith and N. Philpot, Zeeco, Inc.
American Flame Research Committees - International Pacific Rim Combustion Symposium
Advances in Combustion Technology: Improving the Environment and Energy Efficiency
Sheraton, Maui, Hawaii – September 26-29, 2010

2. OUTLINE
• Background and Introduction
• Flare Tests
• Model Setup and Methodology
• Simulation Results
– Low Flow Conditions
– High Flow Conditions
• Observations and Conclusions
Slide 2
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

3. Slide 3
Elevated Multi-Tip Gas Flare Ignition
Slide 3
• Nominal Firing Rate = 350 Tons
Per Hour (TPH)
• Max Firing Rate – 1350 TPH
• Mostly Natural Gas (Mwt = 18)
• Experienced Pressure Wave during
ignition
• Conducted Tests to quantify
ignition phenomena:
− Microphones used to measure
pressure wave
− High Speed Video used to capture
flame during ignition
• Test results reported elsewhere
(summarized below)
• Test video shows ignition behavior
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

4. Slide 4
Test Layout
Slide 4
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

5. Slide 5
Flame during Ignition
Slide 5
Maui, Hawaii
September 27 - 29, 2010
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton

6. Slide 6
Test Results:
Sound Level (pressure wave) and Flame speed estimate
Slide 6
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
• Flame speed estimate from tests is 45 m/s
(Test 1) and 50 m/s (Test 2)
• Maximum pressure generated by spherical
flame propagating at 50 m/s would be ~48
mB (AIChE correlation*)
* Center for Chemical Process Safety, Guidelines for Evaluating the Characteristics of Vapor Cloud
Explosions, Flash Fires and BLEVEs. AIChE (1994).
Maui, Hawaii
September 27 - 29, 2010

7. Model Setup: General Comments
• Transient conservation equations with radiative heat transfer and
combustion chemistry
• Considers soot formation and other multi-phase systems using
Eulerian/Eulerian formulation
• Accurately assess different operation scenarios (wind, flow rate,
fuel type, surroundings)
• Reasonable CPU time requirements on “standard” workstation
• Trade offs for “Engineering” Approach
– Sacrifice generality (large fires only) in favor of quick turnaround with
quantitative accuracy
– Reaction rates and radiation heat transfer models apply only to large fires
– Models intended to make predictions “good-enough” for industrial use
– Model validation for each application to establish accuracy of results
Slide 7
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

8. Combustion Model
• Variant of Said et al. (1997) turbulent flame model
• Relevant Species (model includes relevant reactions)
F = Fuel Vapor (from evaporation or flare tip)
O2 = Oxygen
PC = H20(v) + CO2
C = Radiating Carbon Soot
IS = Non-radiating Intermediate Species
• Eddy dissipation effects and local equivalence ratio effects
• Reactions based on Arrhenius kinetics
C and TA determined for all reactions
Slide 8
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

9. Reaction Rate Model
• Arrhenius rate model
– Consumption of primary reactant increases on reactants mass fraction fRi and
temperature T in volume
– Coefficients C and Activation Temperatures (TA) determined for all reactions
– Where:
Ak = Pre-exponential Factor X1 = Natural Gas Mol Frac
X2 = O2 Mol Frac Ea = Activation Temperature
T = Local Gas Temperature b, c, d = Global Exponents
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 9
Maui, Hawaii
September 27 - 29, 2010

10. Approach (1)
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 10
• Laminar flame global mechanism used as starting point
– Used Activation temperature + mol frac exponents (based on reaction)
– Pre-exponential (Ak) factor adjusted to match turbulent combustion rxn rates
• Turbulent mixing effect on combustion included via LES
– Two coefficients adjust effect (ε = turbulence intensity scale factor;
δ = combustion species mixing time delay)
• Parameters adjusted to to match experimental results
Maui, Hawaii
September 27 - 29, 2010

11. Approach (2)
• Other model variations considered:
– Computational grid size cell number
– Turbulence model (zero equation and one-equation LES)
– Nozzle structure (jet cone vs nozzle surface)
– Numerical upwind differencing
– Time to ignition
• Boundary Conditions
– Hydrostatic pressure on all external boundaries (adjusted to account for wind)
– Initial temperature and composition set to 300 K and air
• Other Assumptions
– Flare gas combustion approximated as described above
– Thermal radiation calculated w/ standard radiation models
– Wind conditions, flare gas inlet temperature and pressure, and radiation
effects set to match measured value
– Flame emissivity = F (gas comp, soot fv, flame size/shape, temp)
Slide 11
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

12. Slide 12
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Nozzle Approximation
Two approaches considered for detailed nozzle
structure:
1.Mass Sources on Nozzle Cone:
• Place source terms on cone surface and inject natural
gas at correct velocity and mass rate as if resolved using
fine cells
• Individual nozzle flow kept exactly identical (in absence
of any flow mal-distribution)
2.Mass Inlet on Nozzle Surface
• Inject fluid through cells representing nozzles
• Total inlet adjusted for correct mass flow
• Individual rates varied (nozzle sizes varied due to
overlap of square cells on circular tips)
3.More general “mass source” approach selected
Maui, Hawaii
September 27 - 29, 2010

13. Slide 13
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Computational Mesh
o Flare dimensions approximated as 3.35m square, computational volume set as 20m
long X 20m wide X 15m high (domain extended ~9m beyond flare edge)
o Domain bottom set at top of elevated flare exit (reduce mesh size)
o Domain separated into two regions
Region 1: Near Tip Region just above nozzle and 7m square by 8m high
• Fixed horizontal cells with equal spacing (80 cells 0.0875m on a side)
• Vertical dimension slowly varied with 0.05m at nozzle face to 0.14m at top of region (90 cells)
• High resolution region had 576,000 cells
Region 2: Buffer Region surrounding Near Tip Region
• Course, stretched cells to provide buffer between boundaries and near tip
• Both horizontal dimensions included 14 cells; vertical dimension included 12 cells
• Provided large distance from edge of domain (pressure boundary) and flame surface to prevent
estimated pressure in igniting flame ball
o During analysis, mesh refined several times to improve calculation results
Maui, Hawaii
September 27 - 29, 2010

14. Slide 14
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Pressure Monitoring Locations
Note: x and z are horizontal positions (x = 0
and z = 0 is flare center) and y is height above
flare tip as shown in graphic)
Maui, Hawaii
September 27 - 29, 2010

15. Model Tuning (1)
• Over 60 CFD runs indicated pressure wave magnitude mostly dependent on
ignition time (combustion kinetics and turbulence had secondary effects)
• Typical pressure pulse of +30 to +40 mB wave followed by negative wave of -10
to -20 mB
• Runs with ignition delay exhibited higher pressures waves
– Combustion parameters varied over significant range but had little effect on predicted
peak pressure wave
• Ignition delay accomplished by:
– Natural gas jets turned on for 0.25 sec prior to igniting pilot
– After ignition, pilot flame grew and ignited flare gas at approximately 1 sec
– Resulting flame ball had significantly higher pressures than nearly all other cases
considered
– Cases #3 and #41 had overpressures ~0.5 atm
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 15
Maui, Hawaii
September 27 - 29, 2010

16. Predicted Pressure for
All Cases at Low Flow Conditions
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 16
Maui, Hawaii
September 27 - 29, 2010

17. Model Tuning (2)
• Final chemical kinetics coefficients selected as providing “best” fit to ignition
tests:
– Ak = 5.0e16, Ta = 20098, b = 0.5, c = 1, and d = 1
• Turbulence parameters selected:
– ε = 0.2; δ = 1e-5
• Kinetics and turbulence parameters not highest values tested (i.e. fastest
kinetics and most rapid mixing)
• Cases with higher values not always result in higher pressures since high
values also leads to combustion in non-ideal mixtures
• Increasing turbulence scale improves mixing and suppresses natural fluid
oscillations in turbulent jet (scale factor not allowed to exceed 2x recommended
value of 0.2)
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 17
Maui, Hawaii
September 27 - 29, 2010

18. Slide 18
Low Flow Results:
Filtering Effect
Slide 18
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
• Time history plot of local gas pressure for typical case @ 4m elevation above nozzle
• LHS figure has each point representing average of 4 time steps (slight filtering)
• Without filtering (RHS figure), isolated pressure peaks for single time steps ( 0.1 ms) predicted considered not representative
of experimental measurements (too fast for test equipment to accurately monitor)
• Filtering used to insure pressure histories representative of large regions and times more consistent with pressure histories
inferred from flame velocity measurements
• Pressure change (max – min) reaches approximately 50 mB (or more) – same as reported in flare tests
• Time between max and min pressure is on order of 16 ms (~60 Hz sound frequency)
Case 18 Case 37
42 mB (0.62 psig)
-10 mB (0.15 psig)
0.77 psig
Maui, Hawaii
September 27 - 29, 2010

19. Slide 19
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Low Flow Results:
Ignition Delay Effect
• Pressure histories from two delayed ignition cases (Case 3 and Case 41)
• Highest Pressure observed on outer edge of growing fire ball
• Minimum pressure observed at center of growing fire ball after high pressure
propagates outward
Case 41Case 3
425 mB (6.25 psig)
125 mB (1.84 psig)
-130 mB (1.91 psig)
-60 mB (0.88 psig)
2.72 psig
8.16 psig
Maui, Hawaii
September 27 - 29, 2010

20. Slide 20
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Low Flow Results:
Pressure Spikes from Ignition Delay
• Highest Pressure on outer edge of growing fire ball
• Minimum pressure at fire ball center after high pressure
region propagates outward
Maui, Hawaii
September 27 - 29, 2010

21. Slide 21
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Low Flow Results:
Horizontal temperature contour through flame ball at 4m
Beginning of Ignition Mid-Point of Ignition Near End of Ignition
• Spatial distance between tick marks on plots is 1m; temporal distance between plots is 30 ms
• Dividing flame propagation distance by time between frames yields flame velocity of 33 m/s
• Experimental flame propagation velocity ~50 m/s (examining video data indicated they failed to subtract
initial ball diameter). Correcting test results yields actual growth rate of 40 – 44 m/s
Maui, Hawaii
September 27 - 29, 2010

22. Slide 22
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Low Flow Results:
Flame ball growth for normal and delayed ignition
Normal Ignition Delayed Ignition
Maui, Hawaii
September 27 - 29, 2010

23. Slide 23
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Low Flow Results:
Normal Ignition Video
Maui, Hawaii
September 27 - 29, 2010

24. Slide 24
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Low Flow Results:
Delayed Ignition Video
Maui, Hawaii
September 27 - 29, 2010

25. Slide 25
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
High Flow Results:
Ignition Delay Effect
• Pressure history from non-delayed (RHS) and delayed (LHS) ignition pressure wave
(4m above flare tip)
• Higher flow wo/ ignition delay caused slightly higher pressure wave (3.53 vs 0.77 psig)
• With Ignition delay, pressure builds until calculation becomes unstable (detonation)
High Rate w/ Ignition DelayHigh Rate wo/ Ignition Delay
160 mB (2.35 psig)
1700 mB (24.98 psig)
-80 mB (1.18 psig)
3.53 psig
Maui, Hawaii
September 27 - 29, 2010

26. Slide 26
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
High Flow Results:
Normal Ignition Video
Maui, Hawaii
September 27 - 29, 2010

27. Slide 27
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
High Flow Results:
Delayed Ignition Video (only two frames at sampling rate)
Maui, Hawaii
September 27 - 29, 2010

28. Conclusions
• Natural Gas Flare Gas Fired through multi-burner tip:
– C3d flare model based on LES mixing model
– Combustion model used EBU type reactions (includes soot)
– 2-zone computational mesh (adjusted to optimize grid)
– Final mesh size ~1.2MM cells
• Simulated low flow (200-350 TPH) and high flow (1350 TPH) conditions
• Compared results to test results
– Pressure wave estimated by AIChE correlation + flame speed estimated from high speed
video (pressure measurements via microphone – not sensitive enough)
– Predictions compared well to data for flame speed and pressure wave
– from 12 tests (2 tip sizes, 3 operating pressures, 2 radiation sample locations)
• Estimated Pressure wave
– Low flow, no ignition delay 0.75 psig, flame speed ~33 m/s (measured 40 m/s)
– Low flow, ignition delay ~ 8 psig possible!
– High flow, no ignition delay 3.5 psig
– High flow, ignition delay resulted in explosion!
Slide 28
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

29. Backup Slides
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 29
Maui, Hawaii September 27 - 29,
2010

30. Radiation Inside Large Fires
• High soot volume fractions make large fires non-transparent
(optically thick) which causes flame to radiate as a cloud
(radiatively diffuse)
• Fire volume defined where soot volume fraction (fi) greater than
minimum volume fraction (fsoot fmin)
• Flame edge (fflameedge) where soot volume fraction = 0.05 ppm
Calculated flame surfaces from 3 time steps from validation against test
Slide 30
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

31. • When fsoot fflameedge then outside “flame” (participating medium
considered)
• View factors from fire to surrounding surfaces calculated at each
time step (includes attenuation by gas and soot media for
flames)
• Re-radiation from surroundings also calculated at each time
• Fire considered black body radiator (εfiresurface = 1)
• Radiation from flame to surroundings assumes Tsurround =
constant
Radiation Outside of Large Fires
Slide 31
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

32. Diffuse Radiation Within Fire
• Calculated indirectly using a Rossland effective thermal
conductivity
– σ = Stefan-Boltzman Constant
– T = local temperature
– βR= local extinction coefficient. Dependent on local species concentrations
• Radiation transport model:
– Predicts radiant flux on external (and internal) surfaces
– Provides source/sinks terms to overall energy equation
Air
R
R k
T
k =
β
σ
3
16 3
Slide 32
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Maui, Hawaii
September 27 - 29, 2010

33. Reactions Involving Fuel
• Incomplete Fuel Combustion (soot producing)
1 kg F + (2.87-2.6S1) kg O2 → S1 kg C + (3.87-3.6S1) kg PC + (50-32S1) MJ
– Combustion Soot Mass Parameter, S1 = 0.005
• Endothermic Fuel Pyrolysis (soot producing)
1 kg F + 0.3 MJ → S2 kg C + (1-S2) kg IS
– Cracking Parameter, S2 = 0.15
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 33
Maui, Hawaii
September 27 - 29, 2010

34. Reactions Not Involving Fuel
• Soot Combustion
1 kg C + 2.6 kg O2 → 3.6 kg CO2 + 32 MJ
• Combustion of Intermediate Species
– Coefficients chosen so that complete combustion of C and IS produce same
species and thermal energy as direct combustion of fuel
American Flame Research Committees - International Pacific Rim Combustion Symposium Advances in
Combustion Technology: Improving the Environment and Energy Efficiency Sheraton
Slide 34
Maui, Hawaii
September 27 - 29, 2010