[American Institute of Aeronautics and Astronautics 41st AIAA/ASME/SAE/ASEE Joint Propulsion...

American Institute of Aeronautics and Astronautics1

Investigation on Flashback propensity of Syngas Premixed Flames

Deepthi Davu*, Rogelio Franco† and Ahsan Choudhuri‡

University of Texas at El Paso, El Paso, Texas, 79968-0521

Robie Lewis§

Department of Energy, Morgantown, West Virginia, 26505-0880

An experimental investigation is presented to delineate the effects of composition andexternal excitation on the flashback propensity of hydrocarbon (CH4-C3H8 and CH4-C2H6 ) and hydrogen (CO-H2 and CH4-H2) fuel blends. The flashback propensity of hydrocarbon fuel blends generally correlates with the flame velocity and stoichiometry of the fuel mixtures. The flashback propensity of hydrocarbon fuel blends is susceptible to the presence of external excitation. Flashback occurs at much leaner conditions in acoustically forced flames. The flashback characteristics of hydrogen fuel blends are generally dominated by the kinetics of the hydrogen. The effects of external excitation on the flashback propensity of CO-H2 flames with more than 5% H2 are not significant. For 0-700Hz forcing range the kinetic time scale of H2 is prevalent over the external forcing time scale.

Nomenclature

d = diameter of tubegF = critical boundary velocity gradientQF = experimentally measured volumetric flow rate at flashback conditionSL = laminar flame speedUu = unburned gas velocity y = distance from the stream boundary α = mixture thermal diffusivity

I. Introduction

ITH an emerging need to address gas turbine combustor issues such as fuel variability and fuel flexibility, fundamental information and data regarding the combustion characteristics of commercial and alternative

fuels are essential. Future generation gas turbine combustors should tolerate fuel compositions ranging from natural gas to a broad range of syngas without sacrificing low emission characteristics1-2. Although older gas turbine engines employ purely nonpremixed combustion systems, current designs of advanced turbine combustors use various degrees of premixing to avoid high temperature NOX forming zones3-4. With the benefit of NOX control from premixed combustion comes a number of concerns, especially combustor flashback. Flashback is a combustion condition at which the gas velocity becomes smaller than the burning velocity and the flame propagates against the gas stream into the burner tube. Flashback is a critical issue for premixed combustor designs, because it not only increases emissions but also causes serious hardware damages. For instance variation in fuel properties may cause a “well-tuned,” dry, low NOX, LP or LPP gas turbine designed for natural gas or No. 2 diesel fuel to suffer significant

* Graduate Research Assistant, Combustion and Propulsion Research Laboratory, Department of Mechanical Engineering, Student Member AIAA.† Graduate Research Assistant, Combustion and Propulsion Research Laboratory, Department of Mechanical Engineering, Currently Engineer, Engineering Systems Development Division, NASA KSC, Member AIAA.‡ Assistant Professor, Department of Mechanical Engineering, 500 West University MS 0521, and Member AIAA§ Project Manager, Gasification and Combustion Projects Division, National Energy Technology Laboratory, 3610 Collins Ferry Road, P. O. Box 0880, WV 26505-0880.

W

41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit10 - 13 July 2005, Tucson, Arizona

AIAA 2005-375

AIAA 2005-3585

Copyright © 2005 by Dr. Ahsan Choudhuri. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit10 - 13 July 2005, Tucson, Arizona

AIAA 2005-3585

Copyright © 2005 by Dr. Ahsan R. Choudhuri. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


operational (flashback, autoignition, excessive oscillations) and performance (high emissions and fuel consumptions) penalties.5

A considerable amount of research has been done on flashback characteristics of pure fuels and their response to combustion oscillation conditions6-9. But the synergetic impacts of the composition and combustion oscillations on flashback propensity of fuels such as natural gas and syngas are largely unknown. Lewis and von Elbe10 defined a term ‘boundary velocity gradient’ to explain the flashback phenomena. The critical value of ‘g’ at which the flashback condition is observed is denoted by gF. From experimentally determined flow rates, Lewis and von Elbe calculated gF for natural gas-air mixture at room temperature and pressure for various cylindrical tube diameters. Usually calculated values of gF for the pure fuel agree fairly with experimental measurements. For a particular fuel-air mixture if SL and α are known it is possible to estimate the flashback propensity of that particular mixture. However, how well this estimation works for commercial and alternate fuels such as natural gas and syngas is yet to be determined. The unavailability of experimental measurements of syngas flashback propensity makes it difficult to verify the accuracy of calculated values of gF from flame speed data.

Motivated by this iseeue, this study was aimed at generating gF values for different compositions of hydrocarbon fuel blends and syngas. The objective of the study is to understand the effects of compositions and combustion oscillations on the flashback propensity of fuel blends and syngas. The paper presents critical boundary velocity gradient maps (gF ) of different compositions of hydrocarbon fuel blends ( methane, propane, ethane and hydrogen) syngas (hydrogen and carbon monoxide)with and without the presence of combustion oscillations.

II. Background and Motivations

Flashback involves complex interactions between burning velocity, local flow-field quantities and combustion instabilities. Flashback in premixed flames is generally attributed to two mechanisms: (i) flame propagation in boundary layers and (ii) flashback due to combustion instabilities.

In the first mechanism, the flame propagates upstream when its normal velocity exceeds the local velocity. This mechanism is more likely to occur in the vicinity of the walls where the velocity field vanishes. Lewis and von Elbe10 studied flame-wall interaction and used phenomenological considerations of flame quenching to explain the occurrence of flashback in boundary layers. For a flame stabilized over a circular burner, Lewis and von Elbe defined a term ‘boundary velocity gradient’ to explain the flashback phenomena. For a sufficiently large burner, the gas velocity profile at the flame may be considered linear and the boundary velocity gradient, g, can be expressed as

y

Ug u

F

where y is the distance from the stream boundary and Uu is the unburned gas velocity. If the gas velocity (Uu) is reduced, g decreases and flame equilibrium position shift closer to the burner rim. On further decrease of the gas flow, a condition is reached at which the gas velocity at some points becomes smaller than the burning velocity and the flame propagates against the unburned gas stream. This is referred to as flashback, and the critical value of g at which this condition is first realized is denoted by gF. From experimentally determined flow rates, Lewis and von Elbe10calculated the value of gF for natural gas-air mixture at room temperature and pressure for various cylindrical tube diameters.

The critical value**, gF, primarily depends on laminar burning velocity (SL) and the mixture thermal diffusivity (). It is difficult to estimate the laminar flame speed of commercial and alternative fuels, which are actually mixtures of many fuel components. The laminar flame speed is a characteristics kinetic constant of the mixture. It strongly depends upon the combustion reaction kinetics, mixture compositions, temperature and pressure. In many aspects, fuel blends have radically different kinetic behavior from those of the original fuels.

The interactions of different chemical kinetics of each primary fuel of the blends along with the flow dynamics significantly alter the combustion characteristics. There are not sufficient reaction paths discernible in a chemical **

PdLS

gF where SL is laminar flame speed and dp is the penetration distance. For cylindrical burner tubes dp is 1/3 the quenching distance dq.

One can thus write qdLS

3Fg for cylindrical tubes. The experimental verification of the above expression yielded the relation

168.1

qdLS

125.14Fg where dq is the quenching distance. Again the quenching distance LS

b2qd

where is the mixture thermal

diffusivity and b is burner scaling constant generally has a value greater than 2.


system comprising only CO and O2 that lead to branched chain reaction and combustion, as occurs with H2 and O2. However, a small contamination of CO-O2 mixture with hydrogen, water vapor, or hydrocarbon is sufficient to link the two chemical systems together in a branched chain reaction mechanism. In addition, the presence of H2O (another constituents of syngas) performs the role of H2 in the kinetics of the CO-O2 reaction in that reaction (O+H2O 2OH) replaces reaction (O+H2OH+H). Due to this alteration of reaction pathways, past studies with moist carbon monoxide found that the flame speed increased significantly with the addition of a very small amount of hydrogen or hydrogen containing fuels.10

In many cases, flashback is observed in situations where flame propagation in the boundary layer is not possible.6-8 Coats12 proposes another mechanism of upstream flame propagation, in which combustion instabilities are involved. This kind of behavior was visualized by Thibaut and Candel13 in a premixed combustor in which the flame was stabilized by a back-ward facing step. The authors have shown that for some regimes instabilities induce large oscillation in the flow-field. The flame moves back and forth in the combustion chamber, and propagates upstream periodically during the pulsation cycles. Flashback regimes studied by Thibaut and Candel13 occur during strong combustion instabilities. They have also shown that low frequency perturbations are the most prone to induce flashback. Although several investigations have shown the effects of combustion oscillation on laminar and turbulent flame propagation, the synergetic impact of combustion instabilities is yet to be identified. The paper also delineates the effects of externally driven excitation on the flashback propensity of syngas at different mixture compositions.

III. Experimental Techniques

The flashback burner system shown in Fig.1 has four primary components: (a) mixing manifold, (b) flow excitation hub (base, speaker and speaker housing), (c) flow conditioner (honeycomb, honeycomb housing and wire mesh), and (d) burner tube assembly (converging nozzle, glass tubes, glass tube adapters, adjustable supports, and c-clamps). The converging nozzle section seamlessly merges with the adapters to accommodate glass tubes of different diameters (10.6 mm, 7 mm and 6 mm). The fuel and air enter into the manifold through four alternate injection holes. The fuel-air mixture then passes through the flow conditioning section to eliminate injection induced flow irregularities and to insure laminar flow through the burner tube. Since the burner the system is also designed to analyze the flashback with external excitation, the bottom part of the injection manifold opens to a flow excitation hub section. The flow excitation segment consists of a thin copper membrane vibration source fed by a 100W speaker. In the forced flow configuration, various synthesized signals are fed to the speaker by an amplifier and a compute controlled audio signal synthesizer combination. In the present study three audio frequencies (300Hz, 500Hz and 700Hz) were used. The burner is located in a vertical combustion chamber which has a window opening over one side for introduction of lighter. The top of the combustion chamber was open to atmosphere through an exhaust duct.

The flame was ignited at flow rate higher than the expected critical flow rate. Once ignited, the flames were visible on the video monitor. During an experiment, the flow rate was reduced in small increments while keeping the composition constant until the flame propagated back into the tube. A high-speed direct video imaging system was used to confirm the flashback condition. The critical boundary velocity gradient gF for different compositions of hydrocarbon fuel blends and syngas were calculated using the following relation:10

3

4

d

Qg F

F

where QF is experimentally measured volumetric flowrate at flashback condition and d is the tube diameter. The volumetric flow rate for a particular tube diameter and mixture composition at which the flame propagates down the burner tube were defined as the critical flow rate QF. In order to draw the flashback propensity map, critical volumetric flow rate for flashback at different mixture compositions in cylindrical tubes of various diameters were measured. The detailed of the experimental system and methodologies can be found elsewhere.11

IV. Results and Discussions

A series of experiments are conducted using the flashback test rig to study the flashback propensity of hydrocarbon fuel blends and syngas at different level of mixture compositions and combustion oscillation conditions.


A. Visual Observation and Qualification of the Test ApparatusThe direct video images at a typical flashback condition are presented in Fig.2. The flame gets attached to parts

of the inner walls of the tube because the slow locally unburned velocity nears the wall. The partial flame inside the tube starts heating up the tube tip by convection, radiation and conduction and the unburned gasses get preheated.To avoid the tube heating after each measurement the tube was cooled to the room temperature. Despite same flame trends for all fuel blends, the flame appears light at the lean limits and appears bright at the rich limit. Which signifies the higher and lower fuel flow rates respectively.

The critical velocity gradients (gF) for a natural gas composition of 81.8% CH4, 17.7% C2H6 and 0.5% N2 were measured to reproduce the data provided by Lewis and von Elbe.10 The gF values measured in the present study are plotted with the Lewis and von Elbe’s10 measurements in Fig. 3. The measured data show good agreement with the previously reported gF values.

B. Composition Effects on gF

Fig. 4 shows the critical velocity gradients (gF) of 100% methane at 10.6mm tube diameters. The 100% methane data are used as a baseline condition to compare with the gF values of hydrocarbon fuel blends. Also shown on the same figure are the effects of propane concentration on the flashback propensity of methane-propane fuel blends. There is a modest increase in the critical boundary velocity gradient with the increase in propane concentration in the mixture. With the addition of 25% propane in the mixture, the maximum gF value increases from 400 s-1 to 490s-1. This is due to the increase in laminar burning velocity of the mixture with the addition of propane. The laminar burning velocities of methane and propane are 40 cm/s and 44 cm/s respectively. The maximum value of gF for 100% methane occurs at 7% fuel which shifts to 15% of fuel with the addition of 25% propane in the mixture. This is due to the change in the mixture stoichiometry with the addition of propane.

The effects of ethane (C2H6) concentration on the flashback propensity of methane-ethane fuel mixtures are shown in the Fig. 5. A sharp increase in maximum gF value occurs with the addition of small amount of ethane in the mixture. The maximum critical velocity gradient of methane changes from 400 s-1 to 490 s-1 with the addition of 5% ethane only. Although the laminar flame velocity of propane and ethane is comparable, the initial effects of ethane addition is markedly different in compared to propane addition. This is mostly likely due to the effects of mixture Lewis number. However, the change in maximum gF is less dramatic with the higher concentration of ethane in the mixture.

The critical velocity gradients of methane-hydrogen mixtures are shown in Fig. 6. It is clearly evident that even a small quantity of hydrogen significantly changes the flashback behavior of fuel mixtures. However, it is interesting to note that, in methane-hydrogen mixtures, the flashback propensity is solely determined by the hydrogen and, therefore, flashback propensity shows an asymptotic behavior for mixtures with more that 5% hydrogen concentration.

Fig. 7 shows the critical velocity gradients for carbon monoxide-hydrogen (CO-H2) mixtures at different hydrogen concentrations in the mixture. The flashback behavior of CO-H2 changes nonlinearly with the increase in hydrogen contents in the mixture. The effect of hydrogen addition is especially significant for the rich mixtures. The area under the flashback propensity curve increase abruptly with the increase in hydrogen contents in the mixture. As discussed earlier, the presence of hydrogen in the mixture creates the necessary branched chain reactions to aggravate the flame propagations. The presence of H2 supplies the necessary active radicals and atoms such as OH, O and H and their diffusion rates into the unburned gas determines the magnitude of flame propagation velocity. As hydrogen concentration in the mixture exceeds certain concentrations the H2-O2 mechanism dominates the kinetic process and flame velocity primarily depends on the kinetics of hydrogen only. However, unlike the CH4-H2

mixtures the domination of H2 kinetics appear at higher H2 concentration (> 10%) in CO-H2 mixture due to the inert nature of CO.

C. Effects of Burner Tube Diameter on gF

Fig. 8 shows the effects of tube diameter on the gF values of 100% CH4 flames. The area under the curve increases with the increase in tube diameter. However, the effects of tube diameter are specially pronounced close to the peak gF values. The tube diameter effects diminish at the lean and rich regimes of the mixture. However, the gF

curves behave strangely for hydrocarbon and hydrogen-hydrocarbon fuel blends. From Figs.9 (a-d) for instance, it was found that for 95%-5% CH4-C3H8 mixture the area under the gF curves shrinks with the increase in tube diameter. This may be due to the combined effects of flame quenching and turbulence at the larger tube diameter. However, currently the authors are unsure why these effects are only significant for fuel blends. At this point it would be difficult to decide whether the deviations are due to the lack of precision of experimental data or of any stratification problems at larger tube diameter. Currently the authors are actively working on understanding this


interesting phenomenon. The CO-H2 mixture exhibits the similar behavior of pure fuels i.e. the area under the gF

curve increases with the increase in tube diameter. However, unlike the 100% CH4 flames, the diameter effects are evident throughout the mixture range.

D. Effects of External Excitation on gF

Figs. 10(a-d) show the effects of external excitation on the flashback propensity of 100% CH4 and CH4-C3H8

fuel blends. The flames are forced with three acoustic frequencies (300 Hz, 500 Hz and 700 Hz) to evaluate the effects of external perturbation on the gF values of fuel blends. For 100% CH4 flames the area under the gF curves steadily increase with the increase in excitation frequency. However, the peak of the curve shifts towards the lean side when the 100% methane flame is forced with a frequency of 300Hz. The peak of the curves moves to the stoichiometric location with forcing frequencies of 500 Hz and 700 Hz. The effects of acoustic forcing are a bit different for 95-5% CH4-C3H8 flames. Apparently no appreciable changes were observed with forcing frequencies at these mixture conditions. However, the behavior of the gF curves significantly changes with the change in forcing frequency for 85-15% and 75-25% CH4-C3H8 mixtures. The shifting of gF curves at the lean side is for 300Hzforcing frequency is apparent for both the mixture. However, unlike the 100% CH4 flames, the 500Hz and 700Hz excitation frequencies also cause the gF curves to shift in the lean side. The effect is specially pronounced for 75-25% mixture where the gF curves at forced conditions generally stays in the same lean conditions.

Figs. 11(a-c) show the effects of external forcing on the gF characteristics of CO-H2 flames. When compared to CH4-C3H8 fuel blends, the CO-H2 fuel blend appears less sensitive to the 0-700Hz forcing frequencies. The shifting of the gF curve on the lean side only appears for mixtures with low H2 contents. The nature of the gF curves remainsunchanged with acoustic forcing for mixtures with more than 5% H2 contents.

The presence of external excitations affects hydrocarbon and hydrogen fuel blends differently. In general pure hydrocarbon and hydrocarbon fuel blends flashback at much leaner and wider conditions with the presence of external forcing. However, the effects are much more complex than this simple statement. For instance, the amount of C3H8 present in CH4-C3H8 largely dictates the actual flashback propensity of the fuel blends. Despite a very little difference between the flame velocities of the methane and propane, they may interact completely differently with the acoustic perturbation due to the complex interactions of kinetics and thermophysical properties.

On the other hand hydrogen flame inherently flashbacks at very lean conditions and the presence of external forcing does not significantly change that behavior, at least for the 0-700Hz forcing range. As stated earlier, the kinetics of CO-H2 mixture generally dominated by the hydrogen for mixtures with more than 10% H2 contents. Thus the effects of acoustic forcing on 85-15 and 75-25 CO-H2 are similar to the pure hydrogen. Definitely, there exists a complex relation between the chemical kinetic time scale and forcing time scale of an externally perturbed flame. It appears that this complex relation actually dictate the flashback conditions of multiple fuel mixtures. The authors are currently pursuing another investigation to measure the intermediate radical and atom concentrations at forcing condition to understand the relation. The data will be available in the near future.

V. Concluding Remarks

The study concludes that: In general the flame velocity dictates the flashback propensity of hydrocarbon fuel blends such as methane-

propane and methane-ethane. However, the scaling of the flashback propensity data at different tube diameters and fuel compositions are not straightforward. The mixture stratification significantly affects the flashback propensity at large tube diameters.

The flashback propensity of methane-hydrogen and carbon monoxide-hydrogen blends largely dominates by the flashback characteristics of hydrogen. This is especially evident for mixtures with more than 5% hydrogen content.

External excitations cause the hydrocarbon fuel blends to flashback at much leaner conditions. However, flashback propensity of fuel blends with more than 5% hydrogen concentration in the mixture is largely dictated by the hydrogen characteristics and not by the external excitations.

Acknowledgments

This research was done with the support of the U.S. Department of Energy, under award DE-FG26-04NT42133. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Department of Energy.


References

1 Advanced Gas Turbine System Research, “request for Proposal,” No. AGTSR 98-01, South Carolina Energy Research and Development Center. Clemson, SC.

2 Narula, R.G., “Alternative Fules for Gas Turbine Plants- An Engineering Procurement, and Construction Contractor’s Perspective,” International Gas Turbine and Aeroengine Congress and exhibit, 98-GT-122, ASME, Stockholm, Sweden, 1998

3 Nicol, D.G., Steel, R.C., Marinov, N.M., and Malte, P.C., “ The Importance of Nitrous Oxide Pathway to NOX in Lean-Premixed Combustion,” Journal of Engineering for Gas Turbine and Power, Vol. 117, No.1, ASME, 1995, pp.100-111.

4 Rutar, T., Horing, D.C., Lee, J.C.Y., and Malte, P.C.,”NOX Dependency on Residence Time and Inlet Temperature for Lean-Premixed Combustion in Jet-Stirred Reactors,” International Gas Turbine and Aeroengine Congress and exhibit, 98-GT-433, ASME, Stockholm, Sweden, 1998.

5 Capehart, S.A., Lee, J.C.Y., Williams, J.T., and Malte, P.C, “ Effect of fuel Composition on NOX formation in Lean-Premixed Prevaporization combustion,” International Gas Turbine and Aeroengine Congress and exhibit, 97-GT-336, ASME, Orlando, FL, 1997.

6 Shuller ,T., Durox, D., and Candel, S., “Self-induced combustion oscillations of laminar premixed flames stabilized on annular burners,” Combustion and Flame, Vol. 132, 2003, in press.

7 Joos, F., and Vortmeyer, D., “Self-Excited Oscillations In Combustion Chambers With Premixed Flames And Several Frequencies” Combustion and Flame, Vol. 65,1986, pp. 253-262.

8 Schimmer, H., and Vortmeyer, D., “Acoustical Oscillation In A Combustion System with a Flat Flame,” Combustion And Flame, Vol. 28, 1977, pp.17-24.

9 Fleifil, M., Annaswamy, A. M., Ghoneim, Z. A. and Ghoniem, A. F., “Response of a Laminar Premixed Flame to Flow Oscillations: A Kinematic Model And Thermo acoustic Instability Results,” Combustion And Flame, Vol. 106, 1996, pp. 87-510.

10 Lewis, B., and von Elbe, G., Combustion, Flames and Explosion of Gases, 3rd edition, Academic Press, Orlando, 1987.

11 Franco, R., “Investigation of the Flashback Propensity of Fuel Blends: Compostion Effects: MS Thesis, University of Texas at El Paso, 2005.

12 Coats, C. M., “Comment on Review of Flashback Reported In Prevaporizing/Premixing Combustors," Combustion and Flame, Vol. 37, 1980, pp. 331-333.

13 Thibaut, D. and Candel, S., “Numerical Study of Unsteady Turbulent Premixed Combustion: Application to Flashback Simulation,” Combustion and Flame, Vol. 113, No. 1-2, 1998, pp. 53-65.


Fig 1: Burner Setup Fig 2: Flashback images during the flashback

5

6

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9

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13

0 100 200 300 400 500

% F

'gF' (

s-1)

Lewis

Present

0

100

200

300

400

500

0 4 8 12 16 20

% Fuel

'gF '

(s-1

)

CH4

95-5

85-15

75-25

Fig 3: Comparison between current and Fig 4: Critical boundary velocity gradient of Lewis data for critical velocity gradient of 1943 methane-propane fuel blend with respect to natural gas-air flames in cylindrical tubes of 10.6 mm at percent flue in cylindrical tube of 10.6mm diameter room temperature at room temperature


0

100

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% Fuel

'gF' (

s-1)

CH4

95-5

85-15

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0

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0 4 8 12 16 20% Fuel

'gF' (

s-1)

CH4

95-5

85-15

75-25

Fig 5: Critical boundary velocity gradient of Fig 6: Critical boundary velocity gradient of methane-ethane fuel blend with respect to methane-hydrogen fuel blend with respect to percent fuel in cylindrical tubes of 10.6mm percent fuel in cylindrical tubes of 10.6mm diameter at room temperature diameter at room temperature

0

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0 20 40 60 80

% Fuel

'gF' (

s-1)

95-5

85-15

75-25

Fig 7: Critical boundary velocity gradient of Fig8: Critical boundary velocity gradient of gas Carbon monoxide-hydrogen fuel blend with stream for flashback of pure methane air flames respect to percent fuel in cylindrical tubes of in cylindrical tubes at room temperature 10.6mm diameter at room temperature

0.00

100.00

200.00

300.00

400.00

500.00

0 2 4 6 8 10 12 14

% Fuel

'gF'(

s-1)

10.6 mm

6 mm

2


Fig 9: Critical boundary velocity gradients for flashback of 95% -5% fuel blends of (a)CH4-C3H8 (b)CH4-C2H6 (c)CH4-H2 (d)CO-H2 gas composition in cylindrical tubes of different diameters at room temperature

0

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0 2 4 6 8 10 12 14 16 18

'gF' (

s-1)

(a )

0

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0 2 4 6 8 10 12 14 16 18

'gF' (

s-1)

(b)

0

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% Fuel

'gF' (

s-1)

(c)

0

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% Fuel

'gF' (

s-1)

10.6mm

7mm

6mm

(d)


Fig 10: The synergetic effects of composition and combustion oscillations of (a) pure methane, (b) 95-5, (c) 85-15 and (d) 75-25 on flashback for CH4 and C3H8 fuel blends with varying frequency for a tube of 10.6mm diameter at room temperature.

0

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0 2 4 6 8 10 12 14

'gF'

(s-1

)

(a) 100%

0

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0 2 4 6 8 10 12 14 16 18

'gF'

(s-1

)

(b) 95% CH4-5% C3H8

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0 2 4 6 8 10 12 14 16 18

% Fuel

'gF' (

s-1)

0 HZ

300 Hz

500 Hz

700 Hz

( d ) 75% CH4-25% C3H8


Fig 11: The synergetic effects of composition and combustion oscillations for (a) 95-5, (b) 85-15 and (c) 75-25 of constant composition and varying frequency on flashback of CO and H2 fuel blend and

a tube of 6mm diameter at room temperature.

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-1)

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(c) 75%CO-25%H2

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