Paper # 070LT-0329 Topic: Laminar Flames

8th

U. S. National Combustion Meeting

Organized by the Western States Section of the Combustion Institute

and hosted by the University of Utah

May 19-22, 2013

Laminar Flame Speeds of Natural Gas Blends with Hydrogen at

Elevated Pressures and Temperatures

Drew Plichta1, Olivier Mathieu

1, Eric Petersen

1, Gilles Bourque

2, Henry Curran

3,

Sin ́ad Burke3, Wayne Metcalfe

3, and Felix Güthe

4

1Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA

2 Rolls-Royce Canada

3National University of Ireland, Galway, Ireland

4Alstom, Baden, Switzerland

Recently there has been a push for more fuel flexibility in gas turbine engines. Designers need base-line data, such as laminar flame

speed, to effectively design said engines. Engineers also need these data at a variety of conditions such as elevated pressures and

temperatures. This study examined the effect of fuel blending as well as initial conditions on laminar flame speed. Natural gas-based

fuels that contain high hydrogen content have recently become of interest in the gas turbine industry. This study examined flame speed

data resultant from a blending of natural gas and hydrogen at various initial conditions. A parametric sweep of equivalence ratios, 0.7-

1.3, was conducted at each condition. The hydrogen content was varied from 50-90%. The initial temperature and pressure were also

varied from 300-450 K and 1-5 atm, respectively. Experiments conducted above atmospheric pressure utilized a 1:6 oxygen-to-helium

ratio to curb the hydrodynamic and thermal instabilities that arise when conducting laminar flame speed experiments. A heated,

constant-volume combustion vessel was utilized to experimentally determine flame speed. The experimentally determined flame

speeds were also compared to the latest AramcoMech chemical kinetics models. These chemical kinetics models are being developed

by Curran and coworkers at National University of Ireland Galway. The experimental data matches well with the calculated flame

speed from the mechanism. The discrepancies between the model and experimental data can be mostly justified when taking into

account the uncertainty of the experimental flame speed.

1. Introduction

The need for fuel flexibility in gas turbine engines comes from fuel availability. Processes such as the gasification of coal

or biomass can produce fuels with high hydrogen content. Hydrogen addition is particularly attractive because of the

potential to extend the flammability limits, increase combustion efficiency, and reduce pollutant emissions (Wu et al.,

2011). These fuels can then be used in turbines, but replacing its designed fuel with one that is relatively more or less

reactive can lead to problems such as blowout, flashback, and auto-ignition. While the flame speed of the main

component of natural gas, methane, has been extensively studied in the literature (Hassan et al., 1998; Aung et al., 1995),

the flame speeds of methane and natural gas blends with high hydrogen content at elevated temperatures and pressures

remains to be studied.

Because the flame speed of blended fuels cannot be obtained by linear combination of each blended fuel constituent,

modeling work has been done to predict the flame speeds of methane and hydrogen blends by Chen et al. (2012).

Correlations for similar fuel blends such as ethane and hydrogen mixtures have been studied by Wu et al. (2011). Flame

speeds of Butane-air mixtures with hydrogen addition have been experimentally determined in a study by Tang et al.

2

(2011), and Yu et al. (1986) conducted methane flame speed experiments at atmospheric conditions with a small amount

of hydrogen addition. Hu et al. (2009) studied the entire range of methane and hydrogen blends at room temperature and

pressure, while the present study covered similar binary fuel blends at elevated temperatures and pressures.

2. Methods

The laminar flame speed experiments presented in this study were carried out by two centrally-ignited, constant-volume

combustion vessels. The first was a centrally ignited cylindrical 7075 aluminum vessel with an internal diameter and

lengths of 30.5-cm and 35.6-cm, respectively. The end caps contained a 6.35-cm thick fused quartz viewing window that

when installed allowed for a 12.7-cm viewing port. The combustion event images were captured using a Z-type

Schlieren setup with a FastCam SAE 1.1 high-speed camera with a mercury arc lamp as the light source. Further details

on this design and experimental methods can be found in a study by de Vries (2009).

The second vessel is made of 17-4PH stainless steel equipped with a circumferential heating jacket. This heating jacket

allows for the elevated initial temperatures in this study of 450 K; it has insulation around the entire vessel to provide

temperature uniformity within the vessel. The development and temperature profile are detailed elsewhere in a study by

Krejci et al. (2011). To view the combustion events on this vessel, a modified Z-type Schlieren setup was utilized as

detailed by Plichta et al. (2013).

Table 1 Experimental Conditions for laminar flame speed measurements.

The test matrix was determined by varying the fuel, hydrogen content, temperature, and pressure. The fuel designated

NG2 (natural gas) in the Table 1 is made of the following constituents (%mol): 81.25% CH4, 10% C2H6, 5% C3H8,

nC4H10 2.5%, and nC5H12 1.25%. This mixture was prepared in-house using the partial pressure method. Air was used as

the oxidizer for the atmospheric experiments. To curb hydrodynamic and thermal-diffusive instabilities, a 1:6 O2:He

ratio was used as the oxidizer for the 5-atm experiments. The high diluent-to-oxygen ratio increased flame thickness

thereby suppressing hydrodynamic instabilities, and helium’s higher diffusivity than nitrogen increased Lewis number,

thus decreasing thermal-diffusive instabilities (Rozenchen et al., 2002). The 1:6 ratio was specifically chosen to mimic

the adiabatic flame temperature of that same fuel with air. A parametric sweep of equivalence ratios was conducted at

each condition in Table 1. All percentages presented in this study are on a molar basis.

The procedure for extracting laminar flame speed is well documented in the literature and therefore will only be briefly

discussed subsequently. Constant pressure is one main assumption for the data analysis to find flame speed and is proven

by previous studies conducted in the facility used in this study (de Vries, 2009). The flame radius in this study was also

limited to the range where no flame acceleration was detected as to neglect wall and ignition effects. A post-processing

program using the density gradients to track the flame edge used a Taubin circle fit and six-point radius method to find

the radius as a function of time as depicted in Figure 1, which shows flame images from 50/50 CH4/H2 and air at

standard temperature and pressure. The bottom row shows the detected flame edge with a white line, and fit to that white

line are six red points from which the flame radius is extracted. After the change in flame radius over time is determined,

corrections need to be made for stretching and to convert to unburned flame speed.

Figure 1 depicts a smooth, laminar flame. As these images were taken from a room temperature and pressure experiment,

one would expect such a laminar flame. These images are representative of flame fronts seen in the present study as no

turbulent, cellular fronts developed while the flame was propagating throughout the viewing window. Only large

wrinkles were seen to form and they did not grow therefore not affect the flame speed values. Helium dilution had the

desired effect of stabilizing the flame front.

Fuel %H2 Temperature(K) Pressure(atm) Oxidizer

CH4 50 300 1 Air

CH4 70 450 5 1:6 O2:He

NG2 90 300 5 1:6 O2:He

CH4 90 450 1 Air

3

The change in radius over change in time for spherically expanding flames is found by the six-point method. Next, the

well-known stretch rate, Eq. (1), for spherically expanding flames was applied and then the linear method, Eq. (2), was

used to extract the flame speed. The linear method was chosen because it is accurate to the first order in finding the

laminar flame speed and Markstein lengths (Chen, 2011). The resultant burned flame speed is divided by the unburned-

to-burned density ratio, σ, to calculate the unburned, un-stretched laminar flame speed, .

(

) (

) (1)

( ) ( ) (2)

(3)

Two versions of the chemical kinetics model developed at the National University of Ireland Galway (NUIG) were used

for this study (Metcalfe et al., 2012). The Aramco mech 1.0 was utilized for the mixture containing methane and

hydrogen, while a higher-order mechanism, Aramco mech 1.0 C5-HT, was used because of the constituents of NG2.

A rigorous uncertainty analysis was also performed. Using the methods developed by Kline and McClintock (1953) the

bias and precision uncertainty were coupled to find the total uncertainty. Three main factors contribute to uncertainty-

temperature, pressure, and equivalence ratio. To find the bias limit, a relationship needed to be developed between these

three factors and flame speed. Other places in the literature have developed such correlations by proposing a quadratic

function of equivalence ratio with temperature and pressure raised to a constant value (Elia et al., 2001). The authors

present Eq. (4) to find the correlation between all the relevant parameters and flame speed, where represents

equivalence ratio. and represent standard temperature and pressure, 1 atm and 298 K , respectively.

( ) ( ) (

)( )

(

)( )

(4)

Figure 1 Captured images (top) and post-processed images (bottom)(50/50 CH4/H2 T = 298 K, P =

1atm).

4

This model was applied to each condition and showed that it was of good form. The temperature dependence in Eq. (4)

was ignored for the room-temperature experiments as it became unity. As the experiments were only performed once at

each condition and equivalence ratio the precision limit was taken as the standard deviation of experimental data with the

correlation in Eq. (4). As the partial pressures method was utilized in the present study, the uncertainty in was

calculated by assuming the worst-case scenario of the lower uncertainty of the fuel and the higher uncertainty of the

oxidizer to find the largest estimated discrepancy of

3. Results and Discussion

While hydrogen has been studied at similar conditions by Krejci et al. (2012) and others places in the literature have

studied hydrogen addition, few comparisons between literature values will be made because only one source was found

to have the same degree of hydrogen addition. In each of the following figures, the data points represent experimental

data, and the line represents the results of the chemical kinetics model. Uncertainty is shown at one equivalence ratio in

each figure so that the other data and trends may be readily visible.

Figure 2 shows a comparison of the experimentally determined flame speeds between the pure fuel cases and 50/50

blended case as seen in the present study. It is readily seen that the blended-fuel flame speed portrays a non-linear

relationship between the flame speed of its two pure components. Similar non-linear relationships have been found with

fuel blending in studies by Hirasawa et al. (2002) and Kochar et al. (2011). Ilbas et al. (2006) also shows a plot of the

flame speeds with the blend showing a non-liner profile that is closer to that of methane than hydrogen. This is indicative

of the fact that unreactive methane severely limits the flame speed even with an equal amount of highly reactive

hydrogen present. The peak of the blended fuels falls much closer to that of methane and at a similar equivalence ratio.

Methane is usually flammable from equivalence ratios of 0.7-1.3, while the addition of hydrogen increases the

flammability limit range from 0.5 to 1.4.

1 2 3

0

50

100

150

200

250

300

T=300K / P=1atm

SL

o (

cm

/s)

H2

50/50 CH4/H

2

CH4

Figure 2 Laminar Flame speeds comparison of pure and blended CH4 and H2 at various equivalence ratios at STP.

Figure 3 depicts the close up view of the 50/50 CH4/H2 blended experimental flame speed results. The solid line

represents the Aramco 1.0 C3 chemical kinetics mechanism. The chemical kinetics model agrees very well with the

experimental data. The peak flame speed occurs at ϕ = 1.1, a similar equivalence ratio to that of peak methane flame

speed. The profile of the flame speed curve is accurately predicted while the indicated uncertainty explains the

discrepancy between the model and experimental data. Hu et al. (2009) also performed experiments. There is very good

agreement between both data sets, and the higher profile of the literature values can be explained by the higher initial

temperature as flame speed is highly dependent upon initial temperature.

5

0.4 0.6 0.8 1.0 1.2 1.4

0

10

20

30

40

50

60

70

CH4:50%H

2 + Air

T=300K / P=1atm

SL

o (

cm

/s)

TAMU

Hu et al. (2009)(303 K, .987 atm)

Aramco Mech 1.0

Figure 3 Blended 50/50 CH4/H2 laminar flame speeds compared to model

and literature values.

Figure 4 shows flame speed results for blending of much more hydrogen, 70 %, than that of methane, 30%. These

experiments were conducted at an elevated temperature and pressure, 450 K and 5 atm, respectively. The model agrees

well with the experimental data while slightly over predicting each condition. The calculated uncertainty at this condition

is the size of each data symbol. The largest discrepancy between the data and model come on the rich side where the

difference between the two seems to plateau. Overall, the model predictions prove to be accurate to a fairly good degree.

0.6 0.8 1.0 1.2 1.4 1.625

50

75

100

125

150

175

200

225

30/70 CH4/ H

2 + 1:6 O

2:He

T=450K / P=5atm

SL

o (

cm

/s)

TAMU

Aramco Mech 1.0

Figure 4 30/70 CH4/ H2 blended laminar flame speed compared to chemical kinetics model.

Figure 5 shows the flame speed results of 90% Hydrogen with 10% Methane. The experimental results agree very well

with the chemical kinetics model. The peak is correctly predicted at ϕ = 1.4, and the slope of the flame speed matches

very well at lean and rich conditions. A sample uncertainty bar is shown that can explain the slightly higher profile than

that of the model. Multiple data points are shown on this profile to show that these experiments are very repeatable

within the calculated uncertainty. Each of these mixtures was made directly in the combustion vessel; earlier studies have

been conducted that show good agreement between experiments that were made previously in a mixing tank and allowed

ample time to form homogeneity.

6

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

100

150

200

250

300

350

CH4:90% H

2 + Air

T=450K / P=1atm

SL

o (

cm

/s)

TAMU

Aramco Mech 1.0

Figure 5 10/90 CH4/ H2 blended laminar flame speed compared to chemical kinetics model.

Figure 6 shows the resultant flame speed from experiments conducted 90% H2 and 10% NG at an elevated pressure of 5

atm. As can be seen, the experimental data match the chemical kinetics model well. The model correctly predicts the

peak flame speed at ϕ = 1.2 while also matching the slope of the experimental data as conditions go to lean and rich. The

uncertainty in flame speed is the size of the symbol in Figure 6. With uncertainty in mind, the model still over predicts

the flame speed slightly. Specifically, the model noticeably over predicts at lean conditions while predicting very well at

very rich conditions.

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.220

40

60

80

100

120

140

160

180

200

NG2:90% H2 + O

2:He 1:6

T=300K / P=5atm

TAMU

Aramco Mech 1.0

SL

o (

cm

/s)

Figure 6 10/90 NG/ H2 blended laminar flame speed compared to chemical kinetics model.

7

4. Conclusions

The blended fuel flame speeds in this study have shown a highly non-linear behavior with respect to the flame speed of

each of the pure-fuel constituents. Helium dilution at elevated pressures had the desired effect of maintaining a laminar

flame throughout the experiments so that laminar flame speed could be measured. The blending effects of CH4, H2, and

NG2 were well captured by the chemical kinetics model as it accurately predicted the laminar flame speed as

experimentally determined in this study. Uncertainty analysis yielded values of 1.7 cm/s to 19.2 cm/s, reasonable when

looking at the same order of flame speeds as found in this study.

Acknowledgements

This research was funded in part by Alstom Power, Switzerland and in part by Rolls-Royce Canada.

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