692
Energy & Fuels 2007, 21, 692-698
Natural Gas-Hydrogen-Air Premixed Mixture Combustion with a
Constant Volume Bomb
Zuohua Huang,* Yong Zhang, Ke Zeng, Bing Liu, Qian Wang, and Deming Jiang
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity,
Xi’an, People’s Republic of China
ReceiVed July 8, 2006. ReVised Manuscript ReceiVed September 5, 2006
Natural gas-hydrogen-air premixed combustion was studied in a constant volume bomb over wide ranges
of equivalence ratios and hydrogen fractions and two initial pressures. A two-zone model was used to calculate
heat release rate and combustion durations based on the pressure data. The study shows that, with the increase
of hydrogen fraction in the mixture, the normalized mass burning rate increases while the flame development
duration and the total combustion duration decrease with the increase of the hydrogen fraction in natural
gas-hydrogen blends over various equivalence ratios. A small difference in maximum pressure for various
hydrogen fractions is presented at the equivalence ratios near the stoichiometric equivalence ratio. The maximum
pressure increases with the increase of the hydrogen fraction in the mixture for lean mixture combustion.
Short combustion duration is presented over wide ranges of equivalence ratios with increasing hydrogen fractions
in the mixture for rich mixture combustion. The difference in flame development duration for mixtures with
various hydrogen fractions increases with a decreasing equivalence ratio for lean mixture combustion and
increases with an increasing equivalence ratio for rich mixture combustion. The ratio of the flame development
duration to the total combustion duration increases with an increasing hydrogen fraction in the mixture, and
this reveals the fact that hydrogen addition has a larger influence on the total combustion duration rather than
on the flame development duration.
Introduction
With increasing concern about energy shortage and environmental protection, improving engine fuel economy and reducing
exhaust emissions have become major research topics in
combustion and engine development. Due to limited reserves
of crude oil, development of alternative fuel engines has attracted
more and more concern in the engine community. The introduction of alternative fuels is beneficial to help alleviate the fuel
shortage and reduce engine exhaust emissions. Natural gas is
considered to be one of the favorable fuels for engines. The
natural-gas-fueled engine has been realized in both the sparkignited and compression-ignited modes. Due to the slow burning
velocity of natural gas and the poor lean-burn capability, the
homogeneous mixture spark-ignited engine has the disadvantages of relatively low thermal efficiency at a stoichiometric
equivalence ratio, relatively large cycle-by-cycle variations in
lean mixture combustion, and poor lean-burn capability, and
these will decrease the engine power output and increase fuel
consumption under stoichiometric equivalence ratio conditions
where a three-way catalyst is used.1-2 Although lean mixture
combustion can help to increase engine thermal efficiency,
strong in-cylinder air motion should be introduced, and this will
further decrease the volumetric efficiency for port fuel supplying
mode. Due to these restrictions, the current natural gas engine
* Corresponding author. Address: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of
China. E-mail: [email protected].
(1) Rousseau, S.; Lemoult, B.; Tazerout, M. Combustion Characteristics
of Natural Gas in a Lean Burn Spark-Ignition Engine. Proc. Inst. Mech.
Eng., Part D 1999, 213 (D5), 481-489.
(2) Ben, L.; Dacros, N. R.; Truquet, R.; Charnay, G. Influence of Air/
Fuel Ratio on Cyclic Variation and Exhaust Emission in Natural Gas SI
Engine; No. 992901, SAE: Warrendale, PA, 1999.
is mostly operated under the condition of a stoichiometric
equivalence ratio with relatively low thermal efficiency. Traditionally, to improve the lean-burn capability and flame burning
velocity of the natural gas engine under lean-burn conditions,
an increase in the in-cylinder flow intensity is introduced. This
measure increases the heat loss to the cylinder wall and increases
the combustion temperature.3 One effective method to solve the
problem of the slow burning velocity of natural gas is to mix
the natural gas with the fuel that possesses a fast burning
velocity. Hydrogen is regarded as the best gaseous candidate
for natural gas due to its very fast burning velocity, and this
combination is expected to improve the lean-burn characteristics
and decrease engine emissions.4-5
Up to now, most work on burning velocities concentrated
on methane-air flames,6 hydrogen-air flames,7 or methanehydrogen-air flames8 while little work was reported on natural
gas-hydrogen flames. Liao measured the laminar burning
(3) Das, A.; Watson, H. C. Development of a Natural Gas Spark Ignition
Engine for Optimum Performance. Proc. Inst. Mech. Eng., Part D 1997,
211 (D5), 361-378.
(4) Blarigan, P. V.; Keller, J. O. A. Hydrogen Fuelled Internal Combustion Engine Designed for Single Speed/Power Operation. Int. J. Hydrogen
Energy 2002, 23 (7), 603-609.
(5) Akansu, S. O.; Dulger, A.; Kahraman, N. Internal Combustion
Engines Fueled by Natural Gas-Hydrogen Mixtures. Int. J. Hydrogen Energy
2004, 29 (14), 1527-1539.
(6) Gu, X. J.; Haq, M. Z.; Lawes, M. Laminar Burning Velocity and
Markstein Lengths of Methane-Air Mixtures. Combust. Flame 2000, 121
(1-2), 41-58.
(7) Lamoureux, N.; Djebaili-Chaumeix, N.; Paillard, C. E. Laminar Flame
Velocity Determination for H2-Air-He-CO2 Mixtures Using the Spherical Bomb Method. Exp. Therm. Fluid Sci. 2003, 27 (4), 385-393.
(8) Halter, F.; Chauveau, C.; Djebayli-Chaumeix, N. Characterization
of effects of pressure and hydrogen concentration on laminar burning
velocities of methane-hydrogen-air mixture. Proc. Combust. Inst. 2005, 30
(1), 201-208.
10.1021/ef0603131 CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/19/2007
Natural Gas-Hydrogen-Air Mixture Combustion
Energy & Fuels, Vol. 21, No. 2, 2007 693
Table 1. Composition of Natural Gas
Figure 1. Constant volume combustion bomb.
velocity of natural gas based on the spherical flame pattern and
found that the laminar burning velocity of natural gas was close
to that of methane.9 Yu investigated the burning velocity of
methane-hydrogen mixtures,10 and Law studied the flame
propagation phenomenon of mixtures with 85% of hydrogen
and 15% of methane.11 Their studies showed that the addition
of hydrogen into natural gas could remarkably increase the
burning velocity of the mixture. Ilbas et al. studied the laminar
burning velocities of hydrogen-air and hydrogen-methaneair mixtures at atmospheric pressure and temperature for variable
equivalence ratios in a constant vessel, and their work reveled
the increase in the burning velocity and widening of flammability limits by hydrogen addition.12 Previous research revealed
the effectiveness of the combination of natural gas-hydrogen
to increase the burning velocity. However, previous work only
supplied the information on the selected hydrogen fraction and/
or at the stoichiometric equivalence ratio, and they usually made
the analysis from a flame image. There are still many aspects
that require investigation, especially over wide ranges of
equivalence ratios and hydrogen fractions. Increasing understanding from a different analysis approach such as getting
information from heat release analysis would be a benefit. The
analysis will provide the supplement or new information for
understanding of natural gas-hydrogen-air combustion and/
or supply the guidance for engine operation.
The objective of this paper is to study natural gas-hydrogenair premixed combustion based on the analysis of the heat
release process at various hydrogen fractions and various
equivalence ratios using a constant volume bomb. A two-zone
model is developed to calculate the heat release rates based on
the pressure data.
Experimental Setup and Procedures
Experiments were conducted in a constant volume bomb shown
schematically in Figure 1. The experimental setup used the same
combustion bomb as that in ref 13. The combustion bomb is a
cuboid type with an inside size of 108 mm × 108 mm × 135 mm.
Two sides of this bomb are transparent to make the inside
(9) Liao, S. Y.; Jiang, D. M.; Cheng, Q. Determination of Laminar
Burning Velocities for Natural Gas. Fuel 2004, 83 (9), 1247-1250.
(10) Yu, G.; Law, C. K.; Wu, C. K. Laminar Flame Speeds of
Hydrocarbon + Air Mixtures with Hydrogen Addition. Combust. Flame
1986, 63 (3), 339-347.
(11) Law, C. K.; Kwon, O. C. Effects of Hydrocarbon Substitution on
Atmospheric Hydrogen-Air Flame Propagation. Int. J. Hydrogen Energy
2004, 29 (8), 867-879.
(12) Ilbas, M.; Crayford, A. P.; Yilmaz, I.; Bowen, P. J.; Syred, N.
Laminar burning velocities of hydrogen-air and hydrogen-methane-air
mixtures, an experimental study. Int. J. Hydrogen Energy 2006, 31 (12),
1768-1779.
items
CH4
C2H6
C3H8
N2
CO2
others
volumetric fraction (%)
96.160
1.096
0.136
0.001
2.540
0.067
observable. The combustible mixture is prepared within the chamber
by adding natural gas, hydrogen, and air according to their
corresponding partial pressures. No gas motion was generated in
the combustion chamber. The centrally located electrodes ignite
the mixture, and a standard capacitive discharge ignition system is
used for producing the spark, thus a laminar flame is developed in
the study. The ignition energy is 45 mJ. The pressure is recorded
by a piezoelectric absolute pressure transducer with a resolution
of 0.01 kPa.
The initial temperature was set at 288 K. Pressures were recorded
in the experiment, and initial pressures and temperatures were
constant. A vacuum pump was used to draw out the burned gases
in the bomb and fresh air, and fuel was added into the bomb
separately according to the settings of initial pressures (0.08 and/
or 0.15 MPa) and equivalence ratios (from a lean mixture of 0.7
equivalence ratio to a rich mixture of 1.3 equivalence ratio). The
initial condition was strictly controlled in the experiments to realize
the same initial pressure and temperature. The influence of wall
temperature on mixture temperature was avoided by providing
enough time for the wall to cool down between explosions.
Hydrogen with a purity of 99.995% is used, while the natural
gas composition is listed in Table 1. Considering the formula of
natural gas as CRHβOγ, it can be calculated that R is 1.01523, β is
3.928084, and γ is 0.05086. The combustible mixture in the bomb
can be expressed as (1 - x)CRHβOγ + xH2 + L(O2 + 3.762N2),
and the equivalence ratio of the natural gas-hydrogen-air mixture
is defined as φ ) [(R + (β/4) - (γ + 1)/2)(1 - x) + 1/2]/L.
Calculation Model
A two-zone model is proposed for combustion analysis. The
spherical flame front divides the combustion chamber into the
burned zone and the unburned zone as shown in Figure 2. The
symbols p, T, V, and m represent the pressure, temperature,
volume, and mass of the chamber gases, respectively, and Qr is
the amount of heat release by fuel combustion. The subscript u
and b represent the unburned state and the burned state,
respectively. The assumptions are given in the model.
(1) The gases are regarded as the ideal gases.
(2) Complete combustion finishes very rapidly when the
unburned charge enters the burned zone.
(3) Pressure reaches its equilibrium value instantaneously,
and there is no difference between the unburned zone and
unburned zone.
(4) No gas leakage occurs, and gas temperatures reach their
respective temperature in the burned zone and unburned zone.
(5) The unburned gases are regarded as the mixture of natural
gas and hydrogen.
(6) Gas properties of unburned and burned gases are
calculated by the fraction of the constituent gases.
Figure 2. Schematic diagram of two-zone model.
694 Energy & Fuels, Vol. 21, No. 2, 2007
Huang et al.
The mass conservation equation is written as,
dmu
dmb
)dt
dt
(1)
From energy conservation, the following two equations can
be established
dVu dmu
dQu
d(muuu)
) -P
+
h +
dt
dt
dt u
dt
(2)
d(mbub)
dVb dmb
dQb dQr
) -P
+
hb +
+
dt
dt
dt
dt
dt
(3)
As same pressure is considered in both the burned zone and
the unburned zone, eq 4 can be established.
P)
mbRbTb muRuTu
)
Vb
Vu
(4)
From the above four equations, the following equations can be
derived14
Tu dP
Q̇u
+
dTu P dt muRu
)
dt
1 ∂uu
+1
Ru ∂Tu
(
)(
)
dTu
1 ∂ub
- P dV
+1 dt
Rb ∂Tb
dP 1 ∂ub mb ∂ub Vu
+
+
V
dt Rb ∂Tb
V ∂Tb V
Ru
∂ub
(ub - uu) +
Tu - Tb
Rb
∂Tb
Q̇b + Q̇r + muRu
dmb
)
dt
(5)
(
(
(
)
)
)
dVb
dV
1 dmu 1 dTu 1 dP
+
) Vu
+
dt
mu dt
Tu dt
p dt
dt
(6)
(7)
The model takes into account of both convective and radiation
heat transfer. The coefficient of convective heat transfer is
derived from the plate-plate convective heat transfer correlation
as follows:
λ
Re
Lc
(8)
In which, λ is the gas conductive coefficient, kW/(m2 K); Lc is
the characteristic length; Re is the Reynolds number, Re ) FνLc/
µ; ν is the velocity, m/s; and µ is the viscosity.
The radiant heat transfer flux q̆ is calculated by
q̆ ) Κσ(T4 - Ti4)
Q̇u ) A[R(Tu - Tw) + Κσ(Tu4 - Tw4)]
(10)
Q̇b ) Af[R(Tb - Tu) + Κσ(Tb4 - Tu4)]
(11)
This is the typical Annand’s heat transfer formula, Here, constant
Κ uses the value of 1.5, A is the wall surface area, and Af is the
spherical flame front area, which can be calculated by Af )
(4π)1/3(3Vb)2/3. Tu and Tb are the gas temperature for the
unburned and burned zones, while Tw is the wall temperature.
In the model, the gas temperatures are assumed to be uniform
in the unburned and burned zones, respectively; thus, no
temperature gradient was considered in the model. With respect
to model calculation, dP/dt is obtained from the pressure data
and dV/dt is zero for the constant volume bomb. The internal
energy and gas constants of mixtures ub, uu, Rb, Ru, ∂ub/∂Tb,
and ∂uu/∂Tu are calculated using the formula given by the
literature15 according to the fraction of each species. Thus, the
unknown variables in these thermodynamic equations are mb,
Tb, and Tu. The initial unburned gas temperature Tb uses the
adiabatic flame temperature Tad, and using a fourth-order
Runge-Kutta scheme, the mb, Tb, Tu, and burning rate dmb/dt
can be obtained. During the combustion process, gas compositions and properties are calculated through chemical equilibrium
with 11 species and 7 equations.16
Results and Discussions
Heat Transfer Calculation
R)C
Thus, the transient heat transfer to the wall Q̇u and transient
heat transfer from burned gas to unburned gas Q̇b are determined
by
(9)
In which, σ is the Boltzmann constant, σ ) 5.67 × 1011 kW/
(m2 Κ4).
(13) Liao, S. Y.; Jiang, D. M.; Gao, J. Measurements of Markstein
Numbers and Laminar Burning Velocities for Liquefied Petroleum Gasair Mixtures. Fuel 2004, 83 (10), 1281-1288.
(14) Ma, F. H. Fundamental Study of Premixed Turbulent Combustion.
Ph.D. dissertation, Xian Jiaotong University, Xian, China, 1997.
In this paper, the normalized mass burning rate is defined as
(1/m)(dmb/dt), where m is the total mass of combustible gases.
The normalized mass burning rate is calculated from the
proposed two-zone model described above and reflects the
burning velocity of the mixture during the combustion process.
Figure 3 gives the combustion pressure and the normalized
mass burning rate for the mixture with 60% of natural gas and
40% of hydrogen at an initial pressure of 0.1 MPa. The figure
shows that both the rich mixture and lean mixture give a slow
rate of pressure rise and a low value of peak pressure when
comparing with the stoichiometric mixture. Pressure differences
with varying fuel air equivalence ratios are shown clearly in
the early stage of pressure rise. Meanwhile, the timing where
the pressure reaches its peak value is delayed for both lean and
rich mixtures. In the case of a rich mixture (φ > 1.0), the value
of peak pressure decreases and the timing of peak pressure is
delayed while increasing the equivalence ratio. In the case of a
lean mixture (φ < 1.0), the value of peak pressure decreases
and the timing where the pressure reaches its peak value is
delayed while decreasing the equivalence ratio. High and fast,
the normalized mass burning rate is presented at the stoichiometric equivalence ratio while low and slow the normalized
mass burning rate is demonstrated for the lean and the rich
mixtures. These decreases in the normalized mass burning rates
are considered to be caused by the decrease of flame propagation
speeds for both lean and rich mixtures. Fast burning reduces
the time for reaching its peak pressure value while slow burning
increases this time. The heat transfer will influence the peak
(15) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill Book Company: New York, 1988.
(16) Shiga, S.; Ozone, S.; Machacon, H. T. C.; Karasawa, T. A study of
the combustion and emission characteristics of compressed-natural gas
direct-injection stratified combustion using a rapid compression machine.
Combust. Flame 2002, 129 (1-2), 1-10.
Natural Gas-Hydrogen-Air Mixture Combustion
Figure 3. Combustion pressure and normalized mass burning rate
versus equivalence ratios at 60% NG + 40% H2.
combustion pressure. Since the amount of heat transfer depends
on the gas temperature and time, it is reasonable to assume that
the slow flame propagation speed of the lean mixture will give
a greater fraction of its released energy to coolant than that of
rich mixture and decreases the peak pressure value.
Figure 4 shows the combustion pressure and the normalized
mass burning rate for various hydrogen fractions at an equivalence ratio of 1.0. Small differences in the maximum pressure
for various hydrogen fractions are shown at equivalence ratios
near the stoichiometric equivalence ratio. As the volumetric heat
value of natural gas-air at φ ) 1 is 3132 kJ/m3 and the
volumetric heat value of hydrogen-air at φ ) 1 is 3022 kJ/m3,
they have very close values in volumetric heat release by fuel
combustion. The timing of peak pressure arriving is delayed
by decreasing the hydrogen fraction in the fuel blends. This
indicates that increasing the hydrogen fraction can increase the
flame propagation speed. Figure 4b shows the fast rate and high
value of the normalized mass burning rate for hydrogen-air
mixture combustion and the slow rate and low value of the
normalized mass burning rate for natural gas-hydrogen-air
mixture combustion and natural gas-air mixture combustion.
Hydrogen-air mixture combustion completes at 0.006 s after
the electrode spark. Heat release is not observed or just in its
early stage for natural gas-hydrogen-air mixture combustion
and natural gas-air mixture combustion.
Figure 5 illustrates the combustion pressure and the normalized mass burning rate for various hydrogen fractions and two
initial pressures at an equivalence ratio of 1.0. For both limited
pressures, the results show that the rate of pressure rise and the
normalized mass burning rate increase while increasing the
Energy & Fuels, Vol. 21, No. 2, 2007 695
Figure 4. Combustion pressure and normalized mass burning rate for
different hydrogen fractions at φ ) 1.0.
hydrogen fraction in the fuel blend. However, at an initial
pressure of 0.08 MPa, small variation in the maximum pressure
is observed among the mixtures with different hydrogen
fractions. More fuel will taken into the combustion process at
a high initial pressure, releasing more heat to the chamber and
increasing the gas temperature. The high gas temperature
increases the heat transfer flux value, and long combustion
duration will create greater heat loss from the chamber wall.
When the initial pressure is low, the heat transfer flux value is
low; thus, the effect of heat transfer to the chamber wall on the
peak pressure is decreased. From Figure 5a and b, it can be
seen that the heat release process advances with a decrease in
the initial pressure, which is consistent with previous results.6
A decrease in combustion duration will in turn decrease the
heat loss to the chamber wall. In the case of a large hydrogen
fraction (60% H2), the peak value of the normalized mass
burning rate varies little with the initial pressure. However, in
the case of medium (40% H2) and small hydrogen fractions
(20% H2), the maximum value of the normalized mass burning
rate decreases while increasing the initial pressure.
Figure 6 gives the maximum pressure pmax and the combustion
duration versus equivalence ratios for mixtures with various
hydrogen fractions in the fuel blends. The results show that all
mixtures reach their peak values at equivalence ratios between
1.0 and 1.1, and a small difference in value is observed in this
range. When the equivalence is smaller than 1.0, pmax decreases
and its decreasing rate increases with the decrease of the
equivalence ratio, making large differences in pmax values under
lean mixture conditions. When the equivalence is larger than
1.1, pmax decreases and its deceasing rate increases with the
696 Energy & Fuels, Vol. 21, No. 2, 2007
Figure 5. Combustion pressure and normalized mass burning rate
under various hydrogen fractions and two initial pressures.
increase of the equivalence ratio; this also makes a large
difference in pmax values under rich mixture combustion. While
increasing the hydrogen fraction in the fuel blends, the variation
of pmax versus the equivalence ratio becomes less sensitive. This
behavior indicates that increasing the hydrogen fraction in fuel
blends can increase the flame propagation speed, shorten the
combustion duration, and create less heat loss to wall, and this
is clearly demonstrated in Figure 6b. There exists strong
correlation between the maximum pressure and combustion
duration, that is, a large value of pmax corresponds to a short
combustion duration, revealing the fact that heat transfer plays
an important part in influencing the peak pressure value. This
is consistent with the results obtained by Shiga16 in the study
of natural gas-air combustion using a rapid compression
machine; their study showed that differences in heat loss to the
wall of the combustion chamber are the main reason for the
observed differences in maximum pressure due to combustion.
In this study, a short combustion duration gave a high value of
the observed maximum pressure while long combustion duration
gave a low value of the observed maximum pressure. Besides
the effect of heat loss on the maximum pressure for natural gasair combustion, another reason for this effect would be the
decrease of maximum pressure for natural gas-hydrogen-air
combustion. The figure also shows that high values of pmax and
short combustion duration can be maintained over a wide range
of equivalence ratios in the case of high hydrogen fraction
combustion. This means that an extension of the lean-burn
capability can be achieved with adding hydrogen into natural
gas.
Figure 7 gives the maximum pressure and combustion
duration versus the hydrogen fractions in the fuel blends. In
Huang et al.
Figure 6. Maximum pressure and combustion duration versus equivalence ratios for various hydrogen fractions.
this study, the combustion duration is defined as the time interval
from the ignition start to the timing of peak pressure arriving.
In the case of high initial pressure (p0 ) 0.15 MPa), the value
of peak pressure increases and the combustion duration decreases with the increase of the hydrogen fraction in the fuel
blends. In the case of low initial pressure (p0 ) 0.08 MPa), the
combustion duration decreases with the increase of the hydrogen
fraction in the fuel blends while the value of maximum pressure
shows little variation with hydrogen fraction in the fuel blends.
In order to clarify the influence of hydrogen addition on flame
early development, the flame development duration is used in
this paper and is defined as the time interval from the beginning
of the electrode spark to the timing of 10% accumulated mass
burning.17 Figure 8 shows the flame development duration
versus equivalence ratios for various hydrogen fractions in the
fuel blends. The results show that the addition of hydrogen into
natural gas can decrease the flame development duration and
extend the range of equivalence ratios with short flame
development duration. Flame development duration gives its
shortest value at the equivalence ratio from 1.0 to 1.1 while a
lean or rich mixture increases this duration. Moreover, the figure
also shows that the effectiveness by hydrogen addition on
decreasing of flame development duration is larger for the lean
mixture rather than for the rich mixture. The flame development
durations for natural gas combustion and natural gas-hydrogenair combustion give the shortest values at an equivalence ratio
of 1.0 to 1.1, and lean or rich mixture combustion will increase
(17) Huang, Z.; Shiga, S.; Nakamura, H. A. Basic Study on the Ignition
Position of Natural Gas Direct-Injection Super-Lean Combustion. Combust.
Sci. Technol. 2003, 175 (5), 965-992.
Natural Gas-Hydrogen-Air Mixture Combustion
Energy & Fuels, Vol. 21, No. 2, 2007 697
Figure 9. Ratio of flame development duration to total combustion
duration for various hydrogen fractions.
Figure 9 gives the ratio of the flame development duration
to the total combustion duration. The results show small
variations at equivalence ratios between 0.7 and 1.3 while large
variation is presented at the equivalence ratio between 0.6 and
0.7. For lean mixture combustion, the ratio of the flame
development duration to the total combustion duration increases
with the increase of the hydrogen fraction in the fuel blends.
This reveals the fact that hydrogen addition gives larger
decreasing effectiveness on total combustion duration rather than
on flame development duration.
Conclusions
Figure 7. Maximum pressure and total combustion duration versus
hydrogen fractions at two initial pressures and equivalence ratios.
Figure 8. Flame development duration versus equivalence ratio.
the flame development duration. The flame development duration of hydrogen-air mixture combustion decreases monotonically while increasing the equivalence ratio. This reveals the
fact that the short flame development duration of hydrogenair mixture combustion can be achieved over wide ranges of
the equivalence ratio.
A short combustion duration is seen over wide ranges of the
equivalence ratio with increasing hydrogen fractions in the
mixture. The difference in flame development duration for
mixtures with various hydrogen fractions increases while
decreasing the equivalence ratio for lean mixture combustion
and increases while increasing the equivalence ratio for rich
mixture combustion.
Natural gas-hydrogen-air premixed combustion was studied
in a constant volume bomb over wide ranges of equivalence
ratios and hydrogen fractions and two initial pressures. A twozone model was proposed to calculate the heat release rate and
combustion durations based on the pressure data. The main
results are summarized as follows:
(1) With an increase of the hydrogen fraction in the mixture,
the normalized mass burning rate increases while the flame
development duration and the total combustion duration decrease
at various equivalence ratios and initial pressures.
(2) Small differences in maximum pressure for various
hydrogen fractions are shown near the stoichiometric equivalence ratio. The maximum pressure increases with the increase
of the hydrogen fraction in the mixture for lean and rich mixture
combustion.
(3) A short combustion duration is shown over wide ranges
of the equivalence ratio with an increasing hydrogen fraction
in the mixture. The difference in flame development duration
for mixtures with various hydrogen fractions increases while
decreasing the equivalence ratio for lean mixture combustion
and increases while increasing the equivalence ratio for rich
mixture combustion.
(4) The ratio of the flame development duration to the total
combustion duration increases with an increasing hydrogen
fraction in the mixture. This reveals the fact that hydrogen
addition gives larger decreasing effectiveness for total combustion duration rather than for flame development duration.
Acknowledgment. This study was supported by the National
Natural Science Foundation of China (50636040, 50521604,
50323001). We acknowledge the students of Xi’an Jiaotong
University for their help with the experiment and preparation of
the manuscript. We also express thanks to the colleagues of Xi’an
698 Energy & Fuels, Vol. 21, No. 2, 2007
Jiaotong University for their helpful comments and advice during
the manuscript preparation.
Nomenclature
A ) wall area (m2)
Af ) flame front area (m2)
h ) enthalpy (J)
L ) molar number of air
Lc ) characteristics length (m)
m ) mass of gases (g)
P ) gases pressure (Pa)
Po ) initial mixture pressure (MPa)
q̆ ) radiant heat transfer flux (W/m2)
Q̇b ) heat transfer rate from burned gas to unburned gas (J/s)
Qr ) amount of heat release by fuel combustion (J)
Q̇u ) heat transfer rate from unburned zone to wall (J/s)
R ) gas constant (J/(g K))
Re ) Reynolds number
T ) gas temperature (K)
Huang et al.
Tw ) wall temperature (K)
u ) internal energy (J)
ν ) velocity (m/s)
V ) volume (m3)
t ) time (s)
tfd ) flame development duration (s)
tcom ) combustion duration (s)
x ) volume fraction of natural gas in fuel blend (%)
φ ) equivalence ratio
R ) heat transfer coefficient (W/(m2 K))
λ ) gas conductive coefficient (W/(m2 K))
F ) gas density (kg/m3)
σ ) Boltzmann constant
µ ) viscosity (kg/(m s))
Subscripts
b ) burned gases
u ) unburned gases
EF0603131