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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 Q˙ u + 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
Q˙ b + Q˙ r + 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 q˘ is calculated by
q˘ ) Κσ(T4 - Ti4)
Q˙ u ) A[R(Tu - Tw) + Κσ(Tu4 - Tw4)]
(10)
Q˙ b ) 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 Q˙ u and transient heat transfer from burned gas to unburned gas Q˙ b 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.
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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) q˘ ) radiant heat transfer flux (W/m2) Q˙ b ) heat transfer rate from burned gas to unburned gas (J/s) Qr ) amount of heat release by fuel combustion (J) Q˙ u ) 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
