Experimental and analytical studies on the ignition of methaneacetylene mixtures

41

COMBUSTION AND FLAME 4 9 : 4 1 - 5 0 (1983)

Experimental and Analytical Studies on the Ignition of Methane-Acetylene Mixtures K. S. KRISHNAN, R. RAVIKUMAR Department of Mechanical Engineering. College of Engineering, Trivandrum695 016, India and

K. A. BHASKARAN Department of Mechanical Engineering, IndianInstitute of Technology, Madras 600 036, India

An experimental investigation of the ignition of methane-acetylene--oxygen-argon mixtures was carried out behind reflected shock waves over the temperature range 1400-1900 K and reaction pressures between l and 4 bar. Pure methane, acetylene, and methane-acetylene mixtures (C2H2/CH 4 = 0. I-I) were used in the investigation. The observed ignition delay of acetylene-methane mixtures lie between those of pure methane and acetylene. A 53-step reaction mechanism for the combined oxidation of methane-acetylene mixture was numerically integrated to yield the rate of change with time of the species concentration, temperature, and pressure. The computed ignition delays compare very well with the experimental results, and since the ignition delays for methane-acetylene mixtures lie between those of pure methane and acetylene, it is suggested that a chemical coupling exists between the two oxidation systems.

INTRODUCTION The ignition characteristics of fuels play an important role in the design of combustion systems, and among the lower-order hydrocarbons, methane and acetylene have received maximum attention. Compared to methane, the delay periods of acetylene are much shorter. High-temperature oxidation studies on methane and acetylene have been carried out by several investigators [1-20], but so far, the ignition studies have been mostly confined to a single hydrocarbon or a hydrocarbon with trace additives. Westbrook, however, has made an analytical investigation of methaneethane mixtures [9], and the purpose of the present study was to investigate experimentally the ignition characteristics of methane-acetylene mixtures to derive a reaction mechanism for the Copyright © 1983 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

system which can be used to compute the observed ignition delays. Experiments were conducted for CH4-C2H 202-At mixtures, the relative proportions of fuel varying from pure methane to pure acetylene. The studies were made over a temperature range 1400-1900K at reaction pressures between 1 and 4 bar, and, in all cases, the argon dilution was kept at about 90%. The mixture compositions were chosen so that C~H2 was not merely an additive but a fuel partner. The promotion of ignition of methane by acetylene may take place in two ways. Acetylene has a considerably shorter ignition delay than methane, so that if the two reactions (i.e., that of acetylene and that of methane) are completely uncoupled, the shortening of the induction period is purely an outcome of the heat released by the ignition of acetylene. The second possibility is that the reactions are coupled, in which case the

0010-2180/83/01041 + 10503.00

42

K. S. KRISHNAN ET AL.

reactive free radicals produced in the oxidation of acetylene accelerate the oxidation of methane. An attempt has been made to distinguish between these possibilities.

CH4:

C2H2: EXPERIMENTAL

A stainless steel shock tube of 71 mm i.d. and 14 mm wall thickness was used in the present investigation. The 3-m-long driver and 5-m-long test sections were both connected to a vacuum pump, and all the outlet connections from the shock tube were connected through vacuum valves to an exhaust fan. A pneumatically operated plunger was used to rupture the diaphragm (0.075-mm-thick aluminium foil) for any given driver pressure. The experimental arrangement is the same as that described earlier [ 18]. Incident shock velocities were measured by piezoelectric transducers placed along the test section at known intervals and electronic counters (+0.1-/as accuracy). Reflected shock parameters (pressure, temperature, and density) were computed from the incident shock velocities and the initial conditions, using the three conservation equations and the ideal equation of state assuming vibrational equilibrium, but no reaction before ignition. The molarenthalpies of methane, acetylene, oxygen, and argon were taken from JANAF chemical tables. A computer program was developed for the calculation of the shock parameters. The computations were carried out on an IBM 370/155 digital computer. The onset of ignition, characterized by visible light emissions, was sensed by a photomultiplier which was kept in line with a quartz window provided near the end flange on the tube. Oscilloscopes were also used to check the counter readings. The purity of the oxygen, argon, methane and acetylene used in this study are given below. 0 2"

Ar:

from M/s. Indian Oxygen Ltd., Madras, Grade 1, IS 309-1974; purity, 99% (min.). from M/s. Indian Oxygen Ltd., Madras, Grade 1, IS 5760-1969; purity, 99.97% (min.); oxygen, 10 ppm (max.); hydrogen, 5 ppm (max.); water vapor, 0.0056 mg/ lit. (max.); carbon dioxide, 5 ppm (max.).

from M/s. Philips Petroleum Co., USA; pure grade; methane, 99.10%; nitrogen, 0.60%; propane; 0.01%; ethane, 0.09%; carbon dioxide, 0.20%. from M/s. Indian Oxygen Ltd., Madras; pure grade; purity, 99.28% (with traces of acetone).

Special grade hydrogen with the following specifications was used as the driver gas: H2:

from M/s. Indian Oxygen Ltd., Madras; purity, 99.9% (min.); oxygen, 0.001% (max.); carbon dioxide, 10 ppm (max.); water vapor, 10 mg/m a (max.).

Table 1 gives the composition of the mixtures used in this work.

ANALYTICAL A 53-step reaction mechanism for the oxidation of the methane-acetylene mixture combining the reactions of methane oxidation and acetylene oxidation (given in Table 2) was assembled from recent publications. The methane mechanism is based on the model suggested by Bowman [7], and the acetylene mechanism on that of Jachimowski [16]. Of the 53 reactions, reactions (21)(39) are common to both. The scheme includes a!! the significant reactions of both methane oxidhtion and acetylene oxidation with the latest available rate constant data. The backward rate constant values are k b, and the equilibrium constants are k e. The equilibrium constants k¢ were expressed as k e = aT b exp (c/T), and the constants, a, b, and c were computed. Sixth-order polynomials were fitted for expressing the specific heat data in the temperature range 1000-4000K by the method of least squares. The rate of change with time of concentration, temperature, and pressure were computed by numerical integration of the governing system of differential equations (for the 53 reactions), namely, the equation of state and the equations for conservation of mass and energy. The initial concentrations of reactants CH4, C2Hz, Oz, and Ar were determined by the known stoichiometry; the rest

43

IGNITION OF METHANE-ACETYLENE MIXTURES TABLE 1 Composition of Experimental Gas Mixtures

Mixture 1 2 3 4 5 6 7 8 9 10 11 12 13

CH 4 (%mole)

C2H 2 (%mole)

02 (%mole)

Ar (%mole)

Equivalence ratioa

Acetylene: Methane ratio ([C2H21/[CH4])

3.33 3.00 2.20 1.60 0 2.00 1.00 1.00 0 4.80 3.20 2.40 0

0 0.30 1.10 1.69 3.00 0 0.50 1.00 1.66 0 1.60 2.40 4.80

6.66 6.75 7.15 7.20 7.50 8.00 7.50 9.00 8.33 4.80 5.20 5.40 6.00

90.01 89.95 89.55 89.60 89.50 90.00 91.00 89.00 90.01 90.4 90.00 89.80 89.20

1 1 1 1 1 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0

Pure methane 0.1 0.5 1.0 Pure acetylene Pure methane 0.5 1.0 Pure acetylene Pure methane 0.5 1.0 Pure acetylene

a ~ = (2 ICH4] + 2.5 [C2H2I)/[O21.

of the species were assigned very low values of 10--15 mole/cm a. The species concentrations (in most cases) rose rapidly in a complex manner from the initial (almost) zero values to values of about 10--1210--1° mole/cm a. Thereafter the concentration of species showed an exponential growth in the neighborhood of ignition to reach values of about 10- 9 mole/cm a. At about this time the consumption of fuel and oxygen increased rapidly, resulting in both a temperature and pressure rise. The hydroxyl ground state concentration at this point was about 1 X 10 --9 mole/cm a. Attainment of this value signified the end of delay period in computations. This criterion of OH concentration = 1 X 10--9 mole/ cm a is in accordance with the method used by Beeley et al. [21] and Higgins and Williams [2]. On the other hand, 2 X 10--9 mole/cm 3 was used to characterize experimental ignition delay by Cooke and Williams [22]. Typically, the change from 1 to 2 × 10- 9 mole/cm 3 for the hydroxyl threshold introduces only about a 1-3% change in the delay period. The experimental points obtained based on light emission were also in good agreement with the above criterion, as can be seen from Figs. 1-4. As a countercheck, ignition delays based on a 2.5% temperature rise (signaling detectable

energy release) and the values obtained from the pressure profiles were also considered, and the results obtained were in agreement (within 5%) of those obtained using the OH criterion. The computer program is so developed that it gives the pressure, temperature, concentration of all species, densities, and both the forward and the reverse reaction rates of all the 53 reactions at the required intervals. The significant reactions of the methane, acetylene, and methane-acetylene mechanisms were identified by conducting a separate and detailed sensitivity analysis for each reaction scheme. The results showed that the predicted ignition delays and the growth in concentrations were most sensitive to the following reactions: CH 4 Mechanism (Reactions 1-40) 1. 2. 3. 4. 5. 6.

CH4 + M = C H 3 + H + M CH4 + H = CH a + H 2 CH 4 + OH = CH a + H20 CHa + O z = CHaO + O CHzO + M = HCO + H + M H+O z=OH+O

(R1) (R3) (R5) (R8) (R13) (R34)

C2H2 Mechanism (Reactions 21-53) 1. HCO + OH = CO + H20 2. HCO + H = CO + H 2 3. H C O + M = H + C O + M

(R22) (R24) (R25)

44

K. S. KRISHNAN ET AL TABLE 2 Methane-Acetylene Oxidation Mechanism

No.

Reaction

1 CH4+M=CHa+H+M 2 CH 4 + 0 2 =CH 8 + HO 2 3CH 4 + H = C H a + H 2 4 CH 4 + O = CH a + OH 5 CH 4 + OH = CH a + H20 6 CH 4 + H O 2 =CH 3 + H 2 0 2 7 CH 3 + H C O =CH 4 + C O 8 CH 3 + O 2 = C H 3 0 + O 9 CH 3 + O = C H 2 0 + H l0 CH 3 + OH = CH20 + H 2 !1 C H 3 0 + M = C H 2 0 + H + M 12 CH30 + 0 2 = CH20 + HO 2 13CH20+M=HCO+H+M 14 CH 3 + CH20 = CH 4 + HCO 15 CH20 + OH = HCO + H20 16 CH20 + 0 2 = HCO + HO 2 17 C H 2 0 + O = H C O + O H 18 CH20 + HO 2 = HCO + H202 19 C2H 6 = c H 3 + C H 3

20 C2H6+O =C2H5+OH 21 O 2 + M = O + O + M 22 HCO + OH = CO + H20 23 H C O + O = C O + O H 24 H C O + H = C O + H 2 25 H C O + M = H + C O + M 26 H C O + O 2 = C O + H O 2 27 C O + O + M = C O 2 + M 28H+OH+M=H20+M 29H+H+M=H2+M 30 H + O 2 + M = HO2+ M 31 CO + OH = CO 2 + H 32 HO 2 + H = H20 2 33HO 2 + C O = C O z + O H 34 H + O 2 = O + O H 35 H + H O 2 = O H + O H 36 O + H 2 = O H + H 37 OH +OH = H 2 0 + O 38 H 2 0 2 + O 2 =HO2+HO 2 39 HO 2 + O H = H 2 0 + 0 2 40 C2H 2 + M = C2H + H + M 41 C2H 2 + O 2 = H C O + H C O 42 C2H 2 + H = C2H + H2 43 C2H 2 + OH = C2H + H20 44 C2H 2 + O =CH 2 + C O 45 C2H 2 + O = C2H + OH 46 C2H + 0 2 = HCO + CO 47 C2H + O = C O + C H 48CH 2+O 2=HCO+OH 49CH 2+OH=CH+H20 50 CH 2 + O = CH + OH 51 C H 2 + H = C H + H 2 52CH+O 2=CO+OH 53CH+0 2=HCO+O a Units: cm, kJ, K, mole, s.

Rate constant kt a

Ref.

1.0 X 1017 exp (-358.6/RT) 1.9 X 1011 exp (-244.9/RT) 1.2 X 1014 exp (-49.74/RT) 4.0 X 1014 exp (-58.5/RT) 1.5 X 106T 2.13 exp ( - 1 0 - 2/R T) 2.0 X 1013 exp (-75.24/RT) 3.2 X 1011T o-5 2.4 x 1013 exp (-121.2/R T) 1.3 x 1014 exp (-8.36/RT) 4.0 X 1012 5.0 X 1013 exp (-87.8/RT) 1.0 X 1012 exp (-25]RT) 3.3 X 1016 exp (-339/RT) 1.0 X 1014 exp (-23.24/RT) 7.7 X 1012 exp (-167/RT) 6.0 X 1013 exp (-132.9/RT) 1.8 X 1013 exp (-12.9/RT) 1.0 X 1012 exp (-33.4/RT) 2.5 X 1019T--1 exp (-369/RT) 2.5 X 1013 exp(-26.75/RT) 5.0 X 1015 exp (-480/RT) 1.0 × 1014 1.0 x 1014 2.0 x 1014 1.6 x 1014 exp (-79.4/RT) 4.2 X 1013 exp (-30/RT) 6.3 X 1015 exp (-17.1/RT) 1.5 X 1017T --0.5 2.0 X 1014 exp (-401/RT) 1.6 X 1015 exp (+4.18/RT) 1.3 X 107T 1.3 exp (+3.34/RT) 2.51 X 1013 exp (-2.92/RT) 1.0 X 1014 exp (-96.1/RT) 2.2 X 10 TM exp (-70.18/RT) 2.5 X 1014 exp (-7.94/RT) 1.8 X 1010 exp (-37.2/RT) 5.5 X 1013 exp (-29.26/RT) 4.0 X 1013 exp (-178/RT) 5.0 X 1013 exp (-4.18/RT) 3.6 X 1016 exp (-445/RT) 4.0 X 1012 exp ( - I I 7 / R T ) 7.7 X 1014 exp (-108.7/RT) 6.3 x 1012 exp (-29.3/RT) 6.3 × 1013 exp (-16.72/RT) 3.2 X 1015T -O.6 exp (-71/RT) 1.0 X 1013 exp (-29.3/RT) 5.0 X 1013 1.0 X 1014 exp (-15.5/RT) 2.7 X 1011T0.67 exp (-107.4/RT) 1.9 × 1011T0-68 exp (-104.5/RT) 2.7 X 1011T0.68 exp (-104.5/RT) 1.4 x 1011TO.67 exp (-107.4/RT) 1.0 x 1013

10 8 24 47 48 1 42 27 37 13 27 28 46 30 46 30 46 34 37 31 32 5 45 36 44 38 41 15 25 25 26 25 23 25 25 25 29 34 34 33 29 33 43 43 15 15 15 14 39 35 35 40 16

45

IGNITION OF METHANE-ACETYLENE MIXTURES °

ao

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TABLE 3

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,

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Effect of Adding Acetylene to Methane on the Ignition Delay

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1 Pure methane 3 66.6% CH4 + 33.3% C2H 2 4 50% CH 4 + 50% CzH 2 5 Pure acetylene

1 1

510 110

1

58

1

28

6 Pure methane 7 66.6% CH 4 + 33.3% C2H 2 8 50% CH 4 + 5 0 % C2H 2 9 Pure acetylene

0.5 0.5

350 105

0.5

52

0.5

27

/

10(~

MIX'I 2

x a

4

A

5

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20

No. Mixture

Ignition delay at 1600K, 1 bar (us)

!

$

6

I

7 I04/T CK"I} Fig. 1. Ignition delay data for ¢~= 1 mixtures a t P = 1 bar:

MIX. 1 : MIX. 2:

CH 4 : 0 2 : Ar = 3.33 : 6.66 : 90.01 (%mole). CH 4 : C 2 H 2 : O 2 : A r = 3 . 0 0 : 0 . 3 : 6 . 7 5 : 8 9 . 9 5 (%mole). MIX. 3: CH4 : C2H2 : O2 : Ar= 2.20 : 1.10 : 7.15 : 89.55 (%mole). MIX. 4: CH 4 : C 2 H 2 : O 2 : A r = 1 . 6 0 : 1 . 6 0 : 7 . 2 0 : 8 9 . 6 0 (%mole). MIX. 5: C 2 H 2 : O 2 : A r = 3 . 0 0 : 7 . 5 0 : 8 9 . 5 0 ( % m o l e ). Solid lines give least square fit of the experimental points. Broken lines give least square fit of the calculated points. (Calculations made at 1900, 1800, 1700, 1600, and 1500K.)

4. 5. 6. 7.

H + 02 = OH + 0 CzH 2 + 0 2 = HCO + HCO CzH z + H = CzH + H z CzH 2 + OH = CzH + H 2 0

CH4--C2H 2 Mixtu.re Mechanism (Reactions 1-53)

1. 2. 3. 4. 5. 6. 7. 8.

CH4 + H = CH a + H 2 CH4 + OH --- CHa + H20 HCO + M = H + CO + M H+O z+M=HO 2+M CO + HO2 = CO2 + OH H + O z = OH + O H + HO 2 = OH + OH CzH 2 + 02 = HCO + HCO

(R3) (R5) (R25) (R30) (R33) (R34) (R35) (R41)

RESULTS AND DISCUSSION

(R34) (R41) (R42) (R43)

Figure 1 gives the experimental variation in ignition delay with 1/T for the stoichiometric mix. tures 1-5. The corresponding computed delay TABLE 4 Ignition Parametersa

Mixture I 2 3 4 5

a O" =A exp

CH 4 (%mole)

C2H 2 (%mole)

02 (%mole)

Ar (%mole)

3.33 3.00 2.20 1.60 0

0 0.30 I.I0 1.60 3.00

6.66 6.75 7.15 7.20 7.50

90.01 89.95 89.55 89.60 89.50

(-E/RT).

E (k.l/mole)

A 5.85 X 2.81 × 1.45 × 1.30 X 0.72 ×

10---4 10--2 10--I 10--I 10--I

188 120 81 79 75

46

K. S. KRISHNAN ET AL. °

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MIX" N°.4 P5 ~' | Otto ,* ?. O,tm.

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Fig. 2. Variation of ignition delay with pressure: MIX. 4: CH4:C2H 2 : O 2 : A r = ] . 6 : 7 . 2 : 8 9 . 6 ( % m o l e ) . Solid lines give least square fit of the experimental points. Broken lines give least square tit of the calculated points. (Calcualtions at 1900, 1800, ]700, 1600, and 1500K.)

values are shown on this figure by dotted lines. Similar trends in the variation of delay with lIT were observed for lean (6-9) and rich (10-13) mixtures. From this it can be concluded that the relative compositions of CH 4 and C2H2 in the mixture has a direct effect on the ignition delay. The delay decreases with increasing CzHz content. Also, for a given temperature and pressure, the delay for CH4-C2H z mixtures lies between that of pure CH4 and pure Call2. Table 3 shows the quantitative results of adding acetylene to methane for a few representative mixtures at a pressure of 1 bar and temperature 1600K. The linear variation of delay with temperature observed in all the cases discussed above leads to a relation of the form r --- A exp (E/RT). Table 4 gives values of the parameters A and E for the

stoichiometric mixtures. The value of E decreases with increasing C2Hz content in the mixture, which is evidence for the kinetic coupling of the C2H z and CH 4 reactions. Figure 2 gives the variation of ignition delay with 1/T for initial pressures (P5) of 1, 2, and 3 bar for mixture 4, which contains equal proportions of CH 4 and C2Hz. It will be noted that an increase in the initial pressure decreases the delay period. The calculated temperature and pressure profiles during the induction period are shown in Figs. 3 and 4. The pressure and temperature remain constant during the induction period, followed by a sudden rise, denoting the onset of ignition. The ignition delays based on pressure profiles were higher than those based on OH concentration (2-5%), and the trend was the same for all the mixtures. Since no double ignition is seen in the calculations, it can be inferred that the burning of the two fuels is chemically coupled. At a reaction pressure of 2 bar and an initial temperature of 1800K, the computed concentration prot~iles of different species for CH4-Oz-Ar, C2H2-O2-Ar, and CH4-CzH2-O2-Ar mixtures are shown in Figs. 5, 6, and 7, respectively. It is clear that buildup of concentration of the various species in the mixture is that of neither methane nor acetylene. During the reaction time under consideration (35/as), the growth rates of species from methane is very small, but with acetylene the growth is very fast, and some species (C2H, CH2, CH, and HCO) have passed through a maximum concentration. In the case of the mixture, the concentration of all the species shows a steady growth from the very beginning. Even by 5/as, the concentration of most of the species has exceeded 10-11 mole/cm a. In the case of the acetylene mixture, ignition occurs at about 3 /as, but for the CH4C2H2 mixture it occurs at about 10/as. For the methane mixture, ignition is about 140/as. With acetylene, the species C2H, CH2, CH, and HCO grow to their maximum concentration at about 16 /as and start decreasing rapidly. For the methaneacetylene mixture, even those species which can only come from methane appear much earlier. For example, ethane can come only from the reaction

IGNITION OF METHANE-ACETYLENE

47

MIXTURES

zz~i--

i2oL j 1800I,,"

1600~

~

I 20

0

t 40

I 60 TIME

t 80

I 100

.u sees

Fig. 3. Typical computed temperature profiles, with O = 1,P = 2 bar, T 5 = 1800K: 1 - M I X . 1: C H 4 : O 2 : A r = 3 . 3 3 : 6 . 6 6 : 9 0 . 0 1 ( % m o l e ) . 2 - M I X . 2: CH 4 : C2H 2 : 0 2 : Ar = 3.00 : 0.3 : 6.75 : 89.95 (%mole). 3 - M I X . 14: CH 4 : C2H 2 : 0 2 : Ar = 2.4 : 0.8 : 6.8 : 90.0 (%mole). 4 - M I X . 3: C H 4 : C 2 H 2 : O z : A r = 2 . 2 0 : 1 . 1 0 : 7 . 1 $ : 8 9 . S S ( % m o l e ) . S - M I X . 4: CH 4 : C2H 2 : 0 2 : Ar = 1.60 : 1.60 : 7.20 : 89.60 (%mole). 6 - M I X . S: CzH 2 : 0 2 : Ar = 3.00 : 7.50 : 89.50 (%mole). End of delay periods 1 (148 ~us), 2 (81.3 ,us), 3 (38.09 ~as), 4 (26.24 , s ) , $ (19.86~s), and 6 (2.51 ~s).

-g

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~'0

I

30

1

40

I

I

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50 60 70 TIME ~ $1¢t

, I

80

I

90

I

I00

Fig. 4. Typical computed pressure profiles, with • = 1 , P = 2 bar, T 5 = 1800K: 1 - M I X . 1: 2 - M I X . 2: 3 - M I X . 14: 4 - M I X . 3: 5 - M I X . 4: 6 - M I X . 5:

CH 4 : O 2 : A r = 3 . 3 3 : 6 . 6 6 : 9 0 . 0 1 ( % m o l e ) . CH 4 : C2H 2 : 0 2 : Ar = 3.00 : 0.3 : 6.75 : 89.95 (%mole). CH 4 : C2H 2 : 0 2 : Ar = 2.4 : 0.8 : 6.8 : 90.9 (%mole). CH4:C2H2:O2:Ar=2.20:1.10:7.1S:89.S$(C/cmole). CH 4 : C 2 H 2 : O 2 : A t = 1 . 6 0 : 1 . 6 0 : 7 . 2 0 : 8 9 . 6 0 ( % m o l e ) . C2H 2 : O 2 : A r = 3 . 0 0 : 7 . 5 0 : 7 . 5 0 : 8 9 . 5 0 ( % m o l e ) .

48

K . S. K R I S H N A N

Ol CH~,

-6 -7 HmXTURE I ( g J ; I ) CH~. Oz Ar 3.3 6.6 90.1 ('/. mote )

-8

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-13

CliO

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concentrations of the'intermediate species builc up over a comparable period. As acetylene has a considerably shorter ignitiol delay than methane, it may be the case that tht two reactions (i.e., the oxidation of acetylene anc methane, respectively) are not coupled, so that th( shortening of the induction period is purely ar outcome of the heat released by the ignition o: acetylene. Acetylene ignition occurs at about 3/a: for the conditions considered, and by about 15/a: t h e temperature goes up from 1800K to 2800K Figure 8 gives the concentration profiles obtainec from the calculation for methane ignition (with nc C2H2) but with the temperature profile obtainec for a C2H 2 calculation imposed (thereby exandnin{ the effect of the heat released by acetylene). It i~ clear that the buildup in concentration of the vari ous species is not as fast as that for the mixture

-15

HCO -16

-170

I%

5

l I0

I

15

I

1

20 25 TIME ~ set:.

Fig. 5. Typical c o m p u t e d c o n c e n t r a t i o n m e t h a n e , w l t h T 5 = 1 8 0 0 K , P = 2 bar: MIX. l:

I

I

30

35

~

40. -7

profiles

CO

'%H H

for

CH4:O2:Ar=3.33:6.66:90.01(%mole).

of methane, and its growth in concentration in the methane mixture is extremely slow. In contrast, in the CH4-C2H 2 mixture it builds up rapidly, reaches a maximum value at about 20/as, and then shows a decreasing tendency. Similarly CH a is also built up at about 5/as and smoothly increases. It is evident that some o f the methane reactions are accelerated by the presence of acetylene reactions and that some of the acetylene reactions are retarded by the methane reactions. CH z is produced by reactions (1)--(6) and the reverse of reactions (7)-(10) and (14). The species O, H, OH, HCO, etc., are produced rapidly from the acetylene reactions, and they in turn react with CH 4 to produce CHa, while [CH20] and [HCO] also grow rapidly. The buildup of CH20 in the methane mixture is slow; at a temperature of 1800K and a pressure of 2 bar, the ignition delay is around 140/as and the

o

40

b -II

HCO

=~-I 2 o~

CzH= O= Ar 3.0 7.5 89.S('/,meld

~-13 .~-14 -15 -16

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CzH=

5

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20 25 TIME ~1 =~.

30

35

40

Fig. 6. T y p i c a l computed concentration profiles f o r acetylene, w i t h T 5 = 1800K, P = 2 bar: MIX. 5:

C2H2 : O2 : Ar = 3.0 : 7.5 : 89.5 (%mole).

49

IGNITION OF METHANE-ACETYLENE MIXTURES

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25

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TIME )* sec$ Fig. 7. Typical computed concentration profiles for methane-acetylene mixture, with T 5 = 1800K, P = 2 bar: MIX. 4:

CH4:C2H2:O2:Ar=1.6:1.6:7.2:89.6 (%mole).

Thus it may be concluded that the acceleration in the reaction rate of methane with the addition of acetylene is not due to thermal coupling but is the result of chemical coupling. Further study to establish the nature of the linking reactions is under progress, and the resultswill be published soon.

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35

40

Fig. 8. Comparison of concentration profiles of selected species of mixture (methane + acetylene) and pure methane with C2H 2 temperature effect imposed. Thick lines give methane-acetylene mixture, with initial temperature T 5 = 1800K,P = 2 bar: CH 4 : C2H 2 : 0 2 : Ar = 1.6 : 1.6 : 7.2 : 89.6 (%mole). Thin lines give methane, with initial temperature T 5 = 1800K, P = 2 bar: CH 4 : 0 2 : Ar = 3.33 : 6.66 : 90.01 (%mole). Broken lines give profiles after imposing the temperature effect due to acetylene combustion, temperature effect due to the acetylene mixture ( C 2 H 2 : O 2 : A r = 3.0: 7.5:89.5 %mole) imposed on pure methane with the same conditions.

The authors wish to thank the referees f o r their valuable comments.

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