Reduced combustion mechanism for C₁–C₄ hydrocarbons and its application in computational fluid dynamics flare modeling

Figures & data

Figure 1. Absolute rate of production of C₆H₆.

Table 1. The 50-species list for LU 3.0.1 combustion mechanism.

50 Species

Ar, N2, H, OH, HO2, H2, H2O, O, O2, C, CO, CO2, CH, HCO, CH2, CH2*, CH2O, CH3, CH3O, CH2OH, CH4, CH3OH, HCCO, C2H2, H2CC, CH2CO, C2H3, CH2CHO, C2H4, CH3CHO, C2H5, C2H6, C3H3, aC3H4, C2H3CHO, aC3H5, C3H6, n-C3H7, C3H8, iC4H3, C4H4, C4H52, C4H6, C4H81, p-C4H9, C4H10, C5H5, C6H5, C6H6, C6H5O

Note. C₄H₅2 = CH₃-CC-*CH₂; C₄H₈1 = 1-butene; aC3H4 = Allene; aC₃H₅ = Allyl; iC₄H₃ = CH₂C*-CCH.

Table 2. C₄ species and soot precursors.

Formula	Name
C4 species
C4H10	n-Butane
C4H6	1,3-Butadiene
C4H81	1-Butene
Soot precursors
C2H2	Acetylene
C6H6	Benzene
C2H4	Ethylene

Table 3. Comparison of prediction errors of reduced mechanisms for mole fraction of major species at residence time of 1 sec for C₃H₆ fuel (Wang et al., 2007).

Species	USC II	LU 3.0.1	Abs. error %	LU 1.1	Abs. error %	LU 2.0	Abs. error %
C2H2	1.46E-06	1.32E-06	9.72	1.19E-06	18.53	1.21E-06	17.23
CH4	3.01E-07	3.02E-07	0.23	3.30E-07	9.68	3.91E-07	30.07
CO	5.59E-03	5.70E-03	1.97	6.24E-03	11.68	5.98E-03	7.06
CO2	1.21E-01	1.21E-01	0.20	1.20E-01	0.64	1.21E-01	0.35
H2	1.35E-03	1.37E-03	2.10	1.51E-03	12.31	1.45E-03	7.41
Average abs. error %			2.84		10.57		12.42

Table 4. Comparison of prediction errors of reduced mechanisms for mole fraction of trace species at residence time of 1 sec for C₃H₆ fuel.

Species	USC II	LU 3.0.1	Error, a factor of	LU 1.1	Error, a factor of	LU 2.0	Error, a factor of
C3H6	4.35E-08	4.67E-08	1.07	4.59E-08	1.05	1.48E-07	3.41
HO2	1.19E-07	1.16E-07	1.02	2.06E-07	1.73	2.51E-07	2.10
OH	1.27E-03	1.33E-03	1.05	1.26E-03	1.00	1.22E-03	1.04
CH3CHO	3.16E-12	4.22E-12	1.34	3.58E-11	11.34	4.44E-13	7.11
CH2O	2.11E-07	1.68E-07	1.25	1.75E-07	1.21	2.02E-07	1.04
C2H4	1.81E-08	3.16E-08	1.74	3.84E-08	2.12	2.47E-08	1.36
CH2CHO	8.80E-12	1.10E-11	1.25	1.07E-11	1.21	1.70E-11	1.93
Average error			1.25		2.81		2.57

Table 5. Laminar flame speed (cm/sec)—Comparison of simulation and experimental results for propylene (Davis and Law, 1998).

Equivalence ratio	Experiment	USC II	Abs. error %	LU 3.0.1	Abs. error %
0.7	22	25.7	16.82	27	22.73
0.8	29.5	32.7	10.85	33.7	14.24
0.9	37	38.0	2.70	37.2	0.54
1	42	41.0	2.38	40	4.76
1.1	44.5	41.2	7.42	40	10.11
1.2	44	38.2	13.18	37.1	15.68
1.3	41.5	31.8	23.37	31.3	24.58
1.4	34	23.5	30.88	22.2	34.71
1.5	22	16.4	25.45	16.9	23.18
Average absolute error %			14.78		16.73

Table 6. Laminar flame speed—Comparison of simulation and experimental results for methane (Law et al., 2006).

Equivalence ratio	Experiment	LU 3.0.1	Absolute error %
0.6	10	12.6	26.00
0.7	19	21.1	11.05
0.8	25	28.8	15.20
0.9	32	34.5	7.81
1	35	37.5	7.14
1.1	37	36.9	0.27
1.2	30	32.3	7.67
1.3	28	23.2	17.14
1.4	15	14.39	4.07
Average absolute error %			10.71

Table 7. Laminar flame speed—Comparison of simulation and experimental results for ethylene (Jomaas et al., 2005).

Equivalence ratio	Experiment	LU 3.0.1	Absolute error %
0.7	37	40.05	8.24
0.8	49	50.85	3.78
0.9	58	59.63	2.81
1	63	64.89	3.00
1.1	64	67.25	5.08
1.2	63.5	65.9	3.78
1.3	58	60.6	4.48
1.4	52	52.3	0.58
Average absolute error %			3.97

Figure 2. Laminar flame speed for different equivalence ratios for propylene (Davis and Law, 1998).

Figure 3. Laminar flame speed for different equivalence ratios for Methane (Vagelopoulos et al., 1994; Vagelopoulos and Egolfopoulos, 1998).

Figure 4. Laminar flame speed for different equivalence ratios for ethylene (Jomaas et al., 2005).

Table 8. Ignition delay—Comparison of simulation and experimental results.

10^4/T (K–1)	< Ignition delay (μsec)		Absolute error %
	Experimental	LU 3.0.1
75.76	1285	961.49	25.18
74.07	870	742.7	14.63
72.46	615	570.52	7.23
71.17	550	455.89	17.11
69.69	410	346.57	15.47
68.26	375	261.95	30.15
66.45	185	180.2	2.59
65.36	140	142.16	1.54
63.9	100	102.84	2.84

Figure 5. Ignition delay time for propylene (Qin et al., 2001).

Table 9. Average percentage error of the reduced mechanisms compared with experimental data.

		Average percentage error
Indicator	Chemical	LU 1.0	LU 1.1	LU 3.0.1
Laminar flame speed	Propylene	11.605	8.058	16.73
Adiabatic flame temperature	Ethylene	1.138	0.527	0.2626
Ignition delay	Propylene	30.68	31.25	12.97

Figure 6. Adiabatic flame temperature of methane (Law et al., 2006).

Figure 7. Adiabatic temperature profile of ethylene (Law et al., 2006).

Table 10. Flamelet generation parameters.

Number of grid points in flamelet	64
Maximum number of flamelets	15
Initial scalar dissipation (1/sec)	0.01
Scalar dissipation step (1/sec)	5

Table 11. Grid refinement parameters for flamelet.

Initial number of grid points in flamelets	8
Maximum number of grid points in flamelet	64
Maximum change in value ratio	0.5
Maximum change in slope ratio	0.5

Table 12. Parameters used to generate the PDF table.

Number of mean mixture fraction points	21
Number of mixture fraction variance points	40
Maximum number of species	50
Number of mean enthalpy points	41
Minimum temperature (K)	298

Figure 8. Computational domain and mesh view.

Figure 9. Measured versus predicted black carbon—PDF model.

Figure 10. Measured versus predicted combustion efficiency—PDF model.

Table 13. Comparison of EDC and PDF models.

EDC (eddy dissipation concept)	PDF (probability density function)
Reactions taking place in the flame are governed by the Arrhenius rates	Reactions are governed by a conserved scalar quantity known as mixture fraction
Incorporates detailed chemical mechanisms	Not so rigorous
Molar concentrations are derived from reaction rates, which are calculated using ISAT algorithm	Molar concentrations are derived from the predicted mixture fraction fields
Any number of inlet streams can be defined	Only two inlet streams are allowed, i.e., fuel and oxidizer
Computationally very expensive; requires 5–6 days for convergence	Requires less time for convergence; only 2–3 days

Figure 11. Side view of air-assisted stack showing the inlet surfaces.

Figure 12. Measured versus predicted black carbon—EDC model.

Figure 13. Measured versus predicted combustion efficiency—EDC model.

Table 14. Comparison of PDF and EDC model predictions.

PDF model	EDC model
CE predictions: max. 6%, with an avg. of 2% DRE is always overpredicted	Underpredicts both DRE and CE but in good agreement
Easy to converge	Convergence difficulties at times
Virtually predicts no VOCs	Reasonably predicts VOCs
Soot yield is predicted in the right order of magnitude, within a factor of 2 for TCEQ test cases	Soot yield is predicted in the right order of magnitude, within a factor of 2 for TCEQ test cases
Temperature ranges from 1760 to 1900 K	Temperature ranges from 2000 to 2226 K

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