Numerical investigation and prediction models for methanol-air laminar flame speed

doi:10.11949/0438-1157.20181496

Abstract

Abstract:

The experimental data of the existing laminar flame velocity of methanol were collected and analyzed. The three typical chemical reaction mechanisms describing the oxidation of methanol (Li mechanism, USC Mech-Ⅱ and Burke mechanism) were compared to predict the propagation velocity of laminar flame. Laminar flame speed is an intrinsic property of a combustible mixture. In the present study, the validities and accuracies of three different chemical kinetic models (including Li scheme, USC Mech Ⅱ scheme and Burke scheme) on their predictions of laminar methanol-air flame speed were verified using newly-reported experimental data in the literature. The results show that the tested three kinetic schemes were able to qualitatively characterize the variation of the laminar flame speed of methanol-air mixtures. However, quantitatively, remarkable over-predictions were found between the computed laminar flame speed and experimental ones of fuel-rich mixtures. Consequently, reaction sensitivity analysis and reaction rate analysis were conducted in order to figure out the reason for the discrepancy. As per the kinetic analysis,the H-abstraction of methanol was found to be of great importance to the laminar flame speed. Consequently, the H-abstraction in Li scheme was updated using the newly-reported data, and the predictions of laminar flame speed using the modified Li scheme were greatly improved, especially on the fuel-rich side. In the engineering calculation, a quick estimation of the laminar flame speed with an acceptable accuracy is desired. Motivated by this demand, two empirical prediction correlations for methanol-air laminar flame speed were proposed. The predictions using the two correlations were in good agreement with the reported experimental data at different initial mixture temperatures and pressures. The calculation of the laminar flame speed of methanol-air mixtures using the proposed correlations is much faster than detailed modeling using full kinetic model so that it will save a large amount of computation resources in the engineering calculation.

Key words: methanol, laminar flame speed, reaction kinetics, prediction model, kinetic modeling, simulation

摘要：

收集总结分析了现有的甲醇层流火焰速度的实验数据，比较了三种典型的描述甲醇氧化的详细化学反应机理(Li机理、USC Mech-Ⅱ和Burke机理)对层流火焰传播速度的预测精度。结果表明三种机理均能定性反映甲醇层流火焰速度的变化规律，但在富燃料侧，机理计算值明显高于实验结果。反应动力学分析表明甲醇脱氢反应对层流火焰速度的影响至关重要。根据文献中的最新成果，修正了Li机理中甲醇脱氢反应的速率常数，提高了Li机理对甲醇-空气层流火焰速度的预测精度。针对工程需求，给出了两个甲醇层流火焰速度的快速预测模型，经过校核，所提出的预测模型能够较为准确地预测不同初始温度和压力下甲醇层流火焰速度。

关键词: 甲醇, 层流火焰速度, 反应动力学, 预测模型, 动力学模型, 模拟

CLC Number:

TK 16

Xiehe YANG, Wenfeng SHEN, Yang ZHANG, Hai ZHANG, Jiansheng ZHANG, Junfu LYU. Numerical investigation and prediction models for methanol-air laminar flame speed[J]. CIESC Journal, 2019, 70(8): 3011-3020.

杨协和, 沈文锋, 张扬, 张海, 张建胜, 吕俊复. 甲醇-空气层流火焰速度的数值研究和预测模型[J]. 化工学报, 2019, 70(8): 3011-3020.

Figures/Tables 11

Fig.1 Variations of laminar flame speed of methanol-air mixtures with respect to normalized equivalence ratio at ambient temperature and pressure(p = 101325 Pa, Tu= 298 K)

Fig.2 Variations of laminar flame speed of methanol-air mixtures with respect to normalized equivalence ratio at ambient pressure and elevated temperature (p = 101325 Pa, Tu = (340±3） K)

Fig.3 Variations of laminar flame speed of methanol-air mixtures with respect to normalized equivalence ratio at elevated pressure and temperature

Fig.4 Prediction error using different kinetic mechanisms

Fig.5 Reaction sensitivity of laminar flame speed of methanol-air mixtures at different normalized equivalence ratios and elevated pressure (Tu= 298 K)

Table 1 Kinetic constants for dehydrogenation reaction of CH3OH

反应	参考文献	A	β	E_a/(J/mol)
R1 CH₃OH+OH $? ?$ CH₂OH+H₂O	[17]	7.100×10⁶	1.800	-2494.8
R1 CH₃OH+OH $? ?$ CH₂OH+H₂O	[28]	3.080×10⁴	2.650	-3376.7
R2 CH₃OH+OH $? ?$ CH₃O + H₂O	[17]	1.000×10⁶	2.100	2079.1
R2 CH₃OH+OH $? ?$ CH₃O + H₂O	[28]	1.500×10²	3.030	-3193.8
R3 CH₃OH + H $? ?$ CH₂OH + H₂	[17]	3.200×10¹³	0.000	25512.8
R3 CH₃OH + H $? ?$ CH₂OH + H₂	[29]	3.070×10⁵	2.550	22771.0
R4 CH₃OH + H $? ?$ CH₃O + H₂	[17]	8.000×10¹²	0.000	25512.8
R4 CH₃OH + H $? ?$ CH₃O + H₂	[29]	1.990×10⁵	2.560	43114.3

Table 1 Kinetic constants for dehydrogenation reaction of CH3OH

反应	参考文献	A	β	E_a/(J/mol)
R1 CH₃OH+OH $? ?$ CH₂OH+H₂O	[17]	7.100×10⁶	1.800	-2494.8
R1 CH₃OH+OH $? ?$ CH₂OH+H₂O	[28]	3.080×10⁴	2.650	-3376.7
R2 CH₃OH+OH $? ?$ CH₃O + H₂O	[17]	1.000×10⁶	2.100	2079.1
R2 CH₃OH+OH $? ?$ CH₃O + H₂O	[28]	1.500×10²	3.030	-3193.8
R3 CH₃OH + H $? ?$ CH₂OH + H₂	[17]	3.200×10¹³	0.000	25512.8
R3 CH₃OH + H $? ?$ CH₂OH + H₂	[29]	3.070×10⁵	2.550	22771.0
R4 CH₃OH + H $? ?$ CH₃O + H₂	[17]	8.000×10¹²	0.000	25512.8
R4 CH₃OH + H $? ?$ CH₃O + H₂	[29]	1.990×10⁵	2.560	43114.3

Fig.6 Comparisons of reaction rate constants of methanol H-abstraction reactions

Fig.7 Comparison of species profile of CH2OH and CH3O (p=101325 Pa, Tu=298 K)

Fig.8 Prediction error using modified Li mechanism

Fig.9 Variations of methanol-air mixtures laminar flame speed with respect to different initial mixture temperatures (p = 101325 Pa)

Fig.10 Methanol-air mixtures laminar flame speed at different initial mixture temperatures and pressures

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