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物理化学学报  2019, Vol. 35 Issue (2): 158-166    DOI: 10.3866/PKU.WHXB201802272
论文     
低温燃料化学特性对湍流预混火焰传播的影响
张帆,任哲,钟生辉,尧命发,彭志军*()
Role of Low-Temperature Fuel Chemistry on Turbulent Flame Propagation
Fan ZHANG,Zhe REN,Shenghui ZHONG,Mingfa YAO,Zhijun PENG*()
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摘要:

大分子碳氢燃料的低温化学反应及两阶段点火特性会显著影响火焰的分区及燃烧情况。本文采用数值模拟的方法探究了正庚烷/空气预混混合气在RATS燃具上的湍流火焰传播,与试验结果具有一致性。模拟使用的是44种物质,112步的正庚烷简化动力学机理。使用OpenFOAM的reactingFoam求解器建立了简化模拟流道及出口的三维模型,模拟了在大气环境下,初始反应温度450–700 K、入口速度6 m·s−1与10 m·s−1、焰前流动滞留时间100 ms及60 ms、当量比φ = 0.6的正庚烷/空气混合气湍流火焰燃烧情况。结果发现,标准化湍流燃烧速度与混合气初始温度以及流动滞留时间有关。在低温点火阶段,正庚烷氧化程度受到初始温度与速度的影响,燃料分解并在预热区中产生大量中间物质如CH2O,继而会影响湍流火焰燃烧速度。随着初始反应温度的升高,湍流燃烧火焰逐渐由化学反应冻结区过渡到低温点火区;温度超过一定数值后,燃料不再发生低温反应,此时燃烧位于高温点火区域。

关键词: 低温燃料化学特性两阶段点火正庚烷火焰分区    
Abstract:

In modern advanced internal combustion engines such as homogeneous compression ignition engine (HCCI) and reactivity controlled compression ignition engine (RCCI), turbulence/chemistry interactions have a dramatic influence on the combustion efficiency. In particular, the low-temperature fuel chemistry and two-stage ignition of large hydrocarbon fuels can significantly affect the turbulent flame regimes and propagation. The turbulent flame propagation and flame structure of a turbulent premixed n-heptane/air flame is simulated in a slot, i.e., reactor-assisted turbulent slot (RATS) burner. In the center, a premixed n-heptane/air gas mixture flows out from the burner, exiting into the surrounding atmosphere. In order to maintain a high Reynolds number for the flame, a pilot flame consisting of stoichiometric methane/air is applied. The GRI3.0 mechanism for methane/air mixture is adopted. A three-dimensional (3D) numerical simulation model is established based on OpenFOAM reactingFoam solver. A reduced kinetic mechanism of n-heptane consisting of 44 species and 112 reactions is employed, which is validated against the detailed mechanism with regard to the ignition delay time over a wide range of the initial temperature, equivalence ratio, and pressure. Then, the effects of the reactant temperature (ranging from 450 to 700 K), inlet velocity (6 m·s−1 and 10 m·s−1), and pre-flame flow residence time (100 ms and 60 ms) on the turbulent flame combustion of the n-heptane/air mixture with an equivalent ratio of 0.6, are investigated by performing 3D simulations. Twelve cases are considered and analyzed based on the flow residence time and ignition delay time. The 2D span-wise temperature contour is used to show that when the ignition Da number and fuel reactivity increase, the flame temperature increases and the flame height decreases, indicating a stronger turbulent burning velocity (ST). The results coincide well with experiment results and indicate that the extent of fuel oxidation is affected by the reactant temperature and inlet velocity during the low-temperature ignition stage, since the ratio between the ignition delay time and flow residence time plays an important role. Moreover, a quantitative analysis is performed on the flame front. The intermediate species CH is used to mark the thin reaction zone, and the turbulent burning velocity is obtained. Two branches of turbulent burning velocities verified the upper and lower limits and are consistent with the fitting correlation of ST/SL from a previous semi-implicit expression by Won et al. (2014), in which SL is laminar burning velocity. In the upper limit, the fuel decomposes and produces a large amount of intermediate species like CH2O in the pre-heat zone, which subsequently increases the turbulent burning velocity. While in the lower limit in which the flow residence time is shorter than the first ignition delay time, it shows a smaller turbulent burning velocity and a thin reaction zone, which is in chemically-frozen-flow regime. A transitional regime between the low- to high-temperature ignition regimes is also identified, where the ignition delay time is comparable with the heated flow residence time before the flame was produced. With an increase in the reactant temperature, the turbulent flame gradually changes from the chemically frozen flow regime to the low-temperature ignition regime. When the temperature is higher than a certain value, the low-temperature ignition will not happen again and the flame will be classified as being in the high-temperature ignition regime.

Key words: Low temperature chemistry (LTC)    Two-stage ignition    n-Heptane    Flame regimes
收稿日期: 2017-12-21 出版日期: 2018-02-27
中图分类号:  O643  
基金资助: 国家自然科学基金青年基金(51506146);重大研究计划重点项目(91541205)
通讯作者: 彭志军     E-mail: pengzj@tju.edu.cn
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引用本文:

张帆,任哲,钟生辉,尧命发,彭志军. 低温燃料化学特性对湍流预混火焰传播的影响[J]. 物理化学学报, 2019, 35(2): 158-166, 10.3866/PKU.WHXB201802272

Fan ZHANG,Zhe REN,Shenghui ZHONG,Mingfa YAO,Zhijun PENG. Role of Low-Temperature Fuel Chemistry on Turbulent Flame Propagation. Acta Phys. -Chim. Sin., 2019, 35(2): 158-166, 10.3866/PKU.WHXB201802272.

链接本文:

http://www.whxb.pku.edu.cn/CN/10.3866/PKU.WHXB201802272        http://www.whxb.pku.edu.cn/CN/Y2019/V35/I2/158

图1  (a)燃具示意图;(b)三维数值模拟计算域;(c)计算域网格
图2  计算域底面入口(a)和底面速度入口湍流场(b)
图3  简化机理与ver3.1详细机理在φ = 1 (a)和φ = 0.6 (b)时的滞燃期对比
图4  简化机理压力(a)和部分组分(b)随时间变化
C7H16 O2 N2
φ = 0.6 0.0382 0.2242 0.7376
表1  φ = 0.6正庚烷混合气组分含量(质量分数)
Case T/K U/(m∙s−1) U’/(m∙s−1) τres/ms τ1st/ms SL/(cm∙s−1)
1 450 6 0.75 106.3 / 38.4
2 450 10 1.25 63.8 / 38.4
3 500 6 0.75 106.3 / 51.7
4 500 10 1.25 63.8 / 51.7
5 600 6 0.75 106.3 384.4 84.2
6 600 10 1.25 63.8 384.4 84.2
7 650 6 0.75 106.3 111.9 106.3
8 650 10 1.25 63.8 111.9 106.3
9 670 6 0.75 106.3 77.1 116.4
10 670 10 1.25 63.8 77.1 116.4
11 700 6 0.75 106.3 51.1 133.3
12 700 10 1.25 63.8 51.1 133.3
表2  算例初始参数
图5  450-700 K,当量比φ = 0.6混合气NXC7H16 (a)和CH2O (b)组分随时间变化情况
图6  U = 10 m∙s−1、T = 450-700 K范围内稳定火焰情况
图7  U = 6 m∙s−1、T = 450-700 K范围内稳定火焰情况
图8  670 K与700 K不同入口速度火焰对比
图9  被测标准湍流燃速ST/SL与标准化湍流强度u'/SL的函数关系
图10  中间产物CH2O和OH在初始温度500 K、670 K,流动时间100 ms时的浓度对比
Case T/K τ1st comparison τflow Regime
1 450 / > 106.3 CF
2 450 / > 63.8 CF
3 500 / > 106.3 CF
4 500 / > 63.8 CF
5 600 384.4 > 106.3 CF
6 600 384.4 > 63.8 CF
7 650 111.9 ~ 106.3 LTI
8 650 111.9 > 63.8 CF
9 670 77.1 < 106.3 LTI
10 670 77.1 ~ 63.8 LTI
11 700 51.1 < 106.3 LTI
12 700 51.1 ~ 63.8 LTI
表3  12算例时间尺度关系及火焰分区
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