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Acta Phys. -Chim. Sin.  2019, Vol. 35 Issue (2): 158-166    DOI: 10.3866/PKU.WHXB201802272
ARTICLE     
Role of Low-Temperature Fuel Chemistry on Turbulent Flame Propagation
Fan ZHANG,Zhe REN,Shenghui ZHONG,Mingfa YAO,Zhijun PENG*()
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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 wordsLow temperature chemistry (LTC)      Two-stage ignition      n-Heptane      Flame regimes     
Received: 21 December 2017      Published: 27 February 2018
MSC2000:  O643  
Fund:  the National Natural Science Foundation of China(51506146);the National Natural Science Foundation of China(91541205)
Corresponding Authors: Zhijun PENG     E-mail: pengzj@tju.edu.cn
Cite this article:

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.

URL:

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

Fig 1 (a) Schematic of reactor-assisted turbulent slot burner; (b) computational domain of three-dimensional numerical simulation; (c) the grid mesh in the simulation.
Fig 2 Bottom inlet of the computational domain (a); inlet turbulent velocity (b).
Fig 3 Comparison of ignition delay time for n-heptane mechanism at φ = 1 (a) and φ = 0.6 (b) conditions between reduced and ver3.1 detailed mechanism.
Fig 4 Time-dependent change of pressure (a) and important species (b) of simplified mechanism.
C7H16 O2 N2
φ = 0.6 0.0382 0.2242 0.7376
Table 1 Component mass fraction of n-heptane mixture (φ = 0.6).
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
Table 2 Initial simulation parameters of 12 cases.
Fig 5 Temporal evolution of NXC7H16 (a) and CH2O (b) mass fraction at 450-700 K with equivalence ratio φ = 0.6.
Fig 6 2D temperature distributions at the burner outlet from 3D results for different reactant temperatures (450-700 K) but the same inlet velocity (U = 10 m∙s−1).
Fig 7 2D temperature distributions at the burner outlet from 3D results for different reactant temperatures (450-700 K) but the same inlet velocity (U = 6 m∙s−1).
Fig 8 2D temperature distributions at the burner outlet for two reactant temperatures 670 K, 700 K and two inlet velocities 6 m∙s−1, 10 m∙s−1.
Fig 9 Measured turbulent burning velocity normalized by SL as a function of normalized turbulent intensity, u'/SL.
Fig 10 2D contour of intermediate species (CH2O, OH) mass fraction comparisons for different reactant temperatures 500 K and 670 K at flow residence time of τres = 100 ms.
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
Table 3 Time scale relations and flame regimes for 12 cases.
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