低温燃料化学特性对湍流预混火焰传播的影响
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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Open FOAM的reacting Foam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CH_2O,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。
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 ofstoichiometric 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 3 D simulations. Twelve cases are considered and analyzed based on the flow residence time and ignition delay time. The 2 D 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 CH2 O 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.
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 ofstoichiometric 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 3 D simulations. Twelve cases are considered and analyzed based on the flow residence time and ignition delay time. The 2 D 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 CH2 O 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.
引文
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(2)Bisetti,F.;Chen,J.Y.;Chen,J.H.;Hawkes E.R.Proc.Combust.Inst.2009,32(1),1465.doi:10.1016/j.proci.2008.09.001
(3)Soika,A.;Dinkelacker,F.;Leipertz,A.Combust.Flame 2003,132(3),451.doi:10.1016/S0010-2180(02)00490-X
(4)Wang,J.H.;Wei,Z.L.;Zhang,M.;Huang,Z.H.Sci.China Technol.Sci.2014,57:445.doi:10.1007/s11431-014-5471-y
(5)Ju,Y.Adv.Mech.2014,44,201402.[琚诒光.力学进展,2014,44,201402.]doi:10.6052/1000-0992-14-011
(6)Dryer,F.L.Proc.Combust.Inst.2015,35(1),117.doi:10.1016/j.proci.2014.09.008
(7)Li,B.Studies of Flame Propagation and Extinction Characteristics of Heavy Hydrocarbon Fuels[D].Beijing:Tsinghua University,2014.[李博.高碳碳氢燃料的火焰传播特性和熄灭特性的研究[D].北京:清华大学,2014.]
(8)Zhang,M.;Wang,J.H.;Jin,W.;Huang,Z.H.;Kobayashi,H.;Ma,L.Combust.Flame 2015,162,2087.doi:10.1016/j.combustflame.2015.01.007
(9)Sun,W.;Chen,Z.;Gou,X.;Ju,Y.G.Combust.Flame 2010,157(7),1298.doi:10.1016/j.combustflame.2010.03.006
(10)Guo,J.J.;Li,S.H.;Tan,N.X.;Li,X.Y.J.Eng.Thermophys.2014,11,2298.[郭俊江,李树豪,谈宁馨,李象远.工程热物理学报,2014,11,2298.]doi:0253-231X(2014)11-2298-05
(11)Dooley,S.;Won,S.H.;Chaos,M.;Heyne,J.;Ju,Y.G.;Dryer,F.L.;Kumar,K.;Sung,C.J.;Wang,H.W.;Oehlschlaeger,M.A.;et al.Combust.Flame 2010,157(12),2333.doi:10.1016/j.combustflame.2010.07.001
(12)Lawes,M.;Ormsby,M.P.;Sheppard,C.G.W.;Woolley,R.Combust.Flame 2012,159(5),1949.doi:10.1016/j.combustflame.2011.12.023
(13)Joannon,M.D.;Cavaliere,A.;Faravelli,T.;Ranzi,E.;Sabia,P.;Tregrossi,A.Prog.Energy Combust.Sci.2004,30(4),329.doi:10.1016/j.proci.2004.08.190
(14)Won,S.H.;Windom,B.;Jiang,B.;Ju,Y.G.Combust.Flame 2014,161(2),475.doi:10.1016/j.combustflame.2013.08.027
(15)Windom,B.;Sang,H.W.;Reuter,C.B.;Jiang,B.;Ju,Y.G.;Hammack,S.;Ombrello,T.;Carter,C.Combust.Flame 2016,169,19.doi:10.1016/j.combustflame.2016.02.031
(16)Gou,X.;Sun,W.;Chen,Z.;Ju,Y.Combust.Flame 2010,157,1111.doi:10.1016/j.combustflame.2010.02.020
(17)Krisman,A.;Hawkes,E.R.;Talei,M.;Bhagatwala,A.;Chen,J.H.Proc.Combust.Inst.2017,36(3),3567.doi:10.1016/j.proci.2016.08.043
(18)Krisman,A.;Hawkes,E.R.;Talei,M.;Bhagatwala,A.;Chen,J.H.Combust.Flame 2016,172,326.doi:10.1016/j.combustflame.2016.06.010
(19)Zhang,F.;Yu,R.;Bai,X.S.Proc.Combust.Inst.2015,35(3),2975.doi:10.1016/j.proci.2014.09.004
(20)Zhang,F.;Yao,M.Acta Phys.-Chim.Sin.2016,32(8),1941.[张帆,尧命发.物理化学学报,2016,32(8),1941.]doi:10.3866/PKU.WHXB201604223
(21)Liu,S.;Hewson,J.C.;Chen,J.H.;Pitsch,H.Combust.Flame 2004,137(3),320.doi:10.1016/j.combustflame.2004.01.011
(22)Curran,H.J.;Gaffuri,P.;Pitz,W.J.;Westbrook,C.K.Combust.Flame 1998,114,149.doi:10.1016/S0010-2180(97)00282-4
(23)Tang,Q.L.;Geng,C.;Li,M.K.;Liu,H.F.;Yao,M.F.Acta Phys.-Chim.Sin.2015,31(12),2269.[唐青龙,耿超,李明坤,刘海峰,尧命发.物理化学学报,2015,31(12),2269.]doi:10.3866/PKU.WHXB201510082
(24)Xiao,J.;Zhang,B.;Zheng,Z.L.Acta Phys.-Chim.Sin.2017,33(9),1752.[肖杰,张博,郑朝蕾.物理化学学报,2017,33(9),1752.]doi:10.3866/PKU.WHXB201704273
(25)Zhou,B.;Brackmann,C.;Li,Z.S.;Alden,M.Combust.Flame 2015,162(7),2937.doi:10.1016/j.combustflame.2014.07.020