基于TPDF方法正庚烷喷雾燃烧特性模拟
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摘要
采用输运概率密度函数模型(TPDF)开展了不同初始环境温度、氧体积分数下正庚烷喷雾火焰发展过程模拟研究.以发动机燃烧网络(ECN)试验数据为基准,首先将模拟得到的着火延迟期和火焰浮升长度与均质反应器模型(WSR)模拟结果以及试验结果进行对照,发现氧体积分数较低时,由于TPDF模型能够对小尺度上组分和温度的湍流波动进行精确求解,可获得准确的燃烧特征量.然后基于TPDF方法,系统分析和对比了不同初始环境条件下准稳态火焰结构以及火焰发展过程的燃烧特征,发现在不同的初始温度和氧体积分数条件下,着火均首先出现在浓混合气区域.降低初始环境温度,高当量比区域反应活性下降,导致着火推迟,燃油与空气有更长的时间进行混合;降低温度的同时降低氧体积分数,则导致着火进一步推迟,油、气混合更为充分.着火位置混合气的浓度较高温、高氧体积分数条件有所降低,燃烧消耗了本来就不足的氧气,导致高当量比区域更难发生反应.
Transported probability density function(TPDF)model and well stirred reactor(WSR)model were applied for the simulation of n-heptane spray flame under different initial temperature,oxygen mole fraction conditions.The predicted results from TPDF and WSR were compared to the available experimental data from engine combustion network(ECN)in terms of the ignition delay time(ID)and the flame lift-off length(LOL).Results show that at low oxygen mole fraction,the predicted ID and LOL from TPDF method agree better with experimental data than that from WSR model.This is because of the advantage of TPDF model in solving the small turbulent fluctuations in temperature and composition.Then the influence of initial conditions on the ID,flame LOL,quasi-steady state flame structure and flame development process are systematically analyzed.At different initial temperatures,or oxygen mole fractions,high-temperature kernels first appear in fuel-rich region.By decreasing the initial ambient temperature,the reactivity of the mixture with high equivalence ratio is declined.As a result,the ignition of mixture is delayed,and the mixing time for fuel and air gets longer.Decreasing the initial temperature and oxygen mole fraction,the ID is further delayed.Mixing between fuel and air is much better.The concentration of the mixture ignited first becomes leaner.The inadequate oxygen is consumed during the combustion process,resulting the less reactive mixture in fuelricher regions.
Transported probability density function(TPDF)model and well stirred reactor(WSR)model were applied for the simulation of n-heptane spray flame under different initial temperature,oxygen mole fraction conditions.The predicted results from TPDF and WSR were compared to the available experimental data from engine combustion network(ECN)in terms of the ignition delay time(ID)and the flame lift-off length(LOL).Results show that at low oxygen mole fraction,the predicted ID and LOL from TPDF method agree better with experimental data than that from WSR model.This is because of the advantage of TPDF model in solving the small turbulent fluctuations in temperature and composition.Then the influence of initial conditions on the ID,flame LOL,quasi-steady state flame structure and flame development process are systematically analyzed.At different initial temperatures,or oxygen mole fractions,high-temperature kernels first appear in fuel-rich region.By decreasing the initial ambient temperature,the reactivity of the mixture with high equivalence ratio is declined.As a result,the ignition of mixture is delayed,and the mixing time for fuel and air gets longer.Decreasing the initial temperature and oxygen mole fraction,the ID is further delayed.Mixing between fuel and air is much better.The concentration of the mixture ignited first becomes leaner.The inadequate oxygen is consumed during the combustion process,resulting the less reactive mixture in fuelricher regions.
引文
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[2]周磊,解茂昭,罗开红,等.代数亚格子湍动能模型及非黏性亚格子应力模型在喷雾大涡模拟中的应用[J].内燃机学报,2013,31(3):215-221.
[3]Zhang Y Z,Kung E H,Haworth D C.A PDF method for multidimensional modeling of HCCI engine combustion:Effects of turbulence/chemistry interactions on ignition timing and emissions[J].Proceedings of the Combustion Institute,2005,30(2):2763-2771.
[4]Wehrfritz A,Kaario O,Vuorinen V,et al.Large eddy simulation of n-dodecane spray flames using flamelet generated manifolds[J].Combustion and Flame,2016,167(1):113-131.
[5]Bottone F,Kronenburg A,Gosman D,et al.The numerical simulation of diesel spray combustion with LES-CMC[J].Flow Turbulence and Combustion,2012,89(4):651-673.
[6]Zhou L,Lu Z,Ren Z,et al.Numerical analysis of ignition and flame stabilization in an n-heptane spray flame[J].International Journal of Heat and Mass Transfer,2015,88(1):565-571.
[7]Jaishree J.Lagrangian and Eulerian probability density function methods for turbulent reacting flows[D].USA:Mechanical Engineering,Pennsylvania State University,2011.
[8]Mehta R S.Detailed modeling of soot formation and turbulence radiation interactions in turbulent jet flames[D].USA:Mechanical Engineering,Pennsylvania State University,2008.
[9]王海峰,陈义良,刘明候.湍流扩散燃烧的数值研究--PDF方法和火焰面模型的性能比较[J].工程热物理学报,2005,26(S1):241-244.
[10]朱旻明,陈义良,吕柳艳,等.湍流射流火焰的PDF模拟及误差分析[J].中国科学技术大学学报,2004,34(2):176-182.
[11]Bhattacharjee S,Haworth D C.Simulations of transient n-heptane and n-dodecane spray flames under enginerelevant conditions using a transported PDF method[J].Combustion and Flame,2013,160(10):2083-2102.
[12]Janicka J,Peters N.Prediction of turbulent jet diffusion flame lift-off using a PDF transport equation[J].Symposium(International)on Combustion,1982,19(1):367-374.
[13]Pei Y,Hawkes E R,Kook S,et al.Modelling ndodecane spray and combustion with the transported probability density function method[J].Combustion and Flame,2015,162(5):2006-2019.
[14]Nguyen,Thong V,Pope S B.Monte carlo calculations of turbulent diffusion flames[J].Combustion Science and Technology,1984,1(1):13-45.
[15]Patel A,Kong S,Reitz R D.Development and validation of a reduced reaction mechanism for HCCI engine simulations[C]//SAE Paper.Detroit,Michigan,USA,2004,2004-01-0558
[16]Pickett L M.Engine combustion network[EB/OL].http://www.sandia.gov/ecn/,2012.
[17]Benajes J,Payri R,Bardi M,et al.Experimental characterization of diesel ignition and lift-off length using a single-hole ECN injector[J].Applied Thermal Engineering,2013,58(1/2):554-563.
[18]Bekdemir C,Somers L M T,de Goey L P H,et al.Predicting diesel combustion characteristics with largeeddy simulations including tabulated chemical kinetics[J].Proceedings of the Combustion Institute,2013,34(1):3067-3074.