An analysis of the structure of an n-dodecane spray flame using TPDF modelling
An analysis of the structure of an n-dodecane spray flame using TPDF modelling
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DOI：
10.1016/j.combustflame.2015.11.034
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发表时间：
2016-06
影响因子：
4.4
通讯作者：
Y. Pei;E. Hawkes;M. Bolla;S. Kook;G. Goldin;Yue Yang;S. Pope;S. Som
中科院分区：
文献类型：
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作者：
Y. Pei;E. Hawkes;M. Bolla;S. Kook;G. Goldin;Yue Yang;S. Pope;S. Som
为了了解柴油机的点火和燃烧行为，本研究调查了几个方面的点火和燃烧的ann-dodecane喷雾在一个高压，高温室，称为喷雾A，使用的数据从建模使用的传输概率密度函数（TPDF）的方法。该模型已全面验证了良好的协议，在我们以前的工作对所有可用的实验数据，包括混合分数和速度场在非反应的情况下，和火焰升离长度和点火延迟反应的情况下。这种良好的一致性鼓励进一步研究的数值模型的结果，以帮助了解这种火焰的结构，这有助于补充实验信息，这是非常有限的，由于困难的实验条件下，这种火焰存在。例如，局部混合分数、温度、速度梯度等的定量实验测量在反应情况下尚不可能。模型结果的分析表明，两个阶段的点火被认为是发生在整个环境温度条件下：第一阶段是迅速启动的贫侧的温度高，并依次移动到更丰富，更冷的条件。第一阶段是非常有弹性的湍流，发生在一个非常低的达姆科勒数的区域。点火的第二阶段首先发生在燃料射流头部后面的区域中的富混合物中，在该区域中混合物梯度低，并且似乎受到湍流的强烈影响。相对于均相反应器，它是延迟的贫侧，但在富侧提前，这表明夹带和混合从早期点燃贫区域进入更丰富的混合物是一个重要的减速剂的点火过程。第二阶段的点火前传播在非常高的速度最初，这表明它是一个顺序的点火移动根据梯度的点火延迟和/或停留时间。然而，火焰在低得多的速度的区域中的贫侧上稳定，其中湍流速度波动足够高，使得湍流传输影响传播。它稳定在一个区域的低Damköhler数，这意味着竞争的化学与微观混合也可能参与稳定。的稳定机制进行了调查的运输预算的分析，显示火焰是稳定的自燃，但缓和湍流扩散。火焰指数的进一步分析支持这种稳定机制，并证明在同一火焰中同时存在非预混和预混燃烧模式。流场分析还表明，局部卷吸和膨胀是火焰底部附近的重要流动特征。
With a view to understanding ignition and combustion behaviours in diesel engines, this study investigates several aspects of ignition and combustion of ann-dodecane spray in a high pressure, high temperature chamber, known as Spray A, using data resulting from modelling using the transported probability density function (TPDF) method. The model has been validated comprehensively with good to excellent agreement in our previous work against all available experimental data including for mixture-fraction and velocity fields in non-reacting cases, and flame lift-off length and ignition delay in reacting cases. This good agreement encourages further investigation of the numerical model results to help understand the structure of this flame, which serves to complement the experimental information that is available, which is very limited due to the difficult experimental conditions in which this flame exists. For example, quantitative experimental measurements of local mixture-fraction, temperature, velocity gradients, etc. are not yet possible in reacting cases. Analysis of the model results shows that two-stage ignition is found to occur across the ambient temperature conditions considered: the first stage is rapidly initiated on the lean side where temperatures are high and sequentially moves to richer, cooler conditions. The first stage is extremely resilient to turbulence, occurring in a region of very low Damköhler number. The second stage of ignition occurs first in rich mixtures in a region behind the head of the fuel jet where mixture gradients are low, and appears to be influenced strongly by turbulence. Relative to a homogeneous reactor, it is delayed on the lean side but advanced on the rich side, suggesting entrainment and mixing from the early igniting lean regions into richer mixtures is an important moderator of the ignition process. The second-stage ignition front propagates at very high velocities initially, suggesting it is a sequential ignition moving according to gradients of ignition delay and/or residence time. The flame stabilises however on the lean side in a region of much lower velocity, where turbulent velocity fluctuations are sufficiently high such that turbulent transport influences the propagation. It stabilises in a region of low Damköhler number which implies that a competition of chemistry versus micro-mixing might also be involved in stabilisation. The stabilisation mechanism is investigated by an analysis of the transport budgets, showing the flame is stabilised by autoignition but moderated by turbulent diffusion. Further analysis of the flame index supports this stabilisation mechanism, and demonstrates the simultaneous existence of non-premixed and premixed combustion modes in the same flame. Analysis of the flow fields also reveals that local entrainment and dilatation are important flow features near the flame base.