On the crossover temperature and lower turnover state in the NTC regime
On the crossover temperature and lower turnover state in the NTC regime
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DOI：
10.1016/j.proci.2016.05.046
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发表时间：
2017-01-01
影响因子：
3.4
通讯作者：
Law, Chung K.
中科院分区：
文献类型：
--
作者：
Ji, Weiqi;Zhao, Peng;Law, Chung K.
本文研究了NTC曲线低温区下驻点的热力学和化学动力学行为，即NTC曲线的低周转态，点火延迟期在该区域出现局部极小值。结果表明，点火延迟期随不同压力下的周转温度而变化，周转温度T下限和初始压力P-0之间存在一定的相关性，P-0与EXP(-E-a/T-下限)相似，可以用另一种阿累尼乌斯关系来关联。对于表现NTC的典型正构烷烃，即正丁烷和正庚烷，利用详细的机理对这两种行为进行了系统的研究。研究发现，在不同压力下的较低周转状态下，第一阶段和第二阶段点火延迟期大致相等，总点火延迟期与第一阶段点火延迟期具有相似的温度依赖关系。为了进一步描述第二点火阶段，研究了第一阶段末期的冷焰温升，发现随着初始温度的升高，冷焰温升呈线性下降。然后，在冷火焰温升消失的状态下定义交叉温度。这种状态可以与一个常数a相关联，该常数表示低温化学产生的OH与总燃料消耗之间的比率。结果表明，第二阶段着火延迟与H_2O_2分解的反应速率常数有很好的相关性。在此基础上推导出了两个阶段的动力学解析表达式，较好地再现了不同压力下较低的周转温度，为进一步研究周转状态下的两阶段着火提供了新的思路。将计算结果与文献中正庚烷点火延迟时间的实验数据进行了比较，结果吻合较好。(C)2016年，由燃烧研究所提供。由爱思唯尔公司出版。
This work investigates the thermodynamic and chemical kinetic behavior of the lower stationary point, denoted as the lower turnover state, of the NTC curve near the low temperature regime, where the ignition delay exhibits a local minimum. It is found that the ignition delay varies Arrheniusly with the turnover tem-perature under various pressures, and that the turnover temperature, T lower, and the initial pressure, P-0, are related and can be correlated by another Arrhenius dependence, as P-0 similar to exp(-E-a/T-lower). These two behaviors have been systematically investigated using detailed mechanisms for typical n -alkanes exhibiting NTC, namely n -butane and n -heptane. It is found that the first and second stage ignition delays are approximately equal to each other at the lower turnover states under various pressures, such that the total ignition delay shares similar temperature dependence with the first stage ignition delay. To further characterize the second ignition stage, the cool flame temperature rise at the end of the first stage is investigated and shown to de-crease linearly with increasing initial temperature. A crossover temperature is then defined at the state where the cool flame temperature rise disappears. This state can be correlated with a constant a, which denotes the ratio between OH production from the low-temperature chemistry and the total fuel consumption. It is shown that the second stage ignition delay correlates well with the reaction rate constant for the H2O2 de-composition. Analytical expressions are then derived based on these kinetic insights on the two stages, which well reproduce the lower turnover temperature under different pressures and provide further insights into the two-stage ignition at the turnover states. Comparison of the present results with the literature experimental data of the n-heptane ignition delay time shows good agreement. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.