ABSTRACT

Previous studies on autoignition propagation modes were often performed based on constant-volume configuration. However, the reactant mixture in reciprocating engines always experiences significant variable volume and ever-changing thermodynamic conditions, which may affect autoignition initiation and subsequent development during knocking combustion. In this study, the autoignition reaction wave propagation induced by thermal stratifications was investigated numerically, with addressing the role of reciprocating piston motion and primary flame compression. Compression heating was considered to emulate the compression and expansion caused by reciprocating piston motion, and different combustion boundary conditions and fuel properties were performed to investigate the impact on autoignition propagation modes. The results of hydrogen cases show that similar to constant-volume configurations, various autoignition propagation modes (including thermal explosion, detonation, and deflagration) can be observed. However, the normalized temperature gradients demarcating different autoignition propagation modes change significantly under variable thermodynamic conditions of reciprocating engines. Such an influence can also be embodied in engine combustion phasing. It is found that the intense autoignition involving detonation development prefers to occurring around the Top Dead Center with higher chemical reactivity and energy density. Furthermore, similar studies were further carried out for isooctane and the significant influence from reciprocating piston motion is still observed. Besides, it is found that almost all the autoignition events induced by thermal stratifications develop into deflagration rather than detonation for isooctane. The underlying reasons can be elucidated through the detonation peninsular diagrams for different fuels.

摘要

以前对自燃传播模式的研究通常是基于恒定体积配置进行的。然而，往复式发动机中的反应混合物总是经历显着的体积变化和不断变化的热力学条件，这可能会影响爆震燃烧过程中的自燃启动和后续发展。在这项研究中，通过数值研究了由热分层引起的自燃反应波传播，并解决了往复活塞运动和初级火焰压缩的作用。压缩加热被认为是模拟由往复活塞运动引起的压缩和膨胀，并使用不同的燃烧边界条件和燃料特性来研究对自燃传播模式的影响。氢气案例的结果表明，类似于恒定体积配置，可以观察到各种自燃传播模式（包括热爆炸、爆轰和爆燃）。然而，在往复式发动机的可变热力学条件下，划分不同自燃传播模式的归一化温度梯度发生了显着变化。这种影响也可以体现在发动机燃烧定相中。发现涉及爆炸发展的强烈自燃更倾向于发生在具有更高化学反应性和能量密度的上死点附近。此外，还对异辛烷进行了类似的研究，仍然观察到活塞往复运动的显着影响。此外，发现几乎所有由热分层引起的自燃事件都发展为爆燃而不是异辛烷的爆炸。通过不同燃料的爆炸半岛图可以阐明根本原因。