Numerical Simulations on Autoignition Propagation Modes under Reciprocating Engine-relevant Conditions. Issue 13 (3rd October 2021) - Pan, Jiaying
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.