Ignition characteristics of a temporally evolving n-heptane jet in an iso-octane/air stream under RCCI combustion-relevant conditions

Abstract

The ignition characteristics of a temporally-evolving n-heptane jet in an iso-octane/air stream under reactivity controlled compression ignition (RCCI) combustion-relevant conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species primary reference fuel (PRF)/air reduced mechanism. For the DNSs of RCCI combustion, iso-octane and n-heptane are chosen as two different fuels delivered by the port-fuel and direct-fuel injections, respectively. Therefore, the ignition characteristics of both fuels can be investigated by simulating the ignition of a temporally-evolving n-heptane jet with relative jet velocity, U 0 , within iso-octane/air charge. It is found that the first-stage ignition kernels governed by the low-temperature chemistry first develop primarily within the n-heptane jet near the mixing layer regardless of U 0 , and evolve into low-temperature flames, propagating into relatively fuel-rich mixtures in the n-heptane jet. High-temperature flames also develop first in the n-heptane jet, following the trajectories of low-temperature flames, and then, propagate towards both relatively fuel-lean mixtures of the iso-octane/air charge and fuel-rich mixtures of the n-heptane jet. They keep propagating into fuel-lean mixtures and finally end-gas auto-ignition occurs. It is also found that the first-stage ignition occurs more quickly with increasing U 0 due to enhanced mixing between relatively cold n-heptane jet and hot iso-octane/air charge, and consequently, the second-stage ignition also advances in time with increasing U 0 , which are opposite to the results found in previous DNSs of RCCI combustion. Such ignition characteristics are more likely to prolong the overall combustion duration and reduce the peak of heat release rate with increasing U 0 . In addition, chemical 1 explosive mode analysis (CEMA) identifies important variables and reactions for the low-, intermediate-, and high-temperature chemistries under such RCCI conditions.

Effects of Turbulence and Temperature Fluctuations on Knock Development in an Ethanol/Air Mixture

“…Most previous computational studies are largely limited to a simplified 1-D detonation configuration, thus lacking in the realistic multi-dimensional effects associated with the interactions of temperature and velocity fluctuations that may involve multiple detonation wave interactions (Robert et al 2015a, b). Multi-dimensional DNS studies have mostly been undertaken on the autoignition characteristics only, without realization of detonation developments (Sankaran et al 2005;Chen et al 2006;Hawkes et al 2006;Bansal and Im 2011;Gupta et al 2011;Yoo et al 2011;Yu and Bai 2013;El-Asrag andJu 2013, 2014;Bhagatwala et al 2014Bhagatwala et al , 2015Bansal et al 2015;Yoo et al 2013;Luong et al 2013;Kim et al 2015;Luong et al 2017;Yu et al 2019). In these studies, a key issue was to identify the ignition characteristics, which was further developed into the ignition regime diagram (Im et al 2015;Pal et al 2017a) to predict the occurrence of strong and weak ignition modes in the presence of temperature and turbulence fluctuations.…”

Knock propensity in a thermally inhomogeneous DME/air mixture: a DNS study

“…The initial mean temperature, the level of temperature fluctuations, and its characteristic length scale are varied to identify the developing detonation regime in a thermally inhomogeneous DME/air mixture. All the simulations are performed with initial uniform equivalence ratio, 𝜙 0 , and pressure, 𝑃 0 , of 1 and 40 atm, respectively, which represent the near top-dead-center conditions of an IC engine [1,2,2,21,[53][54][55][56][57][58][59][60][61]. As shown in Fig.…”

Direct numerical simulations of turbulent reacting flows with shock waves and stiff chemistry using many-core/GPU acceleration

“…To mimic pressure rise due to piston motion and front propagation, an inert mass source term is added to the governing equations (1a-1d), following the previous approaches [10,20,[57][58][59]. A schematic of the numerical configuration used for this case is shown in Figure 5.…”

“…There is currently only a limited fundamental understanding of the nature of combustion in an auto-ignitive mixture that is conducive to both spontaneous ignition and diffusion driven flame propagation. Several previous experimental and numerical studies have focused on characterizing the nature of combustion and delineating it into multiple regimes based on the propagation speed of the autoignition front and also based on the impact of turbulence and stratification on such a front [11][12][13][14][15][16][17][18][19][20][21]. Fundamental numerical simulations that can capture the nature and characteristics of autoignition in stratified reactive mixtures are key to improving and applying the appropriate combustion models in device-scale simulations.…”