Direct numerical simulations of ignition of a lean n-heptane/air mixture with temperature and composition inhomogeneities relevant to HCCI and SCCI combustion

Abstract

The effects of temperature and composition stratifications on the ignition of a lean n-heptane/air mixture at three initial mean temperatures under elevated pressure are investigated using direct numerical simulations (DNSs) with a 58-species reduced mechanism. Two-dimensional DNSs are performed by varying several key parameters: initial mean temperature, T 0 , and the variance of temperature and equivalence ratio (T ′ and φ ′) with different T − φ correlations. It is found that for cases with φ ′ only, the overall combustion occurs more quickly and the mean heat release rate (HRR) increases more slowly with increasing φ ′ regardless of T 0. For cases with T ′ only, however, the overall combustion is retarded/advanced in time with increasing T ′ for low/high T 0 relative to the negative-temperature coefficient (NTC) regime resulting from a longer/shorter overall ignition delay of the mixture. For cases with uncorrelated T − φ fields, the mean HRR is more distributed over time compared to the corresponding cases with T ′ or φ ′ only. For negatively-correlated cases, however, the temporal evolution of the overall combustion exhibits quite non-monotonic behavior with increasing T ′ and φ ′ depending on T 0. All of these characteristics are found to be primarily related to the 0-D ignition delays of initial mixtures, the relative timescales between 0-D ignition delay and turbulence, and the dominance of the deflagration mode during the ignition. These results suggest that an appropriate combination of T ′ and φ ′ together with a well-prepared T −φ distribution can alleviate an excessive pressurerise rate (PRR) and control ignition-timing in homogeneous charge compression-ignition (HCCI) combustion. In addition, critical species and reactions for the ignition of n-heptane/air mixture through the whole ignition process are estimated by comparing the temporal evolution of the mean mass fractions of important species with the overall reaction pathways of n-heptane oxidation mechanism. The chemical explosive mode analysis (CEMA) verifies the important species and reactions for the ignition at different locations and times by evaluating the explosive index (EI) of species and the participation index (PI) of reactions.

Prediction of Ignition Regimes in DME/Air Mixtures with Temperature and Concentration Fluctuations

“…Early direct injection may result in uncorrelated T-φ fields because of turbulent mixing and heat transfer. Late direct injection may produce negatively-correlated T-φ fields attributed to the evaporative cooling of fuel vaporization and incomplete mixing [23,27,[29][30][31]68]. As a result, the temperature and equivalence ratio fields have a comparable integral length scale, l T ∼ l φ .…”

Direct numerical simulation of n-heptane/air auto-ignition with thermal and charge stratifications under partially-premixed charge compression ignition (PCCI) engine related conditions

“…For negative T-φ correlation, the ignition mode is similar to HCCI combustion. A further study on a lean n-heptane/air mixture with temperature and charge stratifications shows that with a suitable T-φ correlation one can prevent the excessive PRR and control the ignition timing [31]. With higher initial temperature stratification, the contribution of flame propagation to the total heat release is higher.…”

“…8 shows the budget terms of reaction rate of CO and diffusion rate of CO at LTHR stage (0.16 ms) and HTHR stage (0.41 ms) for case 1. The two budget terms are defined as: [31].…”

“…Both experiments and numerical simulations have shown that fuel properties have a large effect on the onset of ignition and the subsequent combustion process in PCCI engines [31]- [35]. For example, the transition from auto-ignition dominant regime to flame propagation dominant regime depends on the fuels in PCCI engines [26].…”

“…Using two-stage ignition fuel n-heptane, it can be found that initial mean temperature(T 0 )and temperature fluctuation(T')have a significant influence on the spatial reaction zone structures. If T 0 is within the NTC region, the molecular diffusion effect is rather minor, which is different from the single-stage ignition fuel [25] [26][29] [31]. Meantime, the 3 two-stage ignition fuel is sensitive to local equivalence ratio distribution such that the engine load can be extended to higher, while the ignition timing is relatively insensitive to the local equivalence ratio, which renders charge stratification of a single-stage fuel to be less effective for creating a staged combustion event [32] [34].…”