TY - JOUR
T1 - Effects of oxygen concentrations on the ignition and quasi-steady processes of n-heptane spray flames using large eddy simulation
AU - Wei, Haiqiao
AU - Zhao, Wanhui
AU - Lu, Zhen
AU - Zhou, Lei
N1 - KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The work is supported by National Natural Science Foundation of China (Grant No. 91741119, 51606133, 91641203) and Marine Low-Speed Engine Project (Phase I). The work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1(A).
PY - 2018/12/26
Y1 - 2018/12/26
N2 - Exhaust Gas Recirculation (EGR) is a frequently used technique to reduce the production of NOx. The effect of EGR on the early flame evolution, two-stage ignition process and spray flame structures for n-heptane spray flames are investigated using large eddy simulation. The two-stage ignition process is identified based on the formation of key species and early heat release process. Results demonstrate that a longer ignition delay (ID) and flame lift-off length (LOL) under lower oxygen concentration conditions could increase the mixing time for fuel and air. However, the first-stage ignition still initiates in fuel-richer regions for the cases with higher EGR rates due to the lack of oxygen. In contrast, compared to the case with the same initial oxygen content but at a higher gas temperature of 1000 K, the first-stage ignition moves to stoichiometric mixture fraction regions at 900 K. The combustion mode analysis based on hydroxyl and formaldehyde is conducted to distinguish between the low- and high-temperature combustion regions. Most importantly, to study the stabilization mechanism, the chemical explosive mode analysis (CEMA) is conducted based on analysis on the local flow time scale and the chemical time scale. During the early stage of ignition, a balance between reaction and mixing implies that cool flame propagates from the ignition spots through the entire flow field. And during the quasi-steady state, autoignition plays a dominant role.
AB - Exhaust Gas Recirculation (EGR) is a frequently used technique to reduce the production of NOx. The effect of EGR on the early flame evolution, two-stage ignition process and spray flame structures for n-heptane spray flames are investigated using large eddy simulation. The two-stage ignition process is identified based on the formation of key species and early heat release process. Results demonstrate that a longer ignition delay (ID) and flame lift-off length (LOL) under lower oxygen concentration conditions could increase the mixing time for fuel and air. However, the first-stage ignition still initiates in fuel-richer regions for the cases with higher EGR rates due to the lack of oxygen. In contrast, compared to the case with the same initial oxygen content but at a higher gas temperature of 1000 K, the first-stage ignition moves to stoichiometric mixture fraction regions at 900 K. The combustion mode analysis based on hydroxyl and formaldehyde is conducted to distinguish between the low- and high-temperature combustion regions. Most importantly, to study the stabilization mechanism, the chemical explosive mode analysis (CEMA) is conducted based on analysis on the local flow time scale and the chemical time scale. During the early stage of ignition, a balance between reaction and mixing implies that cool flame propagates from the ignition spots through the entire flow field. And during the quasi-steady state, autoignition plays a dominant role.
UR - http://hdl.handle.net/10754/631590
UR - https://www.sciencedirect.com/science/article/pii/S0016236118321653
UR - http://www.scopus.com/inward/record.url?scp=85059015835&partnerID=8YFLogxK
U2 - 10.1016/j.fuel.2018.12.097
DO - 10.1016/j.fuel.2018.12.097
M3 - Article
AN - SCOPUS:85059015835
VL - 241
SP - 786
EP - 801
JO - Fuel
JF - Fuel
SN - 0016-2361
ER -