Experimental and Kinetic Study on the Critical Ambient Density for Auto-ignition of the Diesel Spray under Cold-start Conditions

ABSTRACT

In the present study, the effect of ambient density (${\rho }_a$) on spray two-stage auto-ignition characteristics under cold-start condition was investigated using both optical and numerical methods. By the high-speed Shadowgraph, the LTI (low-temperature ignition delay) and HTI (high-temperature ignition delay) were obtained and followed an increasing trend with the decreasing ${\rho }_a$. Both experimental and simulated results show that HTI is more sensitive to the ${\rho }_a$ than LTI, because the high-temperature chemistry is more suppressed at higher ${\rho }_a\hspace{0.17em}$due to the increased scalar dissipation rate . Moreover, the simulation showed that the high-temperature ignition emerges at the same mixture fraction of 0.065, regardless of changes of ${\rho }_a$, which indicates that ${\rho }_a$ has little effect on the ignition site in the mixture fraction space. Further kinetic analysis found that as the ${\rho }_a$ decreases, the O₂ concentration reduces and the reaction rates of both low- and high-temperature reactions reduce, leading to the increased LTI and HTI. When the HTI increases to such a long time that no ignitable mixture exists in the space due to the continuing diffusion after the end of injection, the spray misfire happens.

KEYWORDS:

• Cold start
• ambient density
• cool flame
• chemical kinetics
• two-stage ignition

Acknowledgments

This study is based upon work supported by National Natural Science Foundation of China [Grant No.:51976011] and China Postdoctoral Science Foundation (Grant No.: 2020M680378).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the China Postdoctoral Science Foundation [2020M680378]; National Natural Science Foundation of China [51976011].