Effects of Ambient Temperature and Oxygen Concentration on Diesel Spray Combustion Using a Single-Nozzle Injector in a Constant Volume Combustion Chamber

Abstract

This work investigates the effects of ambient conditions on diesel spray combustion in an optically accessible, constant volume chamber using a single-nozzle fuel injector. The ambient O₂ concentration was varied between five discrete values from 10% to 21% and three different ambient temperatures (800 K, 1000 K, and 1200 K). These conditions simulate different exhaust gas recirculation (EGR) levels and ambient temperatures in diesel engines. Both conventional diesel combustion and low temperature combustion (LTC) modes were observed under these conditions. A transient analysis and a quasi-steady state analysis are employed in this article. The transient analysis focuses on the flame development from beginning to the end, illustrating how the flame structure changes during this process; the quasi-steady state analysis focuses on the stable flame structure. The transient analysis was conducted using high-speed imaging of both OH* chemiluminescence and natural luminosity (NL). In addition, three different images were acquired using an ICCD camera, corresponding to OH* chemiluminescence, narrow-band flame emission at 430 nm (Band A) and at 470 nm (Band B), and were used to investigate the quasi-steady state combustion process. From the transient analysis, it was found that the NL signal becomes stronger and confined to narrow regions when the temperature and O₂ concentration increase during the development of flame. The OH* intensity is much lower for the 10% ambient O₂ and 800 K conditions compared to the higher temperatures and O₂ levels. This implies the occurrence of LTC under these conditions. Results from the quasi-steady combustion stage indicate that high-temperature reactions effectively oxidize the soot in the downstream locations where only OH* signal is observed. In addition, an area was calculated for each spectral region, and results show that the area of Band A and Band B emissions in these images is larger than the area of OH* emissions at the lower O₂ concentrations while the area of OH* emission is larger than the area of Band A and Band B emissions at higher O₂ concentrations, for a given ambient temperature. Moreover, the mixture stoichiometry was analyzed using a reformulated definition of excess air ratio for diluted combustion, and this shows that more mixing is required to achieve complete combustion for low ambient oxygen concentration conditions where longer and wider flames are observed. This observation is also verified by the flame length estimated from the NL images.

Keywords:

• Diesel engine
• Excess air ratio
• Low-temperature combustion
• Natural luminosity
• OH* chemiluminescence
• Spray combustion

ACKNOWLEDGMENTS

This material is based on work supported by, or in part by, the Faculty Research and Professional Development (FRPD) Fund from the North Carolina State University, the Natural Science Foundation under Grant No. CBET-0854174, and the U.S. Army Research Laboratory and the U.S. Army Research Office under grant W911NF-10-1-0118. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the funding agencies.

Notes

*Molecular weight for air.

*Note: (I: the measurable intensity; I₀: the incident intensity).

*Note: (I: the measurable intensity; I₀: the incident intensity).

*The gain range for Andor ICCD camera is from 0 to 255 and the gain range for Kodak analyzer is from 0 to 1.