Effects of Fuel Unsaturation on Transient Ignition and Flame Development in Sprays

ABSTRACT

Fundamental studies using homogeneous mixtures in shock tube and rapid compression machine (RCM) have shown that the fuel reactivity and ignition characteristics are significantly affected by the presence and number of double bonds in the fuel molecular structure. However, this phenomenon has not been investigated in turbulent reacting sprays under realistic conditions. In this article, we numerically examine the effects of fuel unsaturation and sensitivity on the transient ignition and flame evolution in n-heptane and 1-heptene sprays in the Sandia constant volume reactor. 3D simulations of two-phase reacting flows are performed using a commercial CFD code, CONVERGE. The ignition and combustion chemistry of n-heptane and 1-heptene is modeled using a reduced CRECK mechanism, which has been previously validated against the shock tube ignition delays and reacting spray measurements. Results indicate that, at intermediate temperatures, the ignition in both homogeneous mixtures and sprays is strongly influenced by fuel unsaturation. First of all, the ignition delay is much higher for 1-heptene compared to that for n-heptane, especially at low temperatures. Secondly, ignition in n-heptane sprays occurs in fuel rich mixtures, and is characterized by a two-stage ignition process. On the other hand, ignition in 1-heptene sprays occurs in lean mixtures. Results also indicate the existence of multiple ignition locations in sprays, which is in contrast to ignition in homogeneous mixtures. The flame evolution in sprays correlates strongly with the ignition kernel evolution. Consequently, the n-heptane spray flame contains two reaction zones, namely, a rich premixed zone (RPZ) and a non-premixed reaction zone (NPZ), while the 1-heptene flame is characterized by three reaction zones, i.e., a lean premixed zone in addition to NPZ and RPZ. The difference is a direct consequence of longer ignition delay in 1-heptene spray compared to that in n-heptane spray, which facilitates increased fuel-air mixing in the former. The flame lift-off location correlates with the ignition delay and ignition kernel location. Thus, at lower temperatures (1000 K and 1100 K), the lift-off length of 1-heptene flame is greater than that of n-heptane flame, which is due to longer ignition delays for 1-heptene sprays. In contrast, at higher temperatures (1200 K) the lift-off length of 1-heptene flame is smaller, which is due to the fact that the ignition kernel and, thus, lean premixed zone are located closer to the injector for 1-heptene spray compared to n-heptane spray.

KEYWORDS:

• Diesel spray ignition
• Fuel unsaturation
• n-Heptane
• 1-Heptene
• Liftoff length
• Spray flame

Acknowledgments

The authors greatly appreciate the help provided by P. K. Senecal and his colleagues at Convergent Science in running the Converge code. The authors also wish to thank the reviewers for the many useful comments, especially those pertaining to the effect of fuel sensitivity on ignition.

Notes

¹ OH distribution has often been used to identify the reaction zone in previous studies.