Autoignition Dynamics of N-dodecane Droplets under Normal Gravity

ABSTRACT

Experimental observations of two-stage autoignition dynamics of fiber-supported normal dodecane droplets in air under normal gravity are presented for a range of pressures and temperatures. High-speed shadowgraph imaging of the autoignition process reveals cool-flame and hot-flame front-formation and propagation dynamics. During two-stage ignition, a cool-flame kernel is first formed below the droplet; it then propagates toward the droplet along the fuel-vapor plume, and subsequently a hot-flame kernel is established behind the cool-flame front which rapidly expands, engulfing the droplet and establishing the classical diffusion flame. Results for the cool-flame and hot-flame kernel locations and their propagation speeds are presented for a range of ambient pressures, varying between normal atmospheric pressure and super-critical pressures and temperatures.

KEYWORDS:

• two-stage ignition
• autoignition
• droplet combustion

Acknowledgments

The assistance of Daniel Gotti, Jay Owens, Vittorio Valletta, Anthony Ogorzaley, and Eric Neumann in the design, fabrication, and operation of the experimental facility is gratefully acknowledged. Vasko Nechev helped with some of the experiments and data reduction. The authors are also grateful to Dr. Meridith B.(Med) Colket for a careful review of this paper and many helpful discussions.

Notes

ⁱ Since the shadowgraph measures the changes in the second derivative of the refractive index, the location of highest rate of heat release, i.e., the flame fronts appear as dark lines in the shadow graphs. The darker the flame front is in the shadowgraph the higher is the heat-release rate at that location.

Additional information

Funding

This work was supported by the NASA Space Life and Physical Sciences Research and Applications Program.