Simultaneous Formaldehyde PLIF and High-Speed Schlieren Imaging for Visualization in High-Pressure Spray Flames (Proceedings of the Combustion Institute, Volume 35, Issue 3, 2015, Pages 3167-3174, DOI for Paper)

The high-speed movies available in this page are intended to provide support to a manuscript published at the Proceedings of the Combustion Institute entitled “Simultaneous formaldehyde PLIF and high-speed schlieren imaging for visualization in high-pressure spray flames” by Skeen et al. (2014). The operating conditions for these movies are the Spray A conditions unless specified otherwise, using ECN Spray A injector: 210370. The axial and radial distances are given in millimeters and the times are with respect to the start of injection and are given in microseconds (µs).

Schlieren imaging is based on the principle that gradients in temperature and/or composition can be visualized due to differences in refractive index. For example, the boundary between a vaporizing diesel spray and the ambient gases can be resolved because the vaporizing fuel and ambient gases have different refractive indices and the vaporizing fuel cools the surrounding gases at its boundary resulting in a temperature gradient at the interface. Similarly, temperature changes that occur during low- and high-temperature ignition result in refractive index gradients discernable by this technique.

In previous experiments, the low-temperature ignition in a diesel spray flame was observed as a local “softening” of refractive index gradients near the head of the spray. This phenomenon was attributed to the consumption of fuel vapor resulting in the production of intermediate species and a modest amount of heat release that increased the local temperature to a value closer to that of the ambient gases. Consequently, the local refractive index was closer to that of the surrounding gases and the schlieren effect was less pronounced. Visualization of the high-temperature ignition event was then easily discerned as the steep temperature gradients returned. As formaldehyde (CH₂O) has been observed by planar laser-induced fluorescence (PLIF) imaging during the low-temperature ignition event, it was supposed that the softening of refractive index gradients in the schlieren imaging would be coincident with regions of formaldehyde and that the formaldehyde would be consumed following high-temperature ignition.

The high-speed schlieren movies were acquired at 150,000 frames per second with a 2 µs exposure time and a resolution of 0.21 mm/pixel. A high-intensity, short-pulse (1 µs) LED was used for illumination. Formaldehyde PLIF was excited with a 355 nm laser sheet and images were acquired at select timings with an intensified CCD camera having a 250 ns gate time.

 

Corresponding  Figure(PROCI-D-13-01117)	Raw  Schlieren	Normalized  Schlieren	Time-derivative Schlieren	Normalized Schlieren + Formaldehyde PLIF
Fig. 3 (left panels)	Non-reacting	Non-reacting	Non-reacting	—
Fig. 3 (right panels)	Reacting	Reacting	Reacting	Reacting
Fig. 4 (PLIF 190 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 4 (PLIF 240 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 4 (PLIF 290 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 4 (PLIF 340 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 4 (PLIF 490 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 4 (PLIF 390 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 5 (PLIF 1690 µs)	Reacting	Reacting	Reacting	Reacting
Fig. 5 (PLIF 2090 µs)	Reacting	Reacting	Reacting	Reacting