Pool Flames - Dynamics of Dissipative Structures

Real-time holographic high-speed interferometry is an optical technique for investigating the dynamics of organized structures in pools flames and none reacting flows.

The beam of a 2 W CW Argon laser Atlanta 5 1 4 . 5 nanometers is split into 2 beams by a beam splitter - on the left. The reflected beam is expanded to a diameter of 25 centimeters. As the test beam, it passes through the flame.

The 2nd beam incidence upon the hologram from the left. It forms the reference beam with a diameter of 8 cm. Behind the hologram the reconstructed comparison beam interferes continuously with the test beam.

The hologram was exposed 1/10th s as a laser power of 100 mW. After development it was repositioned exactly.

Whenever the flame distorts the test beam, a pattern of infinite interferometric fringes appears.

To prevent vibrations of the optical elements they´re placed on a granite bench with a pneumatic suspension. The superimposed images of the pool flames and the simultaneously observed fringe patterns were filmed at 600 pictures per second and an exposure time of 1/2400 s. On the right the high-speed city camera.

The pull flames were adjusted with in a cutout in the granite bench.

This is a methanol pool flame of 4.6 cm diameter.

By copying each frame twelve times, the resulting time delay visualizes the seperate phases of the oscillations of the flame.

More details are revealed by the fringe patterns behind the hologram.

In the high-speed cinematographic shots at 600 f/s and .5 ms exposure of a 10 cm diameter pool flame, they're free from movement blur.

With an inert gas, the thermal boundary layer between gas and air leads to approximately parallel fringes with a concave or convex curvature. Downstream, increased fringe spacings indicate decreased density gradients. Density sinks represent a local increase of the mass density due to exchange processes with the surrounding air. Downstream, the dissipation process increases. In the ascending hot air stream, the thermal boundary layer consists of parallel fringes up to a height of nearly 25 cm. Only farther downstream, the layer becomes strongly wrinkled. Here the number of density sinks is much greater than in the Helium stream because of the cooling process in the hot air. At a short distance above the pool rim, there is a relatively high number of density sinks. Due to the small density gradients, the fringe spacing is relatively large. The thermal boundary layer resolves downstream at a short distance from the pool rim.

The high flame temperatures in the hydrogen pool flame leads to a broad thermal boundary layer with symmetrical constrictions. Near the flame axis, there are singular axial density parcels or pairs of them. Downstream, density parcels are formed even within the boundary layer. The flame can be lowered vertically with respect to the test beam. In this case, the density field from a distance of 26 cm downstream of the pool rim is much broader than upstream with only small constrictions. The thermal boundary layer is only locally folded and contains a large number of density sources and sinks. Many more details can be analyzed by 12-fold frame duplication of the high-frequency shots.

The processes appear delayed 300-fold.

Further phase pictures will also be shown in the following sequences.

If the volume flow rate is larger, circular indentations of the boundary layer develop and the number of density parcels increases. The frequency of the oscillations of the boundary layer also increases.

In all hydrogen flames, no density parcels exist at the pool rim with a pool diameter of 4.6 cm. The thermal boundary layer of the methane pool flame is heavily and unsymmetrically wrinkled. Near the pool rim, density parcels are observed. Moreover axial density parcels develop within the luminous part of the flame. Additional density parcels are formed within the boundary layer which oscillates at a relatively low frequency. From 25 cm downstream, the organized structures are broadened strongly and their dynamics and number increase. Here constrictions of the boundary layer are flatter. With a greater pool diameter, deep constrictions of the boundary layer develop; their frequency decreases. The axial density parcels no longer combine to form pairs. The luminous mushroom of the flame leads to a partial rotation of the density parcels. Downstream, in the upper part of the flame, the density field broadens strongly. The folding of the boundary layer is restricted to only a few positions. The dynamics and the number of the density sinks increase strongly; the sink size decreases. Between the density parcels, new boundary layers develop. The methanol pool flames show no yellow particle emission because no soot is formed. The very weak blue emission caused by the flame radicals has a much lower intensity than that of the green laser light. The thermal boundary layer oscillates mono-periodically; it broadens and constricts strongly. The density parcels near the pool rim travel to the flame axis. The ascension of a single axial density parcel is typical. Downstream, the width of the thermal boundary layer is very narrow and also oscillates mono-periodically. Here are many more density sources and sinks than upstream in the lower part of the flame. The fringe spacing increases conspicuously; this means a decrease in the density gradients. The thermal boundary layer is also broadened strongly and oscillates with a lower frequency than in the smaller methanol flame.

The boundary layer is strongly constricted, but the number of the density sources and sinks is small.

Farther downstream in the flame plume, the boundary layer decays. The total density field is broader than e.g. in the n-hexane flames shown later. Relatively narrow ribbon elements and insular elements are frequent. In the region of the flame neck, the following structures are observed: density parcels near the pool rim, axial density parcels, waves of the thermal boundary layer, a fuel boundary layer, and flame mushrooms. The frequency of the thermal boundary layer is higher than the frequency of the fuel boundary layer. The thermal boundary layer oscillates bi-periodically. The region of the flame plume reveals the following structures: sources and sinks of the mass density, association of several density parcels, insular elements, ribbon elements, waves of the thermal boundary layer, and (only seldom) luminous flame fields.

The thermal boundary layer is folded strongly and oscillates tri-periodically in the flame plume. In the flame neck, many density sources and sinks exist between the density parcels near the pool rim. Here a yellow mushroom develops simultaneously with its characteristic density structure. The thermal boundary layer oscillates tri-periodically even in the flame neck. Downstream, the boundary layer is folded locally in many positions; the incommensurable frequencies decrease. The dynamics and the number of the density sinks increase. The lifetime and the size of the sinks decrease. The number of the insular and ribbon elements increases. In the n-hexane pool flame, one can observe the formation and decay of small soot cloudlets especially well in slow motion.