Non-Premixed Swirl Flames

Combustion requires the effective mixing of fuel and oxidizer. For premixed flames, mixing takes place in a separated mixing chamber. This type of burner imposes the danger of a flashback of the flame into the mixing chamber. Another possibility is the mixing of fuel and air within the burner chamber. This prevents the danger of flashbacks, but homogeneous mixing is more difficult to achieve. A method often used in practical burners is the application of the swirling flows to improve mixing.

In the swirl burner natural gas freely propagates into the burner chamber in the axial direction and is surrounded by the combustion air flow which has radial and tangential velocity components (swirl) in addition to the axial flow. The resulting flow field is strongly turbulent. Hot burned gases are transported back to the nozzle by internal recirculation in the flame and thus ensure effective mixing and stable ignition conditions.

Figure 1: laser-based temperature imaging in the swirl flame via Rayleigh scattering

Figure 2: Location of the observed air in the burner. The flame has a thermal power of 150 kW.

This burner was investigated by means of laser spectroscopic imaging methods. As an example the averaged temperature distribution is shown in the right part of the image. On the left there are the temperature distributions obtained from a single laser pulse. The right side shows a temporal average. The method bases on the detection of elastically scattered laser light (Rayleigh scattering). The coldest regions can be found in the swirling combustion air above the nozzle while the hottest zones are located on the burner axis in the inner recirculation zone.

Figure 3: Simultaneous measurement of temperature (Rayleigh scattering), NO and OH (by laser-induced flourescence) in the swirl flame. The flames depicted in horizontal rows were detected simultaneously and thus allow the analysis of concentration correlations.

Additionally, multi-species imaging measurements were carried out in the same burner depicted in fig.2. Here, the temperature was detected via Rayleigh scattering, whilst OH and NO were measured via laser-induced flourescence with excitation at 248 and 226 nm, respectively.

References:

[1] J. Kazenwadel, W. Koban, T. Kunzelmann, and C. Schulz, "Fluorescence imaging of natural gas / air mixing without tracers added," Chem. Phys. Lett. 345, 259-264 (2001).
[2] S. Böckle, J. Kazenwadel, T. Kunzelmann, D.-I. Shin, C. Schulz, and J. Wolfrum, "Simultaneous single-shot laser-based imaging of formaldehyde, OH and temperature in turbulent flames," Proc. Combust. Inst. 28, 279-286 (2000).
[3] S. Böckle, J. Kazenwadel, and C. Schulz, "Laser-diagnostic multi-species imaging in strongly swirling natural gas flames," Appl. Phys. B 71, 741-746 (2000).