Impact of location and flow rate oscillation of the pilot jet on the flow structures in swirling premixed flames

Abstract

The paper investigates the swirling flow structures in an unconfined coannular swirl burner under different conditions, namely, with or without heat release, two locations of the pilot jet, and constant or pulsating mass flow rate in the pilot jet. Six turbulent premixed swirling flames as well as two isothermal turbulent swirling flow cases for reference are studied with the aid of large-eddy simulation. Detailed comparisons with experimental data are performed for velocity statistics, and generally good agreement is achieved. Furthermore, coherent structures are analyzed, and local power spectra are computed. In the nonreactive case with retracted pilot jet, a very strong precessing vortex core (PVC) is observed in both the numerical result and the experiment. The PVC is weak in the nonretracted case. In the reactive cases in which the whole central recirculation zone is enclosed in the high-temperature postflame region, the PVC is almost completely suppressed. In pulsating reactive jets, the vortex structures exhibit a ring shape but not the spiral shape shown in the reactive cases with constant mass flow rate in the pilot jet. The flame response to a sinusoidal variation of the mass flow rate in the pilot jet is studied, and an attempt to quantitatively compare the flame transfer function is performed as well.

Keywords:

• large-eddy simulation
• dynamic thickened-flame model
• flame transfer function
• pulsating flame
• precessing vortex core

Acknowledgements

The support of the German Research Foundation (DFG) through the Collaborative Research Center CRC 606 “Unsteady Combustion” is gratefully acknowledged. The computations were performed on both the HP-XC cluster of SSCK Karlsruhe and the SGI Altix 4700 cluster of ZIH Dresden. The authors thank Dr. Büchner and his coworkers (Karlsruhe University) for providing their experimental data in electronic form.