Laser-diagnostic and numerical study of strongly swirling natural gas flames

Abstract

This study is focused on the experimental and numerical investigation of the reacting flow field of a strongly swirling, unconfined 150-kW natural gas flame. This work is embedded in the TECFLAM swril burner project in which five research groups currently are involved with various tasks. The final goal is the accurate prediction of complex combustion systems with advanced models based on a complete data set of the governing turbulent flow and reaction field. At this point, attention is paid to the influence of the turbulence structure on the mixing process. A two-component Laser-Doppler Velocimetry (LDV) system is used for the determination of mean and fluctuating velocity components. Two-dimensional temperature fields and fuel-gas distributions are measured via Rayleigh scattering. Three-dimensional temperature distributions and flamefront surfaces are obtained via simultaneous measurements of Rayleight scattering and OH Laser-Induced Fluorescence (LIF) in three adjacent planes. An advanced numerical simulation, based on a nonlinear second moment closure is presented to be in good agreement with experimental data. The mean values of axial and circumferential velocity reconfirm a substantial reverse flow surrounded by a curved shear layer. High strain rates yield an intensive turbulent mixing process. It can be concluded from measured temperature fluctuations in this region that reaction takes place in this inner part of the shear layer. The entraining ambient air cannot penetrate through this highly strained area, thus isolating the hot core and providing a stabilizing mechanism to the flame. The 3-D time-resolved measurements of the flamefront give evidence that its structure is disconnected for strongly swirling flames.