Laser-diagnostic and numerical study of strongly swirling natural gas flames
This study is focused on the exptl. 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 swirl 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 detn. of mean and fluctuating velocity components. Two-dimensional temp. fields and fuel-gas distributions are measured via Rayleigh scattering. Three-dimensional temp. distributions and flame front surfaces are obtained via simultaneous measurements of Rayleigh 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 exptl. 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 temp. 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 flame front give evidence that its structure is disconnected for strongly swirling flames.
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