The effects of turbulence on the lean blowout mechanisms of bluff-body flames

Abstract

The lean blowout mechanisms of premixed bluff-body flames are experimentally investigated at various turbulence intensities. Turbulence levels are varied using a novel turbulence generator, which combines static grid and fluidic jet impingement techniques. Three different turbulence levels are probed to study their effects on lean blowout. The three conditions span across the combustion regime diagram, from flamelets to broken reactions. For all three turbulence levels, the lean blowout process is induced through controlled fuel flow rate reduction. The transient blowout process is captured using three simultaneous high-speed diagnostic systems: particle image velocimetry (PIV), stereoscopic PIV (SPIV), and C2 * /CH * species measurements. The two PIV systems are used to resolve the instantaneous velocity and vorticity fields, and the C2 * /CH * species diagnostics allow for global equivalence ratios to be evaluated throughout the duration of blowout. The results show that the dynamics of lean blowout vary with turbulence intensity. At low turbulence levels, the flame experiences a global effect where the flame boundary interacts with the shear layer vorticity. This imparts high strain rates along the length of the flame, leading to blowout. As turbulence levels increase, the blowout mechanism becomes less dependent on flame and shear layer interactions and more driven by flame-turbulence interactions. At high turbulence conditions, flame-eddy interactions within the freestream augment flame stretching via increased flame straining and small-scale flame corrugations. Increased flame stretching disrupts the flame stabilization process, and ultimately results in blowout.

Turbulence-Driven Blowout Instabilities of Premixed Bluff-Body Flames

“…In this study, the equivalence ratio is not a primary parameter of interest; however, temporal equivalence ratio curves were evaluated for each turbulence condition to verify the blowout equivalence ratio, which is presented in Table 1 and will be discussed later. Additional details of the methodology as well as the temporal curves can be found in a previous study (Morales et al 2020b).…”

Four-Line C2*/CH* Optical Sensor for Chemiluminescence Based Imaging of Flame Stoichiometry

“…For instance, the intensity ratio of C 2 */CH* has been shown to produce a monotonic relationship with Φ between 0.6 and 1.4 for various hydrocarbon–air flames [ 11 , 25 , 26 , 27 ]. The developed calibration curves of C 2 */CH* vs. Φ can then be used to track local or transient flame dynamics, such as heat release-rate fluctuations or lean blowout [ 28 , 29 ]. OH* and CH* are also commonly used pairs of radical species for chemiluminescence imaging [ 16 , 25 , 30 , 31 , 32 , 33 ].…”