Numerical investigation of flame-flow interactions in the blue whirl

The blue whirl has attracted considerable attention due to distinctive structure and promising soot-free combustion technology. Recirculation flow effects on blue whirl local flame characteristics remain to be explored. In this study, numerical simulations were conducted to investigate these flame-flow interactions of blue whirl. Three-dimensional reactive Navier-Stokes equations were solved with a calibrated chemical-diffusive model (CDM) for heptane/air mixture. The Grabowski velocity profile is applied to introduce a steady axisymmetric vortex breakdown. The results show that vortex breakdown causes an inverted conical flame structure and deviates the triple point from the leading position. The heat release and density changes at flame surface can lead to extreme values in the local velocity components. Moreover, vortex breakdown effects on the rich premixed flame surface were also investigated. It indicates that the excess fuel species in the bottom rich flame can enter the flame envelope and transport back to this rich branch again by the recirculating flow. This process can increase local equivalence ratio and reduce local heat release, consequently causing a non-monotonic response of flame characteristics on the blue-whirl surface. The local flame properties in blue whirl surface show that flame propagation speed exhibits a similar trend to that of the fully premixed flame regime. In addition, the flame speed on blue-whirl surface has a negative and non-linear correlation with flame curvature. This correlation can also be found in super-unity Lewis number premixed flames. This work provides further insights into the recirculation flow effects of blue whirl. Novelty and significance statement The blue whirl has attracted considerable attention due to its distinctive flame structure and soot-free combustion. A key remaining question concerns the influence of recirculation flow on local flame characteristics. The novelty of this work lies in revealing the three-dimensional vortex breakdown structure and the associated species transport processes in the blue whirl. We also identify a non-monotonic response of flame surface characteristics resulting from vortex breakdown. Moreover, the local flame propagation speed on blue-whirl premixed surface is investigated for the first time. This study holds significance as it significantly advances the current understanding of blue whirl dynamics and provide insights for high-efficiency and low-emission combustion applications in the future.