Aerodynamic flame stabilization at sub-limit global equivalence ratio, exploiting high velocity ratio coaxial jets: An experimental approach

Abstract

This work investigates the flow and flame stabilization characteristics of a novel partially premixed aerodynamic flame stabilization configuration. Flames are established in the interaction region of high velocity ratio coaxial jets, at the location where the local flame speed and the reacting flow velocity match. The isothermal flow and mixing field characteristics are first experimentally investigated at four velocity ratios, ranging from 5 to 9, to delineate the operating conditions that promote the formation of a centrally located recirculation region. Combined with a suitable and freely adjustable fuel injection methodology, that supplies the reacting region with a locally flammable mixture, an inverted bowl-shaped flame is aerodynamically established, at sub-limit global equivalence ratio values, with ignition. These flames have no fixed edge anchoring point and remain relatively stationary throughout the investigated load variations, suffering no heat losses to the burner. Particle Image Velocimetry, Fourier Transform Infrared Spectroscopy, Mie scattering and OH* Chemiluminescence measurements have been conducted to investigate the flow and mixing characteristics, flame topology and turbulence effects as well as displacement speed correlations with turbulence intensity. Results indicated that the flame brush location and topology are demarcated by the flame induced accelerating region and maximum OH* chemiluminescence intensity with local Damköhler ($\mathrm{Da}$) and Karlovitz ($\mathrm{Ka}$) numbers suggesting that flame stability is primarily governed by reaction kinetics rather than turbulence effects. Finally, the local displacement speeds revealed a lineal correlation with the fluctuating velocities, for the flame configurations studied, highlighting characteristic resemblances with the typical low-swirl burner setup.