Fuel-Insensitive Counter-Swirl Burner for Biofuel Spray Combustion

Airblast swirl-stabilized burners are inherently sensitive to fuel physicochemical properties, often exhibiting degraded flame stability when operated with high-viscosity and low–lower heating value (LHV) fuels.  To address this limitation, the present study introduces a counter-swirl burner incorporating a convergent nozzle that enables fuel-insensitive spray combustion while preserving stable flame morphology over a wide range of fuel properties.  Fuel flexibility is demonstrated using Jet A-1, straight Karanja vegetable oil, and glycerol-fuels spanning orders of magnitude in viscosity and calorific value.  The influence of swirl strength on spray flame structure, stabilization, and thermal performance is systematically investigated.   Time-averaged direct imaging and $OH^*$ chemiluminescence measurements show the counter-swirl geometry promotes the formation of anchored premixed reaction zone coupled with a centrally stabilized, jet-like spray flame.  Enhanced swirl intensifies secondary atomization, accelerating droplet evaporation and rapid air–fuel premixing, which supports flame anchoring.  The central confinement of the flame increases its separation from the combustor walls, thereby reducing wall heat dissipation.  This effect is quantified by a temperature difference of approximately 1200 K between the flame core and the non-insulated combustor wall, indicating swirl-induced aerodynamic insulation and enhanced thermal feedback to the incoming reactants, which contributes directly to improved reaction kinetics.  The local temperature and equivalence ratio distribution of the glycerol spray flame were examined, revealing the fuel stratification.  This region attains peak temperatures of approximately 1990 K, approaching adiabatic flame conditions.  Consequently, elevated upstream concentrations of CO and cracked hydrocarbons, followed by rapid depletion within 70 mm downstream, indicate accelerated chemical kinetics.  Overall, the counter-swirl configuration stabilizes the flame along the burner centerline by enveloping it with hot swirling air, enabling lean premixed prevaporized (LPP)-like combustion.  This stabilization mechanism decouples flame behavior from fuel properties, enabling stable, efficient combustion of high-viscosity, low-calorific-value fuels.