Ethanol spray tubular flame established in a swirling air flow

This study proposes an efficient technique to burn ethanol spray in an intense swirling air flow and investigates the mechanism of flame lift-off. On the basis of typical tubular burners for gaseous fuels, ethanol spray was axially induced and mixed with the tangentially injected air under room temperature, yielding the ethanol spray tubular flames under various operating conditions. The results show that from an ultra-lean condition of global equivalence ratio of 0.1 to the rich condition, two typical flames, namely attached and lifted tubular flames, were established. Then, the structure of the ethanol spray tubular flame was specified by temperature measurements, OH-PLIF imaging, indicating its overwhelming aerodynamic and thermal stability with low pollutant emissions. Meanwhile, a flue gas analyzer was used to characterize the gas composition in the hot exhaust. It is found that the burner can achieve very low emissions of both CO and NO_x, illustrating high combustion completeness under both attached and lifted flame conditions. To give a better understanding and make fully utilization of spray tubular flames, a parametric study was carried out to investigate the effects of the tangentially swirling air flow, the flow rate and oxygen concentration of atomizing gas, and the flow rate of liquid ethanol. Generally, an attached flame is lifted under a higher tangential velocity of air flow; increasing oxygen concentration of the atomizing gas flow leads to the height decrease of the flame lift-off, even the reattachment of the lifted flame; by raising the ethanol flow rate to exceed a critical value, the flame also lifts off. Furthermore, the evaporation time of droplets, residence time and chemical reaction time were calculated to quantify the flame lift-off behavior. The lifted flame can be established only when the evaporation time is larger than both the reaction time and the residence time.