Experimental investigation of the effects of inlet high-swirl air preheating and dilution on kerosene flame

•Increased air dilution tolerance with high swirl: the high swirl concept significantly expands the permissible range of air dilution for a kerosene flame by 12 %. Simultaneously, it reduces preheating requirements by 11 % when compared to the same flame under non-swirling conditions.•Enhanced flame stability through air swirling: swirling the co-flow of air improves flame stability, resulting in a more compact flame.•Limitation of the high swirl concept in achieving MILD mode: the high swirl concept alone was unable to transition the kerosene flame into a flameless mode.•Efficiency of the high swirl concept in MILD mode: despite its inability to achieve a flameless mode, the high swirl concept reduces the need for additional air preheating and dilution to transition the kerosene flame into MILD (Moderate or intermediate low oxygen Dilution) mode. This study investigated the effects of co-flow air swirling on the combustion of Kerosene, as relatively heavy fuel, under preheated and diluted air conditions. Various factors affecting the flame structure, such as preheating temperature, dilution, swirl number, and airflow rate were studied using the upgraded MILD (Moderate or Intense Low-Oxygen Dilution) combustion tester of Sharif University of Technology (SMSTR). This tester, which is symmetrically axial, provides co-flow air with the desired temperature, swirl number, and dilution for a liquid fuel spray burner. The flame characteristics was investigated through direct photography and chemiluminescence. The results show that the stability range significantly increases with the air swirling, while the flame length decreases. Enhancement of combustion quality and more complete combustion of fuel were observed with preheating. Preheating resulted in geometric changes in the appearance of the flame, specially reducing flame lift-off and the front view area in high airflow. A decrease in the air oxygen content causes a general change in the structure of the flame, resulting in a shorter and more lifted one compared to non-diluted condition. The chemiluminescence images of the flame also revealed weaker and more uniform emission pattern with the inlet high-swirl air preheating and dilution. By utilizing around 15 % oxygen content dilution, preheating at around 656 K, and a swirl number of 0.8, the combustion zone exhibited similarities to the MILD combustion mode, suggesting the achievement of MILD combustion. The high swirl concept allowed higher dilution limits