This study explores the impact of burner outlet size on the behavior of a swirl burner with three nozzles. Our motivation stems from the need to understand how altering the burner’s hydraulic diameter affects combustion characteristics. The outlet hydraulic diameters ( \(D_h\) ) were 5, 10, and 15 mm, and the bluff body diameter (BBD) was 10 mm. An unconfined premixed n-butane air mixture was tested using PIV at an equivalence ratio of 1, a pressure of 1 bar, a temperature of 300K, a Reynolds number of 4000, and a swirl intensity of S1.5. The findings show that the burner outlet expansion pushed the forward stagnation point upstream. The recirculation length and width suddenly doubled for the \(D_h\) = 10 mm case and were almost constant for bigger outlet hydraulic diameters. Particle image velocimetry showed that the \(D_h\) = 5 mm case had the highest Turbulent Kinetic Energy, resulting in higher stretch rates and vorticity. Under identical mixing conditions, the three flames have very different turbulence. CH* chemiluminescence showed that burner outlet size increased reaction rate. Due to the increased stretch rate and local flame extinction, exhaust gas analysis showed that the C \(_x\) H \(_y\) concentration was higher for the D \(_h\) =5 mm case and lowered with increasing burner outlet size. Entrainment increased for burner outlet hydraulic diameters of 5, 10, and 15 mm. Damkohler numbers under different cases reveal the flame’s internal categorization. The flame structure changed from thin to corrugated flamelets as the burner’s hydraulic diameter increased. This study underscores the complex relationship between burner outlet dimensions and flame dynamics, providing valuable insights into how flames are categorized under different circumstances. These findings carry significant importance for the optimization of combustion processes and the enhancement of efficiency in practical applications.