The impact of dilution air strategy on n-heptane turbulent swirl spray flames

• Dilution air addition studied in n-heptane turbulent swirl spray flames. • PIV and OH-PLIF used to study flow, flame structure, local extinction, and lift-off. • Dilution air addition displaced vortex position, enhancing turbulence intensity. • Flame stability limit improved with the addition of dilution air. This paper investigates the impact of dilution air addition on the stability and flame structure of n-heptane turbulent swirl spray flames. Particle Image Velocimetry (PIV) was used to study the flow fields with and without dilution air. OH Planar Laser Induced Fluorescence (OH-PLIF) measurements were utilized to assess the impact of dilution air addition on flame structure, local extinction, and lift-off. Experimental results were collected at different flow conditions including quenching point (global blow-off), near blow-off, and stable conditions. 1D non-premixed laminar counter-flow flame simulations were also conducted to gain preliminary understanding of the flame structure behaviour and temperature when adding more air with hot products. The numerical results showed that adding 10 % more air on the oxidiser side increased the peak values of the temperature, heat release rate (HRR) and OH mole fraction, with the maximum flame temperature rising by approximately 3.9 %. The flame’s stability limit increased when adding dilution air, and a 2 % addition made the flame persist longer. The flame restabilized with the addition of 5 % dilution air at the blow-off co-flow conditions of the non-diluted flames. OH* chemiluminescence results showed that the addition of dilution air up to 10 % of the co-flow increased the heat release rate by as much as 247 %, indicating a strong enhancement in flame intensity and reaction zone activity. The addition of dilution showed the droplets to spread more widely into the inner recirculation zone (IRZ). PIV showed a clear displacement of the vortex inwards and upwards when adding dilution air, with increased turbulence intensity aligned with shear layers. Without dilution air, the OH flame sheet appeared continuous, while with dilution air, it appeared to be wrinkled, cloud-shaped, and distributed. Statistical analysis of the OH-PLIF results showed that adding 5 % dilution air increased local extinction events by up to 7.5 %, primarily due to the intense aerodynamic strain imposed on the flame front under highly turbulent conditions. OH-PLIF results showed that the addition of dilution air increases the relative OH intensity in the outer flame branch (OFB) at the same horizontal plane downstream. The novel findings on extended operation condition effect by adding dilution air provide the first evidence of enhanced flame stabilization towards lean combustion strategy.