The effect of pressure on the characteristics of pure propane jet diffusion flame at sub-atmospheric pressures

Sub-atmospheric pressure significantly influences the combustion stability of non-premixed jet flames at high-altitude conditions. This work conducts experimental and numerical simulation studies to investigate the flame stability of non-premixed propane jet under pressures ranging from 40 kPa to 101 kPa. The results show that the height of laminar jet flame increases linearly with volumetric flow rate. The flame flickering frequency decreases with increasing Reynolds number, while the flickering amplitude increases with Reynolds number. Pressure reduction advances the onset of the laminar-turbulent transition, yields wider radial flame expansion and facilitates flame liftoff. The height of attached laminar flame exhibits non-monotonic pressure dependence. It slightly decreases with rising pressure below a critical threshold, followed by a rebound increase. At a fixed Reynolds number, the laminar liftoff height increases monotonically with pressure drops, and a parallelogram stability regime is identified for stable laminar lifted flames. Based on edge-flame stabilization theory, a pressure-corrected correlation is proposed to predict the laminar liftoff height over the pressure range of 40–101 kPa. For all tested nozzle diameters, the flame blowout velocity increases with increasing pressure. Numerical results reveal that lower pressure weakens jet momentum dissipation, leading to faster axial flame penetration and greater flame length. By contrast, higher pressure intensifies shear layer instability, triggers more frequent flickering and larger-amplitude oscillations. The soot emissions increase as pressure decreases, whereas NOx formation shows a continuously increasing trend.