Shixiang Liu

Diffusion jet flames exist widely in industrial combustion devices and aerospace combustion at high altitude under sub-atmospheric pressure environments. Ambient pressure strongly alters gas density, molecular diffusion, and air entrainment, hence significantly changing flame height, laminar–turbulent transition, and instability characteristics. However, systematic experimental quantification and unified scaling laws for jet flame in sub-atmospheric pressures remain insufficient. In this work, a comprehensive experimental study covering 291 test cases was carried out in a large reduced pressure chamber (3 × 2 × 2 m3) with propane as a fuel. Three nozzle diameters (1, 2, and 3 mm) and ambient pressures ranging from 40 to 100 kPa were adopted to capture full flame evolution from laminar flow, laminar–turbulent transition to turbulent blowout. Non-dimensional similarity analysis was performed based on momentum–buoyancy characteristic length (Lm) and flow rate characteristic length (Lq). Results show that at fixed environmental pressure, flame height first increases and then decreases with jet exit velocity, and flame height decreases monotonically with reducing ambient pressure at same exit velocity due to weakened air entrainment strength. Small diameter (1 mm) flames may blow out under laminar conditions at pressures below 50 kPa, while blowout only occurs in turbulent regions for 2 and 3 mm diameter nozzles. A normalized correlation incorporating pressure-dependent air entrainment strength (αT) is introduced, which achieves asymptotic convergence of flame height for all nozzle diameters in turbulent near-blowout regions responding to Lm/Lq > 20. The present study provides a fluid physics interpretation of flame height and transition of non-premixed jet flame under sub-atmospheric pressures.