Near field flow dynamic and heat transfer characteristics of curved ceiling jet from tunnel fires under reduced pressures

Sub-atmospheric pressure environments and curved tunnel spaces significantly influence fire behavior and heat transfer mechanisms, thereby posing challenges for fire prevention in tunnels and underground space infrastructure. This study investigates the near field flow dynamic and heat transfer characteristics of fire-induced curved ceiling jet under sub-atmospheric pressures through experimental and simulation methodology. Experiments employed a downscaled model with varied atmospheric pressures (100 kPa∼45 kPa), fire source-ceiling heights (0.25 m∼0.35 m), and fire heat release rates (7.1 kW∼24.0 kW) to analyze flame morphology, flow field characteristics, and ceiling heat flux distribution. Results show significantly increased flame extension length and distinct vortex formation at the flame tip under sub-atmospheric pressure. And numerical simulations visualized vortex generation and evolution within the pulsation cycle. Pressure reduction causes ceiling heat flux to decrease near the fire source. Based on theoretical analysis, a relationship between the ceiling heat flux at the stagnation point and environmental pressures has been revealed. Furthermore, by analyzing the heat transfer mechanisms within the curved ceiling and horizontal ceiling, a unified predictive model for both transverse and longitudinal heat flux under different pressures was established. This study can provide a theoretical basis for fire risk assessment and emergency rescue strategies for tunnel fires under reduced pressures.