Experimental and numerical studies on flame radius, temperature profile and heat flux of axi-symmetric ceiling fires under various sub-atmospheric pressures

Previous studies concerning ceiling fires (fire source located on the ceiling) under normal atmospheric pressure. However, research exploring the behavior of ceiling fires under sub-atmospheric pressures, which may occur at high altitudes, remains absent. This study addresses this gap through experiments carried out in a variable atmospheric pressure chamber and numerical simulations. A horizontal mica plate was used as the ceiling, and numerical study was conducted to compare with the experimental data. Five atmospheric pressures (40, 55, 70, 85 and 100 kPa), various heat release rates, burner diameters and fuel types (methane and propane) were considered. Key parameters obtained included the flame radius, maximum flame thickness, temperature profile and total heat flux beneath the ceiling. Experimentally, the flame radius and maximum flame thickness were estimated based on 50 % flame appearance probability. Numerically, the flame radius was estimated by $\Delta T=500K$ ($\Delta T$ is temperature rise above the ambient) in the isothermal-temperature profile. From the results, the flame radius ($R_f$) increases as atmospheric pressure decreases, with the maximum flame thickness being significantly smaller than the flame radius. Regarding the temperature profile, temperature rise remains nearly constant under various sub-atmospheric pressures in the near field of fire source, and it gradually increases as atmospheric pressure decreases in the far field. The heat flux in the near field of fire source decreases as atmospheric pressure decreases, and it increases as atmospheric pressure decreases in the far field. Then, the prediction models for the temperature rise and total heat flux profiles were obtained, both the non-dimensional temperature rise and heat flux under various sub-atmospheric pressures can be described well by using the flame radius as characteristic length. Additionally, the physical analysis of the air entrainment behavior of ceiling fires under sub-atmospheric pressures was conducted, and the rate of flame air entrainment decreases as atmospheric pressure decreases. Finally, a novel characteristic length $l_{a,p}={\left(\dot{Q}/\left(\left(0.48P^{\ast }+0.52\right)\left(\Delta H_c/s\right)\right)\right)}^{1/2}$ (where $\dot{Q}$ is the heat release rate, $P^{\ast }$ is the atmospheric pressure normalized to 100 kPa, $\Delta H_c$ is heat of combustion of fuel, and s is the stoichiometric air-to-fuel mass ratio) was proposed, and the flame radius was demonstrated proportional to $l_{a,p}$. This is a supplement of the fundamentals and hazard characteristics of ceiling fires under various sub-atmospheric pressures.