Experimental investigation on flame geometry and downstream heat flux from diffusion flames on inclined fuel surface under sub-atmospheric pressures

It is common that fires develop over sloping rooftops or wildfire spread on uphill slopes in plateau. However, fires over inclined fuel surface at sub-atmospheric pressures have not been well addressed in the past. In this work, the effect of sub-atmospheric pressure on flame geometry and downstream heating behavior of fires over inclined fuel surface were experimentally investigated. Ambient pressure P and inclination angle $\theta$ were varied from 30 to 101 kPa and 0°–40°. It was observed that the diffusion flame tends to be dimmer and the laminar portion appears to be larger at lower pressures due to inhibited soot formation and stronger molecular diffusion. Flame length scales including flame projection length and flame attachment length grow with decreased P and increased $\theta$. Based on mass balance principle, flame length scales were derived to be two-thirds power of a modified dimensionless heat release rate considering smaller flow rate of energy at lower pressure. Total heat flux and its components including radiative and convective heat fluxes were found to decrease as pressure and inclinations reduce. Ultimately, a power-law decay between normalized total heat flux and downstream distance was established in flame attachment and plume-like regions covering a wide range of pressures and inclination angles.