Fire scale modeling and effects of buoyant flow on laminar upward flame spread

Over the last few decades, efforts to improve fire safety have emphasized understanding material flammability through small-scale laboratory testing due to safety and cost constraints. However, extending these findings to real-world fire scenarios is challenging, as buoyancy-driven flows—critical to fire behaviors—vary significantly with scales. The objective of this numerical study is to understand the effects of buoyant flow on fire dynamics, specifically in the context of upward flame spread over vertically oriented solid fuels. To approach the problem incrementally, simulations are first performed for inert thermal plume (no combustion), followed by simulations of laminar upward flame spread over a thin solid material. The results show that the boundary layer thicknesses and flame standoff distance increase when gravity and pressure decrease, and scale with p−0.5g−0.25. The buoyant flow magnitude increases with gravity but remains the same when pressure is varied (UB∼p0g0.5). The results of upward flame spread simulations further show that reaction rate, flame temperature, and flame spread rate have positive correlations with both pressure and gravity. Interestingly, while the incident radiative heat flux on the solid surface (q˙fr″) increases with ambient pressure, it is insensitive to gravity. Nevertheless, q˙fr″ has a minor effect on the net heat flux, owing to the dominant contribution of convective heat input. Comparing cases of a constant p2g, distributions of convective and net heat fluxes, solid surface temperatures, and solid mass loss rates are generally similar, resulting in similar overall flame behaviors. However, thermal inertia and flame radiation loss are both lower in a reduced pressure environment. This leads to higher flame temperature and higher convective heat input in a small region near the flame base, weakening the effects of pressure on flame spread. Because of this, when applying p2g pressure modeling, the flame spread rate is higher in reduced pressure than in reduced gravity. The results further show that p1.8g modeling has a better performance in correlating fire dynamics.