Unraveling the surface heat flux heterogeneity in cone calorimetry of cylindrical charring material : Insights from experiments and 2D modeling

Although originally designed for planar samples, cone calorimeter is frequently used for the flammability assessment and pyrolysis modeling of non-planar objects, like electrical cables. Prescribing the boundary conditions for the numerical models of such objects requires knowledge about the radiative and convective heat fluxes along the sample circumference. In this work, we performed gasification and flaming experiments and 2D numerical simulations on birch cylinders. During the gasification of a single-rod, the char front propagated faster vertically than horizontally, while similar rates were observed in the flaming condition. With five rods, ∪- and ∩-shaped char fronts were observed in gasification and flaming conditions, respectively. From the simulations, we extracted detailed heat flux distributions, with main deviation from flat samples being the steep reduction in incident radiation between the top and sides of the cylinders. Moreover, in five-rod configurations, radiative flux to the sides of the central rod increases up to 10 (gasification) and 20 kW m⁻² (flaming). On the outermost rod, the descending flame induces downward-moving waves of heat fluxes with amplitudes 20 kW m⁻² for convection and 15 kW m⁻² for radiation. Simplified expressions for the heat transfer boundary conditions were tabulated for practical engineering applications.