Local convective heat fluxes in concurrent flame spread

Understanding the heat transfer mechanisms that govern the interaction between the flame and the solid fuel surface is essential for predicting and modeling flame spread. This work presents a novel technique to spatially resolve the convective heat transfer during horizontal concurrent flame spread over polymethyl methacrylate (PMMA) sheets. The experiments are conducted in a bench-scale flow duct under three inlet flow velocities. A side-wall radiometer ( s 1 ) and a dual heat flux gauge positioned downstream and adjacent to the fuel surface ( s 2 and s 3 ) are used to separate the radiative contribution from the total heat flux. The temperature at the dual heat flux gauge ( s 2 ) is also measured, and then a convective heat transfer coefficient is determined locally through an empirical Nusselt number correlation. A previously introduced non-dimensional total heat flux correlation is employed, demonstrating a collapse of all data across the experimental conditions. The local contributions of the heat-transfer mechanisms were quantified, showing that radiation is larger over most of the heated-zone domain, while convection becomes larger in the region of the heated zone nearest the pyrolysis front as the flame approaches the heated zone target ( s 2 location). The flame spread rate is obtained experimentally and predicted using the heat transfer contributions. The results exhibit excellent agreement between experiments and model predictions, highlighting that the methodology provides an accurate and consistent framework to predict flame spread behavior under concurrent flow conditions. Novelty and significance statement : This work presents a novel technique to spatially resolve convective heat transfer during horizontal concurrent flame spread over PMMA. Using heat flux measurements and fundamental analysis, the contributions of convection and radiation to the fuel surface are each resolved. An empirical local Nusselt number correlation is proposed allowing for estimation of the local convective heat transfer coefficient. The thermal response of the fuel during flame spread is assessed using the local Biot number to identify thermally thin or thick behavior. This study provides quantitative insight into the coupled radiative and convective heat transfer mechanisms that control horizontal flame spread, key processes in solid-fuel combustion and fire growth. The results offer a transferable framework for model validation and development in material flammability research, and potentially larger-scale fire scenarios (wildfires and structure fires), contributing to improved fire growth prediction and safety design across combustion and fire science disciplines.