Assessment and application of phosphor thermometry for spatially resolved surface temperature measurements during downward flame spread

Flame spread over a solid surface is a critical metric in assessing the fire hazard of a material. At the core of the flame spread problem is heat transfer to and within the solid fuel. Accurate measurement of surface temperature on the burning solid is necessary to describe the heat transfer mechanisms which drive flame spread. This work employs phosphor thermometry to measure the spatiotemporal surface temperature during downward flame spread over polymethyl methacrylate (PMMA) samples. The phosphor Gd3Ga5O12:Cr,Ce is used to measure the surface temperature in a 23 × 23 mm2 area with an image resolution of 410 µm/pixel. CH* chemiluminescence imaging is performed alongside phosphor thermometry to measure the flame spread rate and evaluate the surface temperature relative to the flame position. This work investigates the limitations and considerations required to adequately measure surface temperatures in a flame spread scenario using phosphor thermometry. The optimal phosphor coating thickness to prevent interference with the flame spread process is first investigated. Phosphor coating thicknesses of 6 µm and 4 µm impeded flame spread rate and altered the flame shape – thus proving too invasive for this application. A coating thickness of 2 µm, which provided a phosphor to PMMA surface ratio of 0.55/0.45, had no measurable effect on the flame spread behavior and provided reliable 2D surface temperature measurements. The phosphor measurements presented in this work exhibit a similar reliability to a thermocouple, but provide spatially resolved surface temperatures and provide measurement access to the surface underneath the flame sheet. The findings report 2D spatiotemporal surface temperature measurements ahead of the flame front, at the flame's leading edge, and in the pyrolyzing region immediately beneath the flame. Detailed surface temperature measurements underneath the flame sheet are novel to the use of phosphor thermometry and have not been previously recorded. This study showcases this diagnostic technique in the context of flame spread, and shows the potential of applying these methods to other solid-fuel related research.