Heat transfer to and ignition of ceiling by an impinging diffusion flame

This work is an experimental study of the heat transfer to and the ignition of a ceiling surface exposed by an impinging buoyant jet diffusion flame. Natural gas flames in the range of 2.9 kW to 10.5 kW on a burner of a diameter of 64 mm were used. The shapes of these flames are described with appropriate temperature and species concentration measurements. The gas temperature below and the total heat flux to the stagnation point were found to be functions of the ratio of the ceiling height to the flame height. In the intermittent flame region the stagnation point heat flux was found to be higher than given by You and Faeth, but to approach their results in the plume region. For these flames, the ignition of a PMMA ceiling was found to occur only if the visible flame touches the ceiling at least 50 % of the time, i.e., if the average flame height is bigger than the ceiling height. The critical heat flux was between 20 kW/m2 and 30 kW/m2 depending, e.g., on the size of the flame. These values are higher than the critical radiative heat flux for piloted ignition of the same material. The sustained ignition of PMMA required a surface temperature in the range of 380 °C to 410 °C. After turning the gas burner off, the flames on the surface extinguished repeatably as the surface temperature decreased to 320 °C.