Experiments and modeling of the temperature profile of turbulent diffusion flames with large ullage heights

•Experiments conducted for propane diffusion flames with various ullage heights.•Detailed measurements of axial temperature in upper and down-reaching flames.•Flame shape and length analyzed and burning regions identified.•New models for calculating virtual origin in upper and down-reaching flames.•Global correlations for axial temperature in upper and down-reaching flames. In the last decades, several major fire accidents involving storage tanks occurred, particularly for the tanks with a low fuel level (large ullage height). In case of a tank fire, the axial temperature profile is affected by the ullage because of the restriction in air entrainment by the tank sidewall. This work is aimed at examining the temperature profile of turbulent diffusion flames with large ullage heights. A series of experiments were conducted with different ullage heights (h = 16–42 cm) and fuel supply rates. The temperature profiles both inside and outside the fuel tray were measured and analyzed. Five regions were identified based on flame intermittencies: namely 1) fuel vapor, 2) down-reaching intermittent flame, 3) continuous flame (down-reaching and upper flame), 4) upper intermittent flame and 5) buoyant plume. It was found that for a given fuel supply rate the length of the fuel vapor region increases linearly with the ullage height. The lengths of the fuel vapor and continuous flame regions both increase with the fuel supply rate, whereas that of the down-reaching intermittent flame region shows an opposite trend. Based on the experimental data and dimensionless analysis, new correlations for the virtual origin were proposed, which were then incorporated in the predictions of the temperature profiles in both upper and down-reaching flames. The present results could not only contribute to the understanding the effects of the ullage height on the axial temperature profile in storage fuel tank fires with large ullage heights but are also of practical importance in the thermal hazard assessment of fire accidents involving tank liquid fuels.