Experimental Study of Upward Flame Spread Behaviors over Thermally Thick Vertical PMMA of Various Sample Widths under Normal Wind

The effect of wind normal to the fuel surface on vertically upward flame spread is investigated over thermally thick PMMA slabs of various widths. The 1.2 cm-thick PMMA slabs of three sample width (10, 15, 20 cm) are vertically arranged subject to a horizontal wind (normal to fuel surface) ranging from 0 to 1.8 m/s. Flame size parameters, heat flux distribution and flame spread rate are measured and analyzed quantitatively. It is found that flame shape is much different under the strike of wind and flame spread rate decreases with wind velocity. To interpret the effect of normal wind, mechanism analysis is conducted based on flame spread theory. It is concluded that because of the transverse fuel diffusion, flame extending width increases with wind velocity, flame thickness and sample width in a relation of w e & SIM; & delta;2f u & INFIN; w s . This leads to a lower flame height with increasing wind velocity, from which flame preheating length is shorter, being negative to flame spread. Flame thickness or flame volume increases with wind velocity which indicates a larger fuel burning rate due to the increasing flame heat flux in pyrolysis region. Two different modes are found for pyrolysis heat flux, that heat flux is controlled by buoyancy (flame) induced flow at low wind conditions while it could change to be mainly influenced by forced flow with the increasing wind velocity. Based on the analysis of the controlling mechanisms of flame spread under this configuration, a theoretical model is proposed to predict vertically upward flame spread process in normal wind, from which flame spread rate increases as a power function of time, and decreases with wind velocity. This work provides basic observation on upward flame spread progress in normal wind conditions and a detailed analysis of wind effect mechanisms on vertically upward flame spread behaviors.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.