Understanding of the physics and mechanisms of externally venting flames from enclosure fires with parallel sidewalls at the opening is fundamental to investigating fire spread of U-shape building façade. This work aims to identify the impact of separation distance of sidewalls on façade flame height, heat flux and sustained internal combustion. A series of numerical simulations is conducted in a full-scale compartment with 3.6 m in length/width and 3.1 m in height. An external façade wall is measured 6 m in height and 3.6 m in width. A pyrolysis surface with an area of 3.2x3.2 m 2 is placed in the middle of the enclosure with a theoretical heat lease rate varying from 1 to 8 MW. The semi-empirical correlations, derived from a cubic enclosure of 0.4 m with a HRR lower than 250 kW, are evaluated for a full scale façade fire for identifying the flame height dependence on the separation distance of sidewalls. It is found that an enhanced buoyancy-induced flow between sidewalls leads to an increase of the flame height by a factor of 50% and a rise of the peak of radiation heat flux by a factor of 20% as compared to a flat façade fire. The height of externally venting flame is sensitively reduced by a factor of 40-50% with an increase of the normalized sidewalls distance by opening width up to 3. The predicted flame height is more pronounced with an increase of flame height by a factor of 40% for a full scale façade than for a small scale one due to turbulence development. The predicted internal HRR is in quantitative agreement with the one from the correlation for a Global Equivalence Ratio (GER) below 3. The predicted temperatures are in good agreement with the experimental data except in the intermittent-like regions where an over-prediction of the temperature by a factor of 50% is found due to a reduction in accuracy of LES combustion model.

No abstract