Flame extension area of unconfined thermal ceiling jets induced by rectangular-source jet fire impingement

Highlights: • Experiments are carried out to study the flame extension area of ceiling jets. • Flame extension area tends to be larger for burner with small aspect ratio. • A new unified correlation to predict the flame extension area is established. - Abstract: Flame extension area is a significant parameter of thermal ceiling jet, which directly determines the strong heat transfer area between the thermal plume and the building ceiling. Experiment was conducted to investigate the flame extension area induced by rectangular source fires in this work. Results show that the flame extension area data sets can not be precisely predicted by former correlations. The aspect ratio of rectangular fire source has an influence on flame extension area; the fire source with smaller aspect ratio leads to a larger flame extension area. Finally, a global non-dimensional correlation combined the effects of source-ceiling height, heat release rate, source aspect rations are proposed to predict the flame extension area of flat unconfined ceiling jets. The new proposed correlation can also provide reference for the flame extension area beneath the confined tunnel ceiling during a tunnel fire. This work can supply added results to complement the existing knowledge of flame extension beneath the building roof and it can also provide directions for the fire risk assessment and fire protection design of buildings.