Flame extension lengths beneath a double slope roof induced by gaseous fuel jet flame

[Display omitted] •Flame extension lengths of ceiling jet under double slope roof were studied.•The influence of roof angle on the flame extension length was analyzed.•Previous models were not applicable to double slope roof ceiling jet.•Unified flame extension length model was developed.•The proposed flame extension correlation has a maximum error of approximately 20%. Due to the characteristics of good drainage and stable structure characteristics, the double slope roof is widely used in civil houses, industrial plants, and ancient buildings. However, double slope roof fire accidents occurred frequently. Studying the flame characteristics under the ceiling can provide scientific guidance for building fire protection. The present study investigated the flame extension lengths beneath a double slope roof induced by gaseous fuel jet flame, which had not been quantified in the literature. Results show that the length of flame extension was longer in the ridge direction compared to the slope direction, with a maximum difference of approximately two times. Moreover, their deviation increases with larger HRR and smaller θ, resulting from the fact that the structure of the double slope roof would restrict air entrainment and induce more unburned gaseous fuel to spread along the ridge. Comparing the experimental data with previous models proves that they cannot predict the ceiling flame extension length under the influence of roof angle. A new correlation incorporating the effect of θ was established based on the proposed flame volume assumption to estimate the equivalent HRR at the ceiling, whose accuracy and reliability were further validated by reference data. The model prediction agreed well with the literature data, with a maximum error of around 20%.