Predicting models for critical velocity at the stair entrance under a two-story building fire

The cross-floor spread of fire smoke in a two-story building hinders personal evacuation and leads to casualties. To address this issue, sufficient wind pressure needs to be applied at the stair to prevent smoke from entering the upper space, and the critical velocity is an important parameter worthy of attention. By conducting a series of model experiments and numerical simulations in this study, the critical velocity below the ceiling screen at the stair is investigated, considering the factors of heat release rate (HRR), fire source location, ceiling screen depth and number of stairs. The results show that the critical velocity is proportional to one-third of the HRR, and the proportional coefficient decreases as the fire source is farther from the stair and the ceiling screen depth increases. The prediction models for the critical velocity under fire sources located in front and back of the stairs are proposed, respectively, and the proposed model is validated under three stairs. Additionally, the proposed prediction model is compared with the velocity value required by the relevant standard, and some dangerous situations are identified that require further strengthened ventilation. This study can provide theoretical guidance for ventilation design and emergency response in actual engineering with similar structures.