Inclined Neutral Plane and Smoke Temperature Spatial Distribution in Z-Shaped Passages Under Fire Scenarios

Z-shaped passages are commonly used for connecting underground spaces such as subway entrances and mining laneways to the ground surface, consisting of alternating horizontal and inclined sections. As Z-shaped passages are typically used for evacuation, the unidirectional flow of gases within the passage makes determining the location of the neutral plane (NP) crucial for positioning evacuation exits. This study investigates both longitudinal and vertical temperature distributions and reveals the unique inclination of the neutral plane in Z-shaped passages. Through theoretical analysis and numerical simulations, the study examines ceiling smoke temperature rise and downstream temperature decay under varying heat release rates (HRR) and passage slopes. A new correlation for maximum ceiling temperature rise and a model for downstream temperature decay are established. Additionally, the vertical temperature distribution of smoke in the passage is derived. Finally, by combining the spatial temperature distribution of smoke in the passage, a theoretical model for predicting the neutral plane is developed and validated through comparisons with simulation results. The proposed model demonstrates high accuracy in predicting the inclined neutral plane in Z-shaped passages. These insights provide critical guidance for designing efficient smoke control and evacuation systems in complex underground environments.