Influence of Relative Spatial Layout of Fire and Openings on Neutral Plane Characteristics and Distribution Mechanisms of Mass and Heat Release for Spill Plumes in Compartment Fires

This paper presents a numerical investigation into full-scale double-opening compartment fires. It aims to reveal the underlying mechanisms by which the relative spatial layout of the fire source and openings influences neutral plane characteristics, flow field dynamics, and the allocation of spill plume heat release. The findings indicate that asymmetric boundary constraints in corner-door configurations cause the neutral plane to deform into an “S-shape.” The average neutral plane height decreases significantly as the total heat release rate increases. Moreover, the closer the fire source is located to the door, the more pronounced the descent of the neutral plane becomes. Furthermore, complex internal vortices and momentum losses induced by asymmetric layouts cause classical mass flow rate prediction formulas to overestimate actual values. By introducing a spatial structural factor to calibrate the discharge coefficient, the theoretical calculations of the inflow mass flow rates achieve a high degree of agreement with the simulation results, reducing the relative error to approximately 15%. The results also demonstrate that variations in door and window positions have limited influence on the overall spill-plume mass flow rate. However, different opening configurations modify the spatial aerodynamic characteristics of the flow field, thereby affecting the spatial distribution of external heat release. The incoming airflow from a centrally positioned door directs the fuel gases toward the window, whereas the wall-bounded vortex induced by a corner door entrains fuel to spill out from the door. Once the fire scale exceeds the critical indoor heat release rate, the compartment enters a ventilation-limited regime. This results in increased outward transport of unburned gases and an abrupt rise in the external heat release rate.