Compartment fire behavior with two opposite openings under crosswind

External wind almost always exists around high-rise buildings. Due to the superimposed or competitive effect of wind pressure, buoyancy, and thermal expansion, the fire behaviors in high-rise building enclosures with two openings exhibit more complex dynamic evolution characteristics than those without external wind effects. In situations with such effects, new models are required to systematically analyze the fire behavior evolution mechanism.

In this study, we established a more practical compartment fire scenario and performed numerical analyses using the computational fluid dynamics (CFD) code, fire dynamics simulator (FDS), to study the evolution of the fire behavior of a compartment with two opposite openings. A total of 48 simulations were performed, wherein different wind speeds and heat release rates were considered. The fire source was a propane burner in the center of the compartment. The simulation duration was set at 350 s. The use of numerical simulations opened up the potential for a direct evaluation of a wide range of variables (e.g., the mass rate of inflow and outflow through openings and temperature) and even more complex quantities (e.g., the heat release rate within predefined volumes such as windward side space, leeward side space, and space inside the compartment). To obtain accurate simulations, sensitivity analysis was performed. The impact of crosswind speed on the behavior of fire spill plume on the windward and leeward sides, the temperature inside the compartment, and the flow pattern across the two openings were analyzed. Combined with the energy conservation equation, the average temperature rise model inside the compartment at over-ventilated conditions under the crosswind was established based on dimensional analysis. A critical wind speed (v_c) was determined for the conversion of gas from bidirectional to unidirectional flow according to the pressure differences among the windward side, the compartment, and the leeward side.

This study finds that a uniform mesh size of 2 cm is sufficient to achieve convergence. The results indicate that (1) the fire spill plume on the windward side at under-ventilated conditions gradually disappears as the wind speed (v) increases. Afterward, the fire spill plume on the leeward side also disappears gradually. At the same time, the heat release rate inside the compartment gradually increases. The critical criterion for the occurrence of the fire spill plume is that the global equivalence ratio Φ equals 0.645. When Φ≤0.645, combustion occurs only within the compartment. When Φ>0.645, the fire spill plume appears outside the compartment. (2) As the wind speed increases, the average temperature within the compartment constantly decreases at under-ventilated conditions and constantly increases at over-ventilated conditions. When v < v_c, there is a bidirectional flow through the openings on the windward or leeward side; when v≥v_c, the flow through the openings on both sides is unidirectional. The accuracy of the critical wind speed model is further verified based on an analysis of the mass flow rate through the openings on both sides.

The outcomes and findings of this study will help improve the existing theories of enclosure fire dynamics and provide theoretical and technical support for the fire protection of high-rise buildings.