Numerical Investigation of the Temperature Field Evolution of L-Shaped CFST Columns During Water Spray Cooling After Elevated-Temperature Fire Exposure

To investigate the cooling mechanism and temperature field evolution of L-shaped concrete-filled steel tubular (CFST) columns during post-fire water spray cooling, the ISO-834 standard fire curve was adopted as the fire heating condition. On this basis, the initial temperature field for the cooling stage was established based on fire simulation results for different flame coverage thicknesses. A fluid–structure coupled numerical model that comprehensively accounts for radiation, convection, and conduction was then developed using the computational fluid dynamics (CFD) software Fluent. In addition, the discrete phase model (DPM) was employed to simulate droplet transport and heat mass transfer, and a droplet evaporation model was activated to account for latent heat absorption. Meanwhile, the SST k–ω turbulence model was also adopted to systematically simulate the cooling process of L-shaped CFST columns under different spray positions and spray flow rates. The results indicate that, during the spray cooling process, the spray position and spray flow rate are key factors governing the cooling effectiveness. Owing to the geometric asymmetry of the L-shaped cross-section, different spray positions resulted in markedly different cross-sectional temperature distributions. Among them, Position 3 (outer surface of the horizontal limb) exhibited the best overall cooling performance, with the average cooling rate on the sprayed surface reaching a maximum of 1.011 °C/s; this was followed by Position 1 (outer surface of the vertical limb), whereas Position 2 (internal corner region) exhibited a relatively lower cooling rate. Moreover, an increase in the spray flow rate accelerated the cooling of the sprayed surface and reduced the time required to reach the target temperature. During the initial cooling stage, the concrete core continued to absorb heat from the high-temperature steel tube and adjacent high-temperature regions. When the heat input exceeded the heat output, the internal concrete temperature continued to rise. As cooling proceeded, the concrete temperature began to decrease gradually. The thermal resistance at the steel–concrete interface was primarily responsible for the lag in the concrete temperature response. Within the investigated parameter range, the results can provide a reference for post-fire temperature assessment and spray parameter selection for L-shaped CFST columns.