Characteristics and behaviors of fire plumes in concave vertical structures

Concave vertical structures are widely integrated into modern architectural systems, such as building facades, enclosed ventilation shafts, and recessed corridor components. Owing to their confined geometric boundaries, these structures induce fire plume behaviors that differ fundamentally from those in open spaces. Such behaviors include abnormal plume propagation, flame morphology distortion, and non-uniform temperature distribution, which may accelerate fire spread and increase evacuation and rescue risks. However, existing studies have primarily focused on fire plumes in open or simple confined spaces, with limited systematic investigation of the intrinsic evolution mechanisms of plume characteristics within concave vertical configurations. In particular, the coupling effects between structural dimensions and fire source conditions remain insufficiently understood. To address this gap, this study systematically investigates the evolution laws of key fire plume parameters (namely, average flame height and vertical temperature distribution) in concave vertical structures. Specifically, the objectives are to (a) clarify how the coupling of structural dimensions (width and depth) and fire source position regulates oxygen supply and heat feedback during plume development, (b) reveal the dominant mechanisms driving plume parameter variations in concave configurations, and (c) establish quantitative dimensionless models for characterizing average flame height and vertical temperature distribution. Ultimately, this study aims to provide a theoretical foundation for improving dynamic fire evolution models and formulating targeted firefighting strategies, thereby enhancing fire safety management for buildings incorporating concave vertical components.

A 1∶10 reduced-scale experimental platform was constructed based on the Froude similarity criterion to ensure the scalability of fire dynamic behaviors. The experimental matrix included multiple concave structural configurations. The concave region width was set to 0.3—0.6 m (corresponding to 3—6 m at full scale), depth ranged from 0.3 to 0.9 m (3—9 m full scale), and the test structure height was 1.55 m (15.5 m full scale). Fire experiments were conducted under varying heat release rates (HRRs) of 23.69—65.14 kW, corresponding to 7.4—20.5 MW at full scale, to represent different fire intensities. Key parameters were measured as follows: average flame height was obtained using continuous image acquisition (50 fps) and image processing (to extract flame contours and calculate time-averaged heights); vertical temperature distribution was recorded using K-type thermocouples spaced at 0.1 m intervals, covering heights from 0.1 m to 1.2 m along thecentral axis of the concave structure. After data acquisition, dimensionless analysis was performed to normalize structural and fire parameters. A dimensionless structural factor integrating width, depth, and height was introduced to quantify geometric confinement effects. Regression analysis was then applied to establish predictive models for average flame height and vertical temperature distribution.

The experimental results reveal distinct variation patterns of fire plume parameters under concave vertical constraints: (1) The average flame height increases monotonically with HRR, rising from 0.31 m at 23.69 kW to 0.68 m at 65.14 kW (maximum tested condition). (2) Temperature decreases with increasing height. In the near-field region (≤0.3 m above the source), temperature drops by 40%—50% from the peak value; in the far-field region (>0.3 m), the decrease rate slows to 15%—20%. Structural width affects temperature distribution. Narrower configurations (0.3 m width) produce a more concentrated high-temperature zone confined within 0.2 m above the fire source, whereas wider structures (0.6 m width) exhibit a more dispersed thermal distribution. (3) Further analysis confirms that the coupling between structural dimensions and fire source position governs oxygen supply and heat feedback. Smaller structures restrict oxygen entrainment, leading to localized incomplete combustion, while simultaneously enhancing wall radiant feedback. These two mechanisms jointly modify plume morphology, flame height, and vertical temperature profiles. The introduction of the dimensionless structural factor enabled the development of predictive models for both average flame height and vertical temperature distribution. The models achieved coefficients of determination (R²) exceeding 0.84, demonstrating strong fitting reliability and predictive capability.

This study systematically clarifies the evolution mechanisms of fire plumes in concave vertical structures. The main conclusions are as follows: (1) Variations in fire plume parameters are governed by the coupled effects of structural dimensions and HRR, with oxygen supply restriction and heat feedback enhancement identified as the dominant mechanisms. (2) The proposed dimensionless structural factor effectively quantifies concave geometric confinement effects, and the developed models accurately predict average flame height and vertical temperature distribution. (3) The findings enhance the accuracy of dynamic fire evolution modeling in concave architectural configurations and provide practical guidance for firefighting strategies, such as cooling structural walls in deep concave spaces to mitigate heat feedback effects. Future research should validate these conclusions via full-scale experiments and expand the analysis to different fire source types (e.g., liquid and solid fuels) to improve model generalizability.