A cylinder-frustum model for thermal radiation characteristics of strong-plume tunnel fires

Strong plume fires in tunnels can generate severe thermal hazards due to the interaction between flame behavior and the confined tunnel structure. However, the influence of fire source-to-wall distance on flame extension and thermal radiation characteristics remains insufficiently understood. In this study, experiments were conducted in a 1:6 scaled tunnel model to investigate the flame morphology and radiation characteristics of strong plume fires under different fire source positions, heat release rates, and burner sizes. The evolution of transverse and longitudinal flame extension was analyzed, and the physical mechanisms associated with wall confinement and asymmetric air entrainment were discussed. The results indicate that decreasing the fire source-to-wall distance enhances wall confinement, leading to asymmetric flame spreading and significant variations in flame extension length. Based on the experimental observations, prediction correlations for the characteristic flame extension lengths were developed considering the effects of heat release rate, burner size and fire source-to-wall distance. Furthermore, an equivalent cylinder-frustum radiation model was proposed by representing the flame geometry according to its actual extension characteristics. The model provides reasonable predictions of radiative heat flux and highlights the influence of flame configuration on thermal radiation in tunnel fires. These findings improve the understanding of strong plume fire behavior and provide a theoretical basis for evaluating thermal radiation hazards in confined tunnel environments.