Study on the applicability of scaling laws in tunnel fires under natural ventilation conditions

This article numerically investigates the impact of tunnel scale ratio and fire Heat Release Rate (HRR) on the scaling results of temperature distribution and smoke field in a tunnel under natural ventilation conditions, by preserving Froude (Fr) scaling. Computational fluid dynamics simulations were performed on a 1:8 scale physical tunnel model [1], showing satisfactory agreement between measured data and predicted results. On this basis, four additional numerical models with scales of 1:1, 1:2, 1:4, and 1:16 were built based on Froude scaling principles, considering the scaling of thermal properties of materials and wall thickness. Four HRRs ranging from 3 MW to 50 MW were adopted. The simulation results provide insights into the temperature and flow fields, revealing the influence of tunnel scale ratio, HRR, and wall scaling method on scaling errors. Significant discrepancies found in the smallest tunnel model with a scale ratio of 1:16 were well explained by transitional or laminar flow due to low Reynolds number (Re), which can primarily cause a significant reduction in ceiling jet temperature as well as an increase in smoke arrival time and smoke thickness. Finally, after analyzing scaling errors for practical tunnel fire design scenarios, recommendations are provided for engineering application of selecting appropriate physical scale models for tunnel fire tests.