Temperature distribution and characteristics induced by fire smoke in L-shaped utility tunnels with small curvature radii

A computational fluid dynamics model is established to investigate the behavior of fire smoke spreading in L-shaped utility tunnels with small curvature radii (R). The fire evolution process for tunnels with R = 0–25 m and critical ventilation velocities (V) of 0.5–3.0 m/s is analyzed, and the Richardson number (Ri) is introduced to determine the critical value for the development of fire smoke back-layering. The results show that R and V strongly influence the fire-induced spatiotemporal evolution characteristics in such utility tunnels. Smaller R or higher V are associated with stronger centrifugal forces, which causes the smoke to move downstream of the fire source and increases the instability and risk. The non-dimensional smoke back-layering length shows a linear relationship with ln(Q*/V*3), where Q* is the dimensionless fire source power. Integrated with the V and smoke force in a curved tunnel, the critical Ri value is found to be 6.34 for R = 10 m. This means that smoke back-layering will appear when Ri > 6.34 and disappear when Ri < 6.34. The present work positively provides a tool for the assessment of thermal environment of L-shaped utility tunnel's fire safety in engineering practice.