Experimental study on ceiling maximum temperature and back-layering length in inclined tunnel fires

• The back-layering length in longitudinal sloping tunnels was studied. • The effect of fire location on the induced airflow between was explored. • The impact of fire source location on ceiling maximum temperature was discussed. • A dimensionless model to predict ceiling maximum temperature was derived. • A prediction model for back-layering length was established. To explore the influence of longitudinal fire locations within longitudinal sloping tunnels on the maximum temperature rise of the ceiling ( Δ T max ) and the back-layering length ( L b ) of the tunnel's low entrance under natural ventilation, a series of tests were conducted in a 1/20 scaled tunnel. This research discussed three fire scenarios where the fire occurs in the low tunnel entrance region, the center of the tunnel, and the high tunnel entrance region, respectively. Results show that the velocity of the induced flow ( v in ) increases with slope when the fire occurs at the low entrance or the tunnel center region, while the fire occurs near the high tunnel entrance and the v in is not sensitive to the slope. The slope has a similar impact on the Δ T max and the L b ; as the slope increases, the Δ T max decreases and the L b shrinks, and this decreasing and shrinking trend diminishes with the increase in the distance between the low entrance and the fire center. Besides, models were proposed to predict the Δ T max and the L b at the low entrance caused by different fire locations in a longitudinally sloped tunnel. These findings may help guide the prediction of fire protection in tunnels and the design of safe evacuation.