Effects of Tunnel Length on Combustion Efficiency in Tunnel Fires

Abstract

Combustion efficiency in tunnel fires was measured using a 1:20 scale model tunnel with a 0.25-m height, H, while varying the length of the model tunnel, L, as the experimental parameter. A series of fire experiments was conducted to confirm whether combustion efficiency was affected by the tunnel length, L. A dimensionless tunnel length defined as L* = L/H was selected as L* = 8, 16, 24, 32, 40, and 48, corresponding 40–240 m in full scale. The results showed that combustion efficiency was not affected by the dimensionless tunnel length ranging from L* = 8–32, and its average value was 90%. However, when the dimensionless tunnel length was L* = 40 or more, part of the smoke flowing under the ceiling of the model tunnel descended to the floor, and the smoke descent phenomenon dramatically changed the flow dynamics of the smoke. As a result, the flame on the gas burner became unstable, and ultimately the ghosting flame phenomenon was observed close to the fire source. We qualitatively discussed the cause of the ghosting flame phenomenon, in addition to combustion efficiency.