THERMAL SCIENCE

International Scientific Journal

Zhiwen Wang

,

Yujia Zhao

,

Jingtai Shi

,

Youbo Huang

,

Chengjia Luo

EXPERIMENTAL STUDY ON FLAME CHARACTERISTICS IN A BRANCHED TUNNEL FIRE WITH ASYMMETRIC MAINLINE SLOPES

ABSTRACT
Flame length and flame tilt angle are key parameters for evaluating the tunnel fire hazards and radiative damage. Previous study on flame geometry have mainly focused on the ordinary tunnels, whereas fires in branched tunnels have received little attention. This study conducts a series of small-scale experiments to investigate the flame morphology in a branched tunnel with asymmetric mainline slopes on the two sides of the fire source in the mainline. Three heat release rates including 1.12kW, 1.64kW and 2.8kW, five upstream slopes and downstream slopes of mainline are systematically varied. Results show that the mainline slope significantly affects both flame length and flame tilt angle. The flame inclines toward the higher portal side due to the greater stack effect in that direction. The flame tilt angle and flame length increase monotonically with the tunnel slope as it approaches a unidirectional slope. The induced airflow velocity in sloped mainline causes the flame tilt angle to exceed 35°. Under a relatively large upstream slope, a larger downstream slope weakens both the flame length and the flame tilt angle. A new dimensionless airflow velocity uin* is introduced to refine the existing mathematical formulation for flame length, and a novel prediction model is established. The cosine of the flame tilt angle is well correlated utilizing Q∗1/5Fr1/2. These findings and the developed models provide the validated tools for fire risk assessment and the optimization of tunnel fire safety systems.
KEYWORDS
tunnel fire, flame length, branched tunnel, natural ventilation, induced airflow velocity
PAPER SUBMITTED: 2026-04-13
PAPER REVISED: 2026-05-29
PAPER ACCEPTED: 2026-06-04
PUBLISHED ONLINE: 2026-08-08
DOI REFERENCE: https://doi.org/10.2298/TSCI260403158W
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Experimental study on flame characteristics in a branched tunnel fire with asymmetric mainline slopes

© 2026 Society of Thermal Engineers of Serbia. Published by the Vinča Institute of Nuclear Sciences, National Institute of the Republic of Serbia, Belgrade, Serbia. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International licence