Ceiling-jet thickness and vertical distribution along flat-ceilinged horizontal tunnel with natural ventilation

• The heat loss from the ceiling-jet to tunnel walls gives a great influence to the ceiling-jet thickness.
• Correlations to estimate the ceiling-jet thicknesses along the tunnel axis were developed.
• Both velocity and temperature distributions show the top-hat distribution and depict the bulging shape.
• Correlations to represent the temperature and velocity distributions were developed.

A series of fire tests was conducted in a 10.0 m (L) × 0.75 m (W) × 0.45 m (H) model tunnel with a rectangular cross section, and detailed measurements were taken of the temperature and velocity within a quasi-steady state fire-driven ceiling-jet running along the centre of a ceiling.The ceiling-jet thickness was defined as the distance from the tunnel ceiling to the point where the temperature and/or velocity dropped to half of their maximums. Correlations to represent the variation in the ceiling-jet thickness along the tunnel axis were developed with the aid of a theoretical approach. The coefficients included in these correlations were determined based on the experimental results obtained. It was found that the ceiling-jet thickness derived from the temperature was 1.17 times greater than that from the velocity in the tranquil flow region.In the tranquil region, both the velocity and temperature showed top-hat distributions, with a bulging shape from the apex of the distribution towards the tunnel floor. A cubic function and coordinate transformation were applied to develop empirical formulae for the temperature and velocity distributions, which were represented by the dimensionless distance from the tunnel ceiling and dimensionless temperature rise and/or velocity at a given distance from the fire source. The correlation developed for the temperature distribution was compared with the results of large- and full-scale tunnel experiments, which verified its applicability.