Analysis of experimental data on the effect of fire source elevation on fire and smoke dynamics and the critical velocity in a tunnel with longitudinal ventilation

A 1/3 scaled tunnel model was built to explore the effect of fire source elevation on fire and smoke behavior, and especially the critical velocity, in a tunnel with longitudinal ventilation. An experimental campaign has been carried out with a series of tests in 9 configurations, varying the fire size, ventilation velocity and fire elevation. Results in terms of combustion rate, ceiling temperature and critical velocity are discussed. If the flame reaches the ceiling when the fire source is elevated, the fuel burning rate increases by 15% due to increased heat feedback created by the part of the flame below the ceiling. The location of the maximum ceiling temperature can move towards the downstream region in such conditions. A variable named as the height of the effective tunnel, H', was proposed to substitute the tunnel height, H, in the Li et al. model [1] to consider the effect of fire elevation in predicting critical velocity. The accuracy was illustrated by means of the results of the present tests and those of other research studies. Based on a dimensional analysis, three conditions are identified on the basis of the ratio of the flame height to the height difference between fire surface and the tunnel ceiling, L-fi/H'. This allows to demonstrate the variation of the critical velocity with changing fire elevations. The accuracy was illustrated by means of the results of the present tests and those of other research studies. The increase of fire elevation leads to _1 4 the increase of the dimensionless variables Q* (W/H') (-1/4), L-fi/H' and V* (although this remains sometimes unchanged), rather than only a rise or diminution of critical velocity.