Effect of Beams on Ceiling Jet Behavior and Heat Detector Operation

Effect of obstructions on an unconfined ceiling jet induced by a fire plume has been investigated using small-scale experimental simulations. Parallel beams placed on a ceiling with the ceiling height varied between 0.76 to 1.28 m resulted in a range of beam depth to ceiling height ratio of 0.048 to 0.19. The non-dimensional fire size ranged from 0.003 to 0.027, corresponding to a fire size range of 450 kW to 1160 kW for a 3.5 m ceiling. The results show that the ceiling jet velocity and temperature beyond the obstruction, when normalized by the smooth ceiling jet correlation, can be characterized as a simple function of the ratio of obstruction depth to the ceiling height. The data shows a nearly 80% reduction in the velocity and temperature of the ceiling jet flowing over an obstruction, when compared with the smooth unconfined case, for a beam depth equal to approximately 15% of the fire to ceiling height. It is also shown that a nearly corridor flow condition is attained when the obstruction approaches about 20% of the fire to ceiling height. These drastic changes in the ceiling jet temperature and velocity can result in delayed fire detection when a heat detector is present outside the primary bay.