Visualization Study of the Vortex Behavior and Fluid Layer Formation of a Periodically Pulsating Jet under a Ceiling

The vortex behavior and thickness of the nozzle fluid layer of pulsating jets under a ceiling are studied using the laser-assisted visualization technique. The vortex formation characteristics of jets pulsating at different frequencies are compared with those of continuously issuing jets to investigate the effects of the pulsating frequency on the thicknesses of the nozzle fluid layers under a ceiling. To identify similarities between the features of pulsating isothermal jets and periodically pulsating pool fires, the frequencies of the pulsating jets are determined using the results obtained from previous pool fire studies. The formation and propagation characteristics of vortices in pulsating jets are investigated, along with the effects of the pulsation frequency on the thickness of the nozzle fluid layer. The size of the first primary vortex generated decreases as the pulsation frequency of the jet increases. The thickness of the nozzle fluid layer of pulsating jets is larger than that of continuously issuing jet, although the first primary vortex is smaller. The thickness of the nozzle fluid layer increases with decreasing pulsation frequency in jets with the same Reynolds number. The results of this study show the possibility that the ceiling jet behavior including the thickness of the smoke layer induced by a pulsating real fire plume can be strongly related to the pulsation frequency of the plume.