Abstract:

During the process of density jump in fire smoke, the ambient air will be entrained in, with the smoke layer thicken obviously, and the mass flow increased, moreover, which will increase the difficulty of evacuation and rescue, and affect the effectiveness of smoke control design. In order to clarify the mechanism and harm of the density jump of smoke layer, the influence of the top obstacle on the density jump of the downstream smoke layer during the horizontal spread of fire smoke along the ceiling was studied by numerical simulation. The results show that without the effect of the top barrier, the density jump of smoke cannot be caused by increasing the heat release rate alone. However, the barriers of the ceiling will suddenly change the flow state of the smoke layer, consequently the density jump occurs. And the change of smoke layer thickness and entrainment mass flow rate is related to the height of obstacles. The gradient Froude number is proposed as the dimensionless number to judge the density jump of smoke layer and to measure its effect.

Key words: fire safety, narrow-long space, fire smoke, density jump, stratified shear flow, numerical simulation