Relation between horizontal ventilation velocity and backlayering distance in large closed car parks

Abstract

Due to the ceiling jet phenomenon, smoke above a fire source has a natural tendency to spread under the ceiling in all directions, until a barrier is reached. The present study focuses on smoke control, rather than smoke clearance, in large closed car parks. A particular situation is that the ceiling height is much lower than the horizontal car park dimensions. Also, flame heights can be in the same order of magnitude as the ceiling height, so that flames can penetrate into the smoke layer under the ceiling near the fire source. Smoke control in case of fire in large car parks can be established by horizontal mechanical ventilation. A 'critical ventilation velocity' exists, for which no smoke backlayering occurs, i.e. the car park is maintained smoke-free at one side of the fire source. In many cases, however, backlayering can be allowed to a certain distance. We analyse the results from a large series of CFD-simulations, used as numerical experiments, and illustrate that there is a relation between the horizontal ventilation velocity and the backlayering distance. The backlayering distance varies linearly with the difference between the critical ventilation velocity and the actual ventilation velocity of the incoming fresh air. We perform a parameter study with variation of heat release rate per unit area, fire source area, car park width and car park height. We show that the coefficient in the mentioned linear relationship is independent of the fire source area, the car park height and the car park width, but increases with decreasing heat release rate per unit area. We compare the results for the critical ventilation velocity in car parks to results obtained in tunnel fires. We confirm the observations that the critical ventilation velocity increases with fire source area and heat release rate per unit area, as well as a small influence of the car park width. We observe an increase of the critical ventilation velocity with increasing car park height. Finally, we point out that care must be taken when a smoke control system design is based on volume flow rates, calculated from cold inlet flow velocities, as differences between extraction velocities and incoming air velocities can be substantial.KEYWORDS: large closed car parks, smoke management, horizontal mechanical ventilation, backlayering, CFD, performance-based design, modelling, fluid dynamics NOMENCLATURE LISTING

Numerical Simulations of Flow Fields in Case of Fire and Forced Ventilation in a Closed Car Park

“…The temperature scales from 300 K (in blue) to 900 K (in red). As expected and in line with the results in [8], the lower inlet velocity leads to an increase in back-layering length. Also, with the lower ventilation velocity, the average temperature in the car park is higher.…”