Nele Tilley

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

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.

On the use of small-scale results for the design of ventilation systems in large-scale atria

Many researchers use a set of small-scale atrium experiments to develop formulae for smoke and heat exhaust ventilation systems. They correspond to large-scale atria via Froude-scaling. A similar study can be performed with numerical simulations, where a small-scale setup can be scaled up to compare the results. However, with numerical simulations as well as in experiments, proper scaling is necessary. This paper discusses the dimensionless numbers for scaling. The simulations confirm that scaling based on Froude-number alone seems allowed as long as the flows in all configurations are sufficiently turbulent.

Future directions for video fire detection

To accomplish more valuable and more accurate video fire detection, this paper points out future directions and discusses first steps which are now being taken to improve the vision-based detection of smoke and flames. First, an overview is given of the state-of-the-art detection methods in the visible and infrared spectral range. Then, a novel multi-sensor smoke and flame detector is proposed which combines the multimodal information of low-cost visual and thermal infrared detection results. Experiments on fire and nonfire multi-sensor sequences indicate that the combined detector yields more accurate results, with fewer false alarms, than either detector alone. Next, a framework for multi-view fire analysis is discussed to overcome the lack in a video-based fire analysis tool and to detect valuable fire characteristics at the early stage of the fire. As prior experimental results show, this combined analysis from different viewpoints provides more valuable fire characteristics. Information about 3-D fire location, size and growth rate can be extracted from the video data in practically no time. Finally, directions towards standardized evaluation and video-driven fire forecasting are suggested.