On the fluid dynamics of the make-up inlet air and the prediction of anomalous fire dynamics in a large-scale facility

The present paper is focused on the fluid dynamics of the make-up air at the vents in case of an atrium fire, its influence on the fire-induced conditions and the necessity of properly modelling it to obtain a reliable numerical prediction. For this aim, experimental data from two full-scale atrium fire tests conducted in a 20m cubic facility, with venting conditions involving mechanical smoke exhaust and make-up air velocities larger than 1m/s, and with different fire powers, are presented. Subsequent numerical simulations of these tests have been performed with the code Fire Dynamics Simulator v5.5.3. Two different approaches have been followed to simulate the make-up air inlet fluid dynamics, involving one domain which only considers the inside of the building and another which includes part of the outside. In the former simulations, anomalous phenomena around the fire appear, while the inclusion of part of the exterior domain results in completely different fluid dynamics inside the facility, which agrees better with the experimental data. A detailed analysis of the fluid mechanics at the air inlet vents is conducted to explain these discrepancies. Finally, further simulations are performed varying the make-up area and the exterior extension to assess the appearance of the aforementioned phenomenon. ► Original and new experimental fire tests within a 20m cubic atrium. ► Simulations with FDSv5.3.3 validated experimentally. ► Physical or non-physical nature of the numerical results depending on the modelling of the make-up inlet air.