Confinement Velocity for Smoke in Tunnels – How to Poke a Stick at it

In 2023, NFPA 502 changed from the absolute prevention of backlayering (critical velocity) to allowing some backlayering (confinement velocity). That made sense for tunnel safety. However, it is not clear how to design for that. NFPA 502 does not provide any guidance on how to determine such a confinement velocity and there is no reliable calculation method that can be used for real (full-scale) tunnels. The complexity of determining confinement velocity and the parameters that influence backlayering are explored. Those influences are quantified via a CFD model validated for critical velocity, also understanding the importance of parameters like wall roughness, wall temperature etc. Surprisingly, in some circumstances, confinement velocity is no less than critical velocity. Guidance on how to approach confinement velocity for real projects is also offered.