Critical of critical velocity – An industry practitioner’s perspective

Scale modellers have fuelled increased estimates of critical velocity, which have in turn found favour in client’s requirements, and even the US standard NFPA 502. These increased values are causing problems on projects. Left unchallenged, they cause substantial increases in the required numbers of jet fans in road tunnels, and thereby increase capital cost, operational costs and system complexity. Importantly, during a tunnel fire, the resulting higher velocities potentially degrade any smoke stratification and cause a faster fire growth, possibly also fire spread between vehicles. With those incentives, the technical backing for the changes to the critical velocity values resulting from Annex D of NFPA 502 (2020) has been reviewed. The question is: Were the earlier critically velocity estimates flawed, or is there some systematic problem with the more recent approaches to their calculation? Clearly, as they are different by a factor of around 1.6 for a 50 MW road tunnel fire, the old and the new approaches cannot both be correct. The conclusion is that the important physical effects and flow characteristics for determining critical velocity in real tunnels are different to those in scale model tunnels, especially those used to support the 2020 NFPA 502 Annex D equation, and they do not scale to full size. The 2020 Annex D equation does not predict critical velocity in real tunnels.