A critical velocity episode... who would have thought? Even though I'm not an enthusiast of this approach, I have to admire the new science and researchers striving to improve it. This week I welcome Conrad Stacey and Michael Beyer from Stacey Agnew to talk about their recent developments. We'll take you through the historical development of this concept since its inception in 1958, examining key variables like tunnel height and heat release rate along the way. We go into the Memorial Tunnel experimental project, and we discuss the context and the economic implications of recent updates to the NFPA 502 standards, exploring how changes in required ventilation speeds have impacted tunnel design costs and stirred controversy. Conrad and Michael provide fresh insights into the complexities of tunnel smoke control, the distinction between critical and confinement velocities, and the significance of fire intensity in tunnel environments. Focusing on the historical Memorial Tunnel study, they explain the experiments that have been the source of our current models and how revisiting this data with modern CFD techniques allowed them to analyze this even further. With their newly proposed model for assessing critical velocity, the HRR of the fire is not considered as an important variable anymore, and replaced with the fire intensity (HRR Per-Unit-Area). This follows an observation that it is just the front of the fire that interacts with the incoming air, and thus making fire larger by making it 'longer' does not influence the outcomes - a new feature consistent with their newly defined model and Memorial Tunnel data.You can read their study here: https://www.researchsquare.com/article/rs-4278205/v1Some previous takes on the critical velocity by the team: CRITICAL OF CRITICAL VELOCITY – AN INDUSTRY PRACTITIONER’S PERSPECTIVE Critical velocity and the significance of the imminent retraction of 2020 NFPA 502’s Annex D critical velocity equations Part OneCritical velocity and tunnel smoke control Part 2 - Filling the NFPA 502 void----The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.