On Li, Lei & Ingason’s Critical Velocity Data Treatment and Their Clarification

Li, Lei & Ingason’s “Study of critical velocity and backlayering length in longitudinally ventilated tunnel fires” (Li et al., the “2010 paper”) proposed a formula for the critical velocity to prevent smoke backlayering, giving results that were, on average, 35% higher than was shown by published test data for the most relevant design case. Through the US National Fire Protection Association’s (NFPA) standard 502, it began to have a big impact on tunnel designs. Their methodology and conclusions have been brought into question (Stacey & Beyer, 2020, and in later correspondence with Fire Safety Journal (FSJ)). Following that correspondence, FSJ published a note by the same authors in March 2024 (“FSJ note”) giving what was called a clarification to the 2010 paper. This letter provides background, essential context, and facts omitted in the FSJ note. The technical issues that we identified with their data treatment were brought to Li and Ingason’s attention first in 2020 (via private communications with Li and Ingason, via the Graz Conference, via two NFPA 502 working group meetings) and again in April 2022 after we wrote to the FSJ Editor. This letter provides a much more complete and accurate clarification, both to Li et al. and the FSJ note.