Critical velocity is an important indicator of the smoke control design for longitudinal ventilation tunnel fires.Currently,two calculation methods,i.e.,the critical Froude model(recom-mended by the World Road Association,PIARC)and the piece-wise function model(recommended by National Fire Protection Association,NFPA)are widely adopted in practice.Neverthe-less,the calculation result of the two models shows a significant difference,causing confusion and disagreement regarding tunnel fire safety design.This work analyzes the differences between the PIARC equation and the NFPA equation under various heat re-lease rates,tunnel widths,and tunnel heights,respectively and the reasons are discussed in detail.Then,the smoke control per-formances and feasibilities of these two models are compared with a set of numerical simulations,full-scale and reduced-scale test data.Results show that the PIARC equation fails to describe the correlation between critical velocity and heat release rate due to the fixed Frc value and unrealistic uniform mixing assumption.The PIARC equation will underestimate the critical velocity and the error increases with the tunnel aspect ratio.The NFPA for-mula comprehensively considers the impact of the relationship be-tween the fire plume and the tunnel structure on critical velocity.Therefore,the prediction shows a better smoke control perfor-mance and agrees well with physical test data.Moreover,realistic factors,e.g.,blockage,altitude,fire source rising,and lateral lo-cations will also affect the value of critical velocity and should be considered in future work.
维普
万方数据