The efficacy of natural smoke extraction in longitudinal ventilation through vertical shafts is compromised due to flame throttling effect, hindering smoke dispersal. This study investigates the influence of upstream throttling effects on longitudinal ventilation using theoretical analysis and numerical modeling of natural smoke exhaust system in tunnels. Parameters such as longitudinal ventilation velocity, vertical shaft positioning, and heat release rate were varied to scrutinize their impact. Experimental validation was conducted using scaled tunnel models. The findings reveal a direct proportionality between the equivalent length of flame throttling effect and the dimensionless smoke back-layering length, with the correlation coefficient dependent on factors such as tunnel wall friction coefficient, dimensions, and smoke layer height. Moreover, the throttling effect exhibits correlations with heat release rate and longitudinal ventilation. Notably, the impact of upstream fire throttling escalates with increasing dimensionless heat release rate (Q*≤0.15 ), while beyond Q*>0.15 , the effect remains consistent regardless of fire size. Additionally, flame throttling effect significantly alters the smoke back-layering length in tunnel fires, necessitating its consideration for computational accuracy in modeling. This study provides computational models and methodologies to incorporate the upstream throttling effect for improved accuracy in predicting smoke back-layering length.
 

tunnel fire, throttling effect, longitudinal ventilation, smoke exhaust system, smoke back-layering length