Longitudinal ventilation is widely used to control smoke movement in tunnels in the case of fire;however

the influence of longitudinal ventilation on fire propagation in tunnels has not been thoroughly investigated for the influence includes two contrary aspects.On the one hand longitudinal ventilation can supply abundant oxygen which can enlarge the fire size and increase the probability of fire propagation;on the other hand longitudinal ventilation can carry much of the heat away from tunnels and decrease the probability of fire propagation.This paper investigates the influence of longitudinal ventilation on fire propagation in tunnels.First

the influence on fire size is analyzed;then

the Fire Dynamics Simulator（FDS） is utilized to simulate the fire propagation between vehicles under different ventilation velocities and corresponding fire sizes.The laws of different longitudinal ventilation velocities on fire propagation are achieved and the measures to control fire propagation are proposed.The results show that

along with the increase of ventilation velocity

the heat release rate of fire is greatly increased;as for the influence on fire propagation

when the ventilation velocity is lower than 2 m/s

the distance of fire propagation increases with the velocity′s increase

but when the ventilation velocity is larger than 2 m/s

it has little influence on fire propagation.