System velocity distribution law of central exhaust mode with independent smoke exhaust duct in highway tunnel is investigated in this paper.The fire dynamics simulation software FDS is used to simulate tunnel fire under the central exhaust mode.The simulation cases include bidirectional exhaust condition

unidirectional exhaust condition and different smoke exhaust valve setup plans.The velocity distribution law of smoke exhaust duct and smoke exhaust valves is studied.According to the research

the velocity of smoke exhaust duct and smoke exhaust valves presents symmetric distribution under bidirectional exhaust condition

and presents asymmetric distribution under unidirectional exhaust condition.While the cross section of smoke exhaust duct is fixed

changing the setup plan of smoke exhaust valve has little effect on the velocity of the end of smoke exhaust duct.With the increase of opening area of smoke exhaust valve

the velocity of smoke through exhaust valve

which is near the fan

gradually decreases

but the smoke exhaust rate of this valve gradually increases.The velocity distribution of smoke exhaust duct and smoke exhaust valves depends on the smoke exhaust rate

the setup plan of smoke exhaust valve

the cross section area of smoke exhaust duct

etc.This paper provides a basis and reference for the reasonable design of system velocity of central exhaust mode in highway tunnel.