Performance of smoke confinement during fire accident in subway station

: Fire accidents often lead to tremendous losses and smoke has been reported to be the main cause of these heavy casualties. Accordingly, smoke movement can influence escape and smoke confinement is particular significant. Fire Dynamics Simulator software (FDS) was utilized to carry out complementary full scale numerical simulations of fire accident in a subway station. Different measure of smoke prevention and exhaust were adopted and the difference of smoke confinement among the measure was compared. According to the smoke temperature, hung wall had a good role during early stage of fire accident. However, the performance became worse with smoke accumulation. If the hung wall were adopted, smoke was exhausted from basement 2 and air was supplied into the basement 1 at the same time, the efficacy of smoke confinement was the best. The mechanical exhaust rate had obvious implications for the smoke diffusion.


Introduction
During the urban construction, metro has a rapid increase to relieve traffic congestion. Subways are well known for their advantages such as: high speed, low energy consumption and so on. Most of subway stations belong to underground buildings. Once there is a fire accident, smoke diffuse rapidly, which play a bad role on evacuation and lead to heavy casualties and property losses [1,2] . For example, in 2005, fire broke out at subway station in London, England and resulted in the deaths of 56 people, and an additional 700 injuries. In 2015, because of an electrical wiring problem, there was a fire accident that took place at Doris square subway in Washington, D.C., USA and the subway trainn was quickly filled with smoke, which resulted in the deaths of 1 people, and an additional 81 injuries [3] . According to statistical data, toxic smoke is the leading cause of death.
During fire accident in subway station, due to the smoke flowing and a few emergency exit, the ratio reach to 85% of the people killed [4] . Consequently, major fire

Description of the research object
The research was conducted in a typical subway interchange station, as shown in Fig. 1. , which had three stories underground. The station hall was a cross-shaped concourse in basement 1 and the dimension was 105m L x 40m W x 5m H. Four exits from basement 1 played an important role on leading to open ground. At the same time, passengers could set down and pick up in basements 2 and 3, with the dimensions of 125m L X 24m W X 5m H and 48m L X 24m W X 5m H, respectively. There was one line in basements 2 and 3, respectively and the two lines were vertical. The direction of the line in basement 2 was east-west and the direction in basement 3 was north-south. In addition, there were vents uniformly installed under the ceiling of each basement and the layout of these vents is shown in Fig. 1. The dimensions of the vents were 1m X 1m. Some vents were as inlets and some vents were as outlets. During a fire incident the ventilation system, used during normal operation, had to be able to exhaust the smoke and control the smoke diffusion.
During fire accident, outlets played role on exhaust smoke and inlets played role on sending fresh air. The screen door system was set and the fire accident broken out in the station was researched.

Description of the research method
The Fire Dynamics Simulator (FDS) code was used to simulate the heat and smoke propagation phenomena with different measure of smoke confinement in case of fire [6].

Simulation result and discussion
The fire broke out in platform and FDS software was used to simulate the smoke diffusion in the station.

Conclusions
A numerical simulation were performed to research the smoke diffusion with different smoke confinement modes. In this paper, the influences of the effect of hung wall and mechanical ventilation was discussed. According to simulation results, hung wall really played a good role in preventing smoke diffusion at fire early stage. With the duration of the fire accident, more and more smoke accumulated, which disable smoke confinement. If hung wall was installed and the mechanical ventilation was activated, the efficacy of smoke confinement was best.
It was obvious that the efficacy became better with the increase of the rate of smoke exhaust.