Study on smoke movement for a fire in a metro depot throat area with different side wall heights

The throat area is an important part of the metro traffic that connects the metro depot and the mainline tunnel. The throat area of 37500 m2 was conducted to investigate smoke movement. The influence of side wall height on smoke movement was considered by combining the theoretical analysis with numerical results. Six sidewall heights including 0 m, 3 m, 5 m, 7.5 m, 9 m, 10 m were designed. The sidewall would have a little impact on free diffusion of smoke when the height of the side wall is lower than 7.5 m. Smoke movement may be affected by the adjacent wall, forming the phenomenon of smoke accumulation when the side height is 7.5 m. It should be considered to expand the openness of the side wall or add mechanical auxiliary smoke exhaust equipment in a local position. This work may provide an engineering reference value for fire smoke control in the throat area of the metro depot.


Introduction
The throat area is the region that connects the metro depot and the main line tunnel.In recent years, in order to optimize the use of land resources, the depot with throat area has been superstructured.According to the Environmental Impact Assessment Report requirements, the throat area needs to be equipped with side walls of different heights and louver-shaped air vents on the wall, which brings some challenges to smoke control in the case of fire in the throat area.
In the study of smoke diffusion characteristics, many scholars have carried out a lot of research from different angles [1][2][3][4][5][6].Han et al. [7] studied the effect of tunnel slope and tunnel blockage on the smoke diffusion characteristics of fires in a single-sided closed metro depot.Li et al. [8] studied the temperature variation of the roof of the closed metro depot, and the final longitudinal temperature decay equation that conforms to the measured temperature is obtained.Liu et al. [9] studied the optimal depth of the smoke curtain to prevent the spread of smoke from the platform to the station hall.
In the past, most scholars studied the fire smoke diffusion characteristics of the subway platform and subway hall [10][11][12] besides the metro depots, especially the throat areas.In this study, the smoke diffusion characteristics would be studied by using theoretical concept analysis and FDS simulation to analyze the relationship between pressure neutral surface and smoke flow as well as the visibility distribution in the throat area under different sidewall heights.Thus, the free diffusion characteristics of the smoke in the throat area of the subway depot can be studied.

Numerical model
As the model shown in figure 1(a), side A is the connection section between the metro depot and the throat area which has the train fire.Model B, D both sides and part of C side set exterior wall which is 3 m in figure 1(a) and h is the height of the side wall.The C side is the connection area between the depot and the main line tunnel.A tunnel entrance is set up next to the B side.The length of the A side is 200 m, the length of the B side is 300 m, and the length of the C side is 50 m.In the following narrative, the model is defined as longitudinal along the B side direction and horizontal along A side.

Working conditions and visibility slices
The fire source was set at the O point position in figure 1

Mesh and boundary condition settings
McGrattan [13] suggests that the dimensionless quantity D*/δx ranges of dimensionless quantity should be 4 to 16, where the characteristic diameter of the fire source D* is calculated by the equation ( 1):

Results
When fire lasted for 200 s in figure 3, the smoke has spread to the side wall closest to the fire source.Combining with figure 1, the smoke has spread to the side wall position on both sides A and B at this time (for the cross-sectional visibility distribution map, the right edge corresponds to the B side of the model, and the left edge corresponds to D side of model), however, the smoke spread to the B side wall is affected by the height of the wall.When the height of the side wall is 9 m, the opening space in the upper part of the side wall does not meet the conditions of free diffusion of smoke to the outdoors so that the smoke on the edge of the side wall has shown a downward accumulation.In addition, the smoke from the cross-section of the fire source did not diffuse to the C side wall and the smoke diffusion corresponding to the lateral slice position was still not affected by the height of the side wall in figure 3.There is basically no impact on the evacuation of personnel.It can be seen from figure 4 that when the height of the side wall reaches 9 m, the side wall has some effects on the free diffusion of smoke, compared with the results about the 10 m height of the side wall, it is obvious that the thickness of the smoke layer is greater when the openness is 0%.This indicates that when the height of the side wall is 9 m, although the opening area is not large, it can diffuse a part of the smoke.It also indicates that the height of the side wall of 9 m can no longer fully meet the conditions of free diffusion of smoke to the outdoors, the thickness of the smoke layer inside the area gradually increases.At this time, there is still no impact on the evacuation of people.The influence of different side wall heights on the thickness of the smoke layer is obvious when the fire occurs at 500 s in figure 5.When the side wall height is 7.5 m, compared with the height of the side wall of 3 m and 5 m, the space with visibility close to 30 m in the throat area is getting less and less.The thickness of the smoke layer has increased, indicating that the 7.5 m high side wall has an adverse effect on the smoke diffusion.The 9 m high side wall has an obvious effect on the smoke, which basically does not meet the conditions of free diffusion of the smoke to the external environment.
Through figure 6, when the side wall height is 5 m at 600 s, the smoke diffusion has tended to a stable state and can be considered that the 5 m high side wall can meet the free diffusion of smoke to the external environment.When the side wall height is 7.5 m, with the increase of the fire occurrence time, the visibility distribution at the time of the fire 600 s has a slight sinking phenomenon and the thickness of the smoke layer increases slightly in figure 5a.But at this time, it is less influence.When the height of the side wall is 9 m, the visibility distribution during the fire 600 s is greatly affected, indicating that the influence of the side wall on the free diffusion of smoke is obvious in figure 6 4. Conclusion Through the analyses of the smoke diffusion characteristics of the throat area of the metro depot, main conclusions are drawn as follows.
(1) If the adjacent wall has little impact on the free diffusion of smoke, natural smoke exhaust would occur at side wall upper part when the openness is 25%.Therefore, it has no effect on the evacuation of personnel, which meets the requirements of the specification.
(2) If the adjacent wall would affect smoke movement, the smoke emission at side wall upper part is greater than air inflow when the side height is 7.5 m.When the opening conditions of the side cannot fully meet the conditions of free diffusion of smoke outward, the phenomenon of smoke accumulation would happen.
(a)  The heat release rate is 7.5 MW.Six sidewall heights including 0 m, 3 m, 5 m, 7.5 m, 9 m, 10 m were designed.The openness corresponds to 100 %, 70 %, 50 %, 25 %, 10 %, 0 % respectively.When a fire occurs in a large space, the smoke generally presents the spread of the ceiling jet.The visibility distribution of the longitudinal and transverse sections of the center of the fire source can best reflect the characteristics of the jet of the smoke roof, then reflect the free diffusion characteristics of the smoke in the entire space figure 1(b).

Figure 1 .
Figure 1.Numerical model of throat area space.

1 )
Grid independence test results are shown in figure 2, when the size of the grid is 0.25 m  0.25 m  0.25 m, temperature distribution curve is relatively smooth, comparing with other grid.The grid setting is divided into two regions, the grid size of center longitudinal and transverse section of the fire source are 0.5 m  0.5 m  0.25 m, and the other parts are 1.0 m  1.0 m  0.5 m, as shown in figure 1(b).

Figure 2 .
Figure 2. Vertical temperature distribution at the 30 m on the longitudinal section of the center of the fire source under the conditions of different center grid sizes.In this study, the influence of external wind speed and other factors is not considered.In order to reflect the spread state in the free state of the smoke, the opening part of the model is set to a fixed size while the corresponding boundary condition inside is set to "open".The ambient temperature is set to 20 °C.

Figure 3 .
Figure 3.The fire started for 200 s, visibility distribution map of vertical and horizontal section through the center of the fire source.

Figure 4 .
Figure 4.The fire started for 400 s, visibility distribution map of vertical and horizontal section through the center of the fire source.
Visibility distribution of transverse sections of the fire source (a) Visibility distribution of transverse sections of the Longitudinal section visibility distribution in the center of the fire source (b) Longitudinal section visibility distribution in the center of the fire source

Figure 5 .
Figure 5.The fire started for 500 s, Visibility distribution map of vertical and horizontal section through the center of the fire source.

Figure 6 .
Figure 6.The fire started for 600 s, Visibility distribution map of vertical and horizontal section through the center of the fire source.