Numerical Study on the Efficiency of the Point Smoke Exhaust System in V-shaped Urban Underground Road Tunnel

In order to study the exhaust effect of the point smoke exhaust system in V-shaped urban underground tunnel with different structures, effects of the slope differences on both sides of the slope changing point and the operating mode of the exhaust vent on the smoke spread distance, temperature at the height of 2 m above the road and the ceiling temperature were studied by the numerical method. The research results show that for the symmetric V-shaped tunnel, smoke can be controlled in the required range, but for asymmetric V-shaped tunnel with the slope difference on both sides, smoke cannot be easily controlled, ventilation supply is needed to improve the smoke exhaust efficiency. Point smoke exhaust system cannot effectively reduce the maximum temperature of the ceiling above the fire. When the fire source power is large, for the symmetrical V-shaped tunnel with a small slope, some fire prevention measures must be taken to protect the ceiling partition. For the asymmetric V-shaped tunnel, two exhaust vents opened on the small slope side and four opened on the large slope side are suggested for the occupant safe evacuation in tunnel.


Introduction
With the improvement of urbanization and the increasing number of vehicle, the land for urban transportation is becoming increasingly tight.In order to ease the pressure of transportation, building more and more urban underground road tunnels is becoming an important choice for the urban transportation.
Urban underground tunnels are generally located in the central city.The construction is affected by the existing utility tunnel, subway tunnels and other underground structures.The vertical structure is complex, and it is not a simple horizontal or single-slope tunnel, but it's a combination of different forms of V-shaped tunnels.The smoke spread and control in this kind of tunnel must be different with in the tradition tunnels.For the V-shaped tunnel, the stack effect formed by the presence of slope will have a critical impact on the smoke spread.The different slope on both sides of the V-shaped tunnel will make the stack effect on both sides obviously asymmetric.This makes the smoke control more difficult.
The smoke control system in tunnels generally include longitudinal ventilation smoke control systems and point smoke exhaust system.In China, for the urban traffic tunnels, following the Code for fire protection design of buildings (GB50016-2014) [1] and Code for design of urban underground road engineering (CJ221-2015) [2], for the much longer urban underground roads tunnels or tunnels with serious daily traffic jams, point smoke exhaust systems are suggested to be installed.Jie Li [3] by comparing and analysing the two smoke exhaust methods, concluded that the smoke exhaust effect and disaster prevention capability of the point smoke exhaust system are significantly superior.The point smoke exhaust system can effectively achieve the separation of smoke and people, and has become the first choice for smoke control design in many traffic tunnels [4].At present, domestic and foreign studies on tunnel fires are mainly on horizontal and single-slope tunnels [5][6][7][8][9][10][11], and there are also related studies on V-shaped tunnels, such as Xuepeng Jiang [12] and En Xie [13] on the free diffusion law of smoke and smoke back-layering length of V-shaped slopes.But there is a lack of research on the effect of point smoke exhaust in V-shaped tunnels.
The smoke control effect of V-shaped tunnel is affected by various factors, including the slope difference between the two sides of the V-shaped tunnels, the fire heat release rate, the location of the fire source, the operation mode of the smoke exhaust vent, and the length of the V-shaped tunnel.For getting the better smoke control effect in the V-shaped tunnels, the effect of slope difference on the both sides of the V-shaped tunnel and operation mode of the point smoke exhaust system on smoke control in a symmetric V-shaped tunnel would be studied in this paper by the numerical methods.The results are expected to provide some technical support for the design and safe operation of urban underground road tunnels.

Numerical Studies
In this study, FDS simulation software will be used to do the numerical simulations.The schematic of the numerical tunnel is shown in figure. 1 and figure.2. The tunnel is 13 m wide, 6.5 m high, there is a partition with 0.2 m thickness under the arc-shaped tunnel ceiling.the space between the partition and the tunnel ceiling is used as the smoke duct.The clear height is 2.0 m.The numerical V-shaped of tunnel is 400 m long on each side.The slope on both sides is taken as any combination of two slopes of 1%, 3% and 5%.Due to the tunnel allows large vehicles to pass through, the designed fire power is set as 50 MW in this study.The fire source is located as the slope changing point of the V-shaped tunnel, and it is set as a surface fire source with a length of 5m, a width of 3m and a height of 1m above the ground, located at the slope changing point of the V-shaped tunnel.The smoke exhaust rate of the point smoke exhaust system is set as 200 m 3 /s.The size of each smoke exhausted vent is 3m (length) × 2.5m (width), with a spacing of 60m between 2 smoke exhausted vents.The arrangement of smoke exhausted vents is shown in figure .3. Three exhausted vents at both sides of the fire source would be opened for the smoke exhaust when the fire occurs.From figure.4, it can be seen that for the symmetrical V-shaped tunnel, under the free diffusion and smoke exhaust conditions, smoke can spread symmetrically on both sides of the slope changing point, and the slope of the tunnel has little effect on the smoke spread range, and the smoke spread distance along one side tunnel has exceeded 400m at 300s.When the point smoke exhaust system is operated, the smoke can be controlled within the range of 210m-220m at one side tunnel.the smoke spread length is not greatly affected by the slope in the case of the same fire source power and smoke exhaust rate.From figure.5(a), it can be seen that when there is slope difference between the two sides of the slope changing point, the smoke spreads asymmetrically on both sides tunnel.Due to the stack effect, the smoke gradually spreads to the large slope side, and the smoke on the large slope side quickly fills the whole tunnel, while the spread length of the small slope side gradually shortens with time.When the slope difference between the two sides is 4%, the small slope side is basically smoke free after 350s.When the difference of slope on both sides is the same, both are 2%, it can be seen that there is still more smoke spreading to the large slope side when the slope of the large slope side tunnel is larger.
For the V-shaped tunnel with slope difference on both sides, the smoke spread distance is not well controlled by the point smoke exhaust system.With the different smoke vent opening schemes, the distances of smoke spreading to the large slope side and to the small slope are different.The smoke spreads faster and over a longer distance on the large slope side of the tunnel due to the larger slope and stronger stack effect.On the small slope side, the more number of the exhaust vent opened, the longer the distance the smoke spread due to the smoke exhaust fan's suction effect.From figure.5 (b), it can be seen that when the smoke exhaust vents are opened in the same way on both sides of the fire source, the greater the slope difference between the two side tunnels, the faster the smoke spreads on the large slope side, and the shorter the distance the smoke spreads on the small slope side.In the case of same slope difference, the smaller slope of the smaller slope side, the longer smoke spread on this side tunnel.As can be seen from figure.6, for the symmetrical V-shaped tunnel, the use of point smoke exhaust system compared to free diffusion, can reduce the smoke temperature at a height of 2m within the opening range of the smoke exhaust vent by about 20℃.When no smoke exhaust measures are taken, the temperature outside the opening range of the smoke exhaust vent is still high.After adopting the From figure.7(a), it can be seen that for the V-shaped tunnel with slope difference on both sides, when the point smoke exhaust system is used, the high temperature smoke is discharged from the tunnel through smoke exhaust vents, and the temperature on the large slope side the tunnel decreases, while the temperature on the small slope side maintains the outdoor ambient temperature.The temperature distribution at its 2m height does not differ much under different exhaust vent arrangement.In figure 7(b), for different tunnel forms and different smoke vent opening methods, the temperature is lower than 60℃ at 10m away from the fire source.It is more conducive to the safety evacuation of people.From figure.8, it is observed that for the symmetrical V-shaped tunnel, within 30 m from the fire source, the slope can affect the ceiling temperature.The larger the slope, the lower the ceiling temperature.As the distance from the fire source increasing, the slope effect on the ceiling temperature reduces.The length of the high temperature zone at the tunnel ceiling cannot be reduced by adopting the smoke exhaust system.In addition, at the zone of 30 m away from the fire source, the ceiling temperature can be reduced by smoke exhaust system.This is conducive to the structural safety of the partition.From figure 9(a), it is known that for the V-shaped tunnel where the slope difference exists on both sides, the point smoke exhaust system can reduce the ceiling temperature on the large slope side of the tunnel, and has little effect on the ceiling temperature on the small slope side.The difference in ceiling temperature between different smoke exhaust vent opening methods is not significant.The maximum temperature is reduced when the smoke exhaust vent opening method is 2 vents opening at the small slope side and 4 vents opening at large slope side compared with the opening method of 3 vents opening at the small slope side and 3 vents opening at large slope side.From the perspective of protecting the safety of the ceiling partition and the safety of the people in tunnel, it is better to use the former exhaust vent opening method.From figure 9(b), it is shown that for the same smoke exhaust vent opening method, the extraction effect caused by the stack effect is weaker when the slope is smaller on both sides.The smoke flow rate decreases on the large slope side and increases on the small slope side, so the temperature is lower on the large slope side and higher on the small slope side.

Conclusions
The smoke control effect of the point smoke exhaust system in the tunnel is influenced by the slope of tunnel at both sides of the slope changing point, the opening strategy of the smoke exhaust vent and other factors.Under a given designed fire of 50MW and smoke exhaust rate of 200 m 3 /s, the smoke control effect in symmetrical and asymmetrical V-shaped tunnel was studied by the numerical methods, and the results are shown as following: (1) For the symmetrical V-shaped tunnel, when the fire source is located at the slope change point, with the exhaust vent area of 7.5 m 2 and 60 m vent spacing, three vents are opened on each side of the fire source, the smoke can be controlled within the range of 440m in tunnel when the point smoke exhaust system is operation.Comparing with smoke free spread, the smoke spread distance and the temperature at 2 m above the road can be reduced.However, smoke exhaust does not effectively reduce the ceiling temperature.When the fire power is large, the ceiling temperature above the fire is higher, and the ceiling partition should take some protection measures to prevent burning down.
(2) For the asymmetric V-shaped tunnel with the fire source located at the slope change point, the smoke spreads to the side with larger slope in different smoke exhaust vent opening methods, air supply from the end of the tunnel at this side is needed The smoke spread distance on the large slope side is related to the number of smoke exhaust vent opening at the small slope side tunnel.The maximum temperature of the ceiling partition is larger when three exhaust vents are opened at the both side of fire source.Four exhaust vents on larger slope side and two exhaust vents on the small sloe aside are suggested to be opened for protecting the safety of the ceiling partition and protecting the safety evacuation of the occupants.

Figure 4 .
Figure 4. Smoke spread distance variation in single-side of symmetrical V-shaped tunnel

Figure 5 .
Figure 5. Smoke spread distance in asymmetric V-shaped tunnel 3.2.Longitudinal distribution of temperature at 2 m above the road For different slope combinations, the highest temperature values and the temperature values within 5 m before and after the fire source were removed from the simulation results.The longitudinal distribution of temperature at 2m height at each measurement point is shown in figure.6 and figure.7.As can be seen from figure.6, for the symmetrical V-shaped tunnel, the use of point smoke exhaust system compared to free diffusion, can reduce the smoke temperature at a height of 2m within the opening range of the smoke exhaust vent by about 20℃.When no smoke exhaust measures are taken, the temperature outside the opening range of the smoke exhaust vent is still high.After adopting the

Figure 6 .
Figure 6.Temperature variation at 2m height in symmetrical V-shaped tunnel

Figure 7 .
Figure 7. Temperature variation at 2m height in V-shaped tunnel with slope difference on both sides 3.3.Longitudinal distribution of the ceiling temperature The longitudinal distributions of the tunnel ceiling temperature after removing the maximum temperature are shown in figure.8 and figure.9.From figure.8, it is observed that for the symmetrical V-shaped tunnel, within 30 m from the fire source, the slope can affect the ceiling temperature.The larger the slope, the lower the ceiling temperature.As the distance from the fire source increasing, the slope effect on the ceiling temperature reduces.The length of the high temperature zone at the tunnel ceiling cannot be reduced by adopting the smoke exhaust system.In addition, at the zone of 30 m away from the fire source, the ceiling temperature can be reduced by smoke exhaust system.This is conducive to the structural safety of the partition.

Figure 8 .
Figure 8. Ceiling temperature distribution in symmetrical V-shaped tunnel

Figure. 9
Figure. 9 Ceiling temperature distributions in asymmetric V-shaped tunnel