Study on Smoke Spreading Characteristics of Highway Tunnel Fire

In the event of a fire in a road tunnel, the effect of natural smoke exhaust alone is far inferior to that of mechanical smoke exhaust. Study of the characteristics of the flux of smoke gases inside the tunnel with the use of mechanical exhausts in case of sudden fire in road tunnels by means of specific model simulation methods, specific analysis by setting fixed exhaust spacing and different emission volumes, and the addition of fine water mist is studied. The results show that: when the exhaust vent spacing is 30m, the smoke exhaust volume in 30m³/s, 40m³/s, 50m³/s mechanical smoke exhaust device under the action of temperature, CO concentration and visibility will not affect the safe evacuation of the personnel, but taking into account the control of smoke propagation and economic reasonableness, the smoke exhaust volume of 40m³/s is better than the smoke exhaust volume of 30m³/s and 50m³/s. Under the coupling effect of mechanical smoke exhaust system and fine water mist, the spread of smoke in the tunnel can be controlled within a certain range and the internal temperature of the tunnel can be reduced. It can provide a certain basis for the design of the exhaust vent and the setting of water mist in the future.


Introduction
Since the 20th century, the volume of automobile transportation has been increasing, the standard of highway routes has been raised accordingly, and the number of tunnels year-on-year more.Because of the structural characteristics of the tunnel itself, it is not easy for firefighters to rescue people and vehicles when a fire occurs inside.For this reason, the study of fire accidents in highway tunnels is meaningful.
In earlier literature, the authors have done extensive research on the tunnel fires [1].Using FDS software to build a tunnel model to simulate fire scenarios at different wind speeds, which increases the reliability of personnel arriving at safe locations.Guo Chao et al. [2] studied the situation at two pointof-fire.Explains the differences in ventilation, exothermic rate, the temperature along the higher vertical, and the whole process of carbon monoxide.Finally, it was discovered that two different focal points influenced the heat discharge rate and the burning of the fire.Magdolenová Paulína [3] studied the arrangement of the water mist device and nozzles when the power of the fire source is 15 MW.Cao Zhengmao et al. [4] taking the undersea highway tunnel as the experimental target, a rectangular underwater tunnel gas spread control calculation model was established using FDS software.By analyzing the distribution of fumes and the efficiency of exhaust in different circumstances, the adaptability of the horizontal centralized smoke exhaust method for variable width tunnels is derived.
Jun-huan PENG et al. [5] using the fire simulation tool FDS to analyze the operation of different fans in the proposed semi-perpendicular ventilation tunnel of the southern source road, and to make a concrete basis for the improvement of semi-vertical ventilation conditions of the Southern source tunnel fire.Yu Pei et al. [6] discussed the influence of smoke vent area, spacing, smoke vent opening scheme, and longitudinal ventilation on the effectiveness of smoke venting in double shield highway tunnels.Zhou Jialong [7] conducted a comparative analysis of different jet fan arrangement schemes for smoke control, and also studied the effects of different atomization cone angles, fog fluxes, and fog momentum on the spread of smoke and heat insulation in the escape channel.Effect of smoke control using FDS on smoke production with different tunnel widths, different fire source powers and different exhaust port parameter settings by Junmei Li et al. [8].
To summarize, the previous researchers mainly focus on the longitudinal ventilation and smoke exhaust in tunnel fires, and there are very few studies on the coupling of mechanical smoke exhaust and water mist together.Therefore, this paper adopts pyrosim modeling for the numerical simulation of tunnel fire, and carries out specific research and analysis on the parameters of smoke exhaust spacing and volume as well as fine water mist to observe the extent of its influence on the transmission of fumes in tunnels and temperature and CO concentrations.The smoke generated by the fire was maximized to give more time for evacuees to be rescued.

Numerical Modeling
This paper adopts Pyrosim (Thunderhead Engineering PyroSim), a smoke flow analysis software developed by the National Institute of Standards and Technology (NIST), to establish a tunnel fire model.According to the "Highway Tunnel Design Code I 2014", highway and primary road tunnels with a length of more than 1,000 m, and tunnels with a length of more than 2,000 m in the second to fourth level should be equipped with a mechanical smoke exhaust system [9].
(1) Tunnel model The tunnel selected for the study is a single-bore bidirectional tunnel with a total length of 3316 m for the left line and 3320 m for the right line.When the study tunnel is shorter (less than 1 km), the whole tunnel can be selected as the study object, and if the tunnel under study is too long, it is necessary to select the segmented study [10].The tunnel is too long to be studied, so it is necessary to choose a sectional study.For long-distance highway tunnels, a smoke partition is generally divided by 300 m spacing.Therefore, the tunnel studied is selected 300 m, the section is selected with a square intersection of 12 m width and 7 m height.The tunnel wall boundary is a "Rectangular section".The boundary of the tunnel wall is "CONCRETE", the thickness of the wall is 0.5 m, the exit to both ends of the tunnel is "OPEN", and the exhaust vent is "EXHAUST".This paper does not consider the effects of isolated wind speed interference on the evacuation of fumes and fine-water fog extinguishing systems..The smoke extraction duct is above the traveled way, separated by the top septum, which is at the position of 2m from the top of the tunnel.The size of the exhaust vent is 4m×2m, with a spacing of 30 m.The water mist nozzles are arranged at a horizontal height of 0.5 m from the top septum, 3 m from each side wall of the tunnel, with 7 nozzles in each row spaced 20 m apart in two rows uniformly distributed, within 120 m of each left and right of the fire source, to study the nearest areas most vulnerable to the source of fire.The three-dimensional map of the tunnel is shown in figure 1   (2) Fire source setup A fire in a minibus has a fire power is generally 5 MW, and a medium-sized fire caused by a minivan or bus corresponds to an ignition source power of 10 to 20 MW.The design fire power is conservatively taken as 15 MW.The design fire power is conservatively taken as 15 MW, and the center of the tunnel is set with a fire point of 4 m (long) × 2 m (width) × 1 m (high) considering the most unfavorable working conditions.The fire source is set as t 2 super fast fire, the fire growth coefficient is 0.1876, when the power is 15 MW, the combustion reaches the stable stage after 283 s, and the simulate fire occurrence time set to 360s. (

3) Grid division
The results were validated by the National Institute of Standards and Technology (NIST) and were more accurate when the grid size was set between D*/16 and D*/4, and the calculation time is reasonable.The grid characteristic diameter D * is: where: Q is the rate of heat release from the fire source, kW; ρ∞ ambient air density, kg/m3 ; from, cp is the specific heat capacity of air, kJ/(kg-K); T∞ is the ambient air temperature, K; g is the acceleration of gravity, m/s 2 .D * is calculated to be 2.84 m, and set grid size in the range of 0.18 m to 0.71 m.Considering the accuracy of the simulation and the operation efficiency of the computer, this paper adopts the segmented grid method, set the grid in the 100m downstream above the fire source to 0.25m x 0.25 m x 0. 25 m, and the remaining grid set to 0.5m x 0.5m × 0. 5 m.
(4) Measurement point arrangement Slices are arranged in the tunnel along the longitudinal centerline, in order to monitor the tunnel longitudinal temperature, CO concentration, visibility and other influencing factors, the layout of the measurement points in the human eye characteristics of the height level, that is, from the ground at 2m intervals of 15m, the monitoring object for the temperature, visibility and CO concentration.

Analysis and Discussion of Simulation Results
(1) Analysis of longitudinal spread of smoke In 20L/min of fine water mist and exhaust vent spacing of 30m mechanical smoke exhaust device coupling role, as shown in figure 2 for the smoke volume were 30m³/s, 40m³/s, 50m³/s time in 50s and 100s smoke spread distribution.
From figure 2, one can well perceive that the high-temperature smoke spreads along both sides of the tunnel as the time increases.Figure 2(a) shows the smoke spreading situation under three kinds of mechanical smoke exhaust devices with different smoke volume at 50s.Mechanical evacuation began during this time, as shown in the figure, by two exhaust devices located nearest to the source of the fire, and the smoke from the ceiling is sucked into the exhaust pipe through the exhaust vent, which reduces the spread of smoke from the ceiling.The exhaust volume of 40 m 3 /s of fumes flows to 25m to the left and to the right of the source of fire, and the volume of 30m 3 /s and 50m 3 /s of smoke flows into 35m left and right of that source.Figure 2(b) for the time in 100s in three different smoke exhaust volume of mechanical smoke exhaust device under the smoke spread, time in 100s when the smoke volume of 30m³/s smoke spread to 85m upstream and downstream of the tunnel, in the time it can be seen that the two nearest to the source of the fire near the smoke outlet smoke pipe has been filled with smoke, from the source of the fire upstream and downstream of the smoke exhaust device began to exhaust smoke at 60m.Smoke volume of 30m³/s and 50m³/s under the smoke spread to the fire upstream and downstream near 70m.Smoke volume set to achieve both reasonable control of smoke and to achieve economic rationalization.Exhaust volume is small for the sudden occurrence of high-power fire to get the smoke effect may be poor, but the exhaust volume is too large to easily affect the actual smoke effect, will make the proportion of smoke discharged from the air to increase, affecting the actual smoke effect.In practical application, longitudinal air velocity can be introduced, and setting the longitudinal air velocity can reduce the length of fumes spreading upward at the place of the fire, which is helpful for the evacuation of the people on the scene and the rescue of the firefighters.(2) Temperature distribution analysis Figure 3 shows the temperature distribution of the longitudinal center section of the tunnel under 30m 3 /s, 40m 3 /s, 50m 3 /s at the time of 100s. Figure 4 for the distance from the fire source upstream and downstream of a total of 240m range, diagram of temperature variation distribution at 2 metres from the surface of the human eye.
From figure 3, it can be seen the spreading distance of the temperature in the tunnel under the effect of three different smoke exhaust volumes is controlled within 70m upstream and downstream of the fire source, and the two exhaust vents closest to the fire source gather a large amount of hightemperature smoke, which gathers a large amount of smoke below the ceiling, and the temperature around the fire source is high.30m³/s exhaust volume is more than the vertical diffusion of hightemperature smoke from the roof with the exhaust volume of 40m³/s, and 50m³/s.The smoke exhaust volume of 30m³/s is more than that of 40m³/s and 50m³/s.
From figure 4, one can well perceive that the temperature exceeds 100℃ at the characteristic height of human eyes within 30m from the upstream and downstream of the fire source under the effect of three kinds of smoke exhaust volume, under the influence of fine-water fog and smoke extinguisher, the overall temperature of the position is controlled below 50°C at 60m from the left and right sides of the place of fire occurrence, and the temperature is not more than 30℃ under the effect of the exhaust volume of 40m³/s and 50m³/s, which has played a good cooling effect.The temperature of the surrounding area of the fire was relatively low and the amplitude of the temperature curve is larger.Fire occurred within 6 min, the fire source 30 m distance away from the location of the human eye feature height at the temperature shall not be higher than 70 ℃.Under this condition, the three types of smoke exhaust volume all meet the temperature requirements for safe evacuation of people, and will not affect people's healthy life.(3) Analysis of CO concentration Figure 5 shows the distribution of CO concentration in the longitudinal center section of the tunnel at 100s when the smoke volume is 30m³/s, 40m³/s and 50m³/s respectively.Figure 6 shows the distribution of CO changes at the height of human eye characteristic level, that is, 2m from the ground, within a range of 240m to the left and of the place of the fire.
From figure 5, one can well perceive that under the effect of three different smoke exhaust volume, the spread distance of CO in the tunnel is controlled within 75m from the left and right sides of the place where the fire occurred, the distribution of the spread of CO concentration is approximately the same as the distribution of the spread of the temperature, higher concentrations of CO distribution above and around the place where the fire occurred, with a sharp decrease of CO concentrations 75m to the left and right of the place of the fire, the smoke exhaust outlet in time to take away a part of the CO, so that the ceiling at the concentration of CO values decreased.
From figure 6, one can well perceive the CO concentration exceeds 100 ppm at the specified altitude within 60m on the left and right side of the place where the fire occurred, under the influence of three types of exhaust emissions, and the CO concentration above the fire source and around the roof is high and exceeds 200 ppm, which will not be harmful to human beings at short notice.Outside of 75m to the left and right of the place where the fire occurred, the CO concentration decreases abruptly and is controlled below 100ppm, which will not affect people left the area around the fire smoothly when the fire occurred.(4) Visibility distribution analysis Figure 7 shows the visibility distribution of the longitudinal center section of the tunnel at 100s when the smoke volume is 30m³/s, 40m³/s and 50m³/s respectively.Figure 8 shows the distribution of CO changes at the height of human eye characteristic level, that is, 2m from the ground, within a total of 240m to the left and right of the place where the fire occurred.
From figure 7, one can well perceive that the visibility spread distance in the tunnel is controlled within 80m to the left and right of the place where the fire occurred under the effect of three different smoke exhaust volumes, and the distribution of visibility is approximately the same as the distribution of smoke spread.Under the action of the four exhaust vents near the place of the fire, the visibility distribution in the middle and lower part of the tunnel shows a wave shape, and the visibility distribution thickness of the 40m³/s smoke exhaust volume is smaller than that of the 30m³/s and 50m³/s exhaust volumes.
From figure 8, the visibility at the characteristic height of the human eye is not greater than 10m, within 40m from the left and right sides of the place where the fire occurred, under the influence of the three exhausts, due to the large number of smoke gathered near the site of the fire cause visibility is very low close to 0m.

Conclusions and Recommendations
(1) Different exhaust volume has not much difference on the spread of smoke and the influence on the safe evacuation of personnel, and only has a greater effect on the temperature and CO concentration near the place of occurrence of the fire.Considering the economy, 40m³/s exhaust volume is better than 30m³/s and 50m³/s exhaust volume.
(2) With a certain spacing between the exhaust vents, the coupling of mechanical smoke exhaust and water mist can effectively reduce the temperature inside the tunnel, giving more time for evacuation and fire rescue.
(3) The available safe evacuation time for personnel can be obtained by taking into account the factors that have an impact on the evacuation of personnel from the fire scene, i.e, the critical time for the smoke temperature, CO concentration and visibility to reach the level that will cause harm to the human body.
(a), and the tunnel simulation schematic is shown in figure 1(b).(a) Three-dimensional drawing of the tunnel model.(b) Cross-section of the tunnel model.

Figure 1 .
Figure 1.Schematic diagram of the tunnel mode.

Figure 2 .
Figure 2. Distribution of longitudinal spread of tunnel smoke.

Figure 3 .
Figure 3. Temperature distribution in the longitudinal section of the tunnel.

Figure 4 .
Figure 4. Temperature variation at the characteristic height of the human eye.

Figure 5 .
Figure 5. Distribution of CO concentration in the longitudinal section of the tunnel.

Figure 6 .
Figure 6.Variation of CO concentration at human eye height with time.

Figure 7 .
Figure 7. Visibility distribution in longitudinal section of the tunnel.

Figure 8 .
Figure 8. Visibility at human eye height over time.