*3.2. Effect of Air-Door Closing Location on the Back-Layering Flow* 3.2.1. Smoke Back-Layering Length

Figure 6 shows the back-layering length in branch 1 and branch 2 for the different schemes when the fire is stable in the ventilation network with a diagonal branch and without smoke control. Under different experimental conditions, the driving force generated by the thermal buoyancy and the force of the longitudinal ventilation are different, so the back-layering length is different. When the ventilation velocity is 1.90 m/s and 1.79 m/s, the back-layering length is shorter, which is 0.25 m and 1.08 m respectively. The smoke flow does not backflow to the diagonal branch and branch 1. When the ventilation velocity is 1.70 m/s and 1.56 m/s, the back-layering flow spreads to the diagonal branch and branch 1; additionally, when the ventilation velocity is 1.70 m/s, the heat release rate is 19.5 kW, the smoke back-layering flow spreads to branch 1 and reaches to 1.50 m from the fire source. Fires at different ventilation velocities, regardless of whether measures are taken to close air-door 1 or 5, after the smoke control, show that the force of the longitudinal ventilation is greater than the driving force generated by the thermal buoyancy, and the back-layering is prevented. It is observed through the glass window that the upstream smoke flows backwards, towards the fire source, with a smoke back-layering length of 0.

**Figure 6.** Back-layering length in the ventilation network with a diagonal branch.

3.2.2. Wind Velocity Variation in Ventilation Network

Figure 7 presents the wind velocity variation in each branch for the different schemes. As can be seen from Figure 7, when the ventilation velocity is large, the fire has little effect on branch 3. For example, the ventilation velocity is 1.90 m/s, the wind velocity of branch 3 is basically unchanged. In the diagonal branch, when the air flow reaches branch 4, the velocity shows a positive value and when the air flow reaches branch 2, the velocity is negative. As seen in Figure 7, the wind velocity of the diagonal branch increases with the development of the fire, which is a result of the increase in the ventilation resistance of branch 2 as well as the air volume of branch 1 to the diagonal branch due to the thermal effect of the fire zone. The wind velocity in branch 4 also increases as the air flow from the diagonal branch is directed towards branch 4. For example, when the ventilation velocity is 1.56 m/s, the combustible material burns more intensely, the smoke flow backs

up towards both the diagonal branch and branch 1, and the wind velocity increases by 19.2% for branch 3, 36% for the diagonal branch and 22% for branch 4.

**Figure 7.** Wind velocity variation for different schemes: (**1**) Air-door 1 closed: (**a**) Branch 3. (**b**) Diagonal branch. (**c**) Branch 4. (**2**) Air-door 5 closed: (**a**) Branch 3. (**b**) Diagonal branch. (**c**) Branch 4.

According to Figure 7, the fire at the different ventilation velocities and closing air-door 1, shows that the wind flow is directed towards branch 1, with branch 3 being essentially windless; the wind velocity increases in the diagonal branch, and branch 4 stabilizes after a rapid reduction in the wind velocity. Based on the previous research results [20], the critical velocity of branch 2, in the different schemes, is shown in Table 4. It can be seen from Table 4 that the wind velocity of branch 2 is greater than the critical velocity after the smoke control. The smoke back-layering flow subsides, and the wind flow of the ventilation network is redistributed and stabilized. Similarly, when air-door 5 is closed, the wind velocity in branch 3 is rapidly reduced and then restored to a stable level, and the wind flow of branch 3 is directed to the diagonal branch and branch 4 is basically windless. For example, if the ventilation velocity is 1.70 m/s, when air-door 1 is closed, the wind velocity of the diagonal branch increases rapidly and then stabilizes at 0.42 m/s, the wind velocity of branch 4 decreases rapidly to 0.40 m/s, and the wind velocity of branch 2 is 1.28 m/s. When air-door 5 is closed, the wind velocity in branch 3 decreases rapidly and then stabilizes at 0.69 m/s, the wind velocity in the diagonal branch increases rapidly and then stabilizes at 0.58 m/s, and the wind velocity in branch 2 is 1.56 m/s. When compared with closing air-door 1, the wind velocity to branch 2 is greater after closing air-door 5. The greater the wind velocity, the more likely the smoke back-layering flow will subside under the action of wind pressure.


**Table 4.** Wind velocity in the ventilation network with a diagonal branch after the smoke control.

#### 3.2.3. Temperature Variation in Ventilation Network

Figure 8 depicts the temperature variation under the roadway roof before and after the smoke control when the ventilation velocity is 1.70 m/s. The temperature variation, over time, for branch 2 on the upstream of the fire source before and after the smoke control is shown in Figure 8(1), and the temperature variation for the diagonal branch before and after the smoke control is shown in Figure 8(2). According to Figure 8(1), after adopting different smoke control schemes, the smoke back-layering flow of the fire source in branch 2 subsides, and the temperature upwind of the fire source decreases rapidly and then returns to an ambient temperature. It can be seen from Figure 8(2) that when the ventilation velocity is 1.70 m/s the temperature of the diagonal branch decreases rapidly, after adopting different smoke control schemes. After closing air-door 5, the wind velocity of the diagonal branch increases with the air flow force of the diagonal branch being greater than the driving force of the smoke flow, which results in the diagonal branch reversing the smoke flow back to the fire source. When closing air-door 1, the smoke back-layering flow from the fire source to branch 2 subsides and no more smoke flow is retrograded to the diagonal branch. After the smoke has retreated to the diagonal branch it is then diluted by the air flow, discharged through branch 4, and the temperature of the diagonal branch decreases.

**Figure 8.** Temperature variation for different schemes when the ventilation velocity is 1.70 m/s: (**1**) Temperature variation at different locations upstream of the fire source: (**a**) Air-door 1 closed. (**b**) Air-door 5 closed. (**2**) Temperature variation in the diagonal branch.
