4.1. Fire Scenario A1
The fire develops rapidly in fire scenario A1. A smoke detector in the fire room activates at 20 s after the fire breaks out. Introducing a safety factor of 2, the tenants are considered to leave the fire room through the door at about 40 s. The smoke detector in the first-floor corridor activates at 60 s.
Figure 7 shows the change curves of heat release rate (HRR) and oxygen concentration in the fire room for scenarios A1 and A2. As can be seen from the figure, the maximum HRR for fire scenario A1 is about 1.9 MW, much less than the set HRR of 3.3 MW. The main reason is the insufficient ventilation. The oxygen concentration in the fire room drops to below 13% after 200 s, which cannot meet the needs of combustion. In fire scenario A2, for which the fire room door is closed, the oxygen concentration drops to 13% at 130 s and the fire is self-extinguished with a maximum HRR of 0.57 kW.
Figure 8 shows the smoke spread at different moments for fire scenario A1. The tenant opens the door 40 s after the fire breaks out. Smoke spreads from the room to the corridor at 50 s. At 100 s, a large amount of smoke spreads to the first-floor corridor, forming a thick layer of smoke. At 150 s, the smoke fills the corridor and staircase on the first floor and begins to spread to the corridor on the second floor. At 200 s, the second-floor corridor is filled with smoke.
Figure 9 shows the FED distribution at different measuring points at 1.7 m height in the corridor for fire scenario A1. The red line value is the limit of FED affecting personnel safety. As can be seen from the figure, the FEDs for points A1 and B1 (locations shown in
Figure 5) of the first floor corridor reach 1 at 265–280 s. The FEDs for points C1, D1, E1, and G1 reach 1 at 310–330 s. The FEDs for monitoring points in the second-floor corridor (A2 to G2 in
Figure 5) reach 1 within 540 s.
Figure 10 shows the corridor temperature distribution at different moments for fire scenario A1. As can be seen from the figure, 100 s after the fire breaks out, the ceiling temperature of the first-floor corridor outside the fire room reaches 100 °C. At 200 s, the temperatures in most areas of the first-floor corridor reach 100 °C. At 300 s, the temperature in the ceiling of the second-floor corridor reaches 100 °C, and at 400 s, the temperatures in most areas of the second-floor corridor reach 100 °C.
Figure 11 shows the temperature distribution at different measuring points at a height of 1.7 m in the corridor for fire scenario A1. The red line value is the limit of temperature affecting personnel safety. As can be seen from the figure, the temperatures at points A1, B1, and C1 in the first-floor corridor reach 100 °C at around 140 s. The temperatures at points D1, E1, F1, and G1 reach 100 °C at around 190 s. The temperatures at the monitoring points in the second-floor corridor reach 100 °C at around 350 s.
Figure 12 shows the visibility distribution of the corridor at different moments for fire scenario A1. As can be seen from the figure, 100 s after fire breaks out, the visibility at the top of the first-floor corridor outside the fire room decreases to 5 m. At 150 s, the visibilities in most areas of the first-floor corridor decrease to 5 m, and the visibility on the ceiling of the second-floor corridor begins to decrease. At 200 s, the visibilities in most areas of the second-floor corridor decrease to 5 m.
Figure 13 shows the visibility distribution of different measuring points at a height of 1.7 m in the corridor for fire scenario A1. The red line value is the limit of visibility affecting personnel safety. As can be seen from the figure, the visibilities for the measuring points in the first-floor corridor fall below 5 m between 80–110 s. The visibilities for the measuring points in the second-floor corridor fall below 5 m between 160–180 s.
Based on the above analysis, the values of FED, temperature, and visibility for monitoring points A1 and G1 at the two exits of the first-floor corridor are taken as the evaluation criteria.
Table 6 gives the available evacuation time for fire scenario A1.
4.3. Analysis of Personnel Safety Evacuation
The fire simulation results show that the smoke detector in the fire room activated at 20 s after fire breaks out. The smoke detector in the corridor detected the fire at around 60 s and 85 s for the scenarios with the door of the fire room opened or closed, respectively. According to the settings, the broadcast alarm of fire to the whole building is made when both the smoke detectors in the fire room and the corridor are activated. Therefore, the fire alarm time is 60 s when the door of the fire room is opened, and 85 s when the door of the fire room is closed.
The sum of the building area to be evacuated is about 400 square meters. According to Equation (6), the time required for start evacuation, tstart, is 5.7 min, or about 340 s. Typical unimpeded horizontal travel speeds are about 1.2 m/s, unimpeded downward speeds are about 0.8 m/s, and upward speeds are about 0.7 m/s. The studied HAHPB has a simple layout and a single evacuation route, and the density of personnel is very low. Therefore, the evacuation process time is determined by the evacuation time of the tenant farthest away from the evacuation exit. It is about 40 m from the furthest point in the guest room at the end of the second-floor corridor to the evacuation door on the first floor. Assuming the speed in the entire evacuation process takes the unhindered downward speed of 0.8 m/s, we get the evacuation process time of 40 m ÷ 0.8 m/s = 50 s. Therefore, the entire evacuation time is 340 s + 50 s = 390 s.
The effect of HAHPB depressurization on the evacuation time has not been addressed in this paper. In normal situations, when people exit the building, they need to enter the transition cabin first, then they need to depressurize the transition cabin to the outdoor pressure. Only then can they escape from the transition cabin to the outside. As the response time of the occupants is not the same during the fire, they cannot reach the transition cabin at the same time, so it will take quite a long time for the transition cabin to depressurize and re-pressurize over and over again. Note that the transition cabin needs to be re-pressurized to open the door which connects the cabin and the other space in the building. To ensure the occupants can enter the transition cabin without waiting, the entire building must be depressurized, which is the subject needing study in the future.
It can be concluded from
Table 6 that when a fire occurs on the first floor, the safe evacuation of the whole building can be satisfied only under fire scenarios A2 and A4, for which the door of the fire room is closed. When a fire breaks out on the second floor, only the second-floor corridor has hazardous visibility under fire scenarios B2 and B4. In fire scenarios B2 and B4, the first-floor corridor is in a smoke-free environment. Therefore, it is considered that in fire scenarios B2 and B4, the personnel on the second floor can escape safely by first stooping through the second-floor corridor and then getting down to the first floor.