*3.3. Exhaust Efficiency*

*3.3. Exhaust Efficiency*  By comparing the mass flow rate of CO in the exhaust duct under different calculation conditions, the efficiency of smoke extraction with different calculation conditions can be judged [30]. When the layout of the smoke vent is Case "4", the increase in total smoke exhaust efficiency is very small with increasing exhaust volume, and the effect of the exhaust rate on total smoke exhaust efficiency is very small, as shown in Figure 11. Under different exhaust volumes, the total exhaust efficiency is more than 93.7% when the layout of the exhaust vent is Case "4". The maximum difference of total smoke exhaust efficiency is less than 1.5% under different exhaust volumes in Case "4". Moreover, the total exhaust efficiency with the same exhaust rate is significantly higher than that of other By comparing the mass flow rate of CO in the exhaust duct under different calculation conditions, the efficiency of smoke extraction with different calculation conditions can be judged [30]. When the layout of the smoke vent is Case "4", the increase in total smoke exhaust efficiency is very small with increasing exhaust volume, and the effect of the exhaust rate on total smoke exhaust efficiency is very small, as shown in Figure 11. Under different exhaust volumes, the total exhaust efficiency is more than 93.7% when the layout of the exhaust vent is Case "4". The maximum difference of total smoke exhaust efficiency is less than 1.5% under different exhaust volumes in Case "4". Moreover, the total exhaust efficiency with the same exhaust rate is significantly higher than that of other smoke vent opening schemes. When the layout of the exhaust vent is Case "3A", the total smoke exhaust efficiency is the lowest compared with other schemes, and the smoke exhaust efficiency is greatly affected by the exhaust rate. When the exhaust volume increases by 20 m3/s, the total smoke exhaust efficiency increases by more than 2.5%. *Fire* **2022**, *5*, x FOR PEER REVIEW 13 of 17 smoke vent opening schemes. When the layout of the exhaust vent is Case "3A", the total smoke exhaust efficiency is the lowest compared with other schemes, and the smoke exhaust efficiency is greatly affected by the exhaust rate. When the exhaust volume increases by 20 m³/s, the total smoke exhaust efficiency increases by more than 2.5%.

**Figure 11.** Total smoke exhaust efficiency under different exhaust rates. **Figure 11.** Total smoke exhaust efficiency under different exhaust rates.

The trend of total exhaust efficiency under different smoke vent opening states is that the higher the exhaust rate, the greater the total exhaust efficiency. However, if the ex-

With the increase in distance from the fire, the smoke exhaust efficiency of the smoke vent downstream decreases rapidly, as shown in Figure 12. The smoke vent with the highest smoke exhaust efficiency is the nearest one on both sides of the fire. In the process of smoke spreading, the smoke will continuously entrain the fresh air in the tunnel, resulting in the decrease in CO content. Therefore, the CO content near the fire source is the highest. For the smoke vent upstream, the farther away from fire, the higher the exhaust velocity, the lower the exhaust efficiency. Moreover, the exhaust volume has little impact on the exhaust efficiency of the vent closest to the exhaust fan. The smoke exhaust efficiency of the other smoke vents upstream increases with the exhaust rate. The smoke is mainly discharged from the two smoke vents nearest to the fire source, and the exhaust volume has little impact on the exhaust efficiency (the maximum difference of the exhaust efficiency with different exhaust rates is less than 4%). When three smoke vents are opened downstream, the smoke vent 150 m away from the fire makes little contribution to controlling the smoke, and the smoke exhaust efficiency is less than 5% under different smoke exhaust

in exhaust volume in Case "5B". This is because the velocity at the smoke vent downstream is far less than that upstream, and the farther away from the fire, the lower the velocity at the smoke vent. As the exhaust volume increases, the smoke may not spread to the smoke vent, which will result in the intake of fresh air instead of smoke. It can be noticed from Figure 12c,d that when the exhaust volume exceeds 260 m³/s, the exhaust

efficiency of the smoke vent farthest downstream from the fire is almost 0%.

rates.

The trend of total exhaust efficiency under different smoke vent opening states is that the higher the exhaust rate, the greater the total exhaust efficiency. However, if the exhaust rate exceeds 240 m3/s, the total smoke exhaust efficiency decreases with the increase in exhaust volume in Case "5B". This is because the velocity at the smoke vent downstream is far less than that upstream, and the farther away from the fire, the lower the velocity at the smoke vent. As the exhaust volume increases, the smoke may not spread to the smoke vent, which will result in the intake of fresh air instead of smoke. It can be noticed from Figure 12c,d that when the exhaust volume exceeds 260 m3/s, the exhaust efficiency of the smoke vent farthest downstream from the fire is almost 0%. *Fire* **2022**, *5*, x FOR PEER REVIEW 14 of 17

**Figure 12.** Efficiency of each exhaust vent under different exhaust rates. (**a**) Case "4", (**b**) Case "5A", (**c**) Case "5B", (**d**) Case "6". **Figure 12.** Efficiency of each exhaust vent under different exhaust rates. (**a**) Case "4", (**b**) Case "5A", (**c**) Case "5B", (**d**) Case "6".

**4. Conclusions**  A set of CFD simulations were performed in single-side point exhaust tunnels with different smoke exhaust rates to explore the impact of the number of smoke vents on smoke control, temperature profile, and smoke exhaust efficiency. The main conclusions are as follows: (1) When there are more than two smoke vents on one side of the fire source far away from the exhaust fan, some smoke exhaust vents will inhale fresh air rather than toxic smoke. When the layout of the exhaust vent is Case "4", the total length of the smoke With the increase in distance from the fire, the smoke exhaust efficiency of the smoke vent downstream decreases rapidly, as shown in Figure 12. The smoke vent with the highest smoke exhaust efficiency is the nearest one on both sides of the fire. In the process of smoke spreading, the smoke will continuously entrain the fresh air in the tunnel, resulting in the decrease in CO content. Therefore, the CO content near the fire source is the highest. For the smoke vent upstream, the farther away from fire, the higher the exhaust velocity, the lower the exhaust efficiency. Moreover, the exhaust volume has little impact on the exhaust efficiency of the vent closest to the exhaust fan. The smoke exhaust efficiency of the other smoke vents upstream increases with the exhaust rate. The smoke is mainly discharged

(3) By analyzing the simulation results of the vault temperature under different HRRs and exhaust rates, an empirical formula of temperature attenuation for Case "4" was

(4) Under the same exhaust volume, the exhaust efficiency is the highest when the layout of the exhaust vent is Case "4". The total smoke exhaust efficiency of the tunnel is more than 93.7% and the maximum difference of the total smoke exhaust efficiency

+0.60eି2.17(

*x*ష*x*ೌೣ <sup>ಹ</sup> )

. The error of the temper-

*x*ష*x*ೌೣ <sup>ಹ</sup> )

at the same smoke exhaust volume as Case "6";

proposed: ∆*T*(*x*)/∆*Tmax*=0.40eି0.147(

ature attenuation model is less than 15%;

spread in single-side point exhaust tunnels is shortest;

is less than 1.5% under different smoke exhaust rates;

from the two smoke vents nearest to the fire source, and the exhaust volume has little impact on the exhaust efficiency (the maximum difference of the exhaust efficiency with different exhaust rates is less than 4%). When three smoke vents are opened downstream, the smoke vent 150 m away from the fire makes little contribution to controlling the smoke, and the smoke exhaust efficiency is less than 5% under different smoke exhaust rates.
