Abstract: To further improve the lateral smoke exhaust efficiency of tunnel fire, taking the Hong Kong-Zhuhai-Macao Bridge submarine immersed tube tunnel as the research object, FDS numerical simulation software is used to carry out isometric modeling. Set the heat release rate of the fire source to 10, 20, 50 MW, the smoke exhaust rate to 4, 6, 8, 10 m/s, and the aspect ratio to 1:2, 1:1, 2:1, 3:1, a total of 48 sets of the scenarios. Install the "side by side distributed smoke exhaust pipe" at each side exhaust vent, the influence of the aspect ratio of the side-by-side distribution smoke exhaust pipe and the heat release rate(HRR) of the fire source on the smoke exhaust efficiency is explained through control variate method, and analyze the influence of the aspect ratio of this pipe on the improvement rate of the lateral smoke exhaust efficiency. The results show that: （1）Under the condition of the same exhaust rate when the aspect ratio of this pipe is small and the aspect ratio increases from 1:2 to 2:1, the vertical interception range of smoke decreases, and the exhaust efficiency gradually decrease. At this time, the amount of smoke absorbed dominates the exhaust efficiency. When the aspect ratio of this pipe is large, and the aspect ratio is increased from 2:1 to 3:1, the pipe becomes flattered making the exhaust efficiency gradually increase. At this time, the concentration of the smoke absorbed dominates the exhaust efficiency. Therefore, the laterals moke exhaust efficiency first decreases and then increases with the increase of the aspect ratio of this pipe and the minimum value is obtained when the aspect ratio is 2:1.（2）Under the same conditions of smoke exhaust rate and aspect ratio of the pipe, the total amount of smoke generated by combustion increases with the increase of the HRR, while the smoke absorption of the pipe quickly reaches the saturated state, the smoke absorbed by the pipe does not increase significantly, but the total amount of smoke generated by combustion increases, so the exhaust efficiency decreases with the increase of the HRR; Furthermore, under the conditions of the same smoke exhaust rate and different aspect ratios of the pipes, since the smoke absorbed by the pipe has not reached saturation when the HRR is small, there are obvious differences in the exhaust efficiency corresponding to the different aspect ratio. With the increase of the HRR, the smoke exhaust volume of the pipe reaches saturation at this time. Both of the smoke absorbed by the pipe and the total amount of smoke generated by combustion tend to be the same, thus making the smoke extraction efficiency tend to be consistent with the increase of the HRR. In this case, the aspect ratio of the pipe is no longer the main factor affecting the exhaust efficiency, and it should be considered to increase the number of pipes to improve the exhaust efficiency substantially. （3）By comparing the smoke extraction efficiency of the original lateral smoke exhaust system, the lateral smoke exhaust efficiency can be increased by a maximum of 4.3 times after the “distributed smoke exhaust pipe” is installed (the HRR is 10 MW, the smoke exhaust rate is 8 m/s, and the aspect ratio of the pipe is 1:2), and the minimum can be increased by 0.5 times. According to the power function fitting, the functional relationship between the aspect ratio and the improvement rate of the lateral exhaust efficiency is obtained. The improvement rate of smoke extraction efficiency gradually decreases with the increase of the aspect ratio, and finally converges to a certain limit value. The data show that when the aspect ratio is greater than 2:1, the improvement rate is very close to the limit. Although there is a maximum improvement rate when the aspect ratio is 2:1, it will affect the tunnel headroom. Considering economy and practicality, setting the aspect ratio to 1:1 is an optimized design solution.

Key words: side-by-side distribution smoke exhaust pipe, aspect ratio, smoke exhaust efficiency, CO mass flow rate, numerical simulation