CN116663104A - Design method for critical large-spacing range of key smoke outlets of tunnel - Google Patents

Abstract

The invention discloses a design method for the critical large distance range of key smoke exhaust outlets of tunnels, which includes: step 1, determining the safe thickness of the smoke layer, and step 2, when there is no smoke exhaust function in the tunnel and the fire smoke spreads freely, the Knowing the thickness of the smoke layer h _L , deriving and calculating the distance L from the fire source here; step 3, when there is a key smoke exhaust function in the tunnel, and when the fire smoke is spread by the suction force of the smoke exhaust, the known smoke layer thickness h _L* , the distance L ^* from the fire source is obtained by derivation and calculation; step 4, determine the minimum critical large distance D ₁ and the maximum critical large distance D ₂ of key smoke exhaust outlets, and obtain the critical large distance range of key smoke exhaust outlets in the tunnel. The calculation method of the invention is simple, can quickly obtain the critical large distance range of key smoke exhaust outlets according to different tunnel sizes, has better practical applicability, and provides theoretical support for smoke control of key smoke exhaust fires in tunnels and personnel evacuation safety.

Description

A design method for the critical large distance range of key smoke exhaust outlets in tunnels

technical field

The invention relates to the technical field of tunnel fire smoke control, in particular to a design method for critical large distance ranges of key smoke exhaust outlets of tunnels.

Background technique

Due to the narrow and closed characteristics of the tunnel, the tunnel has a unique spatial structure and fire characteristics compared with other buildings. Tunnel fires seriously threaten the life safety of drivers, passengers and fire rescuers in tunnels. At the same time, tunnel fires will damage the tunnel structure and cause secondary accidents, resulting in greater loss of life and property. The key smoke exhaust mode is widely used in highway tunnels, but the current key smoke exhaust mode has a small number of smoke outlets and a large number of air leakage, resulting in high construction and operating costs. Therefore, under the premise of ensuring the safety of the evacuation environment, it is extremely important to reduce the cost of tunnel construction and operation.

The key smoke exhaust mode means that by accurately controlling the smoke exhaust valve of the smoke exhaust duct and using the nearest smoke exhaust outlet of the traffic tunnel fire accident point to organize smoke exhaust, the smoke exhaust efficiency of the smoke exhaust system is greatly improved. Large spacing means that the spacing of key smoke exhaust outlets breaks through the limit of the conventional spacing of 60m, and the fire smoke can be discharged in a timely and effective manner while ensuring the safety of the evacuation environment.

In the current standard and actual tunnel projects, most of them adopt the conventional key smoke exhaust control mode with the distance between the smoke exhaust outlets not exceeding 60m, which has disadvantages such as small spacing, high cost, large number of smoke exhaust outlets, and serious air leakage. In order to reduce the cost of tunnel construction and operation, and under the premise of ensuring the safety of personnel evacuation environment, break through the limit of 60m smoke exhaust outlet spacing, theoretically analyze and study the range of critical smoke exhaust outlet spacing, and provide key smoke control and personnel evacuation safety for tunnels Provide theoretical support.

Contents of the invention

In order to solve the problems existing in the background technology, the purpose of the present invention is to provide a design method for the critical large distance range of key smoke exhaust outlets in tunnels. For some tunnels with longer lengths and higher construction costs, the method provided by the present invention can be adopted. The calculation method of the critical large spacing range is used for the design and calculation of the smoke exhaust system. Compared with the conventional smoke exhaust port spacing, the large smoke exhaust port spacing range described in the present invention is better. Under the premise of ensuring the safety of the personnel evacuation environment, it can reduce the cost of tunnel construction and operating costs.

In order to achieve the above object, the present invention provides the following technical solution: a design method for the critical large distance range of key smoke exhaust outlets in tunnels, comprising the following steps:

Step 1. Determine the allowable height of the smoke in the tunnel under the conditions that can ensure the safety of the evacuation environment, and obtain the allowable thickness of the smoke layer h _L = h _{L *} according to the design size of the tunnel;

Step 2. Calculate according to the relationship between the thickness of the smoke layer and the distance from the fire source and the size of the tunnel. Under the condition that there is no smoke exhaust in the tunnel and the fire smoke spreads freely, the distance L from the fire source at the place where the thickness of the smoke layer is h _L is, Take 2L as the minimum critical large spacing D ₁ of key smoke exhaust outlets;

Step 3. According to the relationship between the thickness of the smoke layer and the distance from the fire source, the size of the tunnel, and the power of the fire source, calculate the key smoke exhaust function in the tunnel. Under the condition that the fire smoke is spread by the smoke suction force, the thickness of the smoke layer The distance between h _L* and the fire source is L ^* , and 2L ^* is used as the maximum critical distance D ₂ of the key smoke exhaust outlet;

Step 4. Obtain the critical large distance range D of key smoke exhaust outlets of the tunnel, where D ₁ ≤ D ≤ _{D 2} .

In the design method of the critical large distance range of the key smoke exhaust outlet of the tunnel, in the step 1, the conditions for ensuring the safety of the personnel evacuation environment are:

The visibility at the allowable height of the smoke in the tunnel should meet V _z ≥ 10m, the temperature should meet T _z ≤ 60°C, and the allowable height of the smoke in the tunnel is 2m.

In the design method of the critical large distance range of the key smoke exhaust outlet of the tunnel, in the step 2, the relationship between the thickness of the smoke layer and the distance from the fire source is:

In the formula, the smoke layer thickness h _L = (H-2), D ₁ represents the minimum critical distance between key smoke outlets, L represents the distance from the fire source when it spreads freely, W represents the width of the tunnel, and H represents the height of the tunnel.

Optionally, in step 2, when the fire smoke spreads freely, the distance L from the fire source is:

In the formula, L represents the distance from the fire source when it spreads freely, B represents the temperature attenuation coefficient when it spreads freely, h _L represents the thickness of the smoke layer when it spreads freely, as shown in Figure 1, W represents the tunnel width, and ρ _a represents the temperature at ambient temperature Air density, T _a represents the ambient temperature, T _L represents the smoke temperature at a distance L from the fire source when it spreads freely, g represents the acceleration of gravity, β represents the smoke entrainment coefficient, h ₀ represents the thickness of the smoke layer near the fire source, γ Indicates the mass flow rate attenuation coefficient, and H indicates the tunnel height.

In the step 3, the relational expression between the smoke layer thickness and the fire source distance is:

In the formula, the thickness of the smoke layer h _L = (H-2), D ₂ represents the maximum critical distance between key smoke exhaust outlets, L ^* represents the distance from the fire source when there is a key smoke exhaust function, W represents the width of the tunnel, and H represents the tunnel Height, Q represents the fire source power.

Optionally, in step 3, when there is a focus on smoke exhaust, the distance L ^* from the fire source is:

In the formula, L ^* indicates the distance from the fire source when there is a key smoke exhaust function, B' indicates the temperature attenuation coefficient when there is a key smoke exhaust function, and h _L* indicates the thickness of the smoke layer when there is a key smoke exhaust function, as shown in Figure 2 , W represents the width of the tunnel, ρ _a represents the air density at the ambient temperature, T _a represents the ambient temperature, T _L* represents the smoke temperature at a distance L ^* from the fire source when there is a key smoke exhaust function, g represents the acceleration of gravity, β represents the smoke Gas entrainment coefficient, h ₀ represents the thickness of the smoke layer near the fire source, γ represents the mass flow rate attenuation coefficient, H represents the tunnel height, Q represents the power of the fire source, T _e represents the temperature of the exhaust outlet, and _Ve represents the exhaust smoke volume.

Compared with the prior art, the beneficial effects of the present invention are: the calculation method of the present invention is simple, the verification and comparison method is reasonable, and the range of the critical large distance between key smoke exhaust outlets can be quickly obtained according to different tunnel sizes, and the critical large distance provided by the present invention can be adopted. The calculation method of the spacing range is used for the design and calculation of the smoke exhaust system, which is applicable to tunnels of different sizes that adopt the key smoke exhaust method. Through theoretical analysis, this method obtains the critical large spacing range of key smoke exhaust outlets in tunnels, and then compares it with the numerical simulation calculation results to verify the accuracy of the theoretical calculation formula, highlighting the practical applicability of the calculation method for the critical large spacing range of key smoke exhaust outlets in tunnels , the calculated results are innovative and have practical engineering significance, and provide theoretical support for the smoke control of key smoke exhaust fires in tunnels and the safety of personnel evacuation.

Description of drawings

The accompanying drawings described here are used to provide a further understanding of the application and constitute a part of the application. The schematic embodiments and descriptions of the application are used to explain the application and do not constitute an improper limitation to the application. In the attached chart:

Fig. 1 is a schematic diagram of the development of free-spreading smoke in the tunnel of the present invention;

Fig. 2 is a schematic diagram of the development of smoke under the action of key smoke exhaust in the tunnel of the present invention;

Fig. 3 is the tunnel model schematic diagram that the present invention sets up;

Table 1 is the table of operating conditions under different tunnel parameters of the present invention;

Table 2 is the range of the critical large spacing of the exhaust outlets under different parameters of the present invention;

Detailed ways

The principles and features of the present invention will be described below in conjunction with the accompanying drawings and specific embodiments. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

A calculation method for the critical large distance range of key smoke exhaust outlets in tunnels, referring to Fig. 1 and Fig. 2, including the following steps:

Step 1. When there is no smoke exhaust function in the tunnel and the fire smoke spreads freely, the thickness of the smoke layer h _L is known, and the distance L from the fire source here is derived and calculated; specifically:

The development of smoke in a tunnel can be divided into four stages (Figure 1):

1: The plume rises freely and hits the ceiling

2: radial spread

3: Transition from radial to vertical spreading process

4: One-dimensional horizontal spread;

As shown in Figure 1, the thickness of the smoke layer at x=L from the fire source is h _L , and the mass flow rate of the smoke is m _L . Compared with the actual tunnel length, the length of the smoke development stages 1 to 3 is shorter, and the total mass flow rate of the smoke development stages 1 to 3 can be regarded as a whole, and the longitudinal length can be ignored.

m ₁ is the entrainment volume of the symmetrical plume. According to the formula in the article "Entrainment in fire plumes" by Zukoski et al., the mass flow rate of the smoke development stage 1 can be estimated:

m ₁ =0.063Q ^1/3 H ^5/3 (1)

In the formula, H represents the height of the tunnel, and Q represents the power of the fire source.

In the smoke development stages 1 and 2, after the smoke hits the ceiling, it will intensify the mixing with the surrounding cold air, which will increase the mass flow rate of the smoke and thicken the smoke layer, so the smoke enters the initial mass flow rate of the smoke development stage 4 m _t can be expressed by the following formula (take γ=1.90):

m _t ＝γm ₁ (γ is a coefficient) (2)

Further, according to the symmetry of the development of free-spreading smoke, the mass flow rate of the smoke moving left or right after hitting the wall:

m ₂ =1/2γm ₁ (3)

The smoke mass flow rate obtained at a distance of x=L from the fire source can be expressed as:

m _e =ρ _a βWu (5)

In the formula: m _β represents the mass entrainment rate of the smoke development stage 4, β represents the entrainment coefficient, take β=0.005, W represents the tunnel width, u represents the average vertical spreading speed of the smoke layer, ρ _a represents the air temperature at ambient temperature density.

At the smoke development stage 4, at position x, the longitudinal average spreading velocity u _x can be calculated by the following formula:

In the formula, u _x represents the average longitudinal spread velocity of smoke at position x, T _a represents the ambient temperature, ΔT _x represents the dimensionless temperature rise at position x, g represents the acceleration of gravity, and h _x represents the thickness of the smoke layer at position x.

Combining the above formulas, the smoke mass flow rate at the position x=L from the fire source can be changed to:

In addition, m _L can also be expressed according to the basic calculation formula of mass flow rate:

Simultaneously combine the above formulas (1), (7) and (8), and combine the ideal gas state equation The relationship between the distance L from the fire source and the thickness of the smoke layer h _L can be obtained:

T _L represents the flue gas temperature at a distance L from the fire source during free spread, and ρ _L represents the air density at a distance L from the fire source during free spread;

Further, for the convenience of calculation, the left side of the relational expression is simplified, which can be Simplified to the average smoke layer thickness in the range of 0-Lm/> Among them, h ₀ is approximately the thickness of the smoke layer near the fire source (according to Zhao Shengzhong's "Study on the Transport Law of Smoke in Tunnel Fire under the Effect of Longitudinal Ventilation", h ₀ is taken as 0.3m). Formula (9) can be changed to:

Furthermore, according to the study "Decay of buoyant smoke layer temperature along the longitudinal direction in tunnel fires" by Hu Longhua et al., the dimensionless temperature rise and longitudinal distance at x under the ceiling can be expressed by the following formula:

ΔT _x = ΔT _max e ^-Bx (11)

In the formula, ΔT _x represents the dimensionless temperature rise at position x, ΔT _max represents the maximum temperature rise of flue gas, B represents the temperature attenuation coefficient when it spreads freely, and B=0.025, Q represents the fire source power, and H represents the tunnel height.

Combining the above formulas (10) to (12), the relationship between the distance L from the fire source and the thickness of the smoke layer h _L is obtained:

In the formula, L represents the distance from the fire source during free spread, B represents the temperature attenuation coefficient during free spread, taking B=0.025, h _L represents the thickness of the smoke layer during free spread, W represents the tunnel width, ρ _a represents the air at ambient temperature Density, take ρ _a = 1.2kg/m ³ , T _a represents the ambient temperature, take T _a = 293K, T _L represents the flue gas temperature at a distance L from the fire source when it spreads freely, take T _L = 313K, g represents the acceleration of gravity , take g=9.8m/s ² , β represents the smoke entrainment coefficient, take β=0.005, h ₀ represents the thickness of the smoke layer near the fire source, take h ₀ =0.3m, γ represents the mass flow rate attenuation coefficient, take γ=1.90, H represents the tunnel height.

Step 2. When there is a key smoke exhaust function in the tunnel, and the fire smoke is spread by the suction force of the smoke exhaust, the thickness of the smoke layer h _L* is known, and the distance L ^* from the fire source here is derived and calculated; specifically:

Assuming that the "suck-through" phenomenon does not occur at the smoke exhaust port, the smoke layer can still maintain a relatively stable state like free spreading at this time, and it can be considered that the smoke below the smoke exhaust port is still in the one-dimensional horizontal spreading stage (Figure 2).

The amount of smoke generated at the height H of the tunnel section is taken as the key amount of smoke exhaust, which can be calculated by referring to the "Technical Standards for Smoke Prevention and Smoke Exhaust Systems in Buildings" (GB51251-2017):

In the formula: V _e represents the exhaust volume of key points, T _a represents the ambient temperature, Q represents the power of the fire source, H represents the height of the tunnel, c _p represents the specific heat of air at constant pressure, and ρ _a represents the gas density at ambient temperature.

Further, when the smoke spreads to the smoke outlet, part of the smoke will be discharged, so that the mass flow rate m _L* of the smoke here is the mass flow rate m _L of the smoke when it spreads freely minus the discharged _me . The mass flow rate of flue gas at the distance from the fire source x = L ^* smoke outlet can be expressed as:

m _β =ρ _a βWu (5)

m _e = ρ _e V _e (16)

In the formula: m _β represents the mass entrainment rate of the smoke development stage 4, β represents the entrainment coefficient, W represents the tunnel width, u represents the longitudinal average spreading speed of the smoke layer, _Ve represents the smoke exhaust volume of the smoke exhaust system, ρ _a represents the gas density at ambient temperature, and ρ _e represents the smoke density at the exhaust port.

Since the smoke below the smoke outlet is still in the one-dimensional horizontal spread stage, the longitudinal average spread velocity at the x position can still be calculated by the following formula:

In the formula, u _x represents the average longitudinal spread velocity of smoke at position x, T _a represents the ambient temperature, ΔT _x represents the dimensionless temperature rise at position x, g represents the acceleration of gravity, and h _x represents the thickness of the smoke layer at position x.

Combining the above formulas (1)～(3), (5)～(6), (15)～(16), the smoke mass flow rate m _L* at the position x=L ^* from the fire source can be changed to :

In addition, m _L* can also be expressed according to the basic calculation formula of mass flow rate:

Combining the above formulas (17) to (18), the relationship between the distance L ^* from the smoke outlet to the fire source and the thickness of the smoke layer h _L* can be obtained:

Further, for the convenience of calculation, the left side of the relational expression is simplified, which can be Simplified to the average smoke layer thickness in the range of 0-L ^* m/> Among them, h ₀ is approximately the thickness of the smoke layer near the fire source, and h ₀ is taken as 0.3m. Formula (19) can be changed to:

Furthermore, since the smoke below the smoke outlet is still in the one-dimensional horizontal spread stage, according to the research "Decay of buoyant smoke layer temperature along the longitudinal direction in tunnel fires" by Hu Longhua et al. "Research on Transport Law" shows that the dimensionless temperature rise under the ceiling at the exhaust outlet x can still be expressed by the following formula:

ΔT _x = ΔT _max e ^-B'x (21)

In the formula, ΔT _x represents the dimensionless temperature rise at position x, ΔT _max represents the maximum temperature rise of flue gas, B′ represents the temperature attenuation coefficient when there is a key smoke exhaust effect, Q represents the power of the fire source, and H represents the height of the tunnel.

Combining the above formulas, the relationship between the distance L ^* from the fire source and the thickness of the smoke layer h _L* is obtained:

In the formula, L ^* indicates the distance from the fire source when there is a key smoke exhaust function, B' indicates the temperature attenuation coefficient when there is a key smoke exhaust function, take B'=0.015, h _L* indicates the thickness of the smoke layer when there is a key smoke exhaust function , W represents the width of the tunnel, ρ _a represents the air density at the ambient temperature, take ρ _a =1.2kg/m ³ , T _a represents the ambient temperature, take T _a =293K, T _L* represents the distance from the fire source when there is a key smoke exhaust effect Smoke temperature at distance L ^* , take T _L* = 313K, g means acceleration of gravity, take g = 9.8m/s ² , β means smoke entrainment coefficient, take β = 0.005, h ₀ means smoke near the fire source Layer thickness, take h ₀ =0.3m, γ represents the mass flow rate attenuation coefficient, take γ=1.90, H represents the tunnel height, Q represents the power of the fire source, T _e represents the temperature of the exhaust outlet, take T _e =313K, V _e Indicates the amount of smoke exhaust.

Step 3. Under the condition of ensuring the safety of the evacuation environment, determine the minimum critical large distance D ₁ and the maximum critical large distance D ₂ of key smoke exhaust outlets. When there is no smoke exhaust function in the tunnel and the fire smoke spreads freely, the smoke layer changes. The thickness of the smoke layer is fast, so the place where the smoke layer thickness reaches 2m is close to the fire source, and 2L is taken as the minimum critical distance D ₁ . The thickening speed of the gas layer is slow, so the place where the thickness of the smoke layer reaches 2m is far away from the fire source, and 2L* is taken as the minimum critical large distance D ₂ to obtain the critical large distance range of the key smoke exhaust outlets of the tunnel; specifically:

According to "Fire Engineering Guidelines" and "China Fire Handbook" Volume 3 "Fire Planning · Public Fire Protection Facilities · Building Fire Protection Design", it can be known that in order to ensure the safety of personnel evacuation environment, the visibility at a clear height of 2m should meet V _z ≥ 10m, and the temperature should be Satisfy T _z ≤60°C. Therefore, it is only necessary to ensure that there is no smoke at a clear height of 2m to ensure the safety of the evacuation environment.

Furthermore, when there is no smoke exhaust function in the tunnel and the fire smoke spreads freely, the height of the smoke layer far away from the fire source is 2m (that is, the thickness of the smoke layer h _L =(H-2)m), if set here Exhausting smoke from the key smoke exhaust outlets can ensure the safety of the evacuation environment. Then, according to formula (13) and substituting relevant parameter data, as shown in Figure 1, the minimum critical distance D ₁ = 2L for the key smoke exhaust outlets of the tunnel is obtained;

In the formula, D ₁ represents the minimum critical distance between key smoke outlets, L represents the distance from the fire source when it spreads freely, W represents the width of the tunnel, and H represents the height of the tunnel.

Furthermore, when there is a key smoke exhaust function in the tunnel, and when the fire smoke is spread by the suction force of the smoke exhaust, the height of the smoke layer far away from the fire source is 2m (that is, the thickness of the smoke layer h _L* = (H-2)m ), then the safety of the personnel evacuation environment can be guaranteed at this time. Then, according to formula (23) and substituting relevant parameter data, as shown in Figure 2, the maximum critical distance D ₂ of the key smoke exhaust outlets of the tunnel is obtained = 2L*.

In the formula, D ₂ represents the maximum critical distance between key smoke exhaust outlets, L ^* represents the distance from the fire source when the key smoke exhaust is active, W represents the width of the tunnel, H represents the height of the tunnel, and Q represents the power of the fire source.

Furthermore, under the condition of ensuring the safety of the evacuation environment, the critical maximum distance range of the key smoke exhaust outlets of the tunnel is obtained as:

D ₁ ≤ D ≤ D ₂ (26)

In the formula, D ₁ represents the minimum critical distance between key smoke exhaust outlets, D ₂ represents the maximum critical distance between key smoke exhaust outlets, and D represents the distance between key smoke exhaust outlets.

Through this calculation method, the critical large distance range of key smoke exhaust outlets can be quickly obtained according to different tunnel sizes.

Furthermore, the present invention also includes verification of the aforementioned calculation method, specifically:

(1) According to the critical large distance range of the key smoke exhaust outlets of the tunnel described in step 3, the parameters such as the size of the physical tunnel and the power of the fire source are substituted (Table 1), and the calculation value of the theoretical critical large distance range is obtained by calculating the theoretical formula (Table 2) ;

Table 1

serial number Fire source power/MW Tunnel size (height x width)/m A1-A5 20 5×9/5×10/6×11/6×12/7×13 B1-B5 30 5×9/5×10/6×11/6×12/7×13 C1-C5 40 5×9/5×10/6×11/6×12/7×13 D1-D5 50 5×9/5×10/6×11/6×12/7×13

Table 2

(2) FDS numerical simulation part:

Fire dynamic simulation software (FDS) is a prior art.

Using FDS software, construct tunnel models with a length of 1000m and different widths and heights, as shown in Figure 3. The smoke outlet is located on the roof of the tunnel, with a horizontal length of 2.5m and a vertical length of 6m. The size of the fire source is taken to be 8m in length x 3m in width x 0m in height, and "layer" measuring points are set at 0.5m intervals in the longitudinal center of the model tunnel, which can directly obtain the height of the smoke layer. The ambient temperature and pressure of the tunnel are 20°C and 101kPa, respectively, and the simulation calculation time is 600s. Set the working conditions of different fire source powers and different tunnel sizes as described in Table 1, and through the "layer" measuring point in the FDS software (see Figure 3), the smoke layer height at each simulated measuring point can be directly obtained, and the smoke The distance from the fire source at the gas layer height of 2m can be directly obtained, so the simulated critical large distance range can be obtained (see Table 2), and compared with the theoretically calculated value in step (1):

By comparing the theoretical critical large spacing range and the simulated critical large spacing range, it is obtained that the simulated large spacing range is slightly smaller than the theoretical value, because the theoretical calculation value does not consider the influence of other influencing factors, making the theoretical calculation value slightly larger, but through the error calculation, each The error results of the working conditions are all less than 10%, which meets the error requirements, and it can be verified that the calculation formula of the above-mentioned theoretical critical large spacing range is accurate and can meet the applicability of actual tunnel engineering.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection of the present invention. within range.

Claims (4)

1. A design method for the critical large spacing range of key smoke exhaust outlets in tunnels, characterized in that it comprises the following steps: Step 1. Determine the allowable height of the smoke in the tunnel under the conditions that can ensure the safety of the evacuation environment, and obtain the allowable thickness of the smoke layer h _L = h _{L *} according to the design size of the tunnel; Step 2. Calculate according to the relationship between the thickness of the smoke layer and the distance from the fire source and the size of the tunnel. Under the condition that there is no smoke exhaust in the tunnel and the fire smoke spreads freely, the distance L from the fire source at the place where the thickness of the smoke layer is h _L is, Take 2L as the minimum critical large spacing D ₁ of key smoke exhaust outlets; Step 3. According to the relationship between the thickness of the smoke layer and the distance from the fire source, the size of the tunnel, and the power of the fire source, calculate the key smoke exhaust function in the tunnel. Under the condition that the fire smoke is spread by the smoke suction force, the thickness of the smoke layer The distance between h _L* and the fire source is L ^* , and 2L ^* is used as the maximum critical distance D ₂ of the key smoke exhaust outlet; Step 4. Obtain the critical large distance range D of key smoke exhaust outlets of the tunnel, where D ₁ ≤ D ≤ _{D 2} . 2. The design method of the critical large distance range of the key smoke exhaust outlet of the tunnel according to claim 1, characterized in that, in the step 1, the conditions for ensuring the safety of the personnel evacuation environment are: The visibility at the allowable height of the smoke in the tunnel should meet V _z ≥ 10m, the temperature should meet T _z ≤ 60°C, and the allowable height of the smoke in the tunnel is 2m. 3. The design method for the critical large distance range of key smoke exhaust outlets of tunnels according to claim 1, characterized in that, in said step 2, the relational expression between the thickness of the smoke layer and the distance from the fire source is: In the formula, the smoke layer thickness h _L = (H-2), D ₁ represents the minimum critical distance between key smoke outlets, L represents the distance from the fire source when it spreads freely, W represents the width of the tunnel, and H represents the height of the tunnel. 4. The design method for the critical large distance range of key smoke exhaust outlets of tunnels according to claim 1, characterized in that, in said step 3, the relational expression between the thickness of the smoke layer and the distance from the fire source is: In the formula, the thickness of the smoke layer h _L = (H-2), D ₂ represents the maximum critical distance between key smoke exhaust outlets, L ^* represents the distance from the fire source when there is a key smoke exhaust function, W represents the width of the tunnel, and H represents the tunnel Height, Q represents the fire source power.