A design method for critical maximum spacing of natural smoke exhaust shaft of urban shallow-buried tunnel

Abstract

The application discloses a kind of urban shallow-buried tunnel natural smoke exhaust shaft critical maximum spacing design method, comprising: using the theoretical derivation mode based on the pressure difference inside and outside the shaft to determine the position of smoke deposition point in tunnel, deduce the critical maximum spacing model of shaft;Combining the temperature distribution formula that Li Yingzhen deduces to the highest temperature of tunnel vault, the critical maximum spacing model of shaft is optimized, and the optimized critical maximum spacing model of shaft is obtained;According to existing shaft tunnel suction and penetration determination model (Fr critical / Ri ’ Model), the optimized critical maximum spacing model of shaft is verified theoretically.The application is complete in design and derivation, and has sufficient theoretical basis, has very high fire guidance significance, represents the new direction of future shaft tunnel design, and provides reference basis for tunnel shaft setting rationality and personnel evacuation safety.

Description

Technical Field

[0001] This invention relates to the field of tunnel fire shaft location design, specifically to a method for designing the critical maximum spacing of natural smoke exhaust shafts in shallow urban tunnels. Background Technology

[0002] Tunnel fires have severe consequences and are extremely difficult to rescue, causing devastating damage to life and property. Vertical shaft smoke extraction systems are widely used in various tunnel smoke extraction systems due to their economic and environmental advantages. The rational placement of vertical shafts to ensure the safe evacuation and effective rescue of personnel within the tunnel is currently a top priority in tunnel shaft research.

[0003] Shallow tunnels refer to tunnels or tunnel sections where the overburden layer is less than twice the tunnel span. Due to their minimal geological damage, they are gradually becoming a primary direction for modern urban infrastructure construction. Their proximity to the ground allows shallow tunnels to typically employ a ventilation system combining natural smoke extraction with vertical shafts, effectively utilizing environmental factors and controlling operating costs. Shaft ventilation utilizes the chimney effect created by the pressure difference between the inside and outside of the shaft to induce airflow within the tunnel, causing smoke to move towards the shaft and be discharged, thus ensuring that the environmental parameters within the tunnel meet the requirements for safe evacuation.

[0004] Currently, both standard and practical tunnel engineering practices mostly adopt fixed shaft spacing, without tailoring adjustments based on different tunnel engineering conditions and parameters such as fire source power. In other words, existing research employs a single shaft spacing model, failing to provide personalized adjustments for varying parameters like fire source power, shaft height, and tunnel clearance. Therefore, this study explores an effective method for controlling project costs by determining the critical maximum spacing between two shafts while ensuring normal smoke extraction.

[0005] To explore the necessity of the critical maximum spacing of vertical shafts, it is urgent to clarify the effectiveness of tunnel temperature control and smoke exhaust efficiency in the design of the critical maximum spacing of natural smoke exhaust shafts in urban shallow-buried vertical shaft tunnels. Summary of the Invention

[0006] To achieve a reasonable setting of the critical distance between shafts for fire sources, the present invention aims to provide a design method for the critical maximum distance between shafts. Based on parameters such as the potential fire source power, reasonable shaft height, and corresponding tunnel clearance for different tunnels, the method designs the corresponding critical maximum distance between shafts. This effectively controls tunnel temperature, ensures personnel evacuation safety, and maximizes the use of environmental factors to reasonably control tunnel operating costs.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for designing the critical maximum spacing of natural smoke exhaust shafts in shallow urban tunnels, wherein the critical maximum spacing model of the shafts is designed as follows:

[0009]

[0010]

[0011] Where: T0 is the ambient temperature inside the tunnel; v0 is the wind speed at the outdoor standard height; H is the total height from the shaft exit to the bottom of the tunnel; H0 is the commonly used outdoor standard height; △T max The maximum temperature increase is given by g; g is the acceleration due to gravity; h is the length of the shaft above the tunnel roof; and x is the longitudinal distance between the shaft and the fire source. v H is a dimensionless longitudinal distance from the fire source, approximately 0. ef The effective tunnel height, i.e., the distance from the bottom of the fire source to the tunnel ceiling, is H. t V is the tunnel clearance height, and V is the wind speed inside the tunnel. Q represents the power of the fire source; b fo Let ρ be the radius of the fire source, ρ0 be the ambient air density, u0 be the natural ventilation wind speed, and c be the radius of the fire source. p This is the specific heat capacity of air.

[0012] Optionally, the derivation process of the critical maximum spacing model for vertical shafts includes:

[0013] According to Bernoulli's equation, the flue gas discharge from the tunnel must ensure that the flue gas pressure inside the tunnel is greater than the air pressure at the shaft outlet, as shown in formulas (1) to (4):

[0014]

[0015]

[0016]

[0017] ΔP≥P w (4)

[0018]

[0019] In the formula: ΔP is the dynamic pressure of flue gas discharge; P w ρ0 is the ambient wind pressure at the shaft outlet; ρ0 is the ambient air density .... s ρ is the density of hot flue gas inside the shaft; g is the acceleration due to gravity; h is the height of the shaft; T0 is the ambient temperature inside the tunnel; Ts is the flue gas temperature; v w The wind speed at the shaft outlet is expressed as in formula (5), where H0 is the outdoor standard height; v0 is the wind speed at the outdoor standard height; H is the total height at the shaft outlet; and n is the exponent, which is 1 / 3.

[0020] Combining formulas (1) to (5), the relationship between flue gas discharge pressure is shown in formula (6):

[0021]

[0022] Choosing Li Yingzhen's arch temperature distribution formula, the variation law of tunnel arch temperature distribution with longitudinal distance is shown in formula (7):

[0023]

[0024]

[0025] In the formula: △T represents the temperature increase; △T max For maximum temperature increase; H t x is the tunnel clearance; x is the longitudinal distance between the shaft and the fire source; x v L is the dimensionless longitudinal distance from the fire source; v is the ambient wind speed at the shaft opening; L is the vertical distance from the fire source. f Q is the length of the flue gas counter-current; c For effective heat release rate;

[0026] Substituting formula (7) into formula (6) using Taylor expansion, we obtain the critical maximum spacing interval relationship of the vertical shaft as shown in formula (9):

[0027]

[0028] The critical maximum spacing of the vertical shafts is symmetrically processed to obtain the critical maximum spacing model of the vertical shafts, which is used to express the critical spacing between two vertical shafts.

[0029] Furthermore, the critical maximum spacing model of the vertical shafts is optimized, including:

[0030] Based on the temperature distribution formula (11) derived by Li Yingzhen for the highest temperature at the tunnel arch:

[0031]

[0032] In the formula: Q is the power of the fire source; H t ΔT is the tunnel clearance height. max For maximum temperature increase; b fo V is the radius of the fire source; V is the wind speed.

[0033] In formula (11), ΔT max Substituting into the aforementioned critical maximum spacing model of vertical shafts, the optimized critical maximum spacing model of vertical shafts is obtained as shown in formula (12):

[0034]

[0035] Furthermore, based on the existing vertical shaft tunnel penetration determination model, the optimized vertical shaft critical maximum spacing model is theoretically verified;

[0036] The existing vertical shaft tunnel penetration determination model includes: Fr critical / Ri' model;

[0037] The Fr critical The model includes:

[0038]

[0039] In the formula: V s A represents the flue gas velocity inside the shaft. s denoted as the exhaust port area; d is the flue gas layer thickness; ΔT is the average temperature rise of the flue gas layer; T0 is the ambient temperature; and g is the gravitational acceleration.

[0040] In formula (13), Fr is the ratio between the horizontal radial force of the flue gas and the thermal buoyancy of the flame. When it is greater than the critical Froude number, the flue gas in the tunnel will be sucked through.

[0041] The Ri' model includes:

[0042]

[0043] In equation (14), Δρ is the density difference between the flue gas below the exhaust port and the surrounding air without exhaust, ρ0 is the air density, d is the thickness of the flue gas layer below the shaft when there is no exhaust, h is the height of the shaft, and A s For the cross-section of the shaft, ρ s0 denoted as smoke density inside a tunnel without exhaust vents, v as smoke velocity below the shaft during exhaust, and w as shaft width.

[0044] When Ri' is less than 1.4, boundary layer separation occurs in the flow field inside the shaft; otherwise, absorption breakdown occurs.

[0045] Optionally, step 4, the verification and optimization of the design method, includes:

[0046] Step 4.1, FDS numerical simulation part: Establish tunnel models with / without shafts, with the fire source set longitudinally between the two smoke exhaust outlets. Select the most unfavorable situation of highway tunnel fire. First, simulate the location of the smoke settling point under the natural smoke exhaust condition of the tunnel using the tunnel model without shafts. Based on this, set shafts at the locations of the smoke settling point and the critical fire source shaft spacing calculated by the model, respectively. Extract the model data to obtain the trend and phenomenon of each working condition parameter.

[0047] Step 4.2: Based on the existing vertical shaft tunnel penetration judgment model, analyze and compare the trends and phenomena of the various working parameters extracted in Step 2, and explore the impact of different parameter changes on temperature control and smoke exhaust efficiency, thereby verifying the feasibility and rationality of the vertical shaft critical maximum spacing model.

[0048] Step 4.3: Based on the previous arch heating formula (11), the trends and phenomena of the various working parameters extracted in Step 2 are analyzed and compared again, the critical maximum spacing model of the vertical shaft is optimized, the critical maximum spacing model of the vertical shaft is verified from the theoretical level, and a new design idea is provided for the design of the spacing of the fire source vertical shaft.

[0049] Compared with existing technologies, the advantages of this invention are: it can design the critical maximum spacing of shafts using existing tunnel temperature data, and it can also directly obtain the critical maximum spacing of shafts based on basic parameters such as high-power fire source, shaft height, and tunnel clearance, verifying its rationality and effectiveness, and making it applicable to shaft tunnels under different engineering conditions. This method, based on the theoretical derivation model of determining the location of flue gas settling points within the shaft using the internal and external pressure difference, verifies the rationality of the critical maximum spacing model. It highlights that the critical maximum spacing model can provide corresponding opinions on the critical maximum spacing of shafts based on different engineering conditions of shaft tunnels, and the results conform to existing shaft tunnel penetration judgment standards. It is highly innovative and can provide constructive design opinions for actual engineering processes. The critical maximum spacing model of shafts based on the internal and external pressure difference has higher practical engineering value and represents a new direction for future shaft tunnel construction and design. Attached Figure Description

[0050] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0051] Figure 1 This is a schematic diagram of the technical route of the design method of the present invention;

[0052] Figure 2 This is a schematic diagram illustrating the analysis of flue gas flow field parameters in a vertical shaft tunnel according to the present invention;

[0053] Figure 3 This is a schematic diagram of the tunnel thermocouple arrangement in this invention;

[0054] Figure 4 This is a graph showing the temperature variation of flue gas at different heights in a 10MW shaftless tunnel established in this invention.

[0055] Figure 5 This is a vertical distribution diagram of flue gas temperature at different shaft locations under different tunnel clearances, established in this invention.

[0056] Figure 6 This is a temperature cloud map near the shaft when the shaft height is 0.24Ht under different tunnel clearances in this invention;

[0057] Figure 7 To illustrate the smoke extraction efficiency of vertical shafts under different tunnel clearance heights in this invention, (a) smoke settling point.

[0058] (b) Critical maximum spacing model for vertical shafts;

[0059] Figure 8 Fr corresponds to different tunnel clearance heights and shaft heights in this invention. critical (a) / Ri'(b) Change curve. Detailed Implementation

[0060] The principles and features of the present invention are described below with reference to the accompanying drawings and specific embodiments. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0061] A design method for the critical maximum spacing of natural smoke exhaust shafts in shallow urban tunnels, see [reference]. Figure 1 This includes the following steps:

[0062] Step 1: Propose the concept of a critical maximum spacing model for vertical shafts, and adopt a theoretical derivation model based on the pressure difference between the inside and outside of the shaft to determine the location of the flue gas settling point (see...). Figure 2 , Figure 3 According to Bernoulli's equation, when parameters such as flue gas temperature and density change, the pressure at the location of the flue gas will also change. The flue gas discharge from the tunnel must ensure that the flue gas pressure inside the tunnel is greater than the air pressure at the shaft outlet, as shown in formulas (1) to (4).

[0063]

[0064]

[0065]

[0066] ΔP≥P w (4)

[0067]

[0068] In the formula: ΔP is the dynamic pressure of flue gas discharge; P w ρ0 is the ambient wind pressure at the shaft outlet; ρ0 is the ambient air density .... s ρ is the density of hot flue gas inside the shaft; g is the acceleration due to gravity; h is the height of the shaft, i.e., the length of the shaft above the tunnel roof; T0 is the ambient temperature inside the tunnel; Ts is the flue gas temperature; v wThe wind speed at the shaft exit is expressed as in formula (5). H0 is the outdoor standard height, which is 10m. v0 is the wind speed at the outdoor standard height, which is the wind speed at a height of 10m above the ground, which is 3m / s. H is the total height at the shaft exit, which is the total height from the top of the shaft to the bottom of the tunnel. n is the exponent, which is generally between 1 / 7 and 1 / 2, and is 1 / 3.

[0069] Combining formulas (1) to (5), the relationship between flue gas discharge pressure can be obtained as shown in formula (6):

[0070]

[0071] Furthermore, the temperature distribution formula for the tunnel arch (Li TZ, Ingason H, Fire-induced ceiling jet characteristics in tunnels under different ventilation conditions, SP Report 2015:23, SP Technical Research Institute of Sweden: Boras, Sweden. 2015.) was selected, and the variation law of tunnel arch temperature distribution with longitudinal distance is shown in formula (7):

[0072]

[0073]

[0074] In the formula: △T represents the temperature increase; △T max For maximum temperature increase; H t x is the tunnel clearance; x is the longitudinal distance between the shaft and the fire source; x v L is a dimensionless longitudinal distance from the fire source, approximately 0; v is the ambient wind speed at the shaft opening; f Q is the length of the flue gas counter-current; c The effective heat release rate.

[0075] Furthermore, by substituting formula (7) into formula (6) through Taylor expansion, we can obtain the critical fire source-shaft spacing interval relationship as shown in formula (9):

[0076]

[0077] Based on this, a symmetrical treatment is applied to the critical maximum spacing between the two shafts, and the expression for the critical spacing between the two shafts is:

[0078]

[0079] The above derivation process involves substituting the highest temperature data obtained from the simulation into the critical maximum spacing model of the shafts to obtain the corresponding critical fire-shaft spacing. Furthermore, Li Yingzhen derived a series of temperature distribution formulas for the highest temperature at the tunnel arch, as shown in formula (11):

[0080]

[0081] In the formula: Q is the power of the fire source; H t ΔT is the tunnel clearance height. max For maximum temperature increase; b fo V is the radius of the fire source; V is the wind speed inside the tunnel.

[0082]

[0083] u0 represents the natural ventilation velocity, which is set to 1.09 m / s in this embodiment. p This is the specific heat capacity of air.

[0084] The highest temperature increase (ΔT) in the currently derived formula (10) max Previously, simulated values ​​were directly substituted. In order to verify and optimize the critical maximum spacing model of vertical shafts from a theoretical perspective, the inventors changed ΔT in formula (11). max Substituting into the critical maximum spacing model of vertical shafts (10), the optimized critical maximum spacing model of vertical shafts is obtained as shown in formula (12):

[0085]

[0086] The power of the fire source Q and H ef Substituting basic parameters such as the effective tunnel height (i.e., the distance from the bottom of the fire source to the top wall of the tunnel) and the shaft height h into formula (12), we obtain the corresponding critical maximum spacing model of the critical shaft. In this embodiment, H t =H ef .

[0087] Step 2: Construct tunnel models with and without shafts to verify the design results of the critical maximum shaft spacing; construct a model with shaft height h and tunnel clearance H. t A vertical shaft smoke extraction tunnel model with variables. The model parameters include technical parameters related to the design of the critical maximum spacing model of the vertical shafts, including fire source power, shaft height, and tunnel clearance. See also... Figure 2 Since the vertical shaft smoke exhaust tunnel has a natural ventilation and symmetrical structure, a horizontal mirror was used as the boundary in the middle of the tunnel's transverse section to build half of the tunnel model. The model dimensions are 100m × 11m × H. t The ignition source is located at the longitudinal center of the tunnel, with a power Q of 20MW and dimensions of 4m × 4m. (Rapid t) 2Fire, fire growth coefficient α = 0.0469 kJ / s 2 The specific working conditions are shown in Table 1:

[0088] Table 1 Operating Condition Settings

[0089]

[0090] To simulate the actual external environment for smoke extraction from the tunnel shaft, an additional grid was installed 10 meters around the shaft exit, with the upstream boundary marked "SUPPLY" and a wind speed of v. w The remaining boundary conditions are set to "OPEN". To investigate the temperature changes at the tunnel arch after a fire and their impact on personnel evacuation, two sets of thermocouples were used: horizontal and vertical. The horizontal thermocouples were distributed 0.05m below the tunnel arch, spaced 0.5m apart throughout the tunnel. The vertical thermocouples were distributed at the longitudinal center of the tunnel, horizontally spaced 0.25m apart, starting directly above the fire source. A schematic diagram of the model is shown below. Figure 3 As shown.

[0091] Table 1 shows the "Non" positions, simulating fire smoke scenarios under natural smoke exhaust conditions in shaftless tunnels. This investigates the location where smoke layer mass gradually increases and begins to settle during natural smoke propagation due to entrainment of air along its path. This information serves as the basis for studying the critical maximum shaft spacing model. The smoke temperature variation curves at different vertical heights in a shaftless tunnel with a fire source power of 10MW (enlarged versions of the corresponding temperature rise sections) are shown below. Figure 4 As shown in (a), (b), (c), and (d) in the diagram. If the flue gas temperature at a certain height in a shaftless tunnel increases with distance from the fire source, it indicates that the high-temperature flue gas at that height has settled. The location corresponding to the trough temperature of the flue gas temperature-distance curve at that height, at a distance from the fire source, is the location where the flue gas begins to settle, i.e., the fire source-smoke settling point, which is shown in the diagram. Figure 4 The upper right corner of (a), (b), (c), and (d) in the text.

[0092] Based on this, vertical shafts were installed at both the distance between the ignition source and the flue gas settling point and at the critical maximum distance between the shafts (see Table 1: Non-condition indicates the location of the ignition source and flue gas settling point). The critical maximum distance model of the vertical shafts derived from theory was verified, and the corresponding temperature distribution diagram under the shafts is shown in the figure below. Figure 5 , Figure 6 As shown.

[0093] Figure 5This diagram shows the vertical distribution of flue gas temperature at shaft locations under different tunnel clearances. Without shafts, the highest temperature inside the tunnel ranges from 150-300℃; after shafts are installed, the temperature decreases significantly, by 20-150℃, demonstrating that shafts effectively control the tunnel's internal temperature. It is noteworthy that although the temperature decreases after shaft installation, the reduction is within a certain range, without a step-like temperature drop, indicating no flue gas layer penetration at the temperature level. Figure 5 (a) Taking the vertical temperature variation near the shaft of a tunnel with a clearance of 5m and different shaft heights under the corresponding fire source-shaft critical distance conditions as an example: Without shafts, the thickness of the high-temperature smoke layer (smoke layer with a temperature above 100℃) near the shaft is 2.5m, and the maximum temperature exceeds 250℃; after the shafts are installed, as the shaft height increases, the smoke layer thickness decreases from 2.5m (5m-2.5m=2.5m) to 1.5m (5m-3.5m=1.5m), the maximum temperature at the arch drops from 150℃ to 100℃, and the temperature near the ground (0-3.0m) is below 60℃, so personnel can escape from high temperatures. The vertical temperature variation near the shafts of other tunnel clearances is similar to... Figure 5 (a) Similarity indicates that the formula for the critical distance between the fire source and the shaft can effectively calculate a reasonable critical distance between the fire source and the shaft, so that the shaft does not experience a suction-through phenomenon during the smoke exhaust process.

[0094] The comparative analysis above reveals that the critical maximum spacing model for shafts, considering different shaft heights, fire source power, and tunnel clearances, calculates a critical maximum spacing greater than the spacing of smoke settling points in tunnels without shafts. While the critical maximum spacing increases the tunnel temperature below the shaft to some extent, temperatures below 4.25m remain below 60℃, ensuring safe escape for personnel.

[0095] In addition, in comparison Figure 7(a) and (b) show that when the shaft height and tunnel clearance are the same, the smoke extraction efficiency corresponding to setting the shaft at the smoke settling point is greater than that corresponding to the shaft smoke extraction efficiency of the critical maximum spacing model. It can be seen that the closer the shaft is to the fire source, the higher the smoke extraction efficiency, which proves the point made by Guo Qinghua (Guo Qinghua, Shi Jianfeng, Yan Zhiguo, et al. Influence of fire source-shaft spacing on smoke flow characteristics in naturally ventilated tunnel fires [J]. Fire Science and Technology, 2021, 40(12): 1739-1743.). In addition, comparing the smoke extraction efficiency corresponding to the same shaft height and different tunnel clearances, it was found that the higher the tunnel clearance, the lower the smoke extraction efficiency. Comparing operating conditions with the same tunnel clearance but different shaft heights, it was found that increasing the shaft height significantly reduced the rate of increase in smoke extraction efficiency. This is because the higher the shaft height, the greater the pressure difference between the inside and outside, which enhances the suction force on the tunnel smoke and increases the amount of smoke discharged from the shaft. While discharging tunnel smoke, the shaft also entrains fresh air from the tunnel and removes some fresh air from the lower part of the tunnel. This phenomenon was confirmed in Fan's (CGFan, J.Ji, ZHGao, JYHan, JHSun. Experimental study of air entrainment mode with natural ventilation using shafts in road tunnel fires[J]. International Journal of Heat and Mass Transfer, 2013, 56(1-2):750-757.) quantitative analysis of smoke extraction efficiency.

[0096] Step 3: Substitute the above theoretical and simulation results into the existing standard penetration test model to determine the feasibility of the vertical shaft critical maximum spacing model, and base the decision on the existing vertical shaft tunnel penetration test model (Fr). critical The / Ri' model was used to theoretically verify the design results of the critical maximum spacing of the vertical shaft;

[0097] Existing models for determining the penetration of vertical shaft tunnels include: Hinkley's Froude critical penetration model and Ji Jie's modified Ri', specifically:

[0098] Hinkley studied the adsorption breakdown phenomenon induced by natural smoke exhaust and proposed a modified Froude number to determine the occurrence of adsorption breakdown in natural smoke exhaust systems, namely:

[0099]

[0100] In the formula: V s The flue gas velocity (m / s) inside the shaft; A s The area of ​​the smoke exhaust outlet (m²) 2); d is the thickness of the flue gas layer (m); ΔT is the average temperature rise of the flue gas layer (K); T0 is the ambient temperature (K); g is the acceleration due to gravity (m / s²). 2 In formula (13), Fr represents the ratio between the horizontal radial force of the flue gas and the thermal buoyancy of the flame. When this ratio exceeds the critical Froude number, the flue gas in the tunnel experiences a blow-through phenomenon. Studies have shown that Fr... critical For 1.5, it is suitable for use near the ventilation opening of the smoke exhaust tunnel, Fr critical Version 1.1 applies to ventilation openings located near the tunnel side.

[0101] Ji Jie et al. argued that the Froude number criterion is not applicable to natural smoke exhaust systems with shafts, and defined a dimensionless Ri' number to determine the effects of different shaft heights on the absorption-through and boundary layer separation phenomena within the shaft, as shown in formula (14):

[0102]

[0103] In equation (14): Δρ is the density difference (kg / m³) between the flue gas below the exhaust port and the surrounding air without exhaust. 3 ); ρ0 is the air density (kg / m³) 3 ); d is the thickness of the flue gas layer below the shaft when there is no smoke exhaust (m); h is the height of the shaft (m); A s Shaft cross-section (m) 2 );ρ s0 Smoke density in a tunnel without exhaust vents (kg / m³) 3 V is the flue gas velocity (m / s) below the shaft during exhaust; w is the shaft width (m). When Ri' is less than 1.4, boundary layer separation occurs in the flow field within the shaft; otherwise, absorption-through occurs.

[0104] like Figure 8 As shown, the measured Fr inside the vertical shaft tunnel, calculated using the vertical shaft critical maximum spacing model, is... critical All values ​​are less than 1.5, and all Ri' values ​​are less than 1.4, indicating no absorption-induced penetration phenomenon caused by natural smoke exhaust and meeting the new absorption-induced penetration critical criterion for natural smoke exhaust shaft tunnels proposed by Ji Jie. It is worth noting that when H... t When the shaft height is 5m, the corresponding Ri' value generally increases with the increase of shaft height. However, when the shaft height is 0.5H... t The Ri value rises to 1.0 because when the tunnel clearance is small, the chimney effect caused by increasing the shaft height causes a large disturbance to the smoke layer inside the shaft. That is, as the shaft height continues to increase, the degree of disturbance of the smoke layer by the chimney effect no longer increases, but the disturbance phenomenon is within a reasonable range, which can ensure the effective smoke exhaust of the shaft.

[0105] Step 4: Further verify and optimize the design method, including:

[0106] Step 4.1, FDS numerical simulation part: Establish tunnel models with / without shafts, set the fire source between the two smoke exhaust outlets longitudinally, select the most unfavorable situation of highway tunnel fire, first simulate the location of smoke settling point under natural smoke exhaust conditions in the tunnel using the tunnel model without shafts, based on this, set shafts at the locations of smoke settling points and the locations calculated by the model of critical maximum shaft spacing, respectively, and extract model data to obtain the trends and phenomena of parameters under each working condition;

[0107] Step 4.2: Based on the existing vertical shaft tunnel penetration determination model (Fr) critical The / Ri' model is used to analyze and compare the trends and phenomena of the various working parameters extracted in step 2, and to explore the impact of different parameter changes on temperature control and smoke exhaust efficiency, thereby verifying the feasibility and rationality of the vertical shaft critical maximum spacing model.

[0108] Step 4.3: Based on the previous arch heating formula (11), the trends and phenomena of the various working parameters extracted in Step 2 are analyzed and compared again, the critical maximum spacing model of the vertical shaft is optimized, the critical maximum spacing model of the vertical shaft is verified from the theoretical level, and a new design idea is provided for the design of the spacing of the fire source vertical shaft.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for designing the critical maximum spacing of natural smoke exhaust shafts in shallow urban tunnels, characterized in that, The critical maximum spacing model for the vertical shafts is designed as follows: Where: T0 is the ambient temperature inside the tunnel; v0 is the wind speed at the outdoor standard height; H is the total height from the shaft exit to the bottom of the tunnel; H0 is the commonly used outdoor standard height; △T max The maximum temperature increase is given by g; g is the acceleration due to gravity; h is the length of the shaft above the tunnel roof; and x is the longitudinal distance between the shaft and the fire source. v H is a dimensionless longitudinal distance from the fire source, approximately 0. ef The effective tunnel height, i.e., the distance from the bottom of the fire source to the tunnel ceiling, is H. t V is the tunnel clearance height, and V is the wind speed inside the tunnel. Q represents the power of the fire source; b fo Let ρ be the radius of the fire source, ρ0 be the ambient air density, u0 be the natural ventilation wind speed, and c be the radius of the fire source. p This is the specific heat capacity of air.

2. The design method for the critical maximum spacing of natural smoke exhaust shafts in urban shallow-buried tunnels according to claim 1, characterized in that, The model derivation process includes: According to Bernoulli's equation, the flue gas discharge from the tunnel must ensure that the flue gas pressure inside the tunnel is greater than the air pressure at the shaft outlet, as shown in formulas (1) to (4): ΔP≥P w (4) In the formula: ΔP is the dynamic pressure of flue gas discharge; P w ρ0 is the ambient wind pressure at the shaft outlet; ρ0 is the ambient air density .... s ρ is the density of hot flue gas inside the shaft; g is the acceleration due to gravity; h is the height of the shaft; T0 is the ambient temperature inside the tunnel; Ts is the flue gas temperature; v w The wind speed at the shaft outlet is expressed as in formula (5), where H0 is the outdoor standard height; v0 is the wind speed at the outdoor standard height; H is the total height at the shaft outlet; and n is the exponent, which is 1 / 3. Combining formulas (1) to (5), the relationship between flue gas discharge pressure is shown in formula (6): Choosing Li Yingzhen's arch temperature distribution formula, the variation law of tunnel arch temperature distribution with longitudinal distance is shown in formula (7): In the formula: △T represents the temperature increase; △T max For maximum temperature increase; H t x is the tunnel clearance; x is the longitudinal distance between the shaft and the fire source; x v L is the dimensionless longitudinal distance from the fire source; v is the ambient wind speed at the shaft opening; f Q is the length of the flue gas counter-current; c For effective heat release rate; Substituting formula (7) into formula (6) using Taylor expansion, we obtain the critical maximum spacing interval relationship of the vertical shaft as shown in formula (9): The critical maximum spacing of the vertical shafts is symmetrically processed to obtain the critical maximum spacing model of the vertical shafts, which is used to express the critical spacing between two vertical shafts.

3. The design method for the critical maximum spacing of natural smoke exhaust shafts in urban shallow-buried tunnels according to claim 2, characterized in that, The model for the critical maximum spacing of the vertical shafts is optimized, including: Based on the temperature distribution formula (11) derived by Li Yingzhen for the highest temperature at the tunnel arch: In the formula: Q is the power of the fire source; H t ΔT is the tunnel clearance height. max For maximum temperature increase; b fo V is the radius of the fire source; V is the wind speed inside the tunnel; In formula (11), ΔT max Substituting into the aforementioned critical maximum spacing model of vertical shafts, the optimized critical maximum spacing model of vertical shafts is obtained as shown in formula (12):

4. The design method for the critical maximum spacing of natural smoke exhaust shafts in urban shallow-buried tunnels according to claim 3, characterized in that, Based on the existing vertical shaft tunnel penetration determination model, the optimized vertical shaft critical maximum spacing model is theoretically verified; The existing vertical shaft tunnel penetration determination model includes: Fr critical / Ri' model; The Fr critical The model includes: In the formula: V s A represents the flue gas velocity inside the shaft. s denoted as the exhaust port area; d is the flue gas layer thickness; ΔT is the average temperature rise of the flue gas layer; T0 is the ambient temperature; and g is the gravitational acceleration. In formula (13), Fr is the ratio between the horizontal radial force of the flue gas and the thermal buoyancy of the flame. When it is greater than the critical Froude number, the flue gas in the tunnel will be sucked through. The Ri' model includes: In equation (14), Δρ is the density difference between the flue gas below the exhaust port and the surrounding air without exhaust, ρ0 is the air density, d is the thickness of the flue gas layer below the shaft when there is no exhaust, h is the height of the shaft, and A s For the vertical shaft cross-section, ρ s0 denoted as smoke density inside a tunnel without exhaust vents, v as smoke velocity below the shaft during exhaust, and w as shaft width. When Ri' is less than 1.4, boundary layer separation occurs in the flow field inside the shaft; otherwise, absorption breakdown occurs.

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