Method for calculating length of fire smoke countercurrent layer of longitudinal ventilation tunnel

Abstract

The invention provides a method for calculating the length of a fire smoke countercurrent layer of a longitudinal ventilation tunnel, and belongs to the technical field of tunnel fire protection. According to the longitudinal ventilation tunnel fire smoke countercurrent layer length calculation method, on the basis of test data of smoke countercurrent layers related to domestic and overseas tunnel fire tests, current domestic and overseas smoke countercurrent layer prediction models are analyzed and compared, and a unified smoke countercurrent layer length prediction formula is deduced based on existing achievements and heat transfer theory laws. Through the expression of the function relationship among the length of the fire smoke countercurrent layer, the fire heat release rate, the critical wind speed and other parameters, key parameters can be provided for tunnel ventilation, smoke exhaust, rescue and other designs, and when tunnel longitudinal temperature data is sufficient under the fire working condition, the fire scale can be dynamically inverted, the smoke diffusion condition in the tunnel can be known in time, and the tunnel can be protected. And scientific guidance is provided for disaster prevention and rescue of the tunnel.

Description

Technical Field

[0001] The invention relates to a method for calculating the length of a smoke countercurrent layer in a longitudinal ventilation tunnel fire, and belongs to the technical field of tunnel fire protection. Background Art

[0002] For longitudinally ventilated tunnels, when a fire occurs in the tunnel, the rise of the fire plume is restricted by the tunnel roof. If the ventilation wind speed is less than the critical wind speed (the critical wind speed is the longitudinal ventilation wind speed that makes the length of the fire countercurrent layer 0), the smoke will flow to the upwind and downwind sides of the fire source, forming countercurrent smoke flow and mainstream smoke flow. The countercurrent smoke flow is opposite to the ventilation and smoke exhaust directions, which will not only hinder the smooth discharge of tunnel fire smoke, but also directly threaten the safety of personnel and property in the upwind area of ​​the fire source. Existing studies have shown that there is a functional relationship between the length of the tunnel fire smoke countercurrent layer and parameters such as the fire heat release rate and critical wind speed. Therefore, establishing a calculation method for the length of the longitudinal ventilation tunnel fire smoke countercurrent layer can provide key parameters for the design of tunnel ventilation, smoke exhaust, rescue, etc., and reasonably ensure the safety of tunnel operation; at the same time, it can also invert the fire scale and smoke diffusion under fire conditions, and scientifically guide on-site disaster prevention and rescue work. Summary of the invention

[0003] The purpose of the present invention is to provide a method for calculating the length of the smoke countercurrent layer in a longitudinal ventilation tunnel fire. It can provide key parameters for the design of tunnel ventilation, smoke exhaust, rescue, etc., and can also dynamically invert the fire scale when the longitudinal temperature data of the tunnel is sufficient under fire conditions, timely understand the smoke diffusion in the tunnel, and provide scientific guidance for disaster prevention and rescue in the tunnel.

[0004] The technical solution of the present invention is: by sorting out and collecting test data related to the smoke countercurrent layer in domestic and foreign tunnel fire tests (including full-scale tests and model tests), the current domestic and foreign smoke countercurrent layer prediction models are analyzed and compared based on these test data, and a unified smoke countercurrent layer length prediction formula is derived based on existing achievements and heat transfer laws, and the formula is verified.

[0005] A method for calculating the length of the smoke countercurrent layer in a longitudinal ventilation tunnel fire is as follows:

[0006] 1. Derivation of the prediction formula for the length of the smoke countercurrent layer

[0007] The smoke countercurrent layer can be divided into the development section, transition section, stable section and destruction section. Based on the morphology of the smoke countercurrent layer in the stable state, a calculation model for the length of the smoke countercurrent layer in the fire in the longitudinal ventilation tunnel is derived.

[0008] Empirical fire smoke thickness description formula:

[0009] H s =0.28H (Formula 1)

[0010] Theoretical expression of smoke countercurrent layer thickness (maximum thickness does not exceed 0.25H):

[0011]

[0012] Taking the maximum thickness of the smoke layer as 0.25H, the average counterflow velocity of the smoke counterflow layer near the fire source is:

[0013]

[0014] The flue gas counterflow mass flow rate is:

[0015] m'=ρu BF H s W (Formula 4)

[0016] Ignoring the density change during combustion, we can approximate:

[0017] ρ a =ρ (Formula 5)

[0018] Combining Formula 3, Formula 4 and Formula 5, we have:

[0019]

[0020] When the area around the combustible is relatively dry and there is no other evaporable substance, u0→0 is approximately true. For the convenience of calculation, the mass flow rate can be simplified to:

[0021]

[0022] According to the heat transfer principle, the temperature field variation law under tunnel fire conditions is deduced:

[0023]

[0024] And verified by full-scale test data:

[0025]

[0026] in

[0027]

[0028] The front end of the flue gas counterflow is the flue gas counterflow stagnation point, that is, the flue gas counterflow velocity at this point can be considered to be zero, and at the same time:

[0029] p1=Δp2(Formula 11)

[0030] The pressure difference Δp2 at the front end of the smoke counterflow under tunnel fire conditions is:

[0031] Δp2=ΔρgH s (Formula 12)

[0032] and

[0033]

[0034] When the wind flow passes through the section where the cross-sectional area suddenly decreases, there will be an obvious pressurization effect, and the thickness of the smoke counterflow front also has an obvious gradual change stage. When in a stable counterflow state, the shear effect of the smoke counterflow lower end with the wind flow is ignored. It is assumed that the counterflow layer and the longitudinal wind flow are two layers. Based on this, for the convenience of calculation, it is considered that the wind flow pressure in the stable section acts evenly on the destruction section, that is:

[0035] u a A=u' a A'(Formula 14)

[0036] Converted into the average wind pressure at the calculated section height (0.75H, H), assuming that the change end curve is 45. Straight line, considering the two-dimensional situation, the pressure coefficient k is:

[0037]

[0038] The equivalent wind pressure p1 is:

[0039]

[0040] Arranging formula 11 to formula 16, we have:

[0041]

[0042] Substituting into the calculation expression of ΔT, we get:

[0043]

[0044] From the perspective of heat transfer and combined with a large number of model test results, the maximum temperature calculation formula under tunnel fire conditions is derived:

[0045]

[0046] Combining Formula 18 and Formula 19, the length of the smoke countercurrent layer is expressed as:

[0047]

[0048] In formula 1 to formula 20: A—tunnel cross-sectional area, m 2 ; C—flow coefficient, ranging from 0.6 to 0.7, with an average value of 0.65; D—the length of the part of the smoke flow cross section in contact with the tunnel wall, m; H—tunnel height, m; H s—smoke layer thickness, m; W—tunnel width, m; P—tunnel circumference, m; α'—convection heat transfer coefficient, W / (m 2 K), generally 22.0 to 30.6, this time the middle value 26.3; c p —Specific heat at constant pressure, J / (kg·K); T—Smoke flow temperature, K; μ(t)—time-related function; t—time, s; m'—smoke counterflow mass flow rate, kg / s; u a —longitudinal ventilation speed, m / s; u0—speed at which combustion-generated gases leave the surface of combustibles, m / s; u BF —average velocity of smoke counterflow, m / s; μ—friction coefficient in the tunnel, ranging from 0.0055 to 0.0073, with an average value of 0.0064 (Gaillard, 1973); ρ a —Air density, kg / m 3 ; ρ—combustion gas density, kg / m 3 ; T a —Ambient temperature, K; T max —Maximum temperature of smoke flow, K; φ n —Mixing ratio, which means the ratio of the fire combustion products to the air flow mass flow in the smoke micro-clusters; f—experimental constant, with a value range of 0.0040 to 0.0046 and an average value of 0.0043; f3—conversion coefficient, which means the ratio of the horizontal counterflow velocity to the maximum vertical velocity of the smoke, single-headed: 0.20 to 0.64, with an average value of 0.36, open: 0.15 to 0.23, with an average value of 0.18.

[0049] 2. Verification of the prediction formula for the length of the smoke countercurrent layer

[0050] The calculation method of the smoke countercurrent layer length can be simplified as: L BF =f(K,ΔT max ,H s ,u a …), it can be seen that when the heat convection loss coefficient inside the tunnel is inaccurate and when the value of parameter K is uncertain due to the irregular shape of the tunnel section, the calculation of the smoke counterflow length will produce a large error. However, when a temperature sensor (such as a grating optical fiber system) is installed along the longitudinal direction of the tunnel, there are two sources of parameter K, fitting parameter K and calculating K using basic parameters. The reliability of the two methods is explained below and verified by experiments;

[0051] (1) K is obtained by fitting the longitudinal temperature

[0052] Under tunnel fire conditions, when the longitudinal temperature data provided by the temperature sensor is sufficient, based on the full-scale fire test data of the Baimangxueshan Tunnel, ΔT=Cexp[-K(x-x0)] is fitted through the longitudinal temperature data of the grating fiber, and ΔT is derived using the known fire test parameters. max , H s Then, we can find L BF ;

[0053] From the longitudinal temperature distribution of flue gas counterflow at typical moments, it can be seen that the combustion is stable from 400s to 800s. Based on this, the longitudinal temperature distribution of flue gas counterflow 600s after ignition is fitted, K = 0.01938, and then the flue gas counterflow length L is calculated according to formula 21 BF ;

[0054] (2) By calculating m' and H s , and then calculate L BF

[0055] Based on the widely used test results of Bettis in horseshoe tunnels and combined with the test results completed in Baimangxueshan Tunnel, m' and H are calculated. s , and then calculate L BF .

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] Several typical smoke countercurrent layer length prediction models that are widely used at present are either empirical or semi-empirical formulas; or are established under quasi-static assumptions, which are quite different from the development of smoke countercurrent in real tunnels; or only obtain results consistent with experiments under specific conditions. The existing smoke countercurrent layer prediction models cannot be conveniently applied to the inversion of heat release rate of tunnel fires and the design of tunnel ventilation, smoke exhaust, rescue, etc. The present invention derives a unified smoke countercurrent layer length prediction formula based on widely recognized research results and heat transfer laws at home and abroad. The formula is simple and clear, the basic calculation data is easy to obtain, and it has been verified by multiple full-scale fire tests, and has a good consistency with the test results. In particular, when a temperature sensor system is arranged along the longitudinal length of the tunnel, the full longitudinal distribution characteristics of the system are fully utilized to calculate a more accurate length of the smoke countercurrent layer of the longitudinal ventilation tunnel fire, which can well reflect the scale of the fire. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 Schematic diagram of the composition of the fire smoke countercurrent layer.

[0059] Figure 2 This is the longitudinal temperature distribution diagram of smoke counterflow at typical time points in the fire test.

[0060] Figure 3This is the longitudinal temperature distribution and fitting diagram of smoke counterflow 600s after ignition in the fire test.

[0061] Figure 4 The full-scale fire test verification results of the smoke countercurrent layer length calculation model are presented. DETAILED DESCRIPTION

[0062] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0063] It should be understood by those skilled in the art that unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this invention belongs.

[0064] Example:

[0065] A method for calculating the length of the smoke countercurrent layer in a longitudinal ventilation tunnel fire is as follows:

[0066] 1. Derivation of the prediction formula for the length of the smoke countercurrent layer

[0067] Relevant scholars have made a detailed description of the smoke countercurrent layer. Generally speaking, the smoke countercurrent layer can be divided into the development stage, transition stage, stable stage and destruction stage, such as Figure 1 As shown in the figure, this paper derives a fire smoke countercurrent layer length calculation model suitable for longitudinal ventilation tunnels based on the morphology of the smoke countercurrent layer under stable conditions.

[0068] Hu Longhua (2006) gave a detailed description and theoretical derivation of smoke thickness, and gave an empirical fire smoke thickness description formula in the article (Hu et al., 2008):

[0069] H s =0.28H (Formula 1)

[0070] Zhang Xingkai (1997) derived a theoretical expression for the thickness of the flue gas countercurrent layer from the perspective of heat transfer and believed that the maximum thickness does not exceed 0.25H:

[0071]

[0072] According to the above research results on smoke thickness, considering applicability, the maximum thickness of the smoke layer is taken as 0.25H. At the same time, according to the research results of relevant scholars (Zhang Xingkai, 1997), the average counterflow velocity of the smoke counterflow layer near the fire source is:

[0073]

[0074] The flue gas counterflow mass flow rate is:

[0075] m'=ρu BF H s W (Formula 4)

[0076] Ignoring the density change during combustion, we can approximate:

[0077] ρ a =ρ (Formula 5)

[0078] Combining Formula 3, Formula 4 and Formula 5, we have:

[0079]

[0080] The derivation of the formula is based on the fact that tunnel fires are in a stable flow state. In this case, relevant scholars (Kitahara Ryoya et al., 1985) proved through experiments that when the surroundings of the combustibles are relatively dry and there are no other evaporable substances, u0 is approximately 0, and it can be taken in the calculation For the convenience of calculation, the mass flow rate can be simplified to:

[0081]

[0082] Zeng Yanhua et al. (2004) derived the temperature field variation law under tunnel fire conditions based on the heat transfer principle:

[0083]

[0084] Hu Longhua et al. (2005, 2006, 2007) deduced that the longitudinal temperature distribution of the tunnel under the steady flow state conforms to the exponential law based on the previous research results (Delichatsios, 1981; Kunsch, 1998, 2002), and verified it with full-scale test data:

[0085]

[0086] in

[0087]

[0088] The front end of the flue gas counterflow is the flue gas counterflow stagnation point, that is, the flue gas counterflow velocity at this point can be considered to be zero, and at the same time:

[0089] p1=Δp2(Formula 11)

[0090] According to the research results of Bailey et al. (2002), the pressure difference Δp2 at the front end of the smoke counterflow under tunnel fire conditions is:

[0091] Δp2=ΔρgH s (Formula 12)

[0092] and

[0093]

[0094] Fan Weicheng et al. (1995) proposed that when the wind flow passes through a section with a suddenly reduced cross-sectional area, there will be an obvious pressurization effect, and the thickness of the smoke counterflow front also has an obvious gradual change stage. According to the assumption of the article, when in a stable counterflow state, the shear effect of the smoke counterflow lower end with the wind flow is ignored, and it is assumed that the counterflow layer and the longitudinal wind flow are two layers. Based on this, for the convenience of calculation, it is considered that the wind flow pressure of the stable section acts evenly on the destruction section, that is:

[0095] u a A=u' a A'(Formula 14)

[0096] Converted into the average wind pressure at the calculated section height (0.75H, H) (assuming the change end curve is a 45° straight line, considering the two-dimensional case), the pressure coefficient k is:

[0097]

[0098] The equivalent wind pressure p1 is:

[0099]

[0100] Arranging formula 11 to formula 16, we have:

[0101]

[0102] Substituting into the calculation expression of ΔT, we get:

[0103]

[0104] Li et al. (2011) derived the maximum temperature calculation formula under tunnel fire conditions from the perspective of heat transfer and combined with a large number of model test results:

[0105]

[0106] Combining Formula 18 and Formula 19, the length of the smoke countercurrent layer is expressed as:

[0107]

[0108] In formula 1 to formula 20: A—tunnel cross-sectional area, m 2 ; C—flow coefficient, ranging from 0.6 to 0.7, with an average value of 0.65; D—the length of the part of the smoke flow cross section in contact with the tunnel wall, m; H—tunnel height, m; H s —smoke layer thickness, m; W—tunnel width, m; P—tunnel circumference, m; α'—convection heat transfer coefficient, W / (m2 K), generally 22.0 to 30.6, this time the middle value 26.3; c p —Specific heat at constant pressure, J / (kg·K); T—Smoke flow temperature, K; μ(t)—time-related function; t—time, s; m'—smoke counterflow mass flow rate, kg / s; u a —longitudinal ventilation speed, m / s; u0—speed at which combustion-generated gases leave the surface of combustibles, m / s; u BF —average velocity of smoke counterflow, m / s; μ—friction coefficient in the tunnel, ranging from 0.0055 to 0.0073, with an average value of 0.0064 (Gaillard, 1973); ρ a —Air density, kg / m 3 ; ρ—combustion gas density, kg / m 3 ; T a —Ambient temperature, K; T max —Maximum temperature of smoke flow, K; φ n —Mixing ratio, which means the ratio of the fire combustion products to the air flow mass flow in the smoke micro-clusters; f—experimental constant, with a value range of 0.0040 to 0.0046 and an average value of 0.0043; f3—conversion coefficient, which means the ratio of the horizontal counterflow velocity to the maximum vertical velocity of the smoke, single-headed: 0.20 to 0.64, with an average value of 0.36, open: 0.15 to 0.23, with an average value of 0.18.

[0109] 2. Verification of the prediction formula for the length of the smoke countercurrent layer

[0110] The calculation method of the smoke countercurrent layer length can be simplified as: L BF =f(K,ΔT max ,H s ,u a …), it can be seen that when the heat convection loss coefficient inside the tunnel is not accurate and when the value of parameter K is uncertain due to the irregular shape of the tunnel section, the calculation of the smoke counterflow length will produce a large error. However, when a temperature sensor (such as a grating fiber system) is installed along the longitudinal direction of the tunnel, there are two ways to get the parameter K: fitting the parameter K and calculating K using basic parameters. The following describes the reliability of the two methods and verifies them by experiment.

[0111] (1) K is obtained by fitting the longitudinal temperature

[0112] Under tunnel fire conditions, when the longitudinal temperature data provided by the temperature sensor (such as the grating fiber system) is sufficient, based on the full-scale fire test data of the Baimangxueshan Tunnel, ΔT=Cexp[-K(x-x0)] is fitted through the grating fiber longitudinal temperature data, and ΔT is derived using the known fire test parameters. max , Hs Then, we can find L BF .

[0113] Figure 2 The longitudinal temperature distribution of flue gas counterflow at typical moments can be seen. It can be seen that the combustion is stable from 400s to 800s. Based on this, the longitudinal temperature distribution of flue gas counterflow 600s after ignition is fitted. Figure 3 It can be seen that K = 0.01938, and then the smoke counterflow length L is calculated according to formula 21 BF .

[0114] (2) By calculating m' and H s , and then calculate L BF

[0115] Based on the widely used test results of Bettis in horseshoe tunnels and Hu Longhua et al. in Yuanjiang tunnels in Yunnan, and combined with the test results completed in Baimangxueshan Tunnel, m' and H are calculated. s , and then calculate L BF , the specific calculation results can be found in Figure 4 .

[0116] When two different calculation methods are used, based on the fire test data of Baimang Snow Mountain in Yunnan, the length of the smoke countercurrent layer L BF The calculation results are shown in Table 1.

[0117] Table 1 Results of the calculation model for the length of the smoke countercurrent layer

[0118] <![CDATA[Calculated value (L BF / H)]]> <![CDATA[True value (L BF / H)]]> Calculate K based on smoke flow characteristics 23.25 29.25 Fitting the countercurrent longitudinal temperature distribution to calculate K 27.24 29.25

[0119] It can be seen that the calculation model is better overall. When K obtained by fitting the longitudinal counterflow temperature distribution is substituted into the smoke counterflow calculation model, the calculated value is close to the actual value. The smoke counterflow layer length L calculated purely based on the smoke flow characteristics BF , which is somewhat different from the actual value. After analyzing the calculation results of the Baimangxueshan Tunnel, the possible reason is that the test site is about 3,400 meters above sea level. Due to the low air density, the buoyancy force of the smoke flow is relatively larger, and thus it spreads farther.

[0120] Therefore, when the tunnel is equipped with a temperature sensor (such as a grating fiber system), the full longitudinal distribution characteristics of the system are fully utilized, and the established theory can well reflect the fire information of the fire; however, the smoke flow characteristics of tunnel fires have great uncertainty, and the difference in the calculation results in Table 1 is only caused by the different calculation methods of the K value.

[0121] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for calculating the length of the smoke countercurrent layer in a longitudinal ventilation tunnel fire, characterized in that: Follow these steps:

1. Derivation of the prediction formula for the length of the smoke countercurrent layer The smoke countercurrent layer is divided into development section, transition section, stable section and destruction section. Based on the morphology of the smoke countercurrent layer in the stable state, a calculation model for the length of the smoke countercurrent layer in the fire in the longitudinal ventilation tunnel is derived. Empirical fire smoke thickness description formula: H s =0.28H (Formula 1) Theoretical expression of smoke countercurrent layer thickness, the maximum thickness does not exceed 0.25H: Taking the maximum thickness of the smoke layer as 0.25H, the average counterflow velocity of the smoke counterflow layer near the fire source is: The flue gas counterflow mass flow rate is: m'=ρu BF H s W (Formula 4) Ignoring the density change during combustion, we can approximate: ρ a =ρ(Formula 5) Combining Formula 3, Formula 4 and Formula 5, we have: When the area around the combustible is relatively dry and there is no other evaporable substance, u0→0 is approximately true. For the convenience of calculation, the mass flow rate can be simplified to: According to the heat transfer principle, the temperature field variation law under tunnel fire conditions is deduced: And verified by full-scale test data: in The front end of the flue gas counterflow is the flue gas counterflow stagnation point, that is, the flue gas counterflow velocity at this point can be considered to be zero, and at the same time: p1=Δp2(Formula 11) The pressure difference Δp2 at the front end of the smoke counterflow under tunnel fire conditions is: Δp2=ΔρgH s (Formula 12) and When the wind flows through a section where the cross-sectional area suddenly decreases, there will be an obvious pressurization effect, and the thickness of the smoke counterflow front also has an obvious gradual change stage. When in a stable counterflow state, the shear effect of the smoke counterflow lower end on the windflow is ignored. It is assumed that the counterflow layer and the longitudinal windflow are two layers. Based on this, for the convenience of calculation, it is considered that the wind flow pressure in the stable section acts evenly on the destruction section, that is: u a A=u' a A' (Formula 14) Converted into the calculation section height of 0.75H, the average wind pressure on H, assuming that the change end curve is 45. Straight line, considering the two-dimensional situation, the pressure coefficient k is: The equivalent wind pressure p1 is: Arranging formula 11 to formula 16, we have: Substituting into the calculation expression of ΔT, we get: From the perspective of heat transfer and combined with a large number of model test results, the maximum temperature calculation formula under tunnel fire conditions is derived: Combining Formula 18 and Formula 19, the length of the smoke countercurrent layer is expressed as: In formula 1 to formula 20: A—tunnel cross-sectional area, m 2 ; C—flow coefficient, ranging from 0.6 to 0.7, with an average value of 0.65; D—the length of the part of the smoke flow cross section in contact with the tunnel wall, m; H—tunnel height, m; H s —smoke layer thickness, m; W—tunnel width, m; P—tunnel circumference, m; α'—convection heat transfer coefficient, W / (m 2 K), generally 22.0 to 30.6, this time the middle value 26.3; c p —Specific heat at constant pressure, J / (kg·K); T—smoke temperature, K; μ(t)—time-related function; t—time, s; m'—smoke counterflow mass flow rate, kg / s; u a —longitudinal ventilation speed, m / s; u0—speed at which combustion-generated gases leave the surface of combustibles, m / s; u BF —average velocity of smoke counterflow, m / s; μ—friction coefficient in the tunnel, ranging from 0.0055 to 0.0073, with an average value of 0.0064 (Gaillard, 1973); ρ a —Air density, kg / m 3 ; ρ—combustion gas density, kg / m 3 ; T a —ambient temperature, K; T max —Maximum temperature of smoke flow, K; φ n —mixing ratio, which means the ratio of the mass flow rate of fire combustion products to the air flow in the smoke micro-group; f—test constant, with a range of 0.0040 to 0.0046 and an average value of 0.0043; f3—conversion coefficient, which means the ratio of the horizontal counterflow velocity to the maximum vertical velocity of the smoke flow, single-head: 0.20 to 0.64, with an average value of 0.36, open: 0.15 to 0.23, with an average value of 0.18; 2. Verification of the prediction formula for the length of the smoke countercurrent layer The calculation method of the smoke countercurrent layer length can be simplified as: L BF =f(K,ΔT max ,H s ,u a …), it can be seen that when the heat convection loss coefficient inside the tunnel is inaccurate and when the value of parameter K is uncertain due to the irregular shape of the tunnel section, the calculation of the smoke counterflow length will produce a large error. However, when temperature sensors are installed along the longitudinal direction of the tunnel, there are two sources of parameter K, fitting parameter K and calculating K using basic parameters. The reliability of the two methods is explained below and verified by experiments; (1) K is obtained by fitting the longitudinal temperature Under tunnel fire conditions, when the longitudinal temperature data provided by the temperature sensor is sufficient, based on the full-scale fire test data of the Baimangxueshan Tunnel, ΔT=Cexp[-K(x-x0)] is fitted through the longitudinal temperature data of the grating fiber, and ΔT is derived using the known fire test parameters. max , H s , and then find L BF ; From the longitudinal temperature distribution of flue gas counterflow at typical moments, it can be seen that the combustion is stable from 400s to 800s. Based on this, the longitudinal temperature distribution of flue gas counterflow 600s after ignition is fitted, K = 0.01938, and then the flue gas counterflow length L is calculated according to formula 21 BF ; (2) By calculating m' and H s , and then calculate L BF .

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