Method for calculating fire smoke layer thickness and heat release rate of ancient wooden gallery bridge

Abstract

The invention discloses a method for calculating the thickness and the heat release rate of a fire smoke layer of an ancient wooden gallery bridge. The method comprises the specific steps that 1, geometric structure information, temperature sensor data and material information of the ancient wooden gallery bridge are obtained; 2, calculating the mass flow rate flowing in / out from the opening, the heat conduction coefficient of the material, the surface area in contact with hot gas and the heat loss of the material according to the set construction information and the data of the temperature sensor; and 3, according to the parameters calculated in the steps 1 and 2, calculating the thickness of a smoke layer through mass conservation, and calculating the fire heat release rate of the ancient wooden gallery bridge according to energy conservation. Compared with a traditional fire parameter calculation model, the method considers the influence of geometric structure information of the ancient wooden gallery bridge on the smoke layer thickness and the heat release rate, including the roof gradient, the weather board height and the like, calculation is more accurate, and accurate reference can be provided for the internal situation of the ancient wooden gallery bridge when a fire occurs.

Description

Technical Field

[0001] The present invention belongs to the field of fire protection of architectural heritage, and in particular relates to a method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire. Background Art

[0002] Wooden covered bridges are one of the most common types of bridges in ancient China and hold significant cultural value. In recent years, China has placed high priority on the scientific preservation of bridge artifacts, issuing several key policies that emphasize the importance of research into preventative protection technologies for these artifacts. Due to the flammability of wood, wooden covered bridges are susceptible to fire. When a fire occurs, the presence of smoke makes the interior of the bridge difficult to see, significantly impacting firefighters' access and tourist evacuation. Furthermore, the heat release rate (HRR) is an indicator of fire severity, and accurately determining fire severity is crucial for developing firefighting strategies. However, existing technologies, such as the Zukoski method and the MQH method, often target small, flat-roofed compartments with only a single opening. Due to the unique structural design of ancient wooden covered bridges (sloping roofs, weatherboards, and the number of openings), conventional calculation methods are difficult to directly apply to fire calculations for wooden covered bridges. Currently, no fire calculation model specifically tailored to this unique architectural heritage form is available. Therefore, the roof slope and height of the weatherboard of the wooden bridge are taken into consideration, and a fire calculation method suitable for wooden bridges is developed, so as to achieve preventive protection against fires of ancient wooden bridges and maximize the safety of the cultural relics themselves and the safety of users. Summary of the Invention

[0003] To address these issues, this paper discloses a scientific, accurate, and effective method for calculating the smoke layer thickness and heat release rate during fires on ancient wooden covered bridges. This method can be applied to calculate the smoke layer thickness and heat release rate during fires on ancient wooden covered bridges, providing theoretical support for fire protection of ancient wooden covered bridges.

[0004] To achieve the above object, the technical solution of the present invention is as follows:

[0005] A method for calculating the thickness and heat release rate of the smoke layer in an ancient wooden covered bridge fire comprises the following steps:

[0006] Step 1: Based on the preliminary research data and temperature sensor data, various parameters of the ancient wooden covered bridge fire are obtained, including roof slope θ, windshield height H0, bridge length L, bridge width W, bridge height H, upper smoke layer temperature T g 、Ambient temperature T a , material thickness δ, material thermal conductivity k, material specific heat c and material density ρ.

[0007] Step 2: Based on the data from step 1, calculate the mass flow rate flowing in / out of the opening and Surface area A in contact with hot gas T , material thermal conductivity h k , and heat loss of the material

[0008] Step 3: Based on the calculation results in steps 1 and 2, the smoke layer thickness H is obtained by mass conservation. N , the heat release rate Q of the ancient wooden bridge fire was calculated based on the law of conservation of energy.

[0009] Furthermore, in step 2, the mass flow rate flowing in / out of the opening is considered, taking into account the roof slope and the height of the weatherboard. and The calculation formula is as follows:

[0010] (1) Mass flow in / out along the length of the bridge

[0011]

[0012] Among them C d is the flow coefficient, ρ a , ρ g are the densities of ambient gas and smoke layer gas respectively, and g is the acceleration due to gravity.

[0013] (2) Mass flow in / out from the bridge width direction

[0014]

[0015] Furthermore, in step 2, considering the roof slope and the height of the weatherboard, the surface area A in contact with the hot gas T The calculation formula is as follows:

[0016]

[0017] Furthermore, in step 2, the thermal conductivity of the material h k The calculation formula is as follows:

[0018]

[0019] Among them, t is the time when the fire occurs, t p is the thermal penetration time of the material, and the calculation formula is as follows:

[0020]

[0021] Here, thermal diffusivity α = k / ρc.

[0022] Furthermore, in step 2, the heat loss of the material The calculation formula is as follows:

[0023]

[0024] Furthermore, in step 3, the smoke layer thickness H N The calculation process is as follows:

[0025] (1) According to the law of conservation of mass, the mass flow rate flowing in and out of the opening and Equal, and since each opening satisfies the law of conservation of mass, in order to simplify the calculation, the law of conservation of mass is used for the mass flow in the width direction of the bridge, H N The calculation formula is as follows:

[0026]

[0027] (2) According to the ideal gas state equation, the unknown gas density is converted into the known gas temperature T g 、T a , smoke layer thickness H N The final calculation formula is as follows:

[0028]

[0029] Furthermore, in step 3, the calculation formula for the heat release rate Q of the ancient wooden covered bridge fire is as follows:

[0030]

[0031] in, is the total mass flow rate overflowing from the opening, c p is the specific heat of the gas.

[0032] Furthermore, the total mass flow rate overflowing from the opening The calculation process is as follows:

[0033] (1) Considering the mass flow rate overflowing from the vents on all four sides, The calculation formula is as follows:

[0034]

[0035] (2) According to the ideal gas state equation, the gas density is converted into gas temperature. The calculation formula is as follows:

[0036]

[0037] Furthermore, the calculation formula for the heat release rate Q of the ancient wooden bridge fire is as follows:

[0038]

[0039] The beneficial effects of the present invention are:

[0040] The proposed method considers the impact of geometric properties (including roof slope and weatherboard height) on smoke layer thickness and heat release rate during a wooden covered bridge fire. Compared to existing methods that only consider ordinary rooms, it achieves higher accuracy. This method can provide an accurate theoretical reference for protecting wooden covered bridges from fire, helping to ensure personal safety during fires, develop fire protection strategies to safeguard architectural heritage, and enhance the fire resistance of wooden covered bridges. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The figure is a schematic diagram of the calculation process of the smoke layer thickness and heat release rate of a wooden bridge fire according to the present invention.

[0042] Figure 2 Schematic diagram of a typical wooden bridge fire cross section.

[0043] Figure 3 This is a schematic diagram of the elevation of a typical wooden bridge fire.

[0044] Figure 4 This is a schematic diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0046] Example 1:

[0047] like Figure 4 The embodiment shown in FIG. Figure 1 The calculation process given first measured the various parameters of the wooden covered bridge, including the bridge length of 30m, the bridge width of 5.6m, the bridge height of 5m, the roof slope of 30°, the height of the weatherboard of 1.4m, the thickness of the weatherboard of 20cm, the thickness of the roof panel of 20cm, the thermal conductivity of wood of 0.14W / (m·K), the specific heat of 2.85kJ / (kg·K), and the density of 640.0kg / m 3 .

[0048] Due to the lack of cases of wooden bridge combustion, the computational fluid dynamics software FDS was used to build a wooden bridge model, arrange temperature sensors, obtain temperature data, and use them to compare the calculated results with the simulation results.

[0049] Select three different times to calculate the smoke layer thickness H N , the calculation results are shown in the following table:

[0050] time <![CDATA[T g (K)]]> <![CDATA[T a (K)]]> <![CDATA[H N (m)]]> FDS data (m) error(%) 600s 552.57 293 2.62 2.86 9.2 800s 596.20 293 2.58 2.72 5.4

[0051] Calculate the surface area A in contact with the hot gas T , the calculation results are as follows:

[0052]

[0053] Calculate the material thermal penetration time t p and the thermal conductivity h k , the calculation results are as follows:

[0054] time δ(m) α <![CDATA[t p (t)]]> Remark 600s 0.2 7.68E-08 130285.71 <![CDATA[t<t p ]]> 800s 0.2 7.68E-08 130285.71 <![CDATA[t<t p ]]> 1000s 0.2 7.68E-08 130285.71 <![CDATA[t<t p ]]>

[0055] Calculate the material thermal conductivity h k , the calculation results are as follows:

[0056] time <![CDATA[h k ]]> 600s 135.47 800s 101.60 1000s 81.28

[0057] Calculate heat loss from materials The calculation results are as follows:

[0058] Calculate the total mass flow rate flowing in / out of the opening The calculation results are shown in the following table:

[0059]

[0060] The heat release rate Q of the ancient wooden bridge fire was calculated. The results are shown in the following table:

[0061] In summary, the method for calculating the smoke layer thickness and heat release rate of an ancient wooden bridge fire proposed in the present invention has an average error of no more than 10% between the calculated smoke layer thickness and fire heat release rate and the FDS data, and can more accurately calculate the fire status of the wooden bridge.

[0062] It should be noted that the above content merely illustrates the technical idea of the present invention and cannot be used to limit the scope of protection of the present invention. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications all fall within the scope of protection of the claims of the present invention.

[0063] time Q(kW) FDS data error(%) 600s 45864.20 49471.73 7.9 800s 51795.36 57158.14 10.4 1000s 65432.29 72835.80 11.3

Claims

1. A method for calculating the thickness and heat release rate of the smoke layer in an ancient wooden covered bridge fire, characterized by: Step 1: Based on the preliminary research data and temperature sensor data, various parameters of the ancient wooden covered bridge fire are obtained, including the roof slope θ, the height of the wind and rain board H0, the bridge length L, width W, bridge height H, and the upper smoke layer temperature T g 、Ambient temperature T a , material thickness δ, material thermal conductivity k, material specific heat c and material density ρ; Step 2: Based on the data from step 1, calculate the mass flow rate flowing in / out of the opening and Surface area A in contact with hot gas T , material thermal conductivity h k , and the heat loss of the material Step 3: Based on the calculation results in steps 1 and 2, the smoke layer thickness H is obtained by mass conservation. N , the heat release rate Q of the ancient wooden bridge fire was calculated based on the law of conservation of energy.

2. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 1 is characterized in that: In step 2, the mass flow rate in / out of the opening is calculated, taking into account the roof slope and the height of the weatherboard. and The calculation formula is as follows: (1) Mass flow in / out along the length of the bridge Among them C d is the flow coefficient, ρ a , ρ g are the densities of ambient gas and smoke layer gas respectively, and g is the acceleration of gravity; (2) Mass flow in / out from the bridge width direction 3. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 1 is characterized in that: In step 2, considering the roof slope and the height of the weatherboard, the surface area A in contact with the hot gas T The calculation formula is as follows:

4. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 1 is characterized in that: In step 2, the thermal conductivity of the material h k The calculation formula is as follows: t<t p hour; t>t p hour; Among them, t is the time when the fire occurs, t p is the thermal penetration time of the material, and the calculation formula is as follows: Here, thermal diffusivity α = k / ρc.

5. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 1 is characterized in that: In step 2, the heat loss of the material The calculation formula is as follows:

6. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 1 is characterized in that: In step 3, the smoke layer thickness H N The calculation process is as follows: (1) According to the law of conservation of mass, the mass flow rate flowing in and out of the opening and Equal, and since each opening satisfies the law of conservation of mass, in order to simplify the calculation, the law of conservation of mass is used for the mass flow in the width direction of the bridge, H N The calculation formula is as follows: (2) According to the ideal gas state equation, the unknown gas density is converted into the known gas temperature T g 、T a , smoke layer thickness H N The final calculation formula is as follows:

7. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 1 is characterized in that: In step 3, the calculation formula for the heat release rate Q of the ancient wooden covered bridge fire is as follows: in, is the total mass flow rate overflowing from the opening, c p is the specific heat of the gas.

8. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 4 is characterized in that: Total mass flow rate overflowing from the opening The calculation process is as follows: (1) Considering the mass flow rate overflowing from the vents on all four sides, The calculation formula is as follows: (2) According to the ideal gas state equation, the gas density is converted into gas temperature. The calculation formula is as follows:

9. The method for calculating the thickness of the smoke layer and the heat release rate of an ancient wooden covered bridge fire according to claim 4 is characterized in that: The calculation formula for the heat release rate Q of the ancient wooden bridge fire is as follows:

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