Calculation method for fire spread in urban area
By dividing urban areas into block units, calculating combustion airflow fields, and combining vector superposition and statistical methods, the accuracy and efficiency issues of urban fire spread calculations are solved, enabling accurate prediction of urban fire spread.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-04
- Publication Date
- 2026-03-17
AI Technical Summary
Existing methods for calculating the spread of fires in urban areas suffer from low accuracy or bottlenecks in improving calculation efficiency.
The urban area is divided into blocks of equal size, fire characteristic parameters are set, the combustion airflow field is calculated and the coupled airflow field is determined by the principle of vector superposition. The fire status is dynamically updated by combining thermal radiation and the probability of fire spread, and the fire spread is predicted by statistical methods.
It achieves a balance between accuracy and efficiency in calculating the spread of fires in urban areas, improves the accuracy and scientific validity of the calculation results, and supports real-time prediction of the spread of large-scale fires.
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Figure CN121681986A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the spread of fires in urban areas, and more specifically to a method for calculating the spread of fires in urban areas. Background Technology
[0002] Urban fires triggered by unexpected events such as earthquakes or explosions can spread rapidly, causing extensive damage to urban buildings. Based on city block units and considering typical fire spread mechanisms between buildings, this paper proposes a method for reasonably calculating the spread and damage of fires in building complexes. This method is of great significance for predicting and assessing the extent of fire damage in urban areas and for developing disaster prevention and mitigation plans.
[0003] Existing methods for calculating urban fire spread include statistical algorithms, probabilistic algorithms, and semi-physical and physical algorithms. Among them, methods based on statistical or probabilistic algorithms have high computational efficiency, but the accuracy of fire spread calculation results is low; methods based on semi-physical and physical algorithms have high computational accuracy, but their high computational complexity leads to bottlenecks in improving computational efficiency. Summary of the Invention
[0004] The purpose of this invention is to solve the technical problems of low accuracy of calculation results or bottlenecks in improving calculation efficiency in existing urban fire spread calculation methods, and to provide a calculation method for urban fire spread.
[0005] To achieve the above objectives, the technical solution provided by this invention is as follows: A method for calculating the spread of urban fires, characterized by the following steps: S1. Based on the typical size and type of the urban area, divide the urban area into blocks of the same size and set the fire characteristic parameters of each block at the current moment. S2. Determine the fire status of buildings in each block unit at the current moment based on the type of accident; S3. Based on the fire characteristic parameters and fire status at the current moment, calculate the combustion airflow field generated by each block unit in the target block unit at the current moment, wherein the target block unit is any block unit within the urban area. S4. Based on the combustion airflow field generated by each block unit in the target block unit, and combined with the known environmental wind field of the target block unit, the coupling airflow field of the target block unit is determined by the vector superposition principle, thereby determining the coupling airflow field of each block unit in the urban area at the current moment. S5. Based on the fire characteristic parameters of each block unit at the current time, the fire status of the buildings in each block unit at the current time, and the coupled airflow field of each block unit at the current time, calculate the number of non-fired buildings in each block unit that will start fire at the next preset time. S6. Based on the number of non-fired buildings in each block unit that catch fire at the next preset time and the fire status of buildings in each block unit at the current time, determine the fire status of buildings in each block unit and the fire characteristic parameters of each block unit at the next preset time; take the next preset time as the new current time, return to step S3, until the next preset time meets the preset calculation termination time, end the calculation, and thus complete the calculation of the spread of fire in the urban area.
[0006] Further, in step S1, the fire characteristic parameters include building density. and building distribution function The building distribution function The expression is: ; In the formula, The dimensionless distance is defined as: ; In the formula, Indicates the distance between buildings. This indicates the average size of the building.
[0007] Furthermore, in step S3, the combustion airflow field includes the magnitude of the combustion airflow velocity. and the direction of combustion airflow; The magnitude of the combustion gas velocity The expression is: ; In the formula, Represents characteristic parameters, Represents the area of the block unit. This represents the distance between the centers of the target block cells. This represents the average heat release rate per unit area of the block unit; The combustion airflow direction is towards the center of the block unit.
[0008] Further, in step S5, the number of fires that occurred in the non-fired buildings within the block unit includes the number of fires caused by thermal radiation from the burning buildings within the block unit, and the number of fires caused by the spread of flying embers from the burning buildings within the block unit, adjacent blocks, and next-nearest blocks; the adjacent blocks refer to the four blocks adjacent to the block unit on all four sides, and the next-nearest blocks refer to the four blocks located diagonally opposite the block unit.
[0009] Furthermore, in step S5, the number of fires caused by thermal radiation in the burning buildings within this block unit. The expression is: ; In the formula, This indicates the number of combustible structures with radiative spread capability within this block unit. A coefficient of 4 indicates that a single combustible structure has four radiating surfaces. The probability of fire spread caused by thermal radiation is expressed as: .
[0010] Furthermore, in step S5, the number of fires caused by the spread of a single burning building within this block unit through the spread of a flying ember includes the number of fires in buildings within this block unit caused by the spread of a flying ember from a single burning building, as well as the number of fires in buildings within adjacent and next-nearest block units. Based on the number of fires caused by the spread of a single burning building within this block unit through the spread of a flying ember and the total number of burning buildings within this block unit capable of spreading a flying ember, the number of fires caused by the spread of a burning building within this block unit can be determined. The number of buildings ignited by the spread of sparks from a single burning building The expression is: ; In the formula, This represents the maximum angle formed between the point of impact of the rocket and the direction of the coupled airflow field. This represents the correction coefficient for the probability of fire spread under different airflow conditions. This indicates the maximum spread distance of the flying fire. This represents the effective radius of the fire settling zone at different angles. The probability of fire spread caused by the spread of flying sparks is expressed as: ; The number of buildings in adjacent and next-nearest block units that are ignited by the spread of a single burning building is determined based on the coupled airflow field of the block unit where the burning building is located and the relative positions of the adjacent and next-nearest block units to the block unit where the burning building is located, using a statistical averaging method.
[0011] Furthermore, in step S5, the number of fires in the local block unit caused by the spread of burning buildings in adjacent and sub-adjacent block units through flying fire is determined based on a statistical averaging method, according to the coupled airflow field of adjacent and sub-adjacent block units, the relative positions of burning buildings in adjacent and sub-adjacent block units and the local block unit.
[0012] Furthermore, in step S2, the accident type includes earthquake fire and accidental explosion fire; the fire state includes no ignition, initial ignition, fire growth, complete combustion, fire decay, and combustion termination. If the accident type is an earthquake fire, all buildings of the same type in the entire city area will be initially ignited with the same probability; If the accident type is an accidental explosion fire, the building has a 90% probability of igniting within 1.5 times the fireball radius of the explosion center; within 1.5 to 5 times the fireball radius, the building has a probability of initial ignition with a squared decay until the probability of ignition at the 5 times fireball radius location approaches 0.
[0013] Furthermore, in step S6, the process of determining the fire characteristic parameters of each block unit at the next preset time based on the number of non-fired buildings that catch fire in each block unit at the next preset time is as follows: based on the number of non-fired buildings that catch fire in each block unit at the next preset time, update the number of non-fired buildings in each block unit at the next preset time, thereby determining the density of non-fired buildings in each block unit at the next preset time. The process of determining the fire status of buildings in each block unit at the next preset time, based on the number of non-fired buildings that have caught fire in each block unit at the next preset time and the fire status of buildings in each block unit at the current time, is as follows: the number of non-fired buildings in each block unit at the next preset time is updated based on the number of non-fired buildings that have caught fire in each block unit at the next preset time; the fire status of burning buildings at the next preset time is determined based on the burning duration of buildings that have caught fire and the sum of the characteristic times of the burning states they have experienced; the burning states include initial ignition, fire growth, complete combustion, and fire decay.
[0014] Furthermore, step S6 also includes saving and outputting the calculation results of the urban area fire spread; the calculation results include the coupled airflow field of each block unit at each preset time from the start of the calculation to the preset calculation termination time, the density of non-fired buildings, and the number of buildings in each burning state.
[0015] The beneficial effects of this invention are: 1. The present invention provides a method for calculating the spread of fires in urban areas. By dividing urban areas into block units according to typical size and type, empirical formulas are used to characterize the airflow field, thermal radiation ignition probability, and flying fire spread probability caused by fires. Based on statistical methods, a balance between calculation accuracy and calculation efficiency is achieved in the analysis and prediction of large-scale fire spread, which solves the problems of low accuracy of calculation results or bottlenecks in improving calculation efficiency in existing methods. 2. The present invention provides a calculation method for the spread of urban fires, which uses the principle of vector superposition to calculate the coupled airflow field, avoiding the direct simulation calculation of complex fire dynamics and improving calculation efficiency; 3. The present invention provides a method for calculating the spread of fire in urban areas, which considers heat radiation and fire spread within block units and fire spread between block units, thereby ensuring that the calculation results of fire spread in urban areas are more reasonable. 4. The present invention provides a method for calculating the spread of fire in urban areas, which uses a statistical averaging method to characterize the probability of the spread of flying fire from burning buildings between block units, thereby improving the scientificity and accuracy of the calculation of flying fire spread. 5. The present invention provides a method for calculating the spread of fire in urban areas. By describing the relative positions of buildings within urban block units through a building distribution function, it achieves a reasonable representation of the distribution of buildings in urban areas, provides effective basic data for calculating the spread of fire in urban areas, and thus improves the effectiveness of the calculation. 6. The present invention provides a method for calculating the spread of fire in urban areas, which can dynamically update the fire spread and fire damage status in urban areas based on the initial fire situation, thereby achieving real-time prediction of the spread of fire in large-scale urban areas. Attached Figure Description
[0016] Figure 1 This is a flowchart illustrating an embodiment of a method for calculating the spread of fires in urban areas according to the present invention. Figure 2 This is a schematic diagram of the fire settling zone in step S5 of an embodiment of the calculation method for the spread of urban fires according to the present invention. Figure 3 In step S5 of an embodiment of the calculation method for the spread of urban fires according to the present invention, a diagram showing the relative positional relationship of block units is provided. Figure 4 a, Figure 4 b is a schematic diagram of the fire settlement zone of different block units in step S5 of an embodiment of the calculation method for the spread of urban fires according to the present invention. Figure 5 This is a schematic diagram illustrating the calculation of the number of buildings on fire within adjacent block units in step S5 of an embodiment of the calculation method for the spread of urban fires according to the present invention. Detailed Implementation
[0017] The present invention will be further described below with reference to the accompanying drawings and embodiments.
[0018] This embodiment provides a method for calculating the spread of fires in urban areas. Taking a specific urban scenario as an example, the method calculates the spread of fires in that urban area. Figure 1 As shown, it includes the following steps: S1. Calculation of Area Unit Division and Parameter Setting. Based on the typical size and type of urban area, the urban area is divided into blocks of the same size, and the fire characteristic parameters of each block are set at the current moment; The typical size of urban areas ranges from 0.8km×0.8km to 1.6km×1.6km, and the types of urban areas include residential, commercial, industrial, and vacant land; the fire characteristic parameters of the block unit include the fire spread characteristic parameters of the block unit and the building combustion characteristic parameters. Fire spread characteristic parameters include building density Average building size Number of building floors and building distribution function Among them, the building distribution function The expression is: ; In the formula, The dimensionless distance is defined as: ; In the formula, Indicates the distance between buildings; Building density Defined as: ; In the formula, Indicates the number of buildings in the block unit. Indicates the area of the block unit; Building combustion characteristic parameters include combustion characteristic time and combustion heat release rate; combustion characteristic time includes initial ignition time, fire growth time, complete combustion time, and fire decay time; combustion heat release rate includes the combustion heat release rate per unit area of the building during initial ignition, fire growth, complete combustion, and fire decay.
[0019] S2. Set the fire status of buildings within each block unit. Determine the fire status of buildings within each block unit at the current moment based on the accident type; where accident types include earthquake fire and accidental explosion fire; fire status includes no ignition, initial ignition, fire growth, complete combustion, fire decay, and combustion termination; If the accident type is an earthquake fire, all buildings of the same type within the entire city area will ignite with the same probability. If the accident type is an accidental explosion fire, buildings will ignite with a 90% probability within a radius of 1.5 times the fireball radius from the explosion center. Within a radius of 1.5 to 5 times the fireball radius, buildings will ignite with a probability that decreases with the square of the fireball radius, until the probability of ignition at a location with a fireball radius approaches 0.
[0020] S3. Calculation of combustion airflow field in block unit. Based on the fire characteristic parameters and fire state at the current moment, calculate the combustion airflow field generated by each block unit in the target block unit at the current moment. The target block unit is any block unit within the urban area. The combustion airflow field includes the magnitude and direction of the combustion airflow; the magnitude of the combustion airflow velocity is proportional to the combustion intensity of the block cell at the current moment, and the direction of the combustion airflow points towards the center of the block cell; the magnitude of the combustion airflow velocity... The expression is: ; In the formula, This represents a characteristic parameter, and in this embodiment, it takes the following values: m 2 / j (All variables are in SI units), Represents the area of the block unit. This represents the distance between the centers of the target block cells. This represents the average heat release rate per unit area of the block unit; Average heat release rate per unit area of the block unit The expression is: ; In the formula, This represents the rate of heat release per unit area of a building. Parameters related to the fire status of a building. This indicates the area occupied by the building.
[0021] S4. Calculation of Regional Coupled Airflow Field. Based on the combustion airflow field generated by each block unit in the target block unit, and combined with the known environmental wind field of the target block unit, the coupled airflow field of the target block unit is determined using the vector superposition principle, thereby determining the coupled airflow field of each block unit in the urban area at the current moment; Coupled airflow velocity of the coupled airflow field The calculation formula is: ; In the formula, Represents block unit The fire was in the block unit The combustion airflow velocity generated at that location, Represents block unit , The distance between the center points Indicates ambient wind speed.
[0022] S5. Fire spread calculation for each block unit within the area. Based on the fire characteristic parameters of each block unit at the current moment, the fire status of the buildings in each block unit at the current moment, and the coupled airflow velocity of each block unit at the current moment, calculate the number of non-fired buildings in each block unit that will start fire at the next preset time. The number of fires occurring in buildings that were not previously on fire within the block unit includes the number of fires caused by thermal radiation from burning buildings within the same block unit. The number of fires caused by the spread of flying embers from burning buildings in this block unit, adjacent and next-nearest block units. The adjacent block unit refers to the four block units that are adjacent to this block unit on all four sides, and the next adjacent block unit refers to the four block units located diagonally opposite this block unit.
[0023] Number of fires caused by thermal radiation in combustible buildings within this block unit The expression is: ; In the formula, This indicates the number of burning structures within this block unit that have the capability to spread radiation. This indicates the density of uncooked buildings within this block unit; a coefficient of 4 indicates that a single burning building has four heat radiation surfaces. This indicates the probability of fire spreading due to thermal radiation. Probability of fire spread caused by thermal radiation The expression is: ; In the formula, This indicates the distance from the target point to the outer wall of the burning building.
[0024] Number of fires caused by the spread of flying embers from burning buildings in this block unit, adjacent and next-nearest block units Including the number of fires caused by the spread of flying embers from burning buildings within this block unit. The number of fires caused by the spread of flying embers from burning buildings in adjacent and next-nearest block units. ,Right now: ; Among them, the number of fires in this block unit caused by the spread of flying embers from burning buildings. The expression is: ; In the formula, This represents the total number of burning buildings within the block unit that have the ability to spread fire. This indicates the total number of buildings that caught fire due to the spread of sparks from a single burning building. This indicates the number of buildings in adjacent block units that catch fire due to the spread of sparks from a single burning building. This indicates the number of buildings in the next nearest block unit that caught fire due to the spread of a single burning building's sparks. in, The expression is: ; In the formula, the angle of fire settling is... This represents the maximum angle formed between the point of impact of the rocket and the direction of the coupled airflow velocity. This represents the correction coefficient for the probability of fire spread under different airflow conditions. In this embodiment, the value is the ratio of the fire settling area when the coupled airflow velocity is 0 to the fire settling area under the current coupled airflow conditions. This indicates the maximum spread distance of the flying fire. This represents the effective radius of the fire settling zone at different angles. This indicates the probability of fire spreading due to the spread of flying sparks; Among them, the probability of fire spread caused by the spread of flying sparks. The expression is: ; In the formula, This indicates the distance from the target point to the geometric center of the burning building; Firefall settling angle With coupled airflow velocity The relationship between them is as follows Figure 2 As shown, its expression is: .
[0025] The number of buildings in adjacent blocks that catch fire due to the spread of flames from a burning building is related to the coupled airflow velocity and the relative location of the adjacent buildings within the block containing the burning building. For example... Figure 3 As shown, the block unit where the currently burning building is located is marked as 0, which is the current block unit. Its neighboring block units are marked as 1, 2, 3, and 4, respectively, and the next nearest block units are marked as 5, 6, 7, and 8, respectively. Fire spreads between adjacent block units through flying sparks. The number of buildings in adjacent blocks that caught fire due to the spread of a single burning building within a block cell. This can be achieved using the following algorithm: a1. Based on the airflow velocity of block unit 0, determine the distribution of stray fire landing zones of burning structures within block unit 0. Specifically, this involves considering the location of the burning structures within block unit 0, the magnitude and direction of the airflow velocity, and combining this information with... Figure 2 The schematic diagram of the fire fall zone shown indicates the distribution of fire fall zones in the currently burning building.
[0026] a2. Based on the location of adjacent block units, determine the projected area of the fire impact zone of the burning building within block unit 0 in the adjacent block unit, where the fire impact zone of the burning building within block unit 0 is located in its adjacent block. The specific calculation process for the projected area is as follows: When the coupled airflow velocity At times, such as Figure 4 As shown in Figure a, based on the range of the angle of the fire impact zone of the burning building. and block unit 0 and neighboring block units Public boundary outside the legal direction Determine the relative position. Is it at the angle? Within; if so, then the area where the flying fire lands is defined by taking the burning building as the origin and the polar angle range within... and Sector and adjacent block units Intersecting portions; if not, the area where the stray fire from the burning building lands is in its adjacent block unit. The projected area is zero; When the coupled airflow velocity At times, such as Figure 4 As shown in b, based on the characteristic angle of the stray fire landing zone of the burning building. and block unit 0 and neighboring block units Public boundary outside normal Determine the relative position. Is it at the angle? Within; if so, the impact zone of the flying fire may include a polar angle range centered on the burning building. and Sector area, and or The triangular area intersects with the adjacent block unit; if not, the fire impact zone includes the area with the burning building as the origin and the polar angle range within... and (like but The triangular region intersects with the adjacent block unit.
[0027] a3. Determine the number of ignition points caused by flying sparks from burning buildings within block unit 0 in adjacent block units, based on the probability of spark ignition. Figure 5As shown, the calculation method is as follows: based on the projected area of the burning building's stray fire in adjacent block units determined in step a2, and the ignition probability of stray fire at each projection area location, it is obtained through numerical integration. The specific expression is: ; In the formula, This indicates the total number of areas projected by the flying fire. Indicates the distance between blocks.
[0028] a4. Considering that the burning building may be located anywhere in block cell 0, traverse the locations of the burning building in block cell 0 and provide the number of buildings in neighboring block cells that caught fire due to the burning building, after statistically averaging the area. The statistical average result of the number of buildings in neighboring block cells that caught fire due to the burning building is given. for: ; In the formula, Indicates the location of the burning building within block cell 0. At that time, the number of fires from a burning building that spread to adjacent block units was ignited. , These represent the length and width of block unit 0, respectively.
[0029] The number of buildings in adjacent blocks that catch fire due to the spread of a single burning building within a block cell. The number of buildings that can catch fire in adjacent block units due to the spread of flying fire. Based on the calculation method, along block cell 0 and neighboring block cells The result is obtained by translating a neighboring block cell to its diagonal position along the common edge.
[0030] Number of fires in this block unit caused by the spread of flying embers from burning buildings in adjacent and next-nearest block units The expression is: ; In the formula, Indicates the neighboring block units of block unit 0 The total number of burning buildings in the country with the ability to spread fire. Indicates adjacent block units The number of fires caused by the spread of embers from a single burning building within block cell 0. Indicates the next nearest block unit of block unit 0 The total number of burning buildings in the country with the ability to spread fire. This indicates the number of fires caused by the spread of a single burning building in block cell 0 due to the fire propagation from the fire in block cell 1; Among them, neighboring block units The method for calculating the number of fires caused by the spread of flying embers in block unit 0 of a burning building is as follows: b1. Based on neighboring block units The airflow velocity is used to determine the adjacent block units. Distribution of stray fire impact zones in internally burning buildings; b2. Based on block cell 0 and neighboring block cells The relative position of the adjacent block units is used to determine the neighboring block units. The projected area of the fire impact zone of an internally burning building in block unit 0; b3. Determine neighboring block units based on the probability of spark ignition. The number of ignition points caused by stray fire from an internally burning building within block unit 0; b4. Considering that the burning building may be located in an adjacent block unit Any position in the adjacent block unit The system iterates through the locations of burning buildings and provides neighboring blocks after statistically averaging their areas. The number of buildings that caught fire in block cell 0 caused by a single burning building.
[0031] For steps b1 to b4, the number of buildings in adjacent block units that can be ignited by flying sparks from burning buildings within the aforementioned block unit. Based on the algorithm, by changing the block unit index (0, )for( , This can be achieved by using the next nearest neighbor block unit. Number of flying sparks ignited in block unit 0 of burning buildings The number of buildings that catch fire in block cell 0 caused by a stray fire from a burning building within an adjacent block cell. Based on the calculation method, along block cell 0 and neighboring block cells The result is obtained by translating a neighboring block cell to its diagonal position along the common edge.
[0032] S6. Based on the number of non-fired buildings in each block unit that catch fire at the next preset time, determine the fire characteristic parameters of each block unit at the next preset time. Based on the number of non-fired buildings in each block unit that catch fire at the next preset time and the fire status of buildings in each block unit at the current time, determine the fire status of buildings in each block unit at the next preset time. Take the next preset time as the new current time and return to step S3 until the next preset time meets the preset calculation termination time, then end the calculation, thereby completing the calculation of fire spread in the urban area. The process of determining the fire characteristic parameters of each block unit at the next preset time based on the number of non-fired buildings that catch fire in each block unit at the next preset time is as follows: based on the number of non-fired buildings that catch fire in each block unit at the next preset time, update the number of non-fired buildings in each block unit at the next preset time, thereby determining the density of non-fired buildings in each block unit at the next preset time. The process of determining the fire status of buildings in each block unit at the next preset time, based on the number of unaware buildings catching fire in each block unit at the next preset time and the fire status of buildings in each block unit at the current time, is as follows: Update the number of unaware buildings in each block unit at the next preset time based on the number of unaware buildings catching fire in each block unit at the next preset time; update the number of unaware buildings in each block unit at the next preset time based on the number of unaware buildings catching fire in each block unit at the next preset time; update the fire status of buildings that have already caught fire based on the burning duration of buildings that have already caught fire. and the combustion states it undergoes. The sum of characteristic times ( The value is 1, 2, 3, or 4 (determined by the current combustion state), used to determine the fire state of the burning building at the next preset time. If the building enters the next combustion state (until the burning building finally enters the combustion termination state), then its combustion state remains unchanged; the combustion state includes initial ignition, fire growth, complete combustion, and fire decay.
[0033] Step S6 also includes saving and outputting the calculation results of urban area fire spread; the calculation results include the coupled airflow velocity, density of uncooked buildings, and number of buildings in each combustion state for each block unit at each preset result saving time from the start of the calculation to a preset calculation termination time. The result saving time step from the start of the calculation to the preset calculation termination time is defined as... Define the calculation time step from the current time to the next preset time as . ,but In this embodiment, the result is saved at a specific time step. The time step is 15 minutes. It lasts for 5 minutes.
[0034] This embodiment is based on urban functional blocks and uses probabilistic statistical methods to describe the fire spread and fire evolution process within each block. It overcomes the difficulty of balancing model complexity and computational efficiency in common urban fire spread analysis and is of great significance for realizing large-scale urban fire prediction.
[0035] Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.
Claims
1. A method of calculating the spread of a fire in an urban area, characterized in that, The method comprises the following steps: S1, dividing the urban area into block units of the same size according to the typical size and type of the urban area, and setting the fire characteristic parameters of each block unit at the current time; S2, determining the fire state of the buildings in each block unit at the current time according to the type of the accident; S3, calculating the combustion air flow field generated by each block unit at the target block unit at the current time according to the fire characteristic parameters and the fire state at the current time, wherein the target block unit is any block unit in the urban area; S4, determining the coupling air flow field of the target block unit by using the vector superposition principle according to the combustion air flow field generated by each block unit at the target block unit and the predicted environmental wind field of the target block unit, so as to determine the coupling air flow field of each block unit in the urban area at the current time; S5, calculating the number of unburned buildings in each block unit that catch fire at the next preset time according to the fire characteristic parameters of each block unit at the current time, the fire state of the buildings in each block unit at the current time, and the coupling air flow field of each block unit at the current time; S6, determining the fire state of the buildings in each block unit and the fire characteristic parameters of each block unit at the next preset time according to the number of unburned buildings in each block unit that catch fire at the next preset time and the fire state of the buildings in each block unit at the current time; taking the next preset time as a new current time, returning to step S3, and ending the calculation until the next preset time meets the preset calculation termination time, so as to complete the calculation of the urban area fire spread.
2. The method according to claim 1, wherein: In step S1, the fire characteristic parameters include a building density and a building distribution function , the expression of which is ； wherein represents a dimensionless distance defined as: ； wherein represents the distance between buildings, represents the average size of the buildings.
3. The method according to claim 2, wherein: In step S3, the combustion gas flow field includes a combustion gas flow speed magnitude and a combustion gas flow direction; The combustion gas flow velocity magnitude The expression is: ； In the formula, denotes a characteristic parameter, denotes the area of the block unit, denotes the distance from the center of the target block unit to the center of the block unit, denotes the average heat release rate per unit area of the block unit; The combustion air flow direction points to the center of the block unit.
4. The method according to claim 3, wherein: In step S5, the number of unburned buildings in the block unit that catch fire includes the number of fires caused by heat radiation of the burning buildings in the block unit, and the number of fires caused by the spread of flying fire of the burning buildings in the block unit, the adjacent block units, and the next adjacent block units; the adjacent block units refer to the four block units adjacent to the four sides of the block unit, and the next adjacent block units refer to the four block units located at the diagonal positions of the block unit.
5. The method according to claim 4, wherein: In step S5, the number of fires caused by thermal radiation from the burning building within the block unit The expression is: ； wherein, represents the number of burning buildings in the unit block that have the ability to spread radiation, and the coefficient 4 indicates that a single burning building has four heat radiation surfaces, represents the fire spread probability caused by heat radiation, and is expressed as: 。 6. The method according to claim 5, wherein: In step S5, the number of fires caused by the single burning building in the block unit through fire spread includes the number of fires of the buildings in the block unit caused by the single burning building through fire spread, the number of fires of the buildings in the adjacent and next adjacent block units caused by the single burning building through fire spread; according to the number of fires caused by the single burning building in the block unit through fire spread and the total number of the burning buildings in the block unit having the ability of fire spread, the number of fires caused by the burning building in the block unit through fire spread can be determined. The number of fires caused by the spread of flying embers from a single burning building The expression is: ； wherein, represents the maximum included angle formed by the firebrand landing point and the coupling air flow field direction, represents the firebrand spread probability correction coefficient under different air flow conditions, represents the limit spread distance of the firebrand, represents the effective radius of the firebrand settlement area at different angles, represents the fire spread probability caused by the firebrand spread, and the expression is: ； The number of fires of the buildings in the adjacent and next adjacent block units caused by the single burning building through fire spread is determined according to the coupled air flow field of the block unit where the burning building is located, the relative positions of the adjacent and next adjacent block units to the block unit where the burning building is located, and the statistical average method.
7. The urban area fire spread calculation method according to claim 6, characterized in that: In step S5, the number of fires of the block unit caused by the burning building in the adjacent and next adjacent block units through fire spread is determined according to the coupled air flow field of the adjacent and next adjacent block units, the relative positions of the burning buildings in the adjacent and next adjacent block units to the block unit, and the statistical average method.
8. The urban area fire spread calculation method according to claim 7, characterized in that: In step S2, the accident types include earthquake fire and accidental explosion fire; and the fire states include no fire, initial ignition, fire growth, full combustion, fire decay and combustion termination. If the accident type is earthquake fire, all buildings of the same type in the entire urban area have the same probability of initial ignition. If the accident type is accidental explosion fire, within a range of 1.5 times the fireball radius from the explosion center, the buildings have a 90% probability of fire; within a range of 1.5 times the fireball radius to 5 times the fireball radius, the buildings have a square decay probability of initial ignition, until the fire probability at the position of 5 times the fireball radius tends to 0.
9. The urban area fire spread calculation method according to claim 8, characterized in that: In step S6, according to the number of unburned buildings in each block unit at the next preset time, the process of determining the fire characteristic parameters of each block unit at the next preset time is: updating the number of unburned buildings in each block unit at the next preset time according to the number of unburned buildings in each block unit at the next preset time, so as to determine the density of unburned buildings in each block unit at the next preset time. According to the number of buildings in each block unit that catch fire at the next preset time and the fire state of the buildings in each block unit at the current time, the process of determining the fire state of the buildings in each block unit at the next preset time is: updating the number of buildings in each block unit that do not catch fire at the next preset time according to the number of buildings in each block unit that catch fire at the next preset time; judging the fire state of the burning buildings at the next preset time according to the burning duration of the buildings that have caught fire and the sum of the characteristic times of the burning states experienced by the buildings; the burning states include initial ignition, fire growth, full combustion and fire decay.
10. The urban area fire spread calculation method according to claim 9, further comprising: saving and outputting the calculation results of the urban area fire spread; the calculation results include the coupled air flow field of each block unit at each preset time, the density of the buildings that do not catch fire and the number of buildings in each burning state from the start of the calculation to the preset end time of the calculation.