A method for designing a cable passive cooling system

Abstract

This invention relates to the field of bridge technology and provides a design method for a passive cooling system for cables. The method involves obtaining the fire heat flux density acting on the outer surface of the cable's protective layer, using this fire heat flux density as a heat input benchmark to determine the thermal configuration design parameters of the phase change material layer and the heat dissipation configuration design parameters of the thermosiphon cycle. A comprehensive thermal verification is then performed by combining the fire heat flux density, design buffer time, and the critical failure temperature of the cable material to determine the first dimensional design parameters related to the phase change material layer and the second dimensional design parameters related to the condensation section. These first and second dimensional design parameters are then used as the system design result. This application enables active and efficient cooling that is completely independent of external energy and control. It can self-sense and self-start when a fire occurs and continuously provide protection for the cable using its own physical mechanisms, thereby bridging the technological gap between passive delay and complex active cooling.

Description

Technical Field

[0001] This invention relates to the field of bridge technology, and in particular to a design method for a passive cable cooling system. Background Technology

[0002] Large bridges, especially suspension and cable-stayed bridges, are facing increasingly severe and complex fire risks due to their cable systems, which serve as core load-bearing components. This stems from both the continuous growth in bridge traffic and the diversification of freight transport, as well as the rapid popularization of new energy vehicles. Traditional fuel fires and hazardous chemical transport fires coexist with emerging electric vehicle lithium battery fires and hydrogen fuel cell vehicle fires, creating a diverse range of fire source threats. Lithium battery fires are characterized by internal thermal runaway, characterized by long duration, high temperatures, and a high risk of reignition; while jet fires caused by high-pressure hydrogen leaks have extremely high flame velocity and heat flux. Once these fires occur on the bridge deck, the resulting intense flames and high-temperature smoke will directly impact or strongly radiate heat to the cables above the bridge deck, causing their surface temperature to exceed 500°C in a very short time, seriously threatening the safety of the cable structure, which is primarily made of high-strength steel wire. As is well known, the mechanical properties of high-strength steel wire are extremely sensitive to temperature. When the temperature exceeds 300℃, its strength and stiffness begin to deteriorate significantly. If a part or the whole loses its load-bearing capacity due to overheating, it can easily cause a catastrophic and continuous collapse of the bridge structure, with unimaginable consequences.

[0003] Faced with this significant safety challenge, the current protective measures relied upon by the bridge engineering community still have significant limitations. The mainstream approach is passive fire prevention, such as wrapping the bridge cables with fire-retardant coatings or tapes, which essentially delays heat transfer through insulation materials. However, this method is a "passive defense" that cannot actively remove heat and will eventually be penetrated by heat under prolonged or high-intensity fires; moreover, these materials are prone to aging and detachment under long-term outdoor weathering, making it difficult to guarantee durability and reliability. In recent years, some active fire prevention technologies have been proposed, such as jet-based fire suppression systems. While these systems can actively intervene, they are highly dependent on external power, complex detection and control systems, and a stable water source. In extreme cases where a fire itself may cause a power outage, the reliability of these systems is questionable, and they also suffer from high maintenance costs and significant environmental impact.

[0004] Therefore, it is necessary to propose a design method for a passive cable cooling system to solve or at least alleviate the above-mentioned defects. Summary of the Invention

[0005] The main objective of this invention is to provide a design method for a passive cooling system for cables, in order to solve the technical problems of existing passive fire protection for bridge cables, which cannot actively remove heat, and active fire protection, which is highly dependent on external power, complex detection and control systems, and stable water sources.

[0006] To achieve the above objectives, the present invention provides a design method for a passive cable cooling system, comprising the following steps:

[0007] S1, obtain the fire heat flux density acting on the outer surface of the cable protection layer; wherein, the cable protection layer includes a sheath layer, a phase change material layer and a heat exchange pipeline layer arranged sequentially from the outside to the inside;

[0008] S2, using the fire heat flux density as the heat input reference, determine the thermal configuration design parameters of the phase change material layer; wherein, the phase change material layer has a preset phase change temperature and is configured to undergo a phase change when the temperature reaches the preset phase change temperature and the thermal conductivity increases from a first thermal conductivity to a second thermal conductivity.

[0009] S3, using the fire heat flux density as the heat dissipation requirement benchmark, determine the heat dissipation configuration design parameters of the thermosiphon cycle; wherein, the thermosiphon cycle is realized through the heat exchange pipeline layer filled with working medium, and is configured to start after the thermal conductivity of the phase change material layer increases to the second thermal conductivity, so as to transfer heat to the condensation section.

[0010] S4. Based on the thermal configuration design parameters and the heat dissipation configuration design parameters, combined with the fire heat flux density, design buffer time, and critical failure temperature of the cable material, a comprehensive thermal verification is performed to determine the first size design parameters related to the phase change material layer and the second size design parameters related to the condensation section.

[0011] S5, take the first dimension design parameters and the second dimension design parameters as the system design result.

[0012] Preferably, step S2 includes the following steps:

[0013] S21, using the fire heat flux density as the heat input benchmark, calculate the theoretical lower limit of the heat that the phase change material layer needs to absorb within the design buffer time;

[0014] S22, Calculate the maximum heat that the phase change material layer can absorb from the initial temperature to the completion of the phase change;

[0015] S23, determine the minimum mass required for the phase change material layer based on the theoretical heat lower limit and the maximum heat, and use the minimum mass as the design mass;

[0016] S24, determine the design thickness of the phase change material layer based on the design quality;

[0017] S25, the design quality and the design thickness are used as thermal configuration design parameters of the phase change material layer; wherein, the phase change material layer has a preset phase change temperature and is configured to undergo a phase change when the temperature reaches the preset phase change temperature and the thermal conductivity increases from a first thermal conductivity to a second thermal conductivity.

[0018] Preferably, step S3 includes the following steps:

[0019] S31, using the fire heat flux density as the heat dissipation requirement benchmark, and based on the principle of heat balance, determine the minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section in steady-state working mode;

[0020] S32, based on the minimum heat dissipation, the condensation temperature of the working fluid in the condensing section, the ambient air temperature, and the overall heat transfer coefficient of the condensing section, the heat dissipation configuration design parameters are determined through condensation heat dissipation calculations; wherein, the heat dissipation configuration design parameters are specifically the minimum effective heat dissipation area required by the condensing section;

[0021] The thermosiphon cycle is achieved through the heat exchange pipeline layer filled with the working medium, and is configured such that after the thermal conductivity of the phase change material layer increases to the second thermal conductivity, the thermosiphon cycle is activated to transfer heat to the condensation section and establish a stable cycle. The condensation section is installed higher than the heat exchange pipeline layer, which serves as the evaporation section, so that the condensate of the working medium can flow back by gravity.

[0022] Preferably, step S22 includes the following steps:

[0023] Using formula The maximum heat absorbed by the phase change material layer from its initial temperature to the point where the phase change is complete was calculated. ;in, For the quality of the phase change material layer, This represents the specific heat capacity when the phase change material layer is in a solid state. This refers to the specific heat capacity of the phase change material layer when it is in a liquid state. The phase transition temperature of the phase change material layer. The initial temperature, The latent heat of phase change of the phase change material layer, This is the highest temperature that the phase change material layer is allowed to reach.

[0024] Preferably, step S23 includes the following steps:

[0025] According to the maximum heat When the theoretical lower limit of heat is equal to the given value, the minimum mass required for the phase change material layer is calculated. And the minimum mass is taken as the design mass.

[0026] Preferably, step S24 includes the following steps:

[0027] According to the minimum mass and the average density of the phase change material layer Obtain the phase change material layer volume ;

[0028] Based on the volume of the phase change material layer and use the formula The design thickness of the phase change material layer was calculated. ;in, The diameter of the cable. This refers to the length of the cable exposed to fire.

[0029] Preferably, determining the minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section in steady-state operating mode in step S31 includes the following steps:

[0030] Using formula The minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section under steady-state operating mode was calculated. ;in, The fire heat flux density, This represents the surface area of ​​the cable exposed to fire.

[0031] Preferably, determining the heat dissipation configuration design parameters in step S32 includes the following steps:

[0032] Using formula The minimum effective heat dissipation area required for the condensation section is calculated. and the minimum effective heat dissipation area As the design parameters for the aforementioned heat dissipation configuration; wherein... The overall heat transfer coefficient of the condensation section is... This refers to the condensation temperature of the working fluid in the condensation section. The ambient air temperature.

[0033] Preferably, step S4 includes the following steps:

[0034] S41, Establish a coupled system heat transfer model including the phase change material layer and the thermosiphon cycle; use the thermal configuration design parameters, the heat dissipation configuration design parameters, and the fire heat flux density as inputs to the coupled system heat transfer model, and use the design buffer time and the critical failure temperature of the cable material as verification criteria for the coupled system heat transfer model.

[0035] S42, Based on the heat transfer model of the coupling system, dynamically simulate the temperature rise process of the cable during the design buffer time to obtain the simulated temperature time history curve of the cable.

[0036] S43, according to the verification criteria, determine whether the peak value of the simulated temperature time history curve is less than the critical failure temperature of the cable material;

[0037] S441, when the peak value is less than the critical failure temperature of the cable material, it is determined that the thermal configuration design parameters and the heat dissipation configuration design parameters meet the design requirements, and the current thermal configuration design parameters are determined to be the first size design parameters, and the current heat dissipation configuration design parameters are determined to be the second size design parameters.

[0038] S442, when the peak value is greater than or equal to the critical failure temperature of the cable material, adjust the current thermal configuration design parameters or the current heat dissipation configuration design parameters, and repeat steps S42 and S43 until the simulated temperature time history curve is lower than the critical failure temperature of the cable material throughout the entire fire protection duration.

[0039] Preferably, the ratio of the second thermal conductivity to the first thermal conductivity is not less than 3.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] This invention provides a design method for a passive cooling system for bridge cables. This application can achieve active and efficient cooling of bridge cables without relying on external energy or control. It can self-sense and self-start when a bridge fire occurs, and continuously provide protection for the cables using its own physical mechanisms, thereby bridging the technological gap between passive delay and complex active cooling.

[0042] Specifically, by obtaining the fire heat flux density, the complex and uncertain fire is transformed into a clear and quantifiable engineering design input parameter, providing a benchmark and source for subsequent quantitative calculations, replacing the crude approach of relying on experience-based estimation in existing technologies. Using the fire heat flux density as a benchmark, the thermal configuration parameters of the phase change material layer (such as design mass and design thickness) are determined. These thermal configuration design parameters ensure that the phase change material layer has sufficient heat capacity to absorb the initial fire heat, providing the system with startup time. Simultaneously, the step-like increase in thermal conductivity is designed as a physical switch to trigger the next stage of the heat dissipation cycle, creating conditions for active heat dissipation. By determining the heat dissipation configuration design parameters for the thermosiphon cycle, the heat dissipation system is entirely based on the passive physical principle of thermosiphon. Its operation does not rely on any external power, but is driven solely by the density difference generated by the phase change of the working fluid and the effect of gravity. It does not require external electricity, water sources, or control, achieving truly passive and continuous heat dissipation. This application verifies and optimizes the paper parameters obtained in the preceding steps through dynamic system simulation, ensuring that the finally determined dimensional parameters are the verified optimal cooperative solution. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0044] Figure 1 This is a schematic diagram of a process in one embodiment of the present invention;

[0045] Figure 2 This is a schematic diagram of the overall system deployment in one embodiment of the present invention;

[0046] Figure 3 This is a cross-sectional schematic diagram of the phase change material layer and the heat exchange pipeline layer in one embodiment of the present invention;

[0047] Figure 4 This is an application scenario diagram of the intermediate pipeline in one embodiment of the present invention.

[0048] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings.

[0049] 10. Cable; 20. Sheath layer; 30. Phase change material layer; 40. Heat exchange pipeline layer; 50. Condensation section; 60. Intermediate pipeline. Detailed Implementation

[0050] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0052] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0053] Furthermore, the use of terms such as "first" and "second" in this invention is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. If the combination of technical solutions is contradictory or impossible to implement, such a combination should be considered non-existent and not within the scope of protection claimed by this invention.

[0054] Please see the appendix Figures 1 to 4 The present invention provides a design method for a passive cable cooling system, comprising the following steps:

[0055] S1, Obtain the fire heat flux density acting on the outer surface of the cable protection layer; wherein, the cable 10 protection layer includes a sheath layer 20, a phase change material layer 30, and a heat exchange pipeline layer 40 arranged sequentially from the outside to the inside; First, determine the target fire scenario that the cable 10 needs to protect against, such as a hydrocarbon fuel (e.g., oil) fire that may occur on a bridge. To simplify the design, a typical heat flux density value of 50-100 kW / m² under a standard fire curve (e.g., an HC hydrocarbon curve) is usually used as the design benchmark. As a specific example, a fire heat flux density of 75 kW / m² is used as the design benchmark. This step transforms the complex and uncertain fire into clear and quantifiable engineering design input parameters, providing a benchmark and source for subsequent quantitative calculations, replacing the extensive approach of relying on experience-based estimation in existing technologies.

[0056] S2, using the fire heat flux density as the heat input benchmark, determine the thermal configuration design parameters of the phase change material layer 30; wherein, the phase change material layer 30 has a preset phase change temperature and is configured to undergo a phase change when the temperature reaches the preset phase change temperature, and the thermal conductivity increases from a first thermal conductivity to a second thermal conductivity; using the fire heat flux density obtained in step S1 as input, combined with the design buffer time (for example, set to 20 minutes, i.e., 1200 seconds, based on the time required for safe evacuation or initial rescue), the thermal configuration design parameters of the phase change material layer 30 can be determined by thermal calculation based on the principle of energy conservation. The preferred thermal configuration design parameters are the mass and thickness of the phase change material layer 30, ensuring that the phase change material layer 30 has sufficient heat capacity to absorb the initial fire heat, thus gaining startup time for the system; at the same time, the step increase in thermal conductivity (from the first thermal conductivity to the second thermal conductivity) is designed as a physical switch to trigger the next stage of heat dissipation cycle, creating conditions for active heat dissipation.

[0057] It is worth noting that the phase change material layer 30 has two main functions: first, it buffers and absorbs heat, absorbing a large amount of heat before the thermosiphon cycle is fully established, thus delaying the temperature rise of the cable 10; second, it triggers temperature control, as its phase change process itself is a constant-temperature heat dissipation mechanism.

[0058] S3, using the fire heat flux density as the heat dissipation requirement benchmark, determine the heat dissipation configuration design parameters for the thermosiphon cycle; wherein, the thermosiphon cycle is implemented through the heat exchange pipe layer 40 filled with working medium, and is configured to start after the thermal conductivity of the phase change material layer 30 increases to the second thermal conductivity, so as to transfer heat to the condensation section 50; this step matches the fire heat flux density to the physical dimensions (such as minimum effective heat dissipation area) that the condensation section 50 must possess. The thermosiphon cycle consists of the following parts: the heat exchange pipe layer 40 (wound around the cable 10) as the evaporation section, the independent condensation section 50 (located in a safe area, such as the top of a bridge pier) as the heat dissipation terminal, and the intermediate pipe 60 connecting the two. Its design goal is that after the thermosiphon cycle is started, its heat dissipation capacity is sufficient to continuously and stably resist the heat input of the fire and avoid heat accumulation in the system. The heat dissipation system designed in this step is based entirely on the passive physical principle of thermosiphon. Its operation does not depend on any external power, but is driven solely by the heat from the fire. It does not require external electricity, water supply, or control, thus achieving true passive continuous heat dissipation.

[0059] S4. Based on the thermal configuration design parameters and the heat dissipation configuration design parameters, combined with the fire heat flux density, design buffer time, and critical failure temperature of cable 10 material, a comprehensive thermal verification is performed to determine the first size design parameters related to the phase change material layer 30 and the second size design parameters related to the condensation section 50. This step can verify and optimize the paper parameters obtained in the previous steps through dynamic system simulation to ensure that the finally determined size parameters are the verified optimal cooperative solution.

[0060] S5, take the first dimension design parameters and the second dimension design parameters as the system design result.

[0061] It is also worth noting that this application achieves complete passivity and automatic triggering of the system through the organic combination of phase change material temperature control triggering and thermosiphon self-circulation. For example... Figure 2 As shown, the core of the system consists of three parts: a sheath layer 20 as the outermost layer (e.g., a weather-resistant aluminum alloy perforated sheath), a phase change material layer 30 as the middle layer, a heat exchange pipeline layer 40 as the inner layer, and a condensation section 50 that forms a thermosiphon cycle with the heat exchange pipeline layer 40.

[0062] The system operates based on the following physical principles:

[0063] Phase change material layer 30 triggering mechanism: The phase change material is solid at room temperature and has low thermal conductivity; when the temperature reaches the phase change point, the material undergoes a phase change and absorbs a large amount of latent heat, while its thermal conductivity increases significantly, making it a good heat conductor.

[0064] Thermosiphon self-circulation mechanism: The working fluid in the heat exchange pipeline absorbs heat and evaporates in the heating section, and the density decreases, generating buoyancy; the steam rises to the condensing section 50, releases heat and condenses, and the density increases. Under the action of gravity, it flows back, forming a continuous cycle.

[0065] Heat transfer path: fire heat → weather-resistant aluminum alloy perforated sleeve → phase change material layer 30 → heat exchange pipeline layer as evaporation section → working medium (referring to the working medium used to carry and transfer heat in the thermodynamic cycle) evaporation → steam rises → condensation section 50 releases heat → condensate returns.

[0066] The system consists of:

[0067] Outer layer: Weather-resistant perforated aluminum alloy sheath, providing mechanical protection while allowing heat transfer;

[0068] Phase change material layer 30: Wrapped around the outside of cable 10, the thickness of which is determined by thermal calculation. It uses microencapsulated hydrated salt phase change material with a phase change temperature of 50-90℃ and a latent heat of not less than 150kJ / kg.

[0069] Heat exchange pipeline layer 40: preferably made of copper coil, tightly wound around the surface of cable 10, filled with low boiling point working fluid (ammonia, propane or water), with a filling rate of 60-70%.

[0070] Condensing section 50: Located inside the bridge tower or other safe area, it uses finned tubes to enhance heat dissipation and is positioned higher than the heat exchange piping layer that serves as the evaporation section, so as to utilize gravity return.

[0071] The design objective of this application is to ensure that, under specific fire scenarios (characterized by fire heat flux density), the temperature of cable 10 (especially the internal high-strength steel wires) remains below its critical failure temperature (typically 300-400℃) within the specified design buffer time. This application transforms the complex and uncertain nature of fire into a clear and quantifiable engineering design input parameter by obtaining the fire heat flux density, providing a benchmark and source for subsequent quantitative calculations, replacing the crude approach of relying on experience-based estimation in existing technologies. Using the fire heat flux density as a benchmark, the thermal configuration parameters (such as design mass and design thickness) of the phase change material layer 30 are determined. These thermal configuration design parameters ensure that the phase change material layer 30 has sufficient heat capacity to absorb the initial fire heat, providing the system with startup time. Simultaneously, the step-by-step increase in thermal conductivity (from the first thermal conductivity to the second thermal conductivity) is designed as a physical switch to trigger the next stage of the heat dissipation cycle, creating conditions for active heat dissipation. By determining the heat dissipation configuration design parameters for the thermosiphon cycle, the heat dissipation system is entirely based on the passive physical principle of thermosiphon. Its operation does not rely on any external power, but is driven solely by the density difference generated by the phase change of the working fluid and the effect of gravity. It does not require external electricity, water sources, or control, achieving truly passive and continuous heat dissipation. This application verifies and optimizes the paper parameters obtained in the preceding steps through dynamic system simulation, ensuring that the finally determined dimensional parameters are the verified optimal cooperative solution.

[0072] In a preferred embodiment, step S2 includes the following steps:

[0073] S21, using the fire heat flux density as the heat input benchmark, calculate the theoretical lower limit of the heat that the phase change material layer 30 needs to absorb within the design buffer time; wherein, the theoretical lower limit of heat refers to the minimum amount of heat that the phase change material layer 30 must absorb within the design buffer time to prevent the cable 10 temperature from rising too quickly. The calculation formula is: ;in, This is the lower limit of theoretical heat, in J. Fire heat flux density, unit ; For the fire-exposed surface area of ​​cable 10, unit ; The unit for the design buffer time is seconds (s).

[0074] S22, calculate the maximum heat that the phase change material layer 30 can absorb from the initial temperature to the completion of the phase change; it should be noted that the total heat that the phase change material layer 30 needs to absorb consists of two parts: one is sensible heat: heating from the initial temperature to the phase change temperature; the other is latent heat: the phase change that occurs at the phase change temperature.

[0075] Further, step S22 includes the following steps:

[0076] Using formula The maximum heat absorbed by the phase change material layer 30 from its initial temperature rise to the point of complete phase change was calculated. The unit is J; among which, The mass of phase change material layer 30 is expressed in kg. 30 represents the specific heat capacity of the phase change material layer when it is in a solid state, in J / (kg·K); 30 represents the specific heat capacity of the phase change material layer when it is in a liquid state, in J / (kg·K); The phase transition temperature of phase change material layer 30 is expressed in Kelvin (K), for example, 58°C, and is a material property. The initial temperature is in K, for example, 35°C in a high-temperature summer environment, which is an engineering design value based on climatic conditions; The latent heat of phase change of phase change material layer 30 is expressed in J / kg, for example, 250 kJ / kg, which is a known key energy storage parameter of the material. The maximum allowable temperature for the phase change material layer 30 is expressed in Kelvin (K).

[0077] S23, determine the minimum mass required for the phase change material layer 30 based on the theoretical heat lower limit and the maximum heat, and use the minimum mass as the design mass;

[0078] Further, step S23 includes the following steps:

[0079] According to the maximum heat When the theoretical lower limit of heat is equal to the given value, the minimum mass required for the phase change material layer 30 is calculated. And the minimum mass is taken as the design mass.

[0080] Minimum mass The calculation expression is:

[0081] ;

[0082] S24, determine the design thickness of the phase change material layer 30 based on the design quality;

[0083] Further, step S24 includes the following steps:

[0084] According to the minimum mass and the average density of phase change material layer 30 30 cubic meters of phase change material layer were obtained Specifically, according to 30 cubic meters of phase change material layer were obtained ;

[0085] According to the volume of the phase change material layer 30 and use the formula The design thickness of the phase change material layer 30 was calculated. ;in, For a cable diameter of 10, The length of the cable exposed to fire is 10.

[0086] Specifically, By combining the two formulas, we can obtain the result. .

[0087] S25, the design quality and the design thickness are used as thermal configuration design parameters of the phase change material layer 30; wherein, the phase change material layer 30 has a preset phase change temperature and is configured to undergo a phase change when the temperature reaches the preset phase change temperature and the thermal conductivity increases from a first thermal conductivity to a second thermal conductivity.

[0088] In this embodiment, the design mass is the minimum required mass calculated through thermal balance based on the fire heat flux density and the design buffer time, ensuring how much phase change material is needed to absorb all the fire heat transmitted within the predetermined time; the design thickness is the equivalent uniform coating thickness converted from the calculated mass, combined with the material density and the surface area of ​​the cable 10, which can provide direct reference for engineers and construction parties.

[0089] In a preferred embodiment, step S3 includes the following steps:

[0090] S31, using the fire heat flux density as the heat dissipation requirement benchmark, and based on the principle of heat balance, determine the minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensing section 50 in steady-state operation mode; when the thermosiphon cycle is fully established and reaches steady-state operation mode, the heat absorbed by the system from the fire is equal to the heat dissipated to the environment, otherwise energy will continue to accumulate. The minimum heat dissipation is the heat that the condensing section 50 must continuously transfer away in steady-state operation mode to balance the fire heat input.

[0091] Furthermore, determining the minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section 50 in steady-state operating mode in step S31 includes the following steps:

[0092] Using formula The minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section 50 under steady-state operating mode was calculated. The unit is W; where, The fire heat flux density is expressed in units of... ; For the fire-exposed surface area of ​​cable 10, unit .

[0093] S32, based on the minimum heat dissipation, the condensation temperature of the working fluid in the condensing section 50, the ambient air temperature, and the overall heat transfer coefficient of the condensing section 50, the heat dissipation configuration design parameters are determined through condensation heat dissipation calculations; wherein, the heat dissipation configuration design parameters are specifically the minimum effective heat dissipation area required by the condensing section 50.

[0094] The thermosiphon cycle is achieved through the heat exchange pipeline layer 40 filled with the working medium. It is configured such that after the thermal conductivity of the phase change material layer 30 increases to the second thermal conductivity, the thermosiphon cycle is activated to transfer heat to the condensation section 50 and establish a stable cycle. The condensation section 50 is installed higher than the heat exchange pipeline layer, which serves as the evaporation section, so that the condensate of the working medium can flow back by gravity.

[0095] Furthermore, determining the heat dissipation configuration design parameters in step S32 includes the following steps:

[0096] Using formula The minimum effective heat dissipation area required for the condensation section 50 is calculated. ,unit ; and the minimum effective heat dissipation area As the design parameters for the aforementioned heat dissipation configuration; wherein... The overall heat transfer coefficient for the condenser section is 50, in units of... , The condensation temperature of the working fluid in the condensation section is 50 K. The ambient air temperature is expressed in Kelvin (K). The worst-case scenario can be taken, such as a high temperature of 35°C in summer.

[0097] The function of the condenser section 50 is to safely release the heat removed from the fire zone by the thermosiphon cycle into the environment. Its design must ensure that the system's heat dissipation capacity is greater than or equal to the input heat throughout the entire fire.

[0098] As a preferred implementation method The process of obtaining it is as follows:

[0099] The condenser section 50 dissipates heat through forced convection and radiation. Its total heat dissipation can be expressed as:

[0100] ;

[0101] in: The total heat dissipation area of ​​the condensation section is 50 mm. ;

[0102] Overall heat transfer coefficient The calculation can be obtained by taking values ​​based on experience or by following a preferred implementation method:

[0103] Total thermal resistance of condensing section 50 include:

[0104] (1) Thermal resistance of working fluid condensation heat transfer ;unit ;

[0105] (2) Thermal resistance of pipe wall ;unit ;

[0106] (3) Fin efficiency correction ; Dimensionless;

[0107] (4) External air convection heat transfer thermal resistance, unit ;

[0108] ;

[0109] Condensation heat transfer coefficient of working fluid (For ammonia in a horizontal pipe): Unit ;

[0110] ;in, ;in, For gravitational acceleration, in units , Liquid density, in units ; This is the density of the gas phase, in units of... ; Liquid phase thermal conductivity, in units ; For latent heat of vaporization, unit ; The dynamic viscosity of the liquid phase is expressed in units of... ; The inner diameter of the pipe is in meters (m). Temperature of the inner wall of the heat exchange tube, in K; For the corrected latent heat of vaporization, unit ; The total surface area of ​​the heat exchange tubes in the condensing section, in units of .

[0111] Forced convection heat transfer coefficient on the air side:

[0112] For air sweeping across finned tube bundles, there is an empirical formula:

[0113] ; These are Nusselt numbers, dimensionless; The Reynolds number is dimensionless. It is the Prandtl number, which is dimensionless;

[0114] ; The outer diameter of the pipe; The thermal conductivity of air, in units of ; The air convection heat transfer coefficient, in units of ;

[0115] Where C and m are constants related to the tube arrangement (which can be selected by referring to the constant table of the Zukauskas formula). In this embodiment, a staggered arrangement with high heat transfer efficiency is adopted, that is, the rear tubes are located directly behind the gap between the front tubes, so that the flow direction of the cooling air changes continuously as it flows through, thereby enhancing the flow field disturbance, destroying the thermal boundary layer of the tube wall, and significantly improving the convective heat transfer coefficient on the air side. According to the Zukauskas correlation, in the Reynolds number range of 1000 < Within 2×10^5, take C = 0.35 and m = 0.60.

[0116] Fin efficiency :

[0117]

[0118] ;

[0119] in, For the corrected length of the fins, . Thermal conductivity of fin material, in units ; The thickness is the fin thickness, in meters (m).

[0120] Depend on The minimum effective heat dissipation area required can be solved:

[0121] .

[0122] In a preferred embodiment, step S4 includes the following steps:

[0123] S41, establish a coupled system heat transfer model including the phase change material layer 30 and the thermosiphon cycle; use the thermal configuration design parameters, the heat dissipation configuration design parameters, and the fire heat flux density as inputs to the coupled system heat transfer model, and use the design buffer time and the critical failure temperature of the cable 10 material as verification criteria for the coupled system heat transfer model.

[0124] Specifically, computational heat transfer and computational fluid dynamics principles can be used to establish a transient heat transfer numerical model as the heat transfer model of the coupled system in a simulation software platform (such as ANSYS Fluent). In this model, the phase change material layer 30 is defined as a region with temperature-dependent thermophysical properties, and its thermal conductivity is set as a piecewise function: when the temperature is below the phase change temperature, the thermal conductivity of the phase change material layer 30 is the first thermal conductivity; when the temperature is greater than or equal to the phase change temperature, the thermal conductivity of the phase change material layer 30 is the second thermal conductivity.

[0125] The outer boundary of the coupled system heat transfer model is subject to a time-varying heat flux boundary with the fire heat flux density as input. The initial conditions of the model are set to a uniform initial temperature (e.g., ambient temperature 35°C).

[0126] This step transforms the physical system, which includes phase change triggering and two-phase flow circulation, into a mathematical model that can be numerically solved on a computer. This coupled system heat transfer model can simulate the complete dynamic heat transfer process from the external fire site → sheath layer 20 → phase change material layer 30 → heat exchange pipeline layer 40 (as evaporation section) → condensation section 50 → external environment, and outputs the time history curve of temperature change over time at each point on cable 10.

[0127] S42, based on the heat transfer model of the coupled system, dynamically simulate the temperature rise process of the cable 10 during the design buffer time to obtain the simulated temperature time history curve of the cable 10; through numerical calculation, run the heat transfer model of the coupled system to output how the temperature of the cable 10 evolves over time under the above design parameters and fire conditions. For example, the simulation results may show that: within 0-800 seconds, the temperature of the cable 10 slowly rises to about 200°C (the phase change material layer 30 is in the buffer heat absorption stage); around 800-1000 seconds, due to the phase change of the phase change material layer 30 triggering a thermosiphon cycle, the temperature rise rate slows down significantly; at 1200 seconds, the temperature of the cable 10 reaches a peak of about 280°C.

[0128] S43, according to the verification criteria, determine whether the peak value of the simulated temperature time history curve is less than the critical failure temperature of the cable 10 material; extract the peak value of the simulated temperature time history curve obtained by simulation (such as 280°C in the above example) and compare it with the critical failure temperature of the cable 10 material (such as 300°C).

[0129] S441, when the peak value is less than the critical failure temperature of the cable 10 material, it is determined that the thermal configuration design parameters and the heat dissipation configuration design parameters meet the design requirements, and the current thermal configuration design parameters are determined as the first size design parameters and the current heat dissipation configuration design parameters are determined as the second size design parameters. When the peak value is less than the critical failure temperature of the cable 10 material, for example, 280°C < 300°C, it is determined that the currently input thermal configuration design parameters are safe and may have room for optimization. Accordingly, these parameters can be determined as the final first size design parameters and second size design parameters. Alternatively, for further design optimization, in another embodiment, the currently input thermal configuration design parameters or the heat dissipation configuration design parameters can be further adjusted, and steps S42 and S43 can be repeated until the simulated temperature time history curve is lower than the critical failure temperature of the cable material throughout the entire fire protection duration. Then, the corresponding thermal configuration design parameters are used as the first size design parameters, and the heat dissipation configuration design parameters are used as the second size design parameters.

[0130] S442, when the peak value is greater than or equal to the critical failure temperature of the cable 10 material, adjust the current thermal configuration design parameters or the current heat dissipation configuration design parameters, and repeat steps S42 and S43 until the simulated temperature time history curve is lower than the critical failure temperature of the cable material throughout the entire fire protection duration. If the peak value is greater than or equal to the critical failure temperature of the cable 10 material (for example, the peak value is 320°C, which is greater than 300°C), it indicates that the current design does not meet the safety criteria. In this case, the input parameters need to be adjusted. Further, if the simulation results show that the temperature rise is too rapid: it may be that the buffering capacity of the phase change material layer 30 is insufficient, and the design thickness in the thermal configuration design parameters can be increased (e.g., from 203mm to 250mm). If the simulation shows that the temperature rise cannot be suppressed in the later stage: it may be that the heat dissipation capacity is insufficient, and the minimum effective heat dissipation area in the heat dissipation configuration design parameters can be increased.

[0131] This embodiment addresses the problem that the thickness of traditional passive fireproof layers is often determined empirically, making it impossible to predict their failure time. Through coupled dynamic simulation, it is possible to accurately predict when the temperature of cable 10 will reach a dangerous value under a specific fire condition during the design phase. Through the closed-loop iterative process in step S4, an optimal combination of parameters that meets safety performance requirements while avoiding over-design (optimizing material usage) is found. Simultaneously, through the iteration in step S4, the optimal balance between the timing of thermosiphon cycle triggering and heat dissipation capacity can be found, which has important guiding significance for practical engineering.

[0132] As a preferred example, the ratio of the second thermal conductivity to the first thermal conductivity is not less than 3. In the early stages of a fire, the phase change material layer 30, which has a lower thermal conductivity (first thermal conductivity), can effectively delay the internal transfer of heat, providing the system with buffer time. When the temperature reaches the phase change point and the thermal conductivity jumps to the second thermal conductivity, the phase change material layer 30 quickly transforms from a heat insulator into a highly efficient heat conductor, allowing heat to be transferred at a high rate to the heat exchange pipeline layer 40, which serves as the evaporation section. This provides sufficient heat flow to drive the rapid evaporation of the internal working fluid and the reliable start-up of the thermosiphon cycle. The ratio of the second thermal conductivity to the first thermal conductivity can be set by those skilled in the art according to actual needs.

[0133] To further illustrate the technical solution of this application, the following engineering examples are also provided:

[0134] Given conditions:

[0135] Cable 10 has a diameter D = 0.2m and a fire-exposed length L = 5m.

[0136] Design Fire: Hydrocarbon Curve, Net Heat Flow = 75 ;

[0137] Design buffer time = 20 minutes = 1200 seconds;

[0138] Phase change material layer 30: Sodium acetate trihydrate; latent heat of phase change of phase change material layer 30. = 250 kJ / kg, phase transition temperature of phase change material layer 30 = 58°C, average density of phase change material layer 30 = 1450 kg / m 3 ;

[0139] Ambient air temperature 35°C;

[0140] Based on the above calculations, the design mass is 1132 kg, the design thickness is 95 mm, and the minimum effective heat dissipation area is 269 m². 2 .

[0141] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A design method for a passive cooling system for cables, characterized in that, Includes the following steps: S1, obtain the fire heat flux density acting on the outer surface of the cable protection layer; wherein, the cable protection layer includes a sheath layer, a phase change material layer and a heat exchange pipeline layer arranged sequentially from the outside to the inside; S2, using the fire heat flux density as the heat input reference, determine the thermal configuration design parameters of the phase change material layer; wherein, the phase change material layer has a preset phase change temperature and is configured to undergo a phase change when the temperature reaches the preset phase change temperature and the thermal conductivity increases from a first thermal conductivity to a second thermal conductivity. S3, using the fire heat flux density as the heat dissipation requirement benchmark, determine the heat dissipation configuration design parameters of the thermosiphon cycle; wherein, the thermosiphon cycle is realized through the heat exchange pipeline layer filled with working medium, and is configured to start after the thermal conductivity of the phase change material layer increases to the second thermal conductivity, so as to transfer heat to the condensation section. S4. Based on the thermal configuration design parameters and the heat dissipation configuration design parameters, combined with the fire heat flux density, design buffer time, and critical failure temperature of the cable material, a comprehensive thermal verification is performed to determine the first size design parameters related to the phase change material layer and the second size design parameters related to the condensation section. S5, take the first dimension design parameters and the second dimension design parameters as the system design result.

2. The design method for a passive cable cooling system according to claim 1, characterized in that, Step S2 includes the following steps: S21, using the fire heat flux density as the heat input benchmark, calculate the theoretical lower limit of the heat that the phase change material layer needs to absorb within the design buffer time; S22, Calculate the maximum heat that the phase change material layer can absorb from the initial temperature to the completion of the phase change; S23, determine the minimum mass required for the phase change material layer based on the theoretical lower limit of heat and the maximum heat, and use the minimum mass as the design mass; S24, determine the design thickness of the phase change material layer based on the design quality; S25, the design quality and the design thickness are used as thermal configuration design parameters of the phase change material layer; wherein, the phase change material layer has a preset phase change temperature and is configured to undergo a phase change when the temperature reaches the preset phase change temperature and the thermal conductivity increases from a first thermal conductivity to a second thermal conductivity.

3. The design method for a passive cable cooling system according to claim 2, characterized in that, Step S3 includes the following steps: S31, using the fire heat flux density as the heat dissipation requirement benchmark, and based on the principle of heat balance, determine the minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section in steady-state working mode; S32, based on the minimum heat dissipation, the condensation temperature of the working fluid in the condensing section, the ambient air temperature, and the overall heat transfer coefficient of the condensing section, the heat dissipation configuration design parameters are determined through condensation heat dissipation calculations; wherein, the heat dissipation configuration design parameters are specifically the minimum effective heat dissipation area required by the condensing section; The thermosiphon cycle is achieved through the heat exchange pipeline layer filled with the working medium, and is configured such that after the thermal conductivity of the phase change material layer increases to the second thermal conductivity, the thermosiphon cycle is activated to transfer heat to the condensation section and establish a stable cycle. The condensation section is installed higher than the heat exchange pipeline layer, which serves as the evaporation section, so that the condensate of the working medium can flow back by gravity.

4. The design method for a passive cable cooling system according to claim 2, characterized in that, Step S22 includes the following steps: Using formula The maximum heat absorbed by the phase change material layer from its initial temperature to the point where the phase change is complete was calculated. ;in, For the quality of the phase change material layer, This represents the specific heat capacity when the phase change material layer is in a solid state. This refers to the specific heat capacity of the phase change material layer when it is in a liquid state. The phase transition temperature of the phase change material layer. The initial temperature, The latent heat of phase change of the phase change material layer, This is the highest temperature that the phase change material layer is allowed to reach.

5. The design method for a passive cable cooling system according to claim 4, characterized in that, Step S23 includes the following steps: According to the maximum heat When the theoretical lower limit of heat is equal to the given value, the minimum mass required for the phase change material layer is calculated. The minimum mass is taken as the design mass.

6. The design method for a passive cable cooling system according to claim 5, characterized in that, Step S24 includes the following steps: According to the minimum mass and the average density of the phase change material layer Obtain the phase change material layer volume ; Based on the volume of the phase change material layer and use the formula The design thickness of the phase change material layer was calculated. ;in, The diameter of the cable. This refers to the length of the cable exposed to fire.

7. The design method for a passive cable cooling system according to claim 3, characterized in that, Determining the minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section in steady-state operating mode in step S31 includes the following steps: Using formula The minimum heat dissipation that the thermosiphon cycle needs to continuously transfer to the condensation section under steady-state operating mode was calculated. ;in, The fire heat flux density, This represents the surface area of ​​the cable exposed to fire.

8. The design method for a passive cable cooling system according to claim 7, characterized in that, Determining the heat dissipation configuration design parameters in step S32 includes the following steps: Using formula The minimum effective heat dissipation area required for the condensation section is calculated. and the minimum effective heat dissipation area As the design parameters for the aforementioned heat dissipation configuration; wherein... The overall heat transfer coefficient of the condensation section is... This refers to the condensation temperature of the working fluid in the condensation section. The ambient air temperature.

9. The design method for a passive cable cooling system according to claim 3, characterized in that, Step S4 includes the following steps: S41, Establish a coupled system heat transfer model including the phase change material layer and the thermosiphon cycle; use the thermal configuration design parameters, the heat dissipation configuration design parameters, and the fire heat flux density as inputs to the coupled system heat transfer model, and use the design buffer time and the critical failure temperature of the cable material as verification criteria for the coupled system heat transfer model. S42, Based on the heat transfer model of the coupled system, dynamically simulate the temperature rise process of the cable during the design buffer time to obtain the simulated temperature time history curve of the cable. S43, according to the verification criteria, determine whether the peak value of the simulated temperature time history curve is less than the critical failure temperature of the cable material; S441, when the peak value is less than the critical failure temperature of the cable material, it is determined that the thermal configuration design parameters and the heat dissipation configuration design parameters meet the design requirements, and the current thermal configuration design parameters are determined to be the first size design parameters, and the current heat dissipation configuration design parameters are determined to be the second size design parameters. S442, when the peak value is greater than or equal to the critical failure temperature of the cable material, adjust the current thermal configuration design parameters or the current heat dissipation configuration design parameters, and repeat steps S42 and S43 until the simulated temperature time history curve is lower than the critical failure temperature of the cable material throughout the entire fire protection duration.

10. The design method for a passive cable cooling system according to claim 1, characterized in that, The ratio of the second thermal conductivity to the first thermal conductivity is not less than 3.

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