Live wire fire spread ignition flame retardance RPUF ignition time prediction method and model construction method

Abstract

The invention discloses an ignition time prediction method and a model construction method for live wire fire spread ignition flame retardance RPUF, and belongs to the technical field of building material fire safety. The method comprises the following steps: measuring or collecting thermophysical parameters of the RPUF added with different flame retardants, establishing a comprehensive prediction model of each thermophysical parameter through a nonlinear regression analysis method, and solving based on the comprehensive prediction model to obtain a corresponding thermophysical parameter correction coefficient; constructing an ignition model of the RPUF, and correcting an ignition time tig calculation formula under different ignition models based on each thermophysical parameter correction coefficient obtained by solving; thermophysical parameters, wire ignition condition parameters and wire current of the flame-retardant RPUF are collected and input into a corresponding ignition time calculation formula, and therefore ignition time prediction is achieved. According to the method, the time window for igniting the RPUF material under the condition of the wire fault can be accurately predicted, and key time parameters are provided for building electrical fire prevention and control.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of building material fire safety, and specifically relates to a method for predicting the ignition time of flame spread of electrified wires igniting flame-retardant rigid polyurethane foam (RPUF) and a model construction method. BACKGROUND

[0002] With the continuous improvement of building energy-saving standards in China, rigid polyurethane foam (RPUF) as a kind of high-efficiency thermal insulation material is widely used in building external wall insulation systems due to its low thermal conductivity, high closed cell rate and other excellent properties. However, the flammable nature of this organic polymer material also poses a serious fire hazard. Statistics show that in recent years, about 35% of building external wall insulation material-induced fire accidents are related to electrical line faults, and the proportion of cases where electrified wires ignite insulation materials accounts for more than 60%.

[0003] In the prior art, significant progress has been made in the flame-retardant modification of RPUF, mainly through the addition of intumescent flame retardants (such as ammonium polyphosphate / pentaerythritol system), nanocomposites (such as layered double hydroxides) and other means to improve the flame-retardant grade of the material. Laboratory tests show that the limiting oxygen index (LOI) of RPUF after flame-retardant treatment can be improved to more than 26%, and the vertical burning test can reach UL94 V-0 level. However, these studies focus on the combustion performance testing of the material itself, using standard fire sources (such as cone calorimeters) for evaluation, and do not fully consider the mechanism of the electrified wire, a special ignition source, in actual fire scenarios.

[0004] It is worth noting that electrified wires, under overload or short-circuit conditions, will produce a complex ignition process: first, the wire temperature rises sharply due to the Joule heating effect, igniting the insulating layer; then, the molten insulating material drips and forms a continuous flame; finally, the flame spreads along the wire and radiates to ignite the adjacent RPUF insulation layer. This process involves the coupling of multiple physical fields, including electrical-thermal conversion, material pyrolysis, flame propagation, etc., and its ignition characteristics differ significantly from standard fire sources.

[0005] In addition, the behavior of flame-retardant RPUF in real fires is more complex: on the one hand, the flame retardant changes the pyrolysis path and combustion chemical reaction of the material; on the other hand, the instantaneous high temperature generated by the electrical fault may exceed the effective temperature range of the flame retardant. This complex interaction makes the traditional ignition time prediction method based on a single fire source have a large error.

[0006] Therefore, developing a method specifically for predicting the ignition time of flame spread of electrified wires igniting flame-retardant RPUF not only fills the gap in the prior art, but also provides a quantitative basis for building electrical fire risk assessment, which has important engineering application value for improving building fire prevention design specifications and optimizing flame-retardant material formulations. SUMMARY

[0007] In order to solve the problem of large error of the traditional single fire source-based ignition time prediction method, the application provides an ignition time prediction method and model construction method for electric wire fire spread igniting RPUF with flame retardance. The ignition time calculation model is established by comprehensively considering the electric wire overload heating, flame propagation characteristics and the self thermal physical properties of RPUF with different flame retardance, the ignition time prediction precision of the electric wire fire spread igniting RPUF with flame retardance can be effectively improved by using the model, and the model is especially suitable for building electrical fire risk assessment and flame retardant material fireproof performance optimization design.

[0008] In order to achieve the above-mentioned purpose, the technical scheme provided by the application is as follows: The application provides a model construction method for predicting the ignition time of electric wire fire spread igniting RPUF with flame retardance, comprising: Measuring or collecting the thermal physical parameters of RPUF with different flame retardants, the thermal physical parameters including thermal conductivity, pyrolysis temperature and ignition temperature; According to the thermal physical parameters of RPUF obtained by measurement or collection, a comprehensive prediction model of each thermal physical parameter is established by a nonlinear regression analysis method, and a corresponding thermal physical parameter correction coefficient is obtained based on the comprehensive prediction model; The ignition model of RPUF is constructed, and the ignition time under different ignition models is corrected based on the correction coefficient of each thermal physical parameter obtained by solving t ig The correction formula is as follows: Thermal thin model Thermal thick model Wherein, p The density of the thermal insulation material is kg / m 3 ; c is the specific heat capacity of the thermal insulation material, kJ / (kg·K); k 1 is the correction coefficient of the thermal conductivity of the thermal insulation material; λ 0 is the thermal conductivity of the original thermal insulation material, W / (m·K); k 3 is the correction coefficient of the ignition temperature of the thermal insulation material; The ignition temperature of the original thermal insulation material is K; The room temperature is K; The heat flux density received at the ignition point is W / m 2 ; d The thickness of the electric wire penetrating RPUF in the direction is m .

[0009] According to any one of the schemes of the first aspect of the present application, the comprehensive prediction model of each thermophysical parameter is established by a nonlinear regression analysis method, and the corresponding thermophysical parameter correction coefficient is obtained based on the comprehensive prediction model, comprising: The comprehensive prediction model of the related thermophysical parameter is established by nonlinear regression analysis through the least square method: ; According to the above formula, the corresponding thermophysical parameter correction coefficient is obtained: ; Wherein, is the thermophysical parameter of the original insulation material; w 1, w 2 are the mass fractions of different flame retardants; in the above formula, k x is the thermophysical parameter correction coefficient, wherein the subscript x takes the value of 1-3, respectively representing the thermal conductivity, the pyrolysis temperature and the ignition temperature.

[0010] According to any one of the schemes of the first aspect of the present application, the preheating zone of the electric wire fire spreading to the RPUF is selected as the thermodynamic control body, and the electric wire insertion position is selected as the ignition point. The RPUF in the preheating zone reaches the pyrolysis temperature from room temperature in the process of being heated to the ignition temperature, the energy conservation equation of the unit area and unit time of the ignition point in the preheating zone is: ; Wherein, is the heat conduction heat flow of the metal wire core of the electric wire to the unit area of the preheating zone of the thermosetting insulation material, W / m 2 ; is the convective heat transfer heat flow of the electric wire flame to the unit area of the preheating zone, W / m 2 ; is the thermal radiation heat flow of the electric wire flame to the unit area of the preheating zone, W / m 2 .

[0011] According to any one of the schemes of the first aspect of the present application, the heat conduction model of the electric wire fire spreading to ignite the flame-retardant RPUF can be simplified as a composite cylindrical radial heat conduction system with internal heat source, and the heat flow density between the inner layer and the outer layer is continuous, then the heat conduction heat flow of the metal wire core of the electric wire to the unit area of the preheating zone of the thermosetting insulation material is calculated as follows: ; Wherein, r 1 is the radius of the metal wire core of the electric wire, m; r 2 is the radius of the entire wire, m; r3 is the radius of the wire after the layer of thermosetting material, i.e. flame-retardant RPUF, is attached, m; k1 is the thermal conductivity of the wire insulation layer, W / (m·K); J is the Joule heat flux density generated by the current, W / m 2 ; T c T is the temperature of the wire metal core, K; T is the temperature of the interface between the insulation layer and the thermal insulation material, K; k2 is the correction coefficient of the thermal decomposition temperature of the thermal insulation material; T p0 T is the thermal decomposition temperature of the original thermal insulation material, K.

[0012] According to any of the schemes of the first aspect of the present application, the temperature difference of the wire metal core to the preheating zone of the thermal insulation material can be calculated according to the following formula: ; In the above formula, h c k is the heat transfer coefficient of the wire metal core, W / (m 2 ·K); W f L is the flame width of the wire, m; L P L is the thermal decomposition length of the RPUF, m; λ c k is the thermal conductivity of the wire metal core, W / (m·K); T f T is the temperature of the flame.

[0013] According to any of the schemes of the first aspect of the present application, the Joule heat flux density generated by the current is calculated as follows: ; wherein, and A c ρ and A are the resistivity and cross-sectional area of the wire metal core, respectively, with units of Ω·m and m 2 ; I I is the wire current, with units of A.

[0014] According to any of the schemes of the first aspect of the present application, the convective heat transfer heat flux per unit area of the wire flame to the preheating zone is calculated as follows: ; wherein, k is the convective heat transfer coefficient from the flame to the RPUF+, W / (m 2 ·K); k2 is the correction coefficient of the thermal decomposition temperature of the thermal insulation material; T p0 T is the thermal decomposition temperature of the original thermal insulation material, K.

[0015] According to any of the schemes of the first aspect of the present application, the heat radiation heat flow per unit area of the preheating zone of the electric wire flame is calculated as follows: ; Wherein, is the emissivity of the flame; is the Stefan-Boltzmann constant, is the view factor of the flame to the preheating zone; T f is the temperature of the flame; k2 is the correction coefficient of the pyrolysis temperature of the insulation material; T p0 is the pyrolysis temperature of the original insulation material, K.

[0016] The second aspect of the present application provides a method for predicting the ignition time of RPUF ignited by the spread of electric wire fire, comprising: Data acquisition and measurement, collecting the thermal physical parameters of RPUF, the electric wire ignition condition parameters and the electric wire current; Ignition time prediction, determining the ignition model according to the thickness of RPUF, inputting the collected data into any of the ignition time prediction models constructed by the first aspect of the present application, and calculating to obtain the ignition time prediction result of RPUF ignited by the spread of electric wire fire.

[0017] Compared with the prior art, the present application can achieve the following beneficial effects: (1) The present application provides a method for constructing an ignition time prediction model of RPUF ignited by the spread of electric wire fire, which quantifies the thermal response characteristics of the flame retardant through parameterization method, and characterizes the complex thermal behavior of the flame retardant as key parameters such as thermal conductivity, pyrolysis temperature and ignition temperature; based on these parameters, a comprehensive prediction model of each thermal physical parameter is established by nonlinear regression analysis method, and the corresponding correction coefficient of each thermal physical parameter is solved; based on the correction coefficient of each thermal physical parameter solved, the ignition time of RPUF under different ignition models is corrected by the calculation formula, so as to realize the quantitative evaluation of the fireproof performance of different flame retardant systems, and effectively ensure the ignition time prediction accuracy of RPUF ignited by the spread of electric wire fire. This method not only simplifies the test process, but also provides a theoretical basis for the optimization design of flame retardant formula. t ig

[0018] (2) The present application can accurately predict the time window of RPUF ignited under the condition of electric wire failure by establishing a multi-parameter coupled ignition time prediction model and further optimizing the preheating zone heat flow calculation model, which provides a key time parameter for building electrical fire prevention, helps to improve the building fire prevention design specification, and reduces the risk of electrical fire. ​

[0019] (3) The application innovatively establishes a heat conduction model considering current characteristics when predicting the ignition time of the wire ignition RPUF, introduces the Joule heat generated by the current as an internal heat source into the heat conduction equation, and constructs a composite cylindrical wall heat conduction model with an internal heat source; the model more accurately reflects the coupling mechanism of wire heating and material ignition in the actual fire scene, and significantly improves the reliability of the prediction results.

[0020] (4) The prediction method proposed by the application is not only suitable for RPUF materials, but also can be applied to the ignition time prediction of other building insulation materials through parameter adjustment, has wide applicability and important engineering application value, and provides a new technical means for building fire safety evaluation. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is the experimental device diagram of the embodiment of the application; Figure 2 is the preparation flowchart of the flame-retardant RPUF in the embodiment of the application; Figure 3 is the TG and DTG number curve of the insulation material sample in the embodiment of the application; Figure 4 is the heat transfer model of the ignition RPUF and the flame-retardant mechanism of different flame retardants in the embodiment of the application; Figure 5 is the fitting diagram of the predicted value and the actual value of the thermophysical property parameter in the embodiment of the application; Figure 6 is the partitioning diagram of the flame-retardant dominant region in the embodiment of the application; Figure 7 is the composite cylindrical wall heat conduction model and the flame radiation model in the embodiment of the application; Figure 8 is the infrared image of the insulation material in the embodiment of the application; Figure 9 is the heat flux density of each component received by the preheating area of the wire insulation material of model I in the embodiment of the application; Figure 10 is the heat flux density of each component received by the preheating area of the wire insulation material of model II in the embodiment of the application; Figure 11 is the calculated value of the ignition time of the insulation material under different working conditions in the embodiment of the application; Figure 12 is the fitting diagram of the calculated value and the experimental value of the ignition time in the embodiment of the application; Figure 13 is the comparison of the thermophysical property parameters of the insulation material in the embodiment of the application; Figure 14 is the statistical diagram of the thermophysical property parameters of the insulation material in the embodiment of the application. DETAILED DESCRIPTION

[0022] The present application provides a method for predicting the ignition time of a flame-retardant rigid polyurethane foam (RPUF) ignited by an electrically charged wire fire spread, which proposes a relatively complete prediction scheme in view of the irreversible curing of thermosetting RPUF materials, easy carbonization at high temperatures and other characteristics, and the thermal behavior characteristics after flame-retardant modification. Specifically, the method comprises the following steps: First, the key thermal physical parameters of the flame-retardant RPUF are determined by means of thermogravimetric analysis, thermal conductivity tester and the like, including thermal conductivity, pyrolysis temperature and ignition temperature; Second, a nonlinear regression prediction model is established by using the least square method, the thermal decomposition characteristics of the RPUF under the action of the flame retardant are analyzed, the influence of different flame retardants is distinguished, and the thermal physical property parameter correction coefficients after adding different mass fractions of the flame retardant are determined; Then, a composite cylindrical wall heat conduction model considering the Joule heat effect of the electric current is established, and the metal core, the insulating layer and the RPUF of the wire are regarded as a whole heat conduction system; Finally, a multi-parameter coupled ignition time prediction model is established by comprehensively considering the wire current parameters, the fire spread characteristics and the thermal physical mechanism of the flame-retardant RPUF, and the ignition time is predicted based on the prediction model.

[0023] The present application innovatively takes into account the coupling effect of multiple factors such as electricity-heat in the prediction model, and converts the complex chemical action mechanism of the flame retardant into key thermal physical parameters, while fully considering the influence of the carbonization behavior of the thermosetting RPUF on the heat transfer process, so that the ignition time can be accurately predicted. Compared with the traditional method, the present application solves the technical problem of inaccurate prediction of the process of igniting thermosetting polymer materials by an electrically charged fire source, and the prediction error can be controlled within 15%, which can provide important technical support for building electrical fire risk assessment and optimization design of flame-retardant thermosetting materials.

[0024] To further understand the content of the present application, the present application will be described in detail below in conjunction with specific embodiments. However, it should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present disclosure. In the following detailed description, many specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it is obvious that one or more embodiments can be implemented without these specific details. In addition, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessary confusion of the concepts of the present disclosure.

[0025] Meanwhile, the terms "include", "contain" and the like in the present application indicate the existence of the described features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0026] It should also be understood that, for clarity, certain features of the disclosure can be described in the context of separate embodiments, but can also be provided in combination in a single embodiment. That is, unless explicitly incompatible or specifically excluded, each separate embodiment is considered combinable with any other embodiment, and such combinations are considered to represent another, different embodiment. Conversely, various features of the disclosure described in the context of a single embodiment can also be provided separately or in any subcombination, for the sake of conciseness. Finally, while a particular embodiment can be described as part of a series of steps or part of a more general structure, each step or substructure can itself be considered an independent embodiment.

[0027] The embodiment of the application provides a fire spread ignition time prediction model construction method of a flame-retardant RPUF ignited by an electrified wire, comprising the following steps: Step one, data acquisition The thermal physical parameters of the RPUF added with different flame retardants are measured or collected, and the thermal physical parameters include a thermal conductivity, a pyrolysis temperature and an ignition temperature; wherein the thermal conductivity can be measured by a thermal conductivity instrument, and the pyrolysis temperature and the ignition temperature can be measured by a thermal gravimetric analyzer.

[0028] Step two, solving of thermal physical parameter correction coefficients According to the thermal physical parameters of the RPUF measured or collected, a comprehensive prediction model of each thermal physical parameter is established by a non-linear regression analysis method, and corresponding thermal physical parameter correction coefficients are solved based on the comprehensive prediction model.

[0029] Preferably, the non-linear regression analysis is performed by a least square method, and the comprehensive prediction model of the related thermal physical parameters is established: ; According to the above formula, the corresponding thermal physical parameter correction coefficients are solved: ; Wherein, is the thermal physical parameter of the original thermal insulation material; w 1, w 2 are mass fractions of different flame retardants; in the above formula, k x is a thermal physical parameter correction coefficient, wherein the subscript x takes values 1-3, respectively representing the thermal conductivity, the pyrolysis temperature and the ignition temperature.

[0030] Step three, construction of an ignition model of the RPUF, and correction of the ignition time under different ignition models based on the thermal physical parameter correction coefficients solved t ig The calculation formula is corrected: , hot thin model; , hot thick model; wherein, p is the density of the insulation material, kg / m 3 ; c is the specific heat capacity of the insulation material, kJ / (kg·K); k 1 is a correction coefficient of the thermal conductivity of the insulation material; λ 0 is the thermal conductivity of the original insulation material, W / (m·K); k 3 is a correction coefficient of the ignition temperature of the insulation material; is the ignition temperature of the original insulation material (before correction), K; is the room temperature, K; is the heat flux received at the ignition point, W / m 2 ; d is the thickness of the RPUF in the direction of the wire insertion, m .

[0031] Further, the preheating zone of the RPUF when the wire fire spreads to the RPUF is selected as the thermodynamic control body, and the wire insertion position is selected as the ignition point. The RPUF in the preheating zone reaches the pyrolysis temperature from the room temperature in a limited process, and is then heated to the ignition temperature. The energy conservation equation per unit area per unit time at the ignition point in the preheating zone is: ; wherein, is the heat conduction heat flux per unit area of the wire metal core to the preheating zone of the thermosetting insulation material, W / m 2 ; is the convective heat transfer heat flux per unit area of the wire flame to the preheating zone, W / m 2 ; is the thermal radiation heat flux per unit area of the wire flame to the preheating zone, W / m 2 .

[0032] (1) The calculation and derivation process of the heat conduction heat flux per unit area of the wire metal core to the preheating zone of the thermosetting insulation material is as follows.

[0033] The wire is inserted into the insulation material, and the Joule heat generated by the current directly acts on the copper core, and then the heat is conducted to the insulation layer of the insulation material through the insulation layer. Therefore, the heat conduction model for igniting the insulation material can be simplified as a radial heat conduction system of a composite cylinder with an internal heat source, as shown in Figure 7 (a).

[0034] For the internal heat source of the inner layer (radius r1 to r2), the heat flux density is: ; The one-dimensional steady-state solution of the heat conduction equation is obtained by twice integrating the equation, and the heat flux density expression is: ; In the above formula, is the thermal conductivity of the wire insulation layer, W / (m·K); is the joule heat flux density generated by the current, W / m 2 ; r 1 is the radius of the wire metal core, m; r 2 is the radius of the entire wire, m; T c is the temperature of the wire metal core (specifically copper core in this embodiment), K; is the temperature of the interface between the insulation layer and the thermal insulation material, K; r is a variable.

[0035] The heat flux density of the outer layer (radius r2 to r3) with internal heat source is: ; The one-dimensional steady-state solution of the heat conduction equation is obtained by twice integrating the equation, and the heat flux density expression is: ; In the above formula, r 3 is the radius of the entire wire after the adhesion of the thermosetting material layer, and since the cell structure of RPUF is relatively uniform and the filler is uniformly dispersed, its thermal conductivity can be approximately considered as isotropic. Therefore, the thickness of the thermal insulation material adhesion layer can be approximately equal to its preheating zone length, and the preheating zone length can be determined by an infrared thermal imager, as shown in Figure 8 , m; k2 is the correction coefficient of the thermal decomposition temperature of the thermal insulation material; T p0 is the thermal decomposition temperature of the original thermal insulation material, K.

[0036] Since the heat flux density between the inner layer and the outer layer is continuous, the following equation can be obtained: ; The final heat flux density expression received by the thermal insulation material preheating zone should include the influence of the inner layer heat source on the heat flux density and be corrected based on the continuity condition of the heat flux density between the inner and outer layers. According to the above derivation, it can be expressed as: ; The temperature difference from the copper core to the material preheating zone can be calculated according to the following formula: ; In the above formula, h c is the heat transfer coefficient of the wire metal core, W / (m 2 ·K); Wf is the flame width of the wire, m; L P is the pyrolysis length of the RPUF, m, which can be approximated as half of the flame width plus the maximum thickness of the carbonized layer, i.e. ; λ c is the thermal conductivity of the metal core of the wire, W / (m·K); T f is the temperature of the flame.

[0037] It is further preferred that the Joule heat flux density generated by the current can be calculated as: ; In the above formula, and A c are the resistivity and cross-sectional area of the copper core, respectively. High temperature can cause the resistivity to increase, which can generally be approximated by the following formula: ; In the above formula, T0 is the reference temperature (normal room temperature), which is taken as 20℃ in this embodiment, is the resistivity at the reference temperature; the typical value of the temperature coefficient of resistance of pure copper at is α = 0.00393 / °C; The typical value at 20℃ is 1.68×10 −8 Ω·m.

[0038] Therefore, the heat conduction heat flux per unit area of the preheating zone of the metal core of the wire is: ; It is worth noting that when the current is 0A, the heat conduction model of the ignition insulation material changes to a composite cylindrical radial heat conduction system, at which time the heat conduction calculation formula is as follows: .

[0039] (2) The convective heat transfer heat flux per unit area of the preheating zone of the wire flame is calculated as follows: Since the experiment is carried out in a closed indoor environment, there is no influence of external air flow, etc., so the convective heat transfer process of the ignition RPUF can be approximately considered as vertical plate natural convection: ; In the above formula, is the convective heat transfer coefficient of the flame to the RPUF+, W / (m 2 ·K), which can be calculated according to the following formula: ; In the above formula,k g wherein h is the height of the flame, m; L is the characteristic length, which can be calculated by the minimum value of the flame height and half of the height of the insulation material, m; and h0 is the height of the insulation material, m. Nu The heat transfer coefficient can be calculated according to the following formula: ; In the above formula, Ra is the Rayleigh number; and for natural convection of the flame, Ra can be expressed as: ; In the above formula, α , β and v are the thermal diffusivity, the thermal expansion coefficient and the kinematic viscosity coefficient of air respectively, β can be determined by the temperature of the flame, and the value thereof is the reciprocal of the temperature of the flame, K -1 The formula for calculating the convective heat flux density can be finally obtained as: ; In the above formula, H f is the height of the flame, m; H h0 is the height of the insulation material, m.

[0040] (3) The radiant heat flux density of the flame of the electric wire to the preheating zone of the insulation material can be estimated as: ; In the above formula, is the emissivity of the flame; is the Stefan-Boltzmann constant, is the view factor of the flame to the preheating zone.

[0041] The emissivity of the flame can be calculated according to the following formula: ; In the formula, D is the outer diameter of the entire electric wire, m.

[0042] Further, for the radiant heat flux density of the flame, when the view factor thereof is considered, the shape of the flame of the electric wire can be simplified as an isosceles triangle. As shown in FIG. 8(b), the surface radiant plane microelement area of the flame of the electric wire is Figure 7 , the surface microelement area of the preheating zone of the insulation material is , the connecting line length between the two microelements is R, and the connecting line and the normal n i and n j form the polar angles θ i and​θ j R, θ i and θ j The value of changes with the position of the area of the preheating zone. Therefore, the view factor from the flame surface to the preheating zone surface is: A i and A j The view factor from the flame surface to the preheating zone surface is: .

[0043] Specifically, preferably, the flame radiation plane is regarded as an isosceles triangle with the flame width as the base and the flame height as the height, and the center position of the flame bottom is taken as the origin to establish the calculation formula of the flame view factor, and the view factor from the flame surface to the preheating zone surface is: .

[0044] The embodiment of the present application also provides a fire ignition time prediction method for fire spread of an electrified wire igniting a flame-retardant RPUF, which comprises the following steps: Data acquisition and measurement, collecting the thermal physical parameters of the flame-retardant RPUF, the electrified wire ignition condition parameters and the electrified wire current; Fire ignition time prediction, determining the fire ignition model according to the thickness of the RPUF, inputting the collected data into the fire ignition time prediction model constructed above, and calculating to obtain the fire ignition time prediction result of the electrified wire igniting the RPUF.

[0045] Further, the data acquisition and measurement specifically comprises the following steps: (1) material thermal physical parameter measurement: using a thermal conductivity instrument to measure the thermal conductivity of the RPUF added with different flame retardants; using a thermal gravimetric analyzer to measure the pyrolysis temperature and the fire ignition temperature of the thermal insulation material; (2) ignition condition parameter acquisition: using a K-type thermocouple to cooperate with a data acquisition instrument to monitor the electrified wire flame temperature in real time during ignition; using an infrared thermal imager to assist in measuring the ignition flame temperature and the preheating zone length of the thermal insulation material; using a high-definition camera to record the fire spread process and the flame width and height parameters; (3) electrical parameter control: using a current generator to accurately control the electrified wire current; using a digital AC / DC clamp meter to monitor the electrified wire current value in real time.

[0046] Embodiment 1 This embodiment uses two different models of electrified wires (model I: the copper core diameter is 6 mm, and the insulating layer thickness is 2 mm; model II: the copper core diameter is 8 mm, and the insulating layer thickness is 2 mm) to perform experiments, and the experimental device is as shown in Figure 1 .

[0047] Meanwhile, the embodiment adopts one-step synthesis method to prepare rigid polyurethane foam (RPUF) and rigid polyurethane foam (RPUF+) added with different flame retardants (graphite EG and piperazine pyrophosphate PAPP). The basic materials for preparing the rigid polyurethane foam include polyether polyol (LY-4110) and 33wt% triethylene diamine solution (A33), polymethylene polyphenyl polyisocyanate (PAPI), dibutyl tin dilaurate (LC), silicone foam stabilizer (AK-8805). The synthesis components of RPUF added with different flame retardants are shown in Table 1, and the specific preparation process is shown in Figure 2 It is noted that only one preparation process example of RPUF+ is given here, and the specific process can be made by using any existing process, and the type of flame retardant is not limited to the specific type used in the embodiment.

[0048] Table 1 Formulation composition of thermal insulation material samples

[0049] The embodiment adopts standard test methods to measure the key thermophysical parameters of the flame-retardant RPUF: the thermal conductivity of the RPUF with different flame-retardant formulations is accurately measured by a steady-state heat flow method thermal conductivity instrument; the characteristic parameters such as the thermal decomposition onset temperature, the maximum thermal decomposition rate temperature and the ignition temperature of the material are measured by a thermal gravimetric analyzer (TGA) at a standard heating rate, as shown in Figure 3 Through the above standardized test process, the thermophysical properties of the RPUF with different flame-retardant systems can be accurately obtained, as shown in Table 2, to provide reliable experimental data support for the establishment of the subsequent ignition time prediction model.

[0050] Table 2 Thermophysical parameters of thermal insulation material samples

[0051] Figure 4 The schematic diagram of the wire fire spreading igniting RPUF is shown in FIG. 1, and the following assumptions are made to simplify the calculation process: (1) the thickness change of the insulation PE in the melting process is ignored; (2) the Marangoni convection heat flux density and the heat loss are ignored; (3) the wire is considered as a thermal thin model, and the thermal insulation material is considered as a thermal thick model; (4) it is assumed that the temperature distribution of the cross section of the copper core and the PE is uniform; (5) the axial heat conduction of the PE layer is ignored; (6) the deformation caused by the wire flame igniting the RPUF is ignored; (7) the ignition point is taken at the position where the wire penetrates when igniting the thermal insulation material; (8) the heat loss of the Joule heat in the PE layer is ignored during the process of igniting the thermal insulation material.

[0052] Further taking the above model and materials as examples, the ignition time of the fire spread of the charged wire igniting the flame-retardant RPUF is predicted, specifically including the following steps: Step one, based on the thermal physical property parameter data of RPUF added with different flame retardants measured by the experimental instrument, through the least square method, a comprehensive prediction model of related thermal physical property parameters such as thermal conductivity, pyrolysis temperature and ignition temperature is established through nonlinear regression analysis; ; In the above formula, is the thermal physical property parameter of the original insulation material; w 1, w 2 are the mass fractions of different flame retardants, respectively.

[0053] To ensure the accuracy of the prediction model, the following method is adopted in this example: one data point is randomly selected from single sample and mixed sample for model calculation, and the remaining data is used to verify the prediction accuracy of the model. First, the thermal conductivity is predicted and fitted to obtain the values of A, B, C, D and E, which are 0.0478, -0.0552, 0.128, 0.308 and 0.5, respectively, i.e. the final expression is: ; Similarly, the nonlinear regression fitting curves of pyrolysis temperature and ignition temperature are obtained: ; ; From the above prediction model analysis, in terms of thermal stability, the introduction of flame retardant always shows a positive effect, and the pyrolysis temperature and ignition temperature are improved with the addition of filler, and under the same mass fraction condition, the improvement effect of PAPP on thermal stability is slightly better than that of EG, which is mainly due to its more excellent flame-retardant charring property. Based on the established unified prediction model, by adjusting the mass fraction ratio of EG and PAPP, the thermal physical property parameter evolution law of RPUF under any composite formula can be accurately predicted, and the fitting parameter R 2 is 0.9821, 0.9472 and 0.9947, respectively, which shows that the prediction fitting degree is high, and the corresponding fitting error is shown in Figure 5 . This not only provides theoretical guidance for material formula design, but also significantly reduces the experimental trial and error cost, realizes efficient material performance optimization. The model has good expansibility and can be applied to a wider range of filler types and content systems, providing reliable technical support for the industrialization development of flame-retardant polyurethane foam materials.

[0054] Based on the above formula, for the convenience of subsequent calculation and other ingredient ratio calculation of the insulation material, the above formula is solved to obtain the corresponding thermal physical property parameter correction coefficient, i.e. ; In the above formula, k x is the correction coefficient, where x takes the value of 1, 2, 3, respectively, representing the thermal conductivity, pyrolysis temperature and ignition temperature. That is: ; In the above formula, k 1 is the correction coefficient of the thermal conductivity of the thermal insulation material; k 2 is the correction coefficient of the pyrolysis temperature of the thermal insulation material; k 3 is the correction coefficient of the ignition temperature of the thermal insulation material.

[0055] The effect of the flame retardant on the thermal conductivity of the RPUF shows a nonlinear characteristic. The addition of EG can significantly improve the thermal conductivity of the material by forming a thermal conduction network, while PAPP has an inhibitory effect due to its low thermal conductivity. The interaction of the two further increases the complexity of the regulation of thermal conductivity. To intuitively represent the influence of the ratio of the flame retardant on the thermal conductivity, this paper draws a region control chart based on the prediction model (as shown in Figure 6 ), which clearly shows the change rule of the thermal conductivity relative to the original material under different EG / PAPP ratios through visual means.

[0056] The calculation formula of the ignition time of the flame-retardant RPUF ignited by the charged electric wire fire is determined according to the thickness of the RPUF. In actual application, when the thickness of the material exceeds 0.005 m (5 mm), it is usually considered as a hot thick model; otherwise, it is a hot thin model. Since the thickness of the RPUF in this example reaches 35 mm, the ignition model can be calculated according to the hot thick model: .

[0057] Further, the preheating zone of the RPUF when the electric wire fire spreads to it is selected as the thermodynamic control body, and the position where the electric wire penetrates is selected as the ignition point. The RPUF in the preheating zone reaches the pyrolysis temperature from room temperature T p , and then is heated to the ignition temperature . During this process, the energy conservation equation per unit area per unit time at the ignition point in the preheating zone is: ; In the above formula, where, is the heat conduction heat flow per unit area of the preheating zone of the thermosetting thermal insulation material caused by the metal wire core of the electric wire, W / m 2 ; is the convective heat transfer heat flow per unit area of the preheating zone caused by the electric wire flame, W / m 2 ; The heat flux radiated by the electric wire flame to a unit area of ​​the preheating zone, in W / m². 2 .

[0058] When calculating the heat flux per unit area of ​​the preheating zone of thermosetting insulation material from the metal core of the wire, the thermal conductivity of RPUF can be approximated as isotropic due to its relatively uniform cell structure and uniform filler dispersion. Therefore, the thickness of the insulation material's adhesion layer can be approximately equal to the length of its preheating zone, which can be determined by an infrared thermal imager. Figure 8 As shown, the results are imported into Table 3.

[0059] Table 3 Length of Preheating Zone for Thermal Insulation Material

[0060] The total heat received by the preheating zone of the insulation material was calculated. To systematically study the influence mechanism of flame retardant type on ignition time, this experiment used the controlled variable method to set the key parameters of the ignition flame (including flame height, width, and temperature) to constant values ​​when calculating the thermal feedback, so as to eliminate the interference of flame characteristic differences on the experimental results.

[0061] Table 4 shows the relevant calculation parameters for a calculation example (using a 40A current, a type I conductor, and an RPUF ignition method). Through analysis... Figure 9 , Figure 10 It can be observed that the difference in total heat flux density received by the preheating zone of the insulation material mainly stems from changes in the flame's thermal conductivity, while the contributions of thermal convection and thermal radiation to the heat flux density are relatively small. Further examination of the effect of current intensity shows that as the current increases, Joule heating exhibits a continuous increasing trend, but the conductive heat flux density decreases when the current reaches 60A. And based on... Figure 9 , Figure 10 The specific heat calculation values ​​for each component and the total heat received by the preheating zone of the insulation material are shown. This provides convenience and a basis for calculating the subsequent ignition time.

[0062] Table 4. Specific calculated values ​​for igniting RPUF with Type I wire at a current of 40A.

[0063] Finally, based on the corresponding ignition time calculation formula, the ignition time of the RPUF ignited by the spread of fire from the energized wire can be calculated. Figure 11 The study demonstrated the variation of ignition time under different experimental conditions. The results showed that in the graphite-added region, the ignition time was positively correlated with the graphite content. This was mainly attributed to the fact that the layered structure of graphite could effectively block heat transfer. Figure 4(b) ), delay the pyrolysis process of the material; in the pyrophosphoric acid piperazine addition zone, the ignition time is shortened with the increase of the content of the flame retardant, which is due to the increase of the release of non-combustible gas when the pyrophosphoric acid piperazine is heated, which promotes the pyrolysis reaction of the material, but its flame retardant effect still makes the ignition time higher than that without adding the flame retardant; and in the mixed addition zone, the ignition time is prolonged with the increase of the total content of the flame retardant, which is due to the synergistic effect of the two kinds of flame retardants, which enhances the flame retardant performance of the material. The increase of the current effectively reduces the ignition time of the thermosetting material, because on the one hand, the joule heat generated by the current can increase the combustion intensity of the wire flame; on the other hand, part of the joule heat will be transferred to the thermal insulation material through the wire. Based on Figure 11 the theoretical analysis results, the theoretical ignition time of the material under different working conditions is obtained, and compared with the experimental statistical value, and the results are shown in Figure 12 The comparison results show that the wire flame spread ignition model based on the heat transfer component has high prediction accuracy, and the deviation between the theoretical calculation value and the experimental value is basically controlled within ± 15%, which verifies the reliability of the established model.

[0064] The main reason for the deviation can be attributed to the difference between the idealized treatment of the experimental conditions and the actual combustion process. Specifically, in order to exclude interference factors, the controlled variable method is used to set the key parameters of the ignition flame as constant values. However, in the actual combustion process, due to the influence of air flow disturbance and combustion instability, the flame will produce irregular oscillation, resulting in fluctuations in its characteristic parameters. This inevitable experimental system error is the main source of the deviation between the theoretical value and the experimental value. Nevertheless, the error range of ± 15% is still within the acceptable range, indicating that the established model can better reflect the actual combustion process.

[0065] In order to systematically evaluate the fireproof performance of different flame retardant formulations RPUF, the present application takes type I wire under 40A overload current condition as an example, and the key performance parameters are statistically analyzed and compared. As shown in Figure 13 and Figure 14 The test data includes: material thermal conductivity, pyrolysis temperature, ignition temperature, mass loss rate, preheating zone length, preheating zone received heat flux and ignition time and other key indicators. By establishing the quantitative relationship between these parameters and the ignition time, the scientific evaluation of the fireproof performance of different flame retardant systems can be realized.

Claims

1. A method of constructing a model for predicting the ignition time of a fire-retardant RPUF ignited by the spread of an electrically charged electric wire, characterized by, Comprise: Measuring or collecting the thermal physical parameters of RPUF added with different flame retardants, the thermal physical parameters including thermal conductivity, pyrolysis temperature and ignition temperature; According to the measured or collected thermal physical parameters of RPUF, a comprehensive prediction model of each thermal physical parameter is established by a nonlinear regression analysis method, and the corresponding thermal physical parameter correction coefficient is solved based on the comprehensive prediction model; The ignition model of RPUF is built, and the ignition time under different ignition models is corrected based on the obtained thermal physical property parameter correction coefficient of each thermal physical property parameter t ig The calculation formula is corrected: , hot thin model; , hot thick model; wherein, Rho is the density of the thermal insulation material, kg / m 3 ; c is the specific heat capacity of the thermal insulation material, kJ / (kg·K); k 1 is a correction factor of the thermal conductivity of the thermal insulation material; Lambda 0 is the thermal conductivity of the original thermal insulation material, W / (m·K); k 3 is a correction factor of the ignition temperature of the thermal insulation material; is the ignition temperature of the original thermal insulation material, K; is the room temperature, K; is the heat flux density received at the ignition point, W / m 2 ; d is the thickness in the direction in which the electric wire is inserted into the RPUF, m.

2. The ignition time prediction model construction method according to claim 1, characterized by, The comprehensive prediction model of each thermal physical parameter is established by a nonlinear regression analysis method, and the corresponding thermal physical parameter correction coefficient is solved based on the comprehensive prediction model, comprising: Nonlinear regression analysis is carried out by the least square method to establish a comprehensive prediction model of the related thermal physical parameters: ； According to the above formula, the corresponding thermal physical parameter correction coefficient is solved: ； wherein, thermal physical parameters of the original insulation material; w 1、 w 2 the mass fraction of different flame retardants; in the above formula, k x thermal physical parameters correction coefficient, wherein subscript x takes values 1~3, respectively representing thermal conductivity, pyrolysis temperature and ignition temperature.

3. The method according to claim 2, wherein The preheating zone when the wire fire spreads to RPUF is selected as the thermodynamic control body, and the wire insertion position is selected as the ignition point. The RPUF in the preheating zone is from room temperature The energy conservation equation of the unit area and unit time at the ignition point in the preheating zone during the process of being heated to the ignition temperature after being limited to reach the pyrolysis temperature is: ； wherein, Q is the heat transfer heat flux per unit area of the preheating zone of the electric wire metal wire core to the thermosetting insulation material, W / m 2 ; Q is the heat transfer heat flux per unit area of the preheating zone of the electric wire flame to the thermosetting insulation material, W / m 2 ; Q is the heat transfer heat flux per unit area of the preheating zone of the electric wire flame to the thermosetting insulation material, W / m 2 .

4. The method according to claim 3, wherein The heat conduction model of the fire spread of the charged electric wire igniting the flame retardant RPUF can be simplified as a composite cylinder radial heat conduction system with an internal heat source, and the heat flux density between the inner layer and the outer layer is continuous, so the heat conduction heat flux per unit area of the metal wire core of the electric wire to the preheating area of the thermosetting insulation material The calculation is as follows: ； wherein, r 1 is the radius of the metal wire core of the electric wire, m; r 2 is the radius of the entire wire, m; r 3 is the radius of the wire after the layer of thermosetting material, i.e. flame-retardant RPUF, is attached, m; is the thermal conductivity of the insulation layer of the electric wire, W / (m·K); is the joule heat flux density generated by the electric current, W / m 2 ; T c is the temperature of the metal wire core of the electric wire, K; is the temperature at the interface between the insulation layer and the thermal insulation material, K; k2 is the correction coefficient for the pyrolysis temperature of the thermal insulation material; T p0 is the pyrolysis temperature of the original thermal insulation material, K.

5. The method according to claim 4, wherein The temperature difference between the metal wire core of the electric wire and the preheating area of the thermal insulation material can be calculated according to the following formula: ； In the above formula, h c is the heat transfer coefficient of the wire metal core, W / (m 2 ·K); W f is the flame width of the wire, m; L P is the pyrolysis length of the RPUF, m; Lambda c is the thermal conductivity coefficient of the wire metal core, W / (m K); T f is the temperature of the flame.

6. The method according to claim 4, wherein The joule heat flux density generated by the current is calculated as follows: ； wherein, and A c respectively the resistivity and the cross-sectional area of the metal wire core of the electric wire, units Ω·m and m 2 respectively; I is the electric wire current, unit A.

7. The ignition time prediction model construction method according to any one of claims 3 to 6, characterized by, The convective heat transfer heat flux per unit area of the preheating area to the electric wire flame is calculated as follows: ； wherein, K) ; k2 is a correction factor for the pyrolysis temperature of the insulation material; 2 K) ; k2 is a correction factor for the pyrolysis temperature of the insulation material; T p0 K) ; k2 is a correction factor for the pyrolysis temperature of the insulation material; 8. The ignition time prediction model construction method according to any one of claims 3 to 6, characterized by, The thermal radiation heat flux per unit area of the preheating area to the electric wire flame is calculated as follows: ； wherein, is the emissivity of the flame; is the Stefan-Boltzmann constant, is the view factor of the flame to the preheat zone; T f is the temperature of the flame; k2 is a correction factor for the pyrolysis temperature of the insulation material; T p0 is the pyrolysis temperature of the original insulation material, K.

9. A method of predicting the ignition time of a charged electrical wire fire spread igniting a flame retardant RPUF, characterized by, Comprise: Data acquisition and measurement, collecting the thermal physical parameters of flame-retardant RPUF, the electric wire ignition condition parameters and the electric wire current; Ignition time prediction, determining the ignition model according to the thickness of RPUF, inputting the collected data into the ignition time prediction model constructed in any one of claims 1-8, and calculating to obtain the ignition time prediction result of the electric wire flame spreading igniting RPUF.

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