An evaluation method for the fast cook-off of explosives

Abstract

The present invention discloses a method for evaluating the rapid cook-off of explosives. The specific steps are as follows: establish a rapid cook-off calculation model with an air domain for simulation according to the rapid cook-off test system; the rapid cook-off calculation model with an air domain includes: a test projectile model, an air domain, a fuel inlet, and a fuel outlet; establish a rapid cook-off calculation model without an air domain, and the rapid cook-off calculation model without an air domain only includes a test projectile model; finally, obtain the scatter correspondence relationship between the heat flux boundary and the diameter of the corresponding charge model; according to any diameter of the charge model and the calculated heat flux boundary, load the heat flux boundary onto the outer wall surface of the test projectile model for calculation, obtain the internal temperature change, ignition time, and ignition temperature of the charge model, and evaluate the rapid cook-off characteristics of the explosives; the present invention can optimize the existing rapid cook-off calculation with an air domain and realize the evaluation of the rapid cook-off of explosives without an air domain.

Description

Technical Field

[0001] The present invention belongs to the technical field of explosive safety assessment, and particularly relates to an assessment method for rapid cook-off of explosives. Background Art

[0002] Modern warfare has increasingly high requirements for the thermal safety of weapons and ammunitions. It is necessary to ensure their absolute safety during manufacturing, storage, transportation, and use, without serious accidents such as ignition or even explosion, and without reducing their safety during the service life cycle. Therefore, it is of great significance to study the safety of ammunitions under thermal stimulation.

[0003] The cook-off test is an important means to test and evaluate the thermal safety of ammunitions. According to different heating environments, it is divided into rapid cook-off and slow cook-off. Rapid cook-off mainly simulates the thermal response characteristics of ammunitions in a fire environment. Experiments and numerical simulations are important methods for studying the rapid cook-off of ammunitions. Since the fire test requires a large test site and the test results are greatly affected by environmental conditions, it is mostly used for the experimental research of prototype ammunitions.

[0004] With the development of numerical simulation technology, numerical simulation has gradually become an important research method for rapid cook-off. Many studies have been carried out in China. Wang Shuai et al. [Wang Shuai, Zhi Xiaoqi, Jia Qiulin, et al. Fire test and numerical simulation of explosives based on the mass_flux method [J]. Journal of Ordnance Equipment Engineering, 2020, 41(8): 1-6.] carried out fire tests and numerical simulation calculations based on RBOE explosives. Based on the physical diagram of the fire test device, a calculation model including the projectile body, air domain, fuel inlet, and outlet was established, as Figure 1 shown. The written linear temperature-time history curve and mass flow rate were loaded into the fuel inlet using the mass flow rate inlet boundary condition. The charge area reacted according to the self-heating reaction law, and the reaction kinetic equation was applied to the charge area through a subroutine. Then, the projectile body was heated by high-temperature gas radiation and heat convection. The experimental and simulation flame environment temperature histories are as Figure 2 shown. The mass flow rate was adjusted to make the calculated ignition time basically consistent with the experimental ignition time.

[0005] Defects of existing numerical calculation methods for rapid cook-off:

[0006] (1) Using the mass flow rate and linear temperature-time history to load into the fuel inlet, the calculated and experimental environmental temperature history curves have a poor fit in the heating section and cannot well describe the growth process of the flame temperature.

[0007] (2) For rapid cook-off numerical calculations, an air domain needs to be established. To simulate the flame combustion environment, the size of the air domain needs to be much larger than the size of the projectile body. Therefore, the calculation amount increases exponentially and the calculation efficiency decreases.

[0008] (3) In the existing numerical calculation of the rapid cook-off of the air-containing domain, the temperature is not directly loaded onto the surface of the projectile. Instead, it is heated through the radiation and convection of high-temperature gas, and the calculation results are greatly affected by factors such as grid size and air-domain size, resulting in poor calculation accuracy. Summary of the Invention

[0009] In view of this, the present invention provides an evaluation method for the rapid cook-off of explosives, which can optimize the existing numerical calculation of the rapid cook-off of the air-containing domain and realize the evaluation of the rapid cook-off of explosives without an air domain.

[0010] The present invention is realized through the following technical solutions:

[0011] An evaluation method for the rapid cook-off of explosives, and the specific steps of this evaluation method are as follows:

[0012] Step 1: Build a rapid cook-off test system and conduct a rapid cook-off test;

[0013] Step 2: Establish a rapid cook-off calculation model with an air domain based on the rapid cook-off test system for simulation; the rapid cook-off calculation model with an air domain includes: a test projectile model, an air domain, a fuel inlet, and a fuel outlet;

[0014] Step 3: Perform calculations on the charge models with different sizes in the test projectile model in the rapid cook-off calculation model with an air domain, that is, by changing the diameter of the charge model, and obtain the ignition time A of the charge model under different charge model diameters through calculation;

[0015] Step 4: Establish a rapid cook-off calculation model without an air domain, and the rapid cook-off calculation model without an air domain only includes a test projectile model; the outer wall surface of the shell model of the test projectile model uses a heat flux boundary as the heating boundary; according to this rapid cook-off calculation model without an air domain, under the condition that the diameter of the charge model is the same as that in Step 3, calculate the ignition time B of the charge model, and by adjusting the size of the heat flux boundary, make the ignition time A in Step 3 equal to the ignition time B; finally, obtain the scatter point correspondence between the heat flux boundary and the corresponding diameter of the charge model, and fit the scatter points into a curve;

[0016] Step 5: According to any diameter of the charge model and the heat flux boundary calculated according to the curve in Step 4, load the heat flux boundary onto the outer wall surface of the test projectile model for calculation, obtain the internal temperature change, ignition time, and ignition temperature of the charge model, and evaluate the rapid cook-off characteristics of the explosives.

[0017] Further, in Step 1, the test system includes: a fuel pool, a test projectile, a thermocouple, a bracket, a suspension rod, an ignition device, and a temperature recorder; the test projectile includes: a projectile case and a charge loaded in the projectile case; the test projectile is fixed to the bracket by the suspension rod, the fuel pool is located directly below the test projectile, and the distance between the lowest point of the test projectile and the upper surface of the fuel in the fuel pool is 300 mm. The fuel in the fuel pool is connected to the ignition device; the thermocouple is installed outside the test projectile and is located on the extension line of the axis of the test projectile. The closest distance between the thermocouple and the outer end face of the test projectile is 50 mm, and the temperature recorder is electrically connected to the thermocouple;

[0018] When performing the rapid cook-off test, the ignition device ignites the fuel. The fuel in the fuel pool burns to generate flames and high-temperature gas, and forms a flame field around the test projectile. The charge of the test projectile heats up in the flame field. When the temperature of the flame field is higher than the ignition point of the charge of the test projectile, when the highest temperature of the charge of the test projectile is higher than its ignition point, the charge ignites; the thermocouple measures the ambient temperature of the flame field where the test projectile is located when the fuel in the fuel pool burns, and the temperature recorder records the change of the ambient temperature measured by the thermocouple over time, obtaining the change curve of the ambient temperature of the test over time.

[0019] Further, in Step 2, the test projectile model is exactly the same as the test projectile, including: a projectile case model and a charge model loaded in the projectile case model; the fuel inlet is the high-temperature gas generated by the combustion of the fuel in the fuel pool. The boundary condition of the fuel inlet is the mass flow inlet boundary, and the mass flow inlet boundary uses a temperature piecewise function and a mass flow rate to fit the test ambient temperature; the distance from the lowest point of the test projectile model to the fuel inlet is the same as the distance between the lowest point of the test projectile and the upper surface of the fuel in the fuel pool; the boundary condition of the fuel outlet is the pressure outlet boundary; by adjusting the gas mass flow rate and the temperature piecewise function to simulate the situation of the flame field generated by the combustion of the fuel in the fuel pool, the change curve of the ambient temperature of the test projectile model in the flame field over time is calculated. If the difference in the ambient temperature at the same time point between the calculated change curve of the ambient temperature of the test projectile model in the flame field over time and the change curve of the ambient temperature obtained from the test is within the set range, it indicates that the established calculation model can describe the flame field of the rapid cook-off test system.

[0020] Further, in Step 4, use the single-phase exponential decay function in Origin software to fit the scatter correspondence between the heat flux boundary and the diameter of the corresponding charge model.

[0021] Further, in Step 4, use Matlab software to fit the scatter correspondence between the heat flux boundary and the diameter of the corresponding charge model.

[0022] Furthermore, the expression after fitting the scatter correspondence relationship between the heat flux boundary and the diameter of the corresponding charge model into a curve is as follows:

[0023]

[0024] Among them, Φ is the heat flux boundary, and D is the diameter of the charge model.

[0025] Furthermore, in Step 4, the expression after fitting the scatter correspondence relationship between the heat flux boundary and the diameter of the corresponding charge model into a curve is as follows:

[0026]

[0027] Among them, Φ is the heat flux boundary, and D is the diameter of the charge model.

[0028] Furthermore, before Step 3, it is necessary to perform mesh division on the rapid cook-off calculation model with an air domain established in Step 2. The mesh size of the air domain is divided into 100 mm, and the mesh size of the test projectile model is divided into 2 mm.

[0029] Beneficial effects: The rapid cook-off assessment method proposed by the present invention optimizes the existing rapid cook-off calculation method on the one hand and proposes a new rapid cook-off calculation method on the other hand.

[0030] (1) For the existing rapid cook-off calculation method with an air domain, the mass flow rate inlet boundary of the present invention uses a temperature piecewise function and mass flow rate to fit the test environment temperature, that is, by using the temperature piecewise function to fit the test environment temperature and acting together with the gas mass flow rate, the inlet boundary condition of the rapid cook-off calculation model with an air domain is obtained, solving the problem that the environmental temperature calculated by the existing calculation method has a poor fit with the test environmental temperature.

[0031] (2) The present invention establishes a rapid cook-off calculation model that only considers the test projectile, and uses the heat flux boundary as the boundary condition for rapid cook-off numerical calculation, that is, using the heat flux boundary as the heating boundary, which simplifies the calculation model and improves the calculation efficiency of rapid cook-off numerical simulation; it solves the problems of large calculation amount and complex influencing factors in the existing rapid cook-off calculation model with an air domain.

[0032] (3) The present invention uses the single-phase exponential decay function in Origin software to fit the relationship between the heat flux boundary and the diameter of the charge model, obtaining the relationship formula between the heat flux boundary and the diameter of the charge model, which can reduce the number of tests and calculations. According to the size of the charge model, a suitable heating boundary can be selected for rapid cook-off calculation, greatly improving the numerical calculation efficiency of rapid cook-off.

[0033] (4) The present invention conducts grid convergence analysis. A large-sized grid is adopted for the air domain, and a small-sized grid is used for the test projectile model. Grids of different sizes are used for calculation, and it is ensured that the calculation converges. On the premise of ensuring the calculation accuracy, the calculation efficiency is improved. Description of the Drawings

[0034] Figure 1 It is a calculation model diagram of the background technology containing an air domain;

[0035] Figure 2 It is an environmental temperature diagram of the test and simulation in the background technology;

[0036] Figure 3 It is a composition diagram of the rapid cook-off test system of the present invention;

[0037] Figure 4 It is a curve diagram of the change of the environmental temperature of the test and simulation of the present invention over time;

[0038] Figure 5 It is a rapid cook-off calculation model diagram of the present invention containing an air domain;

[0039] Figure 6 It is a grid convergence analysis diagram. (a) is a curve diagram of the change of the temperature of the charge model over time under different grids in the air domain, and (b) is a curve diagram of the change of the temperature of the charge model over time under different grids in the test projectile model;

[0040] Figure 7 It is a rapid cook-off calculation model diagram of the present invention without an air domain;

[0041] Figure 8 It is a relationship diagram between the heat flux boundary of the present invention and the diameter of the corresponding charge model;

[0042] Among them, 1 - fuel inlet, 2 - test projectile model, 3 - fuel outlet, 4 - air domain, 5 - shell model, 6 - charge model, 7 - outer wall surface, 8 - fuel pool, 9 - thermocouple, 10 - support, 11 - suspension rod, 12 - ignition device, 13 - temperature recorder, 14 - test projectile, 15 - shell, 16 - charge. Detailed Embodiment

[0043] The following combines the drawings and gives embodiments to describe the present invention in detail.

[0044] This embodiment provides an evaluation method for the rapid cook-off of explosives. The specific steps of this evaluation method are as follows:

[0045] Step 1, build a rapid cook-off test system and conduct a rapid cook-off test, such as Figure 3As shown in the figure, the test system includes: a fuel pool 8, a test projectile 14, a thermocouple 9, a support 10, a suspension rod 11, an ignition device 12, and a temperature recorder 13; the test projectile 14 includes: a projectile case 15 and a charge 16 loaded in the projectile case 15, the length-diameter ratio of the charge 16 is 2, and the wall thickness of the projectile case 15 is 6 mm; the test projectile 14 is fixed on the support 10 by the suspension rod 11, the fuel pool 8 is located directly below the test projectile 14, and the distance between the lowest point of the test projectile 14 and the upper surface of the fuel in the fuel pool 8 is 300 mm, and the fuel in the fuel pool 8 is connected to the ignition device 12; the thermocouple 9 is positioned and installed outside the test projectile 14 by a mounting bracket and rock wool, and the thermocouple 9 is located on the extension line of the axis of the test projectile 14, the closest distance between the thermocouple 9 and the outer end face of the test projectile 14 is 50 mm, and the temperature recorder 13 is electrically connected to the thermocouple 9;

[0046] When performing a rapid cook-off test, the ignition device 12 ignites the fuel, the fuel in the fuel pool 8 burns to generate flames and high-temperature gas, and a flame field is formed around the test projectile 14. The charge 16 of the test projectile 14 heats up in the flame field. When the highest temperature of the charge 16 of the test projectile 14 is higher than its ignition point, the charge 16 ignites; the thermocouple 9 measures the ambient temperature of the flame field where the test projectile 14 is located when the fuel in the fuel pool 8 burns, and the temperature recorder 13 records the change of the ambient temperature measured by the thermocouple 9 over time, obtaining the change curve of the ambient temperature of the test over time, as Figure 4 shown by the solid line;

[0047] Step 2, establish a rapid cook-off calculation model including an air domain according to the rapid cook-off test system for simulation, as Figure 5 shown in the figure, the rapid cook-off calculation model including an air domain includes: a test projectile model 2, an air domain 4, a fuel inlet 1, and a fuel outlet 3; the test projectile model 2 is exactly the same as the test projectile 14, including: a projectile case model 5 and a charge model 6 loaded in the projectile case model 5; the fuel inlet 1 is the high-temperature gas generated by the combustion of the fuel in the fuel pool 8, the boundary condition of the fuel inlet 1 is a mass flow inlet boundary, the gas mass flow rate of the fuel inlet 1 is specified, and the boundary temperature adopts a piecewise function, that is, the temperature piecewise function fits the test ambient temperature for calculation. Finally, the mass flow inlet boundary adopts the temperature piecewise function and the mass flow rate to fit the test ambient temperature; the distance between the lowest point of the test projectile model 2 and the fuel inlet 1 is the same as the distance between the lowest point of the test projectile 14 and the upper surface of the fuel in the fuel pool 8, both are 300 mm; the boundary condition of the fuel outlet 3 is a pressure outlet boundary; by adjusting the gas mass flow rate and the temperature piecewise function to simulate the situation of the flame field generated by the combustion of the fuel in the fuel pool 8, the change curve of the ambient temperature of the test projectile model 2 in the flame field over time is calculated, as Figure 4 shown by the dashed line. If this dashed line coincides with the change curve of the ambient temperature over time obtained from the test (i.e.,Figure 4 basically coincide (i.e., the difference in ambient temperature between the dashed line and the solid line at the same time point is within the set range), indicating that the established calculation model can describe the flame field of the rapid cook-off test system;

[0048] Step 3: Mesh the rapid cook-off calculation model with an air domain established in Step 2, and perform mesh convergence analysis; Since the mesh size of the rapid cook-off calculation model with an air domain has a great influence on the calculation results, it is necessary to perform mesh convergence calculations for different regions. Calculate the air domain 4 with mesh sizes of 150 mm, 100 mm, and 70 mm respectively. As the mesh size decreases, the ignition time of the charge model 6 gradually delays and shows a convergence trend, as shown in Figure 6 (a); Calculate the test projectile model 2 with mesh sizes of 2.5 mm, 2 mm, and 1.5 mm respectively. As the mesh size decreases, the ignition time of the charge model 6 gradually delays and shows a convergence trend, as shown in Figure 6 (b); To balance the calculation efficiency and calculation accuracy, divide the mesh size of the air domain 4 into 100 mm, and divide the mesh size of the test projectile model 2 into 2 mm;

[0049] Step 4: According to the rapid cook-off calculation model with an air domain after meshing, perform calculations of different sizes on the charge model 6 in the test projectile model 2, that is, by changing the diameter D of the charge model 6, and obtain the ignition time A of the charge model 6 at different diameters D of the charge model 6 through calculation;

[0050] Step 5: Establish a rapid cook-off calculation model without an air domain, as shown in Figure 7 , the rapid cook-off calculation model without an air domain only includes the test projectile model 2; The outer wall surface 7 of the shell model 5 of the test projectile model 2 uses a heat flux boundary as the heating boundary; According to this rapid cook-off calculation model without an air domain, under the condition that the diameter is the same as that of the charge model 6 in Step 4, calculate the ignition time B of the charge model 6, and by adjusting the size of the heat flux boundary, make the ignition time A in Step 4 equal to the ignition time B; Finally, obtain the relationship between the heat flux boundary and the diameter D of the corresponding charge model 6, as shown in the scatter plot in Figure 8 ;

[0051] Step 6: Use the single-phase exponential decay function in Origin software or Matlab software to fit the scatter plot in Figure 8 , and obtain the relationship formula between the heat flux boundary Φ and the diameter D of the charge model 6 as:

[0052]

[0053] Step 7: According to any diameter D of the charge model 6 and the corresponding heat flux boundary Φ calculated from formula (1), load the heat flux boundary Φ onto the outer wall surface 7 of the test projectile model 2 for calculation, obtain information such as the internal temperature change, ignition time, ignition temperature, etc. of the charge model 6, and evaluate the rapid cook-off characteristics of the explosive.

[0054] In summary, the above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An evaluation method for the fast cook-off of explosives, characterized in that, The specific steps of this evaluation method are as follows: Step 1: Build a rapid cook-off test system and conduct a rapid cook-off test; Step 2: Establish a rapid cook-off calculation model with an air domain based on the rapid cook-off test system for simulation; the rapid cook-off calculation model with an air domain includes: a test projectile model (2), an air domain (4), a fuel inlet (1), and a fuel outlet (3); Step 3: Perform calculations with different sizes on the charge model (6) in the test projectile model (2) in the rapid cook-off calculation model with an air domain, that is, by changing the diameter of the charge model (6), and obtain the ignition time A of the charge model (6) at different diameters of the charge model (6) through calculation; Step 4: Establish a rapid cook-off calculation model without an air domain, and the rapid cook-off calculation model without an air domain only includes the test projectile model (2); the outer wall surface (7) of the shell model (5) of the test projectile model (2) uses a heat flux boundary as the heating boundary; according to this rapid cook-off calculation model without an air domain, under the condition that the diameter of the charge model (6) is the same as that in Step 3, calculate the ignition time B of the charge model (6), and by adjusting the size of the heat flux boundary, make the ignition time A in Step 3 equal to the ignition time B; finally, obtain the scatter point correspondence relationship between the heat flux boundary and the diameter of the corresponding charge model (6), and fit the scatter points into a curve; Step 5: According to any diameter of the charge model (6) and the heat flux boundary calculated according to the curve in Step 4, load the heat flux boundary onto the outer wall surface (7) of the test projectile model (2) for calculation, obtain the internal temperature change, ignition time, and ignition temperature of the charge model (6), and evaluate the rapid cook-off characteristics of the explosive; In Step 1, the test system includes: an oil fuel pool (8), a test projectile (14), a thermocouple (9), a support (10), a suspension rod (11), an ignition device (12), and a temperature recorder (13); the test projectile (14) includes: a shell (15) and a charge (16) loaded in the shell (15); the test projectile (14) is fixed on the support (10) through the suspension rod (11), the oil fuel pool (8) is located directly below the test projectile (14), and the distance between the lowest point of the test projectile (14) and the upper surface of the fuel in the oil fuel pool (8) is 300 mm, and the fuel in the oil fuel pool (8) is connected to the ignition device (12); the thermocouple (9) is installed outside the test projectile (14), and the thermocouple (9) is located on the extension line of the axis of the test projectile (14), and the closest distance between the thermocouple (9) and the outer end face of the test projectile (14) is 50 mm, and the temperature recorder (13) is electrically connected to the thermocouple (9); When conducting the rapid cook-off test, the ignition device (12) ignites the fuel oil. The fuel oil in the fuel pool (8) burns to generate flames and high-temperature gas, forming a flame field around the test bomb (14). The charge (16) of the test bomb (14) heats up in the flame field. When the temperature of the flame field is higher than the ignition point of the charge (16) of the test bomb (14), when the maximum temperature of the charge (16) of the test bomb (14) is higher than its ignition point, the charge (16) ignites; the thermocouple (9) measures the ambient temperature of the flame field where the test bomb (14) is located when the fuel oil in the fuel pool (8) burns, and the temperature recorder (13) records the change of the ambient temperature measured by the thermocouple (9) with time, obtaining the change curve of the ambient temperature of the test with time.

2. The evaluation method for rapid cook-off of an explosive according to claim 1, characterized in that In step two, the test bomb model (2) is exactly the same as the test bomb (14), including: a shell model (5) and a charge model (6) loaded in the shell model (5); the fuel inlet (1) is the high-temperature gas generated by the combustion of the fuel oil in the fuel pool (8). The boundary condition of the fuel inlet (1) is the mass flow inlet boundary, and the mass flow inlet boundary uses a temperature piecewise function and a mass flow rate to fit the test ambient temperature; the distance from the lowest point of the test bomb model (2) to the fuel inlet (1) is the same as the distance between the lowest point of the test bomb (14) and the upper surface of the fuel oil in the fuel pool (8); the boundary condition of the fuel outlet (3) is the pressure outlet boundary; by adjusting the gas mass flow rate and the temperature piecewise function to simulate the situation of the flame field generated by the combustion of the fuel oil in the fuel pool (8), the change curve of the ambient temperature of the flame field where the test bomb model (2) is located with time is calculated. If the difference in the ambient temperature at the same time point between the calculated change curve of the ambient temperature of the flame field where the test bomb model (2) is located with time and the change curve of the ambient temperature obtained from the test is within the set range, it indicates that the established calculation model can describe the flame field of the rapid cook-off test system.

3. A method for evaluating the fast cook-off of an explosive according to any one of claims 1-2, characterized in that, In step four, the single-phase exponential decay function in the Origin software is used to fit the scatter correspondence between the heat flux boundary and the diameter of the corresponding charge model (6).

4. A method for evaluating the rapid cook-off of an explosive according to any one of claims 1-2, characterized in that, In step four, the Matlab software is used to fit the scatter correspondence between the heat flux boundary and the diameter of the corresponding charge model (6).

5. The evaluation method for the fast cook-off of an explosive according to claim 3, characterized in that, In step four, the expression after fitting the scatter correspondence between the heat flux boundary and the diameter of the corresponding charge model (6) into a curve is as follows: Where, Φ is the heat flux boundary and D is the diameter of the charge model (6).

6. The evaluation method for the fast cook-off of an explosive according to claim 4, characterized in that In step four, the expression after fitting the scatter correspondence between the heat flux boundary and the diameter of the corresponding charge model (6) into a curve is as follows: Where, Φ is the heat flux boundary and D is the diameter of the charge model (6).

7. A method for evaluating the fast cook-off of an explosive according to any one of claims 1-2, characterized in that, Before step three, it is necessary to mesh the rapid cook-off calculation model with an air domain established in step two. The mesh size of the air domain (4) is divided into 100 mm, and the mesh size of the test bomb model (2) is divided into 2 mm.

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