Ventilation parameter design method for supercavitating vehicle

Abstract

This invention discloses a method for designing ventilation parameters for a supercavitating vehicle. Referring to the vehicle's shape and speed, the required ventilation volumetric flow rate for generating a suitable cavitation bubble size is calculated using the ventilation rate formula. The gas density inside the bubble is determined based on the environmental pressure and required cavitation number. The combustion surface of the solid propellant grain in the gas generator is designed in conjunction with fuel characteristics. Supercavitating vehicles designed using this invention are launched from underwater UUVs and surface ships, primarily for countering underwater targets. The method of this invention can efficiently obtain the ventilation parameters required for generating the appropriate supercavitation size when the vehicle is operating under variable conditions. Furthermore, through reasonable propellant design, the supercavitation size of the vehicle can be easily changed to adapt to changes in operating conditions. This avoids the problem of the vehicle not being fully covered by a supercavitation size that is too small after changing operating conditions, as well as the waste and even performance degradation caused by a supercavitation size that is too large.

Description

Technical Field

[0001] This invention belongs to the field of supercavitation drag reduction technology, specifically relating to a method for designing ventilation parameters for supercavitating vehicles. Background Technology

[0002] As ship speeds increase, the penetration advantage of traditional underwater weapons, such as torpedoes, in naval warfare is gradually weakening. To effectively counter large surface ships, there is an urgent need to improve the speed of underwater weapons. Underwater weapons utilizing supercavitation drag reduction technology, relying on a unique hydrodynamic layout, inject non-condensable gas into the flow field to generate a supercavitating bubble, significantly reducing drag and achieving speeds exceeding 200 knots, thus attracting considerable attention.

[0003] To generate and maintain supercavitation, a continuous flow of gas is required to propagate through the surrounding flow field. Due to the large volume of gas required, high-temperature combustion gases generated from burning solid propellant are typically used as the gas source. Parameters such as airflow rate, speed, and depth of travel have a decisive impact on the morphology and size of the supercavitation. The morphology of the supercavitation, in turn, determines the hydrodynamic characteristics, control characteristics, and stability of the vehicle. Accurate prediction and control of the supercavitation morphology is one of the key aspects of the overall design of a supercavitating vehicle. Currently, the airflow rate required for ventilated supercavitating vehicles is mostly determined through simulation calculations, which consumes significant computing power and time. Furthermore, the airflow rates obtained from numerical simulations are mostly fixed values ​​applicable to a specific operating condition, unable to adapt to changes in the working environment, and there is no convenient method to provide variable airflow rates. Summary of the Invention

[0004] The purpose of this invention is to provide a method for designing ventilation parameters for supercavitating vehicles, which solves the problems of insufficient supercavitation size preventing the vehicle from being fully covered after changes in operating conditions, as well as the waste and even performance impact caused by excessively large supercavitation size.

[0005] The technical solution adopted in this invention is a method for designing ventilation parameters for a supercavitating vehicle, which is implemented according to the following steps:

[0006] Step 1: Construct a mathematical description method for ventilation volume based on a large amount of data. Referring to the shape and speed of the aircraft, calculate the ventilation volume flow rate required to generate a cavitation bubble of appropriate size according to the ventilation rate formula.

[0007] Step 2: Calculate the gas density inside the supercavitation bubble based on the ambient pressure and required cavitation number of the aircraft. Combine this with the ventilation volumetric flow rate obtained in Step 1 to calculate the ventilation mass flow rate.

[0008] Step 3: Based on the gas flow rate and fuel characteristics, design the combustion surface size of the solid propellant in the gas generator and the gas flow rate that end-face combustion can provide.

[0009] The invention is further characterized by:

[0010] Step 1 is as follows:

[0011] Step 1.1: Calculate the maximum diameter d of the aircraft based on its shape. t With cavitation diameter d c Ratio k1:

[0012]

[0013] Step 1.2: Set the required cavitation size so that the cavitation completely covers the vehicle body, and calculate the cavitation diameter D. max With cavitation diameter d t The ratio k2:

[0014]

[0015] Maximum cross-sectional area S of cavitation c Represented as:

[0016]

[0017] Step 1.3: Calculate the ventilation volumetric flow rate Q using the ventilation rate formula:

[0018]

[0019] Combining formulas (1) and (2), we get:

[0020]

[0021] Combining this with formula (3) and transforming it, we get:

[0022]

[0023] By transforming formula (6), we can obtain:

[0024]

[0025] Formula (7) gives the relationship between volumetric flow rate and velocity, and maximum cross-section of the cavitation bubble. The two parameters k1 and k2 are related to the ventilation rate, Cq / (k1 2 k2 2 ) is a constant value. Substituting this value into formula (7) yields:

[0026] Q = 0.0635VS c (8)

[0027] The specific process for calculating the gas density inside the ventilated supercavitation in step 2 is as follows:

[0028] Since the ventilation uses compressible air, based on the characteristics of gas pressure and density changes with ambient pressure within the cavitation bubble, the gas density within the ventilated supercavitation bubble is expressed as:

[0029]

[0030] in, R is the gas constant, M c P is the molar mass of the fuel gas; c is the gas pressure inside the bubble; T is the gas temperature.

[0031] The process of calculating the ventilation mass flow rate in step 2 is as follows:

[0032] The formula for cavitation number is:

[0033]

[0034] Where p ∞ Environmental pressure varies with the depth of the vessel, ρ is the density of water, and σ is the pressure. c For the specified vacuolation number;

[0035] The formula for calculating the internal pressure of the bubble after deformation is as follows:

[0036]

[0037] Combining formulas (8), (9), and (11), the ventilation mass flow rate formula is expressed as:

[0038]

[0039] In the formula, p ∞ This indicates that the spacecraft is under environmental pressure p. ∞ V represents the flow rate.

[0040] Step 3 is as follows:

[0041] Step 3.1: Calculate the required burn area size for stable navigation of the aircraft:

[0042] When the aircraft is sailing stably, the pressure and temperature inside the gas generator tend to be constant, and the mass flow rate of the discharged gas is also constant. Therefore:

[0043]

[0044] Where, ρ tui ρ is the density of the propellant, s is the size of the burning surface, and r is the burning rate of the propellant;

[0045] Combining equation (13) we have:

[0046]

[0047] Step 3.2: Calculate the required burnup surface size when the aircraft first starts generating supercavitation bubbles:

[0048] When the spacecraft first starts generating supercavitation bubbles, it needs to introduce three times the amount of gas required for stable navigation. Therefore, the initial burner surface size is:

[0049]

[0050] Step 3.3: Design the propellant charge:

[0051] The ventilation mass flow rate provided by end-face combustion The range is:

[0052]

[0053] Where s r This refers to the cross-sectional area of ​​the gas generator;

[0054] When s = s r At this time, fuel can be cast onto the wall at the end-face combustion section. r At that time, after the fuel was poured onto the wall, a flame-retardant layer was applied to the inner wall of the gunpowder tube to prevent it from burning.

[0055] The beneficial effects of this invention are:

[0056] This invention presents a method for designing ventilation parameters for supercavitating vehicles. Through the summarization and derivation of numerical simulation results, a formula for the ventilation rate under varying operating conditions is derived. A variable cross-section propellant loading method is used to control the ventilation volume under different operating conditions. Compared to traditional numerical simulation methods, this invention's method is more convenient and efficient, significantly reducing the investment of computing power and time. Furthermore, while numerical simulation methods generally only simulate stable operating conditions, this invention can quickly determine the required ventilation volume during varying operating conditions and transitions, enabling the supercavitating vehicle to adapt to changes in environmental pressure. Compared to cylinder-based gas supply, the variable cross-section solid propellant loading method in this invention eliminates the need for additional mixing mechanisms to achieve variations in the gas supply volume, is unaffected by changes in external pressure, and features a simple mechanism and stable gas supply. Attached Figure Description

[0057] Figure 1 This invention relates to a design method and implementation approach for ventilation parameters of a supercavitating vehicle, which uses numerical simulation to obtain a diagram showing the characteristics of cavitation morphology as a function of environmental pressure.

[0058] Figure 2 This invention relates to a design method and implementation approach for ventilation parameters of a supercavitating vehicle, which uses numerical simulation to obtain the characteristic diagram of the maximum diameter and length of the cavitation bubble as a function of environmental pressure. ​

[0059] Figure 3 The present invention relates to a design method and implementation approach for ventilation parameters of a supercavitating vehicle, which uses numerical simulation to obtain a graph showing the variation of gas pressure and density inside the bubble with environmental pressure.

[0060] Figure 4 This is a schematic diagram of the overall structure of a supercavitating supercavitating vehicle to which the supercavitating vehicle ventilation parameter design method and implementation approach of the present invention are applicable.

[0061] Figure 5 This is a schematic diagram of an embodiment of the supercavitating vehicle ventilation parameter design method and implementation approach of the present invention;

[0062] Figure 6 This is a cone-shaped cross-sectional view of the supercavitating vehicle ventilation parameter design method and implementation approach of the present invention;

[0063] Figure 7 This is a schematic diagram of the propellant loading in an embodiment of the design method and implementation approach for the ventilation parameters of a supercavitating vehicle according to the present invention. Detailed Implementation

[0064] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0065] The present invention provides a method for designing ventilation parameters for a supercavitating vehicle, which is implemented according to the following steps:

[0066] Step 1: Construct a mathematical description method for ventilation volume based on a large amount of data. Referring to the shape and speed of the aircraft, calculate the required ventilation volumetric flow rate to generate a cavitation bubble of appropriate size according to the ventilation rate formula; the specific process is as follows:

[0067] Step 1.1: Calculate the maximum diameter d of the aircraft based on its shape. t With cavitation diameter d c Ratio k1:

[0068]

[0069] Step 1.2: Set the required cavitation size so that the cavitation completely covers the vehicle body, and calculate the cavitation diameter D. max With cavitation diameter d t The ratio k2:

[0070]

[0071] Maximum cross-sectional area S of cavitation c Represented as:

[0072]

[0073] Step 1.3: Calculate the ventilation volumetric flow rate Q using the ventilation rate formula:

[0074]

[0075] Combining formulas (1) and (2), we get:

[0076]

[0077] Combining this with formula (3) and transforming it, we get:

[0078]

[0079] By transforming formula (6), we can obtain:

[0080]

[0081] Formula (7) gives the relationship between volumetric flow rate and velocity, and maximum cross-section of the cavitation bubble. The two parameters k1 and k2 are related to the ventilation rate, Cq / (k1 2 k2 2 ) is a constant value. Substituting this value into formula (7) yields:

[0082] Q = 0.0635VS c (8)

[0083] Step 2: Calculate the gas density inside the supercavitation bubble based on the ambient pressure and required cavitation number of the aircraft. Combine this with the ventilation volumetric flow rate obtained in Step 1 to calculate the ventilation mass flow rate.

[0084] In practical applications of supercavitating vehicles, changes in flight depth are involved. Formula (8) does not consider the impact of flight depth changes. To explore the relationship between supercavitating morphology and environmental pressure, a numerical simulation method was used to obtain the relationship between cavitation morphology size and environmental pressure. The simulation conditions were a ventilation rate of 0.93 and a Froude number of 148. Numerical simulations were performed on the supercavitation of the vehicle under a series of environmental pressure ranges. The variation of supercavitating morphology with environmental pressure is as follows: Figure 1 As shown. To quantitatively describe the cavitation size, the characteristics of the maximum diameter and length of the supercavitation as a function of ambient pressure are as follows: Figure 2 As shown in the figure, the elliptical shape and free-closing tail of the supercavitation bubble remain unchanged within a series of environmental pressure variations, but the supercavitation size decreases with increasing environmental pressure. When the environmental pressure increases from 0.15 MPa to 0.20 MPa, the maximum supercavitation diameter Dmax decreases from 6.1 dc to 5.7 dc, and the total length of the bubble decreases from 98.7 dc to 84.4 dc. When the environmental pressure continues to increase to 0.25 MPa, the maximum supercavitation diameter and total length decrease to 5.4 dc and 74.76 dc, respectively.

[0085] To investigate the mechanism by which environmental pressure affects cavitation size, a numerical simulation study was conducted. Figure 3 The characteristics of gas pressure and density inside the cavitation bubble as a function of ambient pressure are presented. As the ambient pressure increases, the pressure inside the cavitation bubble also increases accordingly. Since the air being vented is compressible air, its density is:

[0086]

[0087] in, R is the gas constant, M c P is the molar mass of the fuel gas; c The pressure of the gas inside the bubble is T; the gas temperature is T = 298.15 K at room temperature.

[0088] The introduced non-condensable gas is ambient air at room temperature, the same temperature as the surrounding water, and temperature changes caused by flow are negligible. The gas density within the bubble is positively correlated with the bubble pressure; an increase in bubble pressure leads to an increase in gas density. With a constant mass flow rate, an increase in gas density results in a decrease in the volumetric flow rate within the bubble. The size of the supercavitation bubble depends on the volumetric flow rate within it; since the increase in ambient pressure leads to a decrease in the volumetric flow rate, the bubble's shape and size decrease.

[0089] Therefore, the increase in environmental pressure did not change the venting pattern and cavitation venting mechanism of the high-speed ventilated supercavitation. Under constant ventilation volume and speed, the density within the cavitation changes approximately linearly with environmental pressure, and the cavitation size also changes linearly with environmental pressure. This indicates that the cavitation size is determined by the volumetric flow rate of the gas within the ventilated supercavitation. Therefore, for a ventilated supercavitation with the desired cavitation morphology, when environmental pressure changes, by changing the ventilation volume, as long as the volumetric flow rate remains constant, the supercavitation size will remain essentially unchanged.

[0090] The process for calculating ventilation mass flow rate is as follows:

[0091] The formula for cavitation number is:

[0092]

[0093] Where p ∞ Environmental pressure varies with the depth of the vessel, ρ is the density of water, and σ is the pressure. c σ is the specified vacuolation number. c =0.02;

[0094] The formula for calculating the internal pressure of the bubble after deformation is as follows:

[0095]

[0096] Combining formulas (8), (9), and (11), the ventilation mass flow rate formula is expressed as:

[0097]

[0098] In the formula, p ∞ This indicates that the aircraft is under environmental pressure p. ∞ V represents the flow rate, and the ventilation mass flow rate for generating the expected supercavitation morphology and size can be obtained according to formula (12).

[0099] When the vehicle descends at a constant speed from a depth of 10m to 20m, the descent path is approximately a straight line with a slope of -0.1. Figure 5 As shown, the environmental pressure under this working condition can be expressed as:

[0100] P ∞ =(2+0.01Vt) 下 P0, 0≤t 下 ≤0.98

[0101] Among them, t 下 The time taken for the vehicle to descend, initial depth P ∞ =2P0, P when the next row is completed ∞ =3P0, downlink time;

[0102] Therefore, the spacecraft is under environmental pressure p ∞ Flow velocity V, cavitation number σ to be obtained c For a supercavitation bubble of 0.02, the ventilation mass flow rate required to generate the expected supercavitation morphology and size is calculated according to formula (12), and the initial depth is obtained. When the downlink is completed

[0103] Step 3: Based on the gas flow rate and fuel characteristics, design the combustion surface size of the solid propellant grain in the gas generator and the gas flow rate that end-face combustion can provide. The specific process is as follows:

[0104] Step 3.1: Calculate the required burn area size for stable navigation of the aircraft.

[0105] The structure of the gas generator and gas delivery device that produces gas is shown in the attached figure. Figure 6 As shown, when the aircraft is sailing stably, the pressure and temperature inside the gas generator tend to be constant, and the mass flow rate of the discharged gas is also constant. Therefore:

[0106]

[0107] Where, ρ tui ρ is the density of the propellant, s is the size of the burning surface, and r is the burning rate of the propellant;

[0108] Combining equation (13) we have:

[0109]

[0110] Considering the ventilation duration, the solid propellant grains employ end-face combustion. During the descent phase, which involves uniform changes in the burning surface, propellant grains with the same burning rate are selected throughout this process. The end-face area of ​​the propellant grains is varied to adapt to the changes in the burning surface. A burning rate of 3.5 cm / s and a density of 1.8 g / cm³ are chosen. 3 For the solid propellant charge B, the required combustion surface s1 before descent is calculated to be 145.94 cm. 2 The required fuel surface area s2 for stable navigation after submerging is 298.91 cm. 2 The areas between s1 and s2 increase uniformly, and the transition distance perpendicular to the burning surface is l. 过 =t 下max r = 3.43 cm.

[0111] Step 3.2: Calculate the required burnup surface size when the aircraft first starts generating supercavitation bubbles:

[0112] When the spacecraft first starts generating supercavitation bubbles, it needs to introduce three times the amount of gas required for stable navigation. Therefore, the initial burner surface size is:

[0113]

[0114] Step 3.3: Design the propellant charge:

[0115] Because of the ventilation mass flow rate that end-face combustion can provide. The range is:

[0116]

[0117] Where s r This refers to the cross-sectional area of ​​the gas generator;

[0118] When s = s r At this time, fuel can be cast onto the wall at the end-face combustion section. r At that time, after the fuel was poured onto the wall, a flame-retardant layer was applied to the inner wall of the gunpowder tube to prevent it from burning.

[0119] Based on the calculation results in step 3.1, we know that s0 = 3s1 = 437.82cm 2 However, due to the limited internal space of the spacecraft, to make efficient use of space, the size of the burning surface needs to remain relatively constant. Therefore, it is more appropriate to select solid propellant grains with different burning rates at different times. In the initial stage of ventilation, the ventilation volume is relatively large, and a fuel with a burning rate of 5.0 cm / s and a density of 1.8 g / cm³ is selected. 3 For a solid propellant charge A, the required burning surface s0 = 306.48 cm² during this period is calculated. 2 ​The combustion section is designed with an end face, and fuel is poured into the wall to form a hollow cylinder. A flame-retardant layer is then applied to the inner wall of the propellant tube to ensure that the entire combustion process is end face combustion.

[0120] Final loading method as follows Figure 7 As shown.

[0121] This invention discloses a method for designing ventilation parameters for supercavitating vehicles. First, by summarizing numerical simulation results and deriving relevant formulas, the relationship between ventilation volume, vehicle size, and cavitation size is simplified to a constant. This ultimately yields the required ventilation volumetric flow rate to generate a cavitation of suitable size at a given speed. This method is simple and efficient, avoiding the significant computational effort required for extensive simulations. It is also easy to use and modify; when the size of the application changes, only the corresponding dimensionless coefficients need to be modified.

[0122] Secondly, considering the impact of changes in flight depth (i.e., environmental pressure) on ventilated supercavitating vehicles, the method of this invention summarizes the numerical simulation results (as shown in the appendix). Figure 1-3 As shown in the figure, the conclusion is drawn that "if the volumetric flow rate remains constant, the supercavitation size remains essentially unchanged." Through the derivation of relevant formulas, the calculation formula for the ventilation mass flow rate is obtained, preparing for the next step of proposing a gas supply method. This step studies and derives the condition that ensures the cavitation size does not change with pressure; that is, the ventilation volumetric flow rate required for the vehicle's constant-depth navigation, obtained in the first step of this invention, is equal to the ventilation volumetric flow rate required for the vehicle's variable-depth navigation, the difference being the change in the density of the introduced gas. This method directly addresses the essence of cavitation morphology changes under varying pressure environments, effectively solving the problem of cavitation morphology changes during variable-depth navigation, and eliminating the need for numerical simulation calculations for this more complex condition compared to constant-depth navigation.

[0123] Finally, to achieve the ventilation mass flow rate obtained in the second step, this invention employs a combustion variable cross-section solid propellant column gas supply method. The combustion gas flows from the combustion chamber through the equalization chamber and the gas guide pipe to the cavitation unit and cavitation bowl, and is then ejected (as shown in the attached diagram). Figure 6 , 7 By changing the burning rate and burning surface of the solid propellant, the ventilation mass flow rate can be changed quickly and accurately. Compared with gas cylinder supply, the variable cross-section solid propellant supply in this invention does not require an additional mixing mechanism to achieve changes in the gas supply volume. It is not affected by changes in external pressure, has a simple mechanism, and provides stable gas supply. It can achieve both gradual changes in ventilation mass flow rate to adapt to changes in escort depth and sudden changes in ventilation mass flow rate to meet the requirement of three times the ventilation volume at the initial ventilation time.

[0124] In summary, the simulation calculations required during the application of this invention are relatively small, the time required is short, and the implementation method is simple and convenient, which can meet the need for obtaining stable cavitation size when the supercavitating ventilated vehicle is sailing at varying depths.

Claims

1. A method for designing ventilation parameters for a supercavitating vehicle, characterized in that, The specific steps are as follows: Step 1: Construct a mathematical description method for ventilation volume based on a large amount of data. Referring to the shape and speed of the aircraft, calculate the ventilation volume flow rate required to generate a cavitation bubble of appropriate size according to the ventilation rate formula. Step 2: Calculate the gas density inside the supercavitation bubble based on the ambient pressure and required cavitation number of the aircraft. Combine this with the ventilation volume flow rate obtained in Step 1 to calculate the ventilation mass flow rate. Step 3: Based on the gas flow rate and fuel characteristics, design the combustion surface size of the solid propellant in the gas generator and the gas flow rate that end-face combustion can provide. For ventilated supercavitation with the expected cavitation morphology, when the ambient pressure changes, the supercavitation size remains basically unchanged as long as the volumetric flow rate is kept constant by changing the ventilation rate. By simplifying the relationship between ventilation volume, vehicle size, and cavitation size to a constant, the required ventilation volume flow rate to generate a cavitation of a suitable size at a certain speed was finally obtained. By changing the burning rate and burning surface of the solid propellant, the change in ventilation mass flow rate can be achieved quickly and accurately.

2. The method for designing ventilation parameters for a supercavitating vehicle according to claim 1, characterized in that, Step 1 is as follows: Step 1.1: Calculate the maximum diameter of the aircraft based on its shape. d t With cavitation diameter d c proportion : （1）； Step 1.2: Set the required cavitation size so that the cavitation completely covers the vehicle body, and calculate the cavitation diameter. D max With cavitation diameter d t proportion : （2）； Maximum cross-sectional area of ​​cavitation S c Represented as: （3）； Step 1.3: Calculate the ventilation volumetric flow rate using the ventilation rate formula. Q : （4）； in, Cq The ventilation coefficient; d c The diameter of the cavitation unit; Combining formulas (1) and (2), we get: （5）； in, k 1 represents the maximum diameter of the aircraft. d t With cavitation diameter d c ratio; Combining this with formula (3) and transforming it, we get: （6）； By transforming formula (6), we can obtain: （7）； Formula (7) gives the relationship between volumetric flow rate, velocity, and maximum cavitation cross-section. k 1 、k 2. Both parameters are related to ventilation rate. Cq / (k) 1 2 k 2 2 ） It is a constant value. Substituting this value into formula (7) yields: （8）。 3. The method for designing ventilation parameters for a supercavitating vehicle according to claim 2, characterized in that, The specific process for calculating the gas density inside the ventilated supercavitation in step 2 is as follows: Since the ventilation uses compressible air, based on the characteristics of gas pressure and density changes with ambient pressure within the cavitation bubble, the gas density within the ventilated supercavitation bubble is expressed as: （9）； in, , R The gas constant is... M c The molar mass of the combustion gas; This refers to the gas pressure inside the bubble; The temperature is the gas temperature.

4. The method for designing ventilation parameters for a supercavitating vehicle according to claim 3, characterized in that, The process of calculating the ventilation mass flow rate in step 2 is as follows: The formula for cavitation number is: （10）； in Environmental pressure varies with the depth at which the aircraft travels. The density of water, For the specified vacuolation number; The formula for calculating the internal pressure of the bubble after deformation is as follows: （11）； Combining formulas (8), (9), and (11), the ventilation mass flow rate formula is expressed as: （12） In the formula, Indicates the aircraft is under environmental pressure , V Indicates flow rate.

5. The method for designing ventilation parameters for a supercavitating vehicle according to claim 4, characterized in that, Step 3 is as follows: Step 3.1: Calculate the required burn area size for stable navigation of the aircraft. When the aircraft is sailing stably, the pressure and temperature inside the gas generator tend to be constant, and the mass flow rate of the discharged gas is also constant. Therefore: （13）； in, For the density of the propellant, The size of the burning surface, The burning rate of the propellant; Combining equation (13) we have: （14）； Step 3.2: Calculate the required burnup surface size when the aircraft first starts generating supercavitation bubbles: When the spacecraft first starts generating supercavitation bubbles, it needs to introduce three times the amount of gas required for stable navigation. Therefore, the initial burner surface size is: （15）； Step 3.3: Design the propellant charge: The ventilation mass flow rate provided by end-face combustion The range is: （16）； in This refers to the cross-sectional area of ​​the gas generator; when At this time, fuel can be cast onto the wall using an end-face combustion section. At that time, after the fuel was poured onto the wall, a flame-retardant layer was applied to the inner wall of the gunpowder tube to prevent it from burning.

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