Passive thermal protection metal spray pipe structure and method for engine

Abstract

The invention belongs to the field of spray pipes, and particularly relates to an engine passive thermal protection metal spray pipe structure and method. The method for designing the passive thermal protection metal spray pipe of the engine comprises the following steps that a working interface of the metal spray pipe in the spray pipe is determined; correcting the theoretical molded surface of the metal spray pipe; determining on-way wall thickness distribution of the metal spray pipe; and manufacturing the metal spray pipe designed through the steps, wherein the metal spray pipe is used for being installed on the downstream of a spray pipe working interface. According to the passive thermal protection metal spray pipe, high-temperature-resistant metal materials are adopted for replacing expensive ceramic matrix composite materials, and low-cost manufacturing and using of the engine spray pipe can be achieved. In addition, the passive thermal protection metal spray pipe can be suitable for repeated use of the engine, and therefore the single-time use cost of the engine spray pipe is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to the field of nozzles, and in particular relates to a metal nozzle structure and method for passive thermal protection of an engine. Background Art

[0002] Engine nozzles operate under high-temperature combustion gases (total temperature 2500K and above) and are exposed to harsh environments such as high-temperature oxidation, ablation, and erosion. Ceramic-based composite materials are often used for engine nozzles. The molding process of such materials is relatively complex, with a long cycle and high cost. After single or repeated use at high temperatures, the nozzles may suffer from irreversible problems such as ablation, peeling, and bulging on their surfaces, resulting in a high cost per use of the engine nozzles. Metal materials are generally not used as engine nozzles because their heat resistance requirements make it difficult to meet the working conditions of high-temperature combustion gases. Instead, they are usually used as active cooling and heat protection structures for engines. When metal materials are used as active cooling and heat protection structures for engines, the active cooling structures absorb heat under high-temperature combustion gases, causing the kerosene temperature along the engine structure to gradually increase. As the area of the active cooling structure increases, the temperature rise increases sharply, leading to problems such as kerosene coking and carbon deposition, which ultimately affect the normal operation of the engine.

[0003] In order to solve the above problems, a metal nozzle structure and method for engine passive thermal protection are proposed. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a metal nozzle structure and method for passive thermal protection of an engine that makes the metal reliable, low-cost and reusable in a high-temperature nozzle environment.

[0005] The present invention provides a metal nozzle structure for passive thermal protection of an engine, comprising a metal nozzle, the metal nozzle being mounted at the downstream end of a front section of the engine nozzle, the front section of the nozzle and the metal nozzle being combined to form the engine nozzle, the metal nozzle being designed by the following method, the method comprising the following steps: Determine the working interface of the metal nozzle in the nozzle: obtain the axial temperature distribution along the nozzle, and based on the allowable temperature of the metal material, determine the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material as the working interface of the metal nozzle; Correct the theoretical profile of the metal nozzle: determine the theoretical profile of the metal nozzle, use the correction model to correct the theoretical profile, so that the metal nozzle profile in the working state is consistent with the theoretical profile; Determine the wall thickness distribution along the metal nozzle: Based on the working pressure and temperature along the nozzle, as well as the allowable strength of the metal material at the working temperature, use the wall thickness distribution model to determine the wall thickness distribution along the nozzle; The metal nozzle designed through the above steps is manufactured, and the metal nozzle is used to be installed downstream of the nozzle working interface.

[0006] Furthermore, the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material is determined by the following steps: Calculate the heat flux density of the high-temperature gas to the nozzle wall at each position along the nozzle axis; Calculate the net radiation heat flux density between the nozzle outer wall and the external environment; The heat flux density of the high-temperature gas on the nozzle wall at each axial position of the nozzle is compared with the net radiation heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle where the heat flux density of the nozzle wall heat transfer and the net radiation heat flux between the nozzle outer wall and the external environment reach equilibrium is obtained. This position is the axial position where the nozzle temperature is lower than or equal to the allowable temperature of the metal material.

[0007] Furthermore, the heat flux density of the high-temperature gas to the nozzle wall at each position along the nozzle axis is calculated as follows: The heat flux density of the high-temperature gas to the nozzle wall in the axial direction of the nozzle is calculated using a model: ; in, q w is the heat flux density of the high-temperature gas to the nozzle wall in the axial direction of the nozzle; D t is the equivalent throat diameter; is the specific heat of gas at constant pressure (Btu / (lb.℉)); is the gas viscosity, ; M is the molecular weight of the gas mixture; Pr is the Prandtl number; ; k is the specific heat ratio of the gas mixture; p c,ns is the total pressure of the nozzle; g is the acceleration due to gravity; is the characteristic velocity; R is the curvature radius of the nozzle at the throat; A t is the cross-sectional area of the nozzle throat; A is any cross-sectional area along the nozzle; is the correction factor: ; Where: T wg : local wall temperature measured by hot gas; T c,ns : total temperature at the nozzle inlet; T aw : adiabatic wall temperature of gas; : Mach number of gas flow; The adiabatic wall temperature on the gas side is: ; R is the effective recovery coefficient; r is the local restitution coefficient, for laminar flow, For turbulent flow, .

[0008] Furthermore, the calculation of the net radiation heat flux density between the nozzle outer wall and the external environment includes: The net radiation heat flux density between the nozzle outer wall and the external environment is calculated using the model. ; Where, T w is the nozzle outer wall temperature, T 0 is the external ambient temperature, is the effective blackness of the nozzle outer surface, is the Stefan-Boltzmann constant for radiation.

[0009] Furthermore, the heat flux density of the high-temperature gas on the nozzle wall at each axial position of the nozzle is compared with the net radiation heat flux between the nozzle outer wall and the external environment to obtain the axial position of the nozzle when the heat flux density of the nozzle wall heat transfer and the net radiation heat flux between the nozzle outer wall and the external environment reach equilibrium. This position is the axial position where the nozzle temperature is lower than or equal to the allowable temperature of the metal material, including: When the heat flux density of the nozzle wall heat transfer and the net radiation heat flux density between the nozzle outer wall and the external environment reach equilibrium, q w = q r ,at this time, ; According to the allowable temperature of metal materials [T w ] upper limit, the corresponding axial section of the nozzle is the working interface of the metal nozzle.

[0010] Furthermore, the correction model is: ; Where, ( x i , y i ) is a coordinate point on the theoretical surface of the nozzle; ( x j , y j ) is the corresponding modified surface coordinate point; ɑ is the thermal expansion coefficient of the metal nozzle; T 0 is a point on the nozzle surface ( x i , y i ) operating temperature.

[0011] Furthermore, the wall thickness distribution model along the process is ; Where, is the minimum wall thickness at a certain position along the nozzle; is the working pressure at a certain position along the nozzle; is the diameter of the nozzle at a certain position along the nozzle; It is the allowable strength of the metal material at the working temperature along the axial direction of the corresponding nozzle.

[0012] Furthermore, manufacturing the metal nozzle designed by the above steps includes: A metal nozzle is prepared and the surface of the metal nozzle is subjected to blackening heat treatment.

[0013] The present invention also provides a metal nozzle design method for preparing a metal nozzle, the method comprising the following steps: Determine the working interface of the metal nozzle in the nozzle: obtain the axial temperature distribution along the nozzle, and based on the allowable temperature of the metal material, determine the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material as the working interface of the metal nozzle; Correct the theoretical profile of the metal nozzle: determine the theoretical profile of the metal nozzle, use the correction model to correct the theoretical profile, so that the metal nozzle profile in the working state is consistent with the theoretical profile; Determine the wall thickness distribution along the metal nozzle: Based on the working pressure and temperature along the nozzle, as well as the allowable strength of the metal material at the working temperature, use the wall thickness distribution model to determine the wall thickness distribution along the nozzle; The metal nozzle designed through the above steps is manufactured, and the metal nozzle is used to be installed downstream of the nozzle working interface.

[0014] The beneficial effect of the present invention is that the present invention proposes a metal nozzle structure and method for passive thermal protection of an engine. The interface of the metal nozzle is evaluated based on the internal gas heat exchange and external radiation heat dissipation of the nozzle and the allowable temperature of the metal material; and its theoretical profile is corrected to achieve that the nozzle profile in the working state (after thermal deformation) is consistent with the theoretical profile and the working performance of the nozzle is consistent with or close to the theoretical design state. This allows the passive thermal protection metal nozzle to replace expensive ceramic-based composite materials with high-temperature resistant metal materials, which can achieve low-cost manufacturing and use of engine nozzles. In addition, the passive thermal protection metal nozzle can be suitable for repeated use in the engine, thereby effectively reducing the single-use cost of the engine nozzle. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Attachment Figure 1 It is a schematic diagram of the process of the present invention; Attachment Figure 2 Schematic diagram of the structure of the nozzle in the present invention; Attachment Figure 3(a) is the heat flux density diagram of the high-temperature gas heat transfer to the nozzle wall at various positions along the nozzle axis; (b) is the net radiation heat flux diagram between the nozzle outer wall and the external environment; Attachment Figure 4 The figure shows the theoretical profile and the modified profile of the metal nozzle of the present invention. DETAILED DESCRIPTION

[0016] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0017] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0018] In addition, the terms "first," "second," and so on, used in this disclosure are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referenced. Thus, a feature specified as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of this disclosure, "plurality" means at least two, such as two or three, unless otherwise specifically defined.

[0019] In the present invention, unless otherwise specified or limited, the terms "connection" and "fixation" should be understood in a broad sense. For example, "fixation" can mean fixed connection, detachable connection, or integration; it can mean mechanical connection, electrical connection, physical connection, or wireless communication connection; it can mean direct connection or indirect connection through an intermediate medium; it can mean internal communication between two elements or interaction between two elements, unless otherwise specified. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0020] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in this field can implement it. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0021] As attached Figure 1 -Attached Figure 4As shown, the present invention provides a method for designing a passive thermal protection metal nozzle for an engine, comprising the following steps: Determine the working interface of the metal nozzle in the nozzle: obtain the axial temperature distribution along the nozzle, and based on the allowable temperature of the metal material, determine the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material as the working interface of the metal nozzle; Correct the theoretical profile of the metal nozzle: determine the theoretical profile of the metal nozzle, use the correction model to correct the theoretical profile, so that the metal nozzle profile in the working state is consistent with the theoretical profile. It should be noted that the metal nozzle profile in the working state does not need to be completely consistent with the theoretical profile, and the error can be within the allowable error; Determine the wall thickness distribution along the metal nozzle: Based on the working pressure and temperature along the nozzle, as well as the allowable strength of the metal material at the working temperature, use the wall thickness distribution model to determine the wall thickness distribution along the nozzle; The metal nozzle designed through the above steps is manufactured, and the metal nozzle is used to be installed downstream of the nozzle working interface.

[0022] The present invention proposes a design method suitable for passive thermal protection metal nozzles. Based on the internal gas heat exchange and external radiation heat dissipation of the nozzle and the allowable temperature of the metal material, the interface of the metal nozzle is evaluated; and its theoretical profile is corrected to ensure that the nozzle profile in the working state (after thermal deformation) is consistent with the theoretical profile and the working performance of the nozzle is consistent with or close to the theoretical design state. This allows the passive thermal protection metal nozzle to replace expensive ceramic-based composite materials with high-temperature resistant metal materials, which can achieve low-cost manufacturing and use of engine nozzles. In addition, the passive thermal protection metal nozzle can be applied to the engine for repeated use, thereby effectively reducing the cost of the engine nozzle per use.

[0023] As high-temperature combustion gases (total temperature 2500K and above) expand and perform work after passing through the nozzle, the enthalpy of the high-temperature gas gradually decreases, and the nozzle wall temperature gradually decreases as the nozzle profile expands. When the nozzle wall temperature is lower than the metal's critical temperature, metal materials can be used instead of ceramic-based composites. Furthermore, the working interface of the metal nozzle can be evaluated based on the external radiation heat dissipation during nozzle operation and the nozzle's equilibrium temperature.

[0024] Specifically, the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material is determined by the following steps: Calculate the heat flux density of the high-temperature gas to the nozzle wall at each position along the nozzle axis; Calculate the net radiation heat flux density between the nozzle outer wall and the external environment; The heat flux density of the high-temperature gas to the nozzle wall at each axial position of the nozzle is compared with the net radiation heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle where the heat flux density of the nozzle wall heat transfer and the net radiation heat flux density between the nozzle outer wall and the external environment reach equilibrium is obtained. This position is the axial position where the nozzle temperature is lower than or equal to the allowable temperature of the metal material.

[0025] In this embodiment, by accurately determining the starting position of the metal material for safe operation, the wall temperature of the metal nozzle section is strictly controlled below the allowable temperature during the entire operation process, effectively avoiding strength failure, excessive oxidation or ablation of the metal material due to temperature exceeding the limit.

[0026] In one embodiment, calculating the heat flux density of the high-temperature gas at each position along the nozzle axis for heat transfer to the nozzle wall includes: The heat flux density of the high-temperature gas to the nozzle wall in the axial direction of the nozzle is calculated using a model: ; in: q w is the heat flux density of the high-temperature gas to the nozzle wall in the axial direction of the nozzle; D t is the equivalent throat diameter; is the specific heat of gas at constant pressure (Btu / (lb.℉)); is the gas viscosity, ; M is the molecular weight of the gas mixture; Pr is the Prandtl number; ; k is the specific heat ratio of the gas mixture; p c,ns is the total pressure of the nozzle; g is the acceleration due to gravity; is the characteristic velocity; R is the curvature radius of the nozzle at the throat; A t is the cross-sectional area of the nozzle throat; A is any cross-sectional area along the nozzle; σ is the correction coefficient: ; Where, T wg : local wall temperature measured by hot gas; T c,ns : total temperature at the nozzle inlet; T aw : adiabatic wall temperature of gas; : Mach number of gas flow; Gas side adiabatic wall temperature: ; R is the effective recovery coefficient; r is the local restitution coefficient, for laminar flow, For turbulent flow, .

[0027] In this embodiment, the heat transfer theory of the engine high-temperature gas side wall and the external radiation heat dissipation theory of the high-temperature structure are used to calculate the heat transfer heat flux density of the high-temperature gas and the net radiation heat flux density between the nozzle outer wall and the external environment.

[0028] In one embodiment, calculating the net radiation heat flux density between the nozzle outer wall and the external environment includes: The net radiation heat flux density between the nozzle outer wall and the external environment is calculated using the model. ; Where, T w is the nozzle outer wall temperature, T 0 is the external ambient temperature, is the effective blackness of the nozzle outer surface, is the Stefan-Boltzmann constant for radiation.

[0029] In this embodiment, since the convective heat transfer on the outer wall of the nozzle is mainly radiation heat transfer, it is assumed that the external air is a gas of infinite thickness, and its emissivity is 1.

[0030] In one embodiment, reference Figure 3 , the heat flux density of the high-temperature gas on the nozzle wall at each axial position of the nozzle is compared with the net radiation heat flux density between the nozzle outer wall and the external environment, and the axial position of the nozzle is obtained when the heat flux density of the nozzle wall heat transfer and the net radiation heat flux between the nozzle outer wall and the external environment reach equilibrium. This position is the axial position where the nozzle temperature is lower than or equal to the allowable temperature of the metal material, including: When the heat flux density of the nozzle wall heat transfer and the net radiation heat flux between the nozzle outer wall and the external environment reach equilibrium, q w = q r ,at this time, ; According to the allowable temperature of metal materials [T w ] upper limit, the corresponding axial section of the nozzle is the working interface of the metal nozzle.

[0031] In this embodiment, when the heat flux density of the nozzle wall heat exchange reaches equilibrium with the net radiation heat flux between the nozzle outer wall and the external environment, q w = q r , ignoring the difference in area and temperature between the inner and outer walls of the nozzle, then . Based on this, the passive thermal protection metal nozzle can be calculated at the allowable temperature [T w ] When the upper limit is iThe interface position is the working interface, and a metal nozzle can be used from this position to the nozzle outlet end, and the nozzle front section, such as a metal active cooling structure, is still used from this position to the nozzle inlet end.

[0032] Because metal materials have a large thermal expansion coefficient, using a theoretical profile for nozzle structural design will result in deviations from the theoretical profile during operation (after thermal deformation), leading to performance deviations from the nozzle's designed state. Therefore, the nozzle profile must be corrected to ensure that the operating (after thermal deformation) profile is consistent with or close to the theoretical profile.

[0033] In one embodiment, the modified model is: ; Where: x i , y i ) is a coordinate point on the theoretical surface of the nozzle; ( x j , y j ) is the corresponding modified surface coordinate point; ɑ is the thermal expansion coefficient of the metal nozzle; T 0 is a point on the nozzle surface ( x i , y i ) operating temperature.

[0034] In this embodiment, considering the large thermal expansion coefficient of metal materials, Figure 4 The modified model is manufactured in the cold state ( x j , y j ) is applied at the expected operating temperature T 0 and thermal expansion coefficient ɑ Related relationships (1 - ɑT 0), pre-compensating the working thermal deformation. This makes the actual profile ( x i , y i ) can automatically restore to a near-ideal theoretical aerodynamic profile, effectively solving the problem of aerodynamic performance degradation caused by thermal deformation of the metal nozzle, and ensuring that it can maintain flow efficiency and thrust performance close to the design state under high-temperature working conditions.

[0035] Based on the equal strength design method of metal materials at high temperatures, the wall thickness distribution along the nozzle is determined according to the working pressure and working temperature along the nozzle, as well as the corresponding allowable strength of the metal material at the working temperature: In one embodiment, the wall thickness distribution model along the process is ; Where: is the minimum wall thickness at a certain position along the nozzle; is the working pressure at a certain position along the nozzle; is the diameter of the nozzle at a certain position along the nozzle; It is the allowable strength of the metal material at the working temperature along the axial direction of the corresponding nozzle.

[0036] In this embodiment, the model is based on the working pressure at each point along the nozzle. P i , structural dimensions D i And the actual allowable strength of the material at the working temperature at that point , accurately calculate the minimum wall thickness required This method solves the design challenges caused by uneven temperature and pressure distribution along the nozzle: thicker walls are automatically designed in high-temperature, high-pressure areas (where material strength is low) to ensure strength, while thinner walls are allowed in low-temperature, low-pressure areas (where material strength is high) to achieve lightweighting. This variable wall thickness design significantly reduces the weight of the metal nozzle while maintaining its overall structural strength, optimizing material utilization efficiency.

[0037] In one embodiment, manufacturing the metal nozzle designed by the above steps includes: The metal nozzle is manufactured using processes such as die pressing and spinning, due to its central axisymmetric structure, to achieve low-cost manufacturing. A blackening heat treatment can effectively increase the effective blackness of the metal nozzle surface, thereby enhancing its external radiation heat dissipation.

[0038] In this embodiment, a die-casting or spinning process is used, fully utilizing the axially symmetrical structural characteristics of the nozzle to achieve rapid and low-cost forming and manufacturing of the metal nozzle, significantly reducing manufacturing costs and cycle times. A blackening treatment is applied to the metal nozzle surface, effectively increasing its effective blackness and significantly enhancing its radiative heat dissipation capacity under high-temperature operating conditions. This low-cost surface treatment directly improves the passive thermal protection effectiveness of the metal nozzle without adding an additional cooling system or structural complexity, further ensuring its wall temperature control and reusability.

[0039] In one embodiment, the metal nozzle is made of a high-temperature resistant metal material, such as a high-temperature alloy, stainless iron or carbon steel.

[0040] The present invention also provides an engine passive thermal protection metal nozzle, which is designed using the above-mentioned method for designing an engine passive thermal protection metal nozzle.

[0041] In one embodiment, reference Figure 2 The metal nozzle is installed at the downstream end of the engine nozzle front section through a flange connection. The nozzle front section and the metal nozzle are combined to form the engine nozzle. The nozzle front section can be made of ceramic matrix composite materials and can also adopt a metal active cooling structure.

[0042] This metal nozzle structure, manufactured based on the aforementioned design method, offers the following advantages: 1. Utilizes low-cost metal materials instead of expensive ceramic-based composites; 2. Precise interface design and operating temperature control ensure the safety, reliability, and reusability of the metal sections; 3. Pre-compensation for thermal deformation of the profile ensures aerodynamic performance under high-temperature operating conditions; 4. A constant-strength, variable-thickness design achieves lightweight structure; and 5. Blackening enhances radiative heat dissipation. This passive thermally protected metal nozzle structure, combined with the nozzle front section (e.g., a metal active cooling structure), forms a combined nozzle structure that significantly reduces both overall manufacturing cost and the cost per use while ensuring high reliability and performance.

[0043] The above description is merely an embodiment and does not limit the present invention in any way. Any person skilled in the art can, without departing from the scope of the technical solution of the present invention, make many possible changes, modifications, or modifications to the technical solution of the present invention into equivalent embodiments with equivalent changes using the technical content disclosed above. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. A metal nozzle structure for passive thermal protection of an engine, characterized by: The metal nozzle is installed at the downstream end of the front section of the engine nozzle. The front section of the nozzle and the metal nozzle are combined to form the engine nozzle. The metal nozzle is designed by the following method, which includes the following steps: Determine the working interface of the metal nozzle in the nozzle: obtain the axial temperature distribution along the nozzle, and based on the allowable temperature of the metal material, determine the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material as the working interface of the metal nozzle; Correct the theoretical profile of the metal nozzle: determine the theoretical profile of the metal nozzle, use the correction model to correct the theoretical profile, so that the metal nozzle profile in the working state is consistent with the theoretical profile; Determine the wall thickness distribution along the metal nozzle: Based on the working pressure and temperature along the nozzle, as well as the allowable strength of the metal material at the working temperature, use the wall thickness distribution model to determine the wall thickness distribution along the nozzle; The metal nozzle designed through the above steps is manufactured, and the metal nozzle is used to be installed downstream of the nozzle working interface.

2. The engine passive thermal protection metal nozzle structure according to claim 1, characterized in that: The axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material is determined by the following steps: Calculate the heat flux density of the high-temperature gas to the nozzle wall at each position along the nozzle axis; Calculate the net radiation heat flux density between the nozzle outer wall and the external environment; The heat flux density of the high-temperature gas on the nozzle wall at each axial position of the nozzle is compared with the net radiation heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle where the heat flux density of the nozzle wall heat transfer and the net radiation heat flux between the nozzle outer wall and the external environment reach equilibrium is obtained. This position is the axial position where the nozzle temperature is lower than or equal to the allowable temperature of the metal material.

3. The engine passive thermal protection metal nozzle structure according to claim 2, characterized in that: The heat flux density for calculating the heat transfer between the high-temperature gas and the nozzle wall at each position along the nozzle axis includes: The heat flux density of the high-temperature gas to the nozzle wall in the axial direction of the nozzle is calculated using a model: ; in: is the heat flux density of the high-temperature gas to the nozzle wall in the axial direction of the nozzle; is the equivalent throat diameter; is the specific heat of gas at constant pressure (Btu / (lb.℉)); is the gas viscosity, ; M is the molecular weight of the gas mixture; Pr is the Prandtl number; ; k is the specific heat ratio of the gas mixture; is the total pressure of the nozzle; g is the acceleration due to gravity; is the characteristic velocity; R is the curvature radius of the nozzle at the throat; A t is the cross-sectional area of the nozzle throat; A is any cross-sectional area along the nozzle; is the correction factor: ； Where, : local wall temperature measured by hot gas; : nozzle inlet total temperature; : adiabatic wall temperature of gas; : Mach number of gas flow; The adiabatic wall temperature on the gas side is: ； R is the effective recovery coefficient; r is the local restitution coefficient, for laminar flow, For turbulent flow, .

4. The engine passive thermal protection metal nozzle structure according to claim 3, characterized in that: Calculating the net radiation heat flux density between the nozzle outer wall and the external environment includes: The net radiation heat flux density between the nozzle outer wall and the external environment is calculated using the model. ； Where, T w is the nozzle outer wall temperature, T 0 is the external ambient temperature, is the effective blackness of the nozzle outer surface, is the Stefan-Boltzmann constant for radiation.

5. The engine passive thermal protection metal nozzle structure according to claim 4, characterized in that: Compare the heat flux density of the high-temperature gas on the nozzle wall at each axial position of the nozzle with the net radiation heat flux between the nozzle outer wall and the external environment to obtain the axial position of the nozzle when the heat flux density of the nozzle wall heat transfer and the net radiation heat flux between the nozzle outer wall and the external environment reach equilibrium. This position is the axial position where the nozzle temperature is lower than or equal to the allowable temperature of the metal material, including: When the heat flux density of the nozzle wall heat transfer and the net radiation heat flux density between the nozzle outer wall and the external environment reach equilibrium, q w = q r ,at this time, ; According to the allowable temperature of metal materials [T w ] upper limit, the corresponding axial section of the nozzle is the working interface of the metal nozzle.

6. The engine passive thermal protection metal nozzle structure according to any one of claims 1 to 5, characterized in that: The modified model is: ； Where: x i , y i ) is a coordinate point on the theoretical surface of the nozzle; ( x j , y j ) is the corresponding modified surface coordinate point; ɑ is the thermal expansion coefficient of the metal nozzle; T 0 is a point on the nozzle surface ( x i , y i ) operating temperature.

7. The engine passive thermal protection metal nozzle structure according to any one of claims 1 to 5, characterized in that: The wall thickness distribution model along the process is: ； Where, is the minimum wall thickness at a certain position along the nozzle; is the working pressure at a certain position along the nozzle; is the diameter of the nozzle at a certain position along the nozzle; It is the allowable strength of the metal material at the working temperature along the axial direction of the corresponding nozzle.

8. The engine passive thermal protection metal nozzle structure according to any one of claims 1 to 5, characterized in that: Manufacturing the metal nozzle designed through the above steps includes: A metal nozzle is prepared and the surface of the metal nozzle is subjected to blackening heat treatment.

9. A metal nozzle design method, characterized in that: For preparing a metal nozzle, the method comprises the following steps: Determine the working interface of the metal nozzle in the nozzle: obtain the axial temperature distribution along the nozzle, and based on the allowable temperature of the metal material, determine the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material as the working interface of the metal nozzle; Correct the theoretical profile of the metal nozzle: determine the theoretical profile of the metal nozzle, use the correction model to correct the theoretical profile, so that the metal nozzle profile in the working state is consistent with the theoretical profile; Determine the wall thickness distribution along the metal nozzle: Based on the working pressure and temperature along the nozzle, as well as the allowable strength of the metal material at the working temperature, use the wall thickness distribution model to determine the wall thickness distribution along the nozzle; The metal nozzle designed through the above steps is manufactured, and the metal nozzle is used to be installed downstream of the nozzle working interface.

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