A passive thermal protection metal nozzle structure and method for an engine

Abstract

This invention belongs to the field of nozzles, specifically relating to a passive thermal protection metal nozzle structure and method for engines. The method for designing a passive thermal protection metal nozzle includes the following steps: determining the working interface of the metal nozzle; modifying the theoretical profile of the metal nozzle; determining the wall thickness distribution along the nozzle's travel; and manufacturing the metal nozzle designed through the above steps. The metal nozzle is installed downstream of the working interface of the nozzle. This invention achieves the use of high-temperature resistant metal materials instead of expensive ceramic matrix composites in the passive thermal protection metal nozzle, enabling low-cost manufacturing and use of engine nozzles. Furthermore, the passive thermal protection metal nozzle is suitable for repeated use in engines, thereby effectively reducing the per-use cost of engine nozzles.

Description

Technical Field

[0001] This invention belongs to the field of nozzles, specifically relating to a passive thermal protection metal nozzle structure and method for engines. Background Technology

[0002] Engine nozzles operate under high-temperature combustion gases (total temperature 2500K and above), and are subjected to harsh environments such as high-temperature oxidation, ablation, and erosion. Engine nozzles often utilize ceramic matrix composites, whose molding process is complex, time-consuming, and costly. After single or repeated use at high temperatures, the nozzle surface suffers irreversible problems such as ablation, peeling, and bulging, resulting in high per-use costs. Metal materials, due to their inability to meet the high-temperature combustion gas operating conditions, are generally not used for engine nozzles but are typically used in active cooling and heat protection structures. When metal materials are used as active cooling and heat protection structures, the active cooling structure absorbs heat under high-temperature combustion gases, causing the kerosene temperature along the engine structure to gradually rise. As the area of ​​the active cooling structure increases, the temperature rise increases dramatically, leading to problems such as kerosene coking and carbon buildup, ultimately affecting the normal operation of the engine.

[0003] To address the aforementioned issues, a passive thermal protection metal nozzle structure and method for engines are proposed. Summary of the Invention

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

[0005] This invention provides a passive thermal protection metal nozzle structure for an engine, comprising a metal nozzle installed at the downstream end of the front section of an 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:

[0006] Determine the working interface of the metal nozzle: Obtain the axial friction temperature distribution of the nozzle, and determine the working interface of the metal nozzle based on the allowable temperature of the metal material, with the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material.

[0007] Correcting the theoretical profile of the metal nozzle: Determine the theoretical profile of the metal nozzle, and use a correction model to correct the theoretical profile so that the metal nozzle profile in the working state is consistent with the theoretical profile.

[0008] Determine the friction wall thickness distribution of the metal nozzle: Based on the working pressure and temperature along the nozzle and the allowable strength of the metal material at the working temperature, the friction wall thickness distribution model is used to determine the friction wall thickness distribution of the nozzle.

[0009] Manufacture the metal nozzle designed through the above steps. The metal nozzle is used to install downstream of the nozzle working interface.

[0010] Furthermore, the axial position of the nozzle where the temperature is lower than or equal to the allowable temperature of the metallic material is determined by the following steps:

[0011] Calculate the heat flux density of the high-temperature gas transferred to the nozzle wall at each position along the nozzle axis.

[0012] Calculate the net radiative heat flux density between the nozzle outer wall and the external environment;

[0013] The heat flux density of the high-temperature gas transferred to the nozzle wall at various axial positions of the nozzle is compared with the net radiative heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle when the heat flux density transferred to the nozzle wall and the net radiative 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.

[0014] Furthermore, the calculation of the heat flux density of the high-temperature gas transferred to the nozzle wall at various axial positions of the nozzle includes:

[0015] The heat flux density of high-temperature combustion gas transferred to the nozzle wall in the axial direction of the nozzle is calculated using a model. The model is as follows:

[0016] ;

[0017] in, q w The heat flux density of the high-temperature gas transfer to the nozzle wall in the axial direction of the nozzle; D t Equivalent throat diameter; The specific heat of the gas at constant pressure (Btu / (lb.℉)); For the viscosity of gas, ; M The molecular weight of the gas mixture is given by ; Pr is the Prandtl number. k is the specific heat ratio of the gas mixture; p c,ns The total pressure of the nozzle is g; g is the acceleration due to gravity. R is the characteristic velocity; R is the radius of curvature of the nozzle at the throat; A t Let be the cross-sectional area of ​​the nozzle throat; A is any cross-sectional area along the nozzle path.

[0018] For correction factors:

[0019] ;

[0020] In the formula: T wg : Local wall temperature measured by thermal gas; T c,nsTotal temperature at the nozzle inlet; T aw : Insulation wall temperature of gas; : The Mach number of the gas flow;

[0021] The gas-side insulation wall temperature is:

[0022] ;

[0023] R The effective recovery coefficient; r The local coefficient of recovery is used for laminar flow. For turbulent flow, .

[0024] Furthermore, calculating the net radiative heat flux density between the nozzle outer wall and the external environment includes:

[0025] The net radiative heat flux density between the nozzle outer wall and the external environment is calculated using a model. The model is as follows:

[0026] ;

[0027] In the formula, T w Temperature of the nozzle outer wall T 0 represents the ambient temperature. The effective emissivity of the nozzle outer surface. is the Stefan-Boltzmann constant for radiation.

[0028] Furthermore, the heat flux density of high-temperature combustion gas transferred to the nozzle wall at various axial positions of the nozzle is compared with the net radiative heat flux between the nozzle outer wall and the external environment. This yields the axial position of the nozzle where the heat flux density transferred to the nozzle wall and the net radiative 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 metallic material, including:

[0029] When the heat flux density transferred to the nozzle wall reaches equilibrium with the net radiative heat flux density between the nozzle outer wall and the external environment, q w = q r ,at this time, ;

[0030] According to the allowable temperature [T] of the metallic material w The upper limit corresponds to the working interface of the metal nozzle axial section.

[0031] Furthermore, the modified model is as follows:

[0032] ;

[0033] In the formula, (x i , y i () represents a coordinate point on the theoretical surface of the spray system; x j , y j () represents the corresponding corrected surface coordinates. a is the coefficient of thermal expansion of the metal nozzle; T 0 represents a point on the nozzle profile. x i , y i The operating temperature.

[0034] Furthermore, the friction-line wall thickness distribution model is as follows:

[0035] ;

[0036] In the formula, This represents the minimum wall thickness at a certain position along the nozzle's travel. The working pressure at a certain position along the nozzle; The diameter of the nozzle at a certain position along the nozzle's travel; This represents the allowable strength of the metallic material at the operating temperature along the axial path of the corresponding nozzle.

[0037] Furthermore, manufacturing the metal nozzle designed through the above steps includes:

[0038] Prepare a metal nozzle and perform a blackening heat treatment on the surface of the metal nozzle.

[0039] The present invention also provides a method for designing a metal nozzle, for fabricating a metal nozzle, the method comprising the following steps:

[0040] Determine the working interface of the metal nozzle: Obtain the axial friction temperature distribution of the nozzle, and determine the working interface of the metal nozzle based on the allowable temperature of the metal material, with the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material.

[0041] Correcting the theoretical profile of the metal nozzle: Determine the theoretical profile of the metal nozzle, and use a correction model to correct the theoretical profile so that the metal nozzle profile in the working state is consistent with the theoretical profile.

[0042] Determine the friction wall thickness distribution of the metal nozzle: Based on the working pressure and temperature along the nozzle and the allowable strength of the metal material at the working temperature, the friction wall thickness distribution model is used to determine the friction wall thickness distribution of the nozzle.

[0043] Manufacture the metal nozzle designed through the above steps. The metal nozzle is used to install downstream of the nozzle working interface.

[0044] The beneficial effects of this invention are that it proposes a passive thermal protection metal nozzle structure and method for engines. Based on the internal combustion heat exchange and external radiative heat dissipation of the nozzle, as well as the allowable temperature of the metal material, the interface of the metal nozzle is evaluated. Its theoretical profile is then modified to ensure that the nozzle profile in the working state (after thermal deformation) matches the theoretical profile, and that the nozzle's performance is consistent with or close to the theoretical design. Furthermore, by using high-temperature resistant metal materials instead of expensive ceramic matrix composites, the passive thermal protection metal nozzle can achieve low-cost manufacturing and use of engine nozzles. In addition, the passive thermal protection metal nozzle is suitable for repeated use in engines, thereby effectively reducing the cost per use of engine nozzles. Attached Figure Description

[0045] Appendix Figure 1 This is a schematic diagram of the process of the present invention;

[0046] Appendix Figure 2 This is a schematic diagram of the nozzle structure in this invention;

[0047] Appendix Figure 3 (a) is a heat flux density diagram of high-temperature gas transferring heat to the nozzle wall at various positions along the nozzle axis; (b) is a net radiative heat flux diagram of the nozzle outer wall and the external environment.

[0048] Appendix Figure 4 The theoretical and modified profiles of the metal nozzle of this invention are shown. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0050] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0051] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0052] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection, an electrical connection, a physical connection, or a wireless communication connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two elements or the interaction between two elements, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0053] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0054] As attached Figure 1 -Appendix Figure 4 As shown, the present invention provides a method for designing a passive thermal protection metal nozzle for an engine, comprising the following steps:

[0055] Determine the working interface of the metal nozzle: Obtain the axial friction temperature distribution of the nozzle, and determine the working interface of the metal nozzle based on the allowable temperature of the metal material, with the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material.

[0056] Correcting the theoretical profile of the metal nozzle: Determine the theoretical profile of the metal nozzle, and use a 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 perfectly identical to the theoretical profile; the error can be within the allowable tolerance.

[0057] Determine the friction wall thickness distribution of the metal nozzle: Based on the working pressure and temperature along the nozzle and the allowable strength of the metal material at the working temperature, the friction wall thickness distribution model is used to determine the friction wall thickness distribution of the nozzle.

[0058] Manufacture the metal nozzle designed through the above steps. The metal nozzle is used to install downstream of the nozzle working interface.

[0059] This invention proposes a design method for passively thermally protected metal nozzles. Based on the internal combustion heat exchange and external radiative heat dissipation of the nozzle, as well as the allowable temperature of the metal material, the interface of the metal nozzle is evaluated. The theoretical profile is then modified to ensure that the nozzle profile in the working state (after thermal deformation) matches the theoretical profile, and that the nozzle's performance is consistent with or close to the theoretical design. This allows the use of high-temperature resistant metal materials instead of expensive ceramic matrix composites in passively thermally protected metal nozzles, enabling low-cost manufacturing and use of engine nozzles. Furthermore, passively thermally protected metal nozzles are suitable for repeated use in engines, effectively reducing the cost per use of engine nozzles.

[0060] As the high-temperature combustion gas (total temperature 2500K and above) expands and performs work after passing through the nozzle, the enthalpy of the high-temperature gas flow gradually decreases, and the nozzle wall temperature gradually decreases as the nozzle profile expands. When the nozzle wall temperature is lower than the allowable critical temperature of the metal, a metal material can be used to replace the ceramic matrix composite material. At the same time, the working interface of the metal nozzle can be evaluated based on the external radiative heat dissipation during the nozzle operation and the temperature when the nozzle is in equilibrium.

[0061] Specifically, the axial position of the nozzle where the temperature is lower than or equal to the allowable temperature of the metallic material is determined by the following steps:

[0062] Calculate the heat flux density of the high-temperature gas transferred to the nozzle wall at each position along the nozzle axis.

[0063] Calculate the net radiative heat flux density between the nozzle outer wall and the external environment;

[0064] The heat flux density of the high-temperature gas transferred to the nozzle wall at various axial positions of the nozzle is compared with the net radiative heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle when the heat flux density transferred to the nozzle wall and the net radiative 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.

[0065] 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 throughout the entire operation process, effectively avoiding metal material strength failure, excessive oxidation, or ablation caused by exceeding the temperature limit.

[0066] In one embodiment, calculating the heat flux density of the high-temperature gas at various axial positions of the nozzle to the nozzle wall includes:

[0067] The heat flux density of high-temperature combustion gas transferred to the nozzle wall in the axial direction of the nozzle is calculated using a model. The model is as follows: ;

[0068] in: qw The heat flux density of the high-temperature gas transfer to the nozzle wall in the axial direction of the nozzle; D t Equivalent throat diameter; The specific heat of the gas at constant pressure (Btu / (lb.℉)); For the viscosity of gas, ; M The molecular weight of the gas mixture is given by ; Pr is the Prandtl number. k is the specific heat ratio of the gas mixture; p c,ns The total pressure of the nozzle is g; g is the acceleration due to gravity. R is the characteristic velocity; R is the radius of curvature of the nozzle at the throat; A t Let be the cross-sectional area of ​​the nozzle throat; A be any cross-sectional area along the nozzle path; σ be a correction factor.

[0069] ;

[0070] In the formula, T wg : Local wall temperature measured by thermal gas; T c,ns Total temperature at the nozzle inlet; T aw : Insulation wall temperature of gas; : The Mach number of the gas flow;

[0071] Gas-side insulation wall temperature:

[0072] ;

[0073] R The effective recovery coefficient; r The local coefficient of recovery is used for laminar flow. For turbulent flow, .

[0074] In this embodiment, the heat transfer theory of the high-temperature gas sidewall of the engine and the 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 radiative heat flux density between the nozzle outer wall and the external environment.

[0075] In one embodiment, calculating the net radiative heat flux density between the nozzle outer wall and the external environment includes:

[0076] The net radiative heat flux density between the nozzle outer wall and the external environment is calculated using a model. The model is as follows:

[0077] ;

[0078] In the formula, T w Temperature of the nozzle outer wall T 0 represents the ambient temperature. The effective emissivity of the nozzle outer surface. is the Stefan-Boltzmann constant for radiation.

[0079] In this embodiment, since the convective heat transfer on the outer wall of the nozzle is mainly radiative heat transfer, assuming that the outside air is an infinitely thick gas, its emissivity is 1.

[0080] In one embodiment, reference Figure 3 The heat flux density of high-temperature combustion gas transferred to the nozzle wall at various axial positions of the nozzle is compared with the net radiative heat flux density between the nozzle outer wall and the external environment. This yields the axial position of the nozzle where the heat flux density transferred to the nozzle wall and the net radiative 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 metallic material, including:

[0081] When the heat flux density transferred to the nozzle wall reaches equilibrium with the net radiative heat flux between the nozzle outer wall and the external environment, q w = q r ,at this time, ;

[0082] According to the allowable temperature [T] of the metallic material w The upper limit corresponds to the working interface of the metal nozzle axial section.

[0083] In this embodiment, when the heat flux density of the nozzle wall reaches equilibrium with the net radiative heat flux between the nozzle outer wall and the external environment, q w = q r If we ignore the difference in surface area and temperature between the inner and outer walls of the nozzle, then This allows us to calculate the allowable temperature [T] for passively thermally protected metal nozzles. w At the upper limit, the corresponding section A i The interface position; this interface position is the working interface. 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 between this position and the nozzle inlet end.

[0084] Due to the large coefficient of thermal expansion of metallic materials, when designing nozzle structures using theoretical profiles, the nozzle profile under operating conditions (after thermal deformation) will deviate from the theoretical profile, thus deviating from the nozzle's performance under design conditions. Therefore, the nozzle profile needs to be modified to ensure that the nozzle profile under operating conditions (after thermal deformation) is consistent with or close to the theoretical profile.

[0085] In one embodiment, the modified model is:

[0086] ;

[0087] In the formula: ( x i , y i () represents a coordinate point on the theoretical surface of the spray system; x j , y j () represents the corresponding corrected surface coordinates. a is the coefficient of thermal expansion of the metal nozzle; T 0 represents a point on the nozzle profile. x i , y i The operating temperature.

[0088] In this embodiment, considering the large coefficient of thermal expansion of metallic materials, reference is made to... Figure 4 This modified model is achieved through cold-state manufacturing of the profile ( x j , y j Apply to the expected operating temperature T 0 and coefficient of thermal expansion a Correlation (1 - ɑT 0), pre-compensating for thermal deformation during operation. This ensures that the actual profile of the metal nozzle after thermal expansion at the operating temperature ( x i , y i It can automatically recover to a near-ideal theoretical aerodynamic profile, effectively solving the problem of aerodynamic performance degradation caused by thermal deformation of metal nozzles, and ensuring that it can maintain near-design flow efficiency and thrust performance under high-temperature working conditions.

[0089] Based on the constant strength design method for metallic materials at high temperatures, the nozzle's wall thickness distribution along the nozzle's friction path is determined according to the working pressure and temperature along the nozzle's friction path, as well as the allowable strength of the metallic material at the working temperature.

[0090] In one embodiment, the friction-line wall thickness distribution model is:

[0091] ;

[0092] In the formula: This represents the minimum wall thickness at a certain position along the nozzle's travel. The working pressure at a certain position along the nozzle; The diameter of the nozzle at a certain position along the nozzle's travel; This represents the allowable strength of the metallic material at the operating temperature along the axial path of the corresponding nozzle.

[0093] In this embodiment, the model is based on the working pressure borne at each point along the nozzle. P i Structural dimensions D i and the actual allowable strength of the material at that operating temperature. Accurately calculate the minimum wall thickness required. This method solves the design challenges caused by uneven temperature and pressure distribution along the nozzle: in high-temperature and high-pressure regions (where material strength is low), a thicker wall is automatically designed to ensure strength and safety; in low-temperature and low-pressure regions (where material strength is high), a thinner wall is allowed to achieve weight reduction. This variable wall thickness design significantly reduces the weight of the metal nozzle while ensuring the overall structural strength of the metal nozzle and optimizing material utilization efficiency.

[0094] In one embodiment, manufacturing the metal nozzle designed through the above steps includes:

[0095] For the fabrication of metal nozzles, due to their central axisymmetric structure, molding and spinning processes are preferred to achieve low-cost manufacturing. Furthermore, blackening heat treatment of the metal nozzle surface effectively improves its emissivity, thereby enhancing its external radiative heat dissipation.

[0096] In this embodiment, a molding or spinning process is employed, fully utilizing the axisymmetric structural characteristics of the nozzle to achieve rapid and low-cost forming and manufacturing of the metal nozzle, significantly reducing manufacturing costs and cycle time. Blackening the surface of the metal nozzle effectively improves its effective blackness, significantly enhancing its radiative heat dissipation capacity under high-temperature operating conditions. This low-cost surface treatment method directly improves the passive thermal protection performance of the metal nozzle without adding an extra cooling system or structural complexity, further ensuring its wall temperature control and reusability.

[0097] 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.

[0098] The present invention also provides a passive thermal protection metal nozzle for an engine, which is designed using the above-described method for designing a passive thermal protection metal nozzle for an engine.

[0099] In one embodiment, reference Figure 2 The metal nozzle is installed at the downstream end of the front section of the engine nozzle via a flange connection. The front section of the nozzle and the metal nozzle together form the engine nozzle. The front section of the nozzle can be made of ceramic matrix composite material or can adopt a metal active cooling structure.

[0100] This metal nozzle structure, manufactured based on the aforementioned design method, offers the following comprehensive advantages: 1. It utilizes low-cost metal materials to replace expensive ceramic matrix composites; 2. Through precise interface design and operating temperature control, it ensures the safety, reliability, and reusability of the metal section; 3. Through pre-compensation design for thermal deformation of the profile, it guarantees aerodynamic performance under high-temperature operating conditions; 4. Through equal-strength variable wall thickness design, it achieves structural lightweighting; 5. Through surface blackening treatment, it enhances radiative heat dissipation. This passive thermal protection metal nozzle structure, combined with the nozzle front section (such as a metal active cooling structure), forms a combined nozzle structure that significantly reduces overall manufacturing costs and single-use costs while ensuring high reliability and high performance.

[0101] The above description is merely an embodiment and does not constitute any limitation on the present invention. Any person skilled in the art can make many possible variations, modifications, or alterations to the technical solutions of the present invention without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention, without departing from the scope of the present invention, should fall within the protection scope of the present invention.

Claims

1. A passive thermal protection metal nozzle structure for an engine, characterized in that, The system includes a metal nozzle, which 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: Obtain the axial friction temperature distribution of the nozzle, and determine the working interface of the metal nozzle based on the allowable temperature of the metal material, with the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material. Correcting the theoretical profile of the metal nozzle: Determine the theoretical profile of the metal nozzle, and use a 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 friction wall thickness distribution of the metal nozzle: Based on the working pressure and temperature along the nozzle and the allowable strength of the metal material at the working temperature, the friction wall thickness distribution model is used to determine the friction wall thickness distribution of the nozzle. Manufacture the metal nozzle designed through the above steps. The metal nozzle is used to be installed downstream of the nozzle working interface. The axial position of the nozzle where the temperature is lower than or equal to the allowable temperature of the metallic material is determined by the following steps: Calculate the heat flux density of the high-temperature gas transferred to the nozzle wall at each position along the nozzle axis. Calculate the net radiative heat flux density between the nozzle outer wall and the external environment; The heat flux density of the high-temperature gas transferred to the nozzle wall at various axial positions of the nozzle is compared with the net radiative heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle when the heat flux density transferred to the nozzle wall and the net radiative 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.

2. The passive thermal protection metal nozzle structure for an engine as described in claim 1, characterized in that, The calculation of the heat flux density of high-temperature combustion gas transferred to the nozzle wall at various axial positions of the nozzle includes: The heat flux density of high-temperature combustion gas transferred to the nozzle wall in the axial direction of the nozzle is calculated using a model. The model is as follows: ； in: q w The heat flux density of the high-temperature gas transfer to the nozzle wall in the axial direction of the nozzle; D t Equivalent throat diameter; The specific heat of the gas at constant pressure (Btu / (lb.℉)). μ For the viscosity of gas, ; M The molecular weight of the gas mixture is given by ; Pr is the Prandtl number. k is the specific heat ratio of the gas mixture; p c,ns The total pressure of the nozzle is g; g is the acceleration due to gravity. R is the characteristic velocity; R is the radius of curvature of the nozzle at the throat; A t Let be the cross-sectional area of ​​the nozzle throat; A is any cross-sectional area along the nozzle path. σ is the correction factor: ； In the formula: T wg : Local wall temperature measured by thermal gas; T c,ns Total temperature at the nozzle inlet; T aw : Insulation wall temperature of gas; : The Mach number of the gas flow; The gas-side insulation wall temperature is: ； R The effective recovery coefficient; r The local coefficient of recovery is used for laminar flow. For turbulent flow, .

3. The passive thermal protection metal nozzle structure for engines as described in claim 2, characterized in that, The calculation of the net radiative heat flux density between the nozzle outer wall and the external environment includes: The net radiative heat flux density between the nozzle outer wall and the external environment is calculated using a model. The model is as follows: ； In the formula, T w Temperature of the nozzle outer wall T 0 represents the ambient temperature, ε represents the effective emissivity of the nozzle's outer surface, and σ represents the Stefan-Boltzmann constant of radiation.

4. The passive thermal protection metal nozzle structure for an engine as described in claim 3, characterized in that, The heat flux density of high-temperature combustion gas transferred to the nozzle wall at various axial positions of the nozzle is calculated and compared with the net radiative heat flux between the nozzle outer wall and the external environment. This yields the axial position of the nozzle where the heat flux density transferred to the nozzle wall and the net radiative 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 metallic material, including: When the heat flux density transferred to the nozzle wall reaches equilibrium with the net radiative heat flux density between the nozzle outer wall and the external environment, q w = q r ,at this time, ; According to the allowable temperature [T] of the metallic material w The upper limit corresponds to the working interface of the metal nozzle axial section.

5. The passive thermal protection metal nozzle structure for an engine as described in any one of claims 1-4, characterized in that, The corrected model is as follows: ； In the formula: ( x i , y i () represents a coordinate point on the theoretical surface of the spray system; x j , y j () represents the corresponding corrected surface coordinates. a is the coefficient of thermal expansion of the metal nozzle; T 0 represents a point on the nozzle profile. x i , y i The operating temperature.

6. The passive thermal protection metal nozzle structure for an engine as described in any one of claims 1-4, characterized in that, The friction-wall thickness distribution model is as follows: ； In the formula: δ i This represents the minimum wall thickness at a certain position along the nozzle's travel. P i The working pressure at a certain position along the nozzle; D i The diameter of the nozzle at a certain position along the nozzle's travel; σ i [ ] represents the allowable strength of the metallic material at the operating temperature along the axial path of the corresponding nozzle.

7. The passive thermal protection metal nozzle structure for an engine as described in any one of claims 1-4, characterized in that, Manufacturing the metal nozzle designed through the above steps includes: Prepare a metal nozzle and perform a blackening heat treatment on the surface of the metal nozzle.

8. A method for designing a metal nozzle, characterized in that, The method for preparing a metal nozzle includes the following steps: Determine the working interface of the metal nozzle: Obtain the axial friction temperature distribution of the nozzle, and determine the working interface of the metal nozzle based on the allowable temperature of the metal material, with the axial position on the nozzle where the temperature is lower than or equal to the allowable temperature of the metal material. Correcting the theoretical profile of the metal nozzle: Determine the theoretical profile of the metal nozzle, and use a 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 friction wall thickness distribution of the metal nozzle: Based on the working pressure and temperature along the nozzle and the allowable strength of the metal material at the working temperature, the friction wall thickness distribution model is used to determine the friction wall thickness distribution of the nozzle. Manufacture the metal nozzle designed through the above steps. The metal nozzle is used to be installed downstream of the nozzle working interface. The axial position of the nozzle where the temperature is lower than or equal to the allowable temperature of the metallic material is determined by the following steps: Calculate the heat flux density of the high-temperature gas transferred to the nozzle wall at each position along the nozzle axis. Calculate the net radiative heat flux density between the nozzle outer wall and the external environment; The heat flux density of the high-temperature gas transferred to the nozzle wall at various axial positions of the nozzle is compared with the net radiative heat flux density between the nozzle outer wall and the external environment. The axial position of the nozzle when the heat flux density transferred to the nozzle wall and the net radiative 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.

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