Large scale ramjet fuel injection design method based on boundary layer similarity

Abstract

The present application relates to a kind of large-scale ramjet fuel injection design method based on boundary layer similarity, comprising: the basic configuration of ramjet is constructed, and by the scale variation of ramjet and the actual working condition and flow variation faced, the variation of boundary layer relative thickness in ramjet is qualitatively analyzed, and qualitative analysis result is obtained;Obtain flow field related information, capture characteristic quantity, and obtain characteristic scale information according to the basic configuration of ramjet;Boundary layer displacement thickness is calculated and corrected using scale effect phenomenon;According to general jet penetration depth formula, the corrected boundary layer displacement thickness and the jet penetration depth required for design are brought in, and fuel jet dynamic pressure ratio and jet orifice size are solved;The fuel jet dynamic pressure ratio and jet orifice size obtained are applied to the fuel injection design of ramjet, and the transverse jet injection correction scheme for the variation of boundary layer relative thickness is formed.

Description

Technical Field

[0001] This invention relates to the aerospace field, and more particularly to a large-scale ramjet engine fuel injection design method based on boundary layer similarity. Background Technology

[0002] As a power plant with significant application prospects in high-speed flight, the fuel injection scheme of scramjet engines plays a crucial role in engine performance. Currently, transverse jet fuel injection is a common method in scramjet engines, but this scheme has revealed a series of problems during actual operation, especially under low stoichiometric conditions, where it faces numerous challenges.

[0003] At lower equivalence ratios, the injection pressure and flow rate of the fuel jet are relatively low, resulting in a low dynamic pressure ratio between the jet and the incoming flow. In this situation, the injection penetration depth is extremely sensitive to changes in the relative thickness of the combustion chamber boundary layer. When the combustion chamber is scaled up proportionally on an overall scale, the relative thickness of the combustion chamber boundary layer changes accordingly. Under low equivalence ratio conditions, this change causes variations in the fuel jet penetration depth in the scaled-up engine, leading to alterations in fuel mixing, ignition, and combustion characteristics within the combustion chamber. Ultimately, this results in the engine operating conditions deviating from the original design conditions of the baseline engine. This deviation not only reduces engine performance but may also affect its reliability and stability, failing to meet the stringent performance requirements of actual flight missions.

[0004] Currently, scramjet engine combustion chambers widely employ a design scheme combining transverse fuel jet injection with a concave flame stabilizer. During engine operation, fuel is injected into the combustion chamber through a nozzle of a specific diameter at a preset injection pressure. However, existing fuel injection implementation schemes have significant shortcomings. The injection pressure and nozzle diameter of transverse fuel jet injection are usually determined based on experience or relatively simple scaled-down model experiments, without fully considering the boundary layer changes that real engines may encounter during actual operation, and lacking effective correction measures for these changes.

[0005] In fact, boundary layer variations have a significant impact on fuel penetration depth and mixing efficiency under low stoichiometric ratio conditions. Because the dynamic pressure ratio of the jet / incoming flow is low at this point, fuel injection is more susceptible to interference from the incoming flow. For example, changes in boundary layer thickness may cause the fuel jet's penetration trajectory to deviate from the expected path, resulting in uneven fuel distribution within the combustion chamber and consequently reducing the fuel-oxidizer mixing efficiency. This reduced mixing efficiency directly affects ignition and combustion processes, leading to unstable combustion, decreased combustion efficiency, and ultimately negatively impacting engine thrust performance.

[0006] Furthermore, fuel injection deviates from the design operating point due to boundary layer disturbances, easily triggering a series of combustion degradation phenomena, such as flame oscillation. Flame oscillation not only further reduces combustion efficiency, but its long-term effects may also damage the combustion chamber structure, shorten engine lifespan, and increase maintenance costs and operational risks.

[0007] Meanwhile, as the application scenarios of scramjet engines expand and their performance requirements continue to increase, the combustion chamber size may increase according to actual needs. However, existing injection design schemes cannot provide effective reference and guidance for dealing with the scale effects brought about by the increase in combustion chamber size. This makes it difficult to ensure that the engine can operate stably and efficiently at different scales in actual engineering design and application.

[0008] It is evident that existing transverse jet fuel injection schemes for scramjet engines have numerous shortcomings, severely limiting engine performance under real-world operating conditions. Therefore, there is an urgent need to develop a modified transverse jet injection scheme capable of handling variations in boundary layer thickness. This would address the scale effect issues arising from the increased size of the combustion chamber and drive further development of scramjet engine technology. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a design method for fuel injection of large-scale ramjet engines based on boundary layer similarity.

[0010] To achieve the above-mentioned objectives, this invention provides a large-scale ramjet engine fuel injection design method based on boundary layer similarity, comprising the following steps:

[0011] S1. Construct the basic configuration of the ramjet engine, and through the dimensional changes of the ramjet engine and the actual working conditions and flow changes it faces, conduct a qualitative analysis on the changes in the relative thickness of the boundary layer in the ramjet engine, and obtain the qualitative analysis results.

[0012] S2. Obtain flow field related information, capture characteristic quantities, and obtain characteristic scale information based on the basic configuration of the ramjet engine;

[0013] S3. Calculate and correct boundary layer displacement thickness using the scale effect phenomenon;

[0014] S4. Based on the general jet penetration depth formula, substitute the modified boundary layer displacement thickness and the jet penetration depth required by the design to solve for the fuel jet pressure ratio and jet nozzle size;

[0015] S5. The obtained fuel jet pressure ratio and jet orifice size are applied to the fuel injection design of the ramjet engine to form a lateral jet injection correction scheme for changes in the relative thickness of the boundary layer.

[0016] According to one aspect of the invention, in step S1, the step of constructing the basic configuration of the ramjet engine is a ramjet engine having a concave combustion chamber, and the jet nozzle for injecting fuel is located upstream of the concave combustion chamber.

[0017] According to one aspect of the present invention, in step S1, in the step of performing a qualitative analysis on the change of the relative thickness of the boundary layer in the ramjet engine and obtaining the qualitative analysis result, the qualitative analysis result is: after the combustion chamber in the ramjet engine is proportionally increased, the relative thickness of the boundary layer will become thinner, and based on the interaction between the combustion back pressure and the injection, the thinned boundary layer has an effect on the case where the dynamic pressure ratio is less than a preset value, wherein the dynamic pressure ratio is the ratio of the injection dynamic pressure to the incoming dynamic pressure.

[0018] According to one aspect of the present invention, in step S3, the step of calculating and correcting the boundary layer displacement thickness using the scale effect phenomenon is to calculate the boundary layer displacement thickness using the turbulent boundary layer 1 / 7 power law estimation method, and to correct the calculated boundary layer displacement thickness based on the qualitative analysis results.

[0019] According to one aspect of the present invention, step S3, the step of calculating and correcting the boundary layer displacement thickness using the scale effect phenomenon, includes:

[0020] S31. Based on the boundary layer 1 / 7 power law, the Reynolds number at the jet nozzle location is approximately calculated; and the Reynolds number is expressed as:

[0021] ;

[0022] in, Represents the Reynolds number. Indicates fluid density, Indicates fluid velocity. Indicates the characteristic length. Indicates the viscosity coefficient;

[0023] S32. Based on the obtained Reynolds number, the boundary layer displacement thickness is calculated using the wall law and velocity distribution law formulas; whereby the boundary layer displacement thickness is expressed as:

[0024] ;

[0025] in, Indicates the boundary layer displacement thickness. This represents a coefficient, and it is a constant. This represents a coefficient, and it is a constant. The characteristic length is defined as the flow distance from the ramjet engine inlet to the jet nozzle.

[0026] S33. Convert the qualitative analysis results into correction parameters, and use the correction parameters to correct the boundary layer displacement thickness based on empirical formulas; wherein, the correction parameters are flow field characteristic quantities or dimensionless numbers.

[0027] According to one aspect of the invention, in step S3, the step of correcting the boundary layer displacement thickness using the correction parameter based on an empirical formula, wherein the empirical formula is the van Driest and Spalding empirical formula or the Coles formula.

[0028] According to one aspect of the present invention, in step S4, the step of solving for the fuel jet kinematic ratio and jet orifice size by substituting the modified boundary layer displacement thickness and the designed jet penetration depth into the universal jet penetration depth formula, wherein the universal jet penetration depth formula is the Segal universal penetration depth formula, and is expressed as:

[0029] ;

[0030] in, Indicates the depth of jet penetration. This indicates the size of the jet nozzle, i.e., its diameter. Indicates the dynamic pressure ratio. Indicates the jet flow pressure. Indicates the incoming flow pressure. This represents the axial distance relative to the jet centerline. Indicates the molecular mass ratio. Indicates the molecular mass of the jet. The values ​​represent the molecular mass of the incoming flow, and A, B, C, D, E, F, and G represent empirical values ​​and are constants.

[0031] According to one aspect of the present invention, in step S5, the step of applying the obtained fuel jet pressure ratio and jet orifice size to the fuel injection design of the ramjet engine to form a lateral jet injection correction scheme for the relative thickness variation of the boundary layer is, based on the actual equivalence ratio requirement of the ramjet engine, a correction amount for the fuel jet and jet orifice size is formed based on the obtained fuel jet pressure ratio and jet orifice size, and the obtained correction amount is applied to the fuel injection design of the ramjet engine.

[0032] According to one aspect of the present invention, when the relative thickness of the combustion chamber boundary layer changes due to variations in engine dimensions, the present invention can fully ensure that the fuel injection penetration depth remains at the original design point of the reference engine, ensuring that the fuel jet can still maintain the penetration depth of the original design point even when affected by the boundary layer, thus achieving the ability to guarantee engine performance under low fuel equivalence ratio conditions.

[0033] According to one aspect of the present invention, the present invention can fully and effectively achieve the correction of transverse jet injection, effectively reducing the combustion deterioration problem that may be induced by boundary layer changes.

[0034] According to one aspect of the present invention, the present invention adopts a modified scheme to adjust the fuel injection design so as to fit the actual needs and operating conditions of the engine, effectively ensuring that the penetration depth of the fuel jet meets the requirements.

[0035] According to one aspect of the present invention, the present invention adopts an injection correction scheme based on the estimated boundary layer changes, thereby avoiding the drawback of fuel injection results not matching expectations due to changes in the relative thickness of the incoming boundary layer caused by changes in the combustion chamber size, and effectively ensuring the performance stability of the combustion chamber.

[0036] According to one aspect of the present invention, by matching the boundary layer, fuel injection orifice diameter and pressure, the present invention effectively corrects the influence of boundary layer changes on fuel injection under low stoichiometric conditions, and avoids combustion deterioration that may occur due to design errors.

[0037] According to one aspect of the present invention, when a scramjet engine faces the problem of scale effect due to its increased size, the present invention can provide a fuel injection correction scheme, so that the combustion characteristics of the large-scale combustion chamber are comparable to the experimental results of the scaled-down model, thereby guiding the design of the large-scale engine. Attached Figure Description

[0038] Figure 1 This is a flowchart illustrating the steps of the large-scale ramjet engine fuel injection design method based on boundary layer similarity of the present invention.

[0039] Figure 2 This is a partial structural diagram of the basic configuration of the ramjet engine of the present invention. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The embodiments cannot be described in detail here, but the embodiments of the present invention are not limited to the following embodiments.

[0041] like Figure 1 As shown, according to one embodiment of the present invention, a large-scale ramjet engine fuel injection design method based on boundary layer similarity includes the following steps:

[0042] S1. Construct the basic configuration of the ramjet engine, and through the dimensional changes of the ramjet engine and the actual working conditions and flow changes it faces, conduct a qualitative analysis on the changes in the relative thickness of the boundary layer in the ramjet engine, and obtain the qualitative analysis results.

[0043] S2. Obtain flow field related information, capture characteristic quantities, and obtain characteristic scale information based on the basic configuration of the ramjet engine;

[0044] S3. Calculate and correct boundary layer displacement thickness using the scale effect phenomenon;

[0045] S4. Based on the general jet penetration depth formula, substitute the modified boundary layer displacement thickness and the jet penetration depth required by the design to solve for the fuel jet pressure ratio and jet nozzle size;

[0046] S5. The obtained fuel jet pressure ratio and jet orifice size are applied to the fuel injection design of the ramjet engine to form a lateral jet injection correction scheme for changes in the relative thickness of the boundary layer.

[0047] like Figure 2 As shown, according to one embodiment of the present invention, in step S1, the basic configuration of the ramjet engine is a ramjet engine with a concave combustion chamber, and the jet nozzle for injecting fuel is located upstream of the concave combustion chamber.

[0048] According to one embodiment of the present invention, in step S1, the qualitative analysis of the change in the relative thickness of the boundary layer in the ramjet engine and the acquisition of the qualitative analysis results are performed. This qualitative analysis needs to be based on the actual operating conditions and flow changes that the designed ramjet engine will face, thereby accurately obtaining the variation law or trend of the relative thickness of the boundary layer. Specifically, based on the boundary layer similarity principle, changes in the combustion chamber size will cause changes in the relative thickness of the boundary layer, meaning that changes in the combustion chamber size have an impact on the scale effect. Specifically, when the combustion chamber is proportionally increased, the relative thickness of the boundary layer will decrease due to the influence of nonlinear factors that change with scale in the supersonic combustion flow field. However, changes in boundary layer thickness will affect the fuel injection effect within a certain equivalence ratio range, resulting in poor combustion chamber performance. Specifically, due to the interaction between combustion back pressure and fuel injection, this effect of thinning of the relative thickness of the boundary layer will be significant under low equivalence ratio conditions, i.e., when the dynamic pressure ratio of the jet / incoming flow is low. Therefore, the qualitative analysis results obtained are as follows: When the combustion chamber of a ramjet engine is proportionally enlarged, the relative thickness of the boundary layer will become thinner. Based on the interaction between combustion back pressure and injection, the thinner boundary layer has an impact on the case where the dynamic pressure ratio is less than the preset value (that is, the penetration depth of the fuel jet is reduced. This leads to a poorer mixing effect between the fuel and the incoming air, resulting in a decrease in the performance of the combustion chamber and a decrease in engine thrust). Here, the dynamic pressure ratio is the ratio of the jet pressure to the dynamic pressure of the incoming air.

[0049] It should be noted that the low equivalence ratio is related to the basic configuration of the ramjet engine. When the configuration of the combustion chamber and the fuel jet nozzles differs, the low equivalence ratio will change accordingly. In the case of a low equivalence ratio, there is no back pressure during combustion or the back pressure has not yet advanced to the upstream position. At this time, the boundary layer has a significant impact on the injection. Specifically, the low equivalence ratio is the lower limit of the equivalence ratio for a certain ramjet engine configuration. Below this limit, the combustion back pressure is very low, and the fuel jet will be affected by the upstream boundary layer. Above this limit, the combustion back pressure will be very high, and the boundary layer will separate upstream and no longer affect the fuel injection. For example, for a certain basic configuration of a ramjet engine, when the equivalence ratio is below 0.3, the relatively thinner boundary layer will affect the injection. The effect weakens continuously when the equivalence ratio is between 0.3 and 0.4, and there is no effect when the equivalence ratio is above 0.4. Therefore, the low equivalence ratio is the case of 0.3 and below.

[0050] According to one embodiment of the present invention, in step S2, the flow field related information is obtained, characteristic quantities are captured, and characteristic scale information is obtained based on the basic configuration of the ramjet engine. The flow field related information can be obtained experimentally or through numerical calculation. The captured characteristic quantities include fluid viscosity, density, incoming flow velocity, and fuel jet pressure. In this embodiment, in the step of obtaining flow field related information and capturing characteristic quantities, based on the qualitative analysis results obtained in step S1, basic flow field parameter information such as Mach number, total temperature, total pressure, and density can be obtained, and thus characteristic quantities such as Reynolds number Re and shock wave structure can be obtained. Further, in the step of obtaining characteristic scale information based on the basic configuration of the ramjet engine, the characteristic scale is determined based on the engine's basic configuration dimensions. For example, the engine length and inlet size determined by the basic configuration of the ramjet engine can determine a characteristic dimension. This characteristic dimension is the necessary characteristic scale information for calculating the boundary layer and is used in subsequent steps for calculating the boundary layer displacement thickness.

[0051] According to one embodiment of the present invention, in step S3, the step of calculating and correcting the boundary layer displacement thickness using the scale effect phenomenon, involves calculating the boundary layer displacement thickness using the 1 / 7th power law estimation method for turbulent boundary layers, and correcting the calculated boundary layer displacement thickness based on the qualitative analysis results. In this embodiment, since the flow channel inside the ramjet engine has high enthalpy supersonic flow characteristics, the boundary layer thickness calculation is implemented using the 1 / 7th power law estimation method for turbulent boundary layers. Therefore, step S3 includes:

[0052] S31. Based on the boundary layer 1 / 7 power law, the Reynolds number at the jet nozzle location is approximately calculated; and the Reynolds number is expressed as:

[0053] ;

[0054] in, Represents the Reynolds number. Indicates fluid density, Indicates fluid velocity. Indicates the characteristic length. This represents the viscosity coefficient.

[0055] S32. Based on the obtained Reynolds number, the boundary layer displacement thickness is calculated using the wall law and velocity distribution law formulas; whereby the boundary layer displacement thickness is expressed as:

[0056] ;

[0057] in, Indicates the boundary layer displacement thickness. This represents a coefficient, and it is a constant. This represents a coefficient, and it is a constant. The characteristic length is defined as the flow-direction distance from the ramjet engine inlet to the jet nozzle; in this embodiment, according to the 1 / 7 square law of turbulent boundary layer, the coefficient is... and They are respectively a =0.37, b =-0.2.

[0058] S33. The qualitative analysis results are converted into correction parameters, and the boundary layer displacement thickness is corrected using these parameters based on empirical formulas; where the correction parameters are flow field characteristic quantities or dimensionless numbers. By converting the qualitative analysis results into correction parameters, it is not necessary to introduce additional correction parameters during the correction of the boundary layer displacement thickness; the correction can be obtained simply through calculation, making the correction process of this scheme easier and more efficient.

[0059] In this embodiment, the step of converting the qualitative analysis results into correction parameters can be carried out based on a preset small-scale ramjet engine and a proportionally scaled large-scale ramjet engine. Specifically, in the actual flow field, the boundary layer thickness or Reynolds number is larger or smaller than the theoretically calculated value. Therefore, the correction parameters involved in the qualitative analysis results can be determined based on the qualitative experimental or simulation analysis, so as to correct the theoretical calculation results based on the actual flow field.

[0060] According to one embodiment of the present invention, in step S3, the step of correcting the boundary layer displacement thickness using correction parameters based on empirical formulas includes correcting for the Reynolds number change (e.g., increase) induced by the increase in combustion chamber size based on empirical formulas. Specifically, the empirical formulas are the van Driest and Spalding empirical formulas or the Coles formula. The van Driest and Spalding empirical formulas are expressed as follows:

[0061] ;

[0062] ;

[0063] in, Indicates the wall temperature. This indicates the temperature at the outer edge of the boundary layer. This represents the local friction coefficient. Indicates the local Reynolds number. Indicates the recovery factor. This represents the average friction coefficient.

[0064] Coles' formula is expressed as:

[0065] ;

[0066] in, Represents dimensionless velocity. This represents the velocity distribution function within the boundary layer. This represents the dimensionless wall distance. Indicates dimensionless enthalpy. Indicates the wake intensity parameter. Denotes the Kármán constant. Represents the wake function. Indicates the distance from the wall.

[0067] Based on the above settings, and using the classic smooth flat plate boundary layer formula, the influence of average wall friction resistance and temperature is introduced to fully fit the trend of compressible boundary layer changes under supersonic conditions. The velocity profile and thickness development of the boundary layer are fully corrected, making it suitable for flow conditions with high Mach numbers and high total temperature. This makes the correction results of this scheme closer to the actual values ​​of the real flow field, resulting in more accurate and reliable final design results.

[0068] According to one embodiment of the present invention, in step S4, the step of solving for the fuel jet pressure ratio and jet nozzle size by substituting the corrected boundary layer displacement thickness and the designed jet penetration depth into the general jet penetration depth formula involves determining prior conditions based on experience, experiments, or numerical simulations. Specifically, the designed jet penetration depth is determined based on the fuel type, flame stabilizer shape, rated operating conditions, and preset equivalence ratio of the ramjet engine. .

[0069] Furthermore, by increasing the required jet penetration depth Substituting the values ​​into the jet penetration depth formula, the jet penetration depth can be calculated. The corresponding jet flow pressure and nozzle diameter The relationship between them. In this embodiment, the universal jet penetration depth formula is the Segal universal penetration depth formula, and is expressed as:

[0070] ;

[0071] in, Indicates the depth of jet penetration. This indicates the size of the jet nozzle, i.e., its diameter. Indicates the dynamic pressure ratio. Indicates the jet flow pressure. Indicates the incoming flow pressure. This represents the axial distance relative to the jet centerline. Indicates the molecular mass ratio. Indicates the molecular mass of the jet. The values ​​represent the molecular mass of the incoming flow, and A, B, C, D, E, F, and G represent empirical values ​​and are constants.

[0072] According to one embodiment of the present invention, in step S5, the obtained fuel jet pneumatic ratio and jet orifice size are applied to the fuel injection design of the ramjet engine to form a lateral jet injection correction scheme for the relative thickness variation of the boundary layer. Based on the actual equivalence ratio requirement of the ramjet engine, a correction amount for the fuel jet and jet orifice size is formed based on the obtained fuel jet pneumatic ratio and jet orifice size, and this correction amount is applied to the fuel injection design of the ramjet engine. In this embodiment, experiments or numerical simulations can be performed based on the obtained lateral jet injection correction scheme. If the experimental or numerical simulation results do not meet the design requirements, steps S2 to S5 are further performed to iteratively update the lateral jet injection correction scheme, ensuring that the corrected jet penetration depth remains at the same original design point as the reference combustion chamber.

[0073] In this embodiment, when the relative thickness of the boundary layer in the ramjet engine changes, the aforementioned steps S2 to S5 can be repeated to dynamically update the transverse jet injection correction scheme, so as to continuously maintain the working performance of the ramjet engine.

[0074] The above description is merely an example of a specific solution of the present invention. For any devices and structures not described in detail herein, it should be understood that they are implemented using common devices and methods already available in the art.

[0075] The above description is merely one embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A fuel injection design method for large-scale ramjet engines based on boundary layer similarity, characterized in that, Includes the following steps: S1. Construct the basic configuration of the ramjet engine, and qualitatively analyze the changes in the relative thickness of the boundary layer in the ramjet engine through the dimensional changes of the ramjet engine and the actual working conditions and flow changes it faces, and obtain the qualitative analysis results. S2. Obtain flow field related information, capture characteristic quantities, and obtain characteristic scale information based on the basic configuration of the ramjet engine; S3. Calculation and correction of boundary layer displacement thickness using the scale effect phenomenon; including: S31. The Reynolds number at the jet nozzle location is approximately calculated based on the boundary layer 1 / 7 power law. S32. Based on the obtained Reynolds number, the boundary layer displacement thickness is calculated using the wall law and velocity distribution law formulas; S33. The qualitative analysis results are converted into correction parameters, and the boundary layer displacement thickness is corrected using the correction parameters based on empirical formulas. Specifically, qualitative experimental or simulation analyses are conducted based on a pre-defined small-scale ramjet engine and a proportionally scaled large-scale ramjet engine to determine the correction parameters involved in the qualitative analysis results. The correction parameters are then used to correct the Reynolds number change induced by the increase in combustion chamber size, thereby correcting the boundary layer displacement thickness. The correction parameters are flow field characteristic quantities or dimensionless numbers. The empirical formulas are the van Driest and Spalding empirical formulas. The flow field characteristic quantity of the correction parameters is temperature, and the dimensionless number is the friction coefficient. S4. Based on the general jet penetration depth formula, substitute the modified boundary layer displacement thickness and the jet penetration depth required by the design to solve for the fuel jet pressure ratio and jet nozzle size; S5. The obtained fuel jet pressure ratio and jet orifice size are applied to the fuel injection design of the ramjet engine to form a lateral jet injection correction scheme for changes in the relative thickness of the boundary layer.

2. The fuel injection design method for large-scale ramjet engines based on boundary layer similarity according to claim 1, characterized in that, In step S1, the basic configuration of the ramjet engine is a ramjet engine with a concave combustion chamber, and the jet nozzle for injecting fuel is located upstream of the concave combustion chamber.

3. The fuel injection design method for large-scale ramjet engines based on boundary layer similarity according to claim 2, characterized in that, In step S1, the qualitative analysis of the change in the relative thickness of the boundary layer in the ramjet engine is performed, and the qualitative analysis results are obtained. The qualitative analysis results are as follows: after the combustion chamber in the ramjet engine is proportionally increased, the relative thickness of the boundary layer will become thinner. Based on the interaction between combustion back pressure and injection, the thinned boundary layer has an impact on the dynamic pressure ratio being less than a preset value. The dynamic pressure ratio is the ratio of the injection dynamic pressure to the incoming dynamic pressure.

4. The fuel injection design method for large-scale ramjet engines based on boundary layer similarity according to claim 3, characterized in that, In step S31, where the Reynolds number at the jet nozzle location is approximately calculated based on the boundary layer 1 / 7 power law, the Reynolds number is expressed as: in, Represents the Reynolds number. Indicates fluid density, Indicates fluid velocity. Indicates the characteristic length. Indicates the viscosity coefficient; In step S32, where the boundary layer displacement thickness is calculated based on the obtained Reynolds number and using the wall law and velocity distribution law formulas, the boundary layer displacement thickness is expressed as: in, Indicates the boundary layer displacement thickness. This represents a coefficient, and it is a constant. This represents a coefficient, and it is a constant. The characteristic length is defined as the flow distance from the ramjet engine inlet to the jet nozzle.

5. The fuel injection design method for large-scale ramjet engines based on boundary layer similarity according to claim 4, characterized in that, In step S4, the general jet penetration depth formula is used to solve for the fuel jet kinematic pressure ratio and jet nozzle size by substituting the corrected boundary layer displacement thickness and the required jet penetration depth. The general jet penetration depth formula is the Segal general penetration depth formula, expressed as: in, Indicates the depth of jet penetration. This indicates the size of the jet nozzle, i.e., its diameter. Indicates the dynamic pressure ratio. Indicates the jet flow pressure. Indicates the incoming flow pressure. This represents the axial distance relative to the jet centerline. Indicates the molecular mass ratio. Indicates the molecular mass of the jet. The values ​​represent the molecular mass of the incoming flow, and A, B, C, D, E, F, and G represent empirical values ​​and are constants.

6. The fuel injection design method for large-scale ramjet engines based on boundary layer similarity according to claim 5, characterized in that, In step S5, the obtained fuel jet pressure ratio and jet orifice size are applied to the fuel injection design of the ramjet engine to form a lateral jet injection correction scheme for the relative thickness variation of the boundary layer. Based on the actual equivalence ratio requirement of the ramjet engine, a correction amount for the fuel jet and jet orifice size is formed based on the obtained fuel jet pressure ratio and jet orifice size, and the obtained correction amount is applied to the fuel injection design of the ramjet engine.

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