A low-pressure rotor fulcrum load design method for a turbofan engine fan blade loss state

Abstract

The application discloses a low-pressure rotor fulcrum load design method for a fan blade flying-off state of a turbofan engine and belongs to the technical field of aero-engines. The method firstly determines the maximum unbalance force of the fan blade flying-off, obtains the axial distance of the blade centroid and each fulcrum relative to the main and auxiliary mounting sections, and then calculates the unbalance load of each fulcrum. On this basis, the transient load coefficient considering the transient impact effect and the material strain coefficient considering the material high strain rate effect are introduced to correct the load of each fulcrum, obtain the design fulcrum load, and finally optimize the fan blade mass, centroid radius or fulcrum layout based on the strength reserve evaluation. The application can quickly analyze and optimize the low-pressure rotor fulcrum load in the early stage of engine scheme design, and ensure the safety of the supporting structure under the blade flying-off working condition.

Description

Technical Field

[0001] This invention relates to the field of aero-engine technology, specifically to a method for designing low-pressure rotor support loads under the condition of fan blade loss in a turbofan engine. Background Technology

[0002] Blade loss is a typical severe load condition that aero-engines may encounter, referring to the breakage and ejection of part or all of the rotor blades. It can be caused by various factors, including blade or tenon fatigue failure, large bird strikes, or impacts from other foreign objects, and can generate enormous impact loads on the engine support structure. Both domestic and international aero-engine strength design guidelines clearly require that the engine support structure must be able to withstand blade loss loads without overall failure.

[0003] For turbofan engines, the mass of the fan blades located in the low-pressure rotor is much greater than that of the compressor and turbine blades, and the load caused by fan blade misfire is the most severe. A typical low-pressure rotor structure of a turbofan engine is as follows: Figure 2 As shown, the flyaway load of the fan blades is borne jointly by the support structures at supports 1, 2, and 5. Therefore, it is essential to accurately assess and control the loads at each support point during the design of the low-pressure rotor's loads under the flyaway load of the fan blades to ensure the engine support structure remains intact and to guarantee structural safety.

[0004] Currently, existing technologies mostly use simplified whole-machine finite element models for blade fly-off load design, which has the problems of complex models and long calculation time, making it difficult to meet the urgent need for rapid iteration and optimization design of rotor support load in the early stage of engine design. Summary of the Invention

[0005] The purpose of this invention is to provide a design method for the load on the support point of the low-pressure rotor under the condition of fan blade loss in a turbofan engine, so as to realize the rapid calculation and optimization design of the load on each support point of the low-pressure rotor under the condition of fan blade loss, and significantly improve the design efficiency.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for designing low-pressure rotor support loads under fan blade fly-off conditions in turbofan engines, comprising: S1. Determine the maximum unbalanced force F caused by the fan blades flying off; S2. Obtain the centroid of the fan blade with the largest mass and the axial distance of each support point of the low-pressure rotor relative to the main mounting section and the auxiliary mounting section; S3. Based on the maximum unbalanced force and the axial distance, calculate the unbalanced load at each support point of the low-pressure rotor under the blade fly-off state; S4. Determine the transient load coefficients for each support point; S5. Determine the material strain coefficients at each support point; S6. Calculate the design load of each support point under the blade fly-off state based on the unbalanced load, transient load coefficient and material strain coefficient of each support point. S7. Calculate the tensile strength reserve of each support structure based on the design support load; S8. Determine whether the tensile strength reserve of each support structure meets the preset requirements. S9. If not satisfied, optimize the design: If the tensile strength reserve of all support structures does not meet the requirements, adjust the mass or centroid radius of the fan blades and iteratively execute steps S1 to S8; if only some supports do not meet the requirements, adjust the axial distance between the supports that do not meet the requirements and the main mounting section and / or auxiliary mounting section, and iteratively execute steps S1 to S8 until all supports meet the requirements.

[0008] Furthermore, in step S1, the maximum unbalanced force ,in The mass of the fan rotor blade with the largest mass. The radius of the centroid of the fan rotor blade with the largest mass. This represents the highest transient speed of the fan rotor.

[0009] In step S3, the support points of the low-pressure rotor include support point 1, support point 2, and support point 5. The calculation of the unbalanced load of each support point of the low-pressure rotor under the fan blade fly-off state is specifically as follows: based on the force balance equation, the moment balance equation around the main mounting section, and the moment balance equation around the auxiliary mounting section, the radial unbalanced load of each support point under the fan blade fly-off state is obtained by solving them simultaneously.

[0010] In step S4, the transient load coefficient is derived based on energy conservation, reflecting the influence of the supporting structure mass, radial dimension, material elongation, and impact time during the impact process.

[0011] In step S5, the material strain coefficient is introduced into the Johnson-Cook constitutive model to reflect the strength enhancement effect of the material under high strain rate.

[0012] Step S6 multiplies the transient load coefficient, material strain coefficient, and unbalanced load of each support point under the fan blade fly-off state to obtain the final design support point load.

[0013] Step S7 uses the ratio of the material's tensile strength to its maximum equivalent stress as the tensile strength reserve.

[0014] Step S8 requires that the tensile strength reserve of each support structure be greater than 1.0.

[0015] The optimization strategy in step S9 improves the tensile strength reserve by reducing the unbalanced force source (mass or center of mass) or shortening the force transmission path (distance between the fulcrum and the installation section) according to different unsatisfactory conditions, and the iteration converges quickly.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention calculates the unbalanced loads at each support point under the fan blade fly-off state, and considers the transient impact coefficient and material strain rate coefficient. It enables rapid evaluation and optimization of the support point loads in the early stages of engine design, guiding the rational design of the low-pressure rotor fan blades and low-pressure rotor support point layout. This significantly shortens the design cycle and ensures the strength and safety of the engine support structure under the fan blade fly-off state. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart illustrating the design method for low-pressure rotor support load under the condition of fan blade loss in a turbofan engine, according to an embodiment of the present invention. Figure 2 This is a schematic diagram of a typical low-pressure rotor structure of a turbofan engine in an embodiment of the present invention. Detailed Implementation

[0019] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0020] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, in the absence of conflict, the following embodiments and features in the embodiments can be combined with each other. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] This invention provides a method for designing the low-pressure rotor support load under fan blade loss conditions in a turbofan engine. This invention uses the low-pressure rotor of a certain type of turbofan engine as an example. Figure 2The structural layout is shown, including pivot points 1, 2, and 5, as well as the first-stage and second-stage fan rotor blades. The origin of the low-pressure rotor is defined as the starting point, with the reverse direction as the positive direction. The axial coordinates of key axes are obtained: the axial coordinate of pivot point 1 is... The axial coordinate of fulcrum 2 is The axial coordinate of fulcrum 5 is The axial coordinate of the centroid of the largest mass fan blade is The axial coordinate of the main mounting section is The axial coordinate of the auxiliary installation section is .like Figure 1 As shown, the method mainly includes the following steps: Step S1: Calculate the maximum unbalanced force F caused by fan blade slippage. Based on the engine fan rotor blade model, select the fan rotor blade with the largest mass, usually the first-stage rotor blade, and measure its mass. The distance from the center of mass to the axis of rotation is the radius of the center of mass. Determine the highest transient speed of the fan rotor. The maximum unbalanced centrifugal force generated by the loss of fan blades is:

[0022] in, The maximum unbalanced force, N, is the force at which the fan blades fly off. The mass of the largest fan rotor blade, in kg; Let be the radius of the centroid of the fan rotor blade with the largest mass, in meters. is the highest transient speed of the fan rotor, in rad / s.

[0023] Step S2: Calculate the relevant axial distances Calculate the axial distance of each key point relative to the main and auxiliary mounting sections to obtain the centroid of the fan blade with the largest mass and the axial distance of each support point of the low-pressure rotor relative to the main and auxiliary mounting sections.

[0024] Specifically, based on the aforementioned axial coordinates, calculate the axial distance of each key point relative to the main and auxiliary mounting sections: The axial distance between the center of mass of the heaviest fan blade and the main mounting section (m): ; The axial distance between the center of mass of the heaviest fan blade and the auxiliary mounting section (m): ; Axial distance between support point 1 and main mounting section (m): ; Axial distance between support point 1 and auxiliary installation section (m): ; Axial distance between support point 2 and main mounting section (m): ; Axial distance between support point 2 and auxiliary installation section (m): ; Axial distance between support point 5 and main mounting section (m): ; Axial distance between support point 5 and auxiliary installation section (m): .

[0025] Step S3: Calculate the unbalanced loads at each support point of the low-pressure rotor. The low-pressure rotor is supported radially only by supports 1, 2, and 5. In the event of fan blade loss, assume that the entire unbalanced force F is borne by these three supports, and that the main and auxiliary mounting sections provide support reaction moments for constraint. Establish three sets of equilibrium equations: Force balance: ; Torque balance around the main mounting section: ; Torque balance of auxiliary installation section: ; By combining the above equilibrium equations, the radial unbalanced loads at each support point under the fan blade runaway state can be obtained directly:

[0026]

[0027]

[0028] in, Let N be the unbalanced force at fulcrum 1 when the fan blades are lost. The unbalanced force at fulcrum 2 when the fan blades are lost is N; Let N be the unbalanced force at fulcrum 5 when the fan blades are out of control.

[0029] Step S4: Determine the transient load coefficients at each support point. Since the unbalanced force of fan blade fly-off is a steady-state load, blade fly-off is actually a transient impact process. The load borne by the fulcrum is greater than the steady-state unbalanced force. Therefore, it is necessary to determine the transient load coefficient.

[0030] Specifically, the load acting on the fulcrum is entirely absorbed by the supporting structure. For the i-th fulcrum, assuming the transient impact force remains constant during the impact, according to the principle of energy conservation, we can obtain:

[0031]

[0032]

[0033] in, N represents the transient impact load acting on support point i. Let m be the maximum deformation of the structure supported by fulcrum i under transient impact load; Let the mass of the structure supported by the i-th fulcrum be in kg; Let be the velocity generated by the i-th support structure under transient impact load, in m / s; Let N be the unbalanced force at the i-th fulcrum when the fan blades are out of control.

[0034] Will Substituting into the transient formula above, we get:

[0035] Furthermore, Therefore, the formula for the transient load coefficient of each support point can be obtained as follows:

[0036] in, Let be the radial dimension of the structure supported by the i-th fulcrum, in meters (m). t represents the elongation of the structural material supporting the i-th fulcrum; t represents the transient impact time, in seconds.

[0037] Step S5: Determine the material strain coefficients at each support point. Since the mechanical properties of materials differ greatly between quasi-static and dynamic states, and the strain rate affects the mechanical properties of material deformation, the supporting structure material exhibits strength characteristics under high strain rate in the blade fly-off state, and its yield strength is significantly higher than that in the quasi-static state. Therefore, it is necessary to determine the material strain coefficient.

[0038] In this embodiment of the invention, the Johnson-Cook constitutive model is used, and the effects of temperature and strain hardening are ignored. The ratio of dynamic to quasi-static flow stress is then obtained as the material strain coefficient. In this case, the equivalent plastic strain rate can be estimated through the deformation rate. Substituting into the model, we get:

[0039] in, Let be the initial yield strength of the structural material supported by fulcrum i, in MPa; is the strain hardening coefficient of the structural material supported by the i-th support point; The strain hardening index of the structural material supported by the i-th fulcrum; is the strain rate sensitivity coefficient of the structural material supported by the i-th support point; is the thermal softening index of the structural material supporting the i-th fulcrum. The equivalent plastic strain of the structural material supported by the i-th fulcrum; Let be the strain rate of the structure supported by the i-th fulcrum. Let be the quasi-static strain rate of the structural material supported by the i-th support point, with a recommended value of 0.001s. -1 ; The high strain rate of the structural material supported by fulcrum i; The normalized temperature of the support structure at the i-th fulcrum is denoted as .

[0040] For high strain rates, it can be known that:

[0041] Therefore, the formula for calculating the material strain coefficient at each support point is as follows:

[0042] in, Let t be the elongation of the structural material supporting the i-th fulcrum, and t be the transient impact time.

[0043] Step S6: Calculate the design support load for each support point. The unbalanced loads at each support point calculated in step S3 above The initial values ​​are based on the balance of steady-state forces and moments. However, the loss of a fan blade is a transient impact process that occurs over an extremely short period. This process involves both the dynamic amplification effect of the load and the strength-enhancing effect of the supporting structural material under high strain rates. To accurately obtain the load basis for strength assessment, it is necessary to comprehensively consider the influence of both aspects and correct the initial unbalanced load.

[0044] The purpose of this step is to integrate the transient load coefficient and the material strain coefficient to ultimately determine a design fulcrum load that reflects both transient dynamic effects and material nonlinear effects for subsequent strength verification.

[0045] Specifically, the calculation involves obtaining the design load of the i-th support point under the blade fly-off state based on the unbalanced load, transient load coefficient, and material strain coefficient at each support point:

[0046] in, The design load of the i-th support point under the fan blade fly-off state will be used as the direct input load for subsequent stress analysis and tensile strength reserve calculation of each support structure. Let be the transient load coefficient of the i-th support point under the condition of fan blade loss; Let be the material strain coefficient of the i-th support point under the fan blade loss state; Let be the unbalanced force at the i-th fulcrum when the fan blades are out of control.

[0047] Step S7: Calculate the tensile strength reserve of the support structure at each support point. Design support load The maximum equivalent stress of the supporting structure is obtained by applying it to the finite element or mechanical analysis model of each support point. Then, the tensile strength reserve of each support point under the flyaway load of the fan blade is calculated as follows:

[0048] in, To reserve the minimum tensile strength of the structure supported by the i-th support point. Let be the tensile strength of the structural material supporting the i-th fulcrum. The maximum equivalent stress of the structure supported by the i-th fulcrum.

[0049] Step S8: Determine whether the tensile strength reserve of each support structure meets the preset requirements. The safety criterion is that the tensile strength reserve of each support structure is greater than 1.0, i.e., min(n1, n2, n5)>1.0. If this condition is met, the current design scheme is feasible, and the design ends; otherwise, proceed to step S9 for optimization.

[0050] Step S9: Optimize iterative design If the minimum tensile strength reserve of the support structure under the flyaway load of the fan blades does not meet the requirements, optimization is required. The embodiments of the present invention handle the following two types of cases: Scenario 1: The tensile strength reserve of each support structure is less than 1.0. This indicates that the load source is too large. The fan blade design should be adjusted by reducing the mass m of the fan blade or reducing the centroid radius r of the blade to directly reduce the unbalanced force F. Then, iteratively execute steps S1 to S8 until the tensile strength reserve of each support structure is greater than 1.0.

[0051] Scenario 2: The tensile strength reserve of the structure supported by only some supports (such as support No. 2) is less than 1.0. This indicates that the load distribution at that support is too large. The load can be redistributed by adjusting the rotor layout and shortening the axial distance between that support and the main mounting section and / or auxiliary mounting section. Then, steps S1 to S8 are executed iteratively until the tensile strength reserve of the structure supported by each support is greater than 1.0.

[0052] In summary, the method described in this embodiment of the invention does not require the establishment of a complex whole-machine finite element model. It can efficiently complete the design and optimization of the support load in the early stage of the scheme by using the analytical process, effectively ensuring the structural integrity of the engine under the extreme condition of fan blade loss.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the embodiments of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for designing low-pressure rotor support loads under the condition of fan blade fly-off in a turbofan engine, characterized in that, Includes the following steps: S1. Determine the maximum unbalanced force caused by fan blade slippage. ; S2. Obtain the centroid of the fan blade with the largest mass and the axial distance of each support point of the low-pressure rotor relative to the main mounting section and the auxiliary mounting section; S3, based on the maximum unbalanced force Based on the axial distance, calculate the unbalanced load at each support point of the low-pressure rotor under the condition of fan blade fly-off; S4. Determine the transient load coefficients for each support point; S5. Determine the material strain coefficients at each support point; S6. Calculate the design support load of each support under the fan blade fly-off state based on the unbalanced load, transient load coefficient and material strain coefficient of each support. S7. Based on the designed support load, calculate the tensile strength reserve of each support structure; S8. Determine whether the tensile strength reserve of each support structure meets the preset requirements. S9. If not satisfied, optimize the design: If the tensile strength reserve of all support structures does not meet the requirements, adjust the mass or centroid radius of the fan blades and iteratively execute steps S1 to S8; if only some supports do not meet the requirements, adjust the axial distance between the supports that do not meet the requirements and the main mounting section and / or auxiliary mounting section, and iteratively execute steps S1 to S8.

2. The method according to claim 1, characterized in that, In step S1, the maximum unbalanced force Calculate using the following formula: in, The mass of the fan rotor blade with the largest mass. The radius of the centroid of the fan rotor blade with the largest mass. This represents the highest transient speed of the fan rotor.

3. The method according to claim 1, characterized in that, In step S3, the support points of the low-pressure rotor include support point 1, support point 2, and support point 5. The calculation of the unbalanced load of each support point of the low-pressure rotor under the fan blade fly-off state is specifically as follows: based on the force balance equation, the moment balance equation around the main mounting section, and the moment balance equation around the auxiliary mounting section, the radial unbalanced load of each support point under the fan blade fly-off state is obtained by solving them simultaneously.

4. The method according to claim 1, characterized in that, In step S4, the transient load factor of any i-th support point Based on the principle of energy conservation, the calculation formula is as follows: in, Let i be the mass of the structure supported by the i-th fulcrum. Let be the radial dimension of the structure supported by the i-th fulcrum. Let t be the elongation of the structural material supporting the i-th fulcrum, and t be the transient impact time. Let be the unbalanced force at the i-th fulcrum when the fan blades are out of control.

5. The method according to claim 1, characterized in that, In step S5, the material strain coefficient of any i-th support point Based on the Johnson-Cook model, the calculation formula is as follows: in, Let be the strain rate sensitivity coefficient of the structural material supported by the i-th support point. Let be the quasi-static strain rate of the structural material supported by the i-th support point. Let t be the elongation of the structural material supporting the i-th fulcrum, and t be the transient impact time.

6. The method according to claim 1, characterized in that, In step S6, the design support load of the i-th support point. Calculate using the following formula: in, Let be the transient load coefficient of the i-th support point under the fan blade fly-off state. Let be the material strain coefficient at the i-th support point under the fan blade flyaway state. Let be the unbalanced force at the i-th fulcrum when the fan blades are out of control.

7. The method according to claim 1, characterized in that, In step S7, the tensile strength reserve of the i-th support structure Calculate using the following formula: in, Let be the tensile strength of the structural material supporting the i-th fulcrum. The maximum equivalent stress of the structure supported by the i-th fulcrum.

8. The method according to claim 7, characterized in that, In step S8, the preset requirement is the tensile strength reserve of all support structures. All are greater than 1.

0.

9. The method according to claim 1, characterized in that, In step S9, adjusting the mass or centroid radius of the fan blades specifically means reducing the mass of the fan blades or lowering the centroid radius of the fan blades.

10. The method according to claim 1, characterized in that, In step S9, adjusting the axial distance between the fulcrum that does not meet the requirements and the main mounting section and / or auxiliary mounting section specifically means shortening the axial distance between the fulcrum and the main mounting section and / or auxiliary mounting section.

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