A method for suppressing low-frequency combustion instability in cavitation conditions

Abstract

The present invention discloses a method for suppressing low-frequency combustion instability in cavitation conditions, which includes equivalenting an injector to a spring oscillator system to obtain the equivalent frequency f of the injector; the spring oscillator system includes the spring of the injector and an oscillator equivalent to the moving part of the injector; obtaining the combustion instability frequency f0 of the combustion chamber; calculating the difference between the combustion instability frequency f0 and the equivalent frequency f of the injector, and adjusting the parameters of the spring or the moving part of the injector to make the difference less than a preset value, and suppressing low-frequency combustion instability by using an injector including the spring and the moving part with the current parameters.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquid rocket engines, and particularly relates to a method for suppressing low-frequency combustion instability in cavitation conditions. Background Technique

[0002] A liquid rocket engine injects fuel and oxidizer into a combustion chamber. After injection, atomization, and combustion reactions, high-temperature and high-pressure gas is generated, and it is rapidly expanded and accelerated through a nozzle and ejected, generating a thrust opposite to the exhaust direction. However, the thrust generated during the combustion process is not fixed but will oscillate to form a pressure wave with a certain frequency. When the supply frequency of the propellant flow resonates with the frequency of the pressure wave, low-frequency combustion instability phenomena (flow) with a frequency range of approximately 20 - 1000 Hz will occur. Once combustion instability occurs, it will significantly increase the vibration level, damage sensitive guidance components and payloads, and even cause structural component damage and life loss, intensify heat transfer, lead to ablation damage of thermal components, reduce the engine efficiency, and increase the likelihood of failure. When a cavitating venturi is used as a flow stability adjustment element in a liquid rocket engine, the cavitating venturi is often assembled with a conduit in the engine and serves as a propellant supply and delivery system at the same time. The engine conduit is prone to vibration under the excitation of the engine itself and the outside world, which affects the internal flow state of the conduit and causes the flow parameters to oscillate. When the cavitating venturi is used as a flow stability adjustment element and is affected by the conduit vibration, it may cause abnormal operation of the engine flow stability adjustment function and even promote or exacerbate the occurrence of low-frequency combustion instability in the engine. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above defects and provide a method for suppressing low-frequency combustion instability in cavitation conditions, solving the technical problem of low-frequency combustion instability in cavitation conditions of a liquid rocket engine. The present invention can achieve the suppression of low-frequency combustion instability.

[0004] To achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0005] A method for suppressing low-frequency combustion instability in cavitation conditions, comprising:

[0006] S1 Equivalent the injector to a spring-mass system to obtain the equivalent frequency f of the injector; the spring-mass system includes the spring of the injector and the mass equivalent to the moving part of the injector;

[0007] S2 Obtain the combustion instability frequency f0 of the combustion chamber;

[0008] S3 Calculate the difference between the combustion instability frequency f0 and the equivalent frequency f of the injector;

[0009] When the difference ≥ preset value Δf, adjust the parameters of the spring or the moving part of the injector and return to step S1;

[0010] When the difference < preset value Δf, use the injector with the spring and the moving part including the current parameters to achieve the suppression of low-frequency combustion instability.

[0011] Furthermore, in step S2, by analyzing the measured data of the combustion chamber pressure, obtain the combustion instability frequency f0 of the combustion chamber.

[0012] Furthermore, in step S1, the injector is equivalent to a spring oscillator system, and the equivalent frequency f of the injector is obtained according to the following formula:

[0013]

[0014] where K is the spring stiffness of the injector and m is the mass of the moving part of the injector.

[0015] Furthermore, in step S3, the preset value Δf = 100 - 120 Hz;

[0016] In step S3, the parameters include the spring stiffness of the injector or the mass of the moving part of the injector.

[0017] Furthermore, it also includes:

[0018] By adjusting the outlet pressure, conical surface or expansion angle of the cavitation tube, make the difference between the cavitation tube frequency and the combustion instability frequency f0 > the preset value.

[0019] Furthermore, it also includes:

[0020] By adjusting the diameter, length or wall thickness of the conduit, make the difference between the natural frequency of the conduit and the combustion instability frequency f0 > the preset value, and the difference between the natural frequency of the conduit and the equivalent frequency f of the injector > the preset value.

[0021] Furthermore, it also includes:

[0022] Rigidly support the conduit to reduce the amplitude of the forced vibration of the conduit.

[0023] Furthermore, it also includes:

[0024] Set a throttling element at the downstream pressure node of the cavitation tube;

[0025] The throttling element is a single-hole throttling ring or a multi-hole throttling ring.

[0026] Furthermore, when there is a pressure nozzle for monitoring the outlet pressure of the cavitation tube, the arrangement direction of the pressure nozzle is perpendicular to the plane where the axis of the conduit is located.

[0027] The present invention has the following beneficial effects compared with the prior art:

[0028] (1) The present invention creatively proposes a method for suppressing low-frequency combustion instability in cavitation conditions. By equating the variable-area injector to a spring-mass system, the natural vibration frequency of the injector can be effectively calculated, thereby realizing the suppression of low-frequency combustion instability.

[0029] (2) The present invention can realize the suppression of low-frequency combustion instability by adjusting the parameters of the spring or moving parts of the injector. The method is simple and the result is accurate, which can effectively avoid the phenomenon of low-frequency combustion instability.

[0030] (3) The present invention also proposes a method for suppressing low-frequency combustion instability by adjusting the sizes of the cavitation tube and the conduit, setting throttle elements, and when a pressure nozzle needs to be set, the layout method of the pressure nozzle, which can further realize the suppression of low-frequency combustion instability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is the supply structure of the variable-area injector and the propellant cavitation tube for the engine;

[0032] Figure 2 It is the schematic diagram of the pressure nozzle layout structure;

[0033] In the figure, 1 - adjustable cavitation tube mounting base, 2 - adjusting cone, 3 - cavitation tube, 4 - pipeline socket flange, 5 - pipeline reduced diameter section, 6 - throttle element, 7 - pressure nozzle, 8 - conduit, 9 - injector spring, 10 - injector moving part, 11 - sealing structure. DETAILED DESCRIPTION OF THE INVENTION

[0034] The following is a detailed description of the present invention, and the features and advantages of the present invention will become clearer and more definite with these descriptions.

[0035] The special term "exemplary" here means "serving as an example, embodiment or illustrative". Any embodiment described as "exemplary" here does not have to be construed as superior to or better than other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings do not have to be drawn to scale unless otherwise specified.

[0036] When the variable injection area engine with adjustable working conditions has low-frequency combustion instability in the cavitation condition, the present invention suppresses or eliminates the combustion instability problem through special adjustment means, or adopts some design means in the engine design to avoid or weaken the low-frequency combustion instability phenomenon. It can also be extended to suppress or eliminate the cavitation oscillation problem and non-cavitation low-frequency combustion instability problem of the variable injection area liquid rocket engine.

[0037] After the occurrence of mid - low frequency unstable combustion problems, the characteristic frequencies of combustion instability can often be extracted from experiments at this time. The adjustment means adopted include: using the mathematical model of a spring - mass oscillator to calculate the frequency of a variable - injection - area injector, changing the spring stiffness of the injector's moving spring and the weight of the movable parts (for example, for a pintle injector, the movable parts can be the needle valve or the central tube) to adjust the frequency of the variable - injection - area injector, ensuring that the frequency of the variable - injection - area injector is controlled to be far from the cavitation oscillation frequency; adjusting the parameters of the cavitation tube in the engine, including the outlet pressure, conical surface, divergence angle, etc., changing the flow characteristics of the propellant in the cavitation tube, and ensuring that the combustion instability frequency is avoided; adjusting the dimensions of the conduit such as diameter, length, wall thickness, etc., or adding support structures such as clamps to change the support state of the conduit, using finite - element modal calculation to obtain the natural frequency value of the conduit, and controlling the natural frequency of the conduit to avoid the combustion instability frequency.

[0038] During the design process, when there is no combustion instability problem, the following methods can be used to prevent the occurrence of combustion instability problems as much as possible: When setting pressure measurement points at the outlet of the cavitation tube, pay attention to setting them at specific positions and orientations to avoid amplifying oscillations due to measurement factors; continue to set throttle elements at the outlet of the cavitation tube to increase oscillation damping and suppress unstable parameter conditions.

[0039] Specifically, a method for suppressing mid - low frequency combustion instability in a cavitation condition of the present invention includes:

[0040] (1) Equivalent the variable - injection - area injector to a spring - mass oscillator system, which can effectively calculate the natural vibration frequency of the injector. Among them, all movable parts used in the adjustment process in the thrust chamber injector are equivalent to a spring - mass oscillator, the mass of the movable parts themselves (excluding the mass of the spring) is equivalent to the mass of the spring - mass oscillator, and the actual stiffness of the spring used in the movement process is equivalent to the stiffness of the spring - mass oscillator. Then the equivalent frequency calculation formula of the variable - injection - area injector is

[0041]

[0042] In the formula, K is the actual spring stiffness, m is the total mass of all movable parts in the adjustment process (excluding the mass of the spring), and f is the equivalent frequency of the injector;

[0043] The equivalent frequency f can be increased by increasing the actual spring stiffness or reducing the total mass of the movable parts.

[0044] (2) Adjust the outlet pressure of the cavitation tube, adjust the outlet conical surface or the divergence angle of the cavitation tube, and adjust the length - diameter ratio of the outlet pipeline to change the adjustable cavitation tube frequency, and ensure that the adjustable cavitation tube frequency and the combustion instability frequency avoid each other according to the experiment.

[0045] (3) Adjust the dimensions of the duct, such as diameter, length, wall thickness, etc., or add corresponding rigid supports such as clamps to change the overall support state of the duct, which can adjust the natural frequency of the duct itself and reduce the amplitude of the forced vibration of the duct. Obtain the numerical value of the natural frequency of the duct through finite element modal calculation, so as to accurately avoid the frequency of combustion instability and reduce the response of the duct to the excitation of combustion instability. Control the difference between the natural frequency of the duct, the equivalent frequency f of the injector, and the combustion instability frequency f0 to be not less than 100 Hz, so as to suppress or eliminate the mid-low frequency combustion instability problem under cavitation conditions.

[0046] (4) A throttling element, such as a single-hole or multi-hole throttle ring, is arranged in the downstream system of the cavitation tube. A fixed throttle ring or a detachable throttle ring can be used and arranged near the outlet of the cavitation tube. This part is the pressure node part, so oscillation damping can be generated to suppress the mid-low frequency combustion instability.

[0047] (5) When arranging a pressure measuring nozzle at the downstream cylindrical end of the cavitation tube outlet, that is, at the throttle ring, ensure that the arrangement direction of the pressure measuring point is perpendicular to the bending direction of the duct, that is, perpendicular to the plane where the axis of the bent duct is located. The pressure measuring nozzle is arranged close to the outlet of the cavitation tube rather than far away from the outlet of the cavitation tube, which can avoid the risk of amplifying the oscillation when the pressure measuring point is at the pressure peak.

[0048] Example 1:

[0049] In this example, a pin-and-bolt injector is taken as an example. One end of the spring in the variable injection area pin-and-bolt injector is connected to a fixed structure, and the other end is connected to a moving part (the needle valve in this example). The propellant generates different thrusts on the needle valve under different working conditions, pushing the spring to move up and down, changing the injection area, and realizing the adjustment of different flow injection characteristics. In a specific injector structure, the moving part can be a needle valve, a central cylinder, or other parts that affect the injection characteristics of the engine.

[0050] When mid-low frequency combustion instability occurs in the coupling of a certain path of propellant and the injector under cavitation conditions in a certain test, the combustion instability frequency value f0 can be obtained through the analysis of the combustion chamber pressure measurement data in the test. The stiffness K of the injector spring 9 and the mass m of the injector moving part 10 can be measured through relevant component tests, and the equivalent frequency f of the injector can be calculated using formula (1).

[0051] Compare the combustion instability frequency f0 and the equivalent frequency f of the injector, change the stiffness of the injector spring 9 or the mass of the injector moving part 10, so that the equivalent frequency f of the injector avoids a certain bandwidth frequency from the combustion instability frequency f0, which can effectively suppress the mid-low frequency combustion instability phenomenon in the coupling of a certain path of propellant and the injector.

[0052] In this example, the suppression of mid-low frequency combustion instability in the coupling of the oxidizer path and the injector under cavitation conditions:

[0053] (1) The stiffness of the injector spring 9 is 4000 N / mm, and the weight of the injector moving part 2 is 0.5 kg. Using the formula The equivalent frequency f of the injector is calculated to be 450 Hz, that is, low-frequency combustion instability occurs at this frequency;

[0054] (2) The calculation results of the equivalent frequency f of the injectors composed of injector springs 9 with different stiffnesses and injector moving parts 10 with different weights are shown in Table 1:

[0055] Table 1 Variable area injector frequencies under different spring stiffnesses and moving part weights

[0056] Spring stiffness N / mm Weight of moving part kg Variable area injector frequency 3000 0.6 356 3500 0.6 384 4000 0.6 411 4500 0.6 436 5000 0.6 459 3000 0.5 390 3500 0.5 421 4000 0.5 450 4500 0.5 477 5000 0.5 503 3000 0.4 436 3500 0.4 471 4000 0.4 503 4500 0.4 534 5000 0.4 563

[0057] (3) During the experiment, the medium and low-frequency combustion instability frequency was 450 Hz. By increasing the frequency of the variable area injector, after improvement, the spring stiffness was 5000 N / mm, the weight of the moving part was 0.4 kg, and the frequency of the variable area injector was 563 Hz, achieving the improvement goal that the frequency difference is not less than 100 Hz. Therefore, the combustion instability problem can be suppressed or eliminated.

[0058] Example 2:

[0059] In this example, specific position pressure measurement points and throttle rings are set to avoid combustion instability problems as much as possible:

[0060] As Figure 1 and Figure 2 shown, the adjustable variable area injector engine includes an adjustable cavitation tube mounting base 1, an adjusting cone 2, a cavitation tube 3, a pipeline socket flange 4, a pipeline reduced diameter section 5, a conduit 8, an injector spring 9, and an injector moving part 10. Among them, the adjustable cavitation tube mounting base 1 is used to support the cavitation tube 3. The cavitation tube 3 is connected to the conduit 8 through the adjustable cavitation tube mounting base 1. The adjusting cone 2 is arranged at the small end (inlet) of the cavitation tube 3 to adjust the flow rate of the cavitation tube 3. The pipeline reduced diameter section 5 is the downstream section of the conduit 8. The fuel or oxidant is transported to the injector in the thrust chamber through the conduit 8. The injector injects the fuel and oxidant into the combustion chamber in the thrust chamber, and the fuel and oxidant burn in the combustion chamber. The sealing structure 11 is arranged between the cavitation tube 3 and the adjustable cavitation tube mounting base 1 to provide a sealing function and ensure the normal flow of the propellant in the cavitation tube.

[0061] (1) A throttling element 6, such as a single-hole or multi-hole throttle ring, is set in the downstream system of the cavitation tube. A fixed throttle ring or a detachable throttle ring can be used and arranged at the downstream pressure node position, as shown in Figure 1 ;

[0062] (2) When designing the measurement system, the pressure measurement point 7 is arranged at the outlet of the cavitation tube, i.e., the downstream cylindrical end of the throttle ring, rather than in the middle section or the outlet part of the conduit, which can avoid the risk of the pressure measurement point being at the pressure peak and amplifying the oscillation, and ensure that the arrangement direction of the pressure measurement point is perpendicular to the bending direction of the conduit, see Figure 2 .

[0063] The present invention has been described in detail in combination with specific embodiments and exemplary examples, but these descriptions should not be construed as limiting the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications or improvements can be made to the technical solutions and implementation manners of the present invention, and all of these fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

[0064] The content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A method for suppressing low-frequency combustion instability in cavitation conditions, characterized in that, Including: S1 Equivalent the injector to a spring oscillator system to obtain the equivalent frequency f of the injector; the spring oscillator system includes the spring of the injector and the oscillator equivalent to the moving part of the injector; S2 Obtain the combustion instability frequency f0 of the combustion chamber; S3 Calculate the difference between the combustion instability frequency f0 and the equivalent frequency f of the injector; When the difference < preset value Δf, adjust the parameters of the spring or moving part of the injector and return to step S1; by adjusting the outlet pressure, conical surface or expansion angle of the cavitation tube, make the difference between the cavitation tube frequency and the combustion instability frequency f0 > preset value; When the difference ≥ preset value Δf, use the injector with the spring and moving part including the current parameters to achieve the suppression of low-frequency combustion instability.

2. A method for suppressing low-frequency combustion instability in cavitation conditions, characterized in that, In step S2, the combustion instability frequency f0 of the combustion chamber is obtained by analyzing the combustion chamber pressure measurement data.

3. A method for suppressing low-frequency combustion instability in cavitation conditions according to claim 1, characterized in that, In step S1, the injector is equivalent to a spring oscillator system, and the equivalent frequency f of the injector is obtained according to the following formula: Where K is the spring stiffness of the injector and m is the mass of the moving part of the injector.

4. A method for suppressing low-frequency combustion instability in cavitation conditions according to claim 3, characterized in that, In step S3, the preset value Δf = 100 - 120Hz; In step S3, the parameters include the spring stiffness of the injector or the mass of the moving part of the injector.

5. A method for suppressing low-frequency combustion instability in cavitation conditions, characterized in that, Also including: By adjusting the diameter, length or wall thickness of the conduit, make the difference between the natural frequency of the conduit and the combustion instability frequency f0 > preset value, and the difference between the natural frequency of the conduit and the equivalent frequency f of the injector > preset value.

6. A method for suppressing low-frequency combustion instability in cavitation conditions according to claim 5, characterized in that, Also including: Rigidly support the conduit to reduce the amplitude of the forced vibration of the conduit.

7. A method for suppressing low-frequency combustion instability in cavitation conditions, characterized in that, Also including: A throttling element is arranged at the downstream pressure node part of the cavitation tube, and the throttling element is a single-hole throttling ring or a multi-hole throttling ring.

8. A method for suppressing low-frequency combustion instability in cavitation conditions according to claim 1, characterized in that The arrangement direction of the pressure nozzle for monitoring the outlet pressure of the cavitation tube is perpendicular to the plane where the axis of the conduit is located.

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