A flow control method for improving the starting performance of the intake duct by using a pressure relief slit

Abstract

The present invention proposes a flow control method for improving the starting performance of the air inlet by using a pressure relief slit. First, the flow field of the initial configuration of the air inlet during the starting and non-starting process is solved and the starting performance of the air inlet is evaluated; secondly, according to the flow field characteristics of the initial configuration of the air inlet during the starting and non-starting process obtained by the solution, the arrangement scheme of the pressure relief slit is designed, including determining the separation point and reattachment point positions of the main separation zone on the compression surface in the non-starting flow field of the air inlet, and arranging slits behind the separation point of the main separation zone, in the middle of the separation zone, and in front of the reattachment point of the separation zone; finally, the arrangement scheme of the pressure relief slit is determined to improve the performance of the air inlet and widen the working Mach number range of the air inlet. The present invention can effectively improve the starting performance of the air inlet, widen the working range of the air inlet, and has extremely small performance loss and flow field changes under cruise conditions.

Description

Technical Field

[0001] The invention relates to the fields of fluid mechanics and air intake aerodynamic design, and in particular to a flow control method for improving the starting performance of an air intake by utilizing a pressure relief slit, especially based on a combined pressure relief slit. Background Art

[0002] Since humans have successfully achieved powered flight, they have never stopped pursuing higher and faster flights. Hypersonic technology is a major breakthrough in the history of human aviation. The corresponding hypersonic aircraft usually refers to a type of aircraft that has a cruise Mach number greater than 5, is powered by an air-breathing ramjet engine or a combined propulsion engine, and can travel long distances in the atmosphere and across the atmosphere. Hypersonic aircraft have the characteristics of long-distance cruising, launch beyond the standoff zone, global rapid strike, strong penetration capability and high survivability. In military terms, they can carry out rapid strikes on any target in the world and change the battlefield mode; in civilian use, they can greatly shorten the time of intercontinental travel. At the same time, compared with ordinary disposable launch vehicles, they have high carrying capacity and low operating costs, which is of great significance to human exploration and development of space resources. In general, hypersonic technology has great strategic significance in both the economic and military fields, reflecting a country's scientific and technological strength and industrial foundation, and to a certain extent, it can also reflect its military strength and political status. Therefore, the research on air-breathing hypersonic flight technology has become a scientific research position that countries have entered one after another.

[0003] In the hypersonic Mach number range, traditional engines can no longer meet the needs of aircraft, and the relatively simple and reusable air-breathing scramjet engine has become an ideal choice. According to the working principle of the scramjet engine, it can be divided into four main components: the inlet, the isolation section, the combustion chamber and the tail nozzle. As an important component of the scramjet engine, the hypersonic inlet can compress the captured air and provide the airflow that meets the Mach number and pressure requirements to the combustion chamber. For the scramjet engine and even the entire aircraft, the performance of the inlet is crucial. However, during the flight, if the inlet angle of attack, the isolation section back pressure is too high or the incoming flow Mach number is low, the flow capture performance of the inlet will be sharply reduced and the combustion chamber will not work properly. If the inlet does not start, it will not only affect the incoming flow capture and the operation of the combustion chamber, but may also damage the structure of the engine and the inlet.

[0004] The safe, efficient and stable operation of scramjet engines is based on the smooth start of the air inlet. In practical applications, we should try our best to avoid the situation where the air inlet fails to start. If the aircraft fails to start successfully during operation, it may cause immeasurable consequences. For example, in the flight test conducted by the United States and Australia in 2007, the air inlet experienced multiple changes between the start and non-start states, and the combustion chamber eventually failed to work normally; during the second flight test of X51-A conducted by the United States in 2011, the air inlet also failed to start during the fuel conversion, resulting in the early end of the experiment and failure to achieve the test purpose.

[0005] In order to broaden the working range of scramjet engines, the inlet is required to have the ability to work at a wide Mach number, the restart Mach number needs to be reduced, and the starting performance of the inlet is improved. In engineering applications, variable geometry methods are usually used to avoid and reduce the hazards of inlet failure to start. Variable geometry technology usually adjusts the angle and position of the inlet profile or lip to change the contraction ratio of the inlet at different Mach numbers so that it can work normally in a wide Mach number range. At present, common inlet variable geometry methods include variable wedge surface, variable center cone, variable throat and variable lip mask. Although the above means solve the problem that the inlet is difficult to start at low Mach numbers, the introduction of variable geometry mechanism will change the design profile of the inlet and increase the structural complexity. Therefore, for fixed geometry inlets, it is very necessary to design a simple method that can effectively improve the starting performance of the inlet and broaden the working range of the inlet. Summary of the invention

[0006] In order to improve the starting performance of the hypersonic inlet, widen the working range of the inlet, and avoid the increase in the complexity of the mechanism due to the introduction of a variable geometry mechanism, the present invention proposes a flow control method based on a combined pressure relief slit to improve the starting performance of the inlet. The method can effectively improve the starting performance of the inlet, widen the working range of the inlet, and has minimal performance loss and flow field changes under cruise conditions.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] Step 1: Solve the flow field of the initial configuration of the inlet during the start-up and non-start-up process and evaluate the start-up performance of the inlet;

[0009] Step 2: Based on the flow field characteristics of the initial configuration of the inlet duct during startup and non-startup obtained in step 1, design the layout of the pressure relief slits:

[0010] 1) according to the flow field changes of the initial configuration of the inlet duct during the start-up and non-start-up process obtained in step 1, determine the separation point and reattachment point positions of the main separation zone on the compression surface in the non-start-up flow field of the inlet duct;

[0011] 2) Slits are respectively provided behind the separation point of the main separation zone, in the middle of the separation zone, and in front of the reattachment point of the separation zone;

[0012] Step 3: Determine the layout of the pressure relief slits to improve the performance of the inlet and expand the operating Mach number range of the inlet:

[0013] 1) Re-grid the inlet configuration with the slits introduced, taking the pressure relief of the slits into account during mesh division;

[0014] 2) CFD numerical simulation is performed on the inlet configuration with the slit introduced to obtain the flow field corresponding to different working conditions during the start-up and non-start-up process, as well as the inlet performance parameters including the inlet start-up Mach number, non-start-up Mach number, total pressure recovery coefficient, flow coefficient and pressure rise ratio;

[0015] 3) Compare the inlet performance parameters of the inlet after the slit is introduced with those of the initial configuration of the inlet to determine the effect of the combined slit on the improvement of the inlet starting performance and the widening effect of the inlet working Mach number range; wherein the effect of the slit in accelerating the inlet starting process can be obtained by comparing the starting Mach number, the effect of the slit in delaying the inlet non-starting process can be obtained by comparing the non-starting Mach number, and the effect of the slit on widening the inlet working Mach number range can be obtained by comparing the size of the double solution area; if there is no effect, or the effect does not meet the design requirements, return to step 2 and adjust the slit position.

[0016] Furthermore, the specific process in step 1 is:

[0017] 1) Mesh the initial configuration of the air inlet;

[0018] 2) CFD numerical simulation is performed on the intake duct configuration to obtain the flow field corresponding to different working conditions during the start-up and non-start-up process of the intake duct;

[0019] 3) Through flow field post-processing, the changes in inlet performance parameters including inlet starting Mach number, non-starting Mach number, total pressure recovery coefficient, flow coefficient and pressure rise ratio as well as the flow field characteristics of the flow channel in the inlet duct are obtained.

[0020] Furthermore, in step 1, the initial configuration of the inlet is meshed using a structured multi-block grid, and the first layer of the boundary layer grid height is set to 6.25×10 -7 m, the growth rate is fixed at 1.2, and the grid Reynolds number in the cruise state is Re Δ =5, the total number of grids is 104708, of which the number of normal and flow direction grids in the inner flow channel is 125×257.

[0021] Furthermore, in step 1, the RANS equations are solved by the finite volume method for CFD numerical simulation, the turbulence model adopted is the SST turbulence model, the flux format adopts the roe format, and the flux limiter uses the minmod flux limiter.

[0022] Furthermore, in step 1.2 and step 3.2, a single-step quasi-steady calculation method is used to calculate the Mach number range from 2.5 to 6.5.

[0023] Beneficial Effects

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The combined pressure relief slits used in the present invention are arranged with a relatively small width on the compression profile of the air inlet duct. During the starting process of the air inlet duct, the low momentum airflow in the wall separation bubble can be discharged, and the main separation area can be divided into multiple small-scale separations, effectively reducing the size and thickness of the separation bubble, thereby accelerating the starting process of the air inlet duct; during the non-starting process, due to the suction effect of the slit, the formation of large-scale separations on the compression profile is suppressed, thereby delaying the non-starting process of the air inlet duct. In summary, the present invention can improve the starting performance of the air inlet duct and broaden the working range of the air inlet duct;

[0026] 2. The combined pressure relief slit proposed in the present invention is in a pneumatic self-sealing state at the cruise point, which has a small impact on the performance parameters of the air inlet, and the total pressure recovery loss and flow loss caused are both less than 2%;

[0027] 3. The combined pressure relief slit proposed in the present invention belongs to the category of passive flow control, has little effect on the internal layout of the air inlet duct, and will not significantly increase the structural complexity of the air inlet duct;

[0028] The uses of the present invention include but are not limited to improving the starting performance of a dual air intake duct. It can effectively improve the starting performance of air intake ducts of other geometric shapes, as well as axisymmetric air intake ducts, inward-rotating air intake ducts, etc., and broaden their working range, and has wide applicability.

[0029] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0031] Figure 1 It is the basic configuration of hypersonic inlet.

[0032] Figure 2Schematic diagram of the calculation domain of the initial configuration.

[0033] Figure 3 This is the typical flow field of the initial configuration of the inlet in the non-starting state.

[0034] Figure 4 It is the hysteresis loop curve of the initial configuration of the intake duct.

[0035] Figure 5 The combined slit arrangement schemes in the embodiments are shown in Figure 1. (a) is the design configuration 1, and (b) is the design configuration 2.

[0036] Figure 6 A hysteresis circuit is provided for the air intake after the combined pressure relief slit is introduced.

[0037] Figure 7 It is the leakage amount caused by the introduction of combined pressure relief slits under different working conditions. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0039] The flow control method for improving the starting performance of the inlet duct based on the combined pressure relief slit described in this example includes the evaluation of the starting performance and flow field characteristics of the initial configuration of the inlet duct, determining the combined slit configuration scheme based on the flow field characteristics, and evaluating the performance and flow field of the inlet duct configuration after the combined slit is introduced, ultimately achieving the purpose of improving the starting performance of the inlet duct and widening the range of Mach numbers used. The specific steps are as follows:

[0040] Step 1: Solve the flow field of the initial configuration of the inlet during startup and non-startup and evaluate the startup performance of the inlet:

[0041] 1) This embodiment takes the dual hypersonic inlet as an example and uses structured multi-block grids to grid the initial configuration of the inlet. In this embodiment, in order to accurately capture the flow field characteristics, the first layer of the boundary layer grid height is set to 6.25×10 -7 m, the growth rate is fixed at 1.2, and the grid Reynolds number in the cruise state is Re Δ =5, the total number of grids is 104708, of which the number of normal and flow direction grids in the inner flow channel is 125×257. The initial configuration and calculation domain of the hypersonic inlet are as follows: Figure 1 and Figure 2 as shown in .

[0042] 2) CFD numerical simulation is performed by solving the RANS equations by the finite volume method. In this embodiment, the turbulence model used in the numerical simulation process is the SST turbulence model, the flux format adopts the roe format, and the flux limiter uses the minmod flux limiter. The CFD numerical calculation conditions are: P ∞ =9740.0Pa, T∞ =212.0R, α=0°, where P ∞ is the free flow temperature, T ∞ is the free flow pressure, α is the inlet angle of attack. During the self-starting and non-starting process, the calculation adopts the single-step quasi-steady calculation method, and the calculated Mach number range is 2.5 to 6.5.

[0043] Taking the starting process as an example, the specific method of the single-step quasi-steady calculation method is as follows: first, solve the inlet flow field when Ma=2.5 to obtain the initial flow field, then increase the Mach number with a certain step size (the step size is 0.1 in this embodiment) and continue to calculate on the basis of the initial field. After the calculation converges, the flow field result after the Mach number increases can be obtained. It should be noted that when the Mach number is increased again, it is necessary to add on the basis of the previous flow field, which is the single-step quasi-steady calculation method. Repeat this step to obtain the flow field corresponding to each Mach number during the period when the Mach number increases from 2.5 to 6.5 during the self-starting process. The processing method of the non-starting process is similar to that of the starting process. First, solve the inlet flow field when Ma=6.5 to obtain the initial field, then reduce the Mach number with the same step size as the starting process (0.1 in this embodiment) and add on the basis of the previous flow field result, and the flow field corresponding to each Mach number during the period when the Mach number decreases from 6.5 to 2.5 during the non-starting process can be obtained.

[0044] 3) Post-process the flow field calculation results, evaluate and analyze the starting Mach number and non-starting Mach number of the initial configuration of the inlet and the flow field characteristics under the non-starting state. The flow field calculation results of the typical non-starting state of the inlet are as follows: Figure 3 As shown in the figure, it can be seen that there is a large-scale separation zone on the compression surface. After the incoming flow passes through the separation shock wave in front of the separation zone, it is deflected and overflow is formed. The actual incoming flow capture area of ​​the inlet duct is significantly smaller than that of the downstream in the starting state. The total pressure recovery coefficient of the inlet duct corresponding to each Mach number in the starting process and the non-starting process is obtained through flow field post-processing, and the curve of the total pressure recovery coefficient of the inlet duct changing with the Mach number is plotted. Since the inlet duct has a more significant hysteresis effect during the starting and non-starting process, the total pressure recovery coefficient curve does not overlap during the starting and non-starting process, but forms a "hysteresis loop". The hysteresis loop of the initial configuration of the inlet duct in this embodiment is as follows Figure 4 as shown in .

[0045] Depend on Figure 4It can be seen that during the start-up process, the total pressure recovery coefficient of the inlet is at a low level when the Mach number is low, and when the incoming flow Mach number increases to 5.3, the total pressure recovery coefficient increases rapidly. Combined with the flow field of the inlet under this condition, the large-scale separation area near the compression surface has been completely swallowed into the inlet, and a relatively stable shock wave train structure has been formed in the inlet. It can be judged that the inlet is started, and the starting Mach number of the inlet is 5.3. Similarly, it can be judged that the non-starting Mach number of the inlet is 3.6.

[0046] Step 2: Determine the layout of the pressure relief slits based on the flow field characteristics during the start-up and non-start-up processes:

[0047] 1) For the flow field during the start-up and non-start-up of the inlet obtained in step 1, the main flow features in the start-up flow field are extracted. In this embodiment, when the incoming flow Mach number is 2.5, the separation point of the main separation zone on the compression surface of the initial configuration inlet is located at x=386mm, and the attachment point is located at x=465mm.

[0048] 2) Combined with the initial configuration flow field of the hypersonic inlet in this embodiment, the first slit is set at x = 413 mm behind the separation point of the main separation zone, and the second slit is set at x = 433 mm in the middle of the separation zone. Since the reattachment point is located behind the throat, this embodiment provides two solutions. The first one is as follows: Figure 5 As shown in (a), the third slit is located at x = 456 mm after the throat. Figure 5 As shown in (b), the third slit is located at x=454 mm before the throat.

[0049] Step 3: Improve the performance of the inlet and expand the operating Mach number range of the inlet:

[0050] 1) Re-mesh the inlet configuration with the slits introduced.

[0051] 2) CFD numerical simulation is performed on the inlet configuration with the slit introduced to obtain the flow field and inlet performance parameters corresponding to different working conditions during starting and not starting.

[0052] In this embodiment, the hysteresis circuit of the combined slit rear inlet is introduced as follows: Figure 6 As shown in Figure 2, the leakage caused by the introduction of the slit is as follows: Figure 7As shown. Combined with the numerical simulation results, the first slit behind the separation point and the third slit in front of the reattachment point can effectively discharge the low-speed, low-energy airflow in the separation bubble and effectively reduce the size of the separation bubble. The second slit located in the middle of the separation bubble can realize the segmentation of the separation bubble, thereby reducing the thickness of the separation zone. By introducing the pressure relief slit, the process of the separation bubble and the separation shock wave being pushed into the interior of the inlet under the action of the incoming flow is accelerated, thereby accelerating the start of the inlet. During the non-starting process of the inlet, due to the presence of the pressure relief slit, it is not easy to form a large-scale separation bubble on the compression surface, thereby delaying the occurrence of the inlet non-starting. In this embodiment, after the introduction of the slit, the design configuration 1 reduces the start-up and non-starting Mach numbers of the inlet to 3.6 and 3.0, respectively, and the design configuration 2 reduces the start-up and non-starting Mach numbers of the inlet to 3.4 and 2.8, respectively. The range of the double-solution zone of the inlet has also been reduced from 1.7 of the initial configuration to 0.6. This shows that the starting performance of the inlet has been significantly improved and its operating Mach number range has been significantly widened.

[0053] When the starting performance of the intake duct is improved, the starting Mach number and non-starting Mach number of the intake duct are reduced, which means that the intake duct can be started at a smaller Mach number during the starting process, and can not start at a lower Mach number during the non-starting process, so that the starting can be maintained within a larger range.

[0054] Broadening the working Mach number range can be understood from two perspectives. First, when the starting Mach number and the non-starting Mach number of the inlet are reduced, the interval of the double solution zone will also change accordingly. The double solution zone can be simply understood as an interval [non-starting Mach number, starting Mach number]. When this interval is reduced, the working Mach number range of the inlet will also be increased to a certain extent.

[0055] On the other hand, the operating Mach number of the hypersonic inlet itself is relatively high, and its efficiency is not high at a lower Mach number, and its mission range or design range does not include this part of the interval with a relatively low Mach number. Assuming that there is a small part of the operating conditions within the operating range of the hypersonic inlet in the flight trajectory, the inlet is not started. After the slit is introduced, the starting Mach number and the non-starting Mach number of the inlet are reduced to a certain extent, which also means that the operating Mach number range of the inlet is widened to a certain extent.

[0056] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention.

Claims

1. A flow control method for improving the starting performance of an intake duct by using a pressure relief slit, characterized in that: The following steps are involved: Step 1: Solve the flow field of the initial configuration of the inlet during the start-up and non-start-up process and evaluate the start-up performance of the inlet; Step 2: Based on the flow field characteristics of the initial configuration of the inlet duct during startup and non-startup obtained in step 1, design the layout of the pressure relief slits: Step 2.1: According to the flow field changes during the start-up and non-start-up process of the initial configuration of the inlet obtained in step 1, determine the separation point and reattachment point positions of the main separation zone on the compression surface in the non-start-up flow field of the inlet; Step 2.2: Slits are respectively provided behind the separation point of the main separation zone, in the middle of the separation zone, and in front of the reattachment point of the separation zone; Step 3: Determine the layout of the pressure relief slits to improve the performance of the inlet and expand the operating Mach number range of the inlet: Step 3.1: Re-mesh the inlet configuration with the slits introduced. Step 3.2: Perform CFD numerical simulation on the inlet configuration with the slit introduced to obtain the flow fields corresponding to different working conditions during the start-up and non-start-up processes, as well as the inlet performance parameters including the inlet start-up Mach number, non-start-up Mach number, total pressure recovery coefficient, flow coefficient and pressure rise ratio; Step 3.3: Compare the inlet performance parameters of the inlet after the slit is introduced with those of the initial configuration of the inlet, and determine the effect of the pressure relief slit on the improvement of the inlet starting performance and the widening of the inlet working Mach number range; wherein the effect of the slit in accelerating the inlet starting process is obtained by comparing the starting Mach number, and the effect of the slit in delaying the inlet non-starting process is obtained by comparing the non-starting Mach number; and the effect of the slit on widening the inlet working Mach number range is obtained by comparing the size of the double solution area; if there is no effect, or the effect does not meet the design requirements, return to step 2 and adjust the slit position.

2. A flow control method for improving the starting performance of an intake duct by using a pressure relief slit according to claim 1, characterized in that: The specific process in step 1 is: Step 1.1: Mesh the initial configuration of the air inlet; Step 1.2: Perform CFD numerical simulation on the initial configuration of the inlet duct to obtain the flow fields corresponding to different working conditions during the start-up and non-start-up process of the inlet duct; Step 1.3: Through flow field post-processing, the changes in the inlet performance parameters including the inlet starting Mach number, non-starting Mach number, total pressure recovery coefficient, flow coefficient and pressure rise ratio and the flow field characteristics of the flow channel in the inlet are obtained.

3. A flow control method for improving the starting performance of an intake duct by using a pressure relief slit according to claim 2, characterized in that: In step 1, the initial configuration of the inlet is meshed using structured multi-block grids, and the first layer of boundary layer grid height is set to 6.25×10 -7 m, the growth rate is fixed at 1.2, and the grid Reynolds number in the cruise state is Re Δ =5, the total number of grids is 104708, of which the number of normal and flow direction grids in the inner flow channel is 125×257.

4. A flow control method for improving the starting performance of an intake duct by using a pressure relief slit according to claim 2, characterized in that: In step 1, the RANS equations are solved by the finite volume method for CFD numerical simulation. The turbulence model used is the SST turbulence model, the flux format uses the roe format, and the flux limiter uses the minmod flux limiter.

5. The flow control method for improving the starting performance of the intake duct by using the pressure relief slit according to claim 2, characterized in that: In step 1.2 and step 3.2, a single-step quasi-steady calculation method is used to calculate the Mach number range from 2.5 to 6.5.

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