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Modal Simulation of Axial Flow Fan Blades and Its Influence on Aerodynamic Noise
Source: China Bearing Network Time: 2013-10-09
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It is necessary to calculate the oscillation mode when the impeller is drawn, but the blade blade surface is cluttered and cannot be solved by classical theory. Therefore, it is necessary to use the finite element model to calculate. ANSYS is one of the more famous finite element analysis softwares. With a variety of physical field solving functions, it is very convenient to perform modal analysis; the large CAD system software UniGraphics has a rich surface shape function, which is very suitable for the appearance of a chaotic surface entity such as an impeller, and the built-in solid model is imported. ANSYS can perform modal analysis.
Second, the impeller CAD model establishment and interface import 1. The impeller fundamental parameter axial flow fan is the whole injection ABS plastic impeller, the number of blades is 4, the blade is wide, the blade is swept forward. The working speed is 860r/min, the diameter of the hub is 0. 147m, and the outer diameter of the impeller is 0. 42m. .
2. Several models are set up to measure the external phenotypic values ​​of the blade through a three-coordinate measuring instrument, and the lattices are connected into a curved surface, and the surface of the blade is connected by using the surface selection and stitching function of the software UG. Once all the surfaces are stitched, the surface is actively generated. Each surface is an entity of the divide.
The impeller is a cyclic symmetrical layout. To accelerate the finite element analysis process, the cyclic symmetry function of ANSYS is used to solve a 90° fundamental sector. When modeling, the Z axis of the large coordinate system corresponds to the axis of the impeller, and the impeller is well established. The model then cuts a quarter of the impeller model from two planes with an angle of 90° on the hub axis.
3. There are three ways to import UG models into ANSYS. There are two ways to exchange data according to the direct model: First, the CAD model data is transferred to the profiling system through the standardized data interface; otherwise, it is UG through ANSYS. The dedicated interface of the supply is directly read into the prt file of UG; the third is based on GFEMFEA of UG.
Here adopt the second method, click File→Import→UG in the function menu, and then select the part file.
Third, pre-disposal and solution 1. Input physical parameters of physical data input physical parameters of ABS data: density is 1. 2 × 10-6g / mm3, elastic modulus is 2. 3MPa, Poisson's ratio is 0. 38.
2. The selection of the unit type impeller surface is a variable thickness chaotic surface, and the 10-node tetrahedral element solid92 is selected. The unit adopts the secondary displacement mode, which is very suitable for distinguishing the finite element model from the irregular shape. For the two intervals of the fundamental sector Relative to the 90° section of the hub to distinguish the grid, also selected a two-dimensional unit: MESH200 unit, and set the unit shape parameter to "trianglewith6nodes" (MESH200 unit is specifically used to distinguish the grid, supply grid occupancy function , does not participate in unit operations).
3. Divide the grid First, use the MESH200 triangle plane unit to distinguish the grid on one side of the two sections on the hub, and then copy the grid on the surface to another section through the MSHCOPY command. Use the solid92 unit for the entire model. grid.
4. The chasing condition of the impeller is loaded on the axis of the motor through the hub of the hub. The other movements are bound by the impeller except for the transition. Therefore, the freedom of the finite element node X and Z directions of the hub cylindrical device surface is bound (in the big column) In the surface coordinate system), the nodes of the finite element have a coordinate system corresponding to the load direction. Under normal conditions, the node coordinate system corresponds to the big Cartesian coordinate system. It is necessary to use the NROTAT instruction to transform the node coordinates into the cylindrical coordinate system. And then load the displacement constraint on the node.
5. Cyclic symmetric treatment of cyclic symmetry modal solution is a special simplified modal solution for ANSYS to provide cyclic symmetry layout. There are some special pre-disposal before solving.
First, the requirements are to select the nodes on the upper and lower sections of the impeller and set up two sets, named "Low" and "High". Secondly, the macro of CYCGEN works to establish a second sector on the fundamental sector, modal analysis That is, after the completion of these two sectors, if the instruction works without parameters, it will copy the internal coupling and the binding equation to the second sector; if it is the CYCGEN', LOAD' instruction, the load will also be Copy to the second sector. Here work CYCGEN, 'LOAD'.
6. The solution is to use BlockLanczos as the solution method, and the solution frequency scale is set from 20Hz to 200Hz. For solving the cyclic symmetry mode, ANSYS also provides a special solution macro (not directly using the solve command). The command pattern is: CYCSOL, NDMIN, NDMAX , NSECTOR, LOW
The meaning of each parameter is as follows:
NDMIN, NDMAX: The upper and lower scale diameter of the calculation, the minimum NDMIN is 0, the maximum NDMAX pair can be n/2, and the odd maximum is (n-1)/2.
NSECTOR: The number of sectors with cyclic symmetry, here 4.LOW: the group name of the nodes on the lower view profile.
This command corresponds to the menu path: MainMenu>Solution>ModalCyclicSym.
Enter CYCSOL, 0, 2, 4, LOW to solve.
IV. The comparison experiment between the simulation results and the experimental results is carried out by the hammering method under the condition of the impeller device. The stiffness of the bracket is very large, and the influence of the bracket is neglected. The oscillation mode frequency of the layout in the range of 20-200 Hz is determined by the impeller. The experimentally obtained impeller modal frequency values ​​are 58.17 Hz, 83.38 Hz, 88.69 Hz, 154.8 Hz; the simulated modal frequency values ​​are about 62 Hz, 80 Hz, 88 Hz, and 152. 2 Hz. Correspondingly with the simulation results, they have been verified. Because the vibration mode experiment is more chaotic, there is no further vibration mode experiment, and the simulation data will be used later to investigate the vibration mode.
5. Thinking about prestressing and rotating softening Under normal conditions, the impeller is moving. Because of the centrifugal force and aerodynamic load, the impeller is tensilely deformed. The modality can be very different from the stopping condition, so it is necessary to think about it. One of the reasons that affects the frequency change of the rotating part is the influence of the centrifugal force on the prestress of the blade motion, which constitutes the increase of the impeller stiffness and increases the mode frequency under working conditions.
Another reason: rotational softening, rotational softening reduces the modal frequency. The principle can be clarified by a simple spring-mass rotation system, the spring is perpendicular to the axis of rotation, and when the spring is very rigid and the rotational acceleration is small, I think the deformation of the spring is very small.
The effect of negligent spring deformation on the centripetal acceleration of the mass is established as follows:
Kx=Mωs2r
(1) In the formula k, the spring stiffness x is separated from the equilibrium azimuth ωs. The angular velocity of rotation is the radius of the free point of the particle. However, if the stiffness of the spring is not good, the rotation speed is very large, because the influence of centrifugal force makes the spring occur. Large displacement, which together increases the radius of the particle's centrifugal motion, at which point the equilibrium equation is written as:
Kx=Mωs2(r+x)(2)
If it is still expressed in the form of (1), its equilibrium equation can be written as:
(k-Mωs2)x=Mωs2r
When the applied load is applied, the oscillation equation can be written as:
Mx-(k-Mωs2)x=f(t)
Therefore, the stiffness changes from k to (k-Mωs2), which is equivalent to the rotational softening effect. The higher the rotational speed, the greater the density of the rotating object, and the more pronounced the softening effect. The stress stiffening increases the modal frequency. Rotational softening makes the modal frequency low, and the effect of general stress stiffening is too large. Therefore, the influence of the two elements is considered together, so that the modal frequency in the working condition is higher than the modal frequency in the stopped state.
In order to obtain the modal difference between the actual condition and the stop condition, a modal finite element analysis is performed. The process is to apply a transformation angular velocity to the impeller after the third and fourth processes, and open the prestress switch to select the type of analysis. Stress analysis, and a static stress analysis. Then select the type of analysis as modal analysis, and ensure that the pre-stress switch is turned over, together turn the rotary softening option, the following three and five future processes.
The modal frequency of each mode shape of the accounting results changes less than 1 Hz. Therefore, the impeller can select the mode under the stop condition instead of the mode under the motion condition, thinking from the thoughtfulness of the problem, the stress stiffening, the rotation softening Verification is necessary.
6. Analysis of vibration mode and influence on aerodynamic noise In order to investigate the vibration mode, use the command Expand and input parameter 4 to expand the entire impeller to investigate the vibration mode (menu route: MainMenu>GeneralPostprocessing>Expansector). (1) The first-order oscillation frequency is 61. 5Hz, the blade is embodied as a radial torsion, the maximum deformation is at the root of the blade, the entire impeller shape is reflected as 1, 3 blade sway, 2, 4 does not move, 1, 3 blade reverse torsion.
(2) The second-order oscillation frequency is 62 Hz, and the blade is embodied as a torsion pendulum along the radial line. The maximum deformation orientation is at the blade root. The entire impeller mode is reflected as 1 and 3 blades twisting forward, and 2 and 4 blades are twisted backward.
(3) The third-order oscillation frequency is 62. 5 Hz, and the blade is embodied as a torsion pendulum along the radial line. The maximum deformation orientation is at the blade root. The entire impeller mode is represented by four blades that are twisted in the same way.
(4) The fourth-order oscillation frequency is 80. 3 Hz, and the blade still appears to be twisted along the radial line, but the meandering surface appears on the blade profile, and the largest zigzag representation appears at the larger blade radius, and the entire impeller shape is reflected as 1 3 blades are twisted forward, 2, 4 blades are twisted backwards.
(5) The fifth-order oscillation frequency is 80. 5 Hz, the blade is embodied as a torsion pendulum along the radial line, and the meandering surface of the blade profile is presented. The maximum zigzag representation is at the radius of the larger blade, and the entire impeller shape is reflected as 1, 3 The blade is reverse twisted, 2, 4 does not move.
(6) The sixth-order oscillation frequency is 87. 6Hz, the blade is embodied as a torsion pendulum along the radial line, and the meandering surface of the blade profile is presented. The maximum zigzag representation is at the radius of the larger blade, and the entire impeller is represented by four blades. Twist in the same way.
(7) The seventh-order oscillation frequency is 152. 2 Hz, and the motion modes of the blades of 1 and 3 are mainly the meandering waves on the blade profile. The meandering wave has two pitch lines, and the most severe occurrence of the tortuous wave occurs at the outer periphery of the blade. 3 blade movements differ by 180° phase; 2, 4 blades do not move at all, but there is a small amount of warpage at the forward swept tip; the opposite blade changes phase difference by 180°.
(8) The eighth-order oscillation frequency is 152. 6Hz, and the four vane profiles on the blade show a tortuous wave, and the two blades are in the same state of oscillation, while the adjacent blades are 180° out of phase.
It can be seen that the modal oscillation mode of the impeller is mainly composed of four blades with different circumferential oscillation modes. The reason for this is that the forward swept blade stiffness is much smaller than the rigidity of the hub, that is, the blade is “soft†and the hub is “hardâ€; The low-frequency mode of the blade is mainly dominated by the torsion of the blade, while the high frequency is mainly dominated by the tortuosity of the blade. From the influence of noise, the first six modes are more affected because of the pendulum flow field. The large influence constitutes the change of the blade's intake angle of attack, and then constitutes the fluctuation of the blade's external lift. The most severe conditions will occur, and a large aerodynamic noise and a large power drop will occur.
VII. The modal analysis of the impeller mode by finite element method is carried out. The influence of rotational softening and stress strengthening on the modal frequency under the actual working condition of the impeller is considered. It is found that the situation is not the same as the stopping condition. It is better. After the vibration mode analysis, it is considered that the first six-order oscillation of lower frequency has a great influence on the aerodynamic noise, which provides learning for depicting low-noise fans.
The next step is to calculate the impeller field, obtain the force of the blade flow field and make harmonic analysis of the impeller. The aerodynamic acoustic formula is used to guess the size of the noise, in order to obtain the detailed magnitude of the impeller oscillation to the aerodynamic noise.
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