Response Analysis and Experimental Study on Flow-induced Vibration Structure of Steel Barrel Leakage
Zhu Zhisong, Guo Dongjun, Chen Yangyang
Abstract: Objective To explore the basic principle and experimental method of acoustic emission detection technology for steel drum leakage, and establish a simulation model and experimental model of steel barrel leakage. Firstly, the mechanism of acoustic emission signal generated by steel gas leakage and the basic principle of fluid-solid coupling are introduced. Secondly, the steel barrel leakage model is established. The structure response characteristics under flow-induced vibration are simulated by Workbench fluid-solid coupling method. Then, the acoustic emission detection principle is built. Steel drum leakage acoustic emission detection experimental platform, collecting experimental data and analyzing and processing, discussing the relationship between leakage diameter, internal pressure and steel drum leakage characteristic parameters, combined with frequency domain analysis, to obtain characteristic parameters and characteristics that can characterize steel barrel leakage The vibration frequency; finally, the characteristics of the leaked acoustic emission signal obtained are compared with the simulation results. As a result, the excitation frequency of the gas leakage vibration of the steel drum is 22 to 40 kHz, and the peak frequency is about 25 kHz. Conclusion The simulation results of steel drum leakage are basically consistent with the structural response obtained by experimental detection. The simulation and experimental research methods of steel drum leakage provide a basis for the research of steel barrel gas leakage on-line detection technology.
Key words: steel drum leakage; fluid-solid coupling; acoustic emission; structural response
Steel drums have evolved from temporary storage of general materials to current transportation packaging, sales packaging, industrial packaging, etc. From manufacturing to consumption, a flow tool has been formed, which has become a means of storing interior materials for a long time. The steel drum production line has been automated, and its quality is not satisfactory due to raw materials, production facilities, testing, management and other reasons. Leakage is the primary cause of the quality of the steel drum. Most of the leakage occurs at the barrel weld and the bottom of the barrel. At present, the leak detection methods used in steel drum production lines include soap water foam detection method and pressure decay method, and most companies use the former.
The steel drum is a thin-walled structure, and the high-pressure fluid in the steel barrel is sprayed from the leak hole to the high-speed jet to excite and rub the hole wall, and the wall surface of the excitation leak hole generates vibration, and the vibration propagates in the form of stress wave in the barrel body, that is, generates The acoustic emission signal. Although the characteristics of the acoustic emission signal generated when the steel barrel leaks are very complicated, numerical simulation can be carried out by means of simulation software to reasonably express and analyze the influence of the acoustic emission at the leakage hole. When the steel drum leaks, a jet is generated at the leak hole. The jet spray column is the main cause of the acoustic emission signal generated by the steel drum. The characteristics of the spray column determine the intensity and propagation characteristics of the acoustic emission source. The characteristics of the jet spray column depend on the leak hole. The size, shape and internal pressure of the steel drum, etc., therefore, it is necessary to analyze the response characteristics of the steel tank leakage under the fluid excitation structure. In this paper, the finite element fluid-solid coupling (FSI) method is used to analyze the structural response of the leakage induced vibration of the steel drum, and the characteristic frequency is obtained. The leakage of the steel barrel is experimentally studied. Finally, analyze and compare the results of simulation and experiment.
1 Numerical simulation of steel drum leakage
1.1 Fluid-solid coupling basic governing equation
The fluid-solid coupling analysis mainly includes two methods: strong coupling and weak coupling. The strong coupling method integrates the fluid and solid control equations and unifies them directly, and the fluid-solid coupling interface is simplified into an internal solution domain. Although this method is clear and clear, the mesh size of fluid and solid should be highly consistent when solved. Due to the limitation of computing power, it brings great inconvenience to the analysis and calculation. The practical application of engineering is less, and it is mostly used for theoretical research. The weakly coupled solid-domain and fluid-domain calculation models need to be established separately. The flow field force is solved by computational fluid dynamics (CFD), the structural response is solved by computational structural dynamics (CSD), and then the momentum is realized between the two models at the interface. The exchange, the two physics complete the coupling process. In the solution process, the CFD and CSD solutions are performed separately. After each time step is calculated, the data is exchanged, thus maintaining the independence and integrity of the two modules, and the advantages of each field can be exerted. According to the characteristics of steel drum leakage, weak coupling is used for calculation.
Under the action of fluid, the structural dynamic response equation of the steel drum is:
Where: ms is the structural mass; Ks is the stiffness matrix; Cs is the structural damping; S is the structural vibration displacement vector; F is the external excitation force of the steel drum structure, which needs to be calculated for the fluid. Fluid motion is subject to laws of conservation of mass, including mass conservation, momentum conservation, and energy conservation.
The mass conservation equation is the continuity equation, which means that the sum of the masses of fluid flowing out of the control body per unit time should be equal to the mass reduced by the density change during that time. The fluid flow continuity equation is:
Where: u is the velocity of fluid motion; t is time; Ï is fluid density.
Newton's second law adapts to the momentum equation. The momentum equation, also called the fluid dynamics NS equation, refers to the rate of change of the momentum of the fluid microcell to time t equal to the resultant force of the external forces acting on the microcell. Thus, the momentum conservation equation is expressed as:
Where: t is time; u is the fluid velocity vector; is the density; p is the fluid pressure; Î¼ is the dynamic viscosity; F is the unit mass force vector; âˆ‡ is the Hamiltonian; Î” is the Lagrangian operator.
For the same reason, the fluid-solid coupling problem should also follow the basic conservation theory. Therefore, equation (4) needs to be satisfied at the coupling interface:
Where: Ï„ is the stress; d is the displacement; q is the heat flux; T is the temperature. The subscript f indicates a fluid and s indicates a solid.
1.2 Establishment of fluid-solid coupling model
In the Ansys Workbench software, the transient dynamics Transient Structural module is used to numerically simulate the leakage acoustic emission signal of the steel drum. The correlation between the diameter of the leak and the internal pressure and the characteristic value of the flow-induced vibration response of the inner wall of the gas leakage leak of the steel drum is simulated. The flow field unsteady calculation uses Fluent software, the governing equation is the unsteady Reynolds average NS equation, and the turbulence model uses the standard K-Îµ model.
The finite element model of steel drum domain was established in Transient Structural. The material parameters: density Ï is 7800kg/m3, elastic modulus E is 210GPa, Poisson's ratio Î³ is 0.3, and the structural mesh is divided into tetrahedral mesh. Set the parameters of the fluid domain in Fluent, select the ideal gas as the flow medium, and the fluid grid also adopts the tetrahedral mesh. The mesh of the fluid and structure is divided separately, and the grid cell size should satisfy the formula (5).
Where: L is the grid cell size; c is the acoustic propagation velocity in the medium; fmax is the calculated maximum frequency.
The time step of the transient analysis of the domain and the fluid domain should be consistent, both set to 2.5Ã—10-6s. Due to the large amount of coupling calculation, the calculation deadline is set to 0.0025s, which can meet the research needs. The calculation model is a cylindrical thin-walled steel drum with open ends. The wall thickness of the barrel is 1.2mm. The leakage and fluid domain and the domain model of the steel barrel are shown in Figure 1.
Figure 1 Steel barrel leakage model and meshing
1.3 Fluid-solid coupling simulation process
The variable values â€‹â€‹of the grid points of the flow field, such as pressure and velocity, are separately solved in Fluent. The strain and displacement of each node in the structure field are calculated separately in Structural. The data calculated by the flow field and the structure field are transferred and exchanged on the System Coupling module, and the exchange is bidirectional, that is, the pressure load is transmitted from the flow field to the structure. The structural coupling surface of the field. The structural displacement or stress of the structural surface is transferred to the flow field, which affects the flow field distribution. Then the two physical fields are restarted to solve separately until the pressure load and displacement data transmitted on the coupling interface reach the convergence criterion and stop iterating. The final result of the time step is then entered into the calculation of the next time step. In this way, the law of the variation of the flow field and the structure field with time can be calculated. In order to obtain the response law of the flow-induced vibration structure when the steel drum leaks, it is necessary to simulate the steel drum under different leak diameters and different internal pressures.
1.4 Simulation Simulation Results and Analysis
For the steel drum with internal pressure of 0.18MPa and leakage hole diameter of 0.5mm, the stress distribution of the leakage barrel structure and the enlarged view of the leakage hole are shown in Fig. 2. It can be seen from Fig. 2 that the stress response contour of the steel drum structure is distributed along the circumferential direction, and the structural stress response of the steel drum at both ends away from the leak hole is small, and the stress response is more obvious near the leak hole structure, and reaches the maximum at the leak hole. At the leakage hole, the axial distribution of the structural response stress is significantly larger than the circumferential direction, that is, the amount of axial deformation at the leakage hole due to the jet flow is large.
Fig. 2 Response stress distribution of leakage structure of steel drum
In order to verify the influence of the diameter of the leakage hole on the leakage of the steel drum, the influence of the change of the diameter of the leakage hole on the stress intensity at the leakage hole of the steel barrel was analyzed. The diameter D of the leakage hole was set to 0.2, 0.5, 1.0, 1.5 mm, respectively, and the internal pressure was 0.18 MPa. The effect of the change in leakage pore size on the stress intensity of the leaked hole is shown in Fig. 3. It can be seen from Fig. 3 that as the diameter of the leakage hole of the steel drum increases, the stress intensity of the leakage hole increases slowly and does not change significantly. thus
Figure 3 Curve of the diameter of the leak hole and the maximum stress intensity
It can be seen that the diameter of the leak hole has a very small influence on the stress intensity. Observe the observation point on the wall of the leak hole and analyze the vibration response of the particle point with time. The stress intensity spectrum of the leakage vibration structure of the steel drum with different leakage hole diameter is shown in Fig. 4. It can be seen from Fig. 4 that when the diameter of the leakage hole of the steel drum changes, the stress amplitude, frequency and attenuation at the leakage hole do not change significantly, which indicates that the change of the leakage aperture has little effect on the stress amplitude of the particle at the leakage hole. The excitation frequency at the time of leakage is 23 to 35 kHz, and a significant frequency spike occurs at 24.8 kHz.
Figure 4 Stress intensity amplitude and spectrogram of different leak diameters
Since the steel drum is a thin-walled part, the internal pressure of the steel drum should not be too large, otherwise the steel drum will be plastically deformed and cannot be restored to its original state. In order to verify the influence of the internal pressure of the steel drum on the leakage, the internal pressure was set to 0.15, 0.18, 0.21 and 0.24 MPa, respectively, and the influence of the change of internal pressure on the stress intensity at the leakage hole of the steel drum was analyzed. The effect of the change in internal pressure on the stress intensity at the leak hole when the diameter of the leak is 0.5 mm is shown in Fig. 5. It can be seen from Fig. 5 that the greater the internal pressure of the steel drum, the greater the response stress intensity of the steel drum flow-induced vibration structure, but with the increase of the internal pressure, the stress intensity does not increase proportionally, and the increasing trend is coming. The slower.
Figure 5 Curve of internal pressure and maximum stress intensity of steel drum
An observation point on the wall of the leak hole is used to analyze the vibration response of the particle with time. The stress intensity spectrum of the leakage vibration structure of the steel drum under different internal pressures is shown in Fig. 6. As the internal pressure of the steel drum increases, the stress intensity of the particle at the leakage hole gradually increases, but the amplitude changes periodically, and the vibration frequency does not change substantially. It can be seen that for steel drums of the same diameter, the internal pressure of the steel drum has little effect on the frequency of the signal generated by the leakage. The excitation frequency of the leakage structure response of the steel drum under different internal pressures is basically the same as the excitation frequency of different leakage hole diameters. The frequency range is 22-35 kHz, and the obvious frequency peak appears at 24.8 kHz.
Figure 6 Different internal compressive stress intensity amplitude and spectrum
2 Steel barrel leakage analysis
2.1 Detection principle
The leakage defect of the steel cylinder emits a stress pulse wave under the action of pressure, that is, an acoustic emission signal. The stress pulse wave is a kind of mechanical wave. When it propagates to the surface of the material, the acoustic emission sensor receives the acoustic emission signal, converts the mechanical signal into an electrical signal, and then amplifies it to the acquisition card through a preamplifier connected thereto. The capture card then converts it to a digital signal and reads the waveform data to the computer. The detection system device is shown in Figure 7. According to the simulation results, a resonant high-frequency sensor with a center frequency of 150 kHz and an acoustic emission acquisition chassis SAEU2S of Beijing Shenghua are selected.
Figure 7 detection system device
2.2 Results and analysis
The steel drums with different leakage diameters and different internal pressures (see Figure 7) were studied. See Table 1 for the relationship between the diameter of the different leak holes and the maximum amplitude, excitation frequency band and frequency peak of the leakage signal of the steel drum. See Table 2 for the relationship between the maximum amplitude, excitation frequency band and frequency peak of different internal pressure and steel drum leakage signals.
Table 1 Experimental data of leakage of different internal pressure steel drums
Maximum value /V
Excitation band / kHz
Frequency peak / kHz
It can be seen from Table 1 and Table 2 that when the internal pressure is different, the amplitude of the acoustic emission signal has a large change. When the diameter of the leak is different, it has little effect on the amplitude of the signal. The internal pressure and the diameter of the leak hole do not affect the values â€‹â€‹of the excitation frequency band and the peak frequency when the steel drum leaks.
2.2 Comparison of experiment and simulation results
The leakage experimental spectrum of a steel drum with a diameter of 0.5 mm and an internal pressure of 0.18 MPa is shown in Fig. 8. It can be seen from Fig. 8 that the characteristic excitation frequency for characterizing the leakage of the steel barrel is two frequency segments of 22 to 45 kHz and 95 to 115 kHz. However, the signal strength in the 22 to 45 kHz frequency band is much larger than the 95 to 115 kHz high frequency band, and the peak frequency is 25.6 kHz. From the spectrum analysis results of the vibration response analysis of the fluid-solid coupling structure of the front steel drum leakage, the simulated excitation frequency is 20-35 kHz and the peak frequency is 24.8 kHz. The simulation results are in good agreement with the experimental results. The reliability of the simulation study on the mechanism of the acoustic emission signal generated by the fluid-induced vibration of the steel drum by the solid-coupling method.
Figure 8 Spectrum diagram of acoustic emission experimental signal
Numerical analysis and experimental methods were used to study the structural response of the gas leakage induced by steel drum gas leakage. The conclusions are as follows. These conclusions are crucial for understanding the leakage mechanism of steel drums and developing the steel barrel leakage detection system by using acoustic emission technology.
1) When there is a leak hole in the steel drum, the high-speed fluid vibrates the excitation of the wall of the leak hole, which is the main cause of the acoustic emission signal. The jet spray column is the main reason for the leakage acoustic emission signal generated by the steel drum. The characteristics of the spray column determine the strength and propagation characteristics of the acoustic emission source.
2) The fluid-solid coupling simulation model of steel drum leakage is established. The analysis is carried out by means of finite element analysis software: the contour of the leakage structure of the steel drum is distributed along the circumferential direction, and the structural response of the leakage hole is the largest, and the steel drum is far away from the leakage hole. The end structure stress response is smaller. The structural response of the leak at the axial stress distribution is significantly larger than the circumferential distribution.
3) Using the control variable method, the response of the fluid-solid coupling flow-induced vibration structure with different leakage hole diameter and internal pressure is analyzed numerically. As the diameter of the leakage hole of the steel drum increases, there is no significant change in the stress amplitude, the excitation frequency range and the frequency peak at the leakage hole. As the internal pressure of the steel drum increases, the amplitude of the stress intensity at the leakage hole increases gradually, but the vibration frequency does not change substantially. That is, the excitation frequency and peak frequency of the particle at the leakage hole do not follow the diameter and inside of the leakage hole. The change in pressure varies, and 24.8 kHz is its excitation peak frequency.
4) According to the simulation results, the selection and experiment of the experimental equipment, the steel barrels with different pore sizes and internal pressures can be found that the experimental results of the steel barrels agree well with the numerical analysis results. According to the numerical simulation results, the feasibility of steel drum inspection is inferred, and the reliability of the simulation data is verified by the experimental results. Numerical analysis and experimental research provide theoretical basis and experimental scheme for the development of steel drum inspection system in the later stage.
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