Numerical simulation of debris-removal trajectories on the transport mirrors in high-power laser systems Download: 894次
1. Introduction
High-power laser systems (HPLSs) are large optical instruments for high-energy physics and inertial confinement fusion. The transport mirrors of HPLSs direct light from the main amplifiers to the target and impact remarkably upon the quality of laser beams arriving at the target[1]. Current mirrors work at a fluence of several joules per square centimeter, but future operating conditions will require these mirrors to sustain high fluences of up to tens of joules per square centimeter or even higher. Therefore, rigorous cleanliness is critical in preventing damage to the mirror surfaces and prolonging the lifetime of the optical devices[2].
The debris on mirrors includes natural dust falls, metal fragments, and organic fibers left during operations. Numerous methods have been used to keep the optical devices clean, but to date, few experimental results have demonstrated the optical cleanliness of the whole laser facility[3]. Wiping mirrors with a clean cloth is the commonly practiced technique. However, hard debris often scratches transport mirrors because uneven forces are used in wiping away debris. Thus, the Lawrence Livermore National Laboratory designed an
In the present study, the flow field and the removal trajectories of debris with varied types and sizes on the transport mirror are numerically investigated. A device is fabricated based on the simulation results. This device can capture and collect debris from the mirror surface online.
2. The numerical model
Computational fluid dynamics (CFD) is an important technique for investigating flow fields. With computer numerical calculation and image display, CFD can conduct numerical simulations on a system containing relevant physical phenomena, such as fluid flow and heat. Numerical analysis of the flow field with basic physical quantities (such as speed, pressure, temperature and concentration), as well as the distribution of fluid trajectory, is now a popular topic in fluid studies.
2.1. The model to be calculated
A two-dimensional (2D) model satisfies the requirements of understanding the fluid field and the trajectories of contaminant particles. The 2D model in this study included one air knife, one transport mirror and one cover (Figure
The 2D model was implemented using the business software Gambit 2.4.6, used for geometry and mesh generation, and the 20 640 quadrilateral elements were divided by quadrilateral mesh structures[6, 7]. The coupled implicit solver, steady-state calculations and second-order upwind discretization were adopted. The Courant number was 1. The simulation of the gas–solid flow field was calculated using the discrete phase model. The convergence criteria of the mass residuals and energy residuals were less than .
2.2. Boundary conditions and associated settings
The best way to remove a particle is to blow in the direction of the -axis, where the particle can be subjected to maximum thrust (Figure
Boundary conditions were set as follows. The outlet of the air knife was the pressure inlet. The gage total pressure was 9 atm, the supersonic/initial gage total pressure was 7 atm, the inlet temperature was 300 K and the turbulent energy and default dissipation rates were 1.
Boundaries , , , and were the pressure outlets. , , , and . The outlet temperature was 300 K. The turbulent energy and default dissipation rate were 1. The gage total pressures of boundaries , , and were 1 atm. The gage total pressure of boundary was 0.8 atm.
The remaining boundaries were walls, which were non-slip walls. , , and the length of the up-surface of the transport mirror was 400 mm. Dust, aluminum particles and stainless steel particles, with sizes of 10, 20, 40, and 80 , were used as debris, and they were uniformly distributed on the surface of the transport mirror. The densities of dust, aluminum (Al) particles and stainless steel (ss) particles were 1000, 2700 and , respectively.
2.3. Governing equations
The basic equations of fluid analysis, turbulent flow and particle motion were used as the control equations[8]. These basic equations included the equations for the conservation of mass, momentum and energy. The turbulent flow model was the RNG model, which is an improved model of the standard turbulent model of . The particle motion equation was the force-balance equation in Cartesian coordinates.
3. Simulation results and discussion
3.1. Flow field
Figures
3.2. Debris trajectories
Dust particles with four different sizes were considered. For each size, 200 dust particles were evenly placed on the transport mirror up-surface from the left to the right, and a numerical simulation was performed based on the numerical model and settings stated in Section
Fig. 7. Trajectories of dust particles in four sizes. The sizes of the dust particles in (a)–(d) correspond to 10, 20, 40 and .
The simulation results demonstrate that the motion displacements of the majority of the particles are small in the vertical direction, almost slipping along the upper surface of the mirror (type 2). The -coordinate of the upper mirror surface is 2 mm; thus the minimum value of in Figure
Fig. 9. Maximum values of three particle types in four sizes. The sizes of the particles in (a)–(d) correspond to 10, 20, 40, and .
The values of particles increase rapidly, decrease, then increase along the path, and finally decrease (type 1). This event is due to the fact that the region of the 10 particles instantly becomes a negative pressure zone when the air knife is blowing. Then, turbulences (Figure
With the increase in particle sizes, the numbers of particles of type 3 that begin to reach the maximum values, hit the mirror surface and finally rebound off the surface also increase. This phenomenon is due to insufficient pneumatic forces to overcome the additional gravity caused by the increase in the particle sizes in the falling phase, and more particles crash into the mirror surface and rebound off. The maximum value is 19.1 mm and the size of the dust is at the moment when the four sizes of particles are present.
Similar results are also obtained when numerical simulations are conducted for stainless steel and aluminum particles in four sizes. The corresponding maximum values are 23.6 and 24 mm, and their sizes are both .
Comparing the maximum values of the dust, the aluminum particles and the stainless steel particles, the greater the particle density, the greater the maximum value when the particles are of the same size (Figure
4. The device
A device was designed based on the above model of the air knife (Figure
Table 1. The numbers of particles before and after blowing.
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Ordinary glass with the size of was used as a substitute for the transport mirrors. The glass was divided into four pieces to facilitate the measurement of the number of dust particles using a microscope. Each piece had the same size of . Circular measurement areas of with a diameter of 10 mm were set. Positioning centers represented by ‘’ were distributed on the opposites of the measurement surfaces (Figure
The experimental results are shown in Table
However, the numbers of particles in the particle size ranges of and are reduced from 495 to 79 and from 668 to 43 respectively after blowing when a debris collector is used. This result indicates that the removal efficiency of dust particles in the two ranges is 84% and 94%, which is better than the case without a debris collector. Therefore, using a debris collector can improve removal efficiency. Additionally, 9% and 1% of the dust particles in the two size ranges transfer to the surfaces of #2, #3, #4 and #5 transport mirrors, and these percentages are lower than the case without the debris collector. This result suggests that the debris collector can collect dust particulates and reduce the probability that particles transfer to the surrounding environment, and partly validates the simulation results of the debris-removal trajectories.
5. Conclusions
In the present study, numerical simulations of debris-removal trajectories on the transport mirrors of HPLSs are conducted using Fluent commercial software. The trajectories of contaminant particles of different sizes and types on the transport mirror surface are determined. A device is built based on the simulation results to efficiently capture and collect debris from the surface of the mirror online. Ultimately, debris contamination of other optical components is avoided, cleaning time is shortened and the cleanliness of the mirrors is ensured. Only mirrors laid horizontally have been considered in the present work. Although many transport mirrors are laid non-horizontally in practical applications, more studies are required to understand the debris-removal trajectories of horizontal mirrors. Meanwhile, the cover should also be optimized to capture and efficiently collect debris particles, and further verify the simulation results of the debris trajectories.
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Article Outline
Yangshuai Li, Jianqiang Zhu, Xiangyang Pang, Hua Tao, Xiang Jiao, Yongzhong Wu. Numerical simulation of debris-removal trajectories on the transport mirrors in high-power laser systems[J]. High Power Laser Science and Engineering, 2015, 3(1): 010000e5.