Free vortex pneumatic window asymmetric nozzle design Yi Shihe, Jiang Zongfu, Wang Chengyu (School of Aerospace and Materials Engineering, National University of Defense Technology, Changsha, Hunan 073) uses a supersonic free vortex to seal the low-pressure laser cavity. In this paper, the process of determining the free vortex aerodynamic characteristics of aerodynamic windows is proposed. The application of the characteristic line method in the design of asymmetric supersonic nozzles is discussed. The nozzle is used to generate free vortex flow.
High-energy laser systems have a wide range of applications in industry and military, and have become one of the priority technologies in many countries. As the laser output power increases, the crystal window will generate thermal distortion or even burst due to the inevitable partial absorption of the crystal material output window. The use of cooling technology will have some improvement, but for high-power, large-caliber lasers of several tens of megawatts or even megawatts, due to the limitation of heat transfer rate, any type of crystal window is difficult to meet the requirements of laser output. .
The free vortex pneumatic window can be used for the beam output of high energy lasers. As shown in Figure 1, the laser beam is output through a pneumatic window of a supersonic jet that balances the pressure difference between the laser cavity and the environment by the momentum change of the pneumatic window jet gas. The selected working gas requires no absorption of the laser wavelength to ensure complete penetration of the laser energy. The asymmetric nozzle jet used in the free vortex aerodynamic window can adapt to the large angle steering of the jet, and the jet emerging from this nozzle has the characteristics of free vortex velocity distribution. For an ideal non-viscous jet, the streamline sweeps through a radius of curvature R and R over an arc of angle Δθ across the output aperture of the laser cavity. The pressure inside and outside the nozzle outlet is equal to the laser cavity and the ambient pressure, thus avoiding the generation of a strong wave system, thus ensuring the quality of the output beam. Moreover, the large flow steering also makes it possible to reduce the flow of working gas required to balance a certain pressure difference.
1 The selection of the asymmetric nozzle parameters includes the inner and outer radii R and the flow steering angle Δθ (hereinafter referred to as the free vortex section).
For the design of the pneumatic window, it is first necessary to determine the chamber pressure p, the total temperature T, the laser cavity pressure p, the ambient pressure p, and the type of working gas. The working gas is required to be transparent to the laser beam. The value of the inner and outer radii and the airflow steering angle Δθ of the desired free vortex section is then determined. The choice of Δθ is arbitrary, but should be made larger to increase the change in the amount of jet flow, while preventing the ΔθJournal of the National University of Defense Technology from being too likely to affect the laser beam. The value of R can be calculated from the aperture size D and the flow steering angle: the value can be obtained from the value of R and the velocity profile of the free vortex.
2 Asymmetric nozzle design The ideal free vortex section is created by the nozzle, so the nozzle design will start from the nozzle outlet section. As shown in Fig. 3, AB is the nozzle exit plane. The velocity and radius from any point a to e satisfy the condition of the free vortex section, and the speed direction is perpendicular to the exit plane, thus for a given chamber pressure p, total temperature T and For the optical cavity pressure p, the ambient pressure p, the parameters on the AB plane are known.
Only 5 points are taken on the AB plane, and a finer grid can be taken when actually calculating by computer.
After getting the parameters of the above area, the next step is to select the inner and outer contours. Obviously, the inner and outer contours can meet the requirements as long as their slopes are perpendicular to the AB plane at points A and B. It can be seen that if the wall contour is selected to be an arc and the center of curvature is at the exit plane, then the requirement can be met.
If the radius of curvature of the inner and outer arcs is R 2 respectively, the flow parameters of the ACDA region can be determined from the flow parameters and the inner contour line on the AC. The same method determines the flow parameters near the outer contour. When selecting the inner and outer contours, you must ensure that there are no homologous feature lines intersecting, and you should ensure that the Mach number increases monotonically with the streamline.
The arc of the inner and outer contour lines can extend all the way to the left to the E point and the D point. Of course, if necessary, the left part of the BE arc and the AD arc can also adopt other arc lines of curvature radius or other line types. . The ED line goes to the left to enter the single-wave zone, and its area has only the left or right line characteristic line. The flow parameters on the EF line are constant.
The left end of the EF line can be a conventional shrink-expanded nozzle, and the flow parameters of the conventional nozzle flow field on the EF line are non-Easy and the like: a preliminary study of the asymmetric nozzle of the free vortex aerodynamic window The flow parameters of the tube on the EF line. Thus, the conventional nozzle profile line to the left of the EF line, together with the arc on the right, forms the preliminary profile of the asymmetric nozzle that produces free vortex flow.
This design method must also consider the viscous effect. After obtaining the preliminary profile, the nozzle profile must be corrected for the cover layer. The thickness distribution of the boundary layer displacement is obtained according to the method of the reference, and then the nozzle profile is expanded outward by a displacement thickness to obtain the final nozzle profile.
3 Design results According to the above method, p = 1 / 20 atm, Δθ = 60 °, the working gas is air, and the nozzle type line is designed, as shown in Fig. 4. The curve in the figure has been corrected with a boundary layer. The upper and lower shear layers of the jet will distort the laser beam, and the thickness of the boundary layer on the outlet wall of the nozzle will affect the jet shear layer. In order to reduce the distortion of the laser beam, the thickness of the boundary layer of the nozzle outlet wall should be as small as possible. In the design process of the profile curve, the thickness of the boundary layer of the outlet wall of the nozzle is reduced by shortening the length of the nozzle without affecting the aerodynamic parameters.
Can be used for schlieren observation and interferometry. The total flow pressure, the pressure of the optical cavity, the surface pressure of the diffusion section, etc. are measured by a pressure sensor. Figure 6 is a photograph of the chamber pressure and cavity pressure curve of the pneumatic window recorded by the memory oscilloscope. Preliminary tests have shown that the nozzle design method is feasible and the design is successful.
Line photo
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