Numerical analysis of the comparison between annular and linear shunt pipes for chemical lasers 8 Jin Donghuan, Liu Wenguang, Chen Xing, Lu Qisheng, Zhao Yijun (School of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, Hunan, the structure and linear shunt pipes are quite different. The three-dimensional numerical simulation and comparative analysis of each of the above-mentioned shunt pipes were carried out using the CFD method. The results show that the total pressure in the main pipe of the linear shunt pipe is higher than that of the annular pipe distributed on the outer arc of the branch pipe, but lower than that of the branch pipe. An annular pipe on the inside arc; whether it is branch flow or secondary flow in the annular pipe will cause radial velocity on the cross section of the main pipe, making the fluid flow show obvious three-dimensional characteristics; the flow of each branch of the branch pipe flows along the main stream The direction is basically rising, which is opposite to the trend of the total pressure distribution of the main pipe, and is similar to the static pressure distribution trend of the main pipe; in comparison, the flow fluctuation of the branch pipe of the annular branch pipe distributed on the outer circular arc is the smallest, at The most advantage in the uniform distribution of airflow In the middle, the branch distribution pipe is distributed on the inner circular arc. Mostly in chemical engineering, petroleum, ventilation, heating, water conservancy sprinkler irrigation and other projects, a device that evenly distributes gas or liquid is often needed. A distribution pipe consists of a main pipe and It is composed of several branch pipes with the same structure connected to the side of the main pipe. Shunt pipes are also widely used in continuous wave chemical lasers, such as combustion chamber injectors, sub-nozzle arrays, and gas supply pipes for gas curtain nozzles. The general chemical laser of the stage 1 shunt pipe structure 1 uses a linear shunt pipe, and the ring-column chemical laser uses a ring nozzle array and a cylindrical combustion chamber 131, and the matching gas supply pipe is a ring shunt pipe. A significant difference between the shunt pipe and the linear shunt pipe is the shape of the main pipe and the spray direction of the branch pipe. The uniformity of the air flow distribution is the main technical index of the shunt pipe. In chemical lasers, it is directly related to the mixing efficiency of the air flow 19: Jin Donghuan 1981 , Male, doctoral degree and combustion stability, beam quality in the optical cavity area, protective effect of the air curtain, etc. Research on diversion pipeline of more than one-dimensional theory or model.
1 Calculation model and analysis method Table 1 Boundary conditions of the calculation model Setting the boundary name Specific setting Inlet1 ~ Inlet2 pressure inlet, total pressure 303975ba total temperature 300KWall smooth, steel wall surface, constant temperature 300KSymmetry symmetric boundary, normal velocity component and all variables method The gradient is zero Outlet1 ~ Outlet18 pressure The average static pressure of the outlet header section and the distribution of the total pressure along the flow flow in the flow pipe, boundary layer flow '-and each split pipe with separated pbppe are composed of 1 header and 18 According to the symmetry of the pipe structure, the three-dimensional calculation model of each branch pipe is extracted as shown in the figure. (A) Linear shunt pipe: the main pipe is a straight pipe with a circular cross section, the diameter is 5mm, the length is 95mm, the branch pipe is a "flare" injection hole, the throat diameter is 0.5mm, the half angle of the expansion section is 15, and the hole interval is 5mm ; (B) Ring-shaped diverter pipe I: the main pipe is a curved pipe with a circular cross-section, the cross-sectional diameter is 5mm, the rotation radius is 80mm, the angle is 68.04, the axis arc length is 95, and the branch pipes are distributed on the outer arc of the ring pipe. The parameters are the same as the linear shunt pipe; (c) The annular shunt pipe: The difference from the annular shunt pipe I is that the branch pipes are distributed on the arc inside the main pipe. The ring-shaped shunt pipe I is mainly used as the auxiliary air flow supply pipe of the ring-column laser; the ring-shaped shunt pipe is mainly used as the gas supply pipe for the injection air flow of the combustion chamber.
In the process of grid division of the shunt pipeline calculation model, volume decomposition technology is used to divide the entire model into multiple interconnected geometries. Each geometry uses Cooper technology to generate an unstructured volume grid. The total number of grid cells is about 1 million; the working medium is single. The component N2, which is regarded as a compressible ideal gas medium, is calculated using molecular dynamics theory. The intermolecular force model uses Lennard-ones (6, 12) potential. The viscous model uses the Realizablek-e two-equation turbulence model, which has been effectively used for various types of flow simulation, including rotating uniform shear flow, free 2 calculation results including jet and mixed flow, and discussion of the model assumed airflow Each physical quantity in the flow process has nothing to do with time, that is, steady flow, which is consistent with the stable working state of the pipeline. At this time, the sub-sonic flow in the main pipe, the average Mach number Ma gives the average static pressure and total Along the pressure. For linear pipelines, the cross-section refers to a set of equidistant planes parallel to the inlet plane Inlet1 or Inlet2, with a spacing of 5 mm; for circular pipelines, the cross-section refers to the entrance plane Inlet1 or Inlet2 rotating around the center of rotation of the circular pipeline The resulting set of planes with equal angular spacing has an angular spacing of 3.58 °. The abscissa indicates the distance between each plane and Inlet1. For a ring-shaped pipeline, this distance refers to the arc length on the axis of the manifold corresponding to the angular interval of the cross section. It is the same as the interval between the cross sections of the linear pipeline. The distribution trend is consistent with the calculation result. The static pressure gradually increases along the flow direction, which means that after the airflow enters the main pipe, the speed continues to slow down, and the dynamic pressure is converted to static pressure, so that the static pressure increases, that is, the change in static pressure is determined by the momentum exchange. This situation is called It is the "Momentum Exchange Control Model" 171. In the middle of the main pipe, the air velocity is close to zero, and the static pressure and the total pressure are almost equal. In contrast, the total pressure in the main pipe of the linear shunt pipe is higher than that of the annular shunt pipe I, but lower than that of the annular shunt pipe. This is because whether it is a linear pipe or an annular pipe, the side where the branch pipe is distributed, the flow boundary layer is periodically destroyed due to the air flow split, the boundary layer is not fully developed, and the frictional resistance is less 14; The thickness of the lateral boundary layer is obviously greater than that on the side where the branch pipe is distributed, which is the main action area of ​​friction resistance. When the airflow flows in the annular pipe, it is subjected to centrifugal force. In order to maintain the balance of the movement, the pressure on the outside must be greater than the pressure on the inside. State 19. For the annular shunt pipe I, the thinning effect of the boundary layer on the outer wall of the pipe is destroyed by the shunt of the branch pipe, and the thickening effect of the inner wall boundary layer makes the total friction resistance exceed the linear shunt pipe, and the total pressure loss increases; for the annular shunt pipe In general, the thickening effect of the boundary layer on the inner wall of the pipeline is destroyed by the branching of the branch pipe, while the thinning effect of the boundary layer of the outer wall makes the total frictional resistance less than that of the linear shunting pipeline, and the total pressure loss is reduced. The distribution of the velocity boundary layer can be seen from the velocity cloud image on the symmetry plane of the manifold, and the results are consistent with the above analysis. The area of ​​the branch pipe with a medium velocity greater than 20m / s is uniformly colored white, and according to the symmetry of the gas supply at both ends, only half of the total pipe length is shown.
The air flow in the main pipe mainly flows along the direction of the main pipe axis, but the branch pipe of the velocity cloud pattern shunt pipe on the symmetry plane of the main pipe is generally connected to the side of the main pipe. Under the effect of the branch pipe, the air flow needs to turn to enter the branch pipe, which will be on the cross section The radial velocity is generated so that the fluid flow exhibits obvious three-dimensional characteristics. The velocity vector diagram on the cross section of the main pipe 225mm from the entrance plane Inletl is given. In (a) and (b), the branch pipes are distributed on the left side of the section; in (c), the branch pipes are distributed on the right side of the section. It can be seen from the figure that the airflow of the cross section of the main pipe generally flows to the side where the branch pipe is distributed. In addition to the axial velocity, it can be imagined that the flow line of the air flow in the main pipe is spiral, that is, a vortex is generated. structure. This situation is especially obvious in the annular diverter pipe I. Under the effect of the pressure difference between the inside and outside of the main pipe, the air flow flows from the outer wall to the inner wall along the wall surface. Thus, two vortices with opposite rotation directions are formed on the entire cross section, namely Dean vortex, which is the secondary flow phenomenon 9 in the curved pipe, as shown in (b).
The velocity vector on the cross section of the manifold is shown in Figure 1. The direction of the secondary flow generated by the pressure difference between the inside and outside of the manifold and the outlet of the branch pipe. Vorticity is defined as the rotation of the velocity vector, which can be understood as twice the angular velocity of the fluid micro-cylinder rigidly rotating around its center. 101. As can be seen from the vorticity diagram shown, as the airflow flows downstream from the inlet of the manifold , The vortex will gradually be dissipated due to the viscous effect. The direction of the secondary flow generated by the pressure difference between the inside and outside of the main pipe of the annular shunt pipe I is the same as the direction of the vortex flow generated by the branch pipe shunt. The superposition of the two makes the total vortex strength increased; The superposition of the two makes the total vortex intensity decrease.
The main function of the distribution pipe is to distribute the airflow of each branch evenly. The flow distribution of the airflow of each branch of the splitter pipe is given. It can be seen from the above that the flow of each branch of the shunt pipe is basically increased along the flow direction of the main pipe. From the comparison point of view, the flow distribution of the branch pipe is similar to the static pressure distribution trend of the main pipe, but opposite to the total pressure distribution trend of the main pipe. In the shunt pipe model, the mean value of the vorticity curve on the cross section of the main pipe, at this time, the mass flow of the nozzle is proportional to the total pressure of the air flow. The dynamic pressure in the main pipe is mainly along the direction of the main pipe axis, and the axis of the branch pipe is perpendicular to the axis of the main pipe. Then, the total pressure at the entrance of the branch pipe and the static pressure of the main pipe at the corresponding position seem more reasonable. Where the static pressure of the main pipe is high, the total pressure at the inlet of the branch pipe is also high, and the corresponding mass flow of the nozzle is also larger. In addition, the direction of the centrifugal force generated when the air flow flows in the annular duct I is the same as the flow direction of the branch pipe, which promotes the flow of the branch pipe. Near the inlet of the main pipe, the air flow speed is fast, and the generated centrifugal force is large. This promotion effect is obvious; The direction of the centrifugal force generated when flowing in the annular pipe n is opposite to the flow direction of the branch pipe, which has an obstructive effect on the flow of the branch pipe. This blocking effect is also more obvious near the entrance of the main pipe. This results in branch pipes near both ends of the main pipe. The flow rate of the ring-shaped shunt pipe I is larger than that of the linear shunt pipe, and the ring-shaped shunt pipe n is smaller than that of the linear shunt pipe. It can also be seen that near the center of the main pipe, the airflow speed in the main pipe is slow, and the centrifugal force effect is not obvious, and the main pipe static pressure of the annular shunt pipe n is higher than the linear shunt pipe, but the branch pipe flow at the corresponding position of the annular shunt pipe n is higher than the linear The shunt pipe is low. This is because the thickness of the boundary layer of the two branch pipes is different, and the boundary layer of the n branch pipe of the annular branch pipe is thicker than the branch pipe of the outlet flow distribution of the linear branch pipe. On the symmetry plane (Symmetry in the middle) of the branch pipe, the sharp angle structure at the entrance of the branch pipe of the annular branch pipe I is an obtuse angle, the acute angle of the annular branch pipe n, and the right angle of the linear branch pipe. In comparison, the inlet of the branch pipe of the annular branch pipe I is the most natural, the flow from the main pipe to the branch pipe is the smoothest, the branch pipe boundary layer is relatively thin, the linear branch pipe is centered, and the annular pipe n branch pipe boundary layer is relatively thick.
In order to better explain the change of the mass flow of each shunt pipe, the flow change amplitude a is defined as the flow change of each shunt pipe is shown in Table 2. The annular shunt pipe I has a small static pressure fluctuation, and the centrifugal force promotes the branch pipe at the entrance of the main pipe. Flow, so the flow fluctuation amplitude is the smallest, and it has the most advantage in uniformly distributing airflow; the linear flow distribution pipe basically has no centrifugal force, and the static pressure fluctuation directly determines the flow fluctuation, and the flow fluctuation range is centered; the annular flow distribution pipe n has a large static pressure fluctuation range, and the centrifugal force It hinders the flow of the branch pipe at the entrance of the main pipe, resulting in the largest fluctuation of the flow rate.
Table 2 Flow Fluctuation Amplitude Shunt Pipe Type Linear Shunt Pipe Ring Shunt Pipe I Ring Shunt Pipe n In addition, as can be seen from Table 2, the flow fluctuation amplitude of each gas supply pipe is less than 0.5%, mainly for two reasons: one It is the design of the pipeline structure, there is a parameter that can explain this, that is, the ratio of the total cross-sectional area of ​​the branch pipe of the flow distribution pipe to the cross-sectional area of ​​the total pipe, the smaller the value, the higher the uniformity of the flow field is 111, and the value of this article is 0.18; The second is the choice of gas supply mode. Compared with the single-end gas supply mode, the double-end gas supply used in this article means that only half of the branch pipes are supplied to each inlet, which is equivalent to the total cross-sectional area of ​​the branch pipes of each branch pipe. The ratio of the cross-sectional area of ​​the main pipe is reduced to half of the original, and this value becomes 0.09 in this paper, which effectively reduces the fluctuation of the main pipe pressure of the branch pipe and the flow of the branch pipe.
3 Conclusion The shunt pipe is an indispensable gas distribution device in the chemical laser supply system. The change of the pipe structure will have a greater impact on the flow state of the gas flow and the uniformity of distribution.
In this paper, a three-dimensional numerical simulation and comparative analysis of the linear shunt pipe used in general chemical lasers and the two annular shunt pipes used in ring-column lasers are carried out using CFD methods. The main pipe of the branch pipe directly supplies gas to the branch pipe and is the upstream pipe of the branch pipe. As the resistance to sunburn, the total pressure of the main pipe gradually decreases along the flow direction. On the side where the branch pipe is distributed, the gas shunt makes the flow boundary layer not fully developed. When the gas flows in the annular pipe, the pressure difference between the inside and outside of the main pipe will cause the outer wall boundary layer to become thinner and the inner wall boundary layer to become thicker. Under the combined effect of the two, the total pressure in the main pipe of the linear diverter pipe is higher than that of the annular diverter pipe I, but lower than that of the annular diverter pipe. Whether it is the branch flow of the branch pipe or the secondary flow phenomenon in the annular pipe, the radial velocity will be generated on the cross section of the main pipe, so that the fluid flow exhibits obvious three-dimensional characteristics. In addition to the axial velocity, the streamline of the air flow in the main pipe is Spiral, that is, vortex structure. The secondary flow direction generated by the pressure difference between the inside and outside of the main pipe of the annular diverter pipe I is the same as the direction of the swirl flow generated by the branch pipe. The superposition of the two makes the total vortex strength increase; The direction of the secondary flow is opposite to the direction of the vortex generated by the branch flow, and the superposition of the two makes the total vortex intensity decrease.
The branch pipe is a direct manifestation of the shunting effect of the pipeline and has a variety of forms. In this calculation model, the branch pipe is a "horn-shaped" injection hole, and the pipe has reached a supersonic flow state. The calculation results show that the flow of each branch of the branch pipe is basically increased along the flow direction of the main pipe. This is contrary to the trend of the total pressure distribution of the main pipe, and is similar to the static pressure distribution trend of the main pipe, that is, the flow rate of the branch pipe is largely controlled by the static pressure in the main pipe. In addition, the direction of the centrifugal force generated when the air flow flows in the annular pipe I is the same as the flow direction of the branch pipe, which promotes the flow of the branch pipe; the direction of the centrifugal force generated when the air flow flows in the annular pipe is opposite to the flow direction of the branch pipe, which hinders the flow of the branch pipe . Especially near the entrance of the main pipe, the effect of this effect is more obvious. Under the combined effect of these factors, the flow fluctuation amplitude of the annular diverter pipe I is the smallest, which is the most advantageous in uniformly distributing the airflow, the linear diverter pipe is centered, and the annular duct is the worst. In the practical application of shunt pipes, the air distribution effect can be further improved by increasing the cross-sectional area of ​​the main pipe, using multi-end air supply, and unevenly designing the diameter of the branch pipe.
Acknowledgements Thanks to the High Performance Computing Application Research Center of National University of Defense Technology for providing open high-performance computing resources.
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