Study Of Positive Pressure Sealing Method In A Large-diameter And Multi-band Laser Emission Pipe

Laser transmitter system is applied widely in the field of target ranging and target jamming and laser communication, because it has such advantages as fast response speed, strong anti-interference ability, etc. Now, the trend in development of laser transmitter system comes to be large-diameter and multi-band, and that brings a lot of new problems, in which the purification and sealing problem of laser emission pipe is of great significance to improve the quality and energy of laser. This paper proposes the purification positive pressure ventilation sealing method for the large-diameter and multi-band laser emission pipe, which can adapt to any band in laser emission. It not only can purify the pipe and reduce gas heat effect within the pipe, to achieve the purpose of improve the quality and energy of laser, but also can seal the pipe relying on the positive pressure effect of the cleaned gas.Based on the analysis of the working principle of the purification positive pressure ventilation sealing method, this paper illustrates the importance of guaranteeing the sealing for connection gap of the pipe. So, this paper carries out the work of structure design for static seal and dynamic seal and analyses the leakage, shows that static seal design can be a leak free, while the leakage of dynamic seal design is very small, which can be compensated during the purification positive pressure ventilation.Using the method of numerical calculation, this paper makes a study and analysis for the purification positive pressure ventilation sealing method. The continuous phase model to analyse the positive pressure sealing performance of the cleaned gas is established based on the conservation navier-stokes equations and approximation assumption of incompressible flow field; the discrete phase model to describe and track the trajectories of particles in flow field is established based on the DPM model of the fluid dynamics software FLUENT. The laser interacting with the fluid within the pipe has been made an approximate treatment, which is to put the laser heating effect as stable heat source into the calculation of the flow field of pipe. On the basis of the calculation results, it can be find the effect that the laser transmits in the flow field.The basic simulation model of the purification positive pressure ventilation for the large-diameter and multi-band laser emission pipe has been established, and the numerical simulation of continuous phase and discrete phase has been carried out on it. The results show that cleaned gas could resist the air backflow from outside and make a barrier function for the dust particle with kinds of sizes in the air to protect the optical element when inlet velocity of cleaned gas is more than 4 m/s, while the external environment is under the design request of the strong breeze. The numerical simulation results are verified by the experiments, which prove that the positive pressure sealing ventilation can purify the pipe and achieve the design requirements of purification and sealing of the equipment.After the validation of feasibility and effectiveness of the purification positive pressure ventilation sealing method, this paper conducts a study on its actual application to the laser transmitter system with large-diameter and multi-band and establishes its flow field simulation model. The study to the normal velocity and gas flow state in the monitoring surfaces finds that cleaned gas continuously meets air from outside and mixes it up and resists it. When inlet velocity of cleaned gas is more than 3m/s, it can resist air from outside to access to the secondary mirror which is the outermost optical unit; while more than 9m/s, the purification positive pressure ventilation can meet the requirements of dust removal efficiency, which is more than 90%. In addition, while inlet velocity of cleaned gas is larger, the resistance is greater. When inlet velocity of cleaned gas is more than 3m/s, the change of gas temperature in the pipe caused by the laser’s heating effect of the gas and mirror is within 1°C; the purification positive pressure ventilation can largely reduce the temperature rise caused by the gas absorption from laser energy, the drop can reach more than 10°C compared with no ventilation. Finally, the change of gas refractive index in the pipe caused by the change of gas temperature and the influence that laser transmits in the flow field is very small, which can be accepted for the combined long-wave beams and compensated through the structural adjustment.Aim at the performance requirements of purification and sealing for the pipe, this paper firstly has tested and validated the performance of dehumidification and dust removal of the air purification equipment. And then, through the numerical simulation and testing experiment in the indoor environment, it is found that cleaned gas can keep a certain velocity and positive pressure on emission pipe port and the dust removal efficiency for six particle sizes is more than 90% before the location of the secondary mirror unit, all of that means it achieves the design requirements of purification and sealing. The purification positive pressure ventilation sealing method, therefore, is a simple and effective method to purify and seal the pipe.