Study On Working Mechanism Of FDY Yarm Suction Gun

A yarn suction gun sucks running yarns by Venturi effect which generates by high-speedairflow, and then conveys running yarns by the friction between high-speed air and yarnsurface. With the development of chemical fiber production technology, there is a growingneed for the yarn suction gun with the high performance, such as strong suction capacity, lowenergy consumption, abrasion resistance, low noise and so on. Improvement of theperformance has been the major issue in this area. In order to provide a technical support fordesigning a yarn suction gun with high performance, we used CFD software ANSYS-CFX12.1to simulate airflow patterns of a FDY yarn suction yarn with different geometricalparameters, and discussed the relation between the flow patterns and yarn suctionperformance. As a result, the affecting mechanism was clarified and the obtained results are asfollows:(1) The compressed air issued from jet orifices of the nozzle into the yarn passageaccelerates in converging part of Laval tube with sucked ambient air, attains a sonic speednear the throat of the Laval tube and keeps accelerating to a supersonic speed in the divergingpart. However, a normal shock wave occurs and a subsonic flow is generated. Then, the yarnand air are discharged into the atmosphere. Velocity and density in the yarn suction gun nearthe wall are larger than those near the centerline. This ability of high-density andhigh-speed airflow bias toward the vicinity of the wall and a large yarn length in the helicalairflow increases the suction efficiency.(2) The yarn suction force F has a close relationship to the distribution of air velocity,and depends strongly on the air circumferential velocity component vc. A part of kineticenergy will converts into heat energy because the normal shock wave occurs in the Laval tube,which decreases F and increases energy loss. The normal shock wave should be avoided whendesigning the yarn suction gun.(3) Increasing supplied air pressure brings about increases in both air density andsupersonic flow region, which promotes the yarn suction force. However, successively raisingsupplied air pressure causes a decrease in the air velocity in the yarn inhalation tube, whichleads a decrease in yarn capture force. And, the normal shock wave occurs and becomesstrong, which converts kinetic energy into heat energy. In order to avoid kinetic energy losscaused by normal shock and other negative impact, the supplied air pressure should not toolarge.(4) The rational parameters of nozzle were obtained as follows: number of jet orifices N=3, diameter of jet orifice d=2.0mm, angel of jet orifice=75°and passage divergingangle of nozzle=60°. An increase in N constricts free expansion of the injected air amongjet streams and keeps them move forward more stably, which causes increases in air density and circumferential velocity component vcnear the wall; An appropriate diameter of jet orificed promotes the yarn suction force by helping the development of a strong swirling flow andavoiding the occurrence of a normal shock wave. The angel of jet orifice plays an importantrole in generating a swirling flow by controlling the air circumferential velocity component vc.Increases is beneficial to the airflow distribution uniformly. At too large, more vortices orbackflows are found in the gun, which lead to a decrease in F.(5) The Laval tube with rational structure can be obtained when the converging angle ofLaval tube and the diverging angle of Laval tube are90°and6°, respectively. A rationalcontributes the air to accelerate smoothly near the throat, and reduces the backflows and theintensity of the normal shock wave in the Laval tube, which decreases the kinetic energy lossof air. Therefore, the suction efficiency is improved; A reasonable makes the distribution ofthe swirl flow in the yarn propulsion tube more suitable. Larger drag force of air is exerted onthe yarn, the frictional resistance of wall on the yarn is decreased, and the intensity of thenormal shock wave is reduced. This raises the yarn suction efficiency of the yarn suction gun.