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We investigated numerically flow patterns in the air suction gun and dependence of the flow pattern on the supplied air pressure in order to clarify the working mechanism of an air suction gun. The compressed air issued from compressed-air inflow tubes into a yarn passage accelerates with sucked ambient air owing to negative pressure generated by the compressed air, and attains a critical speed near the throat of the de Laval tube and a supersonic speed in the divergent part of the de Laval tube. The supersonic flow generates a normal shock wave and changes into a subsonic flow. Then, the air is discharged into the atmosphere. Since this compressed air has a circumferential component, it forms a helical flow along the wall of the yarn propulsion tube composed of the de Laval tube and the straight tube. Velocity and density of the helical airflow near the wall are larger than those near the centerline. The suction efficiency is promoted greatly owing to this high focusing ability (bias of high speed and density flow toward the vicinity of the wall) and a large yarn length in the helical airflow. Increased supplied air pressure brings about increases in both air density and supersonic flow region, which promotes the yarn suction force.

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The yarn suction efficiency of an air suction gun is closely related to the airflow patterns, which are strongly affected by the geometry of the gun. To obtain basic data for the optimum design of a gun, we investigated the airflow patterns in the gun with different geometrical parameters by numerical simulation and discussed the relation between the flow patterns and yarn suction force Fm. Compressed-air inflow angle plays an important role in generating a helical flow by controlling circumferential velocity component vc in a yarn propulsion tube. This helical airflow greatly promotes yarn suction capacity. Fm has a closer relationship to the distribution of air velocity than air pressure, and strongly depends on vc. The airflow patterns are weakly dependent on a passage diverging angle of nozzle and a converging angle of de Laval tube. A reduction in throat diameter of de Laval tube causes a rapid extension of the supersonic flow area near the throat accompanied by increasing axial velocity component in the de Laval tube. However, it leads to decreases in vc in the yarn propulsion tube and air velocity in the yarn inhalation tube, which hinders the promotion of Fm.

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