Abstract: To elucidate effect of Laval tube structure on the performance of yarn suction gun for FDY, throat diameter and throat length of the Laval tube are determined by analyzing fluid field of the yarn suction gun on theory. By changing the converging angle of Laval tube ? and the diverging angle of Laval tube ?, yarn suction force F and mass flow rate of compressed air G of the yarn suction gun are measured, and then effects of ? and β on the yarn suction efficiency η (η = F/G) are analyzed. The optimal Laval tube is the one with geometrical parameters of ? = 90° and ? = 6°. η can be raised through a rational ?, with which the converging part of Laval tube accelerates airflow and makes the airflow develop into a strong swirling flow. A rational ? contributes to not only acceleration of super-sonic swirling airflow in the diverging part of Laval tube but also low running resistance of air and yarn, thus enhances?η. However, an over-large ? results in a low?η?because it causes normal shock wave to occur early, which stops the air acceleration and leads to a great loss of kinetic energy.

Key words: yarn suction gun, Laval tube, yarn suction force, yarn suction effiiency, converging angle, diverging angle