A numerical model of billet reheating furnace is proposed, which includes heat fluxes calculation around four billet surfaces and two-dimensional conduction calculation inside billet. Radiation and convection heat fluxes on top and bottom surfaces are calculated simultaneously, based on quartic and linear difference between furnace gas and billet surface temperatures, while furnace gas temperature is determined according to thermocouple values along furnace length together with billet surface temperature. Lateral fluxes are also calculated considering angle factor on billets interval. Two-dimensional partial differential equation is acquired for billet conduction to determine temperature distribution, which is discretised and solved by Alternating Direction Implicit and TriDiagonal Matrix Algorithm. Two embedded thermocouple experiments were carried out to verify furnace gas temperature, the effect of billet interval on lateral heat flux calculation as well as billet temperature. It met agreement well with experiments on billet temperature, which could be a better prerequisite for further reheating furnace automatic control.

Software CFX 12.1 was employed to conduct simulation of the fluid field inside yarn suction guns that possess different jet orifice structures and the relation between fluid field distribution and yarn suction performance was analyzed. The effect mechanism of jet orifice structure on properties of yarn suction gun was clarified, achieving reasonable structural parameters: orifice number N was 3, orifice diameter d was 2.0 mm, and orifice angle was 75˚. The results indicated that increment of jet orifice number led to the increase of free diffusion of ejected airflow and enhancement of directivity, meanwhile, the air density near tube wall inside the gun and the circumferential velocity component of air were improved, thereby the yarn suction force was increased. The conflict among ejected airflows was increased by the superabundant jet orifice number, leading to the decreased yarn suction force. Reasonable orifice diameter was beneficial to the formation of strong swirling flow, and can prevent the occurrence of normal shock, resulting in the increased yarn suction force. The angle of jet orifice mainly controlled the circumferential velocity component value of air that causing the change of swirling flow strength thus changed the yarn suction force. The yarn suction efficiency was closely relative to the distribution of airflow; especially the influence of the circumferential velocity component of air was the most significant. The occurrence of normal shock was not beneficial to the enhancement of yarn suction force, which should be avoided as much as possible.

In order to demonstrate the effect of Laval tube structure of yarn suction gun on the yarn suction performance, throat diameter and throat length of the Laval tube were determined by theoretical analysis of the fluid field in the gun. 12 Laval tubes were designed, and yarn suction force (F) and mass flow rate of compressed air (G) were determined. Thereafter, yarn suction efficiency (η) was analyzed, where η=F/G. The rational geometrical parameters were obtained: converging angle of Laval tube α=90˚ and diverging angle of Laval tube β=6˚. The results indicated that there was a benefit to the airflow acceleration in the converging part of Laval tube and the formation of strong swirling flow by choosing the proper α, finally resulting in the improvement of η. The increase of super-sonic swirling airflow in the diverging section of Laval tube and the decrease of running resistance between the airflow and the yarn could be found when the reasonable β was selected, which also improved η. However, normal shock wave appeared in advance, the kinetic energy lost and the acceleration of airflow stopped due to the over-large β, leading to the decrease of η.