The effects of dual‐orifice air‐ring design on blown film cooling

MTIRODUCTION he air cooling system is an integral part of any T blown film line. It greatly affects not only the heat transfer from the molten polymer film but also the stability and the shaping of the bubble. Several researchers (1-5) have examined the importance of the heat transfer in the modeling of film blowing. Film cooling ultimately affects both production rate and final film properties. Two important aerodynamic phenomena are associated with the cooling airflow, namely the Ventwi and Coada effects. The well known Venturi effect is caused when a fluid flows through a constricted area: its speed increases and the pressure drops (Rg. I). In film blowing, the bubble is pulled by the partial vacuum towards the walls of the air ring. The less known coanda effect (6) occurs when a free jet emerges close to a surface: the jet lends to bend, “attach” itself and flow along the surfzce. The surface may be flat or curved and located inclined or offset to the jet (Fig. 2). The Coanda effect is more pronounced near curved surfaces, and blown film bubble surfaces with the cooling air impinging on them at an angle, offer the possibility of appearance of the Coanda effect.