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Measurements were performed to characterize the development of the cooling jets produced by a typical dual-lip air ring used in the blown film manufacturing process. The distribution of the pressure on the bubble surface and the velocity field in the region above the forming cone were measured for a rigid model bubble with a blow-up ratio of 2.5. The measurements indicated that the development of the flow below the top of the forming cone in the air ring could be divided into two regions separated by local maxima in the static and fluctuating pressure. These maxima seemed to be associated with the location where the lower jet attached to the bubble surface. There was a region of negative gauge pressure on the bubble surface below the forming cone that should act to stabilize the bubble against the forming cone in practical applications. Above the forming cone, the lower jet was initially decelerated and then entrained into the upper jet. The upper jet then attached to the bubble surface and finally the flow recovered to a wall jet along the bubble surface. The profiles of the static and fluctuating pressure in the first and third regions above the forming cone collapsed reasonably well when they were scaled using the velocity of the lower jet and the velocity of the upper jet, respectively. The magnitude of the local maxima in the static and fluctuating pressure where the upper jet attached to the bubble depended on the upper jet velocity and the ratio of the upper and lower jet velocities.

Investigation of the Cooling Jets Used in the Blown Film Process