Date of Award

2000

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Supervisor

J. Vlachopoulos

Abstract

The present work is concerned with the numerical simulation of air-cooling in the film blowing process. In many film blowing production lines, film cooling is the limiting stage in achieving higher productivity. At the same time, the cooling air stream affects the stability and the shaping of the molten bubble, as well as the morphology of the final film product. The cooling process ultimately affects both production rate and final film properties. The aerodynamics of the external cooling air and the effect of air-ring design and operational setup on the cooling efficiency were examined, using turbulent airflow simulations based on the k-[varepsilon] theory. Both single and dual air rings designs were studied. The results suggest that the air-ring airflow is dominated by the Venturi and Coanda effects and that the airflow patterns are very sensitive to minor air-ring design modifications. The variation of heat transfer coefficients along the bubble surface has been examined in detail. Additional numerical simulations were performed to evaluate the performance of a typical internal bubble cooling (IBC) configuration. Some of the limitations of typical IBC implementations were identified and attempts were made to improve the designs using the numerical simulation as an optimization tool. The results suggest that numerical simulations can be used to gain valuable insight on the IBC operation. Numerical simulation can be helpful in reducing the number of trial and error steps during the design and implementation of IBC systems. The development of sizeable temperature gradients inside the melt (in the film thickness direction) was investigated using numerical simulation. Typically, large air-cooling rates at the film surface combined with the low thermal conductivity of polymers lead to significant temperature differences between the internal and external film surfaces. The result indicate that the temperature differences in the film thickness direction may be very large and, therefore, important from a design and modeling perspective. A new methodology to calculate the film stresses is proposed, in which the temperature variations in the thickness direction and their effects on melt rheology during the blowing are taken into account. The results indicate that the film reaches the crystallization temperature having stress differences in the thickness direction. At high production rates the stress differences became more pronounced. Since the film stresses are directly related with the crystallization kinetics, it is conjectured that the final film morphology and properties are affected by the predicted stress differences in the thickness direction.

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