Evaluation of Stresses in a Two-Layer Co-Extruded LDPE Melt Blown Film
Despite the fast growth of co-extruded film production, little research work has been devoted to this process. Films of LDPE, ULDPE, LDPE/ULDPE and ULDPE/LDPE were produced at various processing conditions. A continuous experimental set up was used to evaluate the in-plane birefringence during bubble formation. The data collected were used to derive the axial and transverse stresses. A non-isothermal Newtonian model that allows the determination of the kinematics and dynamics of the two-layer film blowing process was then used to calculate the axial and transverse stress. Generally the calculated stresses are in agreement with the experimental values.
An Investigation of Venturi and Coanda Effects in Blown Film Cooling
Abstract Numerical simulation of turbulent air flow tangentially impinging on a blown film bubble has been carried out. The cooling air is assumed to originate from a dual orifice adjustable air-ring. The stream line pattern and heat flux are determined through a finite volume numerical technique using a version of the so called k-∊ modeling. It is shown that as the air flows through the air ring passage relatively large under-pressure is created (Venturi effect), which applies a suction to the bubble. Depending on the flow rates and geometrical configurations the air jet may attach itself (Coanda effect) to either the air ring surface and/or to the bubble. The Coanda effect has a significant influence on local heat flux. The numerical results suggest that the Venturi and Coanda effects influence significantly process stability, production rate and final film properties.
Finite element calculation of time-dependent two-dimensional viscoelastic flow with the explicitly elliptic momentum equation formulation
Abstract The explicitly elliptic momentum equation (EEME) formulation for the upper-convected Maxwell model is extended to the analysis of time-dependent flows. The formulation makes explicit the elliptic operator, which controls the spatial dependence of the velocity field, and the hyperbolic-like character of the time-dependent equations, which gives rise to the wave-like character of transients when inertia is included. Numerical solutions are computed by using finite element approximations for the spatial dependence of the velocity, stress and pressure, coupled with finite difference discretizations of time derivatives. Numerically stable and accurate solutions are demonstrated for the flows between concentric and eccentric rotating cylinders when the condition for change of type of the steady momentum equation from elliptic to hyperbolic is not satisfied. A semi-implicit algorithm is presented in which the normal stresses are computed implicitly from the corresponding components of the constitutive equation and the velocities, pressure and shear stress are computed implicitly from the EEME-continuity pair and the shear component of the constitutive equation at each time step. The coupling between these two calculations is explicit; however, calculations are not constrained to small time steps. An algorithm that decouples all stress components from velocities and pressure is shown to be severely limited to small time steps. These results are consistent with the constraints caused by the mathematical type of the governing equations.