Theoretical studies of Hollow Fiber Spinning
A theoretical analysis of the hollow fiber spinning process is presented. The full two–dimensional conservation equations are solved for both the bore and clad fluids. Simulation results provide steady–state axial and radial profiles of velocity, temperature, and concentration in the spinneret and draw zone. Transient simulations allow the study of hollow fiber spinning instabilities as well. Isothermal simulations reveal that a recirculation region may exist in the bore fluid under certain operating conditions. For a given die geometry, the presence and size of the recirculation region is dependent on the bore–to–clad flow rate ratio and bore–to–clad viscosity ratio. The appearance of the recirculation region leads to a more rapid decrease of fiber radii after die swell and outer radii predictions are in better agreement with experimental observations than predictions from the one–dimensional thin filament analysis. The predicted two–dimensional temperature profiles for a typical solution hollow fiber spinning process are in good agreement with the one–dimensional approximation. However, significant differences exist between the predicted one– and two–dimensional concentration profiles, especially with a concentration dependent mutual diffusion coefficient. An improved one–dimensional analysis is presented based upon the superposition principle that virtually eliminates the differences between the 1D and 2D results. Transient simulations confirm the conclusion of previous one–dimensional linear and non–linear stability analyses. The results indicate that a critical draw ratio exists above which the spinline is unstable when subjected to a finite disturbance. The results also show that the critical draw ratio increases as Reynolds number increases in agreement with predictions for solid fiber spinning. Experimental measurements of fiber size are presented for the isothermal spinning of a hollow Newtonian filament using a silicone oil clad and air bore. For the experiments, the force exerted by gravity on the filament dominates draw down and the 1D and 2D simulations predict almost identical diameter profiles. The observed variation of OD along the spinline agrees qualitatively with the theoretical results. However, some discrepancy exists that is attributed to the effect of non–Newtonian die swell.
School:University of Toledo
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:hollow fiber membrane spinning fluid mechanics separations
Date of Publication:01/01/2007