Influence of Laval nozzles on the air flow field in melt blowing apparatus

Melt blowing combines extrusion of molten polymer through small orifices with stretching of the hot extrudate by hot air jets to create long, small diameter, fibers. Simulations and experiments were performed to examine: (1) the influence of increasing air pressure inside the melt blowing die (P _inlet) on the air jet in a typical melt blowing process and (2) the influence of a Laval nozzle (a converging-diverging nozzle) on the air jet. The baseline case without a nozzle was simulated and examined based on the y-component of the air velocity profile, v _y(y), at the centerline as a function of P _inlet. As P _inlet increases: (1) the air flow goes from subsonic to supersonic and (2) the maximum value of v _y(y) increases with increasing P _inlet, then starts to oscillate with the formation of compression waves in the supersonic region, P _inlet≥15psig. Simulation also showed that a Laval nozzle influences the air flow field by increasing the maximum value of v _y(y) and eliminating the compression wave at a predictable value of P _inlet. Actual density oscillations in the supersonic flow field exiting a melt blowing die, with and without a Laval nozzle, were captured using a Schlieren visualization technique. In both limits the experimental results are in good agreement with the density oscillation in the supersonic flow field of the air jet anticipated by the simulations.