M.S. Thesis Presentation by Jelena Vukasinovic

(Dr. Ari Glezer, advisor)

"Countercurrent Flow in the Gap Between Co-Axial Cylinders"

Abstract

Fluid motion through an annulus having a rotating inner surface is present in numerous engineering applications including rotating filters and separators, screw pumps and extruders and the return flow between a rotating drill and stationary outer casing used in oil and gas exploration. Knowledge of the fluid motion in these geometries can help in the prediction of the torque and heat and mass transfer properties of the flow. Rotating flow field is dominated by the centrifugal instability and the evolution of secondary vortex structures. Therefore, it is important to understand the onset of these instabilities and the ensuing transition to turbulence and their dependence on the primary flow parameters such as the axial and rotational Reynolds number.

The present study has been focused on the experimental investigation of the flow induced by rotation of a doubly helical screw in a stationary co-axial cylinder as the model for the through-hole plating process of multi-layered printed circuit boards. The screw rotation gives rise to countercurrent axial flow in the gap between the screw and the hole to suppress the formation of cation-depleted boundary layers in the central section of the through-hole and also, stimulates the radial fluid mixing. The goal of the present investigation is to gain an insight as to when the centrifugal instability introduces the secondary helical vortex flow in the gap between the rotating screw and stationary cylinder and thereby increases the electrochemical mass transfer during plating process.

The azimuthal and axial velocity distributions in the gap are simultaneously measured using laser Doppler velocimetry for the rotational Reynolds numbers between 50 and 180. For the lowest investigated rotational Reynolds numbers, the flow is laminar and non-vortical. As the Reynolds number increases the flow undergoes a sequence of instabilities that are manifested by the appearance of coherent vortical structures with important ramifications to the plating process. Single helical vortex, wavy counter-rotating helical vortex pair and interpenetrating helical vortex flow are some of the secondary flow states observed experimentally by direct inspection of the streak visualization images and velocity power spectra.