The effect of rim-shroud gap on the spiral rolls formed around a rotating disk

摘要

In this paper, we use the numerical and empirical approaches and investigate the flow around a finite size disk in a finite size outer cylindrical casing. The disk is located at the center of the stationary casing and it is supported by a driving shaft through the disk and the casing. The disk rotates with the driving shaft. Five sizes of disks with different thicknesses and radii are used. The main purpose of this paper is to estimate the effect of the radial gaps on the entire flows. The combination of the thickness of the disk with smaller radius and the inner side of the casing with larger radius forms Taylor-Couette-like vortex flow in the radial gap. The supercritical instability makes the unsteady wavy vortex flow around the disk rim. When the axial gap between the disk surface and the end-wall of the casing is wide, the unsteady wavy flow is convected along the end-wall of the casing and it makes spiral rolls with negative front angles. When the axial gap is narrower and the Reynolds number is small, the instabilities are confined in the radial gap, and a beadlike flow appears around the disk rim. At higher Reynolds number, the turbulent spirals and the mixed spirals with positive and negative front angles are predicted. It is also shown that the phase velocity of the spiral is about or above the half of the velocity of the rotating disk.