Effects of conical lids on vortex breakdown in an enclosed cylindrical chamber

The effects of the conical lids on vortex breakdown have been investigated numerically. The boundaries for the onset of vortex breakdown are plotted in terms of the Reynolds number, the aspect ratio H 1 ∕ R , where H 1 is the height of the side wall, and the height ratio H 2 ∕ H 1 , where H 2 is the height of the axis. The concave-cone lid ( H 2 ∕ H 1 > 1 ) delays or even completely suppresses vortex breakdown, while the convex-cone lid ( H 2 ∕ H 1 < 1 ) precipitates the onset of vortex breakdown. The attached bubbles were found in the chamber with the convex-cone lid at a smaller height ratio. The critical aspect ratio, below which vortex breakdown cannot occur, decreases with the increase of the height ratio. The results indicate that a conical lid can enlarge the vortex breakdown boundaries and this might be of practical interest when using this device for mixing. The results also suggest that the actual shape of the lid has an influence on suppressing or intensifying vortex breakdown.