抄録
Studies of flow-patterns in vertically standing rectangular-sectioned 180 deg bends whose radius ratio 0.6 and 1.5 were made.
In vertically standing such a stronger curved bend, very lower pressure comes out in a zone which is located along the internal wall just the latter half of the bend, so pressure gradient is very favorable and thus tenbs to more accelerate the fluid than in the case of horizontally lying.
The driving power which developes a pair weak vortexes to set in large one is the radial pressure difference caused by centrifugal force, the ratio of curvature of bend and the axial pressure defference caused by pose of the bend.
As well, vertical standing strong curvature makes the location of maximum velocity shift to inner wall from the outer boundary layer. Accordingly, time rate of change of moment of momentum enlarges as fluid particles turn around the head of inner wall of bend more guickly. In this way, more stronger vortex comes into the stream.
Flow visualization by still and cine photography recordings by injected dye stuffes through hypodermic needle at the riser were made.
The dye-streak which starts from the central nozzle of the riser branches off two parts of the first half of the bend by the centrifugal force owing to curved duct.
As the flow progresses to down-stream, two branched vortexes filaments grow more stronger vortexes by the favorable pressure gradient. It takes a form of slender spiral by suffering tensile force depend on rotational motion with the primary flow. This twin vortexes is thinkable forced vortex.
And the other side, injected dye-streak at the corner wall of the riser circulates with large radius taking a form of broad thin belton account of shearing force. This twin vortexes is thinkable free vortex.
In vertically standing 180 deg bend with strong curvature, a pair combined vortexes which constitution made up a forced vortex at the core and a free vortex around the forced vortex, under restraint by the duct shape and at the same time with the inlet flow-rates.
