Experimental and Numerical Study of Swirling ... - Solid Mechanics

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
Chapter 4
experiment has been conducted using a single blade angle, therefore, all the
measurements are conducted at a constant swirl number. Thus it is not
possible to study the upstream effect of this exit contraction on the flow
profile in the cylinder as a function of swirl number, similar to studies
conducted by (Escudier et al. 1985, Escudier at el. 2006). The experimental
results from different cylinder lengths, however, indicate that the flow does
not feel the presence of outlet contraction until cylinder cross-sections very
close to it as in case of z6 with cylinder length 4D. The downstream change in
cylinder length is not detected by the flow at upstream positions and thus the
flow mainly depends on the upstream conditions at the cylinder inlet. The
flow behavior remains the same for a given Reynolds number and whether
or not the variation in the velocity profiles at different Reynolds number is
due to outlet contraction needs further experimental investigation.
For all the cylinder lengths used in this experiment, the vortex core follows a
helical path downstream the flow. The radius of the helix is not constant and
actually increases along the flow direction. However, in all the cylinder
lengths the helical path does not complete one rotation. Experiments
conducted by (Escudier et al., 2006) observed a helical vortex path when the
outlet contraction is made eccentric with respect to cylinder axis but in the
current experiment the outlet is aligned with the cylinder axis. The helix
rotation is clockwise representing a left-handed helical vortex. However,
contrary to the results obtained by (Alekseenko et al., 1999), the helical path
and fluid particles (swirl) have same direction i.e. clockwise.
At z1 the flow has a tangential velocity like Burgers vortex and the wake-like
profile of axial velocity does not show any reverse flow. As mentioned by
(Alekseenko et al., 2007) this represents that the helical path, followed by the
vortex core downstream the cylinder, has a negative pitch. For the cylinder
length 8D, at z10 and positions downwards, the tangential velocity profile
becomes like forced vortex and the axial velocity profile becomes almost
uniform. Besides, the vorticity profiles having Gaussian profile at z1 also
becomes uniform. This demonstrates that for cylinder length 8D at far down
stream positions the negative pitch changes to infinite value indicating a
transition of left-handed helical vortex to right handed helical vortex called
as ‘L-Transition’ and is one of the characteristics of vortex breakdown
(Okulov et al., 2002) (see Section 2.3.6). However, since z13 is close to outlet
for cylinder length of 8D, the complete transition of the left handed helical
vortex to right handed type cannot be seen from experimental results. It
probably requires conducting measurements by keeping the cylinder length
more than 8D. Another interesting feature is observed from the helical vortex
path for 8D cylinder length where instead of completing one revolution the
vortex path tries to re-twist at far downstream cross-sections. Similar results
are obtained from experiments by (Alekseenko et al., 2007) but the difference
is that in their experiment the outlet contraction is made eccentric to
cylinder axis. Further, this re-twisting of helical path, in their case, is
observed in the bottom vicinity (near inlet) whereas in current experiment it
is observed near cylinder outlet. This re-twisting indicates distortion of helix
and change in its direction (Alekseenko et al., 2007). Thus it can be
concluded that in case of cylinder length 8D, at far downstream positions,
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Swirling Flow in a Pipe