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 2
data for V profile in a vortex chamber did not agree to the empirical
formula by Escudier et al. (1982) in the annular region.
2.2 Methods of Swirl Generation
Swirl in the flow is generated by imparting tangential velocities in to the
flow. In the experimental setups the flow passes through the swirl generator
before entering into the main cylinder/ pipe/ tube or chamber where the
measurements are due to be taken. Different types of swirl generation are
shown in figure 2.5.
There are basically two categories of swirl generators as described below:
(1) The flow, after passing through the swirl generator, enters the main
chamber with a combination of tangential and radial component. These
types of swirl generators are mostly employed with guide vanes that are
fixed at a certain angle(s) to obtain a desired swirl and in some cases the
inlet is fitted in a tangential direction to the vortex chamber (Figure 2.5 a
and b ). Another way is that the flow enters the main chamber through
nozzles fitted or ports grooved (as in Large Two Stroke Diesel Engines)
on the periphery of the chamber walls. These nozzles or ports are at an
angle with the radius of the chamber.
(2) The second category contains devices that are axially aligned with the
main chamber. The flow enters the swirl generator in axial direction and
the swirl is introduced by a twisted tape or a propeller or an externally
rotated honeycomb (Figure 2.5 c and d).
The physical design of the swirl generator and the way swirl is introduced in
the pipe/ cylinder plays a very significant role in the resulting confined
swirling flow. For example, in experiments where the swirl is generated by a
rotating honeycomb section fitted axially with the test cylinder/ pipe, the
tangential velocity peak is always observed at larger distance from the
cylinder axis see (Pashtrapanska et al. 2006, Marliani et al., 2003) etc. The
vortex core will be large in size and the size of the annular region will be
small. The reason for this profile is due to the solid body rotation of the
honey-comb section and the flow passing through it following the same
profile. Due to the presence of wall boundary layer the location of peak
tangential velocity is observed at a small distance from the wall. The size of
the vortex is also influenced by the magnitude of radial velocity and in the
case of rotating honeycomb, swirl is introduced in a non-swirling axially
flowing fluid a larger vortex core should be expected. In the experiments
where the swirl is generated by a propeller (Parchen et al. 1998, Algifri et al.
1988) or twisted tape (Young et al. 1978) etc., also mounted axially with the
pipe axis, the tangential velocity profile and the vortex core size depend on
the physical profile of the twisted tape and propeller blades. In case of swirl
generators with guide vanes and a conical shaped center body (Sarpkaya
1974, Faler et al. 1977, Kitoh 1991) etc. the size of the vortex core is
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Swirling Flows