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
Swirling Flows
This chapter gives a brief overview of flows having swirling motion. A general
classification of different swirling flow regimes have been given based on the
tangential velocity profile. Different swirl generation methods have also been
presented. Aspects of swirling flows related to current study have been discussed
based on experimental results available in the scientific literature. Some of the
challenges regarding the numerical modeling of swirling flows with focus on RANS
based models are also discussed.
According to American Heritage Dictionary of English Language, the word
‘swirl’ means ‘‘to move with a twisting or whirling motion’’. In fluid
mechanics the term ‘swirling flow’ can in general be defined as a class of
different flow types that involve an ‘overall’ twisting or whirling motion
because in many flows localized twisting or whirling motion of fluid exist as
a result of mixing or turbulence is observed yet they are not considered as
swirling flows. The main characteristic of swirling flows as defined by Baker
et al. (1974) and Ito et al. (1986) is that the flow has a combination of
tangential/ vortical and axial motions. Thus the presence of ‘vortex’ is an
important characteristic of swirling flows. Vanyo (1993) defines a vortex as a
mass of fluid whose elements are moving in nearly circular path lines about a
common axis. Care must be taken in distinguishing a vortex from vorticity
because vorticity is the rotation of infinitesimal fluid elements (Vanyo, 1993).
Greitzer et al. (2004) has further characterized the swirling flow as can be
having a large variation in static pressure through the vortex core compared
to an essentially uniform static pressure across a thin shear layer or boundary
layer in a non-swirling flow.
In nature, swirling flows can be observed in case of cyclones, dust devils,
whirlpools and tornados etc. In engineering applications, the swirling flows
exist in many engineering applications both as confined (cyclone separators
and swirl tubes, rotary kilns, co-axially rotating cylinders, hydraulic turbine
draft tubes etc.) and un-confined (swirling jets in combustors and tornado
like flow upstream of the intake of a gas turbine etc.) (Escudier et al. 2006,
Lam, 1993). Swirl in the flow enhances heat transfer in heat exchangers,
homogenizes mixtures in casting and reactants in chemical industry, and in
combustion processes breaks fuel droplets and stabilizes the flame (Cazan et
al., 2009). Contrarily, the swirl/ vortices generated in some engineering
applications e.g. at the intakes of liquid pumps, draft tubes of hydraulic
turbines, weirs and draining of reservoirs etc. are by-product of the fluid
motion and can severely affect the main function of the equipment (Lam,
1993). Therefore, swirl and its effect on the flow field containing it is very
important to be studied. As discussed in Chapter 1, the in-cylinder flow
during scavenging process of large two-stroke diesel engine is also a confined
11
Swirling Flows