Experimental and Numerical Study of Swirling ... - Solid Mechanics
Experimental and Numerical Study of Swirling ... - Solid Mechanics 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 There have been a lot of experimental studies on characteristics of turbulent swirling flows in straight pipe/ vortex chamber where swirl is generated using propeller (Algifri et al., 1988) (Parchen et al., 1998), tangential injection (nozzle or holes drilled in tangential direction) (Martemianov et al., 2004) and (Zhang et al. 2006), twisted tape (young et al. 1978) (Islek A. A., 2004) and (Cazan et al., 2009), rotating tube bundle or honeycomb (Marliani et al., 2003 and Pashtrapanska et al, 2006) and fixed vanes (Kitoh, 1991) (Sarpkaya, 1971) (Leibovich et al, 1978) etc. The aforementioned works are very few in a very large amount of experimental measurements available in the scientific literature and a detailed account is beyond the scope of this thesis. The measurement techniques used also vary and include swirl vortex meters, pitot tubes, hot wire anemometry, Laser Doppler Velocimetry and Particle Image Velocimetry etc. For example Kreith et al. (1965) etc. used swirl vortex meters, (Lam H. C., 1993) etc. used five hole pitot tube, Algifri et al. (1988) and Kitoh (1991) etc. conducted measurements using hot wire anemometry, laser Doppler Velocimetry (LDV) (Parchen et al., 1998) (Marliani et al., 2003) etc. and particle image Velocimetry (PIV) (Zhang et al. 2006) etc. 2.3 Some Aspects of Swirling Flows Swirling flows have different distinct aspects compared to non-swirling flows. In this section only some of the aspects are briefly discussed and for a detailed account see (Alekseenko et al. 2007, Gupta et al. 1984, Saffman 1995, Greitzer et al. 2004, Wu et al., 2006) etc. Effects of Coriolis forces are beyond the scope of this thesis and thus are not discussed. 2.3.1 Stream line Curvature In swirling flows with pure rotation i.e. no bulk flow in axial direction, the streamlines are curved as shown in figure 2.6a. With the addition of axial velocity, the streamline pattern resembles to a spring/ axially stretched spiral object (Figure 2.6b). The spiral shape of the streamlines depends on the type of the swirling flow, axial velocity profile, swirl decay, symmetric or asymmetric swirl, wall curvature in case of cylindrical vortex chamber etc. Figure 2.6c gives a sketch of the streamlines in idealized case of decaying solid-body rotation with uniform axial velocity. 18 Swirling Flows
Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Fig gure 2.6: Ske etch of Streamline es for Rot tating Flows (Mo oene A.F F., 2003). In real l turbulent flows the ppresence of aany or a coombination of aforeme entioned facto ors will give a very compleex picture of sstreamlines annd consequ uently change e the overall sstructure of thhe turbulent fflow field. Thhis topic is s discussed in detail in section 2.6 ffor Numericaal modeling of Turbule ent Swirling Flows. F 2.3.2 A swirli ing flow with radial equilibbrium is the simplest class oof swirling flowws which is steady, axisymmetric, hhas radial veelocity zero eeverywhere annd consequ uently has axia al velocity, tanngential velociity and pressuure as a functioon of radia al distance from m the vortex ccenter (Greitzeer et al., 2004) ). In such a floow the con ntinuity, axial and circumfeerential momeentum equatioons are satisfieed except the t radial momentum. In ssuch a flow thhe tangential/ circumferentiial motion n generates cen ntrifugal forcees which balannce or becomee in equilibrium with th he radial press sure gradient (Sloan et al., 1986). This sstrong couplinng between n swirl and the pressure fielld has made thhe modeling oof swirling flowws very com mplex (Ansys Fluent 12.1 usser guide). In expe erimental results of real turbbulent swirlinng flows, the rradial velocity is not zer ro but very small in magnnitude compaared to axial and tangentiial velocity y components (Kitoh, 1991) ) etc. In case oof asymmetricc swirling flowws the radial pressure gradient will nnot be same inn all radial diirections due to asymme etric distributi ion of tangenttial velocity. 2.3.3 Radial Eq quilibriumm 2 1 dp v dr r Swirl Inte ensity andd Decay Chapter 2 (2. 4) In orde er to quantify the amount oor degree of swwirl in a givenn swirling floww, the mo ost common nly used nonn-dimensionall parameter is the ‘Swiirl Parame eter/ Number r’ denoted byy a symbol ‘ S ’. Howeveer, there is nno universal definition/ equation to ccalculate this. . There are mmany definitionns 19 Swirling Flows
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Experi imental <strong>and</strong> <strong>Numerical</strong> N Stud dy <strong>of</strong> <strong>Swirling</strong> g Flow in Scaveenging<br />
Processs<br />
for 2-Stroke<br />
Marin ne Diesel Engin nes<br />
Fig gure 2.6:<br />
Ske etch <strong>of</strong> Streamline es for<br />
Rot tating Flows (Mo oene<br />
A.F F., 2003).<br />
In real l turbulent flows the ppresence<br />
<strong>of</strong> aany<br />
or a coombination<br />
<strong>of</strong><br />
aforeme entioned facto ors will give a very compleex<br />
picture <strong>of</strong> sstreamlines<br />
annd<br />
consequ uently change e the overall sstructure<br />
<strong>of</strong> thhe<br />
turbulent fflow<br />
field. Thhis<br />
topic is s discussed in<br />
detail in section 2.6 ffor<br />
Numericaal<br />
modeling <strong>of</strong><br />
Turbule ent <strong>Swirling</strong> Flows. F<br />
2.3.2<br />
A swirli ing flow with radial equilibbrium<br />
is the simplest<br />
class o<strong>of</strong><br />
swirling flowws<br />
which is steady, axisymmetric,<br />
hhas<br />
radial veelocity<br />
zero eeverywhere<br />
annd<br />
consequ uently has axia al velocity, tanngential<br />
velociity<br />
<strong>and</strong> pressuure<br />
as a functioon<br />
<strong>of</strong> radia al distance from m the vortex ccenter<br />
(Greitzeer<br />
et al., 2004) ). In such a floow<br />
the con ntinuity, axial <strong>and</strong> circumfeerential<br />
momeentum<br />
equatioons<br />
are satisfieed<br />
except the t radial momentum.<br />
In ssuch<br />
a flow thhe<br />
tangential/ circumferentiial<br />
motion n generates cen ntrifugal forcees<br />
which balannce<br />
or becomee<br />
in equilibrium<br />
with th he radial press sure gradient (Sloan et al., 1986). This sstrong<br />
couplinng<br />
between n swirl <strong>and</strong> the<br />
pressure fielld<br />
has made thhe<br />
modeling o<strong>of</strong><br />
swirling flowws<br />
very com mplex (Ansys Fluent 12.1 usser<br />
guide).<br />
In expe erimental results<br />
<strong>of</strong> real turbbulent<br />
swirlinng<br />
flows, the rradial<br />
velocity is<br />
not zer ro but very small<br />
in magnnitude<br />
compaared<br />
to axial <strong>and</strong> tangentiial<br />
velocity y components (Kitoh, 1991) ) etc. In case o<strong>of</strong><br />
asymmetricc<br />
swirling flowws<br />
the radial<br />
pressure gradient<br />
will nnot<br />
be same inn<br />
all radial diirections<br />
due to<br />
asymme etric distributi ion <strong>of</strong> tangenttial<br />
velocity.<br />
2.3.3<br />
Radial Eq quilibriumm<br />
2<br />
1 dp v<br />
<br />
dr r<br />
Swirl Inte ensity <strong>and</strong>d<br />
Decay<br />
Chapter 2<br />
(2. 4)<br />
In orde er to quantify the amount oor<br />
degree <strong>of</strong> swwirl<br />
in a givenn<br />
swirling floww,<br />
the mo ost common nly used nonn-dimensionall<br />
parameter is the ‘Swiirl<br />
Parame eter/ Number r’ denoted byy<br />
a symbol ‘ S ’. Howeveer,<br />
there is nno<br />
universal<br />
definition/ equation to ccalculate<br />
this. . There are mmany<br />
definitionns<br />
19<br />
<strong>Swirling</strong> Flows
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