Experimental and Numerical Study of Swirling ... - Solid Mechanics
Experimental and Numerical Study of Swirling ... - Solid Mechanics Experimental and Numerical Study of Swirling ... - Solid Mechanics
Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figure 2.8: Sketch of f Streamlines for r Swirl Co ombustor test model (E Escudier et al., 1987). fitted with w exponential decay funcctions (Equatiion 2.11). Howwever, for higgh swirl number flows this function is an approxximation (Steeenbergen et aal., 1998). 2.3.4 S() z Upstream m influencee S e o Swirling flows, at cer rtain swirl nummber, begin too exhibit an innfluence on thhe upstream velocity profile p at highh swirl numbers. Escudieer et al. (1985) conducted LDA expe erimentation oon a model off swirl combuustor with watter as the fluid f and no combustion iinvolved. Figuure 2.8 shows the streamlinne plots at t Re=10,600 de erived from thhe LDA measuurements, on tthe effect of exxit contrac ction on the upstream u floww inside the aaforementioneed experimenttal test setu up. Figures 2. .7a & b are ffor a given loow swirl number and figurres 2.8c,d are a for a high swirl numberr flow. It can bbe seen that aat the given loow swirl nu umber the ups stream effect oof an exit conttraction is verry small but att a higher swirl s number there is a signnificant changge in the upstrream streamlinne profiles s. This upstream influence iss often definedd as the criticaality of the floow where downstream d information i ttravels upstreaam and the sswirling flow is called to t be subcritic cal. The flowws below the ccritical swirl nnumber exhibbit small or r no upstream m influence aree defined to bee in supercriticcal regime. The up pstream influe ence of swirliing flow requuires that thee guidelines ffor assumin ng no couplin ng between fluuid componennts or for the selection of thhe type of f boundary conditions c needed in nummerical modells are different compar red to non-swi irling flows (GGreitzer et al., 22004). z D Chapter 2 (2.11) Where S o is the in nitial swirl inttensity. iss the rate of decay and is a function n of friction fa actor for fully developed pippe flow. 22 Swirling Flows
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Chapter 2 2.3.5 Instabilities and Vortex Breakdown in swirling Flows In swirl flows, instabilities (unsteady phenomena) have many aspects to be studied. Generally, in case of mostly laminar swirl flows, a steady flow is subjected to small unsteadiness/ disturbance and the response of the flow is observed in the form of whether there is growth or decay. With the presence of swirl in the flow, the instability associated means that some tangential velocity distributions consistent with the simple redial equilibrium are unstable and not achievable in practice (Greitzer et al., 2004). This method is called the ‘linear stability analysis’ and in case of turbulent flows such introduced ‘small’ fluctuations are smaller than the turbulent fluctuations and any conclusion of growth or decay of the disturbance always has the growth and decay of turbulent fluctuations as the major contributor (Moene, 2003). Other significant stability studies involve studying vortex break down, precession of vortex cores (PVC), growth and decay of Reynolds normal stress components and the anisotropy of the Reynolds stress tensor etc. The vortex breakdown is an important aspect of swirling flows. In general the vortex breakdown has two distinct types: (i) Bubble or axis symmetric type where there is rapid expansion of vortex core in to a near axis symmetric bubble shape (Figure 2.9a) and (ii) Spiral type where the vortex core deforms into a spiral (Figure 2.9b). Sarpkaya (1972) also observed a double helix type. Benjamin (1962) described vortex breakdown as abrupt and drastic change of flow structure and Ludwieg (1961) suggested that it might be a finiteamplitude manifestation of the instability of the core flow. Velte et al., (2010) define vortex breakdown as explosion or abrupt growth of the slender vortex core with different changes in the flow topology. So far a lot of experimental measurements, observations and numerical studies have been conducted to understand this phenomenon but still the physical mechanism of vortex breakdown is not well understood. The phenomenon remains largely in the qualitative, descriptive realm of knowledge (Novak et al., 2000). In case of confined swirling flows, the experiments conducted by Escudier et al. (1982), show that the consequence of a vortex breakdown is a profile shape change for the axial velocity i.e. from jet-like to wake-like with an intermediate stagnation region. Escudier et al. (1982) discusses that the spatial growth rate of instability waves is very different in the two axial velocity profile types. The upstream jet-like profile becomes unstable and instability waves amplify slowly. Then the core flow undergoes a shock-like transition at some downstream position and the waves grow rapidly on a low velocity wake-like profile. Escudier et al. (1982) suggests that the swirl intensity (flow criticality) is the main parameter that determines the basic characteristic of the downstream flow and the effect of instability waves are superimposed over it. Alekseenko at el. (2007) has given pictures of many different vortex breakdown structures both in confined and unconfined swirling flow experiments. 23 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 />
Figure<br />
2.8:<br />
Sketch <strong>of</strong> f Streamlines for r<br />
Swirl Co ombustor test<br />
model (E Escudier et al.,<br />
1987).<br />
fitted with w exponential<br />
decay funcctions<br />
(Equatiion<br />
2.11). Howwever,<br />
for higgh<br />
swirl number<br />
flows this function is an approxximation<br />
(Steeenbergen<br />
et aal.,<br />
1998).<br />
2.3.4<br />
S() z <br />
Upstream m influencee<br />
S e <br />
o<br />
<strong>Swirling</strong><br />
flows, at cer rtain swirl nummber,<br />
begin too<br />
exhibit an innfluence<br />
on thhe<br />
upstream<br />
velocity pr<strong>of</strong>ile p at highh<br />
swirl numbers.<br />
Escudieer<br />
et al. (1985)<br />
conducted<br />
LDA expe erimentation oon<br />
a model <strong>of</strong>f<br />
swirl combuustor<br />
with watter<br />
as the fluid f <strong>and</strong> no combustion iinvolved.<br />
Figuure<br />
2.8 shows the streamlinne<br />
plots at t Re=10,600 de erived from thhe<br />
LDA measuurements,<br />
on tthe<br />
effect <strong>of</strong> exxit<br />
contrac ction on the upstream u floww<br />
inside the aaforementioneed<br />
experimenttal<br />
test setu up. Figures 2. .7a & b are ffor<br />
a given loow<br />
swirl number<br />
<strong>and</strong> figurres<br />
2.8c,d are a for a high swirl numberr<br />
flow. It can bbe<br />
seen that aat<br />
the given loow<br />
swirl nu umber the ups stream effect o<strong>of</strong><br />
an exit conttraction<br />
is verry<br />
small but att<br />
a<br />
higher swirl s number there is a signnificant<br />
changge<br />
in the upstrream<br />
streamlinne<br />
pr<strong>of</strong>iles s. This upstream<br />
influence iss<br />
<strong>of</strong>ten definedd<br />
as the criticaality<br />
<strong>of</strong> the floow<br />
where downstream d<br />
information i<br />
ttravels<br />
upstreaam<br />
<strong>and</strong> the sswirling<br />
flow is<br />
called to t be subcritic cal. The flowws<br />
below the ccritical<br />
swirl nnumber<br />
exhibbit<br />
small or r no upstream m influence aree<br />
defined to bee<br />
in supercriticcal<br />
regime.<br />
The up pstream influe ence <strong>of</strong> swirliing<br />
flow requuires<br />
that thee<br />
guidelines ffor<br />
assumin ng no couplin ng between fluuid<br />
componennts<br />
or for the selection <strong>of</strong> thhe<br />
type <strong>of</strong> f boundary conditions c needed<br />
in nummerical<br />
modells<br />
are different<br />
compar red to non-swi irling flows (GGreitzer<br />
et al., 22004).<br />
z<br />
D<br />
Chapter 2<br />
(2.11)<br />
Where S o is the in nitial swirl inttensity.<br />
iss<br />
the rate <strong>of</strong> decay <strong>and</strong> is a<br />
function n <strong>of</strong> friction fa actor for fully developed pippe<br />
flow.<br />
22<br />
<strong>Swirling</strong> Flows
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