Combustion in a premixed swirl combustor composed of confined concentric jets (5 cm and 10 cm diameter) were studied. The inner flow is fuel and air; the outer flow is air. Both flows may contain swirl either in the same or opposite directions. The combustor operates at one atmosphere without preheat; methane and propane were used as fuels. Related analyses and experiments were also performed for water flows and isotherm air flows. In these studies, a number of important concepts regarding premixing/prevaporized, swirl stabilized combustion were developed. Some of the more significant are discussed

Gouldin, F. C.

Leibovich, S.

English

NASA Technical Reports Server

EFFECT OF SWIRL ON PREMIXED COPIBUSTION, by &. C, Gouldin and %. Heibovf ck 
Many phenomena, fncl uding l arge scale flow recirculation, are observed i n  
swirl i n g  flows. W particularly interesting set  of flows, including closed 
recirculation zones, i s  often observed a t  sufficiently h i g h  Reynolds number and 
intermediate swirl levels. First observed i n  aerodynamic contexts, these 
phenomena Rave been referred t o  as vortex breakdo~n, The presence of a nearly 
stqtionary stagnation p o i n t  on the axis and unsteady flow in the vortex core 
downstream of the front stagnation p o i n t  are comn character3st!cs of vortex 
breakdown. Experiments reveal a number of possible foms for vortex breakdown. 
Swirl flow combustors frequently exhi bi t  similar flow features which we regard 
as additional examples of vortex breakdown. Such flow phenomena include central 
closed recirculation zones, and the "precessing vortex cores" reported by Syred 
and Beer. 
Swirl ing Plow and vortex breakdown have been studied a t  Cornell for over a 
decade. In recent years, attention has been focused on combustion % n  a premixed 
swirl combus tor composed of confined concentric jets (5 cm and 10 cm diameter) . 
The inner flow i s  fuel and a i r ;  the outer flow i s  a f r ,  Both  flows may contajn 
swirl either in the same (co-swirl ) o r  opposite (counter-swirl ) directions. The 
combustor operates a t  one atmosphere w i t h o u t  preheat; methane and propane have 
been used as fuels. Related analyses and experiments have also been performed 
for water flows and isothermal a i r  flows, In these studs"es a number of fmportant 
concepts regard! ng premi xed/prevapori zed, swi r1 stabi l i zed combustion have been 
developed. Some of the more significant will be discussed here. 
Vortex breakdown exhibits a variety of forms. One of two forms, either the 
spiral form or the near axi -symmetric ( "bubble") form, predominate, depend3 ng on 
inlet and boundary conditions. The occurrence of stagnation points i n  the vortex 
core, unsteady flow - usually containing a clear period-ic disturbance - and the 
observed sensitivity t o  inlet and boundary conditions are all  significant t o  the 
combustor designer. Not all vortex breakdowns provide a large central recircu- 
l a t i o n  zone, b u t  fortunately such recirculation zones may n o t  be necessapy for 
flame stabi 1 ization i n  premixed/prevaporized systems. 
The adverse pressure gradient that can be produced i n  a vortex flow by dis- 
sipation of the swirling velocity component i s  frequently cited as the agent 
responsible for  flow recirculation i n  the combustion l i terature. While adverse 
pressure gradients are involved i n  vortex breakdown, i n  cases of greatest interest 
they can be traced t o  kinematic effects i n  essentially inertia dominated flows, 
and are no t  produced by viscosity, or by inherently dissipative effects. The 
effect of an axial pressure gradient i s ,  of course, magnified on the vortex core 
center1 i ne by swl rl , 
Although vortex breakdowns are highly complex phenomena that s t i l l  defy a 
completely satisfactory understanding, they have received considerable attention 
recently, and significant advances have been made. The development of a wave 
mechanism for breakdown by Squire, Benjamin and by Leibovich and his co-workers 
has enabled an explanation of many details of vortex breakdown flow. In this 
picture of vortex breakdown, inertial effects dominate, The wave model of 
Randal1 and Leibovick has been successful i n  predicting some features of  the 
bubble fom of vortex breakdown, such as the location and size of the recircula- 
t f o n  zone. A major shortcoming of this theory i s  i t s  restriction t o  axially 
symetric flow. Thus the spiral form of breakdown i s  n o t  accessible to the 
https://ntrs.nasa.gov/search.jsp?R=19790016839 2020-03-21T22:17:15+00:00Z
theory. Furthermore, experiments convincingly show tha t  the per iod ic  o s c i l l a t i o n s  
present i n  both s p i r a l  and bubble forms of breakdowns are due t o  waves propagating 
i n  azimuth; thus the o s c i l l a t i o n s  a lso  f a l l  outs ide the scope o f  an axisymnetr ic 
analysis . Osci 11 a t ions  observed i n  the r e c i r c u l a t i o n  zone and downstream sub- 
c r i t i c a l  f l ow  have been t raced by Garg and Leibovick t o  hydrodynamic i n s t a b i l i t i e s .  
The emerging p i c t u r e  i s  of an f n e r t i a  dominated f l ow  sens i t i ve  t o  perturbat ion.  
I f  these perturbat ions can t r ave l  upstream, a vor tex  breakdown o f  some form w i l l  
be present. Flow p ro f i l e s  and boundary condi t ions determine wave speed and type, 
hence, the s e n s i t i v i t y  noted above. 
A v a r i e t y  of experiments have been conducted i n  our combustor, inc lud ing 
v isua l  observations w i t h  a sodium t racer ,  blow-out measurements, and temperature 
and gas composition measurements throughout the combustor. A primary i n t e r e s t  
i s  i n  NO, emissions f o r  lean primary mix ture  operations. Expected reduct ions i n  
emissions w i t h  1 eaner mixtures are observed. However, quenching o f  reac t ion  i n  
the mix ing l aye r  between the two j e t s  causes reduced combustion e f f i c i e n c y  and 
high CH4 emissions. Most su rp r i s ing  i s  the observation o f  NO2 i n  the exhaust. 
On the basis of these experiments a p i c t u r e  o f  the combustion process has been 
developed. 
For l iquid- fueled,  d i f fus ion flame combustors, two popular mechanisms, the 
s t i r r e d  reactor  r e c i r c u l a t i o n  zone model and the boundary l a y e r  i g n i t i o n  delay 
model , have been proposed fo r  flame s t a b i l  i zation. Temperature and composition 
measurements and emission spectroscopy studies show t h a t  reac t ion  i s  no t  uniformly 
d i s t r i b u t e d  throughout the r e c i r c u l a t i o n  zone, and the s t i r r e d  reac to r  model i s  
therefore  no t  appropr iate f o r  our combustor. The observation o f  reac t ion  upstream 
o f  the r e c i r c u l a t i o n  zone and o f  i n s e n s i t i v i t y  o f  the lean blow-out 1 im i  t s  t o  
r e c i r c u l a t i o n  zone s ize  are d i f f i c u l t  t o  exp la in  i n  terms o f  the i g n i t i o n  delay 
model . I n  our combustor we be1 ieve t h a t  combustion takes place i n  a t h i c k  pre- 
mixed tu rbu len t  f lame-l i k e  s t ruc ture .  The flame i s  s t a b i l i z e d  i n  the reg ion o f  
the f o w a r d  stagnat ion p o i n t  of the r e c i r c u l a t i o n  zone. Reaction propagates r a d i -  
a l  l y  from t h i s  reg ion whi le  bei  ng convected downstream. Upstream convection of 
products i n  the r e c i r c u l a t i o n  zone i s  no t  essent ia l  t o  flame s t a b i l i t y *  
We be l ieve t h a t  combustion i n  premixed/prevaporized combustors employing a 
f r e e  standing r e c i r c u l a t i o n  zone w i l l  be s i m i l a r  t o  what we observe. I n  these 
combustors, blow-out w i l l  depend on condi t ions i n  the forward stagnat ion region. 
Ef f ic iency w i l l  depend on r a d i a l  flame propagation. P t  i s  the low v e l o c i t y  reg ion  
o f  f low upstream of the r e c i r c u l a t i o n  zone t h a t  makes combustion possible; upstream 
convection o f  ho t  products w i t h i n  the r e c i r c u l a t i o n  zone i s  n o t  essent ia l  . 
FIGURES 
1. Photoara~h o f  typical axisymmetric vortex breakdown observed in 
water flow a t  approximately 2000 Reynolds number. For flow visual i -  
zation, red dye i s  in*jected on the centerline; blue dye i s  injected 
off the center l ine,  Flow i s  l e f t  to r i ah t .  The tube diverges 
s l i ah t ly  as i s  evident i n  the ~ i c t u r e .  
2. Mean Axial Velocity Profiles i n  ldater Flow Containing Axisymmetric 
Vortex Breakdown as Weasured by Laser Doppler Velocimetry. Re = 2560 
based on flow ra te  and tube diameter. Axial positions of prof i les  
are  aiven by the i r  distance from front staanation point. Positive 
values denote positions downstream of the front  staanation point. 
3 .  Time Mean Streamlines Inside Axisymmetric Vortex Breakdown for  Flow 
of Fiaure 2.  Note multicellular flow and multiple staqnation points. 
4. Low Frequency Components of Eneray Spectrum Measured by Laser 
Doppler Velocimetry a t  Three Radial Positions i n  the Recirculation 
Pone. Conditions a re  those of Fiaures 2 and 3. Fluctuations appear 
to  be due to  nonaxisymmetric waves. 
5. Swirl combustor w i t h  pyrex t e s t  section in operation under co-swirl 
conditions (see Table 2 ) .  For iani t ion a continuous d i s c h a r ~ e  spark 
ipni tor  i s  introduced throush the port evident i n  the picture. Dur- 
i n g  probe measurements a s imilar  port i n  the stainless-steel t e s t  
section was pluaoed to  nrevent flow dis tor t ion.  
6. Swirl combustor i n  operation under counter-swirl condi Lions (see Tab1 e 2 ) .  
7. Schematic of swirl combustor showing swirl penerators and t e s t  
section. The approximate location of the recirculation zone i s  
indicated. Axial measurement s ta t ions a re  noted by the l e t t e r s  
a ,  b ,  c ,  e t c ,  
8. Swirl combustor blow-off l imi ts  based on inner flow equivalence 
r a t i o  for  $H4 f i r i n s .  The combustion volume is  estimated t o  
1 . 3  X 8 0 ' ~ m  fo r  a l  l conditions. 
9. Radial traverse fo r  h i p h  co-swirl case. = 0.559, Si = 0.523, 
m i  = 0.79, mOa = 0.25, u i / u 0  = 1.5, uoa s~ 23.3 m/s. 
10. Time mean isotherms ( K )  f o r  co-swirl ( a )  and counter-swirl (b) conditions. 
Ro.. i s  the combustor radius and Do i t s  diameter. 
11. Time mean isopleths o f  CH (ppm) on a dry basis f o r  co-swirl ( a )  and 
counter-swirl (b )  condi t io8s.  
12. Radial profi les  of l i ne  of s ight  CH and CO' emissions from 
combustor f i red on methane (see Table 1 ) .  X ~ D ,  = 0.8. 
TABLE I. Combustor Operatina Conditions 
inner  f l ow equivalence r a t i o  ( I + ~ ) :  0.78 
ove ra l l  equi vat ence r a t i o  (@oa) : 0.23 
ove ra l l  averaae vel  oc i  t y  (uOa) : 24.6 m/s 
r a t i o  o f  i nner  f l ow average v e l o c i t y  
, t o  outer  f l ow averaae ve loc i t y :  1.4 
inner  f l ow s w i r l  number (Si): 0.493 
outer  f l ow s w i r l  number, (So) co-swi rl : 0.559 
outer  f l ow sw i r l  number, counter- 
s w i r l  : -0.559* 
*The minus s ian i s  used t o  denote the counter-swir l  
cond i t ion ,  tanpent ia l  v e l o c i t i e s  i n  the  opposite 
d i r e c t i o n  for  the  two j e t  f lows. As defined, the  
s w i r l  number i t s e l f  i s  pos i t i ve .  
2  S = 1: u v r 2 d r / R 2  l R 2  u  r d r .  u  and v  are the a x i a l  
1 
and tanpent ia l  v e l o c i t i e s  and R and R are  the 
inner  and outer  r a d i i  o f  the j e t  i n  qu$stion. 

Axial Loeat ions 
8 - 30.6 mlp~ 
e . - 6.6 mrn 
r ~ B . Q r n r n  
a +8.0mm 
@ + 15.0 rn m 

Axial Location = 12.0 mm 
Frequency ( Hz) 





Effect of swirl on premixed combustion

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