The qualitative understanding of soot formation in simple models of gas turbine primary-zone combustors is summarized. Soot formation in flame radiation and air pollution was investigated. Results are presented, namely: (1) if the fuel is premixed with air in approximately stoichiometric proportions, the sequence of states that a fluid element undergoes as it burns is quite different from the sequence when liquid or vapor fuel is injected into an air-flow; (2) swirling flows, as are typical or swirl-can combustors, when burning, can amplify small aerodynamic disturbances upstream of the swirl vanes; and (3) different fuels form significantly different amounts of soot. Each of these effects makes major changes in the amount of soot formed in a given combustor

Hoult, D. P.

English

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‘: 
I : 
XII. SOOT FORMATION IN A TURBULENT SWIRLING FLOW 
Davjd P. Hoult 
’ 1 ., 2.r .;,;:)!* +Massachusetts Institute of Tbch,nology.‘l 
Department of ,M&hanical Engineering 
i’ ‘_, ‘, i,.,,* j.’ , : ., :, ,-; . ‘, I ::, 
In a gas,:turbine combustor, fuel and air are“fed separ’ately,a@ com- 
bust&n occurs .psi;ma.rily within a spatially ,Jimited zone. Turbulent .mixing I 
rates in such a sy%&i have ‘important effects on the emission of pollutants . : 
such:,as nitric oxide, carbon monoxide, and soA: Th$s report summarizes 
the qualitative understanding of. soot formation in simple models of gas 
turbine primary-zone combustors. 
Soot formation is important in flame radiation and air pollution. Know- 
ledge of.the concentration of soot particles is necessary to predict flame 
emissivities for’ radiative heat transfer problems. Also, some carcinogens 
are associated with soot particles. Because small soot particles may be 
deposited deep in the lungs, they may be hazardous to human health. Oxides 
of nitrogen (NO,) control strategies, such as lower temperature and/or 
staged combustion tend to increase soot formation (ref. 1). The use of highly 
aromatic fuels such as those made from coal would increase attention on 
soot because these fuels, in some geometries, tend to generate more soot 
than conventional fuels. 
Little is known about turbulent diffusion flames in geometries approxi- 
mating elements of gas turbine combustors. After being surprised by many 
of our results, we are confident of only a few general statements: First, 
if the fuel is premixed with air in approximately stoichiometric proportions, 
the sequence of states that a fluid element undergoes as it burns is quite 
different from the sequence when liquid or vapor fuel is injected into an air- 
flow. Second, swirling flows, as are typical of swirl-can combustors, when 
burning, can amplify small aerodynamic disturbances upstream of the swirl 
vanes. Third, different fuels form significantly different amounts of soot. 
Each of these effects makes major changes in the amount of soot formed in 
a given combustor. 
309 
https://ntrs.nasa.gov/search.jsp?R=19780003132 2020-03-20T12:43:42+00:00Z
BURNERS 
The burners used consist of cylindrical insulated tubes, 3 inches in 
diameter, in one end of which is mounted a 45’-blade-angle swirler and an 
air-assist atomizer (DeLavan model 30609-11) on the axes of symmetry. 
The atomizer airflow is about 5 percent of the total airflow. Although this 
sounds straightforward enough, the aerodynamics of such simple systems 
cannot be specified because of hardware effects upstream of the swirler and 
atomizer. 
Gas and soot samples were collected at different flame positions with 
a stainless-steel, water-cooled probe (fig. XII-l) that was developed by 
Dr. Gilles .Prado of. the Centre des Recherches sur la Physico-Chimie des 
Surface Solides, Mulhouse, France. With this probe, one can sample either 
gases .(CO, CO2, hydrocarbons, NOx, and 02) or soot. When gases are 
collected, the proble is adjusted so that it is water cooled, but no water mixes 
with the quenched gas samples. When soot is collected, the probe is water 
flushed, and the soot is collected on a fiber-glass filter, from which it is ex- 
tracted and weighed. Details of the soot-measuring method are given in ref- 
erence 1. Reproducibility of soot concentrations is about 10 percent. Be- 
cause of -the rapid quenching in the probe (in about 10m4 set), reactions be- 
tween water end soot can be ignored. We have reported the experiment over 
a range of water flush and collection velocities and operate the experiment in 
a state where the amount of soot does not depend on either the water velocity 
or- the gas collection agency. 
PRELIMINARY RESULTS 
Figure XII-2 shows the original burner. Although it looks quite straight- 
forward, it is not. The difficulty comes in trying to describe the aerodynamic 
effects of the fittings that connect to the atomizer. 
The gas constituents as well as soot were measured. Figure XII-3 des- 
cribes the method used. In particular, when concentrations of CO, CO%, 
hydrocarbons, etc. , are known, the local time-averaged stoichiometry cp 
can be estimated. 
310 
II - 
NxHyOz + 4.76 (Air) - AICO + A2C02 + A3H20 + A4H2 + A502 
: 
: .‘. 
+ A6CrB5 + A7 [!“,l 
; 
The first runs appeared to show large variations in q. Eventually, we 
came to believe that these variations were real. As documentation, fig- 
ure XII-4 shows the results of repeated horizontal traverses with the probe. 
The data point with error bars is not part of the data but indicates the error 
bar accuracy of the data. Figures XII-5 and XII-6 show how the nonuniformity 
in cp persists downstream. We have measured significant variations in 50 
at Z/D = 5, even though the cold-flow cone angle of the spray of the air-assist 
atomizer intersects the walls of the burner at about Z/D = 1 (see fig. XII-3). 
Figure XII-7 shows a vertical-traverse measurement of soot concentration 
at Z/D = 1.33. The variations in q across the burner were about a factor 
of 2; the variations in soot are closer to a factor of 10. Figure XII-8 is a 
contour map of soot concentration at Z/D = 2, which shows the same effect. 
Our concern with these results arose from two practical considerations. 
It was apparent that most of the soot was being formed in relatively small 
volumes of the flow. But there was no obvious reason for the particular 
locations of the nonuniformities in q and soot. In addition to these complex- 
ities, we found that the type of fuel makes a major difference. Figure XII-9 
compares the peak, centerline soot concentration of kerosene (isolated point) 
and the soot obtainedusing a blend of benzene and heptane (solid line) to simu- 
late a range of hydrogen concentrations (ref. 2). Pure benzene (7. ‘I-percent 
hydrogen) has a peak soot concentration some two orders of magnitude greater 
than that of kerosene (14.4-percent hydrogen). 
Our attention next was directed to identifying the causes of the observed 
nonuniformities in cp and soot and then eliminating them. The observed 
nonuniformities are probably due to the aerodynamic wakes of the fittings on 
the injector that are upstream of the swirl vanes. Although cold-flow esti- 
mates indicate that these wakes are not likely to be important downstream of 
the swirl vanes, the large density differences associated with the combustion 
process can combine with the radial pressure gradient associated with swirl 
to greatly amplify these nonuniformities. 
311 
We modified t,he apparatus by,, p@cing the upstream parts .a~$ fittings in 
a larger, low-velocity duct. (Eg. I’XIIy7:!O),. fn addition, a, tar+ .w+s added so ,, 
that we could evaporate fuel in it so as to runthe, same.,prer@xed fuel that , .,x1 
is normally ‘run through@% @jector. ..$ such,~..,experi~~,-.~~e. mass flows 
of fuel and air and-the geometry of the flow remain unchanged_,:.,. 
A better traverse was ,built,into thfs apparatus, so: thaJ.,m(?r-,deQiled 
contour,maps .of 9 against r/D .,and ;.8.: could be. easily measured ,. : 
(fig. XII-11). Figure XII-12 shows the resulting distr@rt+-of, q ,- essen- 
tially cp is constant over cross sections of the burner. This result should 
be compared with figures XII-4, XII-5, and XII-6, which show large variations 
in cp far downstream of the injector. 
Figure XII-13 shows how these changes affect the amount of soot produced. 
The upper curve is for. the originaJ:design (fig. XII-?);. The fuel is benzene, 
and the points. areI the .m;urimum .ampunts of soot ,in theburner,. which occur 
around Z/D = 2. The next lower .cur,ve, shows ,&he same, maximum amounts 
of soot in a geometry that is 
’ stream f&w was’cleared up.” 
described.by figure XII-10 and in which the up- 
This ‘same geometry has the capability of pre- 
mixed operation. The lowest curve shows that virtually .no soot &l mg soot/ 
m3) is. produced in the premixe*d’TIow.~ ‘In this’-flow;; ‘the ~r&s%ws~of air 
both through the air-assist atomiier .and.through the-‘;saP’irler are’the same as 
when the fuel goes through the injector. I’ The .&ly~d?Eereice isthat the pre- 
mixed fuel plus air flows through the swirler blades. Hence; ‘the turbulent 
mixing processes should be very similar in aI1 three&asesc,.-, :. _ 
,._ . . . . , , :. -;_ ; _ 
CONC L,USfONS : . . . .’ .; .’ ’ . . . . 
;‘. I, 
Some general trends in so.ot,formati& .QI a turbulent,’ : swirl&flow are 
suggested by the data presented. First; figure XII-9.shows that fuel compo- 
sition makes a major difference in the amount of soot formed. This is con- 
sistent with other work (ref. 3). Second, small changes in upstream geometry 
apparently can make large changes in soot concentration. Third, as others 
have found, running lean reduces the amount of soot formed (ref. 4). 
Although not shown in a figure, our data from the original design agree 
quite well with MacFarlane’s data (ref. 5) up to 6 atmospheres. But it is not 
312 
known if the effects of upstream geometry on. soot formation remain the 
same as the pressure level is raised, ‘and all the data reported here are for 
an atmospheric pressure burner. 
From a practical point of view, the engine designer may not be able to 
use “clean” fuels. However, our results hint at the possibility of substantial 
improvements in performance with fuels having a lower percentage of hydro- 
gen in the blend than current practice by means of improved aerodynamic 
design of the combustor. 
REFERENCES 
1. Prado, G. P.; et al. : Soot and Hydrocarbon Formation in a Turbulent Dif- 
fusion Flame. Presented at the 16th Symposium (International) on Com- 
bustion, (Cambridge, Mass. ), Aug. 15-21, 1976. 
2. Shamsavari, Kamren: Measurement of Soot in a Turbulent Diffusion Flame. 
M.S. Thesis, MIT, 1976. 
3. Schirmer, R. M.: Effect of Fuel Composition on Particulate Emission from 
Gas Turbine Engines. Emissions From Continuous Combustion Systems,, 
Walter Cornelius and William G. Agnew, eds., Plenum Press, 1972, 
pp. 189-208. 
4. Grobman, Jack and Ingebo, Robert D. : Jet Engine Exhaust Emissions of 
High-Altitude Commercial Aircraft Projected to 1990. NASA TM 
x-3007. 
5. Holderness, F. H. ; and MacFarlane, J. J. : Soot Formation in Rich 
Kerosene Flames at High Pressure. Atmospheric Pollution by Aircraft 
Engines. AGARD CP-125, 1973, pp. 18-1 to 18-9. 
313 

WATER-FLUSHED SOOT SAMPLING PROBE 
Sam 
in 
av able Tube 
Sample + Water __) 
Movable Tube 
Gas Samplinq 
Probe 
Figure X11-1. 
TURBULENT DIFFUSION FLAME BURNER 
Swirler 
Figure XII-2 
315 
EXPER I MENTAL METHOD 
PROBE’ 
1. Water-flushed soot sampling probe. 
a. Water spray is zero for gas analysis. 
b. Water spray is mixed with gas sample to in- 
sure accurate soot collection. 
C. Water spray rate is controlled by moving inner 
tube. 
SAMPLING 
1. Probe is located inside burner. Information as per 
r. e, and z obtained from traversing mechanism. 
2. Gas sample is drawn through tube. 
a. For @ experiments, sample is sent to gas 
sampling cart where [NO.]. [HC], [Or], (CO>], 
and [CO] can be obtained. 
b. For soot experiments, sample is pumped 
through a glass wool filter and wet test meter. 
COORDINATES 
1.3 
1.2 
I.1 
1.0 
+ 
0.9 
0.8 
0.7 
0.6, 
_! 
r 
Z 
33 8 
t 
D 
Figure X11-3. 
REPEATABILITY OF NONUNIFORMITY IN 
EQUIVALENCE RATIO 
I-1 
T 
i -I 
8” -4----g-- 0 4, . . 
9’ 0 0 
0 
x, d 
/ -P 
4 Ll.33 
Xl cl 
P / HORIZONTAL 
1’ 
TRAVERSE 
4 SEPARATE 
x/ RUNS 
0 
I I I 1 1r 
.35 .I 5 0 .I5 .35 .5 
r/D 
Figure X11-4. 
316 
CONTOUR MAP OF EQUIVALENCE RATIO 
270 APAn= 15psi 
Figure X11-5. cfl q 1.0 
CONTOUR MAP OF EQUIVALENCE RATIO 
270 
Figure X11-6. 
MEASUREhiENTS OF SOOT GRADIENTS 
p-l.0 
Z/D = 1.33 
SOOT MASS 
LOADING, 
mglm3 
~5-77-792 
180 
0 .1 .2 .3 .4 
RADIAL POSITION, r/d 
Figure X11-7. 
CONTOUR MAP OF SOOT CONCENTRATION 
Figure X11-8. 
318 
EFFECT OF HYDROGEN ON PEAK SOOT CONCENTRATION 
3ooo 
MASS 
2ooo 
LOADING 
OF ' \ I 
PURE 
n-HEPTANE 
6 8 10 12 14 16 18 
HYDROGEN IN BLEND, % BY WT a-77-793 
Figure XII-9. 
FLOW SCHEMATIC 
Gas Samplinq 
Probe 
Atomizing 
Nozzle 
I 
\ 
Refractory 
Lininq 
Fuel - 
Pr imory Air 
Tank 
Swirler Atomizinq 
Air 
Figure XII-lo. 
319 
EXPERIMENTAL LAY-OUT 
Probe Positioner 
Tank 
Set A 
Chimney 
J El urner 
Probe / > 
Set A, 
i-:7 i 
I 1 
r ----I 
ChannL ,a nel I 1  
Figure X11-11. 
CONTOUR MAP OF EQUIVALENCE RATIO WITH NEW GEOMETRY 
90 
I 
I 
270 
(p’ I Air>68.5;; 
Fuel -4.6 m hr 
Figure X11-12. 
320 
COMPARISON OF PEAK SOOT CONCENTRATION WITH 
METHOD OF FUEL INTRODUCTION 
5000 
4000 
- I 
*- 
INJECTION OLD DESIGN 
n 
3000 
E *,O’ 
\ ** ,.- 
F 2ooo 
i 
i 
L I I 1 I I I I ) 
0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 
EQUIVALENT RATIO c$ 
Figure XII-U. 
321 


Soot formation in a turbulent swirling flow

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