The atomization properties of liquid fuels for the potential use in aircraft gas turbine engines are discussed. The significance of these properties are addressed with respect to the ignition and subsequent combustion behavior of the fuel spray/air mixture. It is shown that the fuel properties which affect the atomization behavior (viscosity, surface tension, and density) are less favorable for the broad specification fuels as compared to with those for conventional fuels

Lefebvre, A. H.

Skifstad, J. G.

English

NASA Technical Reports Server

ATOMIZATION OF BROAD SPECIFICATION AIRCRAFT FUELS 
J.G. Skifstad and A S H .  Lefebvre 
Purdue University 
The atomization properties of any l iquid f u e l  f o r  potent ia l  use i n  
a i r c r a f t  gas turbine engines are widely recognized t o  be of prime importance. 
These properties are important because the  ign i t ion  and subsequent combustion 
behavior of t he  fuel-spray/air mixture are strongly affected by the nature 
of the f u e l  spray preparation and distribution. For example, the mean s i ze  
of the droplets i n  the  spray (e.g., Sauter mean diameter, SMD) , the  dis t r ibu-  
t i o n  of droplet s izes  present at any loca l  posit ion i n  the  spray, the loca l  
fuel /a i r  mixture r a t i o ,  the f rac t ion  of fue l  vaporized, and other factors ,  
re la ted t o  t h e  airflow/spray dynamics, f o r  instance, may be expected t o  be 
largely established by the  l iqu id  inject ion devices and the  arrangements for  
the airflow i n  the  primary region of t h e  combustion chamber. Clearly, the 
fue l  properties which a f f ec t  atomization behavior (viscosity,  surface tension 
and density) w i l l  be less favorable f o r  the broad specif icat ion fue l s  under 
consideration here, as compared with those f o r  conventional fuels. 
sure,  other operational fac tors  such as thermal s t a b i l i t y ,  storage and trans- 
port considerations, pumping behavior, and s o  on, must a l so  be considered, 
But the fue l  must ultimately be well-atomized i n  the  engine over a sui table  
range of operating conditions t o  be a t  a l l  acceptable as a f u e l  f o r  a i r c r a f t  
gas turbines. Those conditions necessarily include extremes, such as a l t i t u d e  
re l igh t  and cold start conditions, as w e l l  as normal operating conditions f o r  
the engine. 
To be 
Fuel in jec tors  f o r  spec i f ic  gas turbine engines are designed t o  atomize 
the fue l  and t o  d is t r ibu te  the fue l  i n  t h e  primary air  flow over a range of 
engine operating conditions from idle t o  f u l l  power. These in jec tors  employ 
one o r  more of several  character is t ic  types of inject ion schemes, such as 
simplex, duplex, air-assist, and a i rb l a s t ,  f o r  instance. They vary i n  t h e i r  
reliance on one or more of the basic mechanisms u t i l i zed  t o  effect  breakup 
of the  l iquid in to  f ine  droplets,  and i n  the  nature of the control of the 
processes t o  enable operation over a suff ic ient ly  wide range of fue l  flow 
ra tes .  The technology involved i n  the  development of such injectors  remains 
v i r tua l ly  an empirical art. 
of a suf f ic ien t ly  detailed character t o  ant ic ipate  t h e  trends i n  the  atomiz- 
a t ion  produced by cer ta in  types of injectors  l i ke ly  t o  be employed f o r  the 
more viscous fue l s  of i n t e re s t ,  
Nonetheless, there have been some investigations 
O f  the  numerous correlations available describing t h e  dependence of t h e  
SMD of the spray droplets on the  l iqu id  properties, t h e  operating conditions 
of the atomizer and the properties of the environment, typ ica l  re la t ions f o r  
simplex ( s w i r l )  atomizers and a i rb l a s t  atomizers indicate the SMD increases 
with increasing viscosity and surface tension, and weakly increases with 
l iquid density. There is evidence the  SMD f o r  the s w i r l  atomizer i s  not 
significantly dependent on the  l iqu id  surface tension. 
fo r  an a i r b l a s t  atomizer i l l u s t r a t i n g  the variation of SMD with l iqu id  
Experimental curves 
117 
https://ntrs.nasa.gov/search.jsp?R=19800020817 2020-03-21T03:29:16+00:00Z
viscos i ty ,  surface tension,  and density show t h a t  increases  i n  each of those 
f u e l  propert ies  r e s u l t  i n  l a r g e r  spray droplet  s i ze s .  Variations of t h e  SWD 
with ambient pressure or temperature are a l s o  i l l u s t r a t e d ,  showing t h a t  in-  
creases i n  ambient pressure and decreases i n  temperature y i e ld  smaller drop 
s izes .  The experimental curves are taken from invest igat ions on a i r b l a s t  
atomizers conducted by one of t h e  authors (AHL) and h i s  former colleagues 
at  Cranfield University. The importance of these pa r t i cu la r  r e s u l t s  here 
l ies  i n  t h e  fact t h a t  airblast atomizers appear t o  o f f e r  t he  g rea t e s t  promise 
f o r  f i n e  atomimtion of t h e  heavier fuels .  The r e s u l t s  shown do not exhib i t  
t h e  complete p ic ture ,  however, insofar  as operation over a range of conditions 
is required and the re  are other  f ac to r s  involved, such as the  f u e l  penetration 
and d i s t r ibu t ion  i n  the  a i r f low within a pa r t i cu la r  combustion chamber, 
Complete data f o r  f u e l s  of i n t e r e s t  are simply not avai lable .  
A research program at Purdue (NSG 3258, NASA Lewis  Research Center) is t o  
explore the  atomization behavior of representat ive f u e l  i n j ec to r s  with f u e l s  
simulating t h e  propert ies  of those falling under t h e  broad spec i f ica t ion  fue l  
c l a s s i f i ca t ion .  
ambient pressures up t o  about 20 a t m  and at  ambient gas temperatures up t o  
1000 OK. The gas flow and d i s t r ibu t ion  iondi t ions are t o  be chosen merely 
t o  avoid rec i rcu la t ion  of spray drople t s  i n  the f lowfield within t h e  pressure 
vesse l  and otherwise are t o  be made t o  have minimal effect on t h e  spray 
dynamics Measurements of t h e  spray propert ies  ( l o c a l  measurements of t he  
droplet  s i z e  d i s t r ibu t ion  a t  points  i n  the  near f i e l d  of t h e  in j ec to r )  are 
t o  be made with an  imaging-type spray analyzer donated t o  Purdue by t h e  
Parker-Hannifin Corporation. This device is capable of individual  droplet  
s i z e  measurements from 8 microns t o  512 microns. 
coupled with a PDP l l V O 3  computer i n s t a l l a t ion .  
These f u e l s  will be sprayed i n t o  nitrogen atmospheres a t  
It is an  automated system 
Given t h e  r e s u l t s  of these invest igat ions,  it should be apparent what 
t radeoffs  are l i k e l y  t o  be involved i n  u t i l i z i n g  each of t h e  cha rac t e r i s t i c  
types of f u e l  i n j ec to r s  f o r  t h e  range of f u e l s  of i n t e r e s t  and over t he  range 
of ambient parameters of i n t e r e s t .  These t r adeof f s  w i l l  involve a l l  of t h e  
spray p rope r t i e s  measured, including drople t  s i z e  d i s t r i b u t i o n  da ta ,  s p a t i a l  
v a r i a t i o n s  of t h a t  d i s t r i b u t i o n ,  and the  s p a t i a l  d i s t r i b u t i o n  of t he  spray 
d rop le t  dens i ty .  The ex ten t  and na ture  of atomization p rope r t i e s  of t h e  
broad spec i f i ca t ion  f u e l s  should then be apparent f o r  each type of i n j e c t o r  
and the  more promising a t t r i b u t e s  of i n j e c t o r s  f o r  s p e c i f i c  a i r c r a f t  gas 
turb ine  app l i ca t ions  using these  f u e l s  might then be i d e n t i f i e d  and incor- 
porated i n  new designs. 
118 
SPECIFIC KINEMATIC SURFACE 
GRAVITY V18COSlTY TENSION 
AVIATION KEROSENE 0.79 1.6 X I@ 24X IG3 
DIESEL FUELS 0.83 6 X le* 2 6 X  J6' 
GAS OILS 0.87 3 0 X  16' 2 9 X  le3 
MARINE ENGINE FUELS 0.94 40OXl6' 3 d X  IC3 
(M2/SI (WM) 
FIGURE I .  REPRESENTATIVE LIQUID PROPERTIES 
FOR PETROLEUM FUELS (293 O K )  
(A) SIMPLEX SWIRL ATOMIZER 
LIQUID 
( a)  AIRBLAST ATOMIZER 
FIGURE 2. ATOMIZER CONFIGURATIONS 
119 
INSTABILITIES FREE BOUNDARY INTERACTIONS 
CAPILLARY WAVES GAS PHASE SHEAR 
* SHEAR DRIVEN WAVES ACOUSTIC FIELDS, PULSES 
VORTEX INSTABILITIES * LIQUID/SOLID SEPARATION 
ACOUSTIC INSTABILITIES CAVITATION PHENOMENA 
*TURBULENCE * THERMAL PHENOMENA 
LIQUID PHASE GEOMETRY ENERGY CONSIDERATIONS 
JETS EQUILIBRIUM ENERGY REQ'D U I W  
e SHEETS * RATES OF DEFORMATION ARE 
0 FILMS NECESSARILY HIGH 
* DROPLETS THERMAL LIMITATIONS OF 
* LIGAMENTS * FUELS 
FIGURE 3. ATOMIZATION PHYSICS 
SMD = K.  (FN)-$* SMALL NOZZLES 
FN = WL/&PL 
(A) SIMPLEX SWIRL ATOMIZERS (SIMMONS) 
(E) AIRBLAST ATOMIZERS (LEFEBVRE) 
FIGURE 4. SMD CORRELATIONS 
120 
20 40 60 80 D O  120 140 
LIQUID FLOW RATE& 
0 OD30 kg) 
ABSOLUTE VIscOSITY, kghs x IO3 
x 0.025 
A 0.020 
0 0.015 
- A 0.010 " 
- 
- 
FIGURE 5. EFFECT OF LIQUID VISCOSITY 
ON DROP SIZE FOR A PRE - 
FILMING AIRBLAST ATOMIZER 
(RIZKALLA AND LEFEBVRE 1 
FIGURE 6. EFFECT OF SURFACE TENSION 
ON DROP SIZE FOR A PRE- 
FILMING AIRBLAST ATOMIZER 
(RIZKALLA AND LEFEBVRE 1 
121 
140 
120 
lo- 
! 80- 
0 I 
&so- z 
40- 
20- 
0 
FIGURE 7. EFFECT OF'LIQUID DENSITY 
ON DROP SIZE FOR A PRE - 
flLMlNG AIRBLAST ATOMIZER 
(RIZK AND LEFEBVRE) 
P,=K?N/n? 
T11296.K - 
A 
n ,. - - 103.6 " - I 1219' 
I I I I I 
IO - 
I 2 3 4 5678910 
AIR PRESSURE ( N / d  x Id) 
FIGURE 8. EFFECT OF AMBIENT AIR 
PRESSURE ON DROP SIZE 
FOR AN AIRBLAST ATOMIZER 
(RIZKALLA AND LEFEBVRE) 
122 
c 
: 
~ O O O  500400~ 200 I 
AIR TEMPERATWE,% 
0 
FIGURE 9. EFFECT OF AMBIENT AIR 
TEMPERATURE ON DROP 
SIZE FOR AN AIRBLAST 
ATOM I Z E R 
(RIZKALLA AND LEFEBVRE ) 
IFUEL+ 
CAMERA 
DI ST R 18 UTlON 
URCE 
PRESSURE 
VESSEL 
FIGURE IO. EXPERIMENTAL APPARATUS 
123 
C A # E R A / M T  M C E  
Am) INJECTOR M E  
TRAVERSED UNDER 
GOYPUTER CONTROL 
Ll6HT SOURCE 
* S u m  (FOCUS) 
MSCRI MINATION 
*SCM LINE COUNTS 
* SIZE QATlNO TO 
PROCESS1 N6 
F W E  COUNT PER 
FIGURE I I .  PARKER - HANNlFlN SPRAY 
DROPLET ANALYZER 
124 


Atomization of broad specification aircraft fuels

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