This is a joint research program, pursued by investigators at the University of Tokyo, UCSD, and NASA Lewis Research Center. The focus is on high-pressure combustion of miscible binary fuel droplets. It involves construction of an experimental apparatus in Tokyo, mating of the apparatus to a NASA-Lewis 2.2-second drop-tower frame in San Diego, and performing experiments in the 2.2-second tower in Cleveland, with experimental results analyzed jointly by the Tokyo, UCSD, and NASA investigators. The project was initiated in December, 1990 and has now involved three periods of drop-tower testing by Mikami at Lewis. The research accomplished thus far concerns the combustion of individual fiber-supported droplets of mixtures of n-heptane and n-hexadecane, initially about 1 mm diameter, under free-fall microgravity conditions. Ambient pressures ranged up to 3.0 MPa, extending above the critical pressures of both pure fuels, in room-temperature nitrogen-oxygen atmospheres having oxygen mole fractions X of 0.12 and 0.13. The general objective is to study near-critical and super-critical combustion of these droplets and to see whether three-stage burning, observed at normal gravity, persists at high pressures in microgravity. Results of these investigations will be summarized here; a more complete account soon will be published

Dietrich, Daniel L.

Kono, M.

Mikami, Masato

Sato, Junichi

Williams, Forman A.

English

NASA Technical Reports Server

HIGH-PRESSURE DROPLET COMBUSTION STUDIES
M. Mikami and M. Kono
Department of Aeronautics, University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113, Japan
20216
J. Sato
Research Institute
Ishikawajima-Harima Heavy Industries Co., Ltd.
Toyosu, Koto-ku, Tokyo 135 Japan
D.L. Dietrich
Sverdrup Technology, Inc.
NASA Lewis Research Center, Cleveland, Ohio 44135
and
F.A. Williams
Center for Energy and Combustion Research
Department of Applied Mechanics and Engineering Sciences 0310
University of California San Diego, La Jolla, California 92093
Introduction
This is a joint research program, pursued by investigators at the University of Tokyo,
UCSD, and NASA Lewis Research Center. The focus is on high-pressure combustion of
miscible binary fuel droplets. It involves construction of an experimental apparatus in
Tokyo, mating of the apparatus to a NASA-Lewis 2.2-second drop-tower frame in San
Diego, and performing experiments in the 2.2-second tower in Cleveland, with experimental
results analyzed jointly by the Tokyo, UCSD, and NASA investigators. The project was
initiated in December, 1990 and has now involved three periods of drop-tower testing by
Mikami at Lewis.
The research accomplished thus far concerns the combustion of individual fiber-supported
droplets of mixtures of n-heptane and n-hexadecane, initially about 1 mm in diameter, un-
der free-fall microgravity conditions. Ambient pressures ranged up to 3.0 MPa, extending
above the critical pressures of both pure fuels, in room-temperature nitrogen-oxygen atmo-
spheres having oxygen mole fractions X of 0.12 and 0.13. The general objective is to study
near-critical and super-critical combustion of these droplets and to see whether three-stage
burning, observed at normal gravity, persists at high pressures in microgravity. Results
of these investigations will be summarized here; a more complete account soon will be
published (ref. 1).
Experimental Apparatus and Procedure
Figure 1 shows a schematic diagram of the experimental apparatus. The pressure chamber
has four orthogonal windows that give two views of the combustion process, a bacidit view
from which the droplet size as a function of time is obtained and a second view which shows
311
https://ntrs.nasa.gov/search.jsp?R=19930011027 2020-03-17T07:59:36+00:00Z
the flame surrounding the droplet. During each test, both of these views were recorded
by 16 mm cine cameras. Microgravity conditions were attained by dropping the pressure
chamber and associated experimental equipment in a drop tower at NASA Lewis Research
Center that provides 2.2 seconds of microgravity (_10-s normal).
Before release of the experimental package, a fuel droplet was formed at the end of a
0.125 mm silica fiber from a high-pressure syringe driven by a stepping motor. Following
release, immediately upon entry into microgravity the droplet was ignited. The ignition
source was a hot wire (aluminum alloy, 0.2 mm diameter) formed into a 2 mm diameter
loop that surrounds the droplet. The hot wire was rapidly removed after ignition, leaving
approximately 2 sec of the 2.2 sec to observe combustion. All processes, including droplet
dispensing and ignition, were controlled by a microcomputer.
The initial droplet size was approximately 1 mm in diameter. I This droplet diameter
is nearly an order of magnitude larger than that of the suspending fiber, while also being
small enough that complete burning could be observed in the available microgravity time.
The ambient gas was a nitrogen-oxygen mixture with oxygen mole fractions, X, being 0.12,
except for a small number of tests at X = 0.13. These reduced oxygen concentrations
were employed to aid in visualizing the droplet from the backlit view at high pressure.
All experiments were performed with the ambient gas and the droplet initially at room
temperature.
The binary fuel droplet was composed of n-heptane and n-hexsdecane with mass frac-
tions of hexadecane, Y, ranging from 0 (pure heptane) to 1.0 (pure hexadecane). The
subject fuels have critical temperatures and pressures of 540 K, 2.74 MPa and 722K, 1.53
MPafor heptane and hexadecane respectively. The pressure in the data reported here varied
from 0.4 to 3.0 MPa.
(_enera] _ombustjon Behavior
Figure 2 shows variations of the square of the droplet diameter d with time t at different
pressures, both normalized according to theory by dividing by the square of the initial
droplet diameter do. For fuel mixtures, after the initial transition period and the first
stage of linear behavior, another transition period e_sts (the second stage), after which the
square of the droplet diameter again decreases with time almost linearly (the third stage),
as seen in Figure 2. The cause of such three-stage burning is as follows: In the first straight
segment, the high-volatility fuel, heptane, vaporizes preferentially from the droplet surface
since the droplet temperature is relatively low. Since liquid-phase diffusion is slow relative
to droplet surface regression, the hexadecane mass fraction gradually increases in the surface
region. When the surface concentration of hexadecane reduces that of heptane sufficiently,
the vaporization rate decreases and the flame approaches the droplet (the second transition
period). After the droplet is heated by the flame enough to allow sufficient hexadecane
to vaporize, hexadecane and heptane vaporize vigorously together, in the second straight
segment. Figure 3, which shows both normalized droplet diameter and flame diameter as
functions of time, illustrates how flame contraction follows the onset of the transition period;
this delay is reduced at larger X.
XWhen the droplet iJ suzpended from the £ber_ it Lsnot spherical in shape, but is elongated along the
fiber due to surface tension. The equivalent spherical droplet diameter is computed as the cubic root of the
product of the major diameter (alonKthe fiber) with the square of the m/nor diameter (perpendicular to the
fib_).
312
As seen in Figure 2, this staged type of burning still exists at the supercritical ambient
pressure of 3 MPa. The concentration Y=0.11 in Fig. 2 lies outside the boundary of staged
burning reported earlier by Niioka and Sato. The phenomenon may have been present but
not observable in the normal-gravity experiments because the duration of the second stage
is shortened by the increased rate of heat transfer produced by natural convection, or the
gas-velocity and surface-tension variations under natural convection may have promoted
internal liquid mixing and thereby narrowed the region of staged burning. Stage burning
occurred for all mixtures tested other than pure fuels in the present study.
Homogeneous nucleation is not observed in this experiment. This result is expected
because the ambient gas is diluted with nitrogen, reducing the droplet surface temperature,
so that the local droplet temperature has less of a tendency to exceed the limit of superheat.
Droplet Lifetime
Figure 4 shows the dependence of the normalized droplet lifetime 2, tl, at different ambient
pressures on the initial hexadecane mass fraction.
This figure shows that the droplet lifetime reaches a maximum at an intermediate hex-
adecane mass fraction, at all but the highest pressure. For fixed initial hex_lecane mass
fraction, the droplet lifetime decreases with increasing ambient pressure, except for pure
hexadecane. For hexadecane, the droplet lifetime exhibits a minimum value at 2.5 MPa
rather than the critical pressure of 1.5 MPa, and the burning-rate constant exhibits a max-
imum at 2.5 MPa, as described later. Earlier results indicated that in air the burning-rate
constant achieves its maximum at the critical pressure. Studies of fuel-droplet evaporation,
however, have reported that the evaporation constant reaches its maximum at an ambi-
ent pressure above the critical pressure and at higher ambient pressures when the ambient
temperatures axe lower. Therefore, the occurrence of the minimum droplet lifetime and the
maximum burning-rate constant above the critical pressure, rather than near the critical
pressure, may be caused by delayed attainment of liquid criticality, associated with the
lower temperatures produced by dilution of air in the present experiments. It is noteworthy
that minima are not seen for heptane or any of the mixtures. Most of their histories seem
dominated by heptane, whose critical pressure is significantly higher; the minima could
develop at higher pressures for these droplets.
As shown by Figure 2, the square of the droplet diameter decreases almost linearly with
time in the first and third stages. Figure 5 shows the burning-rate constant in the first
stage, t(z, and in the third stage, 1(3, both here defined from the maximum slope in each
segment. While/(z decreases sharply with increasing initial hexadecane mass fraction at
fixed ambient pressure, /(3 increases somewhat. Both increase with increasing pressure,
except for pure hexadecane, which exhibits a maximum value at 2.5 MPa, as described
above.
Since the first straight segment occupies most of the droplet lifetime for small Y,/(z,
which decreases with increasing Y, has a major influence under these conditions. On the
other hand, for large Y, since the second straight segment occupies most of the droplet
lifetime,/(3, which increases with increasing Y, is dominant.
The burning-rate constant/(z may be approximated as
Kz = [8_/(p_,)]tn(X + B) (1)
2The droplet lifetime is defined as the time _om the initial appeazance of a luminous flame until the
droplet disappears.
313
where the transfernumber B is
B = [YcT+Yo,o vc+wc+/(vowo)]/(1-YcT+) (2)
on the assumption that, in the first straight segment, the droplet surface temperature is
nearly equal to the boiling point of heptane, and hexadecane vaporization is negligible for
small hexadecane mass fraction. Here, YeT+ is heptane mass fraction in the gas phase at
the gas-liquid interface. The heptane mole fraction in the gas phase Xcr+ is written as
Xc_+ = aXc__ = a(l - Xcle-) (3)
by P,.aoult'slaw and the Clansius-Clapeyronrelation,where a < I,and Xcr- and Xcls-
are heptane and hexadecane mole fractionsin the liquidphase at the phase interface.Here
1_ are relatedto Xi as
= x,w,/ xjwj (4)
From these relations, dKl[dYcas_ < 0 can be shown, which is consistent with the decrease
in K1 with increasing initial hexadecane mass fraction.
Similar reasoning could rationalize the Ka dependence only if heptane vaporization
were negligible in the third stage. It seems more likely that, with decreasing Y, the reduced
duration of the third stage delays attainment of its :m_mum Ka.
Onset of the=Second Transition Period
Figure 6 shows data for the ratio of the critical volume, Vc, defined as the volume of the
droplet at the onset of the second transition period, to Vo, the initial droplet volume. These
data also may be interpreted approximately as the volume at the onset of flame contraction,
which follows the onset of the transition period as seen Figure 3. Results at an ambient
oxygen mole fraction of 0.13 axe also shown in Figure 6. The results indicate that the critical
volume increases with the initial hexadecane mass fraction and is almost independent of
ambient pressure and oxygen content.
Two types of curves are shown in Figure 6 for the critical volumes at 1 MPa generated
with different assumptions. The dotted curve is
VdVo = Y(po/p) (5)
on the assumption that the liquid-phase diffusion is infinitely rapid and hexadecane does
not vaporize until the droplet has exhausted all heptane. Here po is the density of the
fuel mixture at room temperature, and p the density of hexadecane at the boiling point of
heptane. The broken curves are
_/Vo = [Y/{Y + 3_(1 - Y)}](Po/P), (6)
which is an equation of Shaw based on the assumption that the llquid-phase diffusion
coeffident D is sufficiently small relative to the burning-rate constant K such that c =
8D/K << 1 can be treated as a small parameter, and hexadecane vaporization is negligible.
If _ is estimated with the assumption that D is the liquid-phase diffusion coefficient of
heptane in hexadecane at the boiling point of heptane, and K is the burning-rate constant
of heptane, then at 1 MPa, ¢ is about 0.05. The experimental results lie between this result
and the infinite-diffusivity result but fall closer to this result at the larger values of Y and
closer to the infinite-diffusivity curve at smaller Y. Reasons for this behavior are discussed
fully in ref. 1.
314
Conclusions
The main conclusions can be listed: Staged burring still exists above the critical pressures
of heptane and hexadecane. Heptane is dominant in gasification in the first stage and
hex_lecane in the third even at high pressure. Droplet lifetimes increase with the initial
hexadecane mass fraction and reach a maximum value at a mass fraction Of about 0.3 for
pressures below 2.5 MPa in sufficiently diluted atmospheres. The maximum burning-rate
constant in the first stage decreases with the initial hexa_lecane mass fraction but that in
the third stage increases. The droplet volume at the onset of the second transition period
is almost independent of the ambient pressure. Although it can be predicted qualitatively
from liquid-phase diffusion-controLled theory, quantitative predictions are uncertain because
of variations in buruing-rate constants and in liquid-phase diffusion coefficients.
Future P]ans
In view of the resultsdescribed above, itwould be of interestto testbehaviors of these
mixtures at higherpressuresand at differentoxygen-nitrogenratios,particularlyat higher
oxygen content,such as in norma_ Mr. However, experimentaldifficultiesin observingthe
dropletscause the successof such investigationsto be uncertain. Other plans under con-
siderationinvolvestudying freedroplets,testingbinary alcoholmixtures at these elevated
pressures(but probably not super-criticalbecauseoftheirhighercriticalpressures),and in-
vestigatingdropletarraystoobtaininformationrelevanttohigh-pressurespray combustion.
In any event,thissuccessfulinternationalcooperationappears to deserveto continue.
Referen ces
1. Mikami, M., Kono, M., Sato, J., Dietrich, D.L., and Williams, F.A., "Combustion of
Miscible Binary-Fuel Droplets at High Pressure under Microgravity," Combustion Science
and Technology, to appear, 1992.
|
Drag eJfic|d
Figure I. Schcn_c diasTLm of thc (:,'Ix=i,nent_l npl_r_us.
v
l.n
0.R
0.6
0.,I
0.2
I I I
OA
X=0.12
_
tO . O P=I.OMPn
0 A 1'=2.0 kll'a
Z_ O O 1'=3.0MPa
u A m
AO
O O
A OO
A O
O A O
° o A O O
°o_ A O0 0
Pt_Cic.ll_) = 1.5.1 MPa o A O O
I'c{C_ll,rJ = 2.74 Ml'a o A Oo A O
0 l I I
0 0.4 0.8 1.2 1.5
l/doz scd,m.=
l_g,,n.2. TL_nporal s_fi_thm ,,fthe _p.tre d the dlvpl,:t
dimlmtm, d, with tiin¢ it differentambient l,reamurcL
315
1.0
¢q
0.5
I I l I
Y=0.22
P----0.SiMl'a
X---0.13
• • oo _00 D/LL
_'°o e e
Oo • o-oo
_ 000000
(d/dD_ o -
--,. O
o
o
0
I I I l
0.4 0.8 1.2 1.6
2.5
g
2.0
6
o +_ 1.54_
1.0
2
rid0 2 sec]mm2
Temporal variations of the square of theFignre 3.
droplet diameter, d, and flame diameter, D, with
time.
I I I I
3.0 - X=0.12 K1 K3 _
P=3.0 Ml'a [] n
I I 1'=2.5MPa V V
- - P=2.0MPa A A
2.3 _ P=i.5MPa _ 0 -
,_ __.,,IA r=l.0 Ml'a O O
E 2.0 _--\
"'": __---7
.
•' 5 ..-----O----'"
+.0 ...---O---'-""
ol I I t II
0 0.2 0.,t 0.6 0.8 1.0
Y
Figasre 5. Dependence of the bunting rate const,'mt
in the first+stage, lql, and in the third stage, I(2,
at different _nbicnt pressures on tim iuitial
he.xadcc_m mass fracl.ion, Y.
0.5
0
0
l I I I I
X---'0.12
"/'::I
0 P=I.0 MPa A P=2.5 MPa "1
• P=I.5 Ml'a I"1 P=3.0 MPa tA P=2.0 MPal I ! I
0.2 0.4 0.6 0.8 1.0
Y
Figure 4. Dependence of the droplet lifetime, tt, at
different ambient pressures on the initial hexade-
cane mass fraction, Y.
I I I .... I-"++_'_-I.- .. -. n
Diffusion controllcd i ,+.'/ //." l
,.o c=O.l_/ /U / .c=0.2, /_'<--._ / /
O.S //_I / ///.... -
/ /\ / ."
/ ,," X=0.12
/+,'// _/ -" O P=l.0MPa
/ ."" I"2P=3.0MPa0.4
l// ,"<" x=013
'"-_..G'_'.."\ • P=O.4,_P,O.2
" Infin_l¢ A I'=0.8 MPa
t/ -
./." difl'usivi|y [] P I.2 Pa
0_(" I I I I
0 0.2 0.4 0.6 0.8 1.0
Y
Figure G. Dependence of the critica] droplet volume,
V,, at ,tiffcrcnt ambient pressures oa the iaiti'al
hexadecaae nna_ fractiou, Y.
316


High-pressure droplet combustion studies

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