Near critical and supercritical burning of fuel droplet

Faeth, G. M.

English

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Second Annual Report
Report Period: Sept. 1, 1967 to Sept. 1, 1968
An Investigation of Near Critical and Super-Critical
Burning of Fuel Droplets
by
G. M. Faeth
Prepared for
National Aeronautics and Space Administration
September 30, 1968
0 77
Contract: NGR 39-009-017
Technical Management
NASA Lewis Research Center
Cleveland, Ohio
Chemistry and Energy Conversion Division
Paul R. Wieber
Mechanical Engineering Department
The Pennsylvania State University
University Park, Pa.
N	 N 6
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Abstract
This report summarizes the work done under NASA contract
NCR 39-009-007 for the period Se-otember 1, 1967 to Se ptember 1,
1968. Principal accomplishments of the report period are briefly
discussed and publications arising from the research are listed.
The most significant achievement has been the verification
of existing supercritical droplet combustion theories for vari-
ous pressures, droplet sizes and ambient oxygen concentrations.
'.'fork was also begun on high pressure drop let gasification which
included the development of a theoretical model of droplet heat-
up in the presence of combustion. Preliminary experimental work
in this area, to date, has included verification of evaporation
models (without combustion) in gas environments at temperatures
typical of combustion processes at atmospheric pressure.
1
Table of Contents
Title Page . . . . . . . . . . . . . . . . . 1
Abstract . . . . . . . . . . . . . . . . . . 2
Table of Contents	 . . . . . . . . . . . . . 3
I. Introduction	 . . . . . . . . . . . . . 4
II. Apparatus . . . . . . . . It . . . . . . 6
III. Sup ei^critical Droplet Combustion . . . 8
IV. Droplet Gasification . . . . . . . . .11
V. future 11ork . . . . . . . . . . . . . . .13
VI. References . . . . . . . . . . . . . . .14
9
I
11
4 9
i
I. Introduction
This second annual re port is a summary of the principal
accomplishments made during the course of research performed
under NASA contract NGR 39-009-007, durinv the period Sent. 1,
I	
1967 to Sept. 1, 1968. The findings of this research are nre-
sented, in more detail in the First Annual Report and several
other technical reports and papers published during the present
report period2,314,
The computation of droplet life histories has become a
basic step in the design of combustion chambers for liquid fuel
rocket engines 5 . At moderate pressures, this involves computing
the heat-up of the droplet from its injection temperature to its
wet bulb temperature (the condition where all the heat reaching
the droplet is utilized for the 'seat of vaporization of the
material gasifying at the surface of the drop) and the rate of
evaporation throughout the droplets lifetime. however, it was
soon recognized, that at sufficiently high pressures, the dron-
let would approach its critical temperature during its lifetime
(near., critical combustion) or possibly exceed the critical tem-
perature during heat-up (super-critical combustion). For super-
critical combustion, a new burning regime must appear and for
near critical combustion or heat-up to the critical temperature,
many of the assumptions ordinarily made in life history com puta-
tions become questionable. These problems gave rise to a need,	 s_
for further study of droplet evaporation and combustion at high
,s
i
47
pressures.
Therefore, the goal of the present research program is
to study droplet Combustion in the pressure regime where the
droplet approaches or exceeds its critical temperature during;
combustion. In this report period, experimental work has been
completed which gives a reasonable verification of previously
proposed super-critical droplet combustion theories 71II . Work
has also begun on the study of droplet gasification at high
pressures. This has included extending steady state combus-
tion theories, valid at the wet bulb state, to include gasif-
ication processes in the heat-up period of a burning droplet.
Some preliminary experimental work on droplet gasification at
low pressures has also been completed and found to be in good
agreement with the theory. This low pressure matching has
served to provide a base line which will allow the influences
of high pressure phenomena to be evaluated more confidently.
I
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a
II.	 Apparatus
The droplet is supported for the bead of a thermocouDle
within a high pressure reaction chamber. The droplet is
ignited by a small hydrogen diffusion flame at pressures
below 100 psia or by a hot wire placed near the droplet at
higher pressures. The reaction chamber is fitted with win-
dows to allow photographing the droplet. In order to pre-
vent the droplet from falling from the thermocouple bead,
due to reduced surface tension near the critical point, the
experiment is conducted under zero-gravity conditions in a
free fall chamber.
The basic construction of the apparatus was completed
in a previous report period and a more detailed description
of the experimental setup may be found in Ref. (1). In the
present report period, the apparatus has undergone further
development, as follows:
1. A back ground lighting system has been installeO which
allows alternate dark field and silhouette photographs
of the droplet on the motion p icture records. Prelim-
..	
R
inary testing has shown that droplet diameters may be
measured during combustion for pressures up to 100 psia.
At pressures greater than this light refraction from
density gradients obscures the shadowgraph outline of
the droplet. Further modification of the optical system
is planned in order to attempt to extend the pressure
{
arange of diameter measurements, but in any event over-
all evaporation lifetimes can be measured with the
present system to yield "Li nformation on evaporation rates.
2. Errors encountered in temperature measurements, par-
ticularly at pressures below 100 Asia, were reduced in
part by the use of the flame ignitor at these lower
pressures. The source of this error was excessive radi-
ation from the hot wire ignitoi, By reducing the thermo-
couple wire size from .1003 to .001 inch, coupled with
the use of a glass shield over the wires where they pass
through the combustion zone, further errors due to con-
duction along the wires have been minimized.
3. Other changes of a more minor nature included the in-
stallation of more accurate high pressure gages, abso-
lute manometers for low pressure testing and reniping
the gas handling system for GOX service.
0
I
Vi
III.	 Super-Critical Droplet Combustion
With increasing ambient pressure, eventually a condition
is reached where the droplet heats up with little evaporation
and gasification occurs primarily by the droplet exceeding
its critical temperature. In this regime, the combustion
process is primarily controlled by the diffusion of the fuel
vapor to the combustion zone. Spalding has developed a theory
for combustion in this regime by modelinp the fuel vapor as a
point source. Posner has modified this theory to allow for
the finite dimensions of the vapor pocket8.
Tests were conducted to check the validity of these
theories employing decane as the fuel, with droplet sizes in
the range 600-1200p in oxygen nitrogen mixtures varying from
air to pure oxygen. The gas pressures employed in these
tests varied from 800 to 2000 Asia. At these pressures, the
temperature traces indicated that there was little evapora-
tion prior to the droplet exceeding its critical temperature.
A super-critical combustion time was defined as the time
between the droplet reaching its critical temperature and
burnout. This parameter was compared with the theoretical
results and found to be in reasonably good agreement with
respect to variations in initial, droplet diameter, pressure
and ambient oxygen concentration 4 . At high ambient oxygen
concentraticns, where the greatest differences exist betwecy
the theories of Spalding and Posner, the Rosner distributed
tsource model was found to Five a more accurate correlation of
the data.
Predicted and experimental flame positions were also com-
pared. These results were found to be in good agreement with
respect to variations in all experimental variables. These
results may be contrasted to the case of the low pressure
auasisteadycombustion theories, where theoretical and exper-
imental flame positions were often in large disagreement9,
I
IV.	 Droplet lasification
Following the completion of work on super-critical drop -
let combustion, project effort was turned to the problem of
droplet heat-up and gasification at high pressures. General
observations to date on this problem have inL icatecl the fol-
lowing results;
1. The pressure levels where droplet Pasification occurs
primarily by the droplet exceeding its critical tem-
perature are in general agreement with Wieber's 6 calcu-
lations,
2. The rate of droplet heat-up, following ignition, increased
as the ambient oxygen concentration was increased.
There are theories available to estimate dro p let trans-
port rates during heat-up for the case of droplet evaporation
without combustion. However, the existing theories of droplet
combustion u p.-% ve limited to stea.cly burning at the wet bulb
state. Therefore, it was necessary to extend these theories
in order to make the model valid for combustion during heat-
up, The theoretical model resulting from this allows the es-
timation of droplet transport rates at any time during; heat-un,
for any ambient temperature and for any ambient oxy gen concen-
tration assuming the existence of a diffusion flame if a com-
bustion process is present (naturally, the role of fuel and
oxidizer can he readily interchanged to consider an oxidizer
drop let in a fuel, rich atmosphere). Convection effects are
icarried in the model via a filri theory annroximation.
With this model completed, the experimental approach
selected was first of all to obtain base line comparisons for
• variety of ambient conditions at low pressures. This allows
• check of the theoretical ap proach and the methods of com put-
in g average properties in the absence of Iiip;'i pressure effects.
With this base line established further testing, will be under-
taken at increased pressures in order to evaluate the correc-
tions needed for high p ressure computations.
To date, theoretical and experimental comparisons have
only been made for droplet evaporation without combustion.
The theory was compared with earlier data on the zero-gravity
evaporation of iso-octane droplets ii., air at tem peratures on
the order of lOOO°F and rpv^^ssurFs of one and two atmospheresl0
with good agreement respecting the temperature and diameter
variations of the droplet.
A second set of comp arisons were made for water and decane
droplets evaporating in the Rases exhausting from a flat flame
burner. The burner was operated stoichiometrically on a carbon
monoxide, oxygen, nitrogen mixture to yield a gas temperature
of 2520 0K. Under these conditions, the oxygen concentration
in the burner combustion products was less than 1 1/2o due to
dissociation, and combustion effects in the drop let boundary
layer may be neglected with little error for decane. The com-
parison between theory and experiment was again very good.
rV.
	
Future 1trork
The accuracy of the vaporization model will be further
tested by a series of runs, on the flat flame burner, with
a
increasing; ambient oxygen concentration in order to investi-
gate the influence of active combustion in the boundary layer.
D"ith these results in hand, further comparisons will be made
at low pressures in the zero-g apparatus in order to evaluate
the influence of a low gas temperature (external to the flame
front)
With the completion of these base line studies further
so that the high
activity will
results of the low
for high pressure
heat corrections,
testing will be undertaken at high pressures
pressure corrections can be evaluated. This
also include theoretical comparisons of the
pressure models with models making allowance
effects (vapor pressure corrections, content
etr_)_
I
References
A. Publications Arising from Research under the Contract.
1. G. M. Faeth D. P. Dominicis and D. R. Olson, "An Investi-
gation of Near Critical and Super-Critical Burning of Fuel
Droplets," First Annual Report, Contract INGR 39-009,007,
NASA CR-72314, Sept. 1967.
2, D. P. Dominicis, "An Experimental Investigation of Near
Critical and Super-Critical Burning; of Bipropellant Drop-
lets," NASA CR-72399, April 1968. (Published simultane-
ously as an ;1.5. Thesis, The Pennsylvania State university)
3. G. fl. Faeth and D. P. Dominicis, ' , An Experimental Study of
Bipropellant Droplet Combustion at High Pressures," Pro-
ceedings of the 4th ICRP Combustion Conference, CPIA Pub-
lication No. 162, Vol. 1, Dec. 1967.
4. G. .L. Faeth, D. P. Dominicis, J. F. Tulpinsky and D. R.
Olson, "Super-Critical Droplet Combustion,'' presented at
the 12th International Symposium on Combustion, July 14-"0,
1968, Poitieis, France.
B. Other References.
5. Priem, R. J., "Propellant Vaporization as a Criterion for
for Rocket Engine Design: Calculation of Chamber Length
to Vaporize a Single .!-Heptane Dro p ," NACA TN 3985, July
1957,
6. Wieber, P. R., "Calculated Temperature Histories of Vapor-
izing Droplets to the Critical Pnint," AIAA Journal, Vol. 1,
No. 12, Dec. 1963, pp. 2764-2770,
7. Spalding, D. B. "Theory of Particle Combustion at High
Pressur 	 ^," ARSJournal^, Vol. 29, Nov. 1959, pp. 828-835.
8. Rosner, D. E., "On Liquid Droplet Combustion at Hig?? Pres-
sures," AIAA Journal, Vol. 5, No. 1, Jan. 1967,-pp. 163-166.
9. Hottel, H. C., Williams, G. C. and Simpson, H. C., "Combus-
tion of Droplets of Heavy Liquid Fuels," Fifth Symposium
(International) on Combustion, Reinhold, New York, 1954,
pp. 101-129.
10, Faeth, G. M., "Droplet Ignition in a Quiescent Air Environ-
ment," Ph.D. Thesis, The Pennsylvania State University(1964)


An investigation of near critical and super- critical burning of fuel droplets Annual report, 1 Sep. 1967 - 1 Sep. 1968

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