The overall objective is the development of a fundamental understanding of the ignition and combustion of aluminum-based slurry (or gel) propellant droplets using a combination of experiment and analysis. Specific objectives are the following: (1) The development and application of a burner/spray rig and single particle optical diagnosis to study the detailed ignition and combustion behavior of small droplets; (2) Understanding the role of surfactants and gellants (or other additives) in promoting or inhibiting secondary atomization of propellant droplets; and (3) The extension of previously developed analytical models and the development of new models to address the phenomena associated with microexplosions (secondary atomization)

Turns, Stephen R.

English

NASA Technical Reports Server

PRESENTATION 2.2.3
N91-28224
IGNITION AND COMBUSTION
OF
METALLIZED PROPE_
645
https://ntrs.nasa.gov/search.jsp?R=19910018910 2020-03-19T17:28:09+00:00Z

IGNITION AND COMBUSTION OF METALLIZED PROPELLANTS
Stephen R. Turns
DepaMment of Mechanical Engineering
I. Research Objectives and Potential Impact on Propulsion
The overall objective of our research is the development of a fundamental
understanding of the ignition and combustion of aluminum-based slurry (or gel)
propellant droplets using a combination of experiment and analysis. Specific
objectives are ,the following:
. The development and application of a burner/spray rig and single
particle optical diagnostics to study the detailed ignition and
combustion behavior of small (10-75 I_m) droplets, typical of those
encountered in practical applications.
2. Understanding the role of surfactants and gellants (or other additives)
in promoting or inhibiting secondary atomization of propellant droplets.
, The extension of previously developed analytical models and the
development of new models to address the phenomena associated
with microexplosions (secondary atomization).
Slurry or gelled propellants in which a solid constituent is suspended in a
combustible liquid have been of interest for propulsion applications for several
decades. Depending upon the application, these propellants are advantageous
because of either their energy content per unit mass or per unit volume. Many solid
chemical elements are attractive for formulating slurry or gelled propellants. For
example, carbon, aluminum, and boron all offer substantial increases in volumetric
heat of combustion compared to hydrocarbon fuels, while on a gravimetric basis, only
boron provides an increased heating value [1].
For rocket applications, specific impulse and mass density control the payload
capability for a fixed vehicle configuration. Thus, high mass density propellant
systems, such as AI/RP-1/02, become attractive to consider as alternatives to more
conventional propellant systems. Zurawski and Green [2] show that an RP-1/60% AI
fuel has the potential of delivering 17% more payload than a pure RP-1 fuel for an
upper-stage vehicle mission.
Several key technical issues impact the use of AI/RP-1 slurries or gels in
propulsion systems. Among these are rheological properties and ignition and
combustion characteristics. In the area of theology, questions concerning the proper
formulation to provide a stable easily pumpable slurry with acceptable spray
characteristics remain unanswered. With regard to ignition and combustion, the issue
of concern is the relatively long residence times associated with these processes and
two-phase flow losses. These issues are the focus of our research. We have found in
previous studies of AI/JP-10 slurries that the additives used to produce desirable
647
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theological properties impact on the ignition characteristics [3]. For example, the
addition of surfactants to a slurry blend tend to promote the break up of burning slurry
droplets into a very large number of smaller droplets. The aluminum in these small
drops readily ignites and burns rapidly. In addition, the particle sizes of the aluminum
oxide products are smaller, thus decreasing two-phase flow losses. Achieving the
research objectives will provide detailed knowledge of the effect of blending agents on
ignition and combustion of slurry fuels. This information will help lead to formulation of
propellants which provide an optimum combination, or an intelligent trade-off, of
rheological and combustion characteristics. Moreover, understanding and being able
to predict ignition times for small droplets will provide guidelines for feasibility and
developmental studies of metallized propellant engine systems prior to the
construction of costly demonstration hardware.
II. Current Status and Results
Our efforts to date have focused primarily on the design, fabrication, set-up, and
calibration of the various experimental systems required to study the ignition and
combustion of 10-100 p.m diameter slurry droplets. The major systems are the
burner/spray rig and the optical diagnostics. Schematics of these systems are shown
in Figs. 1 and 2, respectively. The design objectives for the burner/spray rig were to
provide a laminar, homogeneous post-flame region for slurry ignition; to allow
flashback-free operation over a range of stoichiometries while operating with oxidizers
ranging from air to 100% oxygen; to isolate the post-flame gases from the ambient
atmosphere; to produce a spray of aluminum slurry droplets varying in size from
approximately 10 to 100 p.m in diameter and introduce a portion of this spray into the
post flame region; and to provide for fixed mounting of laser optics through the use of a
traversing burner support. A burner meeting these requirements was designed,
fabricated, and installed in the laboratory. The system is operational, and its detailed
performance is currently being evaluated. The burner head is based on a design
discussed in Ref. [4]. Gaseous fuel enters the base of the burner and passes through
a dispersion ring that evenly distributes the gas around the perimeter of a fuel
chamber. From here, the fuel passes through 72 stainless steel tubes and exits the
burner at the top surface of a honeycomb matrix. The oxidizing gas enters the middle
section of the burner and passes through another dispersion dng above the manifold
plate. This gas then flows up around the 72 fuel tubes and through the open cells of
the honeycomb matrix surrounding these tubes. This configuration results in a small,
laminar, diffusion flame at the exit of each fuel tube. These flames merge rapidly
providing an excellent approximation of a pre-mixed, laminar, flat flame. Slurry
droplets generated in the spray chamber at the bottom of the burner pass through a
tube located along the centerline of the burner and exit into the hot product gases.
The spray nozzle is of the gas atomizing type and was selected based on work with
coal/water slurries [5]. Initial testing with water showed that the system delivers
droplets of the desired size range into the hot product stream.
The principal diagnostic being used to study droplet ignition, burning, and
secondary atomization is a laser-based sizing and velocity measurement system. By
measuring the evolution of the drop-size distribution with distance downstream of the
burner face, we will be able to ascertain whether the droplets burn without disruption
648
DROPLETS J/ HONEYCOMB
MATRIX
N= INLET
OXIDIZER
INLET
GASEOUS
FUEL
INLET
SPRAY
_VAPOR VENT
LIQUID RETURN
IRRY
ATOMIZING GAS
FIG. 1. Burner and spray rig.
POWER
METER
-ION L
LASER 1
10X BEAM
EXPANDER
HE-NELASER
BURNER SIZE/VELOCITY,
; PH2 r_
PH3
M1 L6,
IGNITIO_
FIG. 2. Optical system for particle sizing and velocity measurements.
649
or if they disrupt. The laser light scattering system designed and built to accomplish
these measurements is shown in Fig. 2. In principle, the two-color laser light scattering
technique [6] operates as follows: droplets pass through the 350 p.m diameter waist of
a focused Ar-ion laser beam. Light scattered by the droplets in the forward direction is
related to particle size, with larger particles producing a larger signal. Because of the
Gaussian distribution of light intensity in the laser beam, only data from droplets that
pass through the beam center should be accepted. This eliminates the ambiguity
associated with the light scattered by droplets passing through the edge of the beam.
For example, a large droplet at the edge of the beam would scatter the same amount
of light as a small droplet passing through the center. To accomplish this
discrimination, a second beam from a He-Ne laser is aligned concentrically with the
Ar-ion beam, but focused to a much smaller waist of approximately 80 _m.
Comparison of the time-dependent signals associated with scattering of the He-Ne
and Ar-ion beams determines whether the droplet passed through the central region of
the Ar-ion beam.
The determination of the particle size is accomplished by a calibration of the
scattered intensity associated with light diffracted through pinholes of known size and
by performing measurements in a stream of monodisperse water droplets. Droplets of
known size are generated with a Berglund-Liu droplet generator. Results of a typical
droplet size calibration are shown in Fig. 3. In addition to obtaining information on
droplet size, knowing the beam diameter readily permits the calculation of droplet
velocity from the droplet time-of-flight through the beam. Figure 4 illustrates a typical
voltage-versus-time trace for a 90 _m water droplet passing through the probe volume.
The peak voltage of approximately 5.5 volts from the upper trace provides the size
information (c.f. calibration curve in Fig. 3.). The droplet velocity computed from the
width of the trace is 1.3 m/s. The signal validation pulse associated with the light
scattered from the concentric red beam is the smaller signal centered below the
primary scattering signal.
III. Proposed Work for Coming Year
During the next 12-month period, the following tasks are planned:
Task 1. Completion of software development for data acquisition and signal
processing.
Task 2. Integrated operation of the complete system (burner, spray rig,
particle sizing and velocimetry systems, data acquisition, and data
processing) using pure liquids and aluminum slurry fuels.
Task 3. Perform screening study to establish which slurry formulation should
be used as a baseline.
Task 4. Conduct parametric study using baseline slurry.
Task 5, Begin modeling effort.
650
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0
10
0.1
i-,,.k .... 4--,-
i--.1- -i _l.-,t. ¸, ,,,
!:!:
"T-'"
--.j!
FI-"
..;i: ;_,_, ,,!hi! I ; I 14
10
Diameter (l_m)
FIG. 3. Particle sizing calibration.
1(_'='i__.,l
"--II
"" tlgS__et_LLize__ropellants
, , , , , , , , , , t , , ,
• . ._ ..... _;?',,; .......
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1 ' ' _........
.... . , . .,_ • . , _,
... _... _ ...... . i iii , i
FIG. 4. Voltage-vs.-time trace for water droplet passing through probe volume.
651
Task 1. At present, our data acquisition capability is limited to obtaining
voltage-versus-time records for the scattering signals associated with the transit of a
single particle through the focal volume. Software for a main menu control has been
written which provides set-up of the acquisition parameters for all A/D boards. The
following describes the software that will be completed in the early portion of the
upcoming year. The data collection software performs the signal validation and
records the pulse height and width of the N-ion signal and whether or not the particle
was ignited. Signal validation involves checking for the presence of multiple particles
in the probe volume, peak saturation, pulse shape, and whether the Ar-ion pulse is
completely contained within the data sample taken. The program will only accept
complete single particle signals for analysis, rejecting all other data. The peak and the
leading and trailing edges of the Ar-ion signal also are located at this time. The Ar-ion
signal pulse width is then calculated, and the pertinent information is passed to
storage. This extraction of the critical information from the signal drastically reduces
the disk space required to define a single particle, making possible the storage of a
large number of samples. The data analysis portion of the software reads particle data
from a specified file and performs the necessary sizing and velocity calculations based
on drop-size calibrations of the laser system. After this is done, the results are written
to a second data file for further statistical and graphical analysis.
Task 2. This task is the final shakedown of the complete experimental system.
Initial experiments will be conducted with pure liquids to simplify fuel handling. The
burner operation limits will be investigated to determine maximum and minimum gas
velocities that can be achieved with different mixture fractions and oxygen content of
the oxidizer stream.
Task_. At present, three AI/hydrocarbon gelled fuels are available: a 55 wt. %
AI/RP-1 fuel formulated by Sun Advanced Research and Marketing; a 53.8 wt. %
AI/RP-1 blend formulated by TRW Space & Technology Group; and a 55 wt. % AI/RP-1
gelled fuel from Aerojet TechSystems. Different blending agents were used in each of
these formulations, so each fuel is likely to exhibit different secondary atomization
characteristics. Tests will be conducted on each blend to ascertain which fuel
provides the best secondary atomization characteristics for a limited set of test
conditions. The fuel selected will be used in the following task.
Task 4. Using the baseline fuel, an extensive set of parametric experiments will
be carried out. These experiments will explore the effects of the hot gas environment
on secondary atomization. The parameters to be investigated include: gas
temperature, oxygen concentration, and residence time. A second set of experiments
will be planned to investigate the effects of the minor constituents in the fuel blends on
secondary atomization.
Task 5. In support of the experimental efforts in Task 4, data will be analyzed
using codes which predict droplet ignition and combustion times. Existing codes are
presently being modified for this task. A physical model for crust formation on slurry
droplets during the RP-1 burnout phase will be formulated, and the appropriate
conservation laws applied.
652
IV. References
1. Pinns, M. L., Olson, W. T., Barnett, H. C., and Breitwieser, R., "NACA Research
on S_urry Fuels," NACA Report t 388, 1958.
. Zurawski, R. L., and Green, J. M., "An Evaluation of Metallized Propellants
Based on Vehicle Performance," AIAA paper AIAA-87-1773, presented at the
23rd Joint Propulsion Conference, San Diego, CA, June 29-July 2, 1987.
3. Wong, S.-C., and Turns, S. R., "Disruptive Burning of Aluminum/Carbon Slurry
Droplets," _or'nbust. Sci. and Tech., 66:75-92, 1989.
. Barlow, R. S., Dibble, R. W., Chen, J.-Y., and Lucht, R. P., "Effect of Damkohler
Number on Superequilibrium OH Concentration in Turbulent Nonpremixed
Jet Flames," AIAA paper AIAA-89-0061 presented at the 27th Aerospace
Sciences Meeting, January 9-12, 1989.
. Holve, D. J., and Meyer, P. L., "Coal/Water Slurries: Fuel Preparation Effects
on Atomization and Combustion," Combustion Science and Technology,
52:243-268, 1987.
6. Wang, J. C. F., and Hencken, K. R., "In Situ Particle Size Measurements Using
a Two-Color Laser Scattering Technique," ADDlied ODtics, 25: 653-657, 1986.
653



Ignition and combustion of metallized propellants

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