Impinging injectors are a common type of injector used in liquid propellant rocket engines and are typically used in engines where both propellants are injected as a liquid, e.g., engines using LOX/hydrocarbon and storable propellant combinations. The present research program is focused on providing the requisite fundamental understanding associated with impinging jet injectors for the development of an advanced a priori combustion stability design analysis capability. To date, a systematic study of the atomization characteristics of impinging liquid jets under cold-flow conditions have been completed. Effects of orifice diameter, impingement angle, pre-impingement length, orifice length-to-diameter ratio, fabrication procedure, jet flow condition and jet velocity under steady and oscillating, and atmospheric- and high-pressure environments have been investigated. Results of these experimental studies have been compared to current models of sheet breakup and drop formation. In addition, the research findings have been scrutinized to provide a fundamental explanation for a proven empirical correlation used in the design of stable impinging injector-based rocket engines

Anderson, W. E.

Pal, S.

Ryan, H. M.

Santoro, R. J.

English

NASA Technical Reports Server

N95- 70901
ANALYTICAL AND EXPERIMENTAL STUDIES OF IMPINGING
H.M. Ryan, W.E. Anderson, S. Pal and R.J. Santoro
Propulsion Engineering Research Center
and
Department of Mechanical Engineering
The Pennsylvania State University
University Park, PA 16802
LIQUID JETS
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SUMMARY:
Impinging injectors are a common type of injector used in liquid propellant rocket engines
and are typically used in engines where both propellants are injected as a liquid, e.g., engines
using LOX/hydrocarbon and storable propellant combinations. The present research program is
focused on providing the requisite fundamental understanding associated with impinging jet
injectors for the development of an advanced a priori combustion stability design analysis
capability. To date, a systematic study of the atomization characteristics of impinging liquid jets
under cold-flow conditions has been completed. Effects of orifice diameter, impingement angle,
pre-impingement length, orifice length-to-diameter ratio, fabrication procedure, jet flow condition
and jet velocity under Steady and oscillatingl and atmospheric- and high-pressure environments
have been investigated. Results of these experimental studies have been compared to current
models of sheet breakup and drop formation. In addition, the research findings have been
scrutinized to provide a fundamental explanation for a proven empirical correlation used in the
design of stable impinging injector-based rocket engines.
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E
DISCUSSION:
An extensive series of detailed measurements, including sheet breakup length, drop size
distribution and wavelength of apparent structures, were completed to delineate the atomization
.... ....... = :
processes of two impinging water jets as a function of o_ce diameter (do), impingement angle
(20), pre-impingement length flj), length-to-diameter ratio (L/do), jet velocity COj) and jet flow
condition [1, 2]. These tests were performed under ambient conditions, and the impinging injector
used ¢omiste d Of two precis!on;boreg!_ m_s that could _: _de_pendenfly modified to obtain the
desired geometry. This injector is referred to as the glass tube impinging injector throughout the
text. Recent work has focused on evaluating the functional dependence of the measured variables
(e.g., drop size) on an injector parameter, do/Uj, associated with an empirical correlation used for
engine stability prediction. In addition, the measurement program has been expanded to include
the effects of fabrication technique and chamber pressure on the impinging jet atomization process.
197
https://ntrs.nasa.gov/search.jsp?R=19950002781 2020-06-17T23:45:50+00:00Z
iFinally, a finite difference model is being used to study the effects of jet disturbances and jet-jet
interactions on incipient wave formation on the liquid sheet formed by two opposed impinging jets.
An empirical correlation, used by industry in the design of stable impinging injector-based
rocket engines, relates the highest sustainable instability frequency to the injector parameter do/Uj
(orifice diameter/injection velocity) [3]. According to the stability correlation, an increase in the
injector parameter, do/Uj, leads to an increase in the frequency range over which the engine can
operate stably. A general objective of this work is to develop a rational physical explanation for
this successful empirical correlation; hence, the experimental results obtained with the glass tube
impinging injector are analyzed with regards to the stability correlation. Specifically, the
dependence of drop size distribution and atomization frequency on do/Uj is examined. It is
important to keep in mind that the discussion to follow pertains to cold-flow atomization data, and
that, ideally, the impinging jet atomization characteristics should be evaluated under high-pressure
and combusting-flow conditions, wl_ch w_I1 be the next_step:iJnderiakefi in _S _sem:chprogram.
The measured drop size number distribution, f(dD), is plotted as a function of drop size,
dD, and injector parameter, as shown in Fig. 1. The drop size distribution measurements were
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0.006
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6 & Z,041 ram. x,-O m
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200 400 600 800 1000 1200 1400
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Fig. 1. The number distribution plotted as a function of
drop size and injector parameter.
made using a Phase Doppler Particle Analyzer
along the spray centerline (x=0) at an axial
location (z) of 41 mm from the impingement
point for an orifice diameter of 0.64 mm and an
impingement angle of 60". An increase in
mean drop size is noted with increasing do/Uj, _
thus indicafiiag daaibigger drops may have a
stabilizing effect. Notice that an increase in the
injector parameter leads to broader distributions
as well. Similar mean diameter and distribution
shape trends are observed for different test
cases (e.g., different radial positions and
impingement angles).
The functional dependence of the periodicity of ligaments detaching from the end of the
impinging jet intact sheet on do/Uj was examined also. The measured separation distance between
adjacent detaching ligaments was converted to an atomization frequency simply by dividing the jet
velocity by the separation distance. The assumption that the detached ligament velocity is equal to
the jet velocity needs to be substantiated. The Calculated atomization frequency associated with the
glass tube impinging injector is presented in Fig. 2, along with the highest sustainable combustion
instability frequency predicted by _e Stability correlation. The dependence Of the atomization
frequency on do/Uj is quite similar to the highest sustainable combustion instability frequency;
198
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z
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do/Uj O)
Fig. 2. A comparison between the calculated atomization
frequency associated with the glass tube impinging
injector and the highest sustainable combustion instability
frequency predicted by the empirical correlation [3].
however, the calculated atomization frequency
is approximately twice as great. The similar
dependencies of maximum possible instability
frequency and atomization frequency on do/Uj
is significant in terms of periodic ligament
formation being a key process in combustion
instability phenomena.
In practical combustors, injector
orifices and pre-impingement lengths are short,
and chamber pressures are high.
Consequently, a series of cold-flow
atomization experiments were undertaken in
which sheet breakup length and apparent
disturbances on the liquid sheet surface were
gauged as a function of jet velocity and chamber pressure for an electro-discharge machined (El)M)
impinging injector having an orifice diameter of 0.51 ram, a length-to-diameter ratio and a pre-
impingement length-to-diameter ratio of five, and jan impingement angle of 60". From
instantaneous spray images, the intact sheet breakup length and the separation distance between
adjacent disturbances on the sheet surface were measured.
40
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2_-60"
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..,t-.b.-f!;::[,_..._,1
_O,f_) _'mmt tm_hJ_t, tr
lift &_& A_A
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tO70 _r_r
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0 500 1000 1500 2000 2500 3000 _00 4000
We
Fig.3. Nondimensionalbreakuplengthasa functionof
Weber number,chamberpressureandinjectortype.
A plot of the nondimensional breakup
length, xb/do, as a function of Weber number,
We (=plUj2do/cr), is shown in Fig. 3. Each
symbol in Fig. 3 represents an average of 17
individual breakup length measurements, while
the bars represent the plus/minus standard
deviation of those measurements. The breakup
length decreases with increasing chamber
pressure. In addition, for both the EDM as
well as the glass tube impinging injector, the
nondimensional breakup length is relatively
insensitive to changes in We. Two differences between the glass tube impinging and the EDM
injectors are the orifice length-to-diameter ratio and the pro-impingement length. The glass tube
impinging injector L/do is 80 and lj/do is 40, while the EDM injector L/do and lj/do are both equal
to five. In spite of these geometric differences between the two injectors, the measured
nondimensional breakup lengths are comparable, especially at the higher Weber numbers.
199
Measurements of the separation distance between adjacent disturbances (disturbance
wavelength) on the intact impinging jet sheet surface were made as a function of chamber pressure
and injector type. As shown in Fig. 4, the measured disturbance wavelength is observed to be
independent of both chamber pressure (up to 1070 kPa) and jet velocity (up to 19 m/s) for the
EDM injector. Disturbance wavelength measurements for the glass tube impinging injector were
independent of jet velocity (up to 18.5 m/s) as well; however, measurements were only made
under ambient pressure conditions. At a pressure of 101 kPa, the measured disturbance
wavelengths associated with the glass tube impinging injector are slightly larger than those
measured for the EDM injector, which has a smaller orifice length and pre-impingement length as
compared to the glass tube impinging injector.
4
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SiT • _11
1070 V_V
0 I ! ! ! ! I I
0 500 1000 ISO0 2000 2S00 ._00 3S00 4000
We
Fig. 4. Nondimensional surface disttgbanee wavelength
as a function of Weber number, chamber pressure and
injector type.
The formation of impact waves on the
sheet surface emanating from the impingement
point has been often observed and is a process
which must be clearly understood for the
development of an accurate mechanistic
impinging jet atomization model. To
specifically address this process, modeling
efforts have centered on the use of a finite
difference Navier-Stokes code (RIPPLE) [4] to
investigate the effects of spatial and temporal jet
flow oscillations on the primary breakup of the
impinging jet fan into ligaments.
Directly opposed axisymmetric jets are
chosen for study to simplify the analysis. Harmonic disturbances are imposed on each jet
upstream of the impingement point to simulate jet instabilities. The imposed jet disturbances have a
radial profile that is proportional to sin (m/do), with the maximum axial velocity oscillation at the
jet periphery being 5% of the mean jet velocity, Uj. The computed free surface contours for two
cases, one in which a steady, flat velocity profile exists at both jet inlets, and the second case in
which a sinusoidal velocity profile having a frequency of 1000 s -1 exists at both jet inlets, are
shown in Fig. 5. For the unsteady jet case, the sheet disturbance frequency equaled the frequency
of the imposed oscillation on the jet, and the resultant sheet is qualitatively similar to those
experimentally observed. This model has some promise of clarifying the process of impact wave
formation, and ongoing analytical work is focused on the most appropriate way to simulate pre-
impingement jet oscillations.
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X(cm) X(cm)
, Steady Inlet Conditions 1000 Hz Oscillation
Fig. 5. Computed free surface contours for a steady jet inlet Condition and an inlet condition With harmonic
disturbances imposed on the jet. The top jet enters at the upper left hand corner with a mean velocity of -10 rigs and
the bottom jet enters at the lower left hand comer with a mean velocity of 9.6 m/s. The jet diameter is 10 nun.
A CKNOWLEDGEMENTS:
The authors would like to acknowledge the support of the Air Force Office of Scientific
Research, Air Force Systems Command, USAF, under grant number 91-0336, as well as the
support of the Penn State NASA Propulsion Engineering Research Center under grant NAGW
1356. In addition, the technical information and feedback provided by the personnel of the Aerojet
Propulsion Division is gratefully acknowledged.
REFERENCES:
[1]
[2]
[3]
[4]
Anderson, W.E., Ryan, H.M., Pal, S. and Santoro, R.J., "Fundamental Studies of
Impinging Liquid Jets," A/AA paper 92-0458, 30th Aerospace Sciences Meeting, Reno,
NV, Jan. 6 - 9, 1992.
Ryan, H.M., Anderson, W.E., Pal, S. and Santoro, R.J., "Atomization Characteristics of
Impinging Liquid Jets," AIAA paper 93-0230, 31st Aerospace Sciences Meeting, Reno,
NV, Jan. 11 - 14, 1993 (accepted for publication in Journal of Propulsion and Power/.
Anderson, W.E., Ryan, H.M. and Santoro, R.J., "Combustion Instability Phenomena of
Importance to Liquid Bi-Propellant Rocket Engines," 28th JANNAF Combustion Meeting,
San Antonio, TX, Oct. 28 - Nov. 1, 1991.
Kothe, D.B., Mjolsness, R.C. and Torrey, M.D., "RIPPLE: A Computer Program for
Incompressible Flows with Free Surfaces," Report LA-12007-MS, Los Alamos National
Laboratory, Los Alamos, NM, April 1991.
=
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