A droplet monitoring system is disclosed for analysis of mixed-phase fluid flow in development of gas turbines. The system uses a probe comprising two electrical wires spaced a known distance apart and connected at one end to means for establishing a dc potential between the wires. A drop in the fluid stream momentarily contacting both wires simultaneously causes and electrical signal which is amplified, detected and counted

Baughman, J. R.

Thys, P. C.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON. D.C. 20546
'(NASA-Case-NPO-10985) DROPLET MONITORING N73-20473
:PEOBE Patent (North American Sockwell
Corp.) 5 p CSCL 14B
A
REPLY TO Onclas
ATTN
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NOTE - If this patent covers an invention made by a corporate
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Pursuant to Section 305(a) of the National Aeronautics and
Space Act. the name of the Administrator of NASA appears on
the first page of the patent; however, the name of the actual
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the Specification, following the words "... with respect to
an invention of ...H
Elizabeth A. Carter
Enclosure
Copy of Patent cited above
NASA-HQ
https://ntrs.nasa.gov/search.jsp?R=19730011751 2020-03-17T08:15:33+00:00Z
United States Patent
Low et al.
in] 3,712,132
[45] Jan. 23, 1973
[ 54 ] DROPLET MONITORING PROBE
[76] Inventors: George M. Low, Deputy Adminis-
trator of the National Aeronautics
and Space Administration with
respect to an invention of; James R.
Baughman, 7110 Pomelo Drive,
Canoga Park, Calif. 91304; Paul C.
Thys, 13208 Goleta, Pacoima, Calif.
91331
[22] Filed: Sept. 23, 1970
[21] Appl. No.: 74,759
[52]
[51]
[58]
156]
U.S. Cl 73/194 E, 324/30 R, 324/65 P
Int. Cl... GO If 3/00
Field of Search 73/29, 194 E, 194 F, 432 PS;
340/239 R; 200/61.05; 128/DIG. 13; 324/71
CP, 65 P, 30 R
References Cited
UNITED STATES PATENTS
3,308,648 3/1967 Moulton et al 324/30 R X
3,449,952 6/1969
2,876,414 3/1959
3,286,167 11/1966
3,264,558 8/1966
Brown et al.... 73/194 E
Shaheen 324/30 R
Gwyn 324/30 R
Heeps 324/65 P
Primary Examiner—Charles A. Ruehl
Attorney—John R. Manning, J. H. Warden and Monte
F. Mott
[57] ABSTRACT
A droplet monitoring system is disclosed for analysis
of mixed-phase fluid flow in development of gas tur-
bines. The system uses a probe comprising two electri-
cal wires spaced a known distance apart and con-
nected at one end to means for establishing a DC
potential between the wires. A drop in the fluid stream
momentarily contacting both wires simultaneously
causes an electrical signal which is amplified, detected
and counted.
7 Claims, 4 Drawing Figures
PATENTEOJAN23I973 3.712.132
FIG. I
v>>>v yx/v v/vvv^
16
FIG. 2
12 14
FIG. 4
FIG. 3
STEAM FLOW
_J
FIRST STAGE NOZZLE
FIRST STAGE TURBINE
WHEEL •
SECOND STAGE
NOZZLE
INVENTORS
J. R. BAUGHMAN
By P. C. THYS
ATTORNEYS
3,712,132
1 2
DROPLET MONITORING PROBE determined by the distance between the two probe
noi^iM nc ixi\/cxiTtr»w wires will cause an electrical signal to be developed. An
ORIGIN OF INVENTION amplifier and detector couple the droplet sensing probe
The invention described herein was made in the per- to a counter in order that the number of droplets larger
formance of work under a NASA contract and is sub- 5 than the probe wire spacing in a period of time may be
ject to the provisions of section 305 of the National determined. By varying the probe wire spacing, data on
Aeronautics and Space Act of 1958, Public Law droplet size distribution can be obtained.
85-568 (72 Stat. 435; 42 USC 2457). The novel features that are considered characteristic
of this invention are set forth with particularity in the
BACKGROUND OF THE INVENTION '° appended claims. The invention will best be un-
The present invention relates to a system for analysis derstood from the following description when read in
of mixed-phase fluid flow, and more particularly to conjunction with the accompanying draw.ngs.
monitoring the propagation of droplets in a fluid stream
 BRIEF DESCR1PTiON OF THE DRAWINGS
that is primarily in a gaseous state. 15
The desirability of increasing the efficiency of tur- FIG. 1 is a cross-sectional view of a droplet sensing
bomachinery has made it necessary to increase rota- probe in accordance with the present invention,
tional speed over the years. Continual development of FIG. 2 is a modification of the droplet sensing probe
new, high-strength alloys has made this increase possi- of FIG. 1.
ble. As a result, turbines with tip speeds in excess of 20 FIG- 3 illustrates schematically the manner in which
2,000 ft/sec, are now being used in many applications. droplet sensing probes provided in accordance with the
In applications where vapor cycles are employed, the present invention may be used for droplet propagation
higher speeds emphasize the recurring problem of tur- studies in a steam turbine.
bine blade erosion due to droplet impact. Such applica- FIG. 4 is a block diagram of a droplet monitoring
tions are found in the power industry as well as in the 25 system of the present invention,
field of aircraft and space power systems. nFsrRiPTinixi OF THF PRFFFRRFD
The erosion, resulting from impact between the EMBODIMENT^
rapidly moving blade and the slow, poorly accelerated
drop has been periodically studied, but the A first embodiment of the present invention illus-
phenomenon has never been fully understood, nor has 30 trated in FIG. 1 utilizes two wires 10 and 11 spaced a
it been possible to predict results for any particular known distance apart and supported in a metal sheath
design. The advent of nuclear-powered steam plants, 12 filled with a synthetic plastic or resin 13. The con-
where higher speeds and wetter steam are used, has struction is very much like that of a thermocouple
revived industrial interest, and development of space probe but with the dissimilar metals in the form of wires
power systems using potassium and mercury vapors has disconnected at the end and spaced throughout a sub-
focused attention to this problem of erosion due to stantial length of their exposed ends as shown. The
droplet impact. In particular, attention is being given to synthetic plastic or resin 13 in the sheath 12 is extended
obtaining a better, more basic understanding of droplet and formed into a small ball 14 to support the base of
propagation, leading to a method of predicting their
 40 the wires 10 and 11 with greater spacing than is desired
presence and giving design information which will over the rest of the wires in order to avoid collection of
minimize their erosive effects. fluid in the liquid state between the wires where they
While the emphasis has been on more basic un- enter the insulation. This is desirable because oscilla-
derstanding of droplet propagation in gas turbine tion and breakaway of this fluid in a liquid state causes
developments, such an understanding is equally basic 45 a variation in the droplet count.
to developments in similar fields where high tempera- In operation, a DC potential is established between
ture erosion phenomena requires characterization of ends 15 and 16 of the respective wires 10 and 11. A
the mixed-phase fluid directed against a stationery ele- droplet of a minimum size determined by the spacing
ment such as a baffle or a moving element much like a between the wires 10 and 11 contacting both wires
turbine blade. In the past, the art of fluid state measure- SO simultaneously will cause current to flow through the
ment has not provided apparatus for obtaining droplet wires, thereby providing a signal which can be detected
propagation data on mixed-phase fluid streams. and recorded, as by an oscilloscope or a counter. Thus,
ORIFrT«5 AND SUMMARY OF THF INVFNTION by COUntin8 these S'8nals- dr°Plet Propagation data isOBJECTS AND SUMMARY OF THE INVENTION
 obtained, and by varying the spacing, droplet distribu-
An object of the invention is to provide a droplet 55 tion data can be obtained.
monitoring system for analysis of droplet propagation Tests have shown that probe sensitivity was only
in mixed-phase fluid streams. slightly dependent on the conductivity of the droplet
It is a further object of the invention is to provide a fluid. Even fluids with poor electrical properties have
droplet sensing probe for use in monitoring mixed- produced satisfactory results, such as monoisopropyl-
phase fluid streams. 60 biphenol (a heat transfer fluid used to drive turbo-alter-
These and other objects of the invention are nators), by simply increasing the DC potential applied
achieved by a droplet sensing probe comprising two to the probe. Voltages from 1.5 to 45 VDC have been
wires spaced a known distance apart in a mixed-phase used. Tests have also shown that the diameter of the
fluid stream to determine the size and number of probe wires be no larger than the wire spacing. Good
droplets being propagated. The probe wires are con- results are obtained if the spacing between the wires is
nected at one end to means for establishing a DC approximately equal to the wire diameter, but the best
potential between them. A drop of a minimum size operating point is somewhat dependent on both vapor
3,712,132
3 4
and droplet velocity at the time of impact. Large wire 29 by the transformer 28. Conventional amplifier
diameters relative to the spacing cause complete design techniques may be employed to further increase
bridging of the fluid accumulated on the wires, even sensitivity and the signal to noise ratio of the monitor-
with high velocity streams. Good results have been ob- ing system.
tained using a large spacing (350 microns) and a cor- 5 The amplified signal produced by droplet impacting
respondingly large wire diameter, and equally good the wires of the probe 25 are detected by a detector 30
results have been obtained using wires of small diame- which preferably includes in its output stage a pulse
ter (75 microns) spaced 200 microns apart. For accu- shaping circuit to provide pulses of predetermined am-
racy, calibration of the probe may be accomplished by plitudes to a counter 31. For example, the detector 30
using high speed photography with droplets of known may comprise a conventional differential amplifier fol-
size and number. lowed by a Schmitt trigger circuit which provides a
The unsupported lengths of the wires 10 and 11 may, square wave. When a pulse produced by an impacting
in some applications, result in excessive vibration, such droplet on the probe exceeds a predetermined
as under conditions existing in a steam turbine. To
 ]5 threshold level set by a reference voltage applied to the
eliminate vibration, a modified probe construction is Schmitt trigger circuit, the leading edge of the square
preferred in the form illustrated in FIG. 2, wherein like wave thus produced may then be differentiated to
elements are identified by the same reference numerals trigger a relaxation oscillator which will produce a
as in FIG. 1. The modification consists of a supporting pulse of standard height and width for reliable counting
ring 17 secured by the ball 14 of synthetic plastic or 20 by the counter 31. Thus every droplet signal is counted
resin. The free ends of the wires 10 and 11 are secured unless a second droplet produces a signal prior to decay
to the ring 17 by a glob 18 of synthetic plastic or resin. of the signal produced by a preceding droplet. Calibra-
To avoid collection of fluid in the liquid state where the tion performed in the manner suggested hereinbefore
wires 10 and 11 enter the glob 18, the wires are shows that this may occur about ten percent of the
separated a distance significantly greater than the spac- 25 time. Accordingly, the recorded droplet count should
ing desired at the center of the ring. normally be increased by 10 percent. Droplet counts of
It should be noted that although the ring 17 is shown 600 per sec. have been obtained with this monitoring
open in the ball 14 to facilitate centering the wires 10 system using for the detector the pulse output terminal
and 11 in the plane of the ring, that is not essential. In of a Hewlett-Packard Frequency Meter Model 550B
practice, the wires 10 and 11 may be supported by the 30 having a frequence sensitivity from 3 Hz to 100 KHz
ring in a plane totally outside of the ring. and an amplitude sensitivity from 0.2 to 150 volts RMS.
The manner in which a droplet sensing probe of It should be noted that the approach angle of the
either embodiment may be used is illustrated in FIG. 3 droplets will change the sensitivity of the probe to
which shows schematically a two-stage steam turbine. droplet size since smaller droplets can bridge the space
A first probe 20 is inserted through a wall 21 of the tur- 35 between the probe wires if the approach angle is other
bine into the path of steam flow through the first stage than 90°. Accordingly, if a probe cannot be mounted
nozzle to obtain droplet propagation data flowing into with the plane of the probe normal to the stream, an
the first stage turbine wheel. Since significant conden- obliquity factor mus t .be used .to determine the
sation of steam will result from flow through the first
 4_ minimum size of droplets being detected,
stage; a second probe 22 inserted through the turbine In practice, some difficulty was experienced with
wall 21 into the second stage nozzle may be provided probe wire deterioration while using higher probe
with greater spacing between its wires. By varying the potentials apparently caused by excessive current flow
wire spacing in each of the probes 20 and 22, droplet while the probe wires were completely shorted with
size distribution in the first and second stages of the tur-
 45 fluid. This was corrected by holding the switch 27 open
bine may be obtained. except when conditions were acceptable for counting.
A preferred system for detecting and recording In addition, various materials have been used to in-
droplets will now be described with reference to FIG. 4. crease probe life and improve performance, such as
The two wires of a droplet sensing probe 25 are con- stainless steel and platinum.
nected to a source of DC potential illustrated as a bat- 50 Use of this droplet monitoring system has provided
tery 26 through switch 27 and the primary winding of a some imperical verification of analytical studies of
transformer 28. While the switch is closed, a droplet mixed-phase fluid flows. Other uses and advantages will
contacting both wires of the probe 25 will cause the occur to those skilled in the art in the imperical in-
current to flow through the primary of the transformer, vestigation of a wide spectrum of fluid dynamics
thereby inducing a signal in the second winding which 55 problems, particularly for equipment development of
is connected to an amplifier 29. Amplification of the high energy level power systems,
signal improves sensitivity, but also increases outside Although particular embodiments of the invention
signal interference. However, standard shielding tech- have been described and illustrated herein, it is recog-
niques will isolate and eliminate this outside signal in- nized that modifications and variations may readily
terference. 60 occur to those skilled in the art and consequently, it is
It should be noted that use of the transformer for intended that the claims be interpreted to cover such
coupling the probe 25 to the amplifier 29 is not essen- modifications and equivalents,
tial, but is desirable because the probe 25 will tend to What is claimed is:
produce an independent EMF which, in many cases, is 1. A droplet sensing probe comprising a sheath, and
greater than the voltage signal produced by a droplet two wires passing through said sheath, each electrically
contacting both wires of the probe 25. This EMF is es- isolated from the other and from said sheath by dielec-
sentially a DC voltage that is isolated from the amplifier trie material, said wires extending from one end of said
3,712,132
sheath for the purpose of making electrical connections
thereto, and from an opposite end in a substantially
parallel arrangement over at least a portion of each for
the purpose of sensing a droplet impacting said two
wires simultaneously as a momentary short circuit 5
between said two wires produced by said droplet, said
wires being spaced a predetermined distance apart
throughout said portion of each and at greater
distances over a portion of each next to said sheath and
dielectric material. ' 10
2. A droplet sensing probe as defined in claim 1 in-
cluding non-conducting means extending from said
sheath at said opposite end for supporting said two
wires at ends thereof remote from said sheath.
3. A droplet sensing probe as defined in claim 2 15
wherein said wires are spaced at greater distances than
said predetermined distance over a portion of each
next to said non-conducting supporting means.
4. A droplet monitoring system using a probe com-
prised of two wires for analysis of droplet propagation 20
in a mixed-phase fluid stream of high velocity compris-
ing:
a sheath for supporting said two wires in said stream,
each wire substantially in a plane normal to the
direction of flow of said fluid stream and electri- 25
cally insulated by dielectric material filling said
sheath except at one end thereof in said stream,
said two wires being parallel to each other over a
substantial portion of their uninsulated ends, and
said parallel portions being spaced a predeter- 30
mined distance apart;
a frame rigidly connected to said sheath at the end
thereof from which said uninsulated ends of said
wires protrude, said frame supporting said uninsu-
lated ends of said wires at points remote from said
sheath;
means for energizing one of said two wires at a
potential distinct from the potential of the other
wire;
means for detecting a change in potential between
said two wires produced by a droplet passing
between said uninsulated ends of said wires when
said droplet contacts both wires simultaneously;
and
means responsive to said detecting means for record-
ing each change in potential detected.
5. A droplet monitoring system as defined in claim 4
wherein said uninsulated ends of said two wires are
spaced apart by said supporting frame a distance
greater than said predetermined distance on each side
of said parallel portions thereof.
6. A droplet monitoring system as defined in claim 5
wherein said recording means comprises a counter.
7. A droplet monitoring system as defined in claim 6
wherein said energizing means comprises:
a voltage source having first and second terminals
with a predetermined potential therebetween, and
having said first terminal connected to one wire;
and
a transformer having a primary winding connected
between said second terminal and the other of said
two wires while said system is in use.
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Droplet monitoring probe

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