A principle formerly used in an instrument for cloud detection was further investigated to provide a simple and rapid means for measuring the liquid-water content of clouds at temperatures above and below freezing. The instrument consists of a small cylindrical element so operated at high surface temperatures that the impingement of cloud droplets creates a significant drop in the surface temperature. ? The instrument is sensitive to a wide range of liquid-water content and was calibrated at one set of fixed conditions against rotating multicylinder measurements. The limited conditions of the calibration Included an air temperature of 20 F, an air velocity of 175 miles per hour, and a surface temperature in clear air of 475 F. The results obtained from experiments conducted with the instrument indicate that the principle can be used for measurements in clouds at temperatures above and below freezing. Calibrations for ranges of airspeed, air temperature, and air density will be necessary to adapt the Instrument for general flight use

Perkins, Porter J.

English

NASA Technical Reports Server

/V	 7't 
RM E50fl2a 
NACA 
RESEARCH MEMORANDUM 
FLIGHT INSTRUMENT FOR MEASUREMENT OF LIQUID-WATER CONTENT

IN CLOUDS AT TEMPERATURES ABOVE AND BELOW FREEZING

By Porter I. Perkins 
Lewis Flight Propulsion Laboratory 
Cleveland, Ohio 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHINGTON	
ory CornL 
March 5, 1951	 Aeronautc
https://ntrs.nasa.gov/search.jsp?R=19810068728 2020-03-21T11:49:02+00:00Z
NACA RM E5OJ12a 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RHEARCH MEMORANDUM 
FLIGHT INSTRUMENT FOR MEASUERMENT OF LIQUID -WATER CONTENT 
IN CLOULS AT TEMPERATURES ABOVE AND BELOW FREEZING 
By Porter J., Perkins 
SUMMARY 
A principle formerly used. in an instrument for cloud detection was 
further investigated to provide a simple and. rapid. means for measuring 
the liquid-water content of clouds at temperatures above and below 
freezing. The Instrument consists of a small cylindrical element so 
operated at high surface temperatures that the impingement of cloud 
droplets creates a significant drop in the surface temperature. 
• The instr.unent is sensitive to a wide range of liquid-water con-
tent and was calibrated at one set of fixed conditions against rotating 
multicylinder measurements. The limited conditions of the calibration 
Included an air temperature of 2cP F, an air velocity of 175 miles per 
hour, and a surface temperature in clear air of 4750 F. The results 
obtained from experiments conducted with the instrument Indicate that 
the principle can be used for measurements in clouds at tempera-
tures above and below freezing. Calibrations for ranges of airspeed, 
air temperature, and:air density will be necessary to adapt the 
Instrument f or general flight use. 
Determination of the amount of liquid water contained in an atmos-
pheric cloud. is of considerable value in meteorology, particularly for' 
investigating the necessary requirements for adequate protection of 
aircral't against the icing hazard. In connection with icing meteorology, 
much effort has been directed toward developing a suitable instrument 
for measuring the liquId-water concentration in clouds where the tem-
perature is below the normal freezing point of 32° F. Because water 
below its normal freezing point will, upon striking a moving surface', 
usually freeze and a&here'to that surface, a suitable ice-collecting 
body becomes a convenient means for measuring the amount of water in a 
below-freezing cloud. For many years, this principle has been used in 
an instrument utilizing the ice-collection characteristics of'various 
size cylinders.. The rotating multicylinder technique, as thie method 
is called, is based on a theoretical treatment of the trajectories of 
cloud droplets about various size cylinders with axes normal to the air 
stream (reference 1).
NACA RM E5OJ12a 
Although a value of the liquid-water concentration of clouds can 
be obtained, with a certain 'degree of accuracy by the niulticylinder 
method, the difficulty of manipulating the multicylind.er device and 
the inability to measure the liquid-water concentration in above-
freezing clouds have emphasized the necessity of developing a more 
suitable instrument. Another method formerly used for measuring 
liquid-water concentration in above-freezing clouds employs a. 
capillary-type water-collecting device where the collecting body is 
of porous material and the water is pulled. in from the surface by 
capillary action (reference 2). These methods as well as any other 
water- or ice-collecting techniques involve bulky eq,uipiient and cannot 
be easily adapted to general flight application, particularly with 
regard. to accurate measurement of the amount of water or ice collected. 
An electric or thermal method for indicating the amount of water 
collected on a surface off ers a medium. for using sturdy and reliable 
instruments that can be readily adapted. for flight use. A measure of 
the amount of electric energy necessary to evaporate water as it is 
collected on a surface can be utilized in an instrument to determine 
the liquid-water content of an icing cloud. A more direct and easier 
method,however, is to detect the ambunt of water striking a. heated. 
surface by measuring the degree of cooling caused by the water as it 
evaporates from the surface. The amount or rate of water 'collected can 
then be measured. in . terins of the , temperature change of the collecting 
surface. This principle was originally used as a cloud detector to 
indicate entrance and. exit from a cloud. during flight (reference 3). 
The instrument was not used for the measurement of quantitative values 
of liquid water because the surface temperatures during operation were 
insufficient.	 . 
An Instrument for measuring the liquid-water concentration of 
clouds utilizing the principle of the cooling effect of water striking 
a heated surface is described herein. The instrument, designed for 
flight measurements in above-, or below-freezing clouds, was. calibrated 
at a given set of air conditions for values of liquid-water concentra-
tion by the rotating multicylinder technique in the NACA Lewis icing 
research tunnel. 
Design considerations. - In order to determine the quantity of 
liquid. water in a cloud from the amount collected on a moving body, 
the collection characteristics of the body must be known. Because the 
droplet trajectories around acylind.er with axis noal to. the air
NACA RM E5cJl2a	 -	 3 
stream have been analyzed, a suitable size cylinder was chosen as the 
collecting body for this instrument. Although a sm.all cylinder will 
collect more water in proportion to its size than a larger cylinder 
(collection efficiency), which makes a small cylinder desirable for 
this instrument, a very small cylinder was found by experiment to have 
less cooling effect from water impingement than a large cylinder. A 
5/16-inch-d.iameter cylinder, which would produce enough surface cooling 
for a high degree of sensitivity to water content, was therefore chosen. 
The mass Of the material was made small in order to minimize the 
cooling-time lag at the point on the surface of the cylinder where the 
temperature change was measured. A hollow tube with a wall thiciQaess of 
0.010 inch and. length of about 1 inch Droved. satisfactory for the water-
sensitive part of the instrument. A heating method that would satis-
factorily combine the requirements of small mass and. high surface temper-
atures necessary for the water-sensitive element was found. to be that of 
generating heat by passing electric currents directly through the element. 
In order to minimize high currents necessary for sufficient heatingi, the 
element was constructed. of a high electric-resistance material. The heat-
ing current was obtained through use of a short-circuit-type transformer 
connected as close to the element as possible and designed for 110-volt, 
400-cycle power input. 
The temperature of. the surface of the element was measured by an 
iron-constantan thermocouple embedded in the element at the aerodynamic 
stagnation point. The element temperature was satisfactorily measured 
by several methods including a self-balancing-type recording potentiom-
eter, a film-recording galvanometer, and thermocouple temperature gages. 
Construction. - A photograph of the Instrument Is shown In figure 1. 
and figure 2 illustrates the pertinent details of construction for sup-
port in the free air Y
 stream. The water-sensitive element was welded to 
the end of a thick-wall brass tube about 9 inches long, which was in 
turn fastened to a base mounting plate. A coaxial-tube electric heater, 
surrounding and electrically insulated from the support tube, was used. 
for protection from ice accretions. The transformer was mounted on the 
inside surface of the base plate, and electric connections were made to 
the element through a 1/8-inch-diameter copper wire inside the support 
tube and a ground connection to the base plate. The thermocouple leads 
were also extended through the support tube. 
Power req.ulrements. - The power required for the water-sensitive 
element is small for the conditions Investigated, being about 250-
volt-amperes input to the transformer. The ice-protection heater for 
the support tube requires a high current of about 40 amperes at 
3.3 volts for adequate protection In Icing conditions as severe as
4	 .	 .NACA PM E5Jl2a 
air temperature of 0° F, air velocity of 200 i1es per hour, and. liqui.d.-
water content of 1 gram per cubic meter. 
LIQUID -WATER -00NTEIT CALIBRATION 
The amount of surface cooling caused by liquid water striking the 
heated element varied greatly with. the quantity of liquid water in the 
air stream. The variation of element surface temperature with increas-
ing water content of the air stream is shown In figure 3. The plot 
indicates a large linear drop i surface temperature with.increasing 
water content down to about 150 F. At about this temperature, the 
instrument becomes lesesensitive to increasing water content and. at a 
temperature of about .100° F becomes almost Insensitive to any large 
quantities of liquid water. Observation of the sensitive element 
showed that the element provided su.fflcient heat to maintain a dry 
surface above 150° F, but that' the surface remained wet below that tem-
perature. In order to obtain a maximum variation of surface tempera-
ture with liquid-water content with a consistent temperature trend, it 
is therefore necessary to maintain a completely dry surface, a condition 
that requires a sufficient surface temperature to evaporate all the 
water on impingement. The Operating liquid-water-content range of the 
instrument Is therefore d.etenuined by the heating capacity of the 
instrument to maintain a dry surface. 
The variation of the drop in surface temperature with the liquid.-
water content of the air stream was calibrated in terms of temperature 
by applying the values of liquid-water content as measured by the con-
ventional multicylinder technique. The caUbration. was conducted at 
below-freezing conditions in thetest section of the icing research. 
tunnel where a simulated. Icing cloud is produced through use of accu-
rately calibrated water sprays introduced upstream Of the model. The 
niulticylinder technique was slightly.ni.odified by , inserting only one 
cylinder at a time to insure the same clond distribution over the 
cylindes 'and the liquid-water-content meter. Separate cylinders were 
individually exposed at the same location followed by exposure of the 
liquid-water-content meter also at the same location. By this means, 
a calibration (fig. 4) was obtained .up to liquid-water-content values 
of 1.1 grams per cubic meter. 
The initial surface temperature of the element in water-free 
saturated air was 4750 F. The maximum. liquid-water content caused a 
surface temperature drop of over 2500 F. The resulting surface tem-
perature of 2250 F was well above the minimum temperature necessary 
for complete evaporation on impingement for the conditions of the
NACA PM E5OJ12a	 5 
investigation. 0. Because the temperature of the element was above the 
minimum of 150 F for all conditions of liquid-water content investi-
gated. (fig. 3), a straight-line calibration was assumed through the 
values established by the inulticylinders. The straight-line relation 
provides a convenient calibration constant of 0.0046 gram per cubic 
meter per °F drop in surface temperature from a water-free condition 
for the conditions investigated. 
Values of liquid-water content of a cloud obtained using the 
Instrument should be considered to have an accuracy only within th6 
adcuracy of the measurements using the multicylinder technique (refer-
enàe 4) because the multicyllnd.er technique was used. as a basis for 
calibration. The greatest advantage of the liquid-water-content meter, 
however, lies in its simplicity of stricture and operation for obtain-
ing results equal to those of the far more complicated inulticylinder 
system. 
A second calibration of the heated element was conducted in the 
icing research tunnel under the same conditions using a rotating-disk-
type icing-rate meter to measure the liquid-water content.. This meter 
had been previously calibrated against the multicylinder technique at 
flight icing condition. The rotatIng disk of the icing-rate meter was 
mounted within reasonably close proximity to the liquid-water-content 
meter. With the assumption that the distribution of liquid water was 
uniform within this area, a direct calibration of the heated element was 
made with the icing-rate meter. These results are also shown in fig-
ure 4. This calibration, of course, also contains the errors associated 
with the multicylinder technique. 
Both calibrations were conducted using the same simulated-cloud 
conditions. The multicylinder technique indicated the droplet sizes in 
the simulated cloud varied with increasing quantity of water Injected 
into the tunnel and ranged between Lanuir C to E size distributions 
with a volume median droplet size (reference 5) close to 10 microns 
diameter.
0PATING CEABACTERISTICS 
The surface temperature of the heated element is not only a function 
of the amount of liquid water striking the surface but also depends on 
air temperature, velocity, and density and on the power input or 
surface temperature in clear air. These variables must be kndwn while 
measuring liquid-water content with the instrument. A complete cali-
bration of the instrument requires the determination of the effect of 
each of these variables on surface temperature.
6
	
NACA PJVI E50J12a 
Several characteristics of the instrument were noted during the 
investigation conducted. in the icing tunnel. In order to check the 
effects of above- and below-freezing temperatures on the liquid-water-
content indications, a known value of water content at a below-freezing 
condition was held. constant in the tunnel while the air temperature was 
raised above the freezing point. No change in the surface-temperature 
drop was observed between 200 and 40° F, which indicates a below-
freezing calibration can be used within this' range of temperatures. 
Large changes In 'air temperature need further Investigation because of 
the vapor-pressure effect on the heat transfer from the surface. 
The instrument was exposed to a cloud composed only of Ice 'crystals 
to determine the effects of frozen particles on the surface temperature. 
The surface temperature was unaffected by the presence of the crystals, 
which Indicates a surface-temperature drop occurs only with liquid. 
droplets. A further investigation to explain this phenomenon was not 
undertaken. 
Because the' heated element has some thermal capacity, a certain 
amount of time lag exists during changing conditions. When suddenly 
exposed to the full range of liquid-water content, the surface tempera-
ture changes almost Instantly but requires about 20 seconds to stabi-
lize. For flight measurements In broken or scattered clouds where water 
contents are rapidly changing in the flight path, a heated, element of 
considerably lower thermal capacity is needed for obtaining true values 
of the liquid-water content. 
SD11MARY OF RULTS 
A small cylindrical element capable of being heated in clear air 
to temperatures In the order of 475° F ias constructed and Investigated. 
to determine the feasibility of measuring the lIquId-water content of 
an air mass by observing the temperature , drop of the sensitive element 
during exposure to the wet air stream. The following results were 
obtained from experiments conducted. with the instrument: 
1. The instrument was very sensitive to a wide range of liquid-
water content and could be used for measurements In above- and. below-
freezing clouds. 
2. At the conditions investigated, the surface temperature of the 
sensitive element had. to be maintained above 1500 F during the measuring 
period to assure complete evaporation of' the impinging water.
NACA RM E5OJ12a 
3. A calibration of the instrument at an air. temperature of 20° F, 
an air velocity of 175 miles per hour, and. a surface temperature in 
clear air of 475° F was obtained using rotating multicylinders as a 
basis . of measurement. 
Lewis Flight Propulsion Laboratory, 
National Advisory Committee for Aeronautics, 
Cleveland, Ohio. 
1. Anon.: The Multicylintier Method. The Mt.. Washington Monthly 
Res. Bull., vol. II, no. 6, June 1946. 
2.Vonnegut, B., Cunningham, P.. M., and. Katz, P.. E.: Instruments for 
Measuring Atmospheric Factors Related to Ice Formation on Air-
planes. Tech. Rep. No. 5519, Air Materiel Command (Wright-
Patterson Field), Aug. 1946. 
3. Lewis, William, and Hoecker,WalterM., Jr.: Observations of Icing 
Conditions Encountered in Flight During. 1949. NACA TN 1904, 1949. 
4. Jones, Alun R., and Lewis, William: A Review of Instruments 
Developed for the Measurement of the Meteorological Factors 
Conducive to Aircraft Icing. NACA P.M A9C09, 1949. 
5. Lanu1r, Irving, and Blodgett, Katherine B.: A Mathematical 
Investigation of Water Droplet Trajectories. Tech. Rep. No. 
5418, Air Matertel Commpnd, AAF, Feb. 19, 1946. (Contract No. 
W-33-O38-ac-9151 with Gen. Elec. Co.)
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NACA RM E5OJ12a 9 
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NACA RM E5OJ12a	 li 
	
Thermocouple junction	 5/16" d.iam. , _Water-sens1tive element 
Lperconductor	 ____ 
	
s1atance	 l/2" diam. 
	
• Thi-wall	 /_—Coaxial support heater 
coaxial 
tubing	
,,.rWel&
8" 
/	 ...• 
/ 1/
,_.Base mounting plate 
1nsut1on	
rmsseuppO h	 support 
- ______ - -	
output 
4: 
•ll0-150volt , •	 - •
	 : 
400-cycle input 
Figure 2. - Construction details of NACA liquid-iater-content meter.
fx4 
0
a 
a, 
.4-)
NACA PM E5OJ12a 
5	 10	 .	 15	 20	 25 
Liquid water injected into air stream (arbitraiy units) 
Figure 3. - Effect of lIquid water in air stream on element 
surface temperature. NACA liquid-water-content meter; air 
temperature, -2 F; air velocity, 185 miles per hour; air 
density, 0.072 pound per cubic foot.
300 
_____ ____
Method 
O Multicyliriders
	
- 
0 Icing-rate meter
____ ____ ____ _____ ____ ____ 
H________//__ 
_ _ _ /0 
_ _
0/
_ _ 
_ _ _ 
/_____
. ____
25( 
r4 
0
' 20( 
a, 
+'
15( 
a> §1 bc 
4;) 
a, 
C, 
aS 
Co
5C
.0	 1.0	 1.2 
Liquid-water content, grains/cu in 
NACA PM E5OJ12a	 13 
Figure 14• - Calibration of NACA liquid-water-content meter using rotating 
multicylinders and icing-rate meter. Air temperature, 20° F; air 
velocity, 175 miles per hour; air density, 0.077 pound per cubic foot; 
clear-air surface temperature of heated element, !1750 F. 
NACA-Langley - 3-5-51 - 325 


Flight Instrument for Measurement of Liquid-Water Content in Clouds at Temperatures Above and Below Freezing

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