A vapor generator was developed for use in the NASA Langley Transonic Dynamics Tunnel (TDT). Propylene glycol was used as the vapor material. The vapor generator system was evaluated in a laboratory setting and then used in the TDT as part of a laser light sheet flow visualization system. The vapor generator provided satisfactory seeding of the air flow with visible condensate particles, smoke, for tests ranging from low subsonic through transonic speeds for tunnel total pressures from atmospheric pressure down to less than 0.1 atmospheric pressure

Bruce, Robert A.

Hess, Robert W.

Rivera, Jose A., Jr.

English

NASA Technical Reports Server

NASA Technical Memorandum 101670
A Vapor Generator for Transonic
Flow Visualization
Robert A. Bruce, Robert W. lless, and Jos_ A. Rivera, Jr.
October 1989
N/ A
National Aeronautics and
Space Administration
I..,_gls y I_seerch Center
Hampton, Virginia 23665
(NASA-TM-I01670) A VAPOR GE-_NER_Tr_R F3R
TRANSONIC FLOW VISUALIZATION (NASA) 19 p
CSCL IkB
G3109
N90-13403
unclas
0251_84
https://ntrs.nasa.gov/search.jsp?R=19900004087 2020-03-19T23:43:47+00:00Z

A VAPOR GENERATOR FOR TRANSONIC FLOW VISUALIZATION
Robert A. Bruce, Robert W. ltess, and Jos6 A. Rivera
-i
SUMMARY
A vapor generator system has been developed for use in the NASA Langley
Transonic Dynamics Tunnel (TDT). Propylene glycol is used as the vapor material.
The vapor generator system was evaluated in a laboratory setting and then used in
the TDT as part of a laser light sheet flow visualization system. The vapor generator
provided satisfactory seeding of the air flow with visible condensate particles,
"smoke", for tests ranging from low subsonic through transonic speeds and for
tunnel total pressures from atmospheric pressure down to less than 0.1 amaospheric
pressure.
INTRODUCTION
A vapor generator has been developed for use in the NASA Langley
Transonic Dynamics Tunnel (TDT). The generator is designed to generate particles
by condensation of a vapor injected into the airflow upstream of a wind tunnel model
to aid in visualizing the flow field characteristics. The TDT is a continuous flow,
single return wind tunnel with a 16-ft. square test section (reference 1). The tunnel
is equipped to use either air or R-12 (dichlorodifluoromethane) as the test medium at
tunnel total pressures which vary from near vacuum to atmospheric pressure.
The requirement for the number of particles generated per unit time for flow
visualization increases with air speed. The range in test airspeed for the TDT is from
near zero velocity to Mach 1.2. There are four basic requirements for the vapor
generator. First, a sufficient number of particles must be present in the flow to be
visible at transonic speeds. Second, a vapor material must be used which is non-
toxic and non-carcinogenic. Third, the vapor material must not contaminate or
damage the test facility. Finally, the size of the particles must be controlled. This is
especially important for laser velocimeter, LV, measurements where the particle size
should be of the order of one micron. This last requirement is often difficult to
achieve.
In this paper the vapor generator system is described and the principal of
operation is explained. This is followed by an example of an application of the
system in the TDT.
VAPOR GENERATOR SYSTEM
The vapor generator system, as shown in the schematic of figure 1, is
composed of a pressure supply, a pressure regulator and several metering and
shutoff valves, the reservoir tank containing the material to be vaporized, tubing, the
vaporizer, and a heated hose to transfer the vaporized material to the tunnel insertion
point. The schematic also shows a tube supplying mixing jet gas which was used in
the laboratory setup but not in the tunnel test. Propylene glycol (PG) was selected
and is used as the vapor material because it is non-toxic and non-carcinogenic and
does not contaminate or damage the test facility. Several of the properties and
characteristics of propylene glycol are listed in Table I,
Reservoir Tank System
The pressurized reservoir tank which contains the liquid propylene glycol is
shown in figure 2 along with the pressure regulator, a fluid flow measurement
device, and several valves used to control flow rate and facilitate refilling. The tank
has a five gallon capacity.
Vaporizer Assembly
The vaporizer is shown in the photograph of figure 3 prior to wiring of the ....
heating elements and connection of the copper tubing. The casing or housing is an
aluminum cylinder with end plates. The housing is 8 inches long by 5.75 inches in
diameter. There are twelve cartridge heating elements, six inserted from each end of
the casing as shown in figure 4. The heating elements are within straight tubes
which traverse the housing from end to end. Copper tubing wound around the
straight tubes forms a cylindrical coil around each of the six pairs of heating
elements. One of the final steps in the assembly process was to fill the empty space
in the housing with cast zinc which functions as a thermal mass. To reduce heat loss
during operation the vaporizer is covered with an insulating blanket as shown in
figure 5.
Heating Elements and Temperature Control.- One of the heating elements is
shown in figure 3 outside of the vaporizer case. The 525 watt heating elements are
0.625 inch diameter by 3.75 inch long. Power to the heating elements is provided
through a temperature control system shown in the schematic of figure 6. The
temperature controller located in the control room allows the operator to adjust the
vaporizer system temperature as needed. Normal operation has been to keep the
vaporizer temperature at approximately 425 °F. The vaporizer temperature is sensed
by an iron constantan thermocouple located inside the vaporizer case adjacent to the
cast zinc thermal mass. The power relay is located in the tunnel plenum chamber
adjacent to the vaporizer. For the setup used, as defined by figure 6, the relay
2
supplied 3 phase, 440 Volt power to the vaporizer heating elements as called for by
the:c0n-/xo-ller.'i_he wiring configuration for the twelve heating elements in the
•_'_porizer, usmg the 3 phase, 440 Volt source, is shown as the top schematic in
figure 7. The remaining four schematics in figure 7 show how the same twelve
heating elements would be wired if the source voltage were different, as listed to the
fight of each schematic. On the schematics the heating elements are represented by
the open circles. The wiring connectors for six of the heating elements are at one
end of the vaporizer case and the wiring connectors for the remaining six heating
e!emenLs are at the opposite end of the case.
Copper Tubing Connections.- The 0_25 inch diameter copper tubing inside
the case is coiled in cylindrical form around each of the six heating element pairs for
efficient heating of the vapor material flowing through the tubing. The copper tubing
for each of the six cylindrical coils enters the case at one end, forms the cylindrical
coil around the straight tube containing a pair of heating elements, and then exits at
the oppositeend of the casing. In figure 2 the copper tubing external to the case is
shown prior to connections being made between coils. The six coils of copper
tubing could be Connected in various ways. For the unit described herein the tubing
was connected so that the liquid propylene glycol flows through a single coil around
a pair of heating elements on its first pass through the case. The tubing then is
connected to a "T" fitting allowing the vapor to expand and flow through a pair of
coils on the second pass through the case. The tubing connections then allow the
vapor to expand further and pass through three coils on the third and last pass
through the case. These three tubes are then interconnected to a 1/2 inch diameter
exit tube.
Transfer Hose
A 15 ft long, 1/2 inch diametcr heated hose, that is available commercially, is
used to transfer the vaporized material from the vaporizer to near the tunnel insertion
point. The heated hose has an internal temperature controller to maintain a fixed
temperature of 425 °F. For most tunnel model installations a short length (less than
12 inches) of unheated and uninsulated tubing is also needed to bring the vapor from
the end of the heated hose to the desired insertion point for the model being tested.
PRINCIPAL OF OPERATION
The formation of a condensed vapor "smoke" can be explained with the aid of
figure 8 which presents the equilibrium vapor pressure curve for propylene glycol as
a function of temperature and the partial pi'essure of the propylene glycol vapor. The
information in figure 8 assumes the tunnel gas "test medium" is air at one
atmosphere pressure (2116 psf) and a temperature of 100 °F. The partial pressure of
propylene glycol is based on the molar fraction of propylene glycol in the propylene
3
glycol and air mixture being considered. If pure propylene glycol vapor is being
injected into the tunnel, then its partial pressure is one atmosphere or 2116 psf. If
the vapor being introduced into the tunnel is a mixture of propylene glycol where the
molar fraction represented by propylene glycol was one-half, then the partial
pressure of the propylene glycol vapor would be 1058 psf.
At temperature and partial pressure conditions above the equilibrium vapor
pressure curve of figure 8 the propylene glycol mixture will be a vapor. Also shown
on figure 8 is the supersaturation curve which, for any given temperature, is at a
partial pressure three times greater than that of the equilibrium vapor pressure curve
(Saturation ratio, S=3). In the region between the equilibrium vapor pressure curve
and the supersaturation curve the vapor can condense. However, there is
insufficient energy to create a new interface surface so condensation will Occur only
on pre-existing particles such as dust or aerosol droplets. At temperature and partial
pressure conditions below the supersaturation curve (S>3), spontaneous nucleation
can occur allowing the vapor to condensate to small droplets or particles which are
visible as "smoke". The largest number of particles are formed, and therefore the
optical density of the "smoke" is greatestwhen the injected vapor is mixed rapidly :_
with the cooler tunnel gas to quickly reach the region of spontaneous nucleation.
Point A on figure 8 representspure propylene glycol vapor (partial pressure =
2116 psfa) at the vaporizer temperature of 425 °F. Note that the vaporizer
temperature is approximately 55 °F hotter than the condensation temperature defined
by the equilibrium vapor pressure curve. This allows for someheat loss in the
unheated portion of tubing delivering the vapor to the desired discharge point in the
tunnel. Point B representsconditions at the point of vapor injection into the tunnel
stream based on the assumption that the vapor has cooled to 375 °F by heat losses
through the tube walls. At the point of injection into the tunnel the propylene glycol
vapor mixes with the cooler tunnel gas (air) with two effects. It dilutes the vapor
(reduces the partial pressure of the propylene glycol), and it reduces its temperature.
The curve, which originates at point B and passesthrough points C and D,
representsthe cooling and partial pressurereduction experienced by the initially pure
propylene glycol vapor as it mixes with the cooler tunnel gas. Spontaneous
nucleation or "smoke" generation will occur only after the vapor hasbeen diluted and
cooled past point D on the curve.
When only pure liquid propylene glycol is admitted to the vaporizer, the vapor
pressure of the boiling liquid helps propel the vapor to the discharge point. This
operation tends to be unstable since the vapor flow rate varies directly with the
vaporizer pressure, and the boiling rate tends to drop with increased pressure. This
results in the vapor being discharged in a nonuniform manner as a series of puffs.
The introduction of a small quantity of the samegasas the test medium (air or R-12)
eliminates this puffing by providing a more continuous flow through the vaporizer.
4
The quantity of this dilution gas flow is not critical and it is usually set near the
minimum flow rate which eliminates the puffing. Point A' on figure 8 represents the
casewhere diluent gas-(air) is mixed With the propylene glycol before entering the
_aporizer such that equal molar quantities of propylene glycol and air are at the
vaporizer temperature of 425 °F (the partial pressure of the propylene glycol is i 058
psfa). Condensation of the vapor on the walls of the injection tubing could occur if
the vapor mixture cooled to less than 330 °F before being injected into the tunnel.
Point B, assumes that the propylene glycol and air mixture has cooled only to about
340 °F before !njection into the tunnel airstream. Mixing with the tunnel gas
produces the curve of temperature with partial pressure which originates at point B'
and passes through points C' and D'. As stated previously, spontaneous nucleation
or "smoke" generation will occur only after the vapor has been diluted and cooled
past point D' on the curve.
Tests were conducted in a laboratory setting where propylene glycol vapor
was discharged into still air, with and without an additional flow (jet) of air
impinging on the vapor flow. The tests were performed to determine the effect of
rapid cooling and mixing on resultant particle sizes and numbers. Typical particle
size distributions with and without added cooling air flow from an air jet are
presented in figure 9. When the vapor is discharged directly into still air with no
added cooling air fl0w, the particle size distribution is centered on a diameter of
about 4.7 micrometers with about 450 panicles in that size category. When a jet of
cooling air is impinged on the vapor discharge, the size distribution is shifted to a
larger number of smaller particles. The distribution with mixing is centered on a
diameter of about 1.0 micrometers with about 850 particles in that size category. For
tunnel operations where the vapor is being introduced directly into the flow,
sufficiently rapid mixing and cooling of the vapor may result, dependent on tunnel
airspeed and on the location and configuration of the vapor discharge nozzle, to
make the introduction of additional cooling gas unnecessary.
The range of temperatures and flow rates that were determined to be optimum in
the still atmosphere laboratory environment are listed in Table II. Only the range of
optimum temperature is relatively small. The fluid flow rate may be increased as
necessary and the dilution gas adjusted accordingly. The flow rates listed in the
table were determined using calibrated flow meters in the lab setup. The use of
calibrated flow meters is not necessary for a permanent installation. Locations of
metering valves which may be calibrated at initial installation are shown in figure 1.
APPLICATION
The vapor generator system was utilized as part of a laser light sheet flow
visualization system to study the leading edge vortex of a semi-span clipped delta
wing model in the TDT. Figure 10 is a photograph taken during the wind tunnel
5
test. The wing leading edge sweep was 50.5 degrees and the wing semi-span
measured 27.85 inches. A single discharge nozzle for the propylene glycol vapor
was located at the wing root near the leading edge flow stagnation point. This
resulted in condensed propylene glycol vapor being entrained in the vortex along the
leading edge of the model where it could be illuminated by the laser light sheet. The
model is at 15 degrees angle of attack, the Mach number is 0.92, and the dynamic
pressure is 51.3 psf. The light sheet is oriented streamwise and located at the 50%
span station. The light sheet illuminates the leading edge vortex and the vortex burst
location (the point at which the illuminated area increases). The vapor generator
system was successfully used for this test series with both air and R-12 test
mediums and at tunnel total pressures from atmospheric pressure down to 1/12
atmosphere.
CONCLUDING REMARKS
A vapor generator for transonic flow visualization has been developed and
utilized in the Transonic Dynamics Tunnel. The propylene glycol vapor used is non-
toxic and non-carcinogenic, and the vapor does not contaminate or damage the test
facility. The vapor generator provides sufficient particles for laser vapor screen
transonic flow visualization. The vapor generator was successfully used in air and
R-12 from atmospheric pressure down to 1/12 atmosphere.
REFERENCES
_ .._ ,L_ __- ....... - .........
1. Reed, Wilmer H., III: Aeroelasticity Matters: Some Reflections on Two Decades
of Testing in the NASA I,angley Transonic Dynamics Tunnel. NASA TM-83210,
1981.
6
Table I.- Propylene Glycol Properties and Characteristics.
Item Value
Molecular Weight
Apparent Specific Gravity at 20/20 ° C
Boiling Point at 760 mm. ttg.
Vapor Pressure at 20 ° C
Freezing Point
Viscosity at 20 ° C
Heat of Vaporization at 1 atm.
Flash Point
Solubility in Water
76.1
1.0381
(187.3 ° C) 369 ° F
< 0.1 mm. Hg
-60 ° C
60.5 cp
296 BTU/Ib
214 ° F
Complete
Table II.- Optimum System Conditions From Laboratory Tests.
System Parameter Allowable Range
Vapor Generator Case Temperature
Heated Hose Temperature
Reservoir Tank Pressure
Propylene Glycol Flow Rate
Dilution Gas Flow Rate
Mixing Gas Flow Rate
420°F to 440°F
425°F (fixed)
35 psi
up to 2.5 gph
0.1 to 1.0 cfm
0.75 to 3.0 cfm
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Figure 2. - Reservoir tank system.
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Figure 7. - Wiring configuralions for different voltage sources.
14
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Figure o. - Effect of cooling air flow on particle size.
16
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1. Report No,
Report DocL_mentation Page
2. Government Accession No.
NASA TM-101670
4. Title and Subtitle
A VAPOR GENERATOR FOR TRANSONIC
FLOW VISUALIZATION
7, Author(s}
Robert A. Bruce, Robert W. Hess,
Jos6 A. Rivera, Jr.
9. Performing Organization Name and Address
NASA Langley Research Center
Hampton, VA 23665-5225
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, DC 20546-0001
15. Supplementary Notes
3. Recipient's Catalog No.
5. Report Date
Octobec 1989
6. Performing Organization Code
8. Performing Organization Report No.
10. Work Unit No.
505-63-21-09
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14, Sponsoring ,Agency Code
16. Abstract
A vapor generator system has been developed for use in the NASA
Langley Transonic Dynamics Tunnel (TDT). Propylene glycol is used as the
vapor material. The vapor generator system was evaluated in a laboratory
setting and then used in the TDT as part of a laser light sheet flow
visualization system. The vapor generator provided satisfactory seeding of
the air flow with visible condensate particles, "smoke", for tests ranging
from low subsonic through transonic speeds for tunnel total pressures
from atmospheric pressure down to less than 0.1 atmospheric pressure.
17. Key Words (Suggested by Authorls))
Flow Visualization
Transonic Flow Field
Smoke Generator
___Vapor Generator
19 Security Classif. (of this report)
Uncl as si fi ed
NASA FORM 1626 OCT 86
18. Distribution Statement
Unclassified - Unlimited
Subject Category 09
----1 20. Security Classif. (of this page)
t Unclassified
21. No. of pages
18
22. Price
A03
. I
,i,, --
: ll


A vapor generator for transonic flow visualization

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