[American Institute of Aeronautics and Astronautics 49th AIAA Aerospace Sciences Meeting including...

1

American Institute of Aeronautics and Astronautics

Approved for public release;

distribution is unlimited.

Temperature-Sensitive Paint Calibration Methodology

Developed at AEDC Tunnel 9

Inna Kurits

1 and Joseph D. Norris

2

Arnold Engineering Development Center, Silver Spring, MD 20903-1005

Pratik Bhandari3

Department of Aerospace Engineering

University of Maryland

College Park, MD 20742

Aerodynamic heating is a critical parameter in hypersonic vehicle design. Ground-test

facilities employ a variety of discrete sensors and global techniques to characterize the

heating on the surfaces of a test article. AEDC Hypervelocity Wind Tunnel No. 9 has been

developing a global heat-transfer measurement technique based on temperature-sensitive

paints (TSP). In order to use TSP as a heat-transfer measurement tool, a unique relationship

between its emission intensity and surface temperature has to be established. A TSP

calibration laboratory was developed and built at Tunnel 9. The resulting calibration

process has been shown to yield repeatable calibrations insensitive to setup perturbations.

The calibration laboratory has been instrumental in the development and implementation of

the two-color TSP system at Tunnel 9.

Nomenclature

CCD = Charge coupled device

CEV = Crew exploration vehicle

Iblu = Emission intensity contribution from the “blue” luminophore

Iblu,ref = Emission intensity contribution from the “blue” luminophore at wind-off condition

Ired = Emission intensity contribution from the “red” luminophore

Ired,ref = Emission intensity contribution from the “red” luminophore at wind-off condition

LED = Light emitting diode

M∞ = Freestream Mach number

Re/L = Unit Reynolds number

T = Temperature

Tref = Reference temperature

α = Angle of attack

I. Introduction

he design of hypersonic vehicles is impacted heavily by the aerodynamic heating that the vehicle will encounter

during flight. In addition to providing measurements of aerodynamic forces and surface pressure data,

hypersonic wind tunnels must be able to accurately characterize the heat-transfer rate on a wind tunnel model.

Ground-test facilities traditionally rely on discrete instrumentation (e.g., thermocouples or thin-film gauges) for

heat-transfer rate measurements. Discrete sensors are impractical to use when attempting to measure global heating

patterns over a large surface of a test article. Flow phenomena such as boundary-layer transition locations and shock

boundary-layer interactions, which are of interest for hypersonic vehicle design, may be inadequately characterized

1 Project Engineer, Aerospace Testing Alliance, AEDC White Oak, Silver Spring, MD, Member AIAA

2 Project Engineer, Aerospace Testing Alliance, AEDC White Oak, Silver Spring, MD, Senior Member AIAA

3 Graduate Research Assistant, University of Maryland, College Park, MD, Member AIAA

T

49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition4 - 7 January 2011, Orlando, Florida

AIAA 2011-851

Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner.

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by discrete measurements. This is especially true if there is no prior knowledge of where the phenomena of interest

may occur on the model.

Temperature-sensitive paint (TSP) is one of several global heat-transfer measurement techniques that have the

capability to provide qualitative and quantitative heat-transfer rate measurements over the surface of a test article.

An intensity-based TSP system has been developed for use at the AEDC White Oak Hypervelocity Wind Tunnel 9

(Tunnel 9). In order to use TSP to quantify the temperature rise on the surface of a wind tunnel model, it is first

necessary to calibrate the paint, i.e., to obtain the relationship between the TSP emission intensity and its

temperature.

This paper describes the development and use of the Tunnel 9 TSP calibration laboratory. While the laboratory’s

primary purpose is to provide temperature calibrations of temperature-sensitive coatings, AEDC White Oak has also

developed it as a tool to quantify the TSP system performance and aid in development and evaluation of new paint

formulations. Since this capability is readily accessible, significant developments in the TSP system at AEDC White

Oak have occurred independent of wind tunnel demonstrations. Specifically, the development of new two-color TSP

discussed in this paper has benefitted from this capability.

A. Tunnel 9 Facility Description Tunnel 9 is a unique blowdown facility

that uses pure nitrogen as the working fluid

and currently operates at Mach numbers 7,

8, 10, and 14. An operational envelope

showing Reynolds number equivalent

altitudes vs. Mach number for Tunnel 9

operating conditions is presented in Fig. 1.

The unit Reynolds number range for the

facility is from 0.05×106

/ft (0.16×106 /m) to

48×106

/ft (157×106 /m) and is useful for

high-altitude/viscous interaction simulations

at low Re/L to duplication of flight dynamic

pressures at high Re/L. Usable test periods in

Tunnel 9 range from 0.25 sec to over 15 sec,

depending on the test conditions. The test

section is over 12 ft (3.7 m) long and has a

diameter of 5 ft (1.5 m), enabling testing of

large-scale model configurations that can

include simultaneous force and moment,

pressure, and heat-transfer instrumentation. Tunnel 9 has a dynamic pitch capability that allows moving large test

articles over a 50-deg pitch sweep at rates up to 80 deg/sec. This unique capability allows data to be collected over

full pitch polars during a typical run, thus increasing the tunnel’s productivity.

A schematic of the entire facility is

shown in Fig. 2, and a photo of the Tunnel 9

Mach 10 nozzle and test cell is provided in

Fig. 3.

During a typical run, the vertical heater

vessel is used to pressurize and heat a fixed

volume of nitrogen to a predetermined

pressure and temperature. The test cell and

the vacuum sphere are evacuated to

approximately 1 mmHg and are separated

from the heater by a pair of metal

diaphragms. When the desired temperature

and pressure are reached in the heater, the

diaphragms are ruptured. The gas then flows

from the top of the heater vessel, expanding

through the contoured nozzle into the test

section at the desired freestream test conditions. As the hot gas exhausts from the top of the heater, cooler nitrogen

gas from the pressurized driver vessels enters the heater base. This cold gas drives the hot gas out the top of the

Figure 1. Hypervelocity Wind Tunnel 9 operational envelope.

Figure 2. Tunnel 9 facility schematic.

4

50

100

150

200

12 168 24 28

Simulation

Duplication

Mach Number

Altitu

de, kft

Reentry Bodies

Flight Vehicles

ENDO Interceptors

Present Tunnel 9

Operation Conditions

Flight Envelopes

20

Diffusers

Nozzles

Pressure ControlValves

GasCompressor

Gas DriverVessels

Gas Heaters(Vertical)

Optical ViewingSystem

Test Cells

Isolation Valves

Vacuum Sphere




heater in a piston-like fashion, thereby maintaining constant conditions in the nozzle supply plenum and the test

section during the run. A more complete description of the Tunnel 9 facility and its capabilities can be found in

Ref. 1.

Figure 3. AEDC Hypervelocity Wind Tunnel No. 9 Test Cell and

Mach 10 nozzle.

B. Previous Tunnel 9 TSP Tests

The TSP measurement technique has been used to make qualitative as well as quantitative surfac

measurements on a variety of geometries

“Orion” Crew Exploration Vehicle (CEV) heat shield

conducted by applying TSP to a section of a

Mach 10, Re/L = 20×106/ft flow. The paint successfully survived multiple runs at

number condition and provided detailed

originating from the protruding pitot probes

of the global technique to provide significant insight into the nature of a highly complex flow

a) Wedge with a protruding fin

M∞ = 14, Re/L = 0.5×106 /ft (1.7×106 /m)

Ref. 2

Figure 4. Temperature-sensitive paint images acquired during past Tunnel 9 test programs.

The heat-transfer maps shown in Fig. 4

quantitative data have also been acquired at Tunnel 9 during runs where a test article was continuously pitched

through an angle-of-attack sweep. Figur

3


like fashion, thereby maintaining constant conditions in the nozzle supply plenum and the test

complete description of the Tunnel 9 facility and its capabilities can be found in

AEDC Hypervelocity Wind Tunnel No. 9 Test Cell and

Mach 10 nozzle.

technique has been used to make qualitative as well as quantitative surfac

geometries in Tunnel 9, such as a wedge with a protruding fin2 (Fig. 4a)

“Orion” Crew Exploration Vehicle (CEV) heat shield (Fig. 4b).3,4

More recently, a paint survivability study was

to a section of a pitot calibration rake and acquiring a set of global heating

The paint successfully survived multiple runs at this high Mach and Reynolds

detailed flow visualization of the surface heating due to bow

probes (Fig. 4c). The set of images in Fig. 4 clearly demonstrates the capability

to provide significant insight into the nature of a highly complex flow field.

b) NASA Orion CEV

M∞ = 10, Re/L = 5.0×106 /ft (16.4×106 /m)

Refs. 3 and 4

c) Pitot calibration rake

M∞ = 10, Re/L

(65.6×10

sensitive paint images acquired during past Tunnel 9 test programs.

Fig. 4 were acquired during fixed angle-of-attack wind tunnel runs.


Figure 5 shows a comparison between heat-transfer rates calculated from coaxial

like fashion, thereby maintaining constant conditions in the nozzle supply plenum and the test

complete description of the Tunnel 9 facility and its capabilities can be found in

technique has been used to make qualitative as well as quantitative surface heat-transfer

(Fig. 4a) and NASA’s

a paint survivability study was

global heating images in a

this high Mach and Reynolds

bow-shock interactions

clearly demonstrates the capability

field.

c) Pitot calibration rake

Re/L = 20.0×106 /ft

(65.6×106 /m)

sensitive paint images acquired during past Tunnel 9 test programs.

wind tunnel runs. Accurate


transfer rates calculated from coaxial

4




thermocouple data and TSP data at two locations during a M∞ = 10, Re/L = 2.0 x 106/ft run where a test article was

continuously pitched between -5 and 12 deg at a rate of ~ 19 deg/sec.5 Coaxial thermocouple is a standard heat-

transfer measurement sensor at Tunnel 9 with the measurement uncertainty of ±6%.6 The coaxial thermocouple

measurement uncertainty is shown with black error bars in Fig. 5.

Figure 5. Comparison between coaxial thermocouple and TSP heat-transfer data

at two locations for a pitching run at M∞ = 10, Re/L = 2.0×106 /ft (6.6×106 /m).5

C. TSP Operating Principles 1. One-Color TSP System

AEDC White Oak has been developing the technologies required for an intensity-based TSP heat-transfer

measurement system for Tunnel 9 over the last six years. The basic operating principle can be described as follows.

Luminescent molecules, or luminophores, are combined with a polymer binder, and the resulting mixture is applied

to a test article on top of a thin, white-base layer. The luminophores in the coating are excited by illumination of a

particular wavelength. The de-excitation process is characterized by the emission of photons with a wavelength that

is red-shifted relative to the excitation wavelength. The emission intensity is inversely proportional to the local

temperature of the coating. As the temperature is raised, an increased number of molecules take thermal paths to de-

excitation (instead of emitting photons), which results in a measurable decrease in emission intensity. A unique

relationship between the emission intensity and paint temperature can be established through a calibration process

and allows the paint to be used as a global surface temperature sensor. An in-depth discussion of various fluorescent

processes can be found in Ref. 7.

The emission of the coating is imaged using a photodetector, typically a CCD camera. In the intensity-based

technique, a wind-off image is acquired just prior to a wind tunnel run to provide a reference of the initial spatial

emission distribution while the test article is at a known uniform temperature. A series of images is acquired during

a wind tunnel run to provide the temperature of the surface of the model as a function of time. During data

reduction, the images are mapped to a three-dimensional grid of the test article in order to relate the image data to a

geometric location on the model and to ensure proper reference to run image alignment. Next, the run images are

ratioed by the reference image to eliminate any spatial intensity variations that are not due to temperature changes,

e.g., uneven paint thickness or a spatially nonuniform illumination field. The temperature-time histories of ratioed

emission intensities on the surface of the model can then be used to calculate surface heat transfer. For a more in-

depth discussion of various TSP heat-transfer measurement methods, see Ref. 8.

The description of the Tunnel 9 TSP system and the data reduction methodology is given more consideration in

Refs. 3 and 4.

2. Two-Color TSP System

The one-color TSP system described above can only be used when there are no significant temporal variations in

the illumination field used to excite the luminophores in the coating. The variations can arise from either temporal

instability of the light source or from motion of the test article during a wind tunnel run. The standard mode of

operation at Tunnel 9 involves continuously pitching models through an angle-of-attack sweep during a single run in

order to increase productivity. In this case, even if the light source is assumed to be perfectly stable, a reference

α, deg

No

rma

lize

dS

tan

ton

Nu

mb

er

-10 -5 0 5 10 15-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Coax (1)

TSP (1)

Coax (2)

TSP (2)

5




image acquired at a single fixed position cannot account for a changing illumination pattern on the surface of a test

article as it is moved through a spatially nonuniform illumination field during a pitch sweep.

To account for nontemperature-induced spatial-temporal variations in emission intensity, a two-color

(biluminophore) TSP formulation was introduced. A second type of luminophore with much lower temperature

sensitivity was added to the original formulation to serve as an “on-board” reference. The emission wavelengths of

the two luminophores need to be optically separable from each other and the excitation wavelength. For the TSP

formulation used in Tunnel 9, the excitation wavelength is centered at 365 nm, and the emissions of the two

luminophores are centered at 614 nm (~red) and 450 nm (~blue). The presence of the second luminophore in the

coating allows for the correction of undesirable spatial-temporal variations in emission intensity by using a ratio-of-

ratios technique.8 In this case, the TSP image data are acquired using a camera system that consists of two 16-bit

CCD cameras equipped with different bandpass filters to separate the emission wavelengths of the two

luminophores in the paint. The biluminophore concept has also been used successfully at the NASA Langley

Research Center in their phosphor thermography heat-transfer measurement system.9

Recently, the two-color TSP system was successfully implemented at Tunnel 9. Heat-transfer data were acquired

on a wedge-shaped section of a hypersonic body during a continuous pitch sweep. Some of the test results are shown

in Fig. 5. The full set of results is presented in Ref. 5.

Successful implementation of the two-color TSP system at Tunnel 9 was in part a result of many tests and

studies conducted in the TSP calibration laboratory. This paper describes the TSP calibration laboratory setup and

details some of the experiments which contributed to the two-color paint system implementation at Tunnel 9.

II. TSP Calibration Laboratory

Each TSP formulation has a unique relationship between emission intensity and temperature that needs to be

determined through a calibration process before the paint can be used to measure surface temperature during a wind

tunnel test. The calibration process itself needs to be robust and produce repeatable calibrations insensitive to minor

setup perturbations. In other words, the calibration results should not be influenced by changes in the physical setup

of the experiment (e.g., sample orientation, paint thickness, or thermocouple placement). This robustness is

necessary in order to make the process practical for repeated use with different paints over the several decade

lifespan of the TSP system.

A second important goal of the laboratory was to determine various parameters of each candidate formulation,

such as the temperature sensitivity in a given temperature range. Testing of TSP cameras, illumination sources, and

paint under conditions optically similar to an actual test was also done in order to characterize the complete system’s

performance, not just an individual component. To perform these studies, a special calibration laboratory was

designed and built at Tunnel 9.

A. Hardware

A specially-designed calibration chamber was constructed to conduct paint development studies and

calibrations. The chamber consists of a vacuum cell with several window ports. The sample placed in a special

holder inside the chamber is illuminated with a 365-nm LED through one of the window ports and is imaged

through another. Figure 6 shows the calibration chamber setup. The TSP emission is measured using the same 16-bit

CCD cameras and optical components that would be used during an actual wind tunnel test. Furthermore, excitation

light intensity is maintained at levels similar to those that would be used during an actual wind tunnel test. By

ensuring similarity between the laboratory and the wind tunnel setup, an accurate assessment of the TSP system

performance (e.g., dynamic range) and of TSP formulations can be made under realistic conditions.

Figure 7 shows the schematic of the TSP sample holder assembly. The sample holder is equipped with a

thermoelectric (Peltier) heater that alternately allows for both heating and cooling of the sample by reversing the

electrical current direction. A heat sink is attached to one side of the heater and a copper plate to the other. Thin

layers of thermal paste are used at each connection in order to ensure uniform thermal contact. The copper plate

between the heater and the TSP sample ensures the sample gets a relatively uniform heating input over its area by

washing out any heating gradients originating on the surface of the heater. The sample itself consists of a 1x1 in.

thin aluminum substrate painted with TSP. Any thermally conductive substrate can be used in place of aluminum.

The sample holder is spring-loaded to hold the entire sample and heater assembly together. The temperature on the

surface of the sample is measured using two beaded type-E thermocouples attached to the surface by dabs of thermal

paste.

6




Figure 6. Tunnel 9 TSP Calibration Laboratory.

Figure 7. Schematic of the TSP Calibration Sample Holder Assembly.

B. TSP Calibration Process

1. Sample Installation

Care must be taken to ensure good thermal contact between all parts of the assembly depicted in Fig. 7. A thin

layer of thermal paste between all components ensures good thermal contact, and the spring-load sample holder

ensures the entire assembly stays together throughout the calibration process. The thermocouples are attached to the

front surface of the sample in such a way that the beads are normal to the surface. The leads are bent in such a way

as to help stay out of the camera view as much as possible. The natural stiffness of the thermocouple wire is used to

ensure that the thermocouple bead remains in contact with the surface.

2. Data acquisition

Each calibration starts by cooling the sample to the low temperature of interest, approximately 50°F (283 K) for

Tunnel 9. Once the sample is at the desired low point, the electric current direction is reversed and the sample is

heated slowly to a desired high point, which is approximately 200°F (367 K) for the Tunnel 9 paint formulation and

camera system. A pair of images is acquired every 2 sec to record emission intensities of each of the luminophores

(“red” and “blue”). Each image is acquired with a separate camera filtered at the appropriate wavelength.

LED

Sample

Cameras

Vacuum Line

Beam Splitter

LED

Sample

Cameras

Vacuum Line

Beam Splitter

LED

Sample

Cameras

Vacuum Line

Beam Splitter

TSP

Copper Plate

Heat Sink

Heater

AluminumSubstrate

Thermocouples

7




Temperature data are acquired simultaneously with the image data using a high-speed data acquisition system. The

data system is set up to acquire a burst of data at 25,000 samples/sec every time a pair of images is acquired. The

duration of the burst corresponds to the exposure time of the cameras, which is usually 2 to 5 msec. The exposure

time is selected so that the “red” (temperature-sensitive) emission is close to the camera pixel’s full linear capacity

at the coldest temperature.

All of the thermocouple data from each burst are averaged to get a single temperature reading per pair of

images. The current direction is again reversed when the desired maximum temperature is reached, and the sample is

slowly cooled down to the initial temperature. The entire calibration takes 10 min, 5 min of heating and 5 min of

cooling. The selection of these low heating and cooling rates will be explained in a later section.

3. Data processing

A 4x4 pixel area is selected from the center of the paint sample to use for the calibration data processing. An

average intensity value of this area is obtained from each image of each of the two cameras. As previously

mentioned, all of the temperature data are averaged to obtain a single value for each pair of images at each point in

time. Thus, the data are reduced to a single temperature and two intensity (red and blue) values at every point in

time. The calibration is computed as shown in Eq. (1). During a wind tunnel run, Tref and the corresponding

reference intensity values are determined by the initial model temperature just prior to a run. The right side of Eq.

(1) will be referred to as the “ratio of ratios” in this paper. Also note that all of the calibration curves presented in

this paper will be shown as T in °F (instead of T/Tref) vs. ratio of ratios.

�

��=

(�� ,��⁄ )

(�� ,��⁄ ) (1)

4. Calibration Process Validation

The process described above was applied multiple times to samples of different TSP formulations. Various

heating and cooling rates were tested. It was observed that while multiple calibrations of the same sample produced

repeatable calibrations when heating a sample and when cooling a sample, the heating and the cooling legs of a

single calibration did not always match. This was especially evident in faster rate calibrations. Figure 8 shows two

such calibrations of a single sample. One calibration was 1 min long (30 sec of heating, 30 sec of cooling) and the

second one was 2 min long (1 min of heating, 1 min of cooling). It is evident that while the heating leg of calibration

one matched the heating leg of calibration two and the cooling leg of calibration one matched the cooling leg of

calibration two, the heating and the cooling parts of each calibration did not match each other.

Further investigation discovered that the differences between the heating and the cooling legs of a calibration

disappear completely for very slow calibrations (on the order of 10 min) as is shown in Fig. 9. Therefore, it can be

concluded that the difference between the heating and cooling parts of each calibration changed depending on the

heating rate, i.e., how quickly the sample was heated or cooled.

The mismatch between the heating and cooling legs of a calibration is attributed to a thermal lag of the

calibration setup rather than the thermal-optical behavior of the paint. The paint emission intensity is a function of

the average temperature inside the paint layer. During the calibration process, the paint layer is heated from one side

while the temperature is measured on the surface of the opposite side, as shown in Fig.7. The higher the heating or

cooling rate, the larger the temperature gradient inside the paint layer, and thus the larger the difference between the

average temperature inside the paint layer and the surface temperature measured by the thermocouples. Heat

conduction into the thermocouple bead and wires may also affect the thermocouple measurement.

If the hypothesis is true, then the thermocouples on the surface are reading too low during the heating phase and

too high during the cooling phase of a calibration that is performed at a fast rate. A calibration performed at a slow

enough rate to be considered quasi-steady should fall between the heating and cooling legs of a fast calibration. This

indeed is the case, as illustrated in Fig. 10. To avoid the thermal lag issue, all of the subsequent calibrations are done

at a slow rate. The rate can be determined experimentally by comparing the heating and cooling legs of each

calibration to ensure the calibration is quasi-steady.

8




Figure 8. Heating and cooling legs of two TSP calibrations. Cal 1 was 1 min (30-sec heating, 30-sec

cooling) and cal 2 was 2 min (1-min heating, 1-min cooling).

Figure 9. Heating and cooling legs of a 10-min TSP calibration (5-min heating, 5-min cooling).

Te

mp

era

ture

(°F

) T

em

pe

ratu

re (

°F)

9




Figure 10. Comparison between a quasi-steady calibration and heating and cooling legs of a calibration

done at a fast rate.

For a calibration to be useful, the calibration process needs to be robust, i.e., repeatable and insensitive to setup

perturbations. To check the setup for sensitivity to perturbations, a sample was calibrated twice and then removed

from the sample holder. At a later time, the sample was reinstalled into the chamber for additional calibrations. The

process of sample installation creates a number of changes in the calibration setup. Specifically, the position of the

sample relative to the light source and the cameras, the placement of the thermocouples on its surface, and the

amount of thermal paste between parts of the setup all change.

The sample was calibrated two more times after the reinstallation, resulting in a total of four calibrations. The

heating and cooling portions of each of the four calibrations show excellent agreement, as seen in Fig. 11. This

demonstrates the robustness of the setup to perturbations and its ability to yield consistent calibration results free of

thermal lag effects or errors due to the assembly of the samples.

Te

mp

era

ture

(°F

)

10




Figure 11. Cooling and heating legs of four calibrations of a sample. Calibrations 3 and 4 were

performed after the sample was completely removed from the holder and then re-installed.

III. Results

C. Two-Color TSP Development

The paint development work requires a laboratory setup that allows paint testing and comparison in a controlled

environment. The TSP calibration facility was designed to support this need. A number of paint formulations have

been developed by LeaTech LLC and tested at White Oak as a part of the TSP system development effort. The TSP

used during the tests described in Refs. 2, 3, and 4 consisted of a single luminophore (one-color) suspended in a

polyurethane binder. Recently, a biluminophore (two-color) TSP formulation and associated hardware upgrades

have been implemented at Tunnel 9 (Ref. 5).

The implementation of the two-color TSP system was in part made possible by extensive work performed in the

TSP calibration chamber. To start, the ability of the two-color formulation to correct for temporal changes in the

illumination field had to be demonstrated in the laboratory environment. This was done by setting up a two-color

sample calibration experiment where a single sample was calibrated four times under the following conditions:

a) 100% illumination intensity (illumination source at full power and intensity held constant in time),

b) 80% illumination intensity (illumination source at 80% power and intensity held constant in time),

c) variable illumination intensity 1 (illumination intensity randomly varied during calibration by approximately

±20%), and

d) variable illumination intensity 2 (illumination intensity randomly varied during calibration by approximately

±20%).

The resulting emission intensities of the two luminophores ratioed by their respective reference intensities

during each of the calibrations are shown in Fig. 12 in order to show their individual behavior. From the figure it is

evident that the red luminophore is much more temperature sensitive than the blue luminophore since the slope of

the red intensity in all cases is steeper. It is also observed that the fluctuations in emission intensity due to changes in

incident illumination affect both of the luminophores in the cases of the variable illumination intensity (c and d).

Next, the data processing procedure described in Section II.B was applied to the four data sets, and the resulting

calibrations are shown in Fig. 13. All calibrations show excellent agreement and no sign of the changes in

illumination intensity, thus indicating that the second luminophore and the ratio of ratios technique can successfully

eliminate errors due to fluctuating illumination field.

Te

mp

era

ture

(°F

)




To provide additional evidence of the usefulness of the sec

computed for one of the variable illumination cases

color and the two-color calibrations were then applied to convert the paint emission intensity da

The resulting temperature histories were compared to the temperature measured with the thermocouples on the

surface of the paint sample. This comparison is shown in

calibration produces erroneous results

temperature history.

Figure 12. Emission intensities of the red and blue luminophores

illumination conditions: (a) 100% intensity, (b) 80% intensity, (c) varying intensity 1,

11


To provide additional evidence of the usefulness of the second luminophore, a one-color calibration was

one of the variable illumination cases by using just the data from the red luminophore. Both the one

color calibrations were then applied to convert the paint emission intensity da


surface of the paint sample. This comparison is shown in Fig. 14. From the plot it is evident that the one

roduces erroneous results, whereas the two-color calibration is able to reconstruct

Emission intensities of the red and blue luminophores during four calibrations under different incident

illumination conditions: (a) 100% intensity, (b) 80% intensity, (c) varying intensity 1, and (d) varying intensity 2.

color calibration was

by using just the data from the red luminophore. Both the one-

color calibrations were then applied to convert the paint emission intensity data into temperatures.


. From the plot it is evident that the one-color

color calibration is able to reconstruct an accurate

during four calibrations under different incident

(d) varying intensity 2.




Figure 13. Calibrations computed using the ratio of ratios technique for the four different incident illumination cases:

(a) 100% intensity, (b) 80% intensity, (c) varying intensity 1,

Figure 14. Comparison between temperature measured with thermocouples during a calibration where incident

illumination was varied with temperatures computed using the

Te

mp

era

ture

(°F

)

12


Calibrations computed using the ratio of ratios technique for the four different incident illumination cases:

(a) 100% intensity, (b) 80% intensity, (c) varying intensity 1, and (d) varying intensity 2.

Comparison between temperature measured with thermocouples during a calibration where incident

illumination was varied with temperatures computed using the two-color calibration and the one-

Calibrations computed using the ratio of ratios technique for the four different incident illumination cases:

(d) varying intensity 2.

Comparison between temperature measured with thermocouples during a calibration where incident

-color calibration.

13




D. TSP Formulation Comparison

The TSP system development process involves the calibration and comparison of many different paint

formulations. Some of the parameters of concern are brightness of emission, durability, adhesion characteristics, and

sensitivity to temperature. The latter issue is addressed below.

Temperature sensitivity in a temperature range of interest is an important paint characteristic and is one of the

parameters that defines how small of a temperature change can be resolved with the system. Some paint

formulations may have a better sensitivity at lower temperatures, while others may have a better sensitivity at higher

temperatures. For the TSP system at Tunnel 9, it was important to find a formulation that would exhibit good

temperature sensitivity over an approximately 60 – 200°F (289 – 366 K) range.

Figure 15 shows calibrations for three different two-color paint formulations considered for use at Tunnel 9.

From the behavior of these calibrations, a selection of the most desirable TSP formulation for a particular

application can be made. For instance, Sample 2-2 exhibits the best sensitivity at lower temperatures out of the three

formulations, but becomes much less sensitive at temperatures over 150°F. Sample 5-3, on the other hand, is less

sensitive at lower temperatures, but retains its sensitivity better at higher temperatures as indicated by the shallower

slope of the calibration curve. Since paint formulation represented by Sample 5-3 has better temperature sensitivity

characteristics over the entire temperature range of interest, it is best suited of the three samples tested for use in the

Tunnel 9 TSP system.

Figure 15. Calibration curve comparison for three two-color TSP formulations.

IV. Conclusion

A TSP calibration laboratory was designed and built at AEDC Tunnel 9 to perform TSP calibrations and support

paint development efforts. Multiple calibration tests were performed to demonstrate the robustness of the setup and

of the calibration methodology. The resulting calibration facility was instrumental in the development and

successful implementation of the two-color TSP system at Tunnel 9.

Acknowledgements

The authors would like to thank Marvine Hamner of LeaTech, LLC for her continued support of Tunnel 9 TSP

system development and for developing the paint formulations. The dedication and support of the entire Tunnel 9

staff has made the project successful and are greatly appreciated.

14




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Method,” Journal of Spacecraft and Rockets, Vol. 36, No. 2, March – April 1999, pp. 160-170.

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