Download development of temperature sensitive paints for the high enthalpy

DEVELOPMENT OF TEMPERATURE SENSITIVE PAINTS FOR THE HIGH
ENTHALPY SHOCK TUNNEL GÖTTINGEN, HEG
Jan Martinez Schramm(1), Klaus Hannemann(2), Hiroshi Ozawa(3), Walter Beck(4) and Christian Klein (5)
(1)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, Email: [email protected]
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, Email: [email protected]
(3)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen
(4)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, Email: [email protected]
(5)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, Email: [email protected]
(2)
ABSTRACT
The precise experimental determination of the heat
loads acting on vehicles traveling at hypersonic speeds
is crucial for the design of the vehicle’s thermal
protection system. Computational fluid dynamics
methods provide increasingly powerful possibilities for
the simulation of such hypersonic configurations.
Nevertheless, the difficulties in the accurate modelling
of high-temperature effects in chemically reacting
flows,
boundary-layer
transition
and
shockwave/boundary-layer interactions, mean that groundbased testing will remain an important tool for
evaluating the heating levels encountered by high-speed
vehicles for the foreseeable future. The High Enthalpy
Shock Tunnel Göttingen (HEG) of the German
Aerospace Center (DLR), capable of performing such
testing, is one of the major European hypersonic test
facilities. It was commissioned for use in 1991 and has
been utilized since then extensively in a large number of
national and international space and hypersonic flight
projects.
1. INTRODUCTION
Conventional heat flux sensors, such as thin film gauges
and thermocouples, have been employed successfully in
hypersonic tunnels and also in HEG for a few decades.
More details about HEG and the typical applications can
be found in [6,7,8]. Such gauges are limited in terms of
the spatial resolution they provide and geometrical
constraints may prevent the installation in complex
model geometries. An attractive nonintrusive
alternative, providing global heat-transfer measurements
at potentially much lower costs and efforts, is the use of
temperature sensitive paints, TSP. A TSP consists of a
luminescent molecule, the luminophore, dissolved in a
suitable binder and applied to the surface of the body to
be inspected. When illuminated with light at appropriate
wavelengths, the intensity of the emitted light of the
color is reciprocal dependent on the temperature of the
luminophores in the TSP layer. The luminescence from
the TSP is a function of the local spatial temperature,
and therefore, each pixel on the camera recording the
luminescence acts as a heat sensor. The heat transfer
rate can subsequently be determined from the time
evolution of the temperatures on the surface.
**
Present address: Tokyo Metropolitan University, Asahigaoka 66, Hino, Tokyo 191-0065, Japan, Email: [email protected]
2. TSP TECHNIQUE
A typical TSP consists of the luminescent molecule and
an oxygen impermeable binder. The impermeability to
oxygen is a mandatory prerequisite for chemically
reacting hypersonic flows, where recombination
processes may occur in the boundary layer and on the
surface. The basis of the temperature sensitive paint
method is the sensitivity of the fluorescent intensity of
the luminescent molecules to their thermal environment.
The luminescent molecule is placed in an excited
electronic state by absorption of a photon. The excited
molecule can relax to the ground state through the
emission of a photon. Radiation-less relaxation is also
possible: this process is enhanced by a rise in
temperature of the luminescent molecule; this is known
as thermal quenching. This means in practise that the
luminescence of the paint will decrease with increasing
temperature. Each luminescent molecule functions as a
temperature sensor and the size of the molecule is
orders of magnitude smaller than classical temperature
sensors; thus, in practice the effective limit in spatial
resolution is only restricted by the camera and the
optical setup employed. Unfortunately, the luminescent
intensity distribution is not only a function of the
temperature. In fact the luminescence from the painted
surface varies with illumination intensity, paint layer
thickness, and the distribution of the luminophore in the
binder. Assuming that these parameters do not vary in
time, they can be eliminated by taking the ratio of the
image at the test condition or wind-on image to an
image taken at a known reference condition or wind-off
image. This procedure is often referred to as radiometric
TSP. Important characteristics for the measurement
technique such as fluorescence lifetime and temperature
sensitivity depend on the type of luminophore;
therefore, it is possible to select an optimal luminophore
for the experimental requirements. The need for short
exposure times when using TSP in short duration
flowstherefore requires high intensity luminescence
signals from the luminophore. Therefore, the use of a
reflective base layer (see Fig. 1) is mandatory and a
thick TSP layer increases the signal as well.
Unfortunately, the response time scales with the square
of the paint thickness, so a relatively thin layer
(typically ≤ 10μm) is usually necessary for ultra-fast
TSP. Using classical TSP on long time scales, we can
assume temperature equilibrium between TSP layer and
base layer, and thus the heat conduction process to be
considered when determining the surface heat flux takes
place in the model material. This is not the case for
ultra-fast applications; here the thickness of the TSP
layer has to be much thinner than that of the base layer,
as shown in Fig. 1. Criteria to use an appropriate ratio of
TSP to base layer have already been established and
discussed [16]. More details are given by the authors in
[14]. Relying on these presuppositions the analysis of
the TSP data can follow that of thin-film gauges and
other
conventional
sensors
for
heat-transfer
measurements, as described in [16] and given below.
3. DEVELOPMENT STRATEGY
Recent published research reports either binder-based
TSP’s or anodized aluminum TSP’s. We decided to
work with binder-based TSP’s. The advantages are the
ease in creating a homogeneous TSP layer in terms of
the luminophore molecule distribution in the binder and
the variability of the concentration of the molecules
allowing modification of the temperature sensitivity and
the luminescent intensity. Furthermore, the application
of the TSP can be performed by spraying techniques,
which allows control of the TSP layer thickness and its
application to models of complex geometries; in some
cases even when already installed in the wind tunnel.
4. Selection Criteria
2
∙ ∙
1
The base layer is treated as a uniform semi-infinite
medium, and approximate solutions to the onedimensional heat-conduction equation with appropriate
boundary conditions are sought, allowing the surface
heat-transfer rate to be recovered from the timedependent temperature. The recovered heat flux is
linearly related to the physical properties of the base
layer (density, heat capacity and conductivity) found in
the term
∙ ∙ . This means the accuracy to which
these values are known directly defines the error of the
heat flux.
For the present investigation, it is necessary to create a
TSP solution capable of measuring temperature
variation in times scales of a few microseconds.
Therefore, the optimal TSP luminophore should be
selected together with the corresponding binder and
solvent. Another important fact is that hypersonic high
speed flows are coupled in most case to a strong selfillumination of the test gas, which is superimposed on
the TSP luminescence signal recorded by the camera, or
in some cases can even excite the paint itself (see Fig.
1). This problem will not be covered in this paper. The
following requirements of the TSP luminophore have
been used as a basis to select promising candidates:
-
luminescent lifetime < 1μs
high temperature sensitivity
low or no pressure sensitivity
strong luminescent intensity
absorption wavelength adapted to light sources
emission wavelength different to self-illumination
binder not permeable to oxygen
Several luminophores where selected and tested and
compared. One of the more promising candidates,
namely
Dichlorotris
(1,10-phenanthroline)
Ruthenium(II) hydrate 98%, here we abbreviate it with
Ru(phen), will be presented in this paper.
5.
Figure 1. Schematic of an ultra-fast TSP setup.
Calibration
Physical properties of the pure luminophores or
luminophores dissolved in binders are referenced in the
literature but not for the combinations of luminophore
and binder we decided to use. Therefore, it was
necessary to implement a complete experimental
calibration procedure. The selected luminophore Ru was
calibrated with respect to absorption and emission
wavelengths, temperature and pressure sensitivity and
the effect of dye concentration on the former two. The
results will be discussed below. The spectral
measurements were performed with a commercially
available fluorescence spectrometer. This fluorescence
spectrometer uses a xenon lamp equipped with a
monochromator to excite the sample and detects the
spectrally resolved emitted light using a spectrograph.
The wavelength accuracy used for the excitation was
±1.5 nm and for the recorded emission ±3 nm. The first
step in the calibration process is to measure the
absorption behaviour of the selected luminophores. The
spectrum obtained is given in Fig. 2. The visible region
of the spectrum is indicated by the color bar on top of
the graph. Consequently, the next step in the calibration
process is to excite the selected luminophore with the
appropriate wavelength and to measure at which
wavelengths the luminophore emits. The samples used
with Ru(phen) were illuminated with a blue LED,
whose emission spectrum is given in Fig. 3. The
dissolved luminophore was sprayed onto an aluminium
sample plate of 10 mm square and 1 mm thickness. The
TSP layer was separated from the aluminium body by a
white polyurethane layer in order to increase the
emission intensity of the luminophore. The prepared
samples were installed in a reference chamber in which
pressure of the ambient gas (dry air) and the sample
temperature can be controlled. The normalized and
spectrally resolved intensities of the emission of the
TSP candidate Ru(phen) is shown Fig. 4. Additionally
the absorption spectrum from Fig. 2 and the LED
spectrum from Fig. 3 are shown in the plot. The peak
emission value was used as the normalizing value Iref for
each luminophore. One important result of this
measurement is the difference in wavelength excitation
and emission maximum of the inspected luminophore:
the Stokes shift. The higher this quantity, the better the
exciting light can be separated (distinguished) from the
luminescence from the luminophore in the recorded
image. For Ru(pen) we obtain a Stokes shift of roughly
120 nm. To calibrate the temperature sensitivities and to
quantify the pressure dependencies of the selected
luminophores, the temperature in the reference chamber
was varied within the range from 268 K to 328 K and
the pressure in the range from 0.1 kPa to 200 kPa,
respectively. Results for temperatures of 283 K, 293 K,
303 K, 313 K, and 323 K at 100 kPa are given in Fig. 5,
which demonstrates the temperature sensitivity of the
Ru(phen) luminophore. The pressure dependency was
cross checked by varying the pressure between 20 kPa
and 100 kPa at 303 K. The peak emission value
(T = 293 K; p = 100 kPa) of each luminophore was used
as the normalizing value Iref. As already discussed in the
introduction, we can observe that the highest sample
temperature gives the lowest intensity emission for the
luminophore Ru(phen) tested.
Figure 2. Normalized absorption spectra of Ru(phen).
Figure 3. Normalized emission spectra of a commerceally available high power LED in the visible blue.
Figure 4. Normalized emission spectra of Ru(phen)
when excited with a blue LED and absorption data from
Fig.2 LED emission from Fig 3.
which had been designed for the development and
testing of high speed miniaturized pressure gauges and
thermocouples [3].
Figure 5. Normalized emission intensities at various
temperatures and pressures of Ru(phen).
Figure 6. Temperature sensitivity of Ru(phen).
The final calibration curve for the relative temperature
sensitivity is given in Fig. 6. It is obvious that the
calibration function is not of linear dependence. Special
care has to be taken, when converting the intensity to
real temperatures.
6. IN-SITU CALIBRATION
The application of the selected luminophore Ru(phen)
in a shock tube experiment, which will be described
now, served two purposes. First, the comparison with
temperature measurements using thermocouples and
thin film gauges, which allows the evaluation of the
response time behavior of the luminophores. Secondly,
the in-situ calibration of the physical properties of the
luminophore Ru(phen), which is needed to evaluate the
heat flux. This in-situ-calibration is possible by
assuming the same heat flux levels occur for the
thermocouple/thin film gauge measurement and the TSP
evaluation (see Eq. 1). The assumption of the same heat
fluxes allows a determinion of the physical properties of
the TSP layer, namely of
∙ ∙ . This measurement
was performed in a shock tube, of length of 4.5 m,
Figure 7. Schematic of the small shock tube facility.
The working principle of the tube will be described only
briefly here. More in-depth descriptions on the working
principle of shock tubes and their applications are given
in [10], [11], [5], [2], [17], [15] and [1]. The basic
layout of the diaphragm-driven shock tube used is
shown in Fig. 11. It consists of two chambers of
different but constant cross sections, separated by a
diaphragm. Prior to the test at time t0, both chambers are
filled with gases at different pressures. Initially the
gases are at rest in both chambers. The left chamber (see
Fig. 7), the driver section, is filled with Helium (state
He, (4)) and the right chamber, the driven section, is
filled with dry air (state Air, (1)). After manual rupture
of the diaphragm with a plunger, a shock wave (S)
moves into the driven section and the head (H) of a
centered expansion wave moves into the driver section.
At time tA, the incident shock wave (S) travels down the
driven section with the speed uS leaving the test gas air
in a compressed state (2), while the head (H) of the
centered expansion wave reflects at the driver section
end wall and travels (RH) in the direction of the driven
section. After reflection of the incident shock (S) at the
driven section end wall (RS), the test gas air is brought
to rest (5); the reflected shock, at speed uSR, penetrates
the contact surface (CS) and, in the case of a tailored
interface condition (p5/p2 = p0/p3), brings the contact
surface to rest. The test time for the experiment is of the
order of 250 s and starts prior to the arrival of the
shock (S) at the TSP wall insert (see Fig. 7 and Fig. 8)
and ends, when the reflected shock (RS) passes the wall
insert again. The approximate position of the TSP
coated side wall is shown in red in Fig. 7. At the
opposite side a window insert is installed which allows
observation of the TSP wall insert. The other two
perpendicular sides are equipped with two stainless steel
inserts with 12 thermocouples (TC) and 8 thin film
gauges (TF). The inserts are flat; the circular cross
section of the shock tube (A) merges into an octagonal
cross section of the test section (B). The TSP wall insert
is manufactured from polyurethane/polyol. The surface
of the TSP wall insert is coated by Ru with a mixing
ratio of 3.2 × 10-1 mol/l and a thickness of 0.2 m. The
insert for the thermocouple and thin film
instrumentation are of stainless steel. To allow for a
representative representation of the shock tube flow,
100 runs with the shock tube were performed with the
classical instrumentation. These thermocouple and thin
film gauge signals were sampled at 1 MHz. The TSP
results obtained will be compared to this averaged
dataset of thermocouple and thin films measurements.
The images for the TSP measurement are visualized
using a Phantom v1210 camera from Vision Resarch.
The camera is equipped with a fast option allowing for a
minimum exposure time is 0.46 μs per frame. The
images were recorded with a sampling rate of 250 kHz
and an exposure time of 3.5 μs. High power commercial
LEDs, operating in continuous mode with a wavelength
of 461 nm were employed to excite the luminophore Ru.
To judge the response time of the color, it is advisable
to
compare
the
time-resolved
temperature
measurements from the TSP signals to those with the
thermocouples and thin film gauges in the shock tube.
This comparison is shown in Fig. 8. Here the averaged
time signals at position (x = 38 mm) of the
thermocouple and thin film gauge measurements are
compared to the time resolved TSP signal measured at
the same axial location. The temperature as measured
by the thin film gauges is shown on the y-axis; the
signals from the thermocouples and TSP have been
scaled to allow a direct comparison. The absolute
temperature rises are different because the values of the
heat conductivity for the materials of the gauges and of
the paint are different. We can observe in Fig. 8 that the
TSP luminophore is able to capture the same rise time
as the thermocouple and thin film gauges. To transform
the temperature TSP signals into wall surface heat flux,
we need to know the dominant physical properties of
the heating process. As shown in the introduction, we
assume that the TSP layer itself is infinitesimally thin,
so that the heat conduction process is limited to the base
layer, which in this case is polyurethane/polyol.
Figure 8. Time resolved temperature traces measured
by thermocouples, thin film gauges and TSP.
Figure 9. Wave diagram (x-t) extracted from the two
dimensional TSP images using the symmetry line data.
To check this assumption and to obtain the relevant
∙ ∙ value, the TSP signal, converted to a heat
transfer rate using the TSP calibration data, was
averaged in the x-t region shown in Fig. 9 (0 mm < x <
50 mm; 150 s < t < 250 s). The
∙ ∙ value for
the paint was then chosen so that the measurements of
the thermocouples and thin film gauges agree best with
those from the TSP measurement (smallest deviation at
all x-positions). This comparison is given in Fig. 10.
The optimal agreement was found with a value of
∙ ∙ = 682 ± 82 J/m2·K·s½. Using this value we can
obtain the complete heat flux distribution in space and
time from the TSP measurements. The corresponding xt-wave-diagram is given in Fig. 9. The incident shock
(S) and the reflected shock (SR) can clearly be
observed. Additionally one can determine the incident
shock speed to be uS = 1250 ± 125 m/s, while for the
thermocouple and thin film measurements, one obtains
uS = 1017 ± 102 m/s and uS = 1040 ± 108 m/s,
respectively. The speed of the reflected shock is uRS =
477 ± 18 m/s when determined with the TSP
measurements and uRS = 415 ± 85 and 496 ± 72 for the
thermocouple and thin films signal evaluation,
respectively. Summarizing, we find that these results,
with consideration of the obtained accuracies, are in
reasonably good agreement. To check whether the
assumption of an infinitesimally thin TSP layer is
correct, we compared the obtained
∙ ∙ value to
other calibrations for the base layer material
polyurethane/polyol.
Figure 10. Comparison of heat flux determined by
thermocouple and thin film measurements to the TSP
data on the flat plate wall insert symmetry line.
The material was calibrated by a DLR material
laboratory in Stuttgart. The average
∙ ∙ value
between 30°C and 60°C (temperature range in the small
shock tube) is 715 ± 65 J/m^2·K·√s. In [9] a value of
670 J/m^2·K·√s has been reported. These values, and
additionally values from a material calibration carried
out in Japan using the manufacturer’s data give an
average of 667 ± 33 J/m^2·K·√s in direct comparison
with 682 ± 82 J/m^2·K·√s from the in-situ-calibration
here. Considering the experimental accuracies, we
cannot distinguish between these values, which leads to
the conclusion that the assumption of an infinitesimally
thin TSP layer and a dominant base layer for the heat
conduction is applicable here.
(TSP) for hypersonic high speed flows as generated in
the High Enthalpy Shock Tunnel Göttingen (HEG) of
the German Aerospace Center (DLR). Amongst other
luminophores, Ru(phen) was investigated with respect
to the defined prerequisite requirements , and the results
have been reported here. Overall, Ru(phen)-based TSP
proved to be a good candidate to carry out ultra-fastresponse TSP. The specific characteristics were
determined experimentally: a temperature sensitivity of
approximately 2 %/K (using concentrations of 3.2 × 10-1
mol/l at 293 K), an emission FWHM of 70 nm and a
Stokes-Shift of roughly 120 nm. Commercially
available high power LEDs for Ru(phen)-based TSP
exist and have been used. 4MU as an attractive
alternative has also been tested, but not reported in this
publication. An insitu calibration of the dominant
physical properties for the heat conduction process
using Ru(phen) was performed in a shock tube. This
allowed us to prove the necessary assumptions to
determine quantitative surface heat fluxes from the TSP
measurements. The comparison of shock-speed
measurements in the shock tube obtained with TSP to
measurements with classical instrumentation such as
thermocouples and thin film gauges showed sufficiently
high time response of the Ru(phen)-based TSP.
Summarizing, it could be concluded that the developed
Ru(phen)-based TSP is suitable for ultra-fast
measurements in short-duration facilities like HEG.
Acknowledgements: The authors thank Ivaylo Petkov
from the DLR Institute of Structures and Design in
Stuttgart for calibration of the base layer used, Ingo
Schwendtke from the HEG team at DLR, Göttingen for
technical support on the shock tube, Stephan Hock for
the characterization of the shock tube flow used and
Jeremy Wolfram for the work in the integration routines
to determine heat flux from the visualizations
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