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 8. REFRENCES 1. Alpher RA, White DR (1958) Flow in shock tubes with area change at the diaphragm section. J Fluid Mech 3(5):457-470 2. Anfimov, N (1992) Capabilities for aerogasdynamical and thermal testing of hypersonic vehicles. In: 17th AIAA Aerospace Ground Testing Conference, AIAA 92-3962, Reston 3. Carl, M, Eitelberg, G and Beck, WH (1997) Shock Tube Tests. DLR Internal Report, DLR-IB 223-97 A 12

7. CONCLUSIONS

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The major objective of this study was to establish an ultra-fast response time temperature sensitive paint

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