Thermometry using thermographic

1. Introduction In recent years a new technique has been developed for remote measurements of surface temperature, but also with a potential for spray and even gasflow diagnostics. It has mainly been used in scientific and industrial applications of surface thermometry to complicated geometries, e.g., rotor engines, turbine engines, and also in medicine. Other quantities such as heat flux through a surface have been investigated, because of its high importance to science and engineering community. During the last years, as the applications of thermographic phosphors have expanded, some attempts have also been made in combustion environment. A useful review article [1] could be a good assistance as an introduction to the subject of thermometry. Thermometry based on the use of thermographic phosphors utilises the physical properties of the phosphor particles for assessing temperature. The phosphor particles used in thermometry are usually inorganic materials having the form of a white-brown powder some 1 - 10 µm in diameter. Such a phosphor consists of a host material and a doping agent from which the light is emitted. A large number of different phosphors are produced today. These cover a wide range of temperatures, from cryogenic temperatures up to 1700 C or higher, making them suitable for many different applications. Each phosphor that is selected is highly sensitive within a specific range of temperatures, exhibiting an accuracy in the order of 1-5 C. Once deposited on the surface of interest and excited by a suitable wavelength, mainly UV light, the phosphor particles emit an intense luminescent light. This emission is called phosphorescence or , terms often used interchangeably, although fluorescence usually refers to emissions having a duration of 10-10 -10-7 s and phosphorescence to their having a duration in the order of 10-7-1 s. After excitation of the thermographic phosphor, the subsequent emission is imaged onto a detector. The temperature can then be deduced from the spectral or temporal properties of the recorded signal. This technique provides a high quantum yield, two- dimensional measurements and remote thermometry, as well as a high degree of accuracy. In contrast to pyrometry, it is not influenced by the emissivity of the material, background reflection from the surroundings or light absorption by optical windows or by surrounding gases. These advantages have allowed thermographic phosphors to be used in a wide variety of applications and in harsh environments. Thermographic phosphors (TP) have normally been used for hard non-combustible materials such as steel or concrete. For those surfaces the phosphor can be applied to the surface and only a very thin layer is required ensuring that the phosphor layer does not influence the flow and heat transfer from and to the surface. In this Chapter its application for combustion diagnostics are described.

2. Background Thermographic Phosphors which have temperature dependant emission are mostly inorganic and made of some ceramic material. The thermographic phosphor is composed by a carrier material which is doped with some activator material. The activator is often a rare earth metal. The doping concentration is typically about one percent, which is a small enough concentration for the activator atoms to be isolated from each other by the host matrix. The host material is mostly transparent to radiation, i.e. it is mostly the activators that absorb and emit radiation. Most phosphors used for thermometry are exited by radiation. The energy is absorbed by the rare earth metal in an electron excitation. The electron is then non- radiatively relaxed to a meta-stable energy level, i.e. a level from where no transitions are allowed.

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The emission spectra of any material will change with temperature since the Boltzmann distribution affects vibrational levels in ground- and excited states. Thermographic phosphors have a strong temperature dependency due to several reasons. When reaching a certain temperature, the excited electrons in the activator atoms will populate high enough energy levels to transfer its energy to the surrounding host material. This energy level is called the Charge Transfer State (CTS) of the host material and is described in the energy level diagram in Figure 1.

The Charge Transfer State is located at very low energies (<40*103 cm-1) for e.g. Eu3+ and Yb3+. Energy transferred to the CTS will relax non-radiatively to its lowest energy state. In the case of Eu3+ the CTS will then feed the D5 levels from where relaxation will occur in form of radiation. At high temperatures fast de-excitation via the CTS to the lowest CTS energy state will be more probable and thus shorten the lifetime of the phosphorescence. Measuring lifetime is consequently one way to determine the temperature of the phosphor.

A high temperature will also increase vibrations in the host material crystal lattice. This will broaden the linewidths of the emitting transitions in the phosphorescent process. Thermal expansion in the host lattice will also induce a frequency shift in the phosphorescence spectra, which accordingly can be used for temperature determination.

Energy / 103 cm-1 40

Host CTS

30 3

5D 2 1 0

20

Eu3+-levels

6 10 5 4 7F 3 2 1 0

0

Figure 1. The charge transfer state (CTS) as it affects the de-excitation in the La2O2S:Eu 5 3+ phosphor. Excited electrons at high D-levels in the Eu -ion can relax via the La2O2S CTS to the Eu3+ 7F ground state level.

There are, as mentioned above, a number of physical phenomena that influence the spectral shape of phosphorescence from thermographic phosphors depending on temperature. The

2 changes in the phosphorescence spectra will take form as changes in phosphorescence intensity, changes in phosphorescence lifetime and/or line shifts in the phosphorescence spectra. The two most common changes used for thermometry are lifetime changes and changes in intensity at specific wavelengths.

3. Temperature measurement approaches 3.1 Lifetime method

As described above most thermographic phosphors have a lifetime which is temperature dependent. According to Eq. 1 the intensity will decay exponentially according to

t − τ I =I oe (1) where I0 is the initial emission intensity, t is time and τ is the lifetime of the phosphorescence. τ is the amount of time to which the intensity has decreased to 1/e of the initial emission I0. In order to cover a large temperature interval several different phosphors may be used. Some of these and their corresponding lifetimes are shown in Fig. 2a. An example is shown in Fig 2b where the temporal decays are measured for three different temperatures. The phosphorescence lifetime will decrease with temperature depending on the phosphor material. Temperature can thus be calibrated and determined by calculating the phosphorescence lifetime from the measured intensity decay. This is normally done by fitting the intensity decay to the theoretical model (eq. 1), using a non-linear fitting procedure. The error in temperature from such a measurement can ideally be less than 1%.The lifetime method is usually used for point temperature measurements using a Photo-Multiplier Tube (PMT) for detection.

Figure 2a. Temperature sensitivity of different phosphor materials.

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Figure 2b.Phosphorescence decays measured at different temperatures. The lifetime becomes shorter when the temperature increases.

3.2 Intensity ratio method

The intensity of certain peaks in the phosphorescence spectrum is often temperature dependant and can thus be calibrated to temperature. Simultaneous measurements of two different wavelengths allow an instantaneous temperature measurement. The phosphorescence spectra of the thermographic phosphor YAG:Dy at different temperatures is shown in Figure 3.

Figure 3 - Phosphorescence spectra as a function of temperature. Comparison of the peak at 455 nm with the one at 493 nm is used for determination of temperature.

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The non-radiative deexcitation rate is strongly dependent on temperature resulting in a decrease of emission efficiency and shortening of the emission decay time at high temperature, although not at the same rate for the different emitted wavelengths. The advantage of this technique is that the ratio between the two apparent peak intensities of the phosphor is only dependent on temperature. Following a UV excitation the emission signal, Sj, can be represented by:

Sj = CjTeNj(T)τj I (2)

Where Cj is the detection efficiency for the jth transition Te is the camera exposure time, Nj(T) is the temperature dependent quantum efficiency for the jth transition, τj is the optical filter transmission and I is the intensity of the laser beam. By rationing the signal for the two different transitions, the resulting value is only dependent on temperature as:

S N (T) j+1 = K j+1 (3) S j N j (T)

Where K is a constant

4. Calibration procedures In order to correlate the phosphorescence to temperature, a series of calibration measurements must be performed under well controlled conditions. Figure 4 shows the set-up of a calibration measurement using a laser for excitation, fibres for guiding light, filters for increasing the signal to background ratio, and detectors for acquiring the phosphorescence emission.

Figure 4 Calibration set-up using a spectrometer, ICCD, laser, PMT, thermocouple and the test cell.

During the calibration procedure several phosphors have been calibrated and both temperature measuring approaches were used and sometimes compared. In the experiments low laser intensity, 50 µJ, was enough to obtain phosphorescence. A power dependence test on the intensity of the emission did not show any noticeable variation of the phosphor lifetime to the laser power between 50 µJ and l mJ.

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The phosphor was excited by the third or fourth harmonic of a Nd: YAG laser at 355 and 266 nm, respectively, with a pulse duration of about 7 ns and a repetition rate of 10 Hz. The laser light was focused into a fiber with a diameter of 0.7 mm and then transmitted to the phosphor in the test cell at the other end of the fiber. The subsequent emission was collected by two other fibers, which individually supplied a spectrograph connected to an intensified charged coupled, device (ICCD), and a photomultiplier tube (PMT), for spectrally and temporally resolved measurements, respectively. The test measurements were done inside a cell used for the calibration. A rod of the same material as the cell (inox) was coated at one end with a mixture (50: 50)% of the phosphor and a ceramic cement (cerastil-c) to a thickness of approximately 200 µm. The rod was inserted into the cylindrical chamber of the cell, which had one small input port for the laser beam and two output ports for the emission. The radiation was guided by quartz fibres (Ø=0.7mm). A calibrated thermocouple with an accuracy of ±1 K was used to measure the temperature close to the phosphor inside the cell. The temperature of the wall of the cylinder could be changed between room temperature and 1000 K using heating wires. An example on the output from these calibration experiments using the wavelength ratio approach is shown in Figure 5 giving the calibration curve using the YAG:Dy phosphor when the ratio between the peaks at 455 nm and 493 nm seen in Figure 33 was evaluated for different temperatures. As can be seen the calibration curve is very smooth and is then used for evaluating the experiments taken place at different applications.

Figure 5 - Calibration curve for YAG:Dy using the intensity ratio method. The ratio of the peaks at 455 and 493 nm as a function of temperature.

As described above an alternative, or a complement, to the intensity ratio technique is to measure the life time of the phosphorescence decay. Using the same set-up as above it was also possible to measure the decay as a function of temperature. In Figure 6 are shown two calibration curves using different phosphors. As can be seen it is important to use the one with highest sensitivity in the temperature region of interest. .

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Figure. 6. Calibration curves for two thermographic phosphors

Figure 7 shows the resulting lifetime and intensity ratio calibration curves for one selected phosphor. In this case a phosphor to be used for measurements of temperatures on burning materials, in the temperature range between 300 and 600 C. The precision of temperature determination in the case of the ratio technique was found to be 5-10°C while the lifetime technique showed a higher precision of 1-5 °C. This is generally the case that the lifetime approach has better precision than the intensity ratio approach. One reason for this is that a lifetime is measured in several points whereas the intensity approach is based on a simple ratio.

Fig. 7 Calibration of temperature against the lifetime and spectral intensity ratio of the phosphorescence. The left axis shows the lifetime versus temperature on a logarithmic scale for two different calibrations days. The right axis presents the relative intensity ratio of the peaks of the spectrally resolved emission against temperature.

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5. Two dimensional surface temperature measurements. So far the discussion has been limited to a principal discussion of the two temperature measuring approaches from a point of view that point measurements are to be performed. In practice it is, however, as pointed out in the course at several occasions, very important to perform two-dimensional measurement. Clearly the approach for intensity ratio is straight forward. In this case the measurement object is illuminated by a laser beam and the object can then be imaged by lenses on two different CCD cameras where the key wavelengths of interest are spectrally isolated by interference filters. The two-dimensional temperature data can then be calculated by dividing the two images and comparing each pixel to the calibration curve, with an error of about 5%. An alternative and a cheaper solution allowing one detector to be used is to use a special designed stereoscope shown in Figure 8.

Figure 8. An optical stereoscope producing two images in different colours

After excitation, the incoming phosphorescence emission passes through separate filters before encountering a 45 degrees mirrors and a prism. The result is two identical images on the CCD ship of the same object. The reflection curve of the mirror surface shows a constant reflection in the wavelengths range of the incoming phosphorescence. Interference filters are placed in front of each entrance of the stereoscope in order to detect phosphorescence signals at these wavelengths. For each measurement, the intensity images corresponding to each wavelength are warped using a grid image, then they are subtracted from background and digitally divided by a reference image as;.

I − B Ref R = phos1 1 ⋅ 2 (4) Ref1 I phos2 − B2

Where R, is a matrix holding the ratio values of all the pixels. Iphos1 is the detected image through filter 1, and Iphos2 is the one detected through filter 2. B is the background corresponding to each image. Ref stands for the reference image. The reference image is recorded to compensate from a possible non uniformity of pixel sensitivity in the CCD camera. The reference image is recorded using a uniform light source imaged through the stereoscope using the same running conditions (CCD gain and exposures time etc). The resulting temperature ratio image is finally inferred from a previously recorded calibration curve.

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With this stereoscope it is thus possible to produce two images, at different colours, imaged onto one CCD detector thus eliminating the need for two CCD when using the spectral ratio approach. (The warping procedure as referred to above assures so that the two images on the CCD get a perfect match on a pixel-by pixel basis. This is not always easy without this process since the fibre bundle leading the light from the image intensifier to the CCD detector to a certain extent may be twisted).

Still, as indicated above it is also of interest to use the lifetime approach since that often have a higher precision. This can also be done in 2D by using the fast framing camera described in a previous Lecture. The phosphor was excited by the same source of laser light 266 nm as in the calibration measurements. The phosphorescence images were obtained by eight consecutively gated CCD detectors enable pixel by pixel-lifetime evaluation of the phosphorescence by interpolating an exponential-decay curve to the counts of the corresponding pixel positions of the sequential CCD images. The temperature at each pixel position was evaluated using a calibration procedure of temperature against lifetime (see Fig. 6).

The 2D-temperature imaging technique was tested on a low-density fiber board in a small flame spread scenario. The phosphor was applied to a surface of 4x3cm by pressing the phosphor powder onto the sample so that a thin coating was obtained. The board was placed vertically and heated at the lower part by burning alcohol. The use of alcohol as ignition fuel was convenient since it generated less soot that might interfere with the measurement. The phosphorescence from the surface of the combustible board was recorded with the framing camera. The detectors inside the camera were sequentially gated, as illustrated in Figure 9a with an individual exposure time of 100 µs so that immediately as one detector was deactivated the following detector was activated. This was done to capture as much phosphorescence as possible at high temperature under an observation time of 800 µs. The acquired images were first subtracted from the background and divided by a reference image obtained from the previous camera calibration. Figure 9b shows the experimental set-up used to measure the 2D temperatures during the flame spread experiments.

Fig. 9a The lifetime was extracted by fitting a simple exponential function to the detected

intensities at each pixel position. Then temperature information was deduced from the lifetime. 9b. The experimental set-up for the measurement of flame spread.

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Figure 10 shows the surface temperature at the higher part of the board during different moments of an upward spreading flame. Figures 10a and 10b give the temperature soon after (4 seconds and 18 seconds, respectively) the time of ignition of the alcohol before any strong temperature changes were observed in the upper region. Later, 32 seconds after ignition, the flames have reached the investigated region and regions of hot temperature are seen in Fig. 10c. The flame spread increased faster until it reached maximum temperature as shown in Fig. 10d, corresponding to 48 seconds after ignition. A maximum temperature of 673-773 K and strong gradients of temperature are observed in the figure. Thirty seconds after, Fig. 10e, the flame started to decrease in intensity and local flame extinction started to appear some tens of seconds later as shown in Fig. 10f and 10g. Finally flames were only observed at the upper edges of the board and in Fig. 10h flames do not appear any longer. Just some local areas continued to react, most probably by pyrolysis. In the lower right figure, contour plots of temperatures (from Fig. 10g) are presented with a temperature resolution of 5 K so that gradients can be more clearly seen. The results from these 2-D measurements are in agreement with previous one-point measurements performed on the same type of fiberboards, where the highest flames were observed 42 seconds after ignition and a surface temperature of 710 K was found. The temperature measurements showed the best accuracy and precision at high temperatures (673-773 K). At lower temperatures the properties were less good (or more critical) mainly because the total time window used for the decay measurements was not wide enough. The total exposure time used was 800 µs whereas the lifetime was 2-3 ms at low temperatures.

Fig. 10 Temperature during flame-spread: Two-dimensional surface temperature of a low- density fiber board in a flame-spread scenario. Time shown below each image is time after pool ignition

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6. Temperature measurements of decomposing materials Biocomposites include a wide range of products for different applications, from construction or insulation panels to plastic products based on polymers. The composition boards, including particle boards and fibre boards, especially medium-density fibreboards (MDF) belong to the most common materials for construction, furniture, and interior decoration. The application of these materials, which are relatively new, is constantly growing in the field of furniture manufacturing. The wide range of use of these construction materials in our surrounding is motivated for economical reasons, however, questions rise about safety aspects especially in the case of a flame spread hazard. Much research is going on today to study the materials physical properties, their decomposition in a low oxygen concentration environment such as flame spread scenario. Measurements of particle surface temperature and mass loss during pyrolysis could provide modellers with precise information on the kinetics properties during decomposition, e.g. the activation energy and Arrhenius factor. Surface temperature constitutes an important parameter in models that determine the heat flow, in or out of the solid, and the material ignition temperature. Since surface temperature is at the boundary between the gas and the solid phase, it is a critical parameter for fire modellers. Many attempts have been made to characterise and measure this parameter as accurate as possible. In Figure 11 is shown an experimental set-up used for analyses of surface measurements during pyrolysis.

Fig. 7 A reactor filled with nitrogen gas was used to study the pyrolysis of wood particles. After the excitation with 266 nm, the phosphorescence emission, from the wood particles, was collected by a spectrograph and a photomultiplier.

The surface temperature of individual wood particles was measured inside a high temperature reactor using thermographic phosphors which allowed remote, instantaneous measurements. An laser beam was used to excite the phosphor particles which were deposited on the investigated materials. The emission from the thin coating of phosphor powder was spectrally resolved. The material chosen was birch particles; the pyrolysis of birch particles was thus investigated at a reactor temperature of 733 K. The reactor was stabilized at a constant temperature before the particle was introduced. Then the surface temperature of the particle was measured.

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In Figure 12, results from these measurements are presented.

723

673

623 Temperature( K )

573 0 100 200 Time(seconds)

Fig. 12 Using the ratio technique, the surface temperature during pyrolysis was measured. The bump seen in the figure was taught to come from the drying phase (water evaporation). After 100 seconds, the surface temperature was seen to converge to the reactor temperature.

The measured surface temperature increased to reach a temperature slightly lower than 733 K. It was noticed at a time duration between 50-100 seconds after the introduction of the particle in the reactor, an evaporation phase of the water contained inside the particles occurred. This evaporation phase decrease the surface temperature and thus could alter the pyrolysis rate. From these tests, interest increased to investigate other materials in a wet and dry phase. In a recent work some decomposing materials such as low-density fibre board, medium board, particle board and polymethylmethacrylate (PMMA) were studied along with their mass loss rate.

7. Internal combustion engine valve temperature measurements Thermographic phosphors thermometry have also been used to measure engine valves and transparent piston temperatures in two dimensions as well point wise of a running, optically accessible, gasoline direct injection engine. The engine, fuelled with isooctane, was operated in continuous and skip-fire mode at 1200 and 2000 rpm. A calibration of the phosphorescence lifetime and spectral properties against temperature allowed temperature measurements between 25 and 600°C. Results from the measurements show the potential of the technique for two-dimensional mapping of engine walls, valves and piston temperatures inside the cylinder. The work presented was performed in an optical direct-injected stratified-charge (DISC) engine. The engine is based on a AVL 528 engine with a Volvo 4-valve gasoline cylinder head. A schematic experimental set-up is presented in Figure 13 showing laser beam alignment and signal collection, as used in the spectral method. For the two-dimensional measurements, a CCD-camera was used together with an imaging stereoscope. As described above the imaging stereoscope allows for two color operation via image doubling. The stereoscope was mounted through an adapter on the objective lens of the ICCD camera.

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Valves

Quartz Piston Stereoscope + Filters Spherical lens ICCD

266 nm

Mirror

Figure. 13 The experimental setup for the case of the spectral method, showing the optically accessible engine (to the left) and the CCD-camera with the stereoscope and filters (to the right).

Images (single-shot) of the temperature of the four valves are presented in Figure 114. Each image was recorded at a certain time after start of the engine. As can be seen, the temperature of the exhaust valves started to increase earlier than the temperature of the intake valves. This is due to the burned gases heating the exhaust valves while leaving the combustion chamber. On the contrary, the intake valves were cooled by newly introduced air. From the figures, one could notice that the temperature started usually to rise from the edges of the valves to the center. After 120 s, high heat conduction from the exhaust to the intake valves could be seen. In several tests achieved on the valves, the average temperature is stable, but the temperature distribution was observed to be usually higher in the surrounding edge of the valves.

After 150 seconds, some islands of low temperatures appeared on the exhaust valves. This could be related to soot deposits on the coated surface or on the piston. These deposits could decrease the strength of the transmitted signal and thus increase the errors of the processed temperature images.

Deg. C

500

400 10 s 50 s 70 s 80 s 300

200

100

100 s 120 s 150 s 180 s Figure 14 Temperature images of the valves at different times after start of the engine. Intake valves are seen in the upper part and exhaust valves in the lower part. The laser was fired 30 CAD BTDC. The engine was run at 2000 rpm

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8. Droplets and spray measurements. Sprays are crucial in a number of different industrial areas today, such as combustion and ink jet printing. In particular the development of internal combustion engines to comply with tightening emission legislation has led to an increased interest in sprays. To be more efficient in this work, modelling plays an important role. However, the models for in-cylinder spray break up, atomization, and vaporization are still not satisfactorily describing/predicting several situations. To further develop the models, diagnostic tools are needed, e.g. for studying droplet velocity, droplet size distribution, and droplet temperature. Laser-induced phosphorescence from thermographic phosphors, seeded to distillate water and iso-octane, has been shown to measure temperatures of single falling droplets. The phosphors were excited by the fourth and third harmonics of a Nd:YAG laser, the subsequent emission was evaluated by spectral and temporal investigations of the thermographic phosphors Mg4FGeO6:Mn and La2O2S: Eu, respectively. The spectral and the temporal methods allowed temperature measurements of free falling droplets up to 433 K. Results from both methods are presented with an estimated accuracy of better than 1%.

In order to demonstrate this technique, the method was applied to mono-dispersed droplets. A quantity of phosphors around 1 % (by weight) was added to the investigated liquid. The experimental set-up is shown in Figure 15. A He-Ne laser was used to monitor the passage of the falling droplets. Whenever a droplet crossed the continuous beam a trigger signal was sent to the Nd:YAG laser. A UV beam with a diameter of 10 mm was then sent towards the droplet at the correct spatial position. The laser beam was not focused on the droplet in order to cover the entire droplet with light

Fig. 15 Experimental set- up: A He-Ne laser was used to trigger the acquisition of the phosphorescence light due to the interaction of the UV laser light with the droplet. The phosphorescence signal was stored in a detector for subsequent processing.

For one point temperature measurements the subsequent emission from the droplet was detected by a photomultiplier detector or a spectrograph for the temporal and the spectral measurements, respectively. The temperature of the liquid could be regulated using heating wires around the liquid container. Also 2D temperature measurements could be made and in Figure 16 is shown a single-shot temperature measurement of a droplet.

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Figure 16. Two dimensional temperature measurement using phosphorescence from thermographic phosphor particles seeded to the liquid.

9. Conclusions Thermographic phosphors are successfully used for temperature measurements of reactive, non-reactive surfaces and in droplets. Special interests towards the exploitation of the technique for temperature measurement of internal combustion engines components, gas phase and the liquid phase e.g. droplets, and sprays are driving the investigation further.

Reference

1. S.W. Allison, G.T. Gillies, Remote thermometry with thermographic phosphors: Instrumentation and applications, Review of Scientific Instruments 68 (7), 2615-2650 (1997).

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