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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

Characterisation of BAM:Eu2+ Tracer Particles for Thermographic Particle Image Velocimetry Benoit Fond, Christopher Abram, Frank Beyrau* Department of Mechanical Engineering, Imperial College London, UK * corresponding author: [email protected] Abstract Thermographic particle image velocimetry is a recently developed laser-based measurement technique capable of simultaneous single-shot temperature and velocity imaging in turbulent flows. The technique is based on thermographic phosphors, which are solid materials with temperature-dependent luminescence properties. Thermographic phosphor particles are seeded into the flow as a tracer. The velocity field is measured with ordinary particle image velocimetry, using laser light scattered by the phosphor particles. Following simultaneous UV excitation, these particles emit phosphorescence with a temperature dependent emission spectrum. Two spectrally filtered images of the particle phosphorescence emission are simultaneously recorded to determine the tracer temperature using a ratio-based method. Micrometre-size particles are used so that the tracer temperature and velocity matches that of the surrounding gas. Even though several different phosphors have been used as gas phase tracers, very little is known about their emission characteristics as individual particles in the gas. Data regarding the emission intensity per particle and the signal dependence on, for example, the laser fluence or oxygen concentration, are needed to develop the technique and properly assess its limitations. This paper introduces a benchmark procedure for the characterisation of phosphor particles in the gas phase, based on particle counting. By measuring the instantaneous seeding density, it allows quantification of the emission intensity per particle and the study of the influence of various parameters affecting the signal. The system is used to characterise 2 µm BAM:Eu2+ particles, a phosphor with attractive luminescence properties. Using these phosphor particles, results indicate that only a moderate seeding density of around 1011 particles/m3, similar to that used in conventional PIV experiments, is required to obtain precise temperature measurements. At this level of seeding, the particles have no effect on the gas properties. The phosphorescence signal and measured ratio are tested in pure nitrogen and air. Signal levels and measured temperatures are completely unaffected in this range of oxygen concentrations. The usable temperature range of this phosphor is also explored by measuring the drop in phosphorescence intensity with temperature. This paper demonstrates that the phosphor BAM:Eu2+ is a very suitable tracer for gas-phase temperature and velocity imaging in turbulent flows up to 950 K.

1. Introduction Optical measurement techniques will continue to have a huge impact on our understanding of complex turbulent flow phenomena. Techniques capable of simultaneously imaging velocity-scalar quantities are particularly beneficial, allowing, for example, the direct measurement of turbulent diffusion and providing invaluable insight into the interactions between the turbulent flow field and density fluctuations or reaction chemistry. Recently, a novel technique for simultaneous temperature and velocity imaging has been developed. It is based on thermographic phosphors, which are solid materials with luminescence properties that can be used for remote temperature sensing. These particles are seeded into the flow under investigation. Using a conventional particle image velocimetry (PIV) approach, visible laser light scattered by the particles is recorded to determine the velocity field. Simultaneously, a UV laser is used to excite the particles, and their temperature sensitive phosphorescence emission is recorded to determine the particle temperature using a two-color intensity ratio method. For micrometre-size particles, the particle temperature and velocity match that of the surrounding gas (Fond et al. 2012). The principle of this thermographic PIV technique has been demonstrated for time-averaged (Omrane et al. 2008), and single-shot measurements (Fond et al. 2012; Neal et al. 2013) as well as at kHz-rates for temporally resolved measurements (Abram et al. 2013). This concept is very attractive for several reasons. It requires relatively simple instrumentation in terms of lasers and cameras and only a single seeded tracer. In contrast to gaseous tracers used for laser induced fluorescence (LIF) thermometry (e.g. acetone, toluene), thermographic phosphors are inert, have a high melting point (2200K) and are in general insensitive to pressure and to the gas composition making them particularly suitable as a tracer for reacting flow applications. In recent years, there have been several studies using thermographic phosphors for gas thermometry (Hasegawa et al. 2007a; Hasegawa et al. 2007b; Omrane et al. 2008; Rothamer and Jordan 2012; Fond et al. -1-

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

2012; Someya et al. 2012; Jordan and Rothamer 2013; Abram et al. 2013; van Lipzig et al. 2013; Lawrence et al. 2013, Neal et al. 2013), and different phosphors have been used. Some important luminescence properties of these phosphors are indicated in Table 1. The authors previously used BAM:Eu2+, a phosphor with several major advantages. It has a broad excitation spectrum extending to the near UV, allowing 355 nm excitation. Its subsequent emission is fast (1 µs), in the blue region of the visible spectrum and has a high quantum yield (> 90% (Shionoya et al. 2006)). These factors suggest that BAM:Eu2+ is a promising phosphor, and indeed it has been successfully used for precise temperature measurements. However, for all the phosphors in Table 1, and other phosphors which could prove suitable for gas temperature measurements, there are no quantitative studies on particle phosphorescence in the gas-phase. This is in direct contrast with the well-understood physical processes of organic tracers used for flow measurements, and so quantitative investigations are needed to compare different phosphors and predict signal levels in practical applications. Phosphor

Lifetime

Excitation wavelength (nm)

1 ms

355

3 ms

266

50 µs

980

YAG:Pr3+

190 µs★

266

BAM:Eu2+

1 µs

355

YAG:Dy

3+

Mg4FGeO6:Mn4+ 3+

Y2O2S:Er ,Yb

★

3+

Studies Hasegawa et al. 2007a; Hasegawa et al. 2007b Omrane et al. 2008; Someya et al. 2012 Rothamer and Jordan 2012 Jordan and Rothamer 2013; Neal et al. 2013 Fond et al. 2012; Abram et al. 2013; van Lipzig et al. 2013; Lawrence et al. 2013

Lifetime of the 1D2 → 3H4 emission used in the ratio evaluation (Kamma et al. 2009). Table 1 Phosphors used in gas-phase thermometry studies

The precision of the ratio-based phosphor thermometry technique is almost entirely determined by two factors: the phosphor sensitivity in terms of the change in intensity ratio with temperature and the signal level. While the spectral change can be measured in spectroscopic studies of the bulk material in the form of an aggregated powder, other important characteristics, such as the signal emitted by each particle, cannot be determined from such an investigation because it is difficult to estimate the number of particles contributing to the luminescence intensity. In addition, because of the multiple scattering that is predominant in the bulk powder, the optical properties of aggregated powders are different to those of individual particles in a gas. Therefore, in order to obtain useful data regarding the influence of a given parameter on the signal level, phosphor particles must be investigated in the gas phase. However, when seeding solid particles it is generally very difficult to set or reproduce the seeding density (the number of particles in a given volume). Therefore, a means to directly determine the number of particles contributing to measured phosphorescence signals is required. This paper aims at establishing benchmark experimental procedures for characterising phosphors in the gas-phase. We describe a particle-counting tool based on high-resolution particle images, which is used to measure the instantaneous seeding density in order to determine the emission intensity on a particle basis in the measurement volume. This system is used to study seeded micron-sized BAM:Eu particles for gas thermometry, quantifying the phosphorescence intensity per particle and investigating the influence of several parameters such as the seeding density, excitation energy, oxygen concentration, and the gas temperature. An example of how such measurements can be used to check the sensitivity of the measured temperature to these parameters is given. The seeding density and laser energy required for precise temperature measurements using BAM:Eu2+ are also evaluated.

2. Experimental setup 2.1. Phosphor particles and test cases 2 µm BAM:Eu2+ thermographic phosphor particles (KEMK63/UF-P1, Phosphor Technology) were used -2-

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

as a tracer. The authors previously showed that 2 µm BAM:Eu particles accurately trace the gas temperature and the flow velocity (Fond et al. 2012). A SiO2 nanoparticle coating (Ludox) was applied to reduce interparticle forces and limit particle agglomeration. For the room temperature studies, a jet of air 40 mm in diameter was used. The particles were seeded into the various gas streams using a custom-built reverse cyclone seeder. The seeding density was adjusted by controlling the air velocity in the seeder, which was achieved by changing the pressure drop across a bypass. To heat up the gas, the seeded stream flowed through an 8 mm inside diameter, 2.5 m long stainless steel coil placed inside a high temperature oven. The temperature of the jet was measured using a fine 50 μm diameter type K thermocouple. With this system, it is possible to obtain exit jet temperatures up to 1200 K. To investigate possible effects of oxygen quenching on the phosphorescence emission, this rig can also be alternately supplied with air or pure nitrogen. 2.2. Particle counting The particles were illuminated with the second harmonic of 5 Hz pulsed Nd:YAG laser (Quanta-Ray GCR-150, Spectra-Physics), as shown in Fig. 1. In order to obtain high-resolution Mie scattering images of the particles, a light sheet 160 μm thick was formed in the measurement plane (see Section 2.4). A hardware binned (2 x 2) interline transfer CCD camera (Imager Intense, LaVision) equipped with a 2 X teleconverter (Tamrom AF 2X), a 105 mm f/2.8 lens with the f-stop at f/4, a 532-10 nm (notation CWL-FWHM) interference filter and an O.D. 2 neutral density filter was used to image the particles. A colour-glass (OG515, Schott) was positioned in front of the detection system at 45° to prevent interference arising from the phosphorescence light reflected by the interference filter.

Fig. 1 Experimental setup. DM dichroic mirror, SO sheet optics, GW glass window, IF interference filter, CF coloured glass filter, DBS dichroic beamsplitter

The collection optics provided a magnification of 1.12, and a field of view of 6 x 8 mm. The FWHM of the image of a single particle was less than a binned camera pixel, or 12.9 µm. The laser pulse energy was adjusted so that the peak intensities of the weakest particle signals were well above the noise threshold. This also ensured that any possible light extinction due to the presence of particles on the optical path had a negligible effect on the particle counting procedure. Particle images were then processed using Matlab in order to evaluate the number of local maxima in each image. First, a cut-off filter of 15 counts is applied. The counting algorithm then sorts pixels by descending order of intensities. The position of the maximum is marked and all pixels less than 2 pixels from this maximum were removed from the list, in order to remove the wings of high peak intensity particle images. In this way, maxima are found until all pixels have been evaluated, and the number of local maxima in each image is used as a measure of the number of particles. 2.3. Phosphorescence imaging The phosphor particles were excited using the third harmonic of a 5 Hz pulsed Nd:YAG laser at 355 nm (Quanta-Ray GCR-150, Spectra-Physics), with a pulse duration of 7 ns and a maximum pulse energy of 150 -3-

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mJ. A 680 µm thick light sheet (see Section 2.4) was formed in the same measurement plane probed by the counting camera. The phosphorescence emission from the particles was detected by two hardware-binned (4 x 4) nonintensified interline transfer CCD cameras (Imager Intense, LaVision) with 50 mm f/1.4 Nikon lenses. Two high transmission (> 80 %) interference filters at 440-90 and 464-40 nm and a 455 nm longpass dichroic plate beamsplitter, AR-coated on the reverse side, were used to exploit the temperature dependent shift and broadening of the emission line (see Fig. 2) for the ratio-based temperature measurement. The camera/beamsplitter system was carefully adjusted using micrometer stages to minimize relative distortion and differences in light collection path. The camera exposure time was set to 5 µs, beginning 1 µs before the laser pulse. The field of view of the cameras was 28 x 21 mm.

1

700K 0.5

0.5

filter transmission

emission intensity (arb. units)

1

300K

0 380

400

420

440

460 480 wavelength (nm)

500

520

540

0

Fig. 2 BAM:Eu2+ emission spectrum following 355 nm excitation, normalised to the emission band peak and recorded at 50 K intervals (Fond et al. 2012). The transmission curves of the filters used in this study are superimposed on the spectrum

The image recorded by a camera in the absence of incident light is called the dark image. An offset voltage is applied to the charge converter of the camera to prevent negative values in the digitisation. For the cameras used here, this offset is between 20 and 60 counts. However, even after the cameras were turned on and the indicated temperature reaches the set value of -12 °C, drift in this offset was identified. It was also found that the camera offset can vary by 2 to 3 counts over a recording sequence, due to the absence of temperature regulation of the chip. In order to account for such variations, images were recorded at a rate twice as high as that of the laser, so that the laser only illuminated every second frame. This allows the evaluation of the camera offset between illuminated frames in order to adjust the background subtraction on a frame-to-frame basis. A time-averaged background image was compiled from all non-illuminated images. A factor was then applied for each image subtraction, which is the ratio of the instantaneous spatial average of the intensity of each non-illuminated image to the spatial average of the time-averaged background. The background-subtracted images were then corrected for non-linearity. The linearity response of the cameras was determined using a 3-inch diameter integrating sphere (Labsphere), and a pre-calibrated detector. The irradiance on the camera chip was varied using an aperture at the sphere entrance port, which was illuminated by the emission of a BAM:Eu2+ phosphor pellet following laser excitation at a constant pulse energy. This arrangement covered a suitable range of irradiance on the chip during the short exposure time used in the actual measurements. The uncertainty in the camera calibration is 2 %. A 7 x 7 moving average filter was applied to the images for a final resolution of 600 µm, as measured using a USAF target. Image pairs were divided, and resulting ratio images were themselves divided by an average ratio image obtained at room temperature to correct for non-uniformities in the light collection efficiency. No software mapping was applied before division of the images.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

2.4 Laser energy and sheet profile measurements To accurately size and match the probe volumes, the laser sheet profiles were measured. The 532 and 355 nm beams were overlapped in the measurement plane using long-pass dichroic mirrors. Cylindrical lenses and Galilean telescopes were used to form light sheets and control their thicknesses independently. The laser sheets were reflected using a glass window with an AR coating on the reverse side onto a thin piece of paper in a position that matched the window - measurement volume distance. This paper was imaged using a CCD camera equipped with an 85 mm Nikon lens. The resolution of the imaging system was checked using a USAF target. The images of the sheets reflected from both the front and the back surface of the glass window were found to have the same FWHM thickness, confirming the homogeneity and linear character of the paper fluorescence. A photodiode-based energy-monitoring unit (Energy monitor V9, LaVision) was used to measure the energies of the lasers on a shot-to-shot basis. This system was pre-calibrated using a pyroelectric energy detector (PEM-45K, Radiant Dyes).

3. Results and discussion 3.1. Seeding density To quantify the mean phosphorescence intensity of single 2 µm BAM:Eu2+ particles, the number of particles present in the probe volume was measured using the particle counting system described above. The seeding density was varied while phosphorescence and Mie scattering images were simultaneously recorded simultaneously. Fig. 3 shows the variation of the single-shot single pixel temperature error with the seeding density, measured using an excitation fluence of 23 mJ/cm2. Since the collected phosphorescence intensity is proportional to the seeding density, the signal to noise ratio also increases with the seeding density and the measurement precision improves. For reference, a 10 K deviation corresponds to a 2.4 % change in the normalised ratio. For PIV measurements, the presence of at least 15 particles on average in each interrogation area is recommended (Keane and Adrian 1990), and therefore a minimum seeding density of 1011 particles/m3 is needed for a resolution of 600 µm. With these phosphor particles, the same seeding density generates enough signal to maintain a temperature precision better than 15 K between 300 and 600 K. These measurements permit estimation of the effect of the particles on the bulk gas properties. For a particle number density of 2 1011 particles/m3, the heat capacity of the gas is increased by only 0.2 % at 300 K and by 0.5 % at 1100 K. Based on particle-fluid thermal conductivity calculations proposed by Maxwell (Maxwell 1873), at this level of seeding, the particles have no effect on the thermal conductivity of the gas due to the very little volume fraction occupied by the particles. These results show that these particles have a negligible effect on the gas properties. Phosphors with a longer lifetime or lower quantum yield would require considerably higher seeding densities, increasing the heat capacity of the gas, and possibly quenching chemical reactions.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

temperature precision (K)

25

20

15

10

5

0

0

0.5

1

1.5

2

2.5

seeding density (particles/m3 x 1011)

Fig. 3 Temperature precision as a function of the seeding density

3.2. Excitation fluence The dependence of the phosphorescence intensity on the laser fluence is shown in Fig. 4. The phosphorescence signal departs from the linear regime, and the increase in phosphorescence intensity with the laser fluence is much lower than expected. Using the developed particle counting system, it is possible to characterise this phenomenon with accuracy, as fluctuations in seeding density can be accounted for with improved sensitivity. Below 2 mJ/cm2, the phosphorescence intensity is a linear function of the laser fluence. Above this value its rate of increase with laser fluence gradually drops to finally reach a constant value above 10 mJ/cm2. phosphorescence intensity (arb. units)

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

0

10

20

30

40

50

2

fluence (mJ/cm )

Fig. 4 Phosphorescence emission intensity with increasing laser fluence

3.3. Temperature and oxygen partial pressure Simultaneous measurements of phosphorescence intensity and seeding density were performed in the heated jet configuration described above in order to determine the evolution of the intensity per particle with temperature. As shown in Fig. 5 (left), at 920 K the particle phosphorescence intensity is 7.5 % of the room temperature intensity. If the drop in gas density is also accounted for, then the drop in phosphorescence intensity per unit volume of gas can be determined, as shown in Fig. 5 (right). At 920 K, the intensity per unit volume is 2.5 % the room temperature value. This range of intensities is still within the dynamic range -6-

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

1

intensity per unit volume (arb. units)

intensity per particle (arb. units)

of the camera (12 bits). Maintaining a precision of 25 K at 900 K would require 4 1012 particles, increasing the gas heat capacity by 4 %.

air nitrogen

0.8 0.6 0.4 0.2 0

400

600

800

1

nitrogen 0.8 0.6 0.4 0.2 0

1000

air

400

temperature (K)

600

800

1000

temperature (K)

Fig. 5 Intensity per particle (left) and intensity per unit volume (right) as a function of temperature obtained in heated streams of air or pure nitrogen

Some phosphors, e.g. Y2O3:Eu3+ were reported to be sensitive to the gas composition for reasons that remain unclear (Brubach et al. 2007). Here, the sensitivity of the phosphorescence intensity to the concentration of oxygen was investigated by measuring in pure nitrogen. The results are also shown in Fig. 5. The presence of oxygen up to atmospheric concentrations does not have any effect on the phosphorescence intensity. The deviations between the two cases are within the experimental uncertainty. This is a significant advantage of this tracer over most LIF tracers, which are strongly quenched by oxygen (Schulz and Sick 2005).

3.4. Investigation of possible preferential attenuation effects If the excitation and emission spectra of the thermographic phosphor tracer particles overlap, reabsorption of phosphorescence light can occur, and the transmitted phosphorescence spectrum may be dependent on the number density since part of the spectrum will be more extinguished. This is often true for laser dye molecules. The dependence of the measured temperature on the number density of particles will depend on the spectral overlap, on the absorption cross-section and on the optical path through the seeded flow of interest. In addition, the Mie scattering cross section is wavelength-dependent, and for particles with broad emission spectrum, extinction caused by elastic scattering can alter the transmitted emission spectrum. For BAM:Eu2+, there is a small overlap between excitation and emission spectrum in the region from 400 to 425 nm (Mishra et al. 2002). The effect of the seeding density on the measured temperature was quantified using the two-camera phosphorescence detection and particle counting systems. Results are presented in Fig. 6. The seeding density has no effect on the measured temperature over the indicated range of seeding densities, for a seeded flow above a 40 mm diameter nozzle. A slight decrease can be observed but given the 2 % uncertainty in the linearity of the detector used for the CCD camera calibration, non-linearity can cause a maximum deviation of up to 20 K over the dynamic range of the camera.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

350 340

temperature (K)

330 320 310 300 290 280 270 260 250

0

0.5

1

1.5

2

2.5

seeding density (particles/m3 x 1011)

Fig. 6 Measured temperature as a function of the seeding density

3.5 Sensitivity to oxygen partial pressure and temperature Finally, the sensitivity of the phosphor to the oxygen concentration was investigated over the 300 - 950 K temperature range. The results are presented in Fig. 7. As with the absolute intensity, the intensity ratio response is also unaffected by oxygen concentrations over the tested range. Another study has shown that this phosphor is completely insensitive to pressure up to 1000 bar (Turos-Matysiak et al. 2007). Therefore, this thermometry technique can be used in flows with significant pressure or oxygen concentration variations. With the filter combination used in this study, the sensitivity of the phosphor to temperature decreases after 800 K. This can be attributed to the filters, which do not integrate the blue broadening of the emission spectrum in the region below 395 nm. As an alternative, a 370 nm long-pass filter can be used on the reflection camera, offering a good level of sensitivity at higher temperatures. 0.9 0.85

intensity ratio

0.8 0.75 0.7 particles in air

0.65

quadratic fitting 0.6

particles in nitrogen

0.55 0.5

300

400

500

600

700

800

900

1000

temperature (K)

Fig. 7 Intensity ratio as a function of temperature obtained in a heated stream of air and of pure nitrogen

4. Conclusions A particle counting system was developed which can be used to directly measure the particle seeding density, allowing quantitative measurements of signal levels from phosphor particles in the gas phase. 2 μm BAM:Eu2+ particles were characterised. Results show that with this phosphor, a seeding density comparable to that used in conventional PIV experiments (1011 particles/m3) provide sufficient signal for -8-

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

precise temperature measurements in turbulent flows. Absolute measurements of the particle seeding density also allow estimation of the effect on the bulk gas. These results show that at these seeding densities, the particles have no effect on the gas thermal properties. The phosphorescence intensity is linearly dependent on the seeding density and independent of the oxygen concentration in the gas. These two parameters have no impact on the intensity ratio and therefore on the measured gas temperature. A non-linear dependence of the emission intensity on the laser fluence was found above 2 mJ/cm2. Comparable signal levels can be achieved using the low-energy pulses of high-speed lasers and the technique has been extended to kHz rates, for time-resolved temperature and velocity imaging (Abram et al. 2013). The temperature range of this phosphor was explored by measuring the evolution of the phosphorescence intensity. These measurements show that the temperature range is limited to 950 K by the thermal quenching of the phosphorescence emission. There are an infinite number of possible phosphors with different sensitivities and covering different temperature ranges, so a phosphor can be chosen for a specific application. Other phosphors currently used for gas thermometry must be characterised in a similar fashion, to ensure that excessive seeding densities are not required to achieve a reasonable precision and to check cross-dependencies on other parameters. These experiments describe simple tools that use the same hardware as a typical thermographic PIV setup, which should be used for the study of promising phosphors in order to extend the capabilities of this promising measurement technique.

References Abram C, Fond B, Heyes AL, Beyrau F (2013) High-speed planar thermometry and velocimetry using thermographic phosphor particles. Appl Phys B-Lasers O 111:155-160. Brübach J, Dreizler A, Janicka J (2007) Gas compositional and pressure effects on thermographic phosphor thermometry. Meas Sci Technol 18:764-770. Fond B, Abram C, Heyes AL, Kempf AM, Beyrau F (2012) Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles. Opt Express 20:2211822133. Hasegawa R, Sakata I, Yanagihara H, Särner G, Richter M, Aldén M, Johansson B (2007a) Twodimensional temperature measurements in engine combustion using phosphor thermometry. SAE Paper 2007–01–1883:1797–1803. Hasegawa R, Sakata I, Yanagihara H, Johansson B, Omrane A, Aldén M (2007b) Two-dimensional gasphase temperature measurements using phosphor thermometry. Appl Phys B-Lasers O 88:291-296. Jordan J, Rothamer DA (2013) Pr:YAG temperature imaging in gas-phase flows. Appl Phys B-Lasers O 110:285-291. Kamma I, Kommidi P, Reddy BR (2009) High temperature measurement using luminescence of Pr3+ doped YAG and Ho3+ doped CaF2. Phys Status Solidi C 6:S187-S190. Keane RD, Adrian RJ (1990) Optimization of Particle Image Velocimeters .1. Double Pulsed Systems. Meas Sci Technol 1:1202-1215. Lawrence M, Zhao H, Ganippa L (2013) Gas phase thermometry of hot turbulent jets using laser induced phosphorescence. Opt Express 21:12260-12281. Maxwell J (1873) A treatise on Electricity and Magnetism. The Clarendon Press, Oxford. Mishra KC, Raukas M, Ellens A, Johnson KH (2002) A scattered wave model of electronic structure of Eu2+ in BaMgAl10O17 and associated excitation processes. J Lumin 96:95-105. Neal NJ, Jordan J, Rothamer D (2013) Simultaneous Measurements of In-Cylinder Temperature and Velocity Distribution in a Small-Bore Diesel Engine Using Thermographic Phosphors. SAE Int J Engines 6:300-318. Omrane A, Petersson P, Aldén M, Linne MA (2008) Simultaneous 2D flow velocity and gas temperature measurements using thermographic phosphors. Appl Phys B-Lasers O 92:99-102. Rothamer DA, Jordan J (2012) Planar imaging thermometry in gaseous flows using upconversion excitation of thermographic phosphors. Appl Phys B-Lasers O 106:435-444. Schulz C, Sick V (2005) Tracer-LIF diagnostics: quantitative measurement of fuel concentration, temperature and fuel/air ratio in practical combustion systems. Prog Energ Combust 31:75-121 -9-

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Shionoya SY, W. M. Yamamoto, H. (2006) Phosphor Handbook. second edn. CRC Press. Someya S, Okura Y, Uchida M, Sato Y, Okamoto K (2012) Combined velocity and temperature imaging of gas flow in an engine cylinder. Opt Letters 37:4964-4966. Turos-Matysiak R, Grinberg M, Wang JW, Yen WM, Meltzer RS (2007) Luminescence of BAM under high pressure: the Eu2+ sites. J Lumin 122:107-109. van Lipzig JPJ, Yu M, Dam NJ, Luijten CCM, de Goey LPH (2013) Gas-phase thermometry in a highpressure cell using BaMgAl10O17:Eu as a thermographic phosphor. Appl Phys B-Lasers O 111:469-481.

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