Simultaneous Two-dimensional Temperature and Velocity Measurements in a Gas Flow Applying Thermographic Phosphors

Transcription

1 Simultaneous Two-dimensional Temperature and Velocity Measurements in a Gas Flow Applying Thermographic Phosphors Gordana Jovicic 1, 2, Lars Zigan 1, 2,*, Sebastian Pfadler 1, 2, 3, Alfred Leipertz 1, 2 1: Lehrstuhl für Technische Thermodynamik (LTT) Univeristät Erlangen-Nürnberg, Germany 2: Erlangen Graduate School in Advanced Optical Technologies (SAOT), Univeristät Erlangen-Nürnberg, Germany 3: now at Siemens, Fossil Power Generation Division, Mülheim an der Ruhr * correspondent author: Lars.Zigan@cbi.uni-erlangen.de Abstract A new measurement technique was examined for two-dimensional gas-phase temperature and velocity determination. Due to the strong influence of the activator and its concentration on the phosphor emission behavior, two phosphor types were investigated (yttrium aluminum garnet doped with dysprosium and yttrium aluminum garnet doped with dysprosium and erbium). Calibration was carried out in a heated air jet. The Dy:YAG and Dy:Er:YAG were tested for the same temperature and gas flow velocities range. Phosphor powder with an average particle size of 2 µm was seeded into the heated air flow where it was excited by two pulsed Nd:YAG lasers operated at wavelengths of 355 nm for laser- induced phosphorescence and 532 nm for particle image velocimetry measurements. The spectral intensity ratio method is applied for thermometry. For the inlet temperatures up to 573 K an average inaccuracy of less than 5% could be obtained. The Dy:Er:YAG showed an improved accuracy and precision due to its increased temperature sensitivity and integral phosphorescence emission. Additionally, for the first time simultaneous singleshot temperature and velocity measurements are presented using thermographic phosphors. 1. Introduction For the evaluation of internal combustion engine processes, gas turbines, industrial furnaces etc., knowledge of the process temperature is of great importance. Since all mentioned environments are usually problematic for the application of conventional measurement techniques, laser-based methods are gaining increasing relevance. One such technique is laser-induced phosphorescence which uses thermographic phosphors as tracers for the temperature determination. Thermographic phosphors are special materials consisting of a ceramic host matrix doped with the lanthanide ions. When exposed to an ultraviolet light source they are emitting radiation. The emission comes from the luminescent core (lanthanide ions) called activator. The emitted phosphorescence signal is strongly temperature dependent and this property is utilized for thermometry. Phosphors can be configured according their emissions that are changing in lifetime (emission decay time) and intensity over the temperature range. The major influence on the emission behavior of phosphors has the concentration and type of activator implemented in the host material. This will affect the intensity of the spectral distribution, decay time and the temperature dependence of the emission [1]. Thermographic phosphors have increasingly been applied in the past few decades for measuring the temperature of surfaces. For this the surfaces are prepared with phosphor coatings [5]. The range of temperatures that can be covered is very broad and depends on the phosphor type [8-10]. The method is also applicable for high temperature conditions. Usable signals were detected for temperatures higher than 1000 o C [3, 4]. The phosphor thermometry has been successfully applied in sprays [2, 15] and in the gas phase [14]. Hasegawa et al [7] reported on the experiments which aimed to validate the gas temperature in a steady flow - 1 -

2 seeded with Dy:YAG particles and applied the technique in a proof of principle study inside an internal combustion engine. Yu et al. [17] reported on the consistency of the Dy:YAG emission temperature function in a combustion environment. Additionally, due to the strong red shift of the signal, relative to the irradiated laser wavelength, the measurements are not sensitive to presence of impurities in the gas samples, such as dust particles or droplets which are often the limiting factor for most of the laser-based techniques. Phosphor particles can also be used for particle image velocimetry (PIV) measurements in the gas phase, offering the possibility of simultaneous velocity and temperature determination. This was demonstrated for average temperature and velocity fields by Omrane et al [14]. For mapping a two-dimensional velocity field with high spatial resolution, PIV is a well-established technique. To realize the reliable velocity measurements, a certain quality of the seeding is needed. It is considered that approximately 20 particles per interrogation area are required. The particle size should be 1 µm or smaller to follow the flow properly [12]. However, with the smaller particle size the phosphorescence intensity decreases, therefore a compromise between the emission efficiency and particle flow behavior must be found. 2. Theoretical background The host crystal lattice of thermographic phosphor has in rare cases the luminescent properties and is only the carrier for the lanthanides [6]. The lanthanides have typically not fully occupied 4f electron shell, which is located within the fully occupied 5s and 5p orbital, being in this way completely shielded from the environment. Due to the strong shielding of the rare earths 4f subshell, the interaction with the host crystal remains very low. For this reason, the resulting radiative transitions are producing very sharp lines in the emission spectrum. The electrons associated with lanthanide ions are absorbing radiation and are excited to higher energy levels. While returning to the ground state, energy is released as a vibrational relaxation or luminescence, see Fig.1. Fig. 1. Schematic representation of the simplified energy-level diagram of Dy:YAG. The emission behavior of the phosphors is strongly dependent on the type of the used activator. According to Cates et al. [3] the dysprosium activator has high quantum efficiency which is greatly influenced by its concentration. With an increase of the activator concentration in the host crystal, the probability of a non

3 radiative energy transfer due to collision processes grows and thus leads to lower emission intensity. This behavior is referred to as concentration quenching. For this reason, the concentration of the activators located in the luminescent materials is in the range of a few percent [1]. Furthermore, there are substances known as sensitizers, which can give additional energy to the activator [6]. In the present work two phosphor types were investigated, Dy:YAG and Dy:Er:YAG, using Dysprosium and Erbium as activator or sensitizer, respectively. For the determination of temperature, different approaches can be applied. Typically measured physical variables are intensity, life time (i.e. phosphorescence decay time) and line shift of selected spectral features. In the present work, the intensity ratio method was applied for temperature calculation [5]. After being irradiated with the laser source of 355 nm wavelength, the activator Dy 3+ is excited to a higher electronic energy state n 2 from which the radiative and non-radiative transitions to the 4 I 15/2 and 4 F 9/2 levels are taking place. The 4 F 9/2 energy level is a stable state from which emissions at 497 nm occur and from the 4I 15/2 level the wavelength of 458 nm is emitted, see Fig.1. The emission intensity I i from these two states can be calculated from I ( = Ae hce kt i ) i, (1) where A represents the calibration coefficient that depends on the characteristics of the used equipment in the experiment and also on the phosphor type; h = 6.63 x Js is the Planck constant; c = 3 x 10 8 m/s is the speed of light; E i is the energy of the emitting state i ( 4 I 15/2 and 4 F 9/2 ) and k = 1.38 x J/K is the Boltzmann constant. With the increase of temperature, the occupation of the 4 I 15/2 level increases significantly and its emissions are detectable and can be used for the measurements. The ratio R between the two emission intensities can be calculated from R = I I 458nm 497nm ( = Be hcδe kt i ), (2) where E i is the energy difference between the levels 4 I 15/2 and 4 F 9/2 and B is a calibration coefficient, which has to be determined experimentally. From Eq.2, the corresponding temperature can be calculated. Analogous, the signal intensity ratio can be considered for broader wavelength ranges, concentrated around peaking intensity emission lines (458 nm and 497 nm). 3. Experimetal Characterization of phosphors Samples of Dy:YAG and Dy:Er:YAG phosphor powder with an average particle size of 2 µm were heated in an oven and excited by the third harmonic of the Nd:YAG laser, with a pulse length of 7 ns and a repetition rate of 10 Hz. A thermocouple (type S, shielded for avoiding the radiation effects, with an inaccuracy of ± 6 K) was placed in the vicinity of the phosphor sample. The testing range was up to Τ = 1473 K, with 100 K increments. The laser beam was focused into the sample through an optical access of the oven (in Fig.2 the oven door is opened for displaying the beam path and the phosphor sample). The - 3 -

4 consequent phosphorescence emission was focused to the spectrometer, see Fig.2, for characterizing the temperature dependent phosphorescence spectrum and selecting appropriate optical filters for the 2D gasphase measurements. Fig.2. Experimental setup for temperature calibration in an oven. The resulting spectra for several temperature values are shown in Fig.3.(a,b,c). With the increase of temperature it is noticeable that the intensity of the detected phosphorescence signal from Dy:Er:YAG phosphor is getting significantly higher compared to Dy:YAG (laser power was kept constant, 90 mj/pulse), but showing emission lines at the same wavelength positions. Fig.3. Phosphorescence spectra at different temperatures (a, b, c) and signal intensity ratio-temperature dependence (d)

5 The signal intensity ratio is more pronounced for Dy:Er:YAG, see Fig.3d. The larger slope leads to a higher temperature sensitivity and together with the increased signal strength of Dy:Er:YAG the overall measurement accuracy of the two-dimensional thermometry in the hot gas flows can be increased. Based on the demonstrated behavior and in order to perform measurements with sufficient signal strength in the gas phase, two broad temperature sensitive areas were chosen for calculating the intensity ratio ( blue region nm and red region nm) Temperature measurements in the gas phase For achieving higher measurement accuracy, the calibration of the phosphors is conducted in a heated gas flow. Phosphor particles are seeded in the air and heated within the heating tube. To generate the jet, a tube with the exit diameter d = 7 mm was used. The temperature of the gas was varied with 50 K increments from 293 K up to the maximum value of T 0 = 573 K, measured with the thermocouple which was placed 15 mm below the tube outlet. In the tube axis at 20 mm height an additional measurement location was considered for calibration. Used thermocouples were K-type with an inaccuracy of ± 3 Κ. Depending on the adjusted temperature, the calculated range of Reynolds numbers was Re = Above the tube outlet, a laser sheet of 50 mm height and thickness of ~ 500 µm was formed. The laser energy was 90 mj per pulse. Two Andor i-star ICCD cameras (with 734 Gen II and 734 filmless Gen III intensifiers), mounted on the Scheimpflug adapters for observing the same measurement plane, were used for the detection of the phosphorescence signal, see Fig.4. A selection of corresponding wavelength regions of nm and nm, as identified in the oven calibration was realized with sets of optical filters SP475/SP650/LP355 and SP500/LP475/LP355 for blue and red region, respectively. Fig.4. Experimental setup for measurements in the gas-phase. Series of 50 image pairs, with the spatial resolution of 100 µm/pixel, were recorded for each tested temperature. Background and dark current noise were subtracted. To remove possible errors on the images which are caused by variations in the pixel-to-pixel sensitivity of the cameras CCD chips or by some potential distortions in the optical paths, flat field correction [13] was performed. A 10x10 pixel condition binning was applied and intensity ratios for single-shot images were calculated. Intensity ratios were considered only for pixels with intensity greater than a defined threshold level for the signals on both cameras. To minimize the effects of shot noise, the single images of each pair were numerically filtered with the use of a median filter, with the filtering region of interest (ROI) which had a size of 5x5 pixels

6 After averaging the intensity ratio images, mean temperature fields are derived and calibration curves are created, see Fig.5. Fig.5. Calibration curves for Dy:YAG and Dy:Er:YAG. The calibration curves of the two phosphor types behave very similar. The slope of the Dy:Er:YAG is slightly steeper as for the investigations of the phosphor powder in the oven. To check the obtainable accuracy of LIP thermometry with the jet calibration, temperature values calculated by applying the intensity ratio method were compared to the readings of thermocouple T tc positioned in the ROI marked with the red rectangle in the LIP images, see Fig.6. The ROI had size of 5x5 mm. Temperature measured in the tube was T 0 = 573 K. Single-shot and averaged temperature fields were examined. Fig.6. Single-shot LIP images at T 0 = 573 K for Dy:YAG (a) and Dy:Er:YAG (b); Averaged temperature fields for Dy:YAG (c) and Dy:Er:YAG (d)

7 The thermocouple reading for Dy:YAG measurement was T tc = 447 K. The difference to the inlet temperature was caused with a system which was constructed for the collection of particles and mounted 30 cm above the jet. The system consisted of a hood connected to a fan and a special membrane filter bag. In this way a certain amount of ambient cold air was mixed with the jet, causing its temperature drop and chaotic behavior. The application of intensity ratio method for Dy:YAG resulted in the mean temperature value of 461 K, corresponding to a systematic error of 3.1 %. For the Dy:Er:YAG, thermocouple reading was T tc = 441 K and LIP delivered a value of 433 K, i.e. 1.8 % error. Conversely to the reasonable error values, the standard deviation of temperatures derived from the single-shot data for both phosphors is rather large, see Fig.7. This is primarily caused by the turbulent properties of the air jet and by the heterogeneous air seeding from shot to shot. Additionally, compared to the pointwise thermocouple measurements, the observed region of interest with a size of 25 mm 2 includes larger temperature gradients resulting from the mixing of cold ambient air and hot flow. The accumulated shot-to-shot standard deviation contains cyclic variations and is very large (around 17 %). The ensemble averaged data give a standard deviation of 38.5 K (8.6 %) for Dy:YAG and 19 K (4.3 %) for Dy:Er:YAG. Fig.7. Temperature histograms for Dy:YAG (left) and Dy:Er:YAG (right); White bars are representing single-shot data points and black bars are the averaged temperatures from 50 images within the selected ROI. Single-shot and ensemble-averaged temperature deviation is smaller for Dy:Er:YAG phosphor, which was expected concerning its higher sensitivity and better signal-to-noise ratio behavior. This phosphor is very promising for the thermometry and velocimetry in the gas flow Simultaneous temperature and velocity measurements A set of simultaneous temperature and velocity measurement was performed with Dy:Er:YAG for the operating point T 0 = 323 K (measured in the tube) and Re = A separate laser and camera system were used for the velocity measurements, see Fig.4. Laser beams from two lasers were formed into overlapping sheets and had approximately same thickness of ~500 µm and height of 50 mm. The energy of the PIV laser was set to 50 mj, with the pulse separation of t = 25 µs. A PCO SensiCam camera coupled with narrow band pass filter (NB 532) was used for the signal detection. The pixel resolution was 70 µm/pixel. For the evaluation of velocity, a crosscorrelation algorithm with an interrogation area of pixels was applied. The seeding density in the interrogation area was around 40 particles. Single-shot temperature and velocity field are presented in Fig

8 Fig.8. Single-shot temperature field (left) with corresponding velocity field (right). The thermocouple reading in the marked ROI was T tc = 321 K, while the application of intensity ratio method resulted in the mean temperature value of 308 K, corresponding to an error of 4.7 %. The resolved velocity field was inhomogeneous in the observed ROI, which is a potential source of uncertainty contributing to the large spatial standard deviation. Within a single-shot image, the standard deviation of temperature was around 10 % which is acceptable for single-shot measurements. Nevertheless, this exemplary image pair displays the potential of the measurement technique to resolve temperature and velocity fields, which is not possible with Raman or LIF-techniques [11, 16]. 4. Conclusions The feasibility of simultaneous planar temperature and velocity measurements in the gas phase is demonstrated using thermographic phosphors. With the used experimental system and for the tested conditions, an inaccuracy of smaller than 5 % could be obtained. The studied Dy:Er:YAG phosphor showed an improved accuracy and precision compared to Dy:YAG. In order to reduce the temperature fluctuations and to improve precision, a calibration in a flow cell or another similar system which could provide the constant test conditions should be the next step. The tested temperature range should be extended in future work, together with an optimization of both detection optics (especially spectral filters) and the seeding system, in order to increase the signal-to-noise ratio in the phosphorescence images. Acknowledgements The authors gratefully acknowledge financial support for parts of this work by the German Research Foundation (DFG) which also funds Erlangen Graduate School in Advanced Optical Technologies (SAOT) within the framework of the German Excellence Initiative. Also the Max-Buchner Foundation (Dechema e.v.) is gratefully acknowledged for support. We thank Dr. Miroslaw Batentschuk, Institute for Materials for Electronics and Energy Technology of our University, for his support and advices during the preparation of the phosphor powder. Special thanks go to the students Tobias Ziegler and Sebastian Beer for supporting the experiments and the data post-processing. References 1. Allison S.W, Gillies G.T (1997) Remote thermometry with thermographic phosphors: Instrumentation and applications. Review of Scientific Instruments 68: Brübach J, Patt A, Dreizler A (2006) Spray thermometry using thermographic phosphors. Applied - 8 -

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