Comparison between phase-Doppler anemometry and

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Comparison between phase-Doppler anemometry and Powered By Docstoc
					   Comparison between phase-Doppler anemometry and
  shadowgraphy systems with respect to solid-particle size
               distribution measurements
   Ralf Kapulla*, Mathias Trautmann** , Salih Güntay* , Abdel Dehbi* and
                             Detlef Suckow*
         *
             Laboratory for Thermal Hydraulics, Paul Scherrer Institut, 5232 Villigen, Switzerland
                **
                   Stusdsvik SINA Industrieservice GmbH & Co. KG, Hauptstrasse 1B, 76297
                                    Stutensee-Blankenloch, Germany




                                                  Abstract
             Simple experiments with 5 different batches of certified Latex spheres covering the range
         from 5 to 70 µm dispersed in water have been conducted in order to compare PDA and
         shadowgraphy systems with respect to the measured mean diameter and the size distribution.
         It was found that all the PDA-measured mean diameters agreed with the reference diameters
         within ±6 % while the shadowgraphy-measured diameters agreed to better than ±5 %.
         However, to achieve these PDA results, it was necessary to heavily correct the input-off-
         axis angle used from DANTECs BSA flow software to calculate correct phase conversion
         factors while the physical off-axis angle was kept constant. This off-axis angle correction
         renders subsequent particle flux calculations with DANTECS software impossible. All of
         the PDA-measured distributions are considerably broader than the shadowgraphy-measured
         distributions; the latter ones coincide for larger diameters very well with the ones guaranteed
         by the manufacturer.

1 Introduction
In order to determine the decontamination factor for different aerosol species in a steam gen-
erator (SG) in the framework of the ARTIST project (Güntay et al. 2004), the aerosol size
distribution has to be determined within the SG components. Consequently the available opti-
cal measurement systems, i.e. a phase Doppler anemometer (PDA) and a shadowgraphy system
need to be inter-compared. Very simple experiments with 5 different batches of certified Latex
spheres covering the range from 5 to 70 µm dispersed in water have been conducted in order to
compare the measured size distribution of both systems with the manufacturer-guaranteed size
distribution.

2 Experimental-Setup
Both measurement series for PDA and shadowgraphy were carried out in a small, square Per-
spex pool, with a wall thickness of 2 mm, an edge length of 80 mm and a height of 80 mm.
For every measurement the clean pool was filled with 250 ml of demineralized water and a
defined amount of latex spheres were injected by means of a pipette. To avoid sedimentation
of the spheres at the bottom of the pool, a magnetic stirrer was used to generate a vortical flow.
Characteristics of the five different batches of particles with the certified nominal mean diame-
ter, dm , the nominal standard deviation, σ m , together with the measurement system used by the
manufacturer to determine these values, can be found in Table 1.
     particle manufacturer             label          dm [µm]      σ m [µm]            measured with

     micro particles GmbH         P S − ST − 5.0       5.045         0.078        COULTER Multisizer III
     BS-Partikel GmbH               T e1000 − 25        10.4          0.2        Single optic particle sizing
     micro particles GmbH         P S − ST − 30.0      29.63         0.35         COULTER Multisizer III
     micro particles GmbH         P S − ST − 50.0      48.9           0.6         COULTER Multisizer III
     micro particles GmbH         P S − ST − 70.0      70.6           1.2      Olympus light microscope, with
                                                                                  image analysis software

Table 1: Five different batches of certified Latex spheres used for the measurements. Information
 according to the manufacturer.



                                                     The phase-Doppler measurements have
                                                 been performed by means of three-detector,
                                                 standard DANTEC PDA which is pictured
                                                 in Fig. 1. The operation principle of the
                                                 PDA can be found elsewhere, (Albrecht et
                                                 al. 2003). The parameters of the PDA set-up
                                                 are given in Table 2.
                                                     Under certain circumstances, particle size
                                                 measurements using a PDA instrument can
Figure 1: Optical arrangement of a three-detec- be erroneous due to the so-called ’Gaussian
 tor, standard PDA. Picture adopted from (Al- beam effect’, see (Araneo et al. 2000). A
 brecht et al. 2003)                             general rule of thumb recommends applying
                                                 standard PDA techniques only up to particle
 diameters which do not exceed 1/3 to 1/2 of the beam waist diameter. With the probe vol-
 ume given in Table 2, ≈ 60 µm is an approximate limit for the particle diameter above which
 the Gaussian beam effects becoming important. Consequently, the Gaussian beam effect might
 become an issue for one batch (70 µm) but not below this size.
      Laser                        ArIon          −       focal length, receiver               310       mm
      wavelength                   514.5         nm       physical∗ off-axis angle Φ            30        ◦

      laser power                  20          mW/beam    aperture mask                      type A       −
      focal length, transmitter    800           mm       signal level validation               −2       dB
                                                  ◦
      intersection angle Θ/2       2.765                  high voltage level               700 − 1200    V
      beam expander ratio          1.950          −       Dominating scattering order       refraction    −
      probe volume dx              0.122         mm
      probe volume dy              0.122         mm
                                                          ∗
      probe volume dz              2.578         mm            explained in the text

                      Table 2: Optical parameters used for the measurements.


    Since operation parameters such as laser power, photomultiplier amplification voltage and
 signal gain, can significantly influence the results, i.e. the measured mean diameter and the
 broadness of the distribution, these parameters have been determined for each particle batch
 individually in the asymptotic range as recommended in (Kapulla and Najera 2006), to ensure
 that the result is independent of the operation parameters.
    The shadowgraphy system (LaVision) consists of a pulsed laser backlight and a diffuser
 which illuminate the particles passing the system between the light source and camera. The
 camera is equipped with a far-field microscope and optional magnification lenses, Fig. 2. The
 shadow-pictures , Fig. 3, consists of dark spots representing the size of the dispersed latex
 spheres. After calibration, i.e. relating the dimension of one pixel to a physical size, the shadow-
 pictures are treated with an image analysis software (DaVis from LaVision) which determines
 the diameters of the dark spots in the field of view.
                                 Lens
                                 Light diffuser
                                 Fluorescence plate                           Farfield microscope
                                                                              Magnification lense
                                                                              Camera




                                         Particles

  Figure 2: Backlight illumination and camera set-up for for the shadowgraphy system used.

                                                            To give an order of magnitude, us-
                                   30 µm                 ing a camera (PCO Sensicam qe, 12
                                                         bit) with a resolution of 1376x1040
                                                         pixel, each pixel represents a physical
                                                         area of 1x1 µm2 . Laser-based pho-
                              50 µm                      tographic recordings such as the shad-
                      5 µm
                                         Out of plane
                                                         owgraphy system under consideration
                                                         are often characterized by a high con-
                                          10 µm
                                                         trast granulation pattern which is super-
                                                         imposed on the image. Due to the ran-
                                                         dom nature of this process, small dark
                                                         and bright grains of speckles can be
                        70 µm                            found on the image plane. Since the
                                                         image of small-particle shadowgraphy
Figure 3: Example of a raw shadowgraphy picture pictures will also result in dark clus-
 taken from a mixture of all latex batches used for the ters of a few pixels, clusters originating
 current experiments.                                    from speckle patterns and those from
                                                         small particles might become indistin-
 guishable from each other, resulting in a bias of the measured size distribution towards smaller
 sizes. A second fluorescence plate mounted at the front of the diffuser reduces the background
 noise considerably.

 3 Results
 The comparison of the measured size distributions obtained by means of the PDA and shad-
 owgraphy systems for latex sphere sizes of 5, 10, 30 and 70 µm can be found in Fig. 4.P dis-
                                                                                           All
 tributions have been normalized to one. Whereas the count mean diameter D(1, 0) =            di /N
 (with i = 1, ..., N ) coincides for the PDA and the shadowgraphy system with the manufac-
 turer guaranteed value very well, see also Fig. 5, the size distributions exhibit some interesting
 features.
     Whereas the PDA-system measured size distribution is always considerably broader than
 the manufacturer-guaranteed distribution for the size range considered here, the distribution
 measured with the shadowgraphy system is only broader for the smallest diameter (5 µm). Since
 one pixel corresponds to an area of ≈ 1x1 µm2 in the fluid, the shadow image of a spherical
 particle having a diameter of 5 µm is represented by ≈ 5 dark pixels. This is definitely the
 lower limit the analysis routines as well as the optical system can handle. Therefore, although
 filtered, randomly contributing noise in the image, might add or subtract a few pixel to the dark
 pixel clusters representing the small particles, which immediately shifts the results towards
 lower or larger sizes, resulting in a broadened distribution, Fig. 4 a). The other shadowgraphy-
 measured size distributions coincide with the manufacturer distributions very well, except for
                                       nominal particle diameter 5.045 µm                                       nominal particle diameter 10.4 µm

                                 1.0                                                                      1.0

                                 0.8                                                                      0.8



                normalized [-]




                                                                                         normalized [-]
                                 0.6                                                                      0.6
                                                                   Manufacturer
                                                                   PDA
                                 0.4                               Shadowgraphy                           0.4                             Manufacturer
                                                                                                                                          PDA
                                                                                                                     A                    Shadowgraphy
                                 0.2                                                                      0.2

                                 0.0                                                                      0.0
                                       0          5          10        15         20                            0         10         20        30         40
                                                       diameter [µm]                                                           diameter [µm]               b)
                                                                                  a)
                                       nominal particle diameter 29.63 µm                                        nominal particle diameter 70.6 µm

                                 1.0                                                                      1.0                              Manufacturer
                                                                                                                                           PDA
                                                                                                                                           Shadowgraphy
                                 0.8                                                                      0.8




                                                                                        normalized [-]
              normalized [-]




                                                                  Manufacturer
                                 0.6                              PDA                                     0.6
                                                                  Shadowgraphy

                                 0.4                                                                      0.4

                                 0.2                                                                      0.2
                                              A                                                                                  A

                                 0.0                                                                      0.0
                                       20         25         30        35         40                            50        60         70        80         90
                                                       diameter [µm]                                                           diameter [µm]
                                                                                   c)                                                                      d)


Figure 4: Comparison of measured size distributions for the nominal mean diamters indicated;
 PDA, shadowgraphy and manufacturer.


 the systematic skew of the distribution towards smaller sizes, Fig. 4 Mark A, which becomes
 less severe for larger sizes.
     It should be however noted, that it was necessary to heavily correct the diameter conversion
 factors β 12 (β 13 ) which relate linearly the measured phase difference Φ12 (Φ13 ) between detec-
 tors 1 − 2 and 1 − 3, Fig. 1, to the particle diameter dp : Φ12 = β 12 dp and Φ13 = β 13 dp to
 obtain the correct mean particle by means of the PDA measurements. Using simply the physi-
 cal off-axis angle of 30 ◦ together with the physical refractive index for latex spheres of 1.59 as
 input parameters for DANTECs BSA Software (V4.0) results in diameter conversion factors of
 β 12 = 4.384 (β 13 = 2.192) which would have increased the mean diameters by a factor of 2, i.e.
 without correction, we measured a mean particle diameter of dp = 20.8 µm when dispersing
 particles with dp = 10.4 µm in water. The DOS-based software SizeWare (V2.4) provides a
 module which accounts for the geometry of the transmitting and receiving optics relative to the
 window and the experimental facility if a window exists between the measurement volume and
 the receiving optics, as well as for the refractive index of the fluid which contains the particles.
 This module allows the calculation of corrected diameter conversions factors, i.e. β 12 = 2.527
 (β 13 = 1.263) for the experiments considered here. DANTECs BSA software does not provide
 the diameter conversion factor as a direct input but uses (presumably), (Albrecht et al. 2003):

               √                                       ·³
                                                              √ ¡                                        ¢ ´1/2
   ∆Φ12   = 2 · 2π dp                                              2             Θ                     Θ 1/2
                                                     1 + m − m 2 1 + sin ψ r sin 2 + cos ψ r cos φ cos 2
                λ                                                                                               ¸
                                                    ³         √ ¡                                        ¢ ´1/2
                                                          2                      Θ                     Θ 1/2
                                                   − 1 + m − m 2 1 − sin ψ r sin 2 + cos ψ r cos φ cos 2
                                                                                                                                                                (1)
                               1.2                                                                   1.2
                                                                        PDA                                                          Schadowgraphy


                               1.1                                                                   1.1



             µPDA / µm , [-]




                                                                                       µSch / µm , [-]




                                                                                                                                                      ±5%
                                                                              ± 10 %
                               1.0                                                                   1.0


                               0.9                                                                   0.9


                               0.8                                                                   0.8
                                          0   20        40         60     80                                      0   20        40         60    80
                                                   diameter [µm]                                                           diameter [µm]

                                                                        PDA                                                          Schadowgraphy
                               10                                                                        10
                    σPDA / σm , [-]




                                                                                            σSch / σm , [-]

                                      1                                                                       1


                                          0   20        40         60     80                                      0   20        40         60    80
                                                   diameter [µm]                                                           diameter [µm]


Figure 5: Comparison of PDA-measured and shdowgraphy measured count mean diameter with
 nominal diameter, as well as measured broadness with nominal broadness


 for first order refraction, with the elevation angles ψ r = ψ 1 = −ψ 2 , the off axis angle Θ, Fig.
 1, and the relative refractive index m = mp /mf , to calculate β 12 (β 13 ). Since Mask A was
 chosen to cover the whole range of particles envisaged, i.e. it was decided to leave the elevation
 angles ψ r = ψ 1 = −ψ 2 unchanged, we had to tune Θ or m. Since it was not possible to achieve
 the corrected diameter conversion factors by solely changing Θ from 30◦ to 88◦ , we changed
 additionally mp from 1.59 to 1.50 and obtained finally β 12 = 2.495 (β 13 = 1.248) as the closest
 possible values compared with the suggestion from the SizeWare software.
    For a quantitative comparison of the results we approximated all size distributions with
 Gaussian distribution according to:
                                                    ½               ¾
                                            1            1 (x − µ)2
                               f (dp ) = √      exp −                                           (2)
                                           2πσ           2 σ2
 resulting in the count mean, µ = D(1, 0), as well as the standard deviation, σ, describing
 the broadness of the distribution. The manufacturer distributions are denoted by index m, the
 PDA-measured distributions by index P DA, and the shadowgraphy-measured distributions by
 index Sch. Since the manufacturer guaranteed distributions are considered as the reference, we
 calculated the fractions µP DA /µm and µP DA /µm , Fig. 5, as well as σ P DA /σ m and σ P DA /σ m .
 For all measurements the PDA-measured mean diameter coincides better than ±6 % and for
 the shadowgraphy-measured better than ±5 % with the reference diameter, Fig. 5, that is, with
 respect to the measured mean diameter both systems have comparable accuracy. The PDA-
 measured distribution is 20 times broader for the smallest size (5 µm), Fig. 5. For increasing
 particle sizes, this relation becomes increasingly better, but even for the largest size considered
here (70 µm) it is two times broader than the reference. No definitive statement can be given
to explain this behavior. This is still subject to further discussions with the PDA manufacturer.
The shadowgraphy-measured size distribution, on the other hand, is 10 times broader for the
smallest sizes, Fig. 5, but then decreases quickly and coincides with the reference value in the
range from 30 to 70 µm.
   It must be stressed that the above mentioned corrected off-axis angle deeply influences the
calculated mean particle-volume flux, by means of the BSA-software, since the mean particle-
volume flux is referenced to the projected probe volume, which contains the off-axis angle as
parameter, (Albrecht et al. 2003).

4 Summary
The results can be summarized as follows:
  • The PDA-measured (shadowgraphy-measured) mean particle size is in agreement within
     ±6 % (better than ±5 %) with the one given by the manufacturer for the size range from 5
     to 70 µm.
  • Since the experimental set-up with latex spheres dispersed in water introduces additional
     refraction of the scattered light for the PDA measurements, the diameter conversion factors
     must be corrected. This correction can be performed by means of a module in the DOS
     based software SizeWare from DANTEC,i.e. one ends up with new diameter conversion
     factors. Because the actual (V4.0) PDA software from DANTEC (V4.0) a) does not con-
     tain this module and b) calculates the diameter conversion factors from the geometrical
     set-up, it was necessary to use an off-axis angle of 88◦ (while having an physical off-axis
     angle of 30 ◦ ) as input for DANTECs BSA software in order to obtain the correct mean
     diameters.
  • Furthermore, it must be emphasized, that this corrected off-axis angle also deeply influ-
     ences the calculated mean particle-volume flux, by means of the PDA software, since the
     mean particle-volume flux is referenced to the projected probe volume, which contains
     the off-axis angle as parameter. Consequently the attempts to inter-compare also the PDA
     and shadowgraphy based flux measurements for the Latex spheres with hand-calculations,
     failed completely.
  • Additionally, it was found that for each measurement, the PDA-measured size distribution
     of the Latex spheres is considerably broader – a factor of 4 over-all in terms of the standard
     deviation – than the one obtained by means of the shadowgraphy system as well as the one
     certified by the manufacturer. No definitive statement can be given to explain this behavior.

References
Albrecht, H.-E. and Borys, M. and Damaschke, N. and Tropea, C. (2003). Laser doppler and
   phase doppler measurement techniques. Wiley-Interscience.
Araneo, L., Damaschke, N., Tropea, C.(2000), Measurement and Prediction of the Gaussian
   Beam Effect in the Phase Doppler Technique, In Proc. 10th Int. Symp on Appl. of Laser
   Techn. to Fluid Mechanics, Lisbon, Portugal, Paper 20.6.
Güntay, S., Suckow, D., Dehbi, A., Kapulla, R, (2004), ARTIST: introduction and first results,
   Nuclear Engineering and Design, Vol. 231, pp. 109-120, 2004.
Kapulla, R. and Najera B. (2006), Sensitivity Assessment of a Phase-Doppler Interferometer to
   User-Controlled Settings, Meas. Sci. Technol. 17, pp. 221–227

				
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