Planar Laser-Induced Fluorescence

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					Planar Laser-Induced Fluorescence
Introduction                                                                                                                                                                                                                                             Features
Planar laser-induced fluorescence (planar-LIF) is an               biomedical engineering (e.g. transport of drugs in                                                                                                                                    • Non-intrusive technology
optical measuring technique used to measure instant                biological flows such as in model veins) and fluid                                                                                                                                    • Quantitative, precise and accurate measurements of
whole-field concentration or temperature maps in liquid            dynamics research (e.g. turbulent mixing and heat-
                                                                                                                                                                                                                                                           concentration or temperature fields
flows.                                                             transfer modelling, indoor climate etc.)
Applications can be found in process engineering (e.g.                                                                                                                                                                                                   • In combination with velocity measurements, transport
mixing in stirring vessels, heating and cooling systems),                                                                                                                                                                                                  properties (e.g. Reynolds flux, turbulent diffusion
                                                                                                                                                                                                                                                           coefficients and other parameters) are made experi-
                                                                                                                                                                                                                                                           mentally available
                                                                                                    Detection of                                                                        Camera
                                                                                                    light intensity                                                                     calibration
                                                                                                                                                                                                                                                                                        Data            Data
                                      Measurement                                                                                                                                                            C
                     Y                volume
                            X                                                                                        Optical
                                                                                                                     filter (red)
                      t                                                                                                                                                                                    PLIF
Laser                                                                                                                                                                                                      image
                                Cylindrical lens            Optical
                                                 Flow with     filter
                                           fluorescent dye  (green)
                                              and particles
                                                                                                                                                                          PIV double

Experimental set-up                                                                                                                                                                                                    Linear fit (at E=E0)
                                                                                                                                                                                                                                                         Synchronised planar-LIF and PIV
The basic equipment needed to carry out planar-LIF                                                                                                                                                                                                       measurements
                                                                                                                                 Signal of fluorescence, S (a.u.)

measurements is:                                                                                                                                                              Fcritic
                                                                                                                                                                                                                                                         Combining concentration or temperature with velocity

• A laser source (typically Nd:YAG or Argon-ion lasers)                                                                                                             500                      Linear regime
                                                                                                                                                                                                                                Signal saturation        measurements enables mixing or heat transfer to be
   with the appropriate optics to form a thin sheet of light                                                                                                        400
                                                                                                                                                                                                                                                         studied at the scalar transport level. In this case, a
                                                                                                           380                                                                                                                                           second camera picks up the laser light scattered by
• A fluorescent dye that marks the fluid and that is                                                                                                                300
                                                                                                                                                                                                                                                         small particles through an optical narrow-band filter. The
   traced during the measurements. This chemical                                                                                                                                                     Experimental
                                                                        Signal of fluorescence, S (a.u.)

                                                                                                           340                                                      200
                                                                                                                                                                                                                                                         signal is processed to measure velocities, as in standard
   compound absorbs the laser light energy and re-emits                                                    320                                                      100
                                                                                                           300                                                                                                              Ccritic                      PIV. The two LIF and PIV receivers are brought to
   light at a longer wavelength that can be detected by a                                                                                                             0
                                                                                                           280                                                            0                  5             10          15             20            25   observe the same locations by means of beam splitter
   photodetector. Commonly used dyes for measure-                                                          260    Experimental                                                                         Concentration (µg+l-1)                            optics and geometrical calibration.
   ments in liquids are rhodamine 6G (for concentration                                                    240

   measurements), rhodamine B (for temperature                                                             220
   measurements) and fluorescein disodium (for concen-                                                     180                                                                                     Calibration curve
                                                                                                                                                                                                                                                         Example: Jet mixing in a cross-flow
                                                                                                                                                                                                 (at E=E0 and C=C0)
   tration or temperature measurements)                                                                    160
                                                                                                                                                                                                                                                         A laminar water-jet flow is established in a semi-
 • A CCD camera equipped with a sharp cut-off or                                                             10     20     30                                        40            50         60      70         80    90
                                                                                                                                                                      Temperature (C°)                                                                   enclosed mixing chamber with a water cross-flow loop. A
   narrow-band filter, so that only the fluorescent light is
                                                                                                                                                                                                                                                         Nd:YAG green laser light sheet illuminates the centre
   recorded. This camera acts as an array of light
                                                                   Typical single-pixel temperature and concentration                                                                                                                                    plane of the jet and excites the rhodamine 6G that is
   detectors (pixels)
                                                                   calibration curves                                                                                                                                                                    added to the fluid. The LIF camera collects the bright
                                                                                                                                                                                                                                                         orange light and the images are processed to give
Calibration                                                                                                                                                                                                                                              instant concentration maps. Eddy structures issued from
                                                                   At low concentration levels, absorption phenomena are                                                                                                                                 the jet are clearly identified on the instant concentration
The level of fluorescence is known to vary with the                negligible (AC, AT ≈ 1), which leads to a linear relation-                                                                                                                            map (image 1). Statistical analysis of the data such as
concentration or the temperature and several other
                                                                   ship between the signal (S) and (C,E,α) or (T,β) - with                                                                                                                               average (image 2) and rms (image 3) of concentration
experimental parameters.                                                                                                                                                                                                                                 further provide meaningful information on the mixing
                                                                   (α, β) two constants that characterise all experimental
- With rhodamine 6G as marker and the concentration                parameters. In such conditions, concentration (or                                                                                                                                     process.
  as measured variable, S is defined as:                           temperature) is accurately measured as the amount of
                                                                   light received by the detector. The calibration procedure                                                                                                                                                               1
  S = ƒoptic · AC · (EVc · Qλ ) · C where AC = e-ε L·C                                                                                                                                                                                                    Instant concentration map
                                                                   consists of determining α (or β) at every pixel of the
- With rhodamine B as marker (for constant concen-
  tration, Co) and the temperature as measured
  variable, S is defined as:                                       Signal processing
                                                                   Signal processing consists of converting the recorded                                                                                                                                               Average concentration map
  S = ƒoptic · AT · (EVc · Co ) · Qλ (T)                           raw images of fluorescence to concentration or tempera-                                                                                                                                             Cross-flow
                                                                   ture maps, via the calibration map determined
Where:                                                             previously. For liquid-liquid mixing, one of the fluids is
- C and T are the dye concentration and the                        marked with the dye, whereas the other is fresh fluid.
  temperature, respectively                                        When passing through the laser sheet, the dye is excited                                                                                                                                                                                         3
                                                                   and re-emits fluorescent light. The light intensity is then                                                                                                                                                        Rms. of concentration
- E is the laser light intensity                                   processed as follows to give concentration:
- Qλ is the quantum efficiency of the dye (at the laser
  excitation wavelength λ)                                                                                                                                                         C≅ S                                                                                    Jet
- foptic corresponds to optical factors
                                                                   For temperature measurements, the dye is first mixed
- Vc is the sampling volume
                                                                   with the fluid and the fluorescence level re-emitted
- AC and AT represent absorption phenomena inte-                   varies with the local temperature. In this case, tempera-
  grated on the light path (L) in the fluid characterised

                                                                   ture is determined as follows:
  by absorption index (ε)
                                                                                         (T -Tref ) ≅ (S -Sref )
                                                                                                          β                                                                                                                                              Example of instant and statistical maps of concentration
                                                                   where Sref is the reference fluorescence signal at T=Tref                                                                                                                             obtained with planar-LIF measurements

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