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Application considerations for laser diagnostics

(Lecture 13)

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Combustion diagnostic concept

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Analyzing and post-processing

Results

T = 1800 K

Planning

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Sources of interference ?Background luminosity

?Laser-induced incandescence

?Laser-induced fluorescence ?Scattering

?Extinction (Trapping)

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–“DC background”?Broadband radiation Array–e.g. blackbody radiation from soot

particles

?Narrowband radiation

–e.g. emission from atoms/molecules

in the flame (chemiluminescence)

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UV Visible

Broadband radiation

blackbody radiation

Narrowband radiation

chemilumeniscence

?Detector gating

–Continues radiation suppressed

– e.g. the image intensifier (MCP) of a CCD can be gated ?Optical filters

–Interference filters

–Short-pass filters

–Long-pass filters

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“open”

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time Filter transm.

1 nm (FWHM)

?Polarization

–Background luminosity not polarized but the signal might be (e.g. Raman and Rayleigh scattering)

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260270280290300310320330340

50000

100000

150000

200000

250000

I n t e n s i t y (c o u n t s )

Wavelength (nm)

250

300

350

400

450

500550600

5000100001500020000

25000Fluorescence from acetone

Raman from CH 42:nd order

Raman from CH 4

I n t e n s i t y (c o u n t s )

Wavelength (nm)

Laser-induced incandescence

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?Occurs in particle-laden

flames at high laser

intensity

?The LII signal lasts up

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to 1 ms

Laser-induced fluorescence

?Undesired broadband

fluorescence induced by the

laser

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?Particularly high

fluorescence yields from

PAH (present in rich sooty

flames)

?The risk of getting

interfering fluorescence

increases with decreasing

laser wavelength

Scattering Interfering scattering may come from:?Particles

?Droplets

?Surfaces

?Optical components

Extinction

?Extinction = Absorption + Scattering

?Strong when particles and droplets are present

?For small particles (d/λ< 0.1) absorption dominates.

?For large particles (d/λ> 0.1) scattering also contributes ?Measurements in practical combustion devices are sometimes limited by extinction

–Laser light may not reach the probe volume due to strong extinction.

–LIF signals may not reach the detector due to strong extinction (trapping)

Camera (CCD) noise

Dark Current

Dark current is caused by thermally generated electrons that build up in the pixels of all CCDs.The rate of dark current build up can be reduced by a factor of 100 or more by cooling the CCD.The remaining dark current is subtracted from an image using dark frames

Pixel Non-Uniformity

Each pixel has a slightly different sensitivity to light, typically within 1% to 2% of the average signal. Pixel non-uniformity can be reduced by calibrating an image with a flat-field image. Shot Noise

Since each photon is an independent event, the arrival of any given photon cannot be precisely predicted; instead the probability of its arrival in a given time period is governed by a Poisson distribution.Shot noise is most apparent when collecting a relatively small number of photons.It can be reduced by collecting more photons, either with a longer exposure or by combining multiple frames.

CCD Read Noise(On-chip)

CCD read noise ultimately limits a camera’s signal to noise ratio, but as long as this noise exhibits a Gaussian, or normal distribution, its influence on your images can be reduced by combining frames and standard image processing techniques.

CCD Camera Noise (Off-chip)

A pixel value is read out of a CCD as a tiny voltage, on the order of microvolts per

electron.The camera’s electronics then pass that voltage to an Analog to Digital Converter (ADC) to be converted into a digital pixel value.Before being passed to the ADC the camera must amplify the pixel voltage to a range appropriate for the ADC.All amplifiers introduce noise, whether that amplifier is in a stereo or a CCD camera.

Other perturbations

?Optical breakdown

–Electrons are removed from the

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molecule and ions are formed ?

plasma

?Population distribution perturbation

–May occur if the laser intensity is very

high

–The lower state gets depleted, i.e. a

non-equilibrium situation arises

?Photochemistry

–High laser intensity at short

wavelengths may lead to

photodissociation of molecules

?Beam steering

?Optical damage

About the exam

Q: Laser-based techniques have a number of significant advantages

compared to conventional probe techniques for diagnostics of combustion processes. Unfortunately there are some disadvantages too. Give four

examples of advantages and disadvantages/limitations (four of each)

A:Advanatges Disadvantages/limitations

+ Nonintrusive-Optical access is needed

+ High spatial resolution-Often complex experiments

+ High temporal resolution-High-level operator skill often needed

+ Species-specific-Interpretation of results may be advanced + In-situ measurements-Limited to small molecules

+ Remote measurements-Often expensive

+ No upper temperature limit

+ Non-equilibrium can be probed

Q:What is the main difference between an incoherent laser technique and a coherent laser technique? Give two example

of incoherent techniques and one example of a coherent

method.

A:An incoherent technique gives a signal that is incoherent, i.e. a signal where the signal photons are out of phase with no specific directionality. Often a single laser is used for the laser/matter interaction process. A coherent technique leads to a signal that has a high directionality with signal photons in phase, and it is often the result of the interaction of several laser beams with the medium.

Incoherent techniques: LIF, Raman scattering, Rayleigh scattering Coherent techniques:CARS, Polarization spectroscopy,

Q:In particle-image velocimetry (PIV), particles are seeded into the flow. What considerations are important when choosing suitable seeder particles, and the use of them in the seeding process?

A:Issues that should be discussed in the answer:

?The seeding particles should have a small enough size making them follow the flow,

and also a size big enough to give a strong enough signal.

?They should also be evenly distributed in the flow with a suitable concentration to

make the particle tracking possible.

?They should also be able to stand the high temperatures of flames, and they should

not agglomerate.

?They should not influence the properties of the flow, for example by catalyzing reactions.

Important info about the exam

The exam takes place on Tuesday, March 8, 9:00 –13:00 in room E421.

Calculator and a physics handbook (TEFYMA or equivalent) allowed.

You may answer, with neat handwriting, in Swedish or English.

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