Development of Robust Ultrafast CARS Thermometry and Species Detection (PPT)

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Delft University of Technology

Bohlin, Alexis

Publication date 2018

Document Version Final published version

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Bohlin, A. (2018). Development of Robust Ultrafast CARS Thermometry and Species Detection (PPT).

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Development of Robust Ultrafast CARS

Thermometry and Species Detection

Alexis Bohlin, Ph.D.

Faculty of Aerospace Engineering, Delft University of Technology

Acknowledgement:

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Research activities and areas of impact

Advancing Renewable Aero-Propulsion

• Grand challenge: air-transportation/energy security/combustion

- Reduced emission of pollutants from aircraft NOx, CO, CO₂, UHC, and soot

x y λ

“Deep insight into multiscale chemical interactions can only be obtained from spectroscopic measurements garnered in spatial and temporal correlation.”

Challenge the future

Temperature maps Large Eddy Simulation

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Time- and spatially resolved optical

diagnostics for combustion analysis

• Challenges: Parameter determination in reacting flows

– Major- and transient species detection Particulate chemistry – Temperature field Mixture fraction Flow field

– Spatial- and temporal correlation (multiscale analysis)

CARS imagery in

flames:

• Strategy: Snap-shot coherent Raman imagery

– Simultaneous hyperspectral imaging (x, y, λ) in a single-laser-shot.

– Benchmarking: Accuracy, Precision, Sensitivity, Resolution and Field-of-view.

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Time- and spatially resolved optical

diagnostics for combustion analysis

• Strategy: Snap-shot coherent Raman imagery

– Simultaneous hyperspectral imaging (x, y, λ) in a single-laser-shot.

– Benchmarking: Accuracy, Precision, Sensitivity, Resolution and Field-of-view.

• Challenges: Parameter determination in reacting flows

– Major- and transient species detection Particulate chemistry – Temperature field Mixture fraction Flow field

– Spatial- and temporal correlation (multiscale analysis)

• Objectives: High-fidelity experiments in combustion systems

Experiments informs theory and vice versa

Device validation

Development of predictive engineering models

- Flameless Combustor

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Actual

temperature

Inaccuracy ~2-3% Single shot precision ~4-5%

• Most accurate technique for thermometry in reacting flows

(wide range of operational conditions).

Why should we use CARS?

Background nanosecond CARS

Evaluated temperature Evaluated temperature Actual temperature

Advanced nanosecond CARS

Improved Accuracy – Spectroscopic modelling (Raman linewidths, ...)

Inaccuracy ~0% ? Single shot precision ~4-5%

Improved Precision – Experimental setup (Laser system, ...)

Goal!

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1-2 mm

True temperature Evaluated temperature

v=0 v=1 Internuclear distance E ne rgy 0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0 3 6 9 12 15 18 21 24 27 30 33 36 39 T=300 K T=1700 K Rot. Q. Number J Fr ac tional P o pulat ion T=300 K T=1700 K

N

₂

• Vibrational CARS, Rotational CARS

• Nanosecond CARS characteristics:

– Non-intrusive, in-situ probe

– High temporal resolution (~10 ns)

– High spatial resolution (~100 µm x 100 µm x 1-2 mm)

• Most accurate technique for thermometry in reacting flows

(wide range of operational conditions).

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< 0.5 mm

• Two-beam femtosecond/picosecond CARS

– Picosecond temporal resolution

(Near collision independent - Raman linewidths)

– Improved spatial resolution

(40 µm x 40 µm x 0.5 mm)

– 1D and 2D imaging capabilities

Inaccuracy < 2-3%

Single shot precision ~1%

• Vibrational CARS, Rotational CARS

1-2 mm

True temperature Evaluated temperature

• Most accurate technique for thermometry in reacting flows

(wide range of operational conditions).

• Nanosecond CARS characteristics:

– Non-intrusive, in-situ probe

– High temporal resolution (~10 ns)

– High spatial resolution (~100 µm x 100 µm x 1-2 mm)

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Measurement object

Spectrometer

CCD

Lens

Short Pass Filter

Lens Lens

ω_probe

ω_pump/Stokes ω_CARS

Two-beam femtosecond/picosecond CARS

Simplified generic phase-matching- and impulsive excitation scheme.

Broadband Laser Narrowband Laser fs pump Time delay / ps fs Stokes 0 ps probe Molecular response Vector mismatch Raman shift Beam crossing angle (θ) All parallel beams

Phase-matching (momentum conservation) Energy conservation p ump S to ke s p ro b e CA RS

Laser driven transitions (Q and S)

Spectroscopy in the time-domain

Δk = kphysical– kgeometrical> 0

Molecular internal energy levels

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N2spectra at two different temperatures

0 200 400 600 800 1000 1200

0 1

# Channel num ber

Air (79% N2and 21% O2) at room temperature

Measurement object

Spectrometer

CCD

Lens

Short Pass Filter

Lens Lens

ω_probe

ω_pump/Stokes ω_CARS

Two-beam femtosecond/picosecond CARS

Broadband Laser Narrowband Laser

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Examples of coherent Raman spectra for some

combustion relevant species

• Specific selection rules (transitions)

ro-vibrational O-, Q-, S-branch (Δ𝑣 = 1, Δ𝐽 = 0, ±2), pure-rotational O, S-branch (Δ𝐽 = ±2)

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Direct coherent Raman temperature imaging

and wideband chemical detection

Fuel + oxidizer _N

2 / Air

10 l/min N₂/ Air

Premixed burner principle

Burner design (Michelsen group, Sandia)

• Canonical sooting

hydrocarbon flat-flame used to benchmark the new techniques. 10 mm Photo: M. Campbell HAB=2mm T~1750 K HAB=1mm T~800 K Ethylene/air φ=2.35

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Morell Nozzle Rod Stabilization Air+CH4 N₂ N₂ 120 82 77 182 50 Wall Premixed V-stabilized flame

Burner design (Dreizler group, TU Darmstadt)

Photo: C. Jainski

• Motivation

Flame-wall interaction plays a key role in the formation of pollutants in a combustion chamber, such as UHC and CO.

Side wall quenching burner

- 1D-CARS temperature- and chemical imaging

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• Automatically overlapped pump/Stokes fields, temporally and spatially, makes the technique more robust and higher pulse energy available.

• Spatial sectioning (probe volume):

~ 40 μm (Beam waist) x 40 μm (Coherent point-spread function) x 0.5 mm (Interaction length).

Two-beam 1D-CARS near-wall imaging

Relay imaging

Cylindrical lenses

Cylindrical lenses Spherical lens

Measurement

Location Entrance SlitImaging Spectrometer

Probe Pump/Stokes Short-pass filter Beam stop y x z x Top View Side View Razor blade

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Multiparameter spatio-thermochemical

probing of flame-wall interactions

• The excellent imaging resolution

allows for thermochemical states

of the thermal boundary layer to

be probed to within ~40 μm of

the interface.

• Concurrent detection of N

₂

, O

₂

,

H

₂

, (CO), CO

₂

, and CH

₄

is

achieved.

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FWI at enhanced turbulence intensities

(Work-in-progress)

• Single-shot spatially dependent statistics of the 1D flame-front gradient / thickness / position become possible (improving heat transfer models)

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Single-shot hyperspectral CARS in the gas-phase

Wideband chemical imaging Temperature imaging

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PBS CL BD BD M G L3 M M CCD RF HWP L2 L1 Nd:YAG, 30mJ@532 nm, 70ps, 20 Hz Ti:Sapphire, 3mJ@800 nm, 45fs, 1 kHz

Rotational quantum number J = 4 5 6 7 8 9 10 11 12 13 14 15 16

O₂ O₂ O₂ O₂

• Tunable spectral dispersion, enabling multispecies detection and probing of a larger 2D field.

• Vector diagram to orientate each location of the spatially resolved measurement.

Simultaneous planar imaging and

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Dispersive Fourier Transform detection

of short pulsed CARS/CSRS signals

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Synchronized ps/fs laser system

for time-resolved non-linear

optical spectroscopy/microscopy

Storage 0.5x1m 0.6 m 3.45m 4.7m Ultrafast Amplifier 35% 2.5 mJ S H B C fs -c om p. 65% 4.5 mJ 0.6m Microscope 1.3m 0 .5 -m spe ctrom eter

Femtosecond laser (ultrafast amplifier) 7 mJ/pulse @ ~780-810 nm (~35 fs) Picosecond laser (SHBC)

2.0 mJ/pulse @ 400 nm (~10 ps)

Snap-shot chemiluminescence flexible hyperspectral imagery

Acknowledgement:

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It is equally fun to buy an air-treatment system,

as it is to buy a vacuum cleaner

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Courtesy of Dr. Arvind Gangoli Rao

Distributed auto-ignition combustion modes

with reduced NOx emission

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Conclusions

• Two-beam femtosecond/picosecond CARS

-

Relevant for 0D, 1D, and 2D temperature measurements in flames when high-fidelity information is needed (inaccuracy <2-3%, precision ~1%)

-

Single-shot quantitative measurements for major species in combustion are within reach (species specific dephasing times, spectroscopy models)

• Can this advanced laser diagnostics technique be employed

for measurements in engines?

-

Technical challenges for the stability of operation (facility temperature and humidity control, propagating TL-beams through optical ports)

• This ultrafast 1D-CARS technique has been successfully

employed at:

1. Flame-wall interaction burner (head-on and side-wall quenching) 2. Sooty flames provided on a McKenna burner

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