PowerPoint Presentation

Thermometry using Laser Induced Thermal Grating Spectroscopy (LITGS)Joveria Baig

Motivation

Optical techniquesLaser Induced Grating SpectroscopyThermometry using LITGS

Spatial Averaging in LITGS

Sensitivity of LITGS in complex temperature fields

Thermometry in burner flame

OutlookOutline

MotivationThermometry:Accurate and precise Spatially resolved Reaction rates are dependent on temperature by the Arrhenius equation:

where k is the reaction rate, A is a pre-factor and T is the absolute temperatureUnderstanding the process of combustion will help:Reduce impact of harmful pollutantsIncrease efficiency of combustion to reduce amount of fuel used The world still heavily relies on combustion of fossil fuels as a primary source of energy. Reduce impact of harmful gases emitted during the process of combustion by reducing the amount of fuel consumedPrevent depletion of fossil fuels by making combustion more efficient

Note: Mention about non-invasive nature of LITGS, no physical probe 3Optical techniquesGeneration of fourth signal field as function of three input fieldsPower series expansion of polarization relates the three source fields through third order electric susceptibility tensor:

Conservation of momentum and energy dictates the phase matching criteria Four Wave MixingNon-linear component of polarization facilitate coherent re-radiation of photon after excitationFourth field generation from polarization of mediumGeometry of propagation vectors dictated by conservation; Energy conservation frequency sum is zero; Momentum conservation propagation vector sum is zero- phase matching conditionNot followed in LITGS since optical fields are not conserving, transfer to medium

5Signal Formation:Two coherent beams interfere to form intensity fringes at the intersection. Molecular excitation, followed by collisional quenching causes a grating to form in the gas. Bragg scattered probe beam forms the LITGS signal. PumpPumpThermal GratingProbeLITGS signal

Laser Induced Thermal Grating SpectroscopyAt points of constructive interference where high intensity fringes are formed, energy from the pulses is absorbed by molecular species with an appropriate frequency response corresponding to the fourth harmonic of the Nd:YAG output radiation. The energy that is absorbed is temporarily stored in excited states. Collisional quenching

6Acoustic waves formed by fast release of energy from the excited moleculesStationary wave due to change in temperatureChange in bulk gas density and hence refractive index

LITGS

Bragg scattered probe beam can be used to monitor the grating evolution

Thermometry using LITGS

Alternative optical techniquesCoherent Anti-stokes Raman Spectroscopy (CARS)

1 2 3 4 Population GratingMoving population gratingProbe grating at any wavelengthDegenerate Four Wave Mixing (DFWM)

Population Grating 1 2 3 4 Resonantly enhanced by real transition Probe grating at same wavelengthStationary population grating fast decayLaser Induced Fluorescence (LIF)

Temperature measurement from intensity of fluorescent signal

Laser

FluorescencefluorescenceabsorptionAbsorption SpectroscopyDoppler broadened line width can give information about temperature TechniqueAdvantagesLimitationsDFWMSensitivity to minor species Complex experimental setupCARSBetter spatial resolutionCan generate signals in N2

- Relatively complex experimental setup- Complicated data analysis

LIFTwo dimensional distributions can be obtainedDirect dependence on signal intensityAbsorption SpectroscopySimple and robustPoor spatial resolution due to line of sight natureComparison with LITGSSpatial Averaging in LITGS

Spatial AveragingPresence of multiple temperatures in the probe volume (in non-uniform temperature fields) can significantly change the shape of LITGS signalChange green and axis labels font size12Pump beam:Quadrupled Nd:YAG laser (266nm)Energy of 15 mJ

Probe beam:300mW Continuous wave diode pumped Solid State laser

LITGS Experimental SetupTo test the effect of two temperatures in the probe volume

Hot flow connected to heating element, cold flow at room temperature

Translation stages to adjust the position of the flow system relative to the optical table

Dual Flow ExperimentMeasurement volume encompasses both flows14Validation

Model developed for calculating LITGS signal for a uniform temperature field Single temperature LITGS model fits well with the experimental dataDual temperature model developed to simulate LITGS signal in a probe volume containing two different temperaturesSensitivity in complex Temperature fieldsTwo different annular temperature distributions modelledHot-cold-hot flowCold-hot-cold flow

Hot 430KCold 270KDifferent Temperature Distributions

ColdHotHotLabel hot and cold in figure17Two different annular temperature distributions modelledHot-cold-hot flowCold-hot-cold flow

Hot 430KCold 270KDifferent Temperature Distributions

ColdColdHot

Comparison

Mark locations on the figure and appropriate graphsConclusion LITGS works to resolve differences in more complicated situations, so we have confidence it will work in a more challenging environment19LITGS in Glder burner flame

ObjectiveUn-burnt ethylene (flame front)Burnt gas (hot region)Evaluate what happens in a single 2D slice at different heights along in the flame Reconstruction of temperature distribution in 3DDescribes what we want to achieve get spatial resolution within objectUse picture of flame, show vertical slices?21

LIGS signal at different positions show presence of multiple temperature

Frequency beating like behavior seen

Figure showing temperature distributionInner circle (cold) 270KOuter ring (hot) 430KModelShow that this is transverse section of a flame22Power spectrum shows two peak frequencies corresponding to presence of two temperatures in the distribution

Power SpectrumExplain how power spectrum is obtained by FFT giving spectral information, frequency information, relating to temperatures23Thermometry in standardized laboratory flame as a precursor to more complicated combustion processes

Co-flow laminar ethylene-air diffusion flow

Experimental Setup

Experimental data from flame7 cm12 mmxxxxxxxxx Probe region has to be greater than the flame diameter

Coarse grid of 2D slice through the flame

Measurements require ethylene hence constrained by flame frontLocations from where experimental data was obtained for fitting is shown by red crosses

Fitting RoutineRephrase lsqcurvefit last comment on the table26Results

Name of ParameterValueInner width (w1)4.8 mmEnd of gradient (w2)5.18mmEnd of hot region radii (w3)5.50mmOutermost radii (w4)6.00mmInner temperature (T1)/K1350 KOuter temperature (T2)/K1930 KAt x=0, z=0 in flame Fast decay of the signal: Presence of high temperature Weighted LITGS of multiple temperatures in probe volume

Change to mm27OutlookDeveloped understanding of spatial averaging in LITGS

Applied to axi-symmetric flame environment

Successfully recovered temperature distribution with significantly enhanced spatial resolution by combining this new understanding of spatial averaging with object symmetry in a novel fitting approach using data from multiple chords

ConclusionTested by comparison of model with experiments in dual flow systemTalk about symmetryTakes information from several chords and combine them 29Acquire experimental data at closer intervals to achieve better fitting with the current model

Model to be made more precise by optimizing parameters such as Reynolds number, quench times, branching ratio etc for each temperature

Combine with other techniques such as Chemilumiscence to get more information about flame

Incorporate details of probe volume shape

Future WorkThank you.

Questions?Collisional quenching