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

2. Motivation Optical techniques ▫Laser Induced Grating Spectroscopy ▫Thermometry using LITGS Spatial Averaging in LITGS Sensitivity of LITGS in complex temperature fields Thermometry in burner flame Outlook Outline

3. Motivation Thermometry: 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 temperature Understanding the process of combustion will help: Reduce impact of harmful pollutants Increase 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.

4. Optical techniques

5. Generation of fourth signal field as function of three input fields Power 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 Mixing

6. Signal 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. Pump Thermal Grating Probe LITGS signal Laser Induced Thermal Grating Spectroscopy

7. Acoustic waves formed by fast release of energy from the excited molecules Stationary wave due to change in temperature Change in bulk gas density and hence refractive index LITGS

8. Bragg scattered probe beam can be used to monitor the grating evolution Thermometry using LITGS Λ

9. Alternative optical techniques Degenerate Four Wave Mixing (DFWM) Laser Induced Fluorescence (LIF) Coherent Anti-Stokes Raman Spectroscopy (CARS) Absorption Spectroscopy Coherent Anti-stokes Raman Spectroscopy (CARS) ħω 1 ħω 2 ħω 3 ħω 4 Population Grating Moving population grating Probe grating at any wavelength Degenerate Four Wave Mixing (DFWM) Population Grating ν ν‘ ħω 1 ħω 2 ħω 3 ħω 4 Resonantly enhanced by real transition Probe grating at same wavelength Stationary population grating – fast decay Laser Induced Fluorescence (LIF) Temperature measurement from intensity of fluorescent signal LaserLaser Fluorescence fluorescence absorption Absorption Spectroscopy Doppler broadened line width can give information about temperature

10. TechniqueAdvantagesLimitations DFWM Sensitivity to minor species Complex experimental setup CARS -Better spatial resolution -Can generate signals in N2 - Relatively complex experimental setup - Complicated data analysis LIF Two dimensional distributions can be obtained Direct dependence on signal intensity Absorption Spectroscopy Simple and robust Poor spatial resolution due to line of sight nature Comparison with LITGS

11. Spatial Averaging in LITGS

12. Spatial Averaging Presence of multiple temperatures in the probe volume (in non-uniform temperature fields) can significantly change the shape of LITGS signal

13. Pump beam: Quadrupled Nd:YAG laser (266nm) Energy of 15 mJ Probe beam: 300mW Continuous wave diode pumped Solid State laser LITGS Experimental Setup

14. To 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 Experiment

15. Validation Model developed for calculating LITGS signal for a uniform temperature field Single temperature LITGS model fits well with the experimental data Dual temperature model developed to simulate LITGS signal in a probe volume containing two different temperatures

16. Sensitivity in complex Temperature fields

17. Two different annular temperature distributions modelled ‘Hot-cold-hot’ flow ‘Cold-hot-cold’ flow Hot 430K Cold 270K Different Temperature Distributions Cold Hot

18. Two different annular temperature distributions modelled ‘Hot-cold-hot’ flow ‘Cold-hot-cold’ flow Hot 430K Cold 270K Different Temperature Distributions Cold Hot

19. Comparison

20. LITGS in Gülder burner flame

21. Objective Un-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 3D

22. LIGS signal at different positions show presence of multiple temperature Frequency beating like behavior seen Figure showing temperature distribution Inner circle (cold) 270K Outer ring (hot) 430K Model

23. Power spectrum shows two peak frequencies corresponding to presence of two temperatures in the distribution Power Spectrum

24. Thermometry in standardized laboratory flame as a precursor to more complicated combustion processes Co-flow laminar ethylene- air diffusion flow Experimental Setup

25. Experimental data from flame 7 cm 12 mm x xx xx xx x x - 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 front Locations from where experimental data was obtained for fitting is shown by red crosses

26. Temperature distribution Estimate Initial parameters Create a temperature map corresponding to input parameters Compute LITGS signal for each temperature on the temperature map Generating LITGS signal Calculate the weighting of each temperature in the LIGS section to be modelled at different locations Generate LIGS by calculating a weighted sum of multiple temperatures present in the LIGS section Retrieving parameters Import experimentally acquired data Run the least square routine until the parameters are optimized i.e. the error between the model and experiment is minimized Fitting Routine

27. Results Name of ParameterValue Inner width (w1)4.8 mm End of gradient (w2)5.18mm End of hot region radii (w3) 5.50mm Outermost radii (w4)6.00mm Inner temperature (T1)/K1350 K Outer temperature (T2)/K1930 K At x=0, z=0 in flame Fast decay of the signal:  Presence of high temperature  Weighted LITGS of multiple temperatures in probe volume

28. Outlook

29. Developed 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 Conclusion

30. Acquire 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 Work

31. Thank you. Questions?

32. Collisional quenching