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Thermometry using Laser Induced Thermal Grating Spectroscopy (LITGS) Joveria Baig Outline • 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 Motivation The world still heavily relies on combustion of fossil fuels as a primary source of energy. Reaction rates are dependent on temperature by the Arrhenius equation: 𝑘= Understanding the process of combustion will help: • Reduce impact of harmful pollutants • Increase efficiency of combustion to reduce amount of fuel used −𝐸𝑎 𝐴𝑒 𝑅𝑇 where k is the reaction rate, A is a pre-factor and T is the absolute temperature Thermometry: • Accurate and precise • Spatially resolved Optical techniques Four Wave Mixing • 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: 𝑃=𝜒 1 𝐸1 + 𝜒 2 𝐸1 𝐸2 + 𝜒 3 𝐸1 𝐸2 𝐸3 + ⋯ • Conservation of momentum and energy dictates the phase matching criteria ∆𝜔 = 𝜔1 + 𝜔2 + 𝜔3 + 𝜔4 = 0 ∆𝑘 = 𝑘1 + 𝑘2 + 𝑘3 + 𝑘4 = 0 Laser Induced Thermal Grating Spectroscopy 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 LITGS signal Pump Probe LITGS • 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 Thermometry using LITGS • Bragg scattered probe beam can be used to monitor the grating evolution 𝑓= 𝜆𝑝𝑟𝑜𝑏𝑒 = 2Λsin(𝜃) 𝑐𝑠 = 𝛾𝑘𝐵 𝑇 𝑚 𝑚𝛬2 2 𝑇= 𝑓 𝛾𝑘𝐵 𝑐𝑠 𝑓= 𝛬 θ = angle of incidence of pump beam 𝑚 = ratio of mass to specific γ heats Λ 1 τ Alternative optical techniques Degenerate Coherent Anti-stokes Four Wave Raman Mixing Spectroscopy (DFWM) (CARS) Laser Induced Fluorescence (LIF) ν‘ ħω1 Degenerate Four Wave Mixing (DFWM) Laser Induced Fluorescence (LIF) Coherent Anti-Stokes Raman Spectroscopy (CARS) Absorption Spectroscopy fluorescence Population Grating absorption Population Grating ħω2 ħω3 ħω4 ħω1 ħω2 ħω3 ν ħω4 Resonantly enhanced by real transition Probe grating at same wavelength Probe grating at any wavelength Stationary population grating – fast Moving population grating decay Fluorescence Absorption Spectroscopy • Temperature Doppler broadened measurement from line widthof can give intensity information about fluorescent signal temperature Comparison with LITGS Technique DFWM CARS LIF Absorption Spectroscopy Advantages Limitations Sensitivity to minor species Complex experimental setup - Better spatial resolution - Can generate signals in N2 - Relatively complex experimental setup - Complicated data analysis Two dimensional distributions can be obtained Direct dependence on signal intensity Simple and robust Poor spatial resolution due to line of sight nature Spatial Averaging in LITGS Spatial Averaging Presence of multiple temperatures in the probe volume (in non-uniform temperature fields) can significantly change the shape of LITGS signal LITGS Experimental Setup Pump beam: • Quadrupled Nd:YAG laser (266nm) • Energy of 15 mJ Probe beam: • 300mW Continuous wave diode pumped Solid State laser Dual Flow Experiment • 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 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 Sensitivity in complex Temperature fields Different Temperature Distributions Hot Hot 430K Cold 270K Cold Two different annular temperature distributions modelled • ‘Hot-cold-hot’ flow • ‘Cold-hot-cold’ flow Hot Different Temperature Distributions Hot 430K Cold 270K Two different annular temperature distributions Cold modelled • ‘Hot-cold-hot’ flow Hot • ‘Cold-hot-cold’ flow Cold Comparison LITGS in Gülder burner flame Objective Burnt gas (hot region) Un-burnt ethylene (flame front) -Evaluate what happens in a single 2D slice at different heights along in the flame - Reconstruction of temperature distribution in 3D Model • 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 Power Spectrum Power spectrum shows two peak frequencies corresponding to presence of two temperatures in the distribution Experimental Setup • Thermometry in standardized laboratory flame as a precursor to more complicated combustion processes • Co-flow laminar ethyleneair diffusion flow Experimental data from flame - Probe region has to be greater than the flame diameter - Coarse grid of 2D slice through the flame 7 cm xxx xxx xxx 12 mm - Measurements require ethylene hence constrained by flame front Locations from where experimental data was obtained for fitting is shown by red crosses Fitting Routine • Estimate Initial parameters • Create a temperature map corresponding to input parameters Temperature • Compute LITGS signal for each temperature on the temperature map distribution • Calculate the weighting of each temperature in the LIGS section to be modelled at different locations Generating • Generate LIGS by calculating a weighted sum of multiple LITGS signal 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 Results At x=0, z=0 in flame • Fast decay of the signal: Presence of high temperature Weighted LITGS of multiple temperatures in probe volume Name of Parameter Value 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)/K 1350 K Outer temperature (T2)/K 1930 K Outlook Conclusion • 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 Future Work • 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 Thank you. Questions?