Temperature and composition spots in a turbulent flow are detected and time-resolved using Laser Induced Thermal Grating Spectroscopy (LITGS). A 355 nm wavelength PIV laser is operated at 0.5–1 kHz to generate the thermal grating using biacetyl as an absorber in trace amounts. In a open laminar jet, a feasibility study shows that small (≃ 3%) fluctuations in the mean flow properties are well captured with LITGS. However, corrections of the mean flow properties by the presence of the trace biacetyl are necessary to properly capture the fluctuations. The actual density and temperature variation in the flow are determined using a calibration procedure validated using a laminar jet flow. Finally, travelling entropy and composition spots are directly measured at different locations along a quartz tube, obtaining good agreement with expected values. This study demonstrates that LITGS can be used as a technique to obtain instantaneous, unsteady temperature and density variations in a combustion chamber, requiring only limited optical access.

Temperature and composition spots in a turbulent flow are detected and time-resolved using Laser Induced Thermal Grating Spectroscopy (LITGS). A 355 nm wavelength PIV laser is operated at 0.5 -1 kHz to generate the thermal grating using biacetyl as an absorber in trace amounts. In a open laminar jet, a feasibility study shows that small ( 3%) fluctuations in the mean flow properties are well captured with LITGS. However, corrections of the mean flow properties by the presence of the trace biacetyl are necessary to properly capture the fluctuations. The actual density and temperature variation in the flow are determined using a calibration procedure validated using a laminar jet flow. Finally, travelling entropy and composition spots are directly measured at different locations along a quartz tube, obtaining good agreement with expected values. This study demonstrates that LITGS can be used as a technique to obtain instantaneous, unsteady temperature and density variations in a combustion chamber, requiring only limited optical access. NOMENCLATUREc Speed of sound [m/s] c p Specific heat capacity at constant pressure [J/kg/K] c p Specific heat capacity at constant pressure [J/mol/K] f LITGS oscillation frequency [Hz] m Mass flow rate [slpm-kg/s] p Pressure [P] p atm Atmospheric pressure [Pa] p s Partial pressure of the saturated vapour [Pa] Q Power [W] R Gas constant [J/mol/K] t Beam thickness [mm] T Temperature [K] X Molar concentration [-] X s Molar concentration in saturated conditions [-] W Bulk molecular weight [g/mole] δ Dilution ratio [-] γ Ratio of specific heat [-] λ Laser wavelength [m] Λ Grating spacing [m] ρ Density [kg/m 3 ] σ f Standard deviation of the LITGS frequency [Hz] θ Crossing angle (pump beams) [rad] θ B Bragg angle [rad] τ Oscillation period [s] 1 De Domenico GTP-18-1380 ASME © https://creativecommons.org/licenses/by/4.0/ ∆T TC Temperature increase measured with thermocouple ∆T LIT GS Temperature increase measured with LITGS [ ] ab Property of the air + biacetyl gas mixture [-] [ ] a Property the air-only flow [-] [ ] i Property secondary gas (CO 2 , Ar, He) [-] [ ] 0 Property of the reference point [-] INTRODUCTIONCombustion noise has become a major research interest within the aerospace community. Stricter emission regulations forced the introduction of lean premixed prevaporised combustors, which produce less NOx but burn more unsteadily, generating more noise and creating the potential for combustion instabilities. Fluctuations in the heat release rate of a flame produce isentropic pressure waves (direct noise), as well as pockets of different temperature, density and composition. Once produced, these entropy and composition spots are advected with the mean flow until they reach the first stage of the gas turbine, where they are accelerated, generating sound waves (indirect noise) [1][2][3][4][5][6][7]. These acoustic waves are partially reflected upstream in the combustor, where they may couple with the combustor acoustics, triggering a low frequency combustion instability often called...

Laser-induced grating spectroscopy (LIGS) is an optical diagnostic technique for gas-phase thermometry in challenging environments where physical probes are undesirable. The Portable In-line LIGS for Optical Thermometry (PILOT) instrument is a novel self-contained, compact device capable of tracer-free LIGS measurements at 400 Hz. It can be mounted in any orientation and includes internal alignment capability, adjustable path length matching for the pump beams, and an energy/power attenuation mechanism for the pump/probe beams. Characterization of the instrument demonstrated that it can produce accurate (<0.37% in ambient air) and precise (±0.7% in ambient air) spatially- and temporally-resolved temperature measurements, and is now ready to be deployed in research facilities.

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