High Frequency Measurement of Temperature and Composition Spots With LITGS

Abstract

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...

Compositional and entropy indirect noise generated in subsonic non-isentropic nozzles

“…Simulations from Rodrigues, Busseti & Hochgreb (2020) and experiments from De Domenico et al. (2019 b ) show that the injected plume mixes sufficiently very close to the injection location, allowing for a quasi-one-dimensional approximation. Following the analysis of Broadwell & Breidenthal (1984), it can be demonstrated that the full mixing penetration occurs before the spot enters the nozzle.…”

On the dispersion of entropy waves in turbulent flows

“…Wang et al 2019;Yoon 2020), with their contribution to the instabilities (e.g. Eckstein & Sattelmayer 2006;Schulz et al 2019) and with their measurements (Wassmer et al 2017;De Domenico et al 2019). Most of the modelling efforts to date have been about entropy-wave dispersion in fully developed turbulent duct flows (Sattelmayer 2002;Morgans, Goh & Dahan 2013;Morgans & Duran 2016;Giusti et al 2017;Fattahi, Hosseinalipour & Karimi 2017;Christodoulou et al 2020;Hosseinalipour et al 2020), where shear dispersion was identified as the dominant mechanism.…”

“…2017; De Domenico et al. 2019). Most of the modelling efforts to date have been about entropy-wave dispersion in fully developed turbulent duct flows (Sattelmayer 2002; Morgans, Goh & Dahan 2013; Morgans & Duran 2016; Giusti et al.…”

A comparative analysis of the evolution of compositional and entropy waves in turbulent channel flows

“…However, so far, there is no systematic investigation focused on the decay of compositional waves. Existing studies of compositional noise have speculated rather differently about the evolution of these waves [22,[57][58][59] and a conclusive view is yet to emerge. The current work aims to address this issue through conduction of high fidelity numerical simulations of the propagation process of a compositional wave in turbulent channel flows.…”