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.

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.…”

Experimental investigation of the hydrodynamic effects upon convecting entropy waves in nozzle flows

“…Separating the direct and indirect noise emissions is an important difficulty in interpreting experimental data on indirect noise [29]. Historically, this has led to underestimation of indirect noise, especially in the real-scale combustors [23,30].…”

“…Historically, this has led to underestimation of indirect noise, especially in the real-scale combustors [23,30]. The classical two-microphone technique [25][26][27] and elongating convecting time of the entropy wave [29,32] were employed to separate two types of direct and indirect sound. The recent experimental works of Domenico et al [29,33,34] were performed in somewhat the same test rig of Bake et al [24,25].…”

“…The classical two-microphone technique [25][26][27] and elongating convecting time of the entropy wave [29,32] were employed to separate two types of direct and indirect sound. The recent experimental works of Domenico et al [29,33,34] were performed in somewhat the same test rig of Bake et al [24,25]. They stated that despite the weaker amplitude of indirect noise in comparison to the direct one in some nozzle conditions, contribution of each type of noise to the combustor acoustics is difficult to identify when they are merged.…”

“…They stated that despite the weaker amplitude of indirect noise in comparison to the direct one in some nozzle conditions, contribution of each type of noise to the combustor acoustics is difficult to identify when they are merged. Domenico et al [29,32] used a long convective time of the entropy wave to separate the noises. They also derived an analytical model using the experimental data and embedded a pressure loss coefficient in the entropy to acoustic wave transfer function [34].…”