Measurement and modeling of the generation and the transport of entropy waves in a model gas turbine combustor

Abstract

Indirect combustion noise is caused by entropy spots that are accelerated at the first turbine stage. These so-called entropy waves originate from the equivalence ratio fluctuations in the air–fuel mixture upstream of the flame. As entropy waves propagate convectively through the combustion chamber, they are subject to diffusion and dispersion. Because of the inherent difficulty of accurately measuring the burned gas temperature with sufficient temporal resolution, experimental data of entropy waves are scarce. In this work, the transfer function between equivalence ratio fluctuations and entropy fluctuations is modeled by a linearized reactor model, and the transport of entropy waves is investigated based on a convection-diffusion model. Temperature fluctuations are measured by means of a novel measurement technique at different axial positions downstream of the premixed flame, which is forced by periodic fuel injection. Experiments with various flow velocities and excitation frequencies enable model validation over a wide range of parameters.

On the dispersion of entropy waves in turbulent flows

“…This post-processing is illustrated in figure 6(a), which showsT(x = 0.1 m, t), its power spectral density (PSD) and its fundamental component for T j = 514 K. Similarly, the experimentalT(χ ) are averaged in the y direction and their Fourier amplitudes are deduced for each pulsed-jet frequency and temperature. The propagation and dispersion of entropy waves has shown to adhere well to the local Strouhal number St = fL/ū in the experimental campaign by Wassmer et al (2017). Assuming the same is true for the current experiment, the comparison between experiments and simulations is performed by plotting the amplitudes of these coherent temperature fluctuations as a function of St, which is done in figure 6(b).…”

“…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.…”

Onset of flame-intrinsic thermoacoustic instabilities in partially premixed turbulent combustors

“…These waves are convected downstream along with the base flow. Their dissipation and dispersion are modeled using the expression given in Wassmer et al 33 They verified that the expression correctly predicts the convecting behavior of entropy waves observed in experiments. Furthermore, at the entrance to the nozzle (location 6), we apply the boundary condition given by Marble and Candel 22 for short unchoked nozzles to account for the reflection of acoustic waves due to the incident acoustic and entropy waves.…”

“…cc =Pe p À Á À Á relating the entropy fluctuations at locations 5 and 6 (equation 14). This form of the transfer function was proposed recently by Wassmer et al, 33 which models the dissipation and dispersion of the entropy waves, as they move from station 5 to 6. Pe is the Peclet number, whose numerical value is suggested to be 30 by Wassmer et al 33 Secondly, in the present investigation, it is observed that the nozzle exit is unchoked and hence the boundary condition for short unchoked nozzle is used from Marble and Candel, 22 which is given as equation (15).…”

High Frequency Measurement of Temperature and Composition Spots With LITGS

“…Non-invasive, high frequency techniques to measure temperature and composition variations in reacting flows include acoustic source-receiver measurements, interferometry, laser absorption spectroscopy, Rayleigh scattering, CARS (Coherent Anti-Stokes Raman Spectroscopy) and LIGS (Laser Induced Grating Spectroscopy). Acoustic source-receiver measurements allow tomographic reconstruction of the temperature field, but involve a complex set-up with many microphones and electrodes connected to a combustor section [26]. Interferometric and laser absorption techniques can achieve good accuracy, but are also limited to line of sight measurements [27,28].…”