Effects of thermoacoustic waves on combustion

Abstract

This thesis presents the result of an investigation on the effect of acoustic waves on turbulent flames with particular focus on practical applications such as gas turbine engines. The objective is to study how pressure waves affect turbulent combustion, and how to account for this effect when performing numerical simulation of industrial devices. In particular, the thesis was aimed towards the set up of an extended version of the Eddy Dissipation Concept (EDC) for the modeling of turbulent combustion able to cach the interaction between pressure waves and combustion.

The starting point was the evaluation of the amplitude and the frequency of the thermo-acoustic waves observed in gas turbine engines. Numerical simulation of both turbine and compressor stages provided the necessary data and knowledge which have been used to study and model the effect of pressure fluctuations on turbulent combustion. On the basis of the fluid dynamic data, two different kinds of perturbations were studied: large pressure fluctuation that will modify the density field, and acoustic waves.

The effect of acoustic waves on turbulent combustion was studied experimetally. In particular, an iso-thermal turbulent flow was perturbed by acoustic waves and the turbulence spectra, whit and whitout acoustic waves, were sampled. On the basis of the experimental results and by using the local entropy production as a measure of the turbulent mixing, a preliminary extended version of the EDC model was tested.

The effect of large pressure variations was instead evaluated numerically. In particular, combustion in the EDC model occurs in reactors often modelled as isobaric and perfectly stirred. To investigate the reactor sensibility, the effect of different pressure levels was tested, as well as the effect of different reactor models.

Reactor modeling proved also to be an interesting way to model noise production due to combustion. The difference of pressure between the inside and the outside of the reactor was used to model a non isobaric reactor that can be used to simulate flame noise.