Browsing by Author "Filosa, Antonio"

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Numerical investigation of thermo-acoustic instabilities using detailed chemistry approaches
(2017) Filosa, Antonio; Aigner, Manfred (Prof. Dr.-Ing.)
The numerical investigation of turbulent reacting flows in gas turbine related configurations is nowadays of high interest. This contributes to reduce the number of tests required for the design and for the optimization of the combustion chamber. New combustion concepts must be developed in order to meet the requirements concerning the pollutant emission in a wide range of conditions. One of the trade-off for achieving low emissions is represented by instabilities especially in the lean premixed combustion, which can lead to structure vibration, enhancement of heat transfer, blow-off and flame flash back. Combustion instabilities are selfexcited pressure fluctuations which occur during unsteady combustion, where pressure and heat release oscillations interact in the combustion chamber. Here, in particular acoustic oscillations drive the heat release rate to fluctuate and thus to feed energy to the acoustic field. In order to gain more knowledge on self-excited oscillations in a combustion process and to study the possible effects that this may generate in the burner, an academic model combustor was designed to represent a combustion-driven Rijke tube by Jim Kok et al.. This combustor was investigated under the EU-funded project "LIMit cycles of thermoacOUstic oscillationS in gas turbINE combustors", abbreviated as "LIMOUSINE". Due to its geometry, self-excited oscillations of the pressure field can occur in the combustor as a result of the closed feedback between acoustics and combustion. In the present work a numerical study of this combustor was performed. Experimentally, the acoustic behaviour of the combustor was determined under stable and unstable conditions, recording the pressure oscillations at different positions. The flame front and the combustion region were detected by mean of the OH* chemiluminescence technique. Additionally gas-phase temperature values were taken using the Coherent Anti-Stokes Raman Scattering (CARS) technique. With the intent to predict accurately the dynamics behavior of the LIMOUSINE combustor, several numerical tools consisting of various detailed chemistry combustion models (Fractal Model FM, Eddy Dissipation Concept Model EDC), and ad-hoc thermal methods (Abe et al. AKNt, Huag and Bradshaw HB model) were implemented in the DLR combustion code THETA. The models were validated first with simple test-cases for steady and then for unsteady conditions. For the numerical verification of the combustion models, simulations were performed considering a jet flame test case (H3-Flame) and a real scale swirl combustor for small industrial gas turbines (G30-Dry Low Emission Combustor). To elucidate the performance of the thermal models instead, various computations were carried out to predict the heat transfer in a cavity, in a pipe expansion, in a backward facing step and also in an oscillating flow. The latter was investigated in order to prove the heat transfer enhancement in unsteady conditions. The numerical results have shown an improvement of the accuracy of the heat transfer when the thermal models are used. In order to simulate the acoustic behaviour of the LIMOUSINE combustor under thermoacoustic oscillations, various numerical simulations were performed. First, a simple calculation was run with global chemistry (Eddy Dissipation Model EDM). Later computations with detailed chemistry (Eddy Dissipation Concept Model EDC) and with the thermal model (Huag and Bradshaw HB model) were carried out. The computation with the EDC combustion model shows an improvement in the determination of the acoustic characteristics (in terms of acoustic frequency and amplitude of oscillations) compared to the case with the EDM. In detail, a main frequency of 250Hz and 185Hz was found with the EDM and EDC respectively. The latter is in good agreement with the experimental value of 181Hz. Furthermore, simulations at a different operative condition were performed using the EDC in conjunction with HB (Huag and Bradshaw HB model). The main goal was to assess the influence of detailed chemistry and unsteady heat transfer on the acoustic behaviour. The results show again that the use of detailed chemistry is necessary to simulate accurately the acoustics of the combustor. Also the unsteady heat transfer is better predicted by considering a non-constant turbulent Prandtl number using the Huag and Bradshaw HB thermal model.