Investigations Of The Dynamics Of Multi-flame And Its Effect On Thermoacoustic Stabilities

Combustion instabilities are often encountered in modern gas turbine combustors,especially with the improving requirements on low-NOx emissions.New challenges for thermoacoustic stabilities have been raised by new combustion designs such as pilot-stabilized premixed flames and circumferentially placed multi-flames in annular combustors,due to the complex coupling between the flames and acoustics,forming a multi-input dynamic system.Based on these understandings,this thesis applies experiments,theoretical analysis and numerical simulation,to investigate the multi-flame dynamics and its effects on the thermoacoustic instabilities systematically.The findings will support the efforts in improving the stabilities of clean combustion.The flame dynamics of both pilot-stabilized premixed flames and circumferentially placed multi-flames in annular combustors are studied in this thesis,using theoretical methods and experimental measurements.The characteristics of thermoacoustic instability modes in the combustors are studied with analytical and numerical tools,aiming at revealing the effects of several asymmetries in combustors.The flame dynamics of piloted premixed flame is investigated under main flame forcing,pilot flame forcing,and dual-forcing of both pilot and main flames.A model is established to describe the dynamic interactions between pilot and main flames,which showed the process clearly.It was also found in the experiments that a higher response to the main force when the flame configuration switched from non-piloted to piloted.Indirect forcing by the forced pilot flame will be introduced to the main flame when only pilot flame is forced,but the amplitude of flame response was low because of the low power introduced by the pilot flame.However,the interaction process will enhance the effect of the forced pilot flame under the dual-forcing case,where a more significant effect of the forced pilot to mains can be found.The model can describe the flame dynamic process.Modeling and measurements were conducted for a forced planar Bunsen flame.The flame is generally set as an asymmetric flame,and the forcing can come from both axial and the transverse directions.As the model reveals,the flame response is vanishing when the flame is symmetric,while a band-pass characteristic is found when the flame is asymmetric and subjected to transverse forcing only.The flame response under simultaneous axial and transverse forcing is equal to the sum of responses to axial forcing only and transverse forcing only,given the fact that the flame response is in the linear range.The experiment results on flame dynamic process and flame transfer function can agree with the analysis.The analysis on flame dynamics is applied to thermoacoustic network model.Analysis on the circumferential thermoacoustic modes is conducted using the network model,to discuss the effects of transverse mean flow,asymmetries in the distribution of axial flame response,and the transverse flame response.The azimuthal modes in the combustor is found to have a more ”spinning” trend when transverse forcing is introduced.The results are validated by 3-D Helmholtz solver.