Abstract
Fully compressible Large Eddy Simulations (LES) are performed to investigate the effect of wall boundary conditions on the nonlinear response of turbulent premixed flames. For the unforced flames, the flame length and flame angle from the present LES results in the isothermal case are in better agreement with experimental results than those in the adiabatic case. For the forced flames, both amplitudes and phases of nonlinear flame response at moderate frequency are well reproduced in the isothermal case. While in the adiabatic case, the amplitudes are well reproduced, the prediction of phases is slightly underestimated. At high frequency, LES results from the isothermal case provide reasonable agreement with the experimental results in general, while the adiabatic case gives unreasonable prediction of nonlinear flame response. Results show that wall boundary conditions affect the spatial distributions of heat release fluctuations by changing the temperature field, which is affected by the evolutions of the flame structure and flow field in the nonlinear oscillation cycle. At moderate frequency, heat release fluctuations in different parts of the combustor are in phase, and wall boundary conditions have limited influence on the global flame response, while at high frequency, wall boundary conditions have significant influence on the global flame response since the heat release fluctuations in different parts of combustor are out of phase. Accurate prediction of flame response at high frequency needs accurate calculation of the wall temperature.
Highlights
Combustion instability is a major challenge to the development of low-emission advanced gas turbines, especially in the lean premixed combustors.1 The mechanism of combustion instability in practical combustion devices involves very complex interactions between acoustic, combustion, and hydrodynamic
The flame dynamics at f = 310 Hz and A = 0.62 are well reproduced by the present Large Eddy Simulations (LES) in the isothermal case, while in the adiabatic case, the first mushroom-shaped flame structure is well characterized, but separated from the main flame structure at 60○, which results in unreasonable nonlinear flame response and underestimated prediction of phases
The fully compressible LES method is used to investigate the effect of wall boundary conditions on the nonlinear response of turbulent premixed flames at different forcing frequencies and amplitudes
Summary
Combustion instability is a major challenge to the development of low-emission advanced gas turbines, especially in the lean premixed combustors. The mechanism of combustion instability in practical combustion devices involves very complex interactions between acoustic, combustion, and hydrodynamic. Kraus et al. used a large eddy simulation (LES) method to study the effect of wall boundary conditions on the self-excited combustion instability and found that the isothermal wall boundary condition can give better prediction of instability modes, but the predicted amplitudes of pressure fluctuation still had obvious deviation from the experimental data. Numerical results in the linear region at 160 Hz agreed well with experimental data, while they deviated a lot at 40 Hz and in the nonlinear region at 160 Hz. Han and Morgans used the low-Mach number LES method to investigate the flame response at 40 Hz, 160 Hz, 240 Hz, and 310 Hz. The results agreed well with experiments at 40 Hz and 240 Hz but were underestimated in the nonlinear region at 160 Hz and gave unreasonable nonlinear response at 310 Hz. Hajialigol and Mazaheri used the compressible LES method to further study the effect of the convective thermal boundary condition on the flame response and found that the predicted amplitudes of flame response at high forcing amplitudes were higher than those in the adiabatic wall boundary condition.
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