Abstract
ABSTRACTThe response of three flames with different degrees of premixedness (fully premixed, non-premixed with radial, and non-premixed with axial fuel injection) to acoustic oscillations is studied experimentally. The flames were imaged using OH* chemiluminescence and OH planar laser-induced fluorescence at 5 kHz. In addition to a flame kinematics analysis, the amplitude dependence of the transfer function was calculated. The dominant spatial structures of the heat release and their periodicity were examined using the proper orthogonal decomposition (POD) method. The Non-Premixed system with Radial fuel injection (NPR) showed the highest response to acoustic forcing, followed by the fully premixed and the Non-Premixed system with Axial fuel injection (NPA). In addition, the response of the non-premixed system with radial fuel injection was greater than that of the fully premixed system for various bulk velocities , global equivalence ratios , forcing amplitudes , and forcing frequencies . In the fully premixed system, the heat release modulation was mainly through flame surface area modulation, while in the NPR system, both the flame area and the equivalence ratio modulations were found to be important mechanisms of the heat release oscillations. About 70% of the energy of the total fluctuations in the NPR case was contained in the first four POD modes, a percentage that decreased with overall equivalence ratio, but only this dropped to about 40% for the NPA flame. The frequency spectra of the coefficients of the POD modes exhibited peaks at the forcing frequency, with increasing broadband contributions in higher modes and for the NPA flame.
Highlights
Combustion instabilities constitute a major challenge for the development of modern gas turbines, because they are associated with the generation of unsteady combustion, and because they lead to considerable system degradation
An experimental study was performed to investigate the response of three flames with different degrees of premixedness to acoustic oscillations
The nonpremixed with radial fuel injection (NPR) system showed the highest response to acoustic forcing, followed by the fully premixed and the Non-Premixed system with Axial fuel injection (NPA) system, which exhibited a very low response
Summary
Combustion instabilities constitute a major challenge for the development of modern gas turbines, because they are associated with the generation of unsteady combustion, and because they lead to considerable system degradation. In the search for the optimization of efficiency and longevity of gas turbines, several different operational modes have been employed to date. Lean flames present the advantage of reduced fuel consumption and NOx production; they are susceptible to combustion instabilities (Candel, 2002). The accurate prediction of the flame response is difficult because the heat release fluctuation is affected by many parameters. Various driving mechanisms of instabilities have been suggested and most importantly the flame–vortex interactions, equivalence.
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