Self-Excited Instability Regimes of Confined Turbulent Premixed Jet Flames

2020-03-04T13:23:55Z (GMT) by Timo Buschhagen
The demand for more efficient and cleaner terrestrial gas turbine engines for energy generation has accelerated with stringent emission regulations. Most terrestrial gas turbine engine configurations utilize lean premixed jet stabilized flames for energy extraction. These highly turbulent flames provide the necessary thermal power densities and low NOx emissions but are prone to combustion instabilities. This work studies the stability of a canonical premixed turbulent jet flame to changes in operation condition at elevated pressure. The underlying coupling mechanisms leading to the excitation of different modes are of interest. Different fuel injection schemes are considered, to delineate the influence of system coupled excitation and flow field intrinsic mechanisms that lead to self-excitation of longitudinal and transverse modes in an axisymmetric jet flame.

The fundamental longitudinal mode sensitivity to operation conditions was investigated in a technically-premixed configuration, for which the fuel injector is exposed to the system dynamics. Global equivalence ratio fluctuations paired with flame-vortex interactions were observed to sustain the 1L mode, for which leaner operating conditions lead to an increase of the limit-cycle amplitude.

To solely focus on the shear layer dynamics involved in the feedback loop of the 1L instability a "fully-premixed" configuration (FPC) of the combustor was designed. The elimination of potential equivalence ratio fluctuations lead to the excitation of longitudinal as well as transverse, and spinning modes, which can be associated with specific burner operation envelope regions. 1L mode coupled flame dynamics indicate axisymmetric emission patterns in OH* emission imaging corresponding to axisymmetric instabilities in the shear layer. Transverse modes correlate with an asymmetric shear layer roll-up process and a flapping motion of the flame. Spinning modes are characterized by high levels of limit-cycle amplitude and a single wave is observed that travels in the annulus of the recirculation zone. From the high speed imaging an azimuthal wave speed of up to 90% of the Chapman-Jouguet velocity for the natural gas - air mixture is computed.

The transverse mode is found to be sensitive to changes in chamber pressure and injector velocity. For a baseline injector velocity at which 1T mode excitation occurs, an increase in system pressure lead to an increase of the 1T mode amplitude. The 1T mode excitation is found to be sensitive to the injector velocity, where the highest amplitude are observed for a Strouhal number band of 1.6-1.7 based on the injector diameter.

A linear stability analysis (LSA) of the underlying base flow field is performed in order to assess if the underlying shear layer instability modes determine the selection of the instability regime for a given flame condition. Two flow perturbation modes are supported by the flow field, Mode 1 is associated with the recirculation zone domain, where preferred mode frequencies favors coupling with longitudinal acoustics chamber modes. Mode 2 resides in the mixture jet, for which preferred mode frequencies match local 1T acoustic eigenfrequencies of the chamber. It is found that for 1T mode dominated operation cases, the recirculation zone associated mode is stabilized leading to to the excitation of the 1T mode in the injector near field. For operation cases showing predominantly longitudinal combustion instability, the recirculation zone mode shows elevated growth rates in the injector near field paired with preferred frequencies that are compatible with the longitudinal acoustic eigenmodes.

These observations indicate that the preferred shear layer perturbation modes impact the selection of the combustion instability regime for a given flame operation condition. These results can be utilized to build a model framework for the design of jet flame type burners, to avoid geometries and operation regions that show a high potential for transverse mode excitation by the underlying flow field.