Radial staging for the control of combustion instabilities in a mesoscale multinozzle array

Abstract

In response to energy system decarbonization, a small-scale multitube nozzle architecture capable of operating on high-hydrogen carbon-neutral fuels is expected to be widely used for Dry Low Emissions (DLE) gas turbine combustion systems. In this study, self-excited combustion instabilities in a mesoscale multinozzle array (6.5 mm i.d.) have been experimentally investigated in a lean-premixed tunable combustor operating with preheated methane and air. Sixty identical swirl injectors are divided into two groups - outer and inner stages - to form radially stratified reactant stoichiometry for the control of self-excited instabilities. Several measurement techniques, including dynamic pressure, global heat release rate, acoustic velocity, and emission gas sampling, are used to investigate the influence of radial fuel staging on self-excited instabilities and thermal NO$_X$/CO formation. In addition, CH* chemiluminescence imaging and OH Planar Laser-Induced Fluorescence (OH PLIF) are obtained to describe the global/local flame structures and dynamics of mesoscale multinozzle flames subjected to asymmetric fuel split condition.

The presence of radial fuel staging has a remarkable influence on the development of self-excited instabilities such as bifurcation patterns, limit cycle frequencies, and amplitude of pressure and heat release rate oscillations. In sixty mesoscale methane/air flames, strong self-excited oscillations occur under even split and outer enrichment conditions. Whereas inner fuel staging effectively mitigates high-amplitude oscillations, the production of thermal NO$_X$ and CO tends to increase relative to outer enrichment cases. It is caused by the presence of a high-temperature region in the center flames, which is generated by the combined effects of richer fuel streams and thermal insulation on the inner stage. By contrast, the flame temperature of outer fuel staging is presumed to be uniformly distributed, and pollutant emissions are therefore reduced. The results show that the optimization between instability amplitudes and emissions output should be carefully considered when fuel staging strategy is introduced in practical gas turbine engines.

CH* chemiluminescence images reveal that overall flame dynamics are strikingly different depending on the fuel split condition. Under even split and outer enrichment cases, large-scale flame surface deformations are induced by vigorous detachment/attachment movements, consistent with the aforementioned measurements of self-excited instabilities. The use of the inner fuel staging method was found, however, to limit the growth of large-amplitude heat release rate fluctuations since the center flames are securely anchored during the whole period of oscillation. OH PLIF measurements reveal that individual flames take on a turbulent jet-like flame structure, and their interactions strongly depend on the direction of radial fuel staging. The leaner streams merge into a single jet structure, whereas the richer reactant jets penetrate far downstream without noticeable interactions between adjacent flame fronts. The fuel staging-induced local flame structure eventually changes the collective motion of sixty flames, which plays a major role in the development of limit cycle oscillations. The present study suggests that the distribution pattern of the nozzle array, in combination with the radial fuel staging scheme, is the key to the control of the instabilities.