As gas turbine firing temperatures increase, it becomes increasingly difficult to maintain low NOx emissions. Thermal NOx formation is relatively slow, so can be countered by reducing combustor residence time. This however exacerbates the emissions of incomplete combustion at part load. Axial staging of fuel in the combustion system, such as that within GE’s “Axial Fuel Staging” or Ansaldo Energia’s “Constant Pressure Sequential Combustion” allows both sufficient residence times for the control of part load emissions as well as limiting the residence time at the highest temperatures, which are confined to the downstream part of the combustion. system.

Here a novel concept for the stabilization of the flame within the downstream part of the combustor, that is configured to operate as part of a sequential system, is proposed and investigated. Within such a system, the flame in question is stabilized by autoignition. Despite this, the flame has to be anchored in some fashion, in order to avoid large excursions in flame location, due to perturbations in, for example, inlet temperature. Presently employed sequential systems employ a dump expansion for flame anchoring. Here however, instead of utilizing flow reversals to provide flame anchoring of the auto-ignition flame, the flow in the premixing section of the combustor is accelerated to a high, but still subsonic, Mach number and then decelerated. The heat release is designed to take place in the deceleration section. It is shown that stable combustion can be achieved, without the need for reliance on fluid mechanic phenomena, such as propagation of reaction from bluff-body stabilizers or on the transport of downstream heat and radicals. This allows combustion to take place at rates governed by the chemical kinetics, rather than being rate limited by fluid mechanic phenomena. Under these circumstances, CO reaches its equilibrium in \<1msec, which allows for compactness and minimal NOx.

The above is demonstrated by way of analytical and numerical investigations. An integral approach is employed to investigate the tradeoff between premixing time, turbulence length scale and pressure drop to show the feasibility of premixing fuel and oxidant at high velocities, while keeping pressure loss at acceptable levels. 1D analyses are conducted to investigate the compressible flow and combustion within the system to characterize the auto-ignition progress, combustion stability and burnout, as well as showing that Rayleigh losses are acceptable. Finally, 3D CFD is used to show that a design can be devised for which the overall pressure drop for the combustor can be kept to ca. 4%.

In recent years there has been interest in integrating the combustion system with the nozzle guide vanes (NGV) as this allows for better air flow management, through removal of the key leakage plane between the combustor and the NGV, as well as for a more compact system. However, releasing heat within a standard NGV configuration, can be unstable and lead to high pressure drop. The integration of the proposed concept as a combined combustor vane is also investigated.  The results show that the concept is well suited to an integrated combustor/NGV system.


A Novel Approach to the Stabilization of Auto-Igniting Flames Within a Gas Turbine Sequential Combustor, Through the Control of Static Temperature Variation Along the Premixing and Flame Zones

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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