Session: 04-12: Ignition I

Paper Number: 82592

82592 - Stabilization of Auto-Igniting Flames Within a Gas Turbine Sequential Combustor, Through the Control of Static Temperature Variation - Detailed Numerical Investigation 

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. The present paper focuses on the downstream stage of a sequential combustion system.

 

In a previous publication (GT2020-14225) the authors introduced a novel stabilization concept for a sequential flame. Though reaction within such a flame is initiate through auto-ignition, the flame has to be anchored, 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. In the proposed concept however, instead of utilizing flow reversals to provide flame anchoring of the 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, where the resultant static temperature gradient provides anchoring for the flame. This method of stabilization does not rely on fluid mechanic phenomena, such as propagation of reaction from bluff-body stabilizers or the transport of heat and radicals from the flame zone.  Reactions can then proceed 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. Additionally, the relatively clean aerodynamics makes the concept particularly suited to an integrated combustor-nozzle guide vane arrangement, thereby eliminating the leakage plane between combustor and guide vane.

 

In the previous publication, the potential of the concept was demonstrated through integral and 1D approaches, where it was shown that fuel/air mixing could be achieved within approximately 1% pressure drop, and a suitable arrangement could limit Rayleigh losses to approximately 3%.

 

In the present work, detailed numerical investigations are conducted to demonstrate an actual design of the concept that is shown to meet emissions and pressure drop requirements. This is done by way of CFD modelling. The CFD, for which a fully compressible formulation is necessarily adopted, incorporates a reheat combustion model that addresses the autoignition and heat release processes. In contrast to previously published models, the present model allows for reactions occurring at high Mach numbers, where significant differences exist between static and total quantities. The CFD shows that sufficient fuel/air premixing can be achieved at pressure drops of ca. 1% provided that the premixer is optimized to allow sufficient macro fuel distribution, while incorporating mixing devices in the main flow path that are designed to achieve the desired turbulence characteristics. In order to limit the pressure drop the mixing devices have to be relatively small, but are well within the capabilities of additive manufacture technology. The results also confirm that acceptable pressure losses for the system can be achieved, provided that the flame is located in the diffuser section, such that the pre-flame Mach number is approximately 0.3.

 

The dynamic characteristics of the system have also been investigated. This was performed by firstly determining the flame transfer function through CFD, where the inlet boundary condition to the premixer is perturbed by imposing a broadband excitation to both the mass flow and temperature. The flame transfer function is then implemented into an acoustic network model which is used to  investigate the system thermoacoustic characteristics. The network model is then used to identify key design and operating parameters that influence the thermoacoustic behaviour and the system stability.

Presenting Author: Fernando Biagioli Infosys Ltd

Presenting Author Biography: Fernando Biagioli has MSc and PhD degrees from Rome University. He has spent 30 years in the Gas Turbine industry, having initially focused on combustion modelling and then taking on development roles in combustion, within Alstom and GE. He has worked on the implementation of new low emissions combustion technologies, for multi-fuel and fuel flexible systems, including Hydrogen, as well as for both standard and reheat combustion applications. He is presently a member of the Combustion at Infosys Turbomachinery &amp; Propulsion.

Authors:

Fernando Biagioli Infosys Ltd
Holger Luebcke Infosys Ltd
Ammar Lamraoui Infosys Ltd
Khawar Jamil Syed Infosys Ltd
Andre Theuer Infosys Ltd
Ennio Pasqualotto Infosys Ltd