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Flameless combustion is one of the most promising technologies that can meet the stringent demands of reduced pollution and increased reliability in future gas turbine engines. Although this new combustion technology has been successfully applied to industrial furnaces, there are inherent problems that prevent application of this promising technology in a gas turbine combustor. One of the main problems is the need for recirculating large amount of burnt gases with low oxygen content, within limited volume, and over a wide range of operating conditions. In the present paper, thermodynamic analysis of a novel combustion methodology operating in the flameless combustion regime for a gas turbine combustor is carried out from the first principles, with an objective to reduce oxygen concentration and temperature in the primary combustion zone. The present analysis shows that unlike in the conventional gas turbine combustor, transferring heat from primary combustion zone to secondary (annulus) cooling air can substantially reduce oxygen concentration in reactants and the combustion temperature, thus reducing NOx formation by a large margin. In addition, to reduce the peak temperature, the proposed methodology is conceptualised / designed such that energy from fuel is released in two steps, hence reducing the peak flame temperature substantially. The new proposed methodology with internal conjugate heat transfer is compared vis-a`-vis to other existing schemes and the benefits are brought out explicitly. It is found that transferring heat from the combustion zone reduces oxygen concentration and increases carbon-dioxide concentration in the combustor, thus creating an environment conducive for flameless combustion. In addition, a schematic of a practical engineering design working on the new proposed methodology is presented. This new methodology, which calls for transfer of heat from the primary combustion zone to alternative air streams, is expected to change the way gas turbine combustors will be designed in the future.

A promising method of reducing NOx emissions in combustion systems is the Flameless oxidation (FO), which is based on significant dilution of the oxygen concentration in the reactant stream and elevating its temperature to above auto ignition level. The present work is aimed at developing an FO based combustor for a sequential combustion turbofan engine, where the primary combustor is fuelled with H2 and the secondary combustor with hydrocarbon (jet or bio-jet) fuel. The work was performed within the framework of the European project AHEAD (www.ahead-euproject.eu). Being situated between the high pressure and the low pressure turbines, the inlet conditions to the FO combustor are non-conventional. CHEMKIN simulations revealed the theoretical feasibility of a combustion system to operate in the FO mode of combustion under the specific Take off and Cruise operating conditions. Several design iterations were conducted to find an appropriate geometrical configuration that would allow for such a system to operate in a stable manner. The design iterations were followed by intensive CFD simulations (FLUENT) and a final design was a achieved where the predictions indicated nearly uniform internal temperature distribution with low mass fraction of CO (14.4ppm) and NOx (0.5 ppm) at the exhaust. A separate experimental verification study was performed and confirmed the ability of the CFD model to predict the behaviour of such a combustion configuration within the hybrid turbofan engine and its results will be published elsewhere.

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