Towards flameless combustion in gas turbine engines

This thesis provides insights on the effect of the air and fuel inlet configurations in enhancing flameless combustion in a single stage gas turbine combustion chamber, by numerically investigating different geometries. This investigation is to aid in the design of a combustion chamber to bring the successful application of flameless combustion (FC) in gas turbine engines one step closer to realization. The interest in FC originates from the projection that in the upcoming decades fossil fuels will remain the dominant form of aviation fuels. Within this perspective, flameless combustion is considered one of the most promising technologies that is able to meet the stringent demands of reduced pollution in the near future. FC occurs at very low oxygen concentrations, approximately between 3%-13%, and at temperatures above auto-ignition of the fuel. Under these conditions the combustion process takes place in a distributed reaction zone, has relatively uniform temperatures and practically no visible flame. The potential benefits of applying flameless combustion in gas turbine engines are a significant reduction in Nitrogen Oxide (NOx ) and Carbon Monoxide (CO) emissions, improved pattern factor and lower sound pressure levels. Based on the knowledge that has been published throughout literature an initial new baseline design is proposed. The baseline design has a cylindrical cross-section with a forward flow configuration equipped with 12 air nozzles and 24 fuel nozzles. The air nozzles are positioned on a circle with a radius of 35.56 mm concentric with the axis of symmetry. The diameter of the combustion chamber is 100 mm and its length is 340 mm. This baseline design served as a starting point for a three-dimensional computational fluid dynamics investigation. As computational resources and time for this investigation are relatively limited, two numerical models have been employed. First, a relatively simplified but computationally light model is used to investigate the different geometries. This model is based upon the mixture fraction model ( f -PDF) and convergence is reached within 2 days. Subsequently, the geometry that performs the best in this geometrical investigation is simulated by a skeletal model. This numerical model is based upon the Eddy Dissipation Concept (EDC) for the turbulence-chemistry interaction with the DRM19 skeletal chemical reaction mechanism. As it requires 10 days to reach convergence it is only used to simulate the best performing geometry. Both models employ the RANS-based realizable k −2 turbulence model. They are validated against the experimental data of a combustion developed by the research group at the Instituto Superior Técnico. It is observed that the f -PDF model predicts a thinner reaction zone with higher local peak temperatures, lower O2 content and higher CO2 concentrations. In comparison, the skeletal model predicts a more distributed reaction zone that is in better agreement with experimental data. However, both models result in unsatisfactory predictions of CO concentrations. Moreover, it has been observed that the prescribed fixed temperature of 1400 K at the walls leads to a consistent overprediction of the temperatures at the exhaust and in the recirculation zone. The geometrical investigation basically consists of three parts. The first part is aimed at investigating the effect of the radial position of the air nozzle on the recirculation zone and the reaction zone. Using the f -PDF model, it was found that the total pressure loss and combustion efficiency of the combustor is independent of the air and fuel inlet configuration. It was shown that the radial position has a strong influence on the velocity field and thus on the recirculation zone, reaction zone and emissions of NOx and CO. When the nozzles are located further from the wall, closer to the axis of symmetry, the total recirculation increases by as much as 26%. The higher total recirculation has a beneficial effect on the reaction zone, most likely resulting in a more distributed reaction zone. Furthermore, it was observed that a single nozzle results in a deeper penetration of the jet into the combustion chamber and simultaneously forming a larger recirculation zone. For a single jet the recirculation zone extends to about 90% of the combustor length while for 12 separate nozzles it extends to about 32%. As a result, the total recirculation ratio is doubled in the case of a single jet. In general, it is found that separating the total air mass flow over multiple nozzles is not beneficial. However, when the air nozzles are located relatively close to one another they merge into one jet that is able to penetrate deep into the combustor. When the 12 air nozzles are positioned in a circumference of 15.0 mm radius concentric with the axis of symmetry, the highest total recirculation ratio (Ktot =0.2639) is obtained. As a result, the lowest emissions, NOx = 14.90 ppm and CO = 39.12 ppm at 15% O2 are predicted. In