Pollutant emissions from combustion systems created increasing environmental and health concerns. Regulations controlling pollutant emissions steadily become stricter. Nitrogen oxides (NOx) are currently regarded as the most harmful pollutants emitted during fuel-air combustion. Existing low NOx techniques all have significant drawbacks, namely, low combustion stability, complex layout and large combustor volume. The strict pollutant emission policies and the drawbacks of existing low NOx techniques motivate research on reduction of pollutant emissions from gas turbines.
The formation of pollutants in combustors is strongly affected by processes of internal heat transfer and mixing by means of hot spots formation modification. The current research intended to investigate the impact of tangential swirl on the heat transfer and mixing in gas turbine combustors with regard to NOx emission reduction. A novel combustor concept, based on flame cooling by pure heat transfer between the primary and secondary flows, was tested in course of the research.
A comprehensive review of swirl-combustion interaction, carried out in course of the research, revealed three major routes of swirl impact on NOx formation. The swirl impact routes of heat transfer rate augmentation and flame elongation have a potential of NOx emission reduction, while the swirl impact route of hot core formation may lead to NOx emission increase. These routes were investigated at the present study by means of analytical investigation as well as heat transfer and combustion experiments using a small-scale gas turbine combustor model. For evaluation of swirl impact, the experiments were performed for swirl flow and axial flow layouts. A novel method for convective heat transfer measurements was developed in course of the experiments.
The heat transfer experiments demonstrated that the increase of convective heat transfer coefficient due to swirl introduction is insufficient in regard to NOx emission reduction in gas turbine combustors. No remarkable effect of flow rate or temperature difference (which in a swirl flow could modify the flow pattern) on convective heat transfer coefficient was observed.
The nearly constant overall equivalence ratio in the combustion experiments matched the most stable combustion regime and was equal to about 0.15. The equivalence ratio in the primary zone was near unity. The combustion experiments demonstrated an increase of NOx emission by a factor of about 2, due to the introduction of swirl. This proved that the hot core formation due to the swirling of the flow is dominant for gas turbine combustors with tangential swirl.
