|Ph.D Student||Kutikov Daniel|
|Subject||Enhancement of Internal Heat Transfer in Low NOx Gas|
|Department||Department of Aerospace Engineering||Supervisor||PROFESSOR EMERITUS Yeshayahou Levy|
|Full Thesis text|
Regulations controlling pollutant emissions from combustion systems steadily become stricter. Currently nitrogen oxides (NOx) are regarded as the most harmful pollutants emitted during combustion of fossil fuels with air. The strict pollutant emission policies and the drawbacks of existing low NOx techniques motivate research on reduction of pollutant emissions from gas turbines.
Combustion in general and the formation of pollutants in particular are strongly affected by processes of internal heat transfer and mixing in gas turbine combustors. Numerous studies suggest that introduction of swirl to the flow can gain control over heat transfer and mixing. Moreover, diverse swirl flow patterns are currently utilized for different purposes in numerous combustion systems. Consequently, swirl application was considered a promising technique for current research. The current research intended to revise the processes of heat transfer and mixing in presence of tangential swirl inside gas turbine combustors with regard to NOx emission reduction.
A novel concept of low NOx combustor was tested in course of the research. In contrast to the existing low NOx methods of primary zone reactants dilution, the new concept was based on flame cooling by pure heat transfer between the primary and secondary flows without mixing. The new concept was believed to yield low NOx combustion accompanied by a minimal amount of drawbacks. Advantages of the new concept (simple layout and small combustor volume) might have allowed application of low NOx combustors not only for ground-based power plants, but also for airborne turbojet engines.
The research included a qualitative analysis of swirl-combustion interaction and swirl-induced convection effects. The analysis led to introduction of a comprehensive approach to convective heat transfer evaluation for swirled annular flow. The analysis also revealed three major routes of swirl impact on NOx formation. The swirl impact routes of heat flux 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. The routes were tested in course of heat transfer and combustion experiments.
The heat transfer and combustion experiments in the current research were performed with a small-scale gas turbine combustor model. 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 is insufficient for significant NOx emission reduction in gas turbine combustors. The combustion experiments demonstrated NOx emission increase by a factor of about 2 due to hot core formation by the swirling flow. No significant effect of swirl on flame elongation was observed. Moreover, the study highlighted the need to perform a parametric study in order to reveal designs and operating conditions that could minimize the effect of the hot central core.
The main conclusion of the research is that the design of swirl-based combustor should be critically performed to avoid situations of an opposite effect to the desired NOx emission reduction.