Experimental investigation of flows related to gas turbine combustion

University dissertation from Fluid Mechanics

Abstract: The notion of "global warming" has become a major public-domain concept as an environmental threat to our future on earth. Independently of the accuracy of existing prediction of combustion emissions on the climate on earth, the concept of reducing combustion emissions has been an important political driving force in imposing more and more severe restrictions on harmful emissions from combustion. To meet the high standards on environment pollution as well as for improved combustion efficiency and reducing costs of produced power, industrial gas turbines (GT) have been equipped with lean premixed combustion systems. Such lean flames operate very close to the extinction limit and are sensitive to different perturbations which may result in combustion instability. Occurrence of combustion instabilities can lead to increase of emission levels, limits the operation range of the GT and can lead to the destruction of combustor components. Further improvements in GT combustion is an active field of research and development (R&D). This thesis reflects some of the current direction of R&D of industrial GT. The thesis is based on experimental investigations of turbulent flows related to gas turbine combustion applications. The goal of the studies is to understand some of the basic mechanisms underlining these processes and thereby contribute to improve future GT designs. Part of the thesis regarding flameless combustion evaluates a novel concept for the gas turbine burner. The investigations include the effect of preheat temperature, air mass flow rate on the combustion dynamics and emissions. The influences of geometrical parameters like the combustion chamber diameter and the exhaust nozzle contraction have also been investigated. Using the OH' chemiluminescence the transition of the flamelet to the flameless (distributed reaction) regime has been documented. Outside of the flameless regime the combustion can be unstable and the combustion instability has been characterized. The thesis presents also a new approach towards flame stabilization in industrial gas turbines using the rich premixed pilot. Results at atmospheric conditions show very good flame stability and at the same time combustion emissions are kept at a very low level. The pilot's mass flow and equivalence ration, fuel mixture profile and swirl number have been identified as key parameters for burner operation and their impact on stability limits and emission levels has been documented. The rich pilot is recognized as the crucial part of the flame stabilization process. A jet in a crossflow is often used in gas turbine among other applications for injection of liquid and gaseous fuel a means for enhanced break-up and evaporation as well as quick mixing. The purpose of the investigation has been to study the mechanisms and structures that control the mixing process. The experiments employ combined PIV/LIF laser technique for simultaneous measurements of velocities and concentration within a plane. The study concerns the impact of injector shape and its orientation on the jet trajectory and the turbulent fluxes. The turbulent fluxes of the injected tracer are of special importance since the data can be used directly for assessing turbulence models for the tracer (scalar) and since such measurements are very scarce in the literature. The experimental results provide data for validation of computational tools.