The operating range of heavy duty gas turbines featuring lean premix combustion to achieve low Nox emissions may be limited by thermoacoustic oscillations. The most promising way to extend the operational envelope of the gas turbine is to modify the burner outlet conditions which itself strongly affect the flame response on acoustic perturbations. The objective of the present paper is the analysis and prediction of the flame response of premixed swirl flames which are typical for gas turbine combustion. The flame response has been determined experimentally by measuring the velocity fluctuations of a forced pulsated burner flow with hot wire probes and the resulting heat release fluctuations OH radiation. The experimentally determined flame response function for the swirl premixed flame follows almost a time lag law. Hence, reasonable agreement has been found between measurements and calculations using a time lag model.
For the suppression or reduction of self-sustained combustion instabilities, modifications of the burner outlet conditions, that strongly influence the dynamic flame response, seem to be the most promising way. Therefore, to derive a detailed physical understanding of the feedback mechanisms the dynamic flame response characteristics, quantified by flame transfer functions, are required in dependence of flame type and operation conditions of the combustor. In the present paper measurements of flame transfer functions of an industrial, full-scale prototype gas turbine burner are discussed. For the detection of periodically-unsteady OH radical radiation (response of the flame) two different UV detection systems were compared. Because the concentration of electronically-excited OH radicals in the reaction zone and therefore, of the measured UV radiation intensity, is strongly depending on volumetric reaction density and local flame temperatures, the UV radiation intensity commonly used for the quantification of the heat release can be misinterpreted. Hence, two different concepts of fuel gas/air mixture formation have been realized in the experiments to separate and to physically interpret the influence of the mixture formation and its quality on the UV radiation intensity of the determined flame transfer functions. The derived understanding of the complex interactions of mixture mass flow oscillations, fluctuations of the mixture composition and the periodic combustion of ring vortices at a full-scale burner is an essential requirement for the interpretation of flame dynamics based on measurements of the UV radiation intensity.
The prediction and the systematic suppression of self-sustained combustion instabilities in combustors for gas turbine applications still suffer from incomplete physical understanding of the feedback mechanisms and lack of experimental data of the dynamic flame characteristics of Lean-Premixed swirl flames. Hence, the experimental determination of the flame transfer functions of LP swirl flames was achieved using a mixing unit to generate a time-independent and spatial homogeneous mixture of natural gas and combustion air at the burner exit. The determined LP flame dynamics are strongly affected by the formation and in-phase reaction of coherent vortex structures, well known as drivers of combustion instabilities, that have been visualized with an phase-correlated imaging technique. The results discussed in this paper lead to a basic understanding of the frequency-dependent dynamics of LP swirl flames on periodic disturbances and especially, of the influence of the preheating temperature and the air equivalence ratio on the amplitude responses and phase angle functions. Based on the measurements and theoretical considerations concerning the burning velocity of steady-state premixed flames a physical model and — derived from it — scaling laws for the prediction of unstable operation modes in dependence of main operation parameters of the flame were formulated and validated by measurements.
One of the main objectives of combustion research in field of gas turbine application during the last decades was and still is the reduction of pollutant emissions. The most promising technology to reduce these pollutants turned out to be Lean Premixed (LP) and Lean Premixed Pre-Vaporized (LPP) combustion. However, serious problems concerning combustion-driven instabilities occurred with the implementation of the LP/LPP-concept. Today, prediction and systematic suppression of self-sustained combustion instabilities is an issue still unsolved, due to incomplete understanding of the physical feedback mechanism and the lack of models for dynamic flame response, i.e. frequency dependent characteristics of LP/LPP swirl flames. In that context, the purpose of the current paper is the establishment of a physical model to describe frequency dependent flame dynamics concerning burning velocity of steady-state premixed flames. Derived from that basic understanding, scaling laws for the prediction of unstable operation conditions will be established in dependence on main operation parameters such as thermal load, mixture temperature, air equivalence ratio and especially of fuel and operating pressure. Therefore, a new swirl-burner has been designed, offering the feasibility to choose the type of fuel, to adjust the swirl number for main and pilot burner and the burner exit geometry steplessly and to vary preheating temperatures, air equivalence ratios and thermal loads in a range of industrial relevance for gas turbine applications. To establish a periodical modulation of the mixture mass flow of the main L(P)P flame at the burner outlet sinusoidally in-time with well-defined frequencies and amplitudes, a pulsating unit was used. Using a mixing/ pre-vaporizing unit to create a time-independent and spatial homogeneous mixture of natural gas/ kerosene vapor and combustion air at the burner outlet, flame transfer functions of LP- and LPP swirl flames depending on main operating parameters were determined. The discussed results then lead to stability map for a given combustion system depending on the main operation parameters based on the knowledge of only one fully-described parameter combination leading to an instable condition. Based on this scaling procedure and confirmed by further experimental work the prediction of stability limits depending especially on the type of fuel, the swirl number and the operating pressure will be possible.
This paper presents the reacting flow field and the temperature distribution of two different airblast nozzles, namely co and counter swirl. To support the interpretation of the obtained results, previous measurements of the isothermal flow and mixture field of methane and combustion air are summarized. Velocities within the turbulent flow field were measured by using 3D-LDA, measurements of the field distribution of temperature were performed by means of thermocouple probes. The results show that the counter swirl arrangement provides formation of an additional vortex in the immediate vicinity of the nozzle, which has been observed as well within the isothermal as within the reacting flow. Furthermore, a dampening effect of the tangential velocity profiles towards turbulent exchange of momentum has been observed within the counter swirl configuration. Both effects cause preferential mixing of the fuel with the inner combustion air flow, thereby performing higher concentrations of methane in the near nozzle mixture field. As a consequence the counter swirl flow field exhibits larger areas of near-stoichiometric composition of fuel and air, resulting in an elevated temperature level within the stabilization zone at otherwise identical operation conditions. Therefore, application of a counter swirl nozzle allows a higher thermal load than the co swirl configuration, which offers a satisfying explanation for the wider operating range of the counter swirl burner.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.