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Abstract

The precessing vortex core (PVC) is the dominant coherent structure of swirling jets, which are commonly applied in gas turbine combustion. It stems from a global hydrodynamic instability that is caused by internal feedback mechanisms in the jet core. In this work, we apply open and closed-loop forcing in a generic non-reacting jet to control this mechanism and the PVC. Control is exerted by two oppositely facing, counter-phased zero-net mass flux jets, which are introduced radially into the flow through a thin lance positioned on the jet center axis. By using this type of forcing, the instability mode m = 1, corresponding to the PVC, can either be excited or damped. This markedly affects the PVC oscillation frequency and amplitude. The passive influence of the actuation lance on the mean flow field properties and the coherent flow dynamics is studied first without forcing. PIV and hot-wire measurements reveal an effect on the mean flow, but no qualitative changes of the PVC dynamics. Lock-in experiments are conducted, in which the synchronization behavior of the PVC with the forcing is determined. Here, two different cases are considered. First, actuation is applied at different streamwise positions in order to identify the region of highest receptivity towards external forcing. This region of lowest lock-in amplitude is shown to coincide with the location of the wavemaker, shortly upstream of the vortex breakdown bubble. Second, the lock-in behavior at a fixed axial position and various forcing frequencies ff is studied. A linear correlation between the lock-in amplitude and the deviation of the forcing frequency from the natural oscillation frequency |ff – fn| is observed. Closed-loop control is then applied with the aim to suppress the PVC. The actuator lance is positioned in the wavemaker region, where the flow is most receptive. Magnitude and phase of the natural flow oscillation associated with the PVC are estimated from four hot-wire signals using an extended Kalman filter. The estimated PVC signal is phase-shifted and fed back to the actuator. PIV measurements reveal that feedback control achieves a reduction of the PVC oscillation energy of about 40%.

In the present study a new combustor design for swirl-stabilized premixed combustion is presented and analyzed. By means of axial air injection into the mixing tube, the flow field inside the mixing tube and combustion chamber is altered for the purpose of increasing flame stability, resulting in low lean blow-out limits and flashback safety. For this reason, the combustor design is suited for fuel-flexible combustion over a wide range of reactivities including those of pure hydrogen and natural gas. Furthermore, the combustor design is suitable for high amounts of steam injection, significantly decreasing NOx emissions. The assessment of the flow field properties is conducted under isothermal and reacting conditions and the results are in good agreement. The fuel-air-mixing quality is examined by water tunnel experiments using laser induced fluorescence and show low levels of unmixedness. Flame properties are assessed by means of OH planar laser induced fluorescence and OH*-chemiluminescence. Emission probing of major combustion species was also conducted.

It is generally accepted that combustion of hydrogen and natural gas mixtures will become more prevalent in the near future, to allow for a further penetration of renewables in the European power generation system. The current work aims at the demonstration of the advantages of steam dilution, when highly reactive combustible mixtures are used in a swirl-stabilized combustor. To this end, high-pressure experiments have been conducted with a generic swirl-stabilized combustor featuring axial air injection to increase flashback safety. The experiments have been conducted with two fuel mixtures, at various pressure levels up to 9 bar and at four levels of steam dilution up to 25% steam-to-air mass flow ratio. Natural gas has been used as a reference fuel, whereas a mixture of natural gas and hydrogen (10% hydrogen by mass) represented an upper limit of hydrogen concentration in a natural gas network with hydrogen enrichment. The results of the emissions measurements are presented along with a reactor network model. The latter is applied as a means to qualitatively understand the chemical processes responsible for the observed emissions and their trends with increasing pressure and steam injection.

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Articles you may be interested inThe particle size magnifier closing the gap between measurement of molecules, molecular clusters and aerosol particles AIP Conf.A new method for determination of particlesize distribution function from small angle scattering data A new method (electronic cascade impaction) of measuring the particle size distribution of aerosols is presented, and an instrument we call an electronic cascade impactor (ECI) which uses the method is described. Aerosol particles are charged in a unipolar charger and enter a multistage cascade impactor. Each collection stage and the final filter is isolated electrically from other parts of the impactor and connected to an electrometer detector. Particles deposit on the collection stages according to their aerodynamic diameters, and from the associated electrical current the deposition rate onto each stage and the particle size distribution can be determined. Calibration data for the ECI are presented. The ECI has been used to study atmospheric aerosol dynamics in real time, and typical data are given. The ECI has proven to be relialbe in several hundred hours of laboratory and field use and other versions employing the same general method should prove useful in a wide variety of measurement situations.PACS numbers: 92.60.Mt, 82.70.Rr 516Rev. Sci.

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