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One of the most important processes in a gas turbine combustor, influencing to a large extent the efficiency of the entire combustion process, is the mixing between a swirling annular jet (primary air) and the non-swirling inner jet (fuel). To study this fundamental flow geometry an experimental facility has been built which allows independent flow rate adjustment of the central (mean stream) and co-axial jet flow m c /m m and furthermore, good optical access for laser-based flow measurement techniques. Other important flow parameters include the Reynolds number, the swirl intensity (S) and the combustor confinement, expressed in terms of an area expansion ratio (ER). The work presented focuses on features of the swirling flow in the concentric annuli part of the inlet system. The laser Doppler technique has been used to measure the velocity profile and the gradient of the velocity in the annular cross section. The circumferential velocity profile follows the so-called free-vortex flow type, being characterized by an increase in the mean angular momentum with radius of curvature. The outcome of the experiment is that the axial velocity profile becomes increasingly asymmetric with increased swirl intensity. The velocity increases from the inside to the outside of the annular flow (with a decreasing gradient at the inner wall) corresponding to an intensified radial movement towards the outer wall due to imposed swirl. The numerical investigations, especially those accounting for the complete swirl generation system and using a second-moment closure reproduced all important mean flow and turbulent features in good agreement with available experimental data. In addition, both the modelling and the Large Eddy Simulations of equilibrium, fully-developed swirling flow in the annular inlet pipe performed separately in course of the inflow data generation, revealed some interesting departures with respect to the sign of the axial velocity gradient. |
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