The present paper is focused on the investigation of the dynamics of the flow downstream of an Ultra Low NOx (ULN) injection system, designed to reduce NOx emissions and combustor axial length. Two rectangular flame tubes have been experimentally investigated: one aimed at simulating an unconfined exit flow, and another with the same transverse dimensions of the combustor annular sector, to simulate the confined flow field. The effects induced by the realistic flame tube presence are investigated comparing the flow field with that generated in the unconfined case. Particular attention is paid to the vortex breakdown phenomena associated with the flow generated by the two co-rotating swirlers constituting the injection system.
Two different and complementary measurement techniques have been adopted to characterize the aerodynamics of the vortex breakdown. The hot-wire investigation results reveal the frequencies associated with the precession motion due to the vortex breakdown. The Particle Image Velocimetry technique has been coupled with Proper Orthogonal Decomposition (POD) for data post-processing in order to reconstruct the swirling motion generated by the injection system. The property of POD, which consists of splitting temporal from spatial information of the flow field in analysis, allows the distinction between deterministic and random fluctuations without the need of an external trigger signal. This feature is fundamental for the better understanding of an highly-swirling flow.