a b s t r a c tThe precessing vortex core (PVC) is a coherent flow structure that is often encountered in swirling flows in gas turbine (GT) combustors. In some swirl combustors, it has been observed that a PVC is present under non-reacting conditions but disappears in the corresponding reacting cases. Since numerous studies have shown that a PVC has strong effects on the flame stabilization, it is desirable to understand the formation and suppression of PVCs in GT combustors. The present work experimentally studies the flow field in a GT model combustor at atmospheric pressure. Whereas all non-reacting conditions and detached M-shaped flames exhibit a PVC, the PVC is suppressed for attached V-shaped flames. A local linear stability analysis is then applied to the measured time-averaged velocity and density fields. For the cases where a PVC appeared in the experiment, the analysis shows a global hydrodynamic instability that manifests in a single-helical mode with its wavemaker located at the combustor inlet. The frequency of the global mode is in excellent agreement with the measured oscillation frequency and the growth rate is approximately zero, indicating the marginally stable limit-cycle. For the attached V-flame without PVC, strong radial density/temperature gradients are present at the inlet, which are shown to suppress the global instability. The interplay between the PVC and the flame is further investigated by considering a bi-stable case with intermittent transitions between V-and M-flame. The flame and flow transients are investigated experimentally via simultaneous highspeed PIV and OH-PLIF. The experiments reveal a sequence of events wherein the PVC forms prior to the transition of the flame shape. The results demonstrate the essential role of the PVC in the flame stabilization, and thereby the importance of a hydrodynamic stability analysis in the design of a swirl combustor.

This study presents a numerical model of a pressure-fed system with flow passage opening devices (FPODs) designed for an air vehicle with a high degree of maneuverability. The FPOD is a mechanical device that connects two separate fuel reservoirs and functions as a valve allowing liquid fuel to flow while minimizing the movement of pressurizing gas from upstream fuel tanks into downstream fuel tanks. A reduced-order model for the fuel motion in an annular fuel tank was developed to configure the depth and inclination angle of the free fuel surface on the cross-sectional plane of an annular fuel tank under accelerating conditions during flight. Furthermore, a newly proposed model that reflects the dynamic characteristics of the FPOD is used to determine the fluid type that is transported through the device. A simulation example shows that the full numerical model captures changes of the fuel transport condition over time in a complete pressure-fed system of annular fuel tanks with FPODs subject to acceleration.