Making Better Swirl Brakes Using Computational Fluid Dynamics: Performance Enhancement From Geometry Variation
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AbstractA fluid with a large swirl (circumferential) velocity entering an annular pressure seal influences the seal cross-coupled dynamic stiffness coefficients, and hence it affects system stability. Typically comprising a large number of angled vanes around the seal circumference, a swirl brake (SB) is a mechanical element installed to reduce (even reverse) the swirl velocity entering an annular seal. SB design guidelines are not readily available, and existing configurations appear to reproduce a single source. By using a computational fluid dynamics (CFD) model, the paper details a process to engineer a SB upstream of a 16-tooth labyrinth seal (LS) with tip clearance Cr=0.203mm. The process begins with a known nominal SB* geometry and considers variations in vane length (LV*=3.25mm) and width (WV*=1.02mm), and stagger angle (*=0 deg). The vane number NV*=72 and vane height HV*=2.01mm remain unchanged. The SBLS operates with air supplied at pressure PS=70bar, a pressure ratio PR=exit pressure Pa/PS=0.5, and rotor speed =10.2 krpm (surface speed R=61 m/s). Just before the SB, the preswirl velocity ratio=average circumferential velocity U/shaft surface speed (R) equals =0.5. For the given conditions, an increase in LV allows more space for the development of vortexes between two adjacent vanes. These are significant to the dissipation of fluid kinetic energy and thus control the reduction of . A 42% increase in vane length (LV=4.6mm) produces a 43% drop in swirl ratio at the entrance of the LS (exit of the SB), from E=0.23 to 0.13. Based on the SB with LV=4.6mm, the stagger angle varies from 0deg to 50deg. The growth in angle amplifies a vortex at 70% of the vane height, while it weakens a vortex at 30% of HV. For =40deg, the influence of the two vortexes on the flow produces the smallest swirl ratio at the LS entrance, E=0.03. For a SB with LV=4.6mm and =40deg, the vane width WV varies from 0.51mm to 1.52mm (50% of WV*). A reduction in WV provides more space for the strengthening of the vortex between adjacent vanes. Therefore, a SB with greater spacing of vanes also reduces the inlet circumferential velocity. For WV=0.51mm, E further decreases to 0.07. Besides the design condition (=0.5), the engineered SB having LV=4.6mm, =40deg, and WV=0.51mm effectively reduces the circumferential velocity at the LS entrance for other inlet preswirl ratios equaling =0 and 1.3. Rather than relying on extensive experiments, the CFD analysis proves effective to quickly engineer a best SB configuration from the quantification of performance while varying the SB geometry and inlet swirl condition.
