MAKING BETTER SWIRL BRAKES USING COMPUTATIONAL FLUID DYNAMICS: PERFORMANCE ENHANCEMENT FROM GEOMETRY VARIATION

A 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 sixteen-tooth labyrinth seal ( LS) with tip clearance C-r = 0.203 mm. The process begins with a known nominal SB* geometry and considers variations in vane length (L-V* = 3.25 mm) and width (W-V* = 1.02 mm), and stagger angle (theta* = 0 degrees). The vane number N-V* = 72 and vane height H-V* = 2.01 mm remain unchanged. The SB-LS operates with air supplied at pressure P-S = 70 bar, a pressure ratio PR = exit pressure P-a / P-S = 0.5, and rotor speed Omega = 10.2 krpm (surface speed Omega R = 61 m/s). Just before the SB the pre-swirl velocity ratio = average circumferential velocity U / shaft surface speed (OR) equals alpha = 0.5. For the given conditions, an increase in L-V 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 alpha. A 42% increase in vane length (L-V = 4.6 mm) produces a similar to 43% drop in swirl ratio at the entrance of the LS (exit of the SB), from alpha(E) = 0.23 to 0.13. Based on the SB with L-V = 4.6 mm, the stagger angle theta varies from 0 degrees to 50 degrees. The growth in angle amplifies a vortex at similar to 70% of the vane height while it weakens a vortex at 30% of H-V. For theta = 40 degrees, the influence of the two vortexes on the flow produces the smallest swirl ratio at the LS entrance, alpha(E) = -0.03. For a SB with L-V = 4.6 mm and theta = 40 degrees, the vane width W-V varies from 0.51 mm to 1.52 mm (+/- 50% of WV*). A reduction in W-V 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 W-V = 0.51 mm, aE further decreases to -0.07. Besides the design condition (alpha = 0.5), the engineered SB having L-V = 4.6 mm, theta = 40 degrees and W-V = 0.51 mm effectively reduces the circumferential velocity at the LS entrance for other inlet pre-swirl ratios equaling alpha = 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.