Measurements of leakage, power loss and rotordynamic force coefficients in a hybrid brush seal

Abstract

This thesis presents measurements of power loss and leakage in a hybrid brush seal (HBS) for increasing pressure differentials and over a range of rotor speeds. The test HBS, Haynes-25 bristle pack [~850 bristles/cm] and 45o lay angle, is 166.4 mm in diameter and integrates 20-arcuate pads connected with thin EDM-webs to the seal casing. The measured drag power at low rotor speeds (< 11 m/s at 1,300 rpm) decreases as the pressure differential across the seal increases. At a fixed rotor speed, a significant drop in drag torque (and drag power) ensues as the supply pressure increases, thus demonstrating a gas film separates the rotor from the seal pads. A constant operating temperature (~24oC) at the rotor/seal interface during tests with shaft rotation also indicates the absence of intermittent contact between the seal pads and rotor. Flow rate measurements at room temperature (25oC) show an improved sealing ability with a leakage reduction of about 36%, when compared to a 1st generation shoedbrush seal. The HBS predicted effective clearance (~50 μm) is a small fraction of that in an equivalent one-tooth labyrinth seal. Identified HBS direct stiffness coefficients decrease (~15%) as function of rotor speed for an increasing supply pressure condition (Pr = 1.7 and 2.4). The identified cross-coupled stiffness is at least one or two orders of magnitude smaller than the direct stiffness coefficient. The cross-coupled mass is negligible for all tested rotor speeds and supply pressures. The HBS energy dissipation mechanism is characterized in terms of a loss factor ( γ) and dry friction coefficient ( μ). The direct HBS viscous damping coefficient is strongly dependent on the excitation frequency, while showing minimal dependence on rotor speed or supply pressure. The HBS novel configuration incorporates pads contacting on assembly the shaft; and which under rotor spinning; lift off due to the generation of a hydrodynamic pressure. Experimental results obtained show that hybrid brush seals (HBS) are a viable alternative to overcoming the major drawbacks of labyrinth seals; namely excessive leakage and potential for rotordynamic instability. Additionally, during operation a gas film in HBS eliminates rotor and bristle wear, as well as thermal distortions; which are commonly known limitations of conventional brush seals.