In the design of high-speed rotating machinery, engineers are seeking ways to enhance rotordynamic stability margins while minimizing vibration levels. An attractive option to improve stability margins is the use of annular seals. These seals are versatile because they are effective sealing elements, possess both stiffness and damping characteristics, and are easily incorporated into a pump. Design of an effective seal requires insight into the parameters that can influence its rotordynamic characteristics. Analysis indicates that changes in clearance and taper can significantly affect direct stiffness as well as other seal characteristics. With this in mind, test results are presented for seals with identical round-hole pattern stator surface treatments at two constant radial clearances and one tapered-seal configuration. Testing is performed with a realistic, high inlet fluid preswirl condition that enables an evaluation of a given seal’s stabilizing capacity. All test results presented are for a centered seal condition at multiple shaft speeds and pressure levels. Experimental trends are consistent with theory in that gains in direct stiffness are realized with both a decrease in a seal’s radial clearance and with a convergent-taper. Also, the stabilizing capacity of the seals was significantly influenced by the configurations tested.

Annular seals are known to enhance rotordynamic stability margins and minimize vibration response levels in high-speed rotating machinery. Theoretical predictions for the rotordynamic characteristics of annular seals exist but additional experimental data is needed to properly anchor these results. NASA’s Marshall Space Flight Center (MSFC) has developed an annular seal test rig and facility to experimentally characterize axially-fed annular seals. The objective of MSFC’s annular seal test rig is to obtain the rotordynamic coefficients (direct and cross-coupled stiffness, damping, and added mass) for a variety of high Reynolds number annular seals. The MSFC test rig supports centered-seal testing with inlet pressures up to 138 bars (2000 psi) and flow rates of over 946 liters per minute (250 gpm). The rig’s shaft is powered by a 186 kilowatt (250 horsepower) steam turbine capable of rotational speeds of over 20,000 revolutions per minute (rpm). A description of the identification process used to obtain rotordynamic coefficients is given as well as procedures for ensuring quality data. Experimental results for a smooth annular seal with an L/D = 0.5 is presented. Excellent agreement between experimental and theoretical results is obtained.

Annular seals are known to enhance rotordynamic stability margins and minimize vibration response levels in high-speed rotating machinery. Theoretical predictions for the rotordynamic characteristics of annular seals exist but additional experimental data is needed to properly anchor these results. NASA’s Marshall Space Flight Center (MSFC) has developed an annular seal test rig and facility to experimentally characterize axially-fed annular seals. Annular seals with deliberately roughened stators (i.e. damping seals) have been shown analyticalty to increase stability margins of rocket engine turbomachinery by reducing the seal’s whirl frequency ratio. The capabilities of MSFC’s annular seal test rig have been enhanced to allow high fluid inlet preswirl testing that is more representative of actual turbopump seal bounder conditions. The purpose of this paper is to describe the effect of this realistic preswirl on the stabilizing capability of both damping and smooth seals. Centered seal results are presented for both a smooth annular seal and a damping seal. These results were obtained for a range of seal pressure differentials, shaft rotational speeds, and two levels of inlet fluid preswirl.