To reduce manufacturing cost and time, a hole-pattern seal incorporating holes of larger diameter (12.19 mm (0.48 inches)) has been proposed. Experimental leakage and rotordynamic coefficients for this new seal design are presented. This experimental data was compared to theoretical results generated by ISOTSEAL a program developed by Kleynhans and Childs. Finally, the performance of this new hole-pattern seal was compared to a hole-pattern seal tested by Wade. The experiments are configured to investigate the influence of changes in pressure ratio, preswirl, rotor speed, and clearances on seal characteristics. Due to stator stability issues, the peak inlet pressures had to be varied to allow for testing. Consequently, to study the effect of inlet preswirl and clearance, data were non-dimensionalized or normalized. Cross-coupled coefficients were relatively frequency-independent while direct coefficients were functions of excitation frequency. For all test cases, the seal developed negative direct stiffness at low frequencies. Tests showed that pressure ratio had minimal effect on rotordynamic coefficients. Non-dimensional cross-coupled stiffness increased with increasing preswirl causing the seal to become less stable with increasing preswirl. Cross coupled stiffness increased with increasing running speed. Two clearances: 0.1 mm (4 mils) and 0.2 mm (8 mils) were tested. The results demonstrated that non-dimensionalized stiffness is greater for the smaller clearance. The larger clearance develops larger normalized direct damping values, and has enhanced stability. Rotordynamic predictions are poor for cross-coupled coefficients. Generally, ISOTSEAL over-predicts direct stiffness and under-predicts direct damping. Negative stiffness was not predicted by ISOTSEAL. Predictions do improve for the smaller clearance. ISOTSEAL does a good job of predicting non-dimensional leakage. Non-dimensionalized direct and effective stiffness were greater for the "old" hole-pattern seal tested by Wade. However, the "new" seal generally developed greater normalized direct damping and exhibited a lower cross-over frequency. Non-dimensionalized leakage was greater for the seal tested here. Production of this new seal proved to be more difficult than originally thought. The price of the new seal cost approximately the same as an original hole-pattern seal.

This thesis presents the results of high supply (up to 84 bar) pressure testing of hole-pattern annular gas seals performed at the Texas A & M Turbomachinery Laboratory in College Station, TX. The test variables were chosen to determine the influence of pressure ratio, rotor speed, and negative preswirl on seal performance. Preswirl signifies the circumferential fluid flow entering a seal, and negative preswirl indicates a fluid swirl in the direction opposite of rotor rotation. Changes in pressure ratio had only small effects on most rotordynamic coefficients. Cross-coupled stiffness showed slightly different profiles through the mid-range of excitation frequencies. Pressure ratio showed some influence on direct and cross-coupled damping at low excitation frequencies. Rotor speed significantly affected both cross-coupled stiffness and cross-coupled damping. As rotor speed increased, the magnitude of cross-coupled rotordynamic coefficients increased due to the positive fluid swirl induced by rotor rotation. For the low rotor speed, negative inlet preswirl was able to overpower the positive rotor induced fluid rotation, producing a negative cross-coupled stiffness. This outcome showed that, for hole-pattern seals, positive fluid swirl does indeed produce positive cross-coupled stiffness coefficients and negative fluid swirl produces negative cross-coupled stiffness coefficients. The addition of negative preswirl greatly reduced cross-coupled rotordynamic coefficients, while direct rotordynamic terms were unaffected. Cross-over frequency signifies the excitation frequency where effective damping transitions from a negative value to a positive value with increasing excitation frequency. Peak effective damping was increased by 50 percent and cross-over frequency reduced by 50 percent for high-negative preswirl versus zero preswirl results. This led to the conclusion that a reverse swirl could greatly enhance the stability of hole-pattern balance piston seals. A two-control-volume model that uses the ideal gas law at constant temperature (ISOT) was used to predict rotordynamic coefficients and leakage. This model predicted direct rotordynamic coefficients well, but greatly under predicted cross-coupled rotordynamic coefficients especially at high negative preswirls. The model predicted seal leakage well at low pressure ratios, but showed increasing error as the pressure ratio was increased. These results showed that the prediction model could not adequately estimate cross-coupled rotordynamic coefficients for a hole-pattern seal with negative inlet preswirl and requires modification to do so.