Vannarsdall, Michael Lloyd (2011-08). Measured Results for a New Hole-Pattern Annular Gas Seal Incorporating Larger Diameter Holes, Comparisons to Results for a Traditional Hole-Pattern Seal and Predictions. Master's Thesis.
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.