Static and Rotordynamic Characteristics for a New Hole Pattern Annular Gas Seal Design Incorporating Larger Diameter Holes

Abstract

To reduce manufacturing cost and time, a new largerdiameter holepattern seal incorporating hole diameters of 12.27 mm, versus prior hole diameters of 3.175 mm has been proposed. The 12.27 mm holediameter seal had substantially better stability performance with higher effective damping and a markedly lower crossover frequency. It had negative direct stiffness coefficients at low frequency, while the 3.175 mm holediameter seal did not. Predictions for the rotordynamic coefficients of this new seal were made based on a twocontrolvolume model developed by Kleynhans and Childs in 1997. The two control volumes consisted of a throughflow controlvolume and a controlvolume B that extended from the surface of the stator at the top of the holes to the bottom of holes. Predictions agreed poorly with measured results, because the model used, assumes gas flows only radially within controlvolume B. With the large holediameters axial and circumferential flow is readily accomplished. Compared to the prior 3.175 mm holediameter seals, the 12.27 mm holediameter seal design leaked approximately 37.5% more which probably precludes its commercial application. Leakage for the seal was well predicted. Although the larger hole diameters were initially proposed to reduce costs, the fabrication was more challenging than originally thought. The larger holes could not be manufactured with a single pass. Hence, manufacturing costs and time were not reduced.