1339. Studies of exit pressure recovery coefficient and its effects on dynamic characteristics of annular water seals.

Rotordynamic instability and vibration due to fluid forces within annular seals are well-known phenomenons that can occur in pumps as well as in turbines. Traditional theoretical predictions for the fluid reaction forces and equivalent dynamic characteristics of annular seals are computationally efficient compared with CFD method. Exit pressure loss coefficient is one of the main factors that influence the precisions of theoretical analysis. In this paper, exit pressure recovery coefficients at three different static eccentricity ratios under different operating conditions are investigated using CFD method. The numerical result shows that exit coefficient increases exponentially with the ratio of circumferential velocity to axial velocity at all these three eccentricity ratios. Besides, an analysis method for annular plain seals with the introduction of exit pressure recovery coefficient varied with operating conditions is proposed based on the previous analysis results. Comparisons are made between theoretical predictions calculated by the two methods with and without exit coefficient over a wide range of pressure drops and running speeds. The result suggests that damping coefficients are much more sensitive to the change of the exit boundary conditions compared to stiffness coefficients. In addition, the new prediction results of cross-coupled stiffness and direct damping coefficient show relatively smooth change with the increase of velocity ratio than the previous results. [ABSTRACT FROM AUTHOR]