Experimental and CFD analysis of static and dynamic rotor stabilities in three-annular seals

At the sealing pressures of 1.25, 2.5, 5, 7.5 or 10 bar and at a rotation speed of zero to 8000 rpm, using experiment and CFD simulation, the static and dynamic stabilities of two three-annular seal basic designs were investigated. Those were the design with identical radial clearances at each annulus (ThASIC) and the one with 2 times larger radial clearance at the second annulus (ThASDC) in the presence of two axial dimensions for the connecting cavities of 1 and 3 mm. A possible mode of self-excited radial oscillations of a non-rotating shaft in a three-annular seal has been available from the experiments, and the explanation of this phenomenon occurrence has been given. The influences of the axial sizes of the connecting cavities and the clearance of the second annulus on the velocity distributions along the length and circumference of the seal have been studied. The analysis of the response amplitudes for a possible imbalance and wall motion orbit depending on the frequency of rotation with direct synchronous whirl of the shaft has shown that with an increase in the sealing pressure, the critical rotation speed and amplitude increases in the ThASDC design and the critical rotation speed decreases in the ThASIC design. The smallest whirl amplitude and a more centered position are typical for the ThASDC design with the enlarged connecting cavity of l_2,4= 3 mm. The analysis of each annulus contribution to the magnitudes of hydrodynamic forces has been carried out, and depending on the sealing pressure and the seal design, the dynamic stiffness and damping coefficients have been got. The effect of the friction factor f_sθ on the circumferential flow for different three-annular seal designs has been obtained. Its impact on the dynamic stability of the rotor (ThAS-RR design) and stator (ThAS-SR and ThAS-SRS designs) ribs, which were located in the connecting cavities of the three-annular seal, has been studied. The dynamic coefficients of stiffness, damping, and leakage for five different designs (ThASIC, ThASDC, ThAS-RR, ThAS-SR, and ThAS-SRS) of the three-annular seals have been compared. Due to the flow hydrodynamics analysis, the formation mechanisms for the decentering and destabilizing forces in the three-annular seals have been explained.