Rotordynamic-Coefficients and Static (Equilibrium Loci and Leakage) Characteristics for Short, Laminar-flow Annular Seals

Abstract

Test results are presented for laminar-flow seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short (L∕D≅0.21), with a diameter of 117mm and a clearance-to-radius ratio 0.0007. A smooth seal, a seal with one central groove, and a seal with three grooves were tested. Groove geometries employed are representative of industrial practice for compressors with a groove-depth to clearance ratio on the order of 6. Tests were conducted at 4000, 7000, and 10,000rpm shaft speed with delta pressures across the seals of 21, 45, and 69bars. For all cases, the flow was laminar. The seals were tested from a centered position out to an eccentricity ratio of 0.7. Static data included leakage and equilibrium loci for a range of loads. Direct and cross-coupled stiffness and damping coefficients and direct mass coefficients were determined from dynamic tests. For the smooth seal, comparisons between measurements and predictions were reasonable for the direct and cross-coupled stiffness and damping coefficients; however, measured added mass coefficients were roughly ten times larger than predicted. Predictions for the grooved seals from a “deep-groove” model that assumed zero pressure oscillations in the grooves greatly over predicted the influence of the grooves. In a centered position, smooth and grooved seals had comparable leakage performance. At higher eccentricity ratios, the grooved seals leaked modestly more. For eccentricity ratios less than approximately 0.3, the grooved seals and the smooth seal had qualitatively similar static and dynamic characteristics. In terms of cross-coupled stiffness coefficients, the grooved seals were less stable than the smooth seal at eccentricity ratios greater than approximately 0.5 but had significantly lower cross-coupled coefficients at reduced eccentricity ratios between zero and 0.3. A grooved centered seal is more stable than a smooth centered seal. The smooth seal had higher damping than the grooved seals and had moderately better centering capabilities.

Effect of a Reduced Oil Flow Rate on the Static and Dynamic Performance of a Tilting Pad Journal Bearing Running in Both the Flooded and Evacuated Conditions

“…The test rig is operational at the Turbomachinery Laboratory since well over a decade [13]. References [7,9] fully describe the bearing test rig depicted in Figure 1.…”

Influence of rub groove on rotordynamics associated with leakage air flow through a labyrinth seal

“…(11) and circumferential momentum Eq. (12). Accordingly, the set of four equations is rearranged in the matrix form,…”

“…Rhode and Adams [10] numerically quantified variation of leakage air flow through the straight-through seal with the axial shifting of the labyrinth tooth in respect to rub grooves; this situation is resulted from different thermal expansion rates of the rotor and stator in the transient operation of the engines [11]. Recently, Childs et al [12] and Graviss [13] claimed that variation of leakage flow through the liquid laminar-flow annular seal due to rub grooves would have significant influence on the rotordynamic coefficients. However, a literature survey reveals that no study was made of influence of rub grooves on dynamics and stability of the rotor subjected to leakage air flow through the labyrinth seal.…”

Experimental Investigation on Rotordynamic Characteristics and Rotor System Stability of a Novel Negative Dislocated Seal

“…In comparison, the frequencydomain method has higher accuracy and signal-to-noise ratio (S/N) [21]. Childs et al developed a frequencydomain identification model to conduct a large number of investigations on the rotordynamic characteristics of different seals [22][23][24][25][26]. As for the study on seal rotordynamic characteristics with the frequency-domain method, the single-point excitation method is applied, by the excitation at the center of cylinder to acquire vibration response and then obtain impedance function.…”