Browsing by Subject "SEAL"
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Item Improving performance and rotordynamic characteristics of injection compressors via much longer balance-piston and division-wall seals(2009-05-15) Rodrigues Rodrigues, MargaritaPredictions are presented for a selected compressor using longer hole-pattern seals with L/D ratios from 0.5 to 2.5. Results were obtained for back-to-back and in-line compressors with the seal located at mid-span and at 82% of rotor span respectively, considering different seal lengths, radial seal clearances, as well as constant clearance and convergent-tapered seal geometries. Predictions of the synchronous rotordynamic coefficients and leakage were estimated using a code developed by Kleynhans and Childs with zero preswirl and constant pressure ratio of 0.5. This code does not include moment coefficients; which can affect the results. Results of all configurations show an increase of stiffness and damping coefficients with increasing seal length. In addition, a significant reduction in leakage (approximately 47 percent) as L/D increases is exhibited for constant clearance and convergent-tapered hole-pattern seals. For the back-to-back compressor, the stability analysis predicts that the system is stable for all speeds and L/D ratios. In fact, the rotor cylindrical-bending mode becomes more stable with lengthening the seals, for both constant clearance and convergent-tapered hole-pattern seals. For constant clearance seals (Case A), the synchronous response at mid-span show a critical speed at 8,000 rpm (cylindrical-bending mode) for all L/D ratios, while a reduction of 85 percent in the peak response is exhibited as L/D increases. Case B, in which the radial clearance is increased as L/D increases to have the same leakage as case A, slightly increases the synchronous response of the model compared to case A. For convergent-tapered seals (Case C), the synchronous response at mid-span shows a higher critical speed (9,000 rpm) for all L/D ratios, and a larger reduction (89 percent) in peak response with increasing L/D, compared to Case A. However, the magnitude of the peak response is larger for convergent-tapered seals than that for constant clearance seals, for all L/D ratios. For in-line compressor, the stability analysis predicts two critical speeds at 6,000 (conical mode) and 18,000 rpm (first bending mode) respectively. Both modes are predicted to be stable for all speed and L/D ratios. Synchronous response at the mid-span for Case A shows the peak response at the first critical speed is slightly reduced as L/D increases while the response at the second critical speed is increased for most of the cases. In addition, the second critical speed is reduced from 18,000 to 13,000 rpm, which is not a concern because it remains above the running speed. This was also the trend for convergent-tapered hole-pattern seal. In addition, the increase of radial clearance in Case B slightly increases the amplitude of vibration, compared to Case A.Item Test versus predictions for rotordynamic and leakage characteristics of a convergent-tapered, honeycomb-stator/smooth-rotor annular gas seal(2009-05-15) Van Der Velde Alvarez, Daniel EduardoThis thesis presents the results for measured and predicted rotordynamic coefficients and leakage for a convergent-tapered honeycomb seal (CTHC). The test seals had a diameter of 114.968 mm (4.5263 in) at the entrance, and a diameter of 114.709 mm (4.5161 in) at the exit. The honeycomb cell depth was 3.175 mm (0.125 in), and the cell width was 0.79 mm (0.0311 in). Measurements are reported with air as the test fluid at three different speeds: 10,200, 15,200, and 20,200 rpm; with a supply pressure of 69 bar (1,000 psi), with exit-to-inlet pressure ratios from 20% to 50%, and using two rotors that are 114.3 mm (4.500 in) and 114.5 mm (4.508 in) respectively; this enables the same seals to be tested under two different conditions. The q factor, which is just a simple way to quantify taper is defined as the taperangle seal parameter and is calculated using the inlet and exit radial clearance. Two taper-angles parameters were calculated; q = 0.24 for the 114.3 mm (4.500 in) rotor, and q = 0.386 for the 114.5 mm (4.508 in) rotor. The q = 0.24 condition was compared to a constant clearance honeycomb seal (CCHC q = 0) because both sets of data were taken with the same rotor diameter. The direct stiffness, effective stiffness, and direct damping coefficients were larger for q = 0.24. The CTHC q = 0.24 eliminates the direct negative static stiffness obtained with CCHC ( q = 0). The cross-coupled stiffness and damping also were larger for q = 0.24, especially at low frequencies. Effective damping is one of the best indicators in determining the stability of a roughened stator annular gas seal. The frequency at which it changes sign is called the cross-over frequency. In applications, this frequency needs to be lower than the rotorsystem?s first natural frequency. Otherwise, the seal will be highly destabilizing instead of highly stabilizing. The magnitude of effective damping and the cross-over frequency also increases with q for all frequencies. Constant clearance honeycomb seals have less leakage than convergenttapered honeycomb seals. CTHC ( q = 0.24), has approximately 20 percent more leakage than CCHC ( q = 0). The experimental results for rotordynamic characteristics and leakage were compared to theoretical predictions by the two-control-volume developed by Kleynhans and Childs. All rotordynamic coefficients were reasonably predicted for all cases. The model does a better job predicting the cross-coupled stiffness and damping coefficients rather than the direct stiffness and damping coefficients. Also, the two-control-volume model predicts the dynamic characteristics of CCHC ( q = 0) better, and does not predict well the effective stiffness and damping for CTHC q = 0.386.