| dc.description.abstract | Modern SFD designs are short in axial length to limit weight and part count and supplied with a low lubricant feed pressure to reduce operating costs related to lubricant storage and pumping power. O-rings (ORs) reduce lubricant side leakage, increasing the viscous damping within a constrained physical space and provide a modest centering support stiffness to the rotor. Continuing a long-term project characterizing SFDs for air breathing engines, the thesis details comprehensive measurements of the forced performance of an OR sealed damper (OR-SFD), with a film land length L=25.4 mm, 127 mm in diameter (D), and a radial clearance c=0.279 mm. The damper, with a slenderness ratio L/D = 0.2. undergoes centered whirl motions with amplitudes r=0.05c to 0.45c, over ω = 10 Hz to 130 Hz (max. squeeze film velocity vs=rω=102.5 mm/s). Lubricant ISO VG 2 supplied at 0.69 bar(g) fills an upstream oil plenum and flows through a single orifice with a check valve midway of the damper length (1/2L). Measurements of dynamic loads, along with the ensuing displacements and accelerations identify the parameters of the test structure, ORs and SFD. This research effort is the first to identify ORs force coefficients over a range of orbit amplitudes and assess its effects on the dynamic performance of the OR-SFD. The ORs force coefficients remain nearly invariant within the identification frequency range; however, they showcase significant orbit amplitude dependence. At r/c = 0.05 the OR centering stiffness (KOR) doubles the static stiffness (KOR,static), and as r/c→ 0.45, KOR approaches ½KOR,static, likely due to the extensive elastic deformation and slow recovery in the rings’ polymeric structure bonds. At r=0.05c and 0.10c, the ORs viscous damping coefficient (COR) contributes to ~10% of the total in the lubricated system (CL), while for r/c > 0.25, it contributes to just 3% of CL. For small orbit amplitudes (r ≤ 0.25c), the experimental SFD added mass (MSFD) and viscous damping (CSFD) coefficients are nearly equivalent to theoretical magnitudes for a fully sealed damper (no side leakage). However, as the orbit size grows to r → 0.45c, MSFD drops nearly 75% and COR decreases by ~40%. The reduction in force coefficients is due to the onset of both lubricant cavitation and air ingestion occurring for vs ≥ 24.5 mm/s. A prediction model delivers squeeze film added mass and viscous damping coefficients which are on average, 10% larger than those derived experimentally. Measured film dynamic pressures evidence both oil vapor cavitation and air ingestion, and video recordings depict a bubbly mixture in the lubricant return line and through the damper top end. Peak-peak film pressures for operation at vs ≥ 34 mm/s show the gas content prevents the generation of peak pressures proportional to vs. Moreover, pk-pk pressures inside the journal oil delivery plenum follow the same trend as those in the film land, showing the mechanical check valve installed in the journal allows for lubricant backflow. A novel approach enables the estimation of the gas volume fraction (GVF), which rapidly increases with vs. The simple procedure draws into a deflated balloon the material contents in the film, weighs the sample and identifies its volume to produce an estimation of the GVF. The research findings reveal more details on the effect of ORs to the forced performance of a damper and their limited ability to prevent air ingestion when operating at large squeeze velocities and a low lubricant feed pressure. | |
