Model and Experimental Verification of the Dynamic Forced Performance of a Tightly Sealed Squeeze Film Damper Supplied With a Bubbly Mixture

2019 ◽  
Vol 142 (1) ◽  
Author(s):  
Luis San Andrés ◽  
Bonjin Koo
Keyword(s):  
Sliding Friction ◽  
Volume Fraction ◽  
Dynamic Pressure ◽  
Mechanical Energy ◽  
Added Mass ◽  
Single Frequency ◽  
Squeeze Film ◽  
Radial Clearance ◽  
Force Coefficients ◽  
Supply Pressure

Abstract Practice and experiments with squeeze film dampers (SFDs) sealed with piston rings (PRs) show the lubricant exits through the PR slit, i.e., the gap made by the PR abutted ends when installed, forced as a jet during the portion of a rotor whirl cycle generating a positive squeeze film pressure. In the other portion of a whirl cycle, a subambient dynamic pressure ingests air into the film that mixes with the lubricant to produce a bubbly mixture. To reduce persistent air ingestion, commercial air breathing engines utilizing PRSFDs demand of a sufficiently large lubricant supply pressure (Ps), and hence a larger flow rate that is proportional to the journal squeeze velocity (vs = amplitude r × frequency of motion ω). The stringent requirement clearly limits the applicability and usefulness of SFDs. This paper presents a computational physics model for a sealed-end SFD operating with a mixture and delivers predictions benchmarked against profuse laboratory test data. The model implements a Reynolds equation adapted for a homogeneous bubbly mixture, includes temporal fluid inertia effects, and uses physics-based inlet and outlet lubricant conditions through feed holes and PR slit, respectively. In the experiments for model validation, a SFD damper, 127 mm in diameter D, film land length L = 25.4 mm (L/D = 0.2), and radial clearance c = 0.371 mm, is supplied with an air in ISO VG2 oil bubbly mixture of known gas volume fraction (GVF), zero (pure oil) to 50% in steps of 10%. The mixture supply pressure varies from Ps = 2.06 bar-g (30 psig) to 6.20 bar-g (90 psig). Located in grooves at the top and bottom of the journal, a PR and an O-ring (OR) seal the film land. The OR does not allow any oil leakage or air ingestion; hence, the supplied mixture discharges through the PR slit into a vessel submerged within a large volume of lubricant. Dynamic load tests with a single frequency ω, varying from 10 Hz to 60 Hz, produce circular centered orbits (CCO) with amplitude r = 0.2c. The measurements record the exerted forces and journal motions and an analysis delivers force coefficients, damping and inertia, representative of the exerted frequency range. The model predicts the pressure field and evolution of the GVF within the film land and, in a simulated process replicating the experimental procedure, delivers representative force coefficients. For all Ps conditions, both predictions and tests show the SFD added mass coefficients significantly decrease as the inlet GVF (βs) increases. The experimentally derived damping coefficients do not show a significant change, except for tests with the largest concentration of air (βs = 0.5). The predicted damping differs by 10% with the test derived coefficient which does not readily decrease as the inlet GVF (βs) increases. The added mass coefficients, test and predicted, decrease with βs, both being impervious to the magnitude of supply pressure. The test PRSFD shows a quadrature stiffness due to the sliding friction between the PR being pushed against the journal. An increase in supply pressure exacerbates this unique stiffness that may impair the action of the squeeze film to dissipate mechanical energy. The comprehensive test results, first of their kind, demonstrate that accurate modeling of SFDs operating with air ingestion remains difficult as the flow process and the paths of its major components (air and liquid) are rather complex.

Model and Experimental Verification of the Dynamic Forced Performance of a Tightly Sealed Squeeze Film Damper Supplied With a Bubbly Mixture

2019 ◽  
Author(s):  
Luis San Andrés ◽  
Bonjin Koo
Keyword(s):  
Sliding Friction ◽  
Volume Fraction ◽  
Dynamic Pressure ◽  
Mechanical Energy ◽  
Added Mass ◽  
Single Frequency ◽  
Squeeze Film ◽  
Radial Clearance ◽  
Force Coefficients ◽  
Supply Pressure

Abstract Practice and experiments with squeeze film dampers (SFDs) sealed with piston rings (PRs) show the lubricant exits through the PR slit, i.e. the gap made by the PR abutted ends when installed, forced as a jet during the portion of a rotor whirl cycle generating a positive squeeze film pressure. In the other portion of a whirl cycle, a sub ambient dynamic pressure ingests air into the film that mixes with the lubricant to produce a bubbly mixture. To reduce persistent air ingestion, commercial air breathing engines utilizing PRSFDs demand of a sufficiently large lubricant supply pressure (Ps), and hence a larger flow rate that is proportional to the journal squeeze velocity (vs = amplitude r × frequency of motion ω). The stringent requirement clearly limits the applicability and usefulness of SFDs. This paper presents a computational physics model for a sealed ends SFD operating with a mixture and delivers predictions benchmarked against profuse laboratory test data. The model implements a Reynolds equation adapted for a homogeneous bubbly mixture, includes temporal fluid inertia effects, and uses physics based inlet and outlet lubricant conditions through feed holes and PR slit, respectively. In the experiments for model validation, a SFD damper, 127 mm in diameter D, film land length L = 25.4 mm (L/D = 0.2), and radial clearance c = 0.371 mm, is supplied with an air in ISO VG2 oil bubbly mixture of known GVF, zero (pure oil) to 50% in steps of 10%. The mixture supply pressure varies from Ps = 2.06 bar-g (30 psig) to 6.20 bar-g (90 psig). Located in grooves at the top and bottom of the journal, a piston ring (PR) and an O-ring (OR) seal the film land. The OR does not allow any oil leakage or air ingestion; hence the supplied mixture discharges thru the PR slit into a vessel submerged within a large volume of lubricant. Dynamic load tests with a single frequency ω, varying from 10 Hz to 60 Hz, produce circular centered orbits with amplitude r = 0.2c. The measurements record the exerted forces and journal motions and an analysis delivers force coefficients, damping and inertia, representative of the exerted frequency range. The model predicts the pressure field and evolution of the gas volume fraction (GVF) within the film land and, in a simulated process replicating the experimental procedure, delivers representative force coefficients. For all Ps conditions, both predictions and tests show the SFD added mass coefficients significantly decrease as the inlet GVF (βs) increases. The experimentally derived damping coefficients do not show a significant change, except for tests with the largest concentration of air (βs = 0.5). The predicted damping differs by 10% with the test derived coefficient which does not readily decrease as the inlet GVF (βs) increases. The added mass coefficients, test and predicted, decrease with βs, both being impervious to the magnitude of supply pressure. The test PRSFD shows a quadrature stiffness due to the sliding friction between the PR being pushed against the journal. An increase in supply pressure exacerbates this unique stiffness that may impair the action of the squeeze film to dissipate mechanical energy. The comprehensive test results, first of their kind, demonstrate that accurate modeling of SFDs operating with air ingestion remains difficult as the flow process and the paths of its major components (air and liquid) are rather complex.

Experimental Force Coefficients for Two Sealed Ends Squeeze Film Dampers (Piston Rings and O-Rings): An Assessment of Their Similarities and Differences

2018 ◽  
Vol 141 (2) ◽  
Author(s):  
Luis San Andrés ◽  
Bonjin Koo ◽  
Sung-Hwa Jeung
Keyword(s):  
Pressure Field ◽  
Dynamic Pressure ◽  
Added Mass ◽  
The Other ◽  
Squeeze Film ◽  
Test Results ◽  
Piston Rings ◽  
Force Coefficients ◽  
Supply Pressure ◽  
Squeeze Film Dampers

Squeeze film dampers (SFDs) in aircraft engines effectively aid to reduce rotor motion amplitudes, in particular when traversing a critical speed, and help to alleviate rotor whirl instabilities. The current work is a long-term endeavor focused on quantifying the dynamic force performance of practical SFDs, exploring novel design damper configurations, and producing physically sound predictive SFD models validated by experimental data. Piston rings (PRs) and O-rings (ORs), commonly used as end seals in SFDs for commercial and military gas turbine engines, respectively, amplify viscous damping in a short physical length and while operating with a modicum of lubricant flow. This paper presents experimental force coefficients (damping and inertia) for two identical geometry SFDs with end seals, one configuration hosts PRs, and the other one ORs. The test rig comprises a stationary journal and bearing cartridge (BC) hosting the SFD and supported on four elastic rods to emulate a squirrel cage. The damper film land length, diameter, and clearance are L = 25.4 mm, D = 5L, and c = 0.373 mm (D/c = 340), respectively. A supply feeds ISO VG 2 oil to the film land at its middle plane through either one hole or three holes, 2.5 mm in diameter, 120 deg apart. In the PRSFD, the lubricant exits through the slit opening at the ring butted ends. The ORs suppress oil leakage; hence, lubricant evacuates through a 1 mm hole at ¼ L near one journal end. The ORs when installed add significant stiffness and damping to the test structure. The ORSFD produces 20% more damping than the PRSFD, whereas both sealed ends SFDs show similar size added mass. For oil supplied at 0.69 bar(g) through a single orifice produces larger damping, 60–80% more than when the damper operates with three oil feedholes. A computational model reproducing the test conditions delivers force coefficients in agreement with the test data. Archival literature calls for measurement of a single pressure signal to estimate SFD reaction forces. For circular centered orbits (CCOs), the dynamic pressure field, in the absence of any geometrical asymmetry or feed/discharge oil condition, “rotates” around the bearing with a speed equal to the whirl frequency. The paper presents force coefficients estimated from (a) measurements of the applied forces and ensuing displacements, and (b) the dynamic pressure recorded at a fixed angular location and “integrated” over the journal surface. The first method delivers a damping coefficient that is large even with lubricant supplied at a low oil supply pressure whereas the inertia coefficient increases steadily with feed pressure. Predictions show good agreement with the test results from measured forces and displacements, in particular the added mass. On the other hand, identified damping and inertia coefficients from dynamic pressures show a marked difference from one pressure sensor to another, and vastly disagreeing with test results from the first method or predictions. The rationale for the discrepancy relies on local distortions in the dynamic pressure fields that show zones of oil vapor cavitation at a near zero absolute pressure and/or with air ingestion producing high frequency spikes from bubble collapsing; both phenomena depend on the magnitude of the oil supply pressure. An increase in lubricant supply pressure suppresses both oil vapor cavitation and air ingestion, which produces an increase of both damping and inertia force coefficients. No prior art compares the performance of a PRSFD vis-à-vis that of an ORSFD. Supplying lubricant with a large enough pressure (flow rate) is crucial to avoid the pervasiveness of air ingestion. Last, the discussion on force coefficients obtained from two distinct methods questions the use of an oversimplifying assumption; the dynamic pressure field is not invariant in a rotating coordinate frame.

Experimental Force Coefficients for Two Sealed Ends Squeeze Film Dampers (Piston Rings and O-Rings): An Assessment of Their Similarities and Differences

2018 ◽  
Author(s):  
Luis San Andrés ◽  
Bonjin Koo ◽  
Sung-Hwa Jeung
Keyword(s):  
Pressure Field ◽  
Dynamic Pressure ◽  
Added Mass ◽  
The Other ◽  
Squeeze Film ◽  
Test Results ◽  
Piston Rings ◽  
Force Coefficients ◽  
Supply Pressure ◽  
Squeeze Film Dampers

Squeeze film dampers (SFDs) in aircraft engines effectively aid to reduce rotor motion amplitudes, in particular when traversing a critical speed, and help to alleviate rotor whirl instabilities. The current work is a long term endeavor focused on quantifying the dynamic force performance of practical SFDs, exploring novel design damper configurations, and producing physically sound predictive SFD models validated by experimental data. Piston rings (PRs) and O-rings (ORs), commonly used as end seals in SFDs for commercial and military gas turbine engines, respectively, amplify viscous damping in a short physical length and while operating with a modicum of lubricant flow. This paper presents experimental force coefficients (damping and inertia) for two identical geometry SFDs with end seals, one configuration hosts PRs, and the other one ORs. The test rig comprises a stationary journal and bearing cartridge (BC) hosting the SFD and supported on four elastic rods to emulate a squirrel cage. The damper film land length, diameter and clearance are L = 25.4 mm, D = 5L, and c = 0.373 mm (D/c = 340), respectively. A supply feeds ISO VG 2 oil to the film land at its middle plane through either one hole or three holes, 2.5 mm in diameter, 120° apart. In the PR-SFD, the lubricant exits thru the slit opening at the ring butted ends. The O-rings suppress oil leakage; hence, lubricant evacuates through a 1 mm hole at ¼ L near one journal end. The O-rings when installed add significant stiffness and damping to the test structure. The ORSFD produces 20% more damping than the PR-SFD, whereas both sealed ends SFDs show similar size added mass. For oil supplied at 0.69 bar(g) through a single orifice produces larger damping, 60% to 80% more than when the damper operates with three oil feedholes. A computational model reproducing the test conditions delivers force coefficients in agreement with the test data. Archival literature calls for measurement of a single pressure signal to estimate SFD reaction forces. For circular centered orbits, the dynamic pressure field, in the absence of any geometrical asymmetry or feed/discharge oil condition, “rotates” around the bearing with a speed equal to the whirl frequency. The paper presents force coefficients estimated from (a) measurements of the applied forces and ensuing displacements, and (b) the dynamic pressure recorded at a fixed angular location and “integrated” over the journal surface. The first method delivers a damping coefficient that is large even with lubricant supplied at a low oil supply pressure whereas the inertia coefficient increases steadily with feed pressure. Predictions show good agreement with the test results, in particular the added mass. On the other hand, identified damping and inertia coefficients from dynamic pressures show a marked difference from one pressure sensor to another, and vastly disagreeing with test results from the first method or predictions. The rationale for the discrepancy relies on local distortions in the dynamic pressure fields that show zones of oil vapor cavitation at a near zero absolute pressure and/or with air ingestion producing high frequency spikes from bubble collapsing; both phenomena depend on the magnitude of the oil supply pressure. An increase in lubricant supply pressure suppresses both oil vapor cavitation and air ingestion which produces an increase of both damping and inertia force coefficients. No prior art compares the performance of a PR-SFD vis-à-vis that of an OR-SFD. Supplying lubricant with a large enough pressure (flow rate) is crucial to avoid the pervasiveness of air ingestion. Lastly, the discussion on force coefficients obtained from two distinct methods questions the use of an oversimplifying assumption; the dynamic pressure field is not invariant in a rotating coordinate frame.

On the Experimental Dynamic Force Performance of a Squeeze Film Damper Supplied Through a Check Valve and Sealed With O-Rings

2021 ◽  
Author(s):  
Luis San Andrés ◽  
Bryan Rodríguez
Keyword(s):  
Dynamic Pressure ◽  
Excitation Frequency ◽  
Check Valve ◽  
Forced Response ◽  
Single Frequency ◽  
Squeeze Film ◽  
Dynamic Force ◽  
Viscous Damping ◽  
Test Rig ◽  
Force Coefficients

Abstract In rotor-bearing systems, squeeze film dampers (SFDs) assist to reduce vibration amplitudes while traversing a critical speed and also offer a means to suppress rotor instabilities. Along with an elastic support element, SFDs are effective means to isolate a rotor from its casing. O-rings (ORs), piston rings (PRs) and side plates as end seals reduce leakage and air ingestion while amplifying the viscous damping in configurations with limited physical space. ORs also add a centering stiffness and damping to a SFD. The paper presents experiments to quantify the dynamic forced response of an O-rings sealed ends SFD (OR-SFD) lubricated with ISO VG2 oil supplied at a low pressure (0.7 bar(g)). The damper is 127 mm in diameter (D), short in axial length L = 0.2D, and the film clearance c = 0.279 mm. The lubricant flows into the film land through a mechanical check valve and exits through a single port. Upstream of the check valve, a large plenum filled with oil serves to attenuate dynamic pressure disturbances. Multiple sets of single-frequency dynamic loads, 10 Hz to 120 Hz, produce circular centered orbits with amplitudes r = 0.1c, 0.15c and 0.2c. The experimental results identify the test rig structure, ORs and SFD force coefficients; namely stiffness (K), mass (M) and viscous damping (C). The ORs coefficients are frequency independent and show a sizeable direct stiffness, KOR ∼ 50% of the test rig structure stiffness, along with a quadrature stiffness, K0∼0.26 KOR, demonstrative of material damping. The lubricated system damping coefficient equals CL = (CSFD + COR); the ORs contributing 10% to the total. The experimental SFD damping and inertia coefficients are large in physical magnitude; CSFD slightly grows with orbit size whereas MSFD is relatively constant. The added mass (MSFD) is approximately four-fold the bearing cartridge mass; hence, the test rig natural frequency drops by ∼50% once lubricated. A computational physics model predicts force coefficients that are just 10% lower than those estimated from experiments. The amplitude of measured dynamic pressures upstream of the plenum increases with excitation frequency. Unsuspectedly, during dynamic load operation, the check valve did allow for lubricant backflow into the plenum. Post-tests verification demonstrates that, under static pressure conditions, the check valve does work since it allows fluid flow in just one direction.

Squeeze Film Damper With a Mechanical End Seal: Experimental Force Coefficients Derived From Circular Centered Orbits

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Luis San Andrés ◽  
Adolfo Delgado
Keyword(s):  
Dry Friction ◽  
Dynamic Pressure ◽  
Test System ◽  
Single Frequency ◽  
Squeeze Film ◽  
Mechanical Seal ◽  
Damping Force ◽  
Squeeze Film Damper ◽  
Force Coefficients ◽  
Aircraft Application

The paper presents parameter identification measurements conducted on a squeeze film damper (SFD) featuring a nonrotating mechanical seal that effectively eliminates lubricant side leakage. The SFD-seal arrangement generates dissipative forces due to viscous and dry-friction effects from the lubricant film and surfaces in contact, respectively. The test damper reproduces an aircraft application that must contain the lubricant for extended periods of time. The test damper journal is 2.54cm in length and 12.7cm in diameter, with a nominal clearance of 0.127mm. The damper feed end opens to a plenum filled with lubricant, and at its discharge grooved section, four orifice ports evacuate the lubricant. In earlier publications, single frequency force excitation tests were conducted, without and with lubricant in the squeeze film land, to determine the seal dry-friction force and viscous damping force coefficients. Presently, further measurements are conducted to identify the test system and SFD force coefficients using two sets of flow restrictor orifice sizes (2.8mm and 1.1mm in diameter). The flow restrictors regulate the discharge flow area and thus control the oil flow through the squeeze film. The experiments also include measurements of dynamic pressures at the squeeze film land and at the discharge groove. The magnitude of dynamic pressure in the squeeze film land is nearly identical for both sets of flow restrictors, and for small orbit radii, dynamic pressures in the discharge groove have peak values similar to those in the squeeze film land. The identified parameters include the test system damping and the individual contributions from the squeeze film, dry friction in the mechanical seal and structure remnant damping. The identified system damping coefficients are frequency and motion amplitude dependent due to the dry-friction interaction at the mechanical seal interface. Squeeze film force coefficients, damping and added mass, are in agreement with simple predictive formulas for an uncavitated lubricant condition and are similar for both flow restrictor sizes. The SFD-mechanical seal arrangement effectively prevents air ingestion and entrapment and generates predicable force coefficients for the range of frequencies tested.

Identification of Force Coefficients in a Squeeze Film Damper With a Mechanical End Seal: Centered Circular Orbit Tests

2007 ◽  
Author(s):  
Luis San Andre´s ◽  
Adolfo Delgado
Keyword(s):  
Dry Friction ◽  
Dynamic Pressure ◽  
Test System ◽  
Single Frequency ◽  
Squeeze Film ◽  
Mechanical Seal ◽  
Damping Force ◽  
Squeeze Film Damper ◽  
Force Coefficients ◽  
Aircraft Application

The paper presents parameter identification measurements conducted on a squeeze film damper (SFD) featuring a non-rotating mechanical seal that effectively eliminates lubricant side leakage. The SFD-seal arrangement generates dissipative forces due to viscous and dry-friction effects from the lubricant film and surfaces in contact, respectively. The test damper reproduces an aircraft application that must contain the lubricant for extended periods of time. The test damper journal is 2.54 cm in length and 12.7 cm in diameter, with a nominal clearance of 0.127 mm. The damper feed end opens to a plenum filled with lubricant, and at its discharge grooved section, four orifice ports evacuate the lubricant. In prior publications (ASME Paper GT2006-90782, IJTC2006-12041), single frequency force excitation tests were conducted, without and with lubricant in the squeeze film land, to determine the seal dry-friction force and viscous damping force coefficients. Presently, further measurements are conducted to identify the test system and SFD force coefficients using two sets of flow restrictor orifice sizes (2.8 mm and 1.1 mm in diameter). The flow restrictors regulate the discharge flow area, and thus control the oil flow through the squeeze film. The experiments also include measurements of dynamic pressures at the squeeze film land and at the discharge groove. The magnitude of dynamic pressure in the squeeze film land is nearly identical for both sets of flow restrictors, and for small orbit radii, dynamic pressures in the discharge groove have peak values similar to those in the squeeze film land. The identified parameters include the test system damping and the individual contributions from the squeeze film, dry friction in the mechanical seal and structure remnant damping. The identified system damping coefficients are frequency and motion amplitude dependent due to the dry friction interaction at the mechanical seal interface. Squeeze film force coefficients, damping and added mass, are in agreement with simple predictive formulas for an uncavitated lubricant condition and are similar for both flow restrictor sizes. The SFD-mechanical seal arrangement effectively prevents air ingestion and entrapment and generates predicable force coefficients for the range of frequencies tested.

Experimental Performance of an Open Ends, Centrally Grooved, Squeeze Film Damper Operating With Large Amplitude Orbital Motions

2014 ◽  
Vol 137 (3) ◽  
Author(s):  
Luis San Andrés ◽  
Sung-Hwa Jeung
Keyword(s):  
Large Amplitude ◽  
Gas Turbines ◽  
Mechanical Energy ◽  
Forced Response ◽  
Dynamic Loads ◽  
Squeeze Film ◽  
Radial Clearance ◽  
Force Coefficients ◽  
Squeeze Film Dampers ◽  
Static Eccentricity

Aircraft engines customarily implement squeeze film dampers (SFDs) to dissipate mechanical energy caused by rotor vibration and to isolate the rotor from its structural frame. The paper presents experimental results for the dynamic forced performance of an open ends SFD operating with large amplitude whirl motions, centered and off-centered. The test rig comprises of an elastically supported bearing with a damper section, 127 mm in diameter, having two parallel film lands separated by a central groove. Each film land is 25.4 mm long with radial clearance c = 0.251 mm. The central groove, 12.7 mm long, has a depth of 9.5 mm (38c). An ISO VG 2 lubricant flows into the groove via three 2.5 mm orifices, 120 deg apart, and then passes through the film lands to exit at ambient condition. Two orthogonally placed shakers apply dynamic loads on the bearing to induce circular orbit motions with whirl frequency ranging from 10 Hz to 100 Hz. A static loader, 45 deg away from each shaker, pulls the bearing to a static eccentricity (es). Measurements of dynamic loads and the ensuing bearing displacements and accelerations, as well as the film and groove dynamic pressures, were obtained for eight orbit amplitudes (r = 0.08c to ∼0.71c) and under four static eccentricities (es = 0.0c to ∼0.76c). The experimental damping coefficients increase quickly as the bearing offset increases (es/c → 0.76) while remaining impervious to the amplitude of whirl orbit (r/c → 0.51). The inertia coefficients decrease rapidly as the orbit amplitude grows large, r > 0.51c, but increase with the static eccentricity. A comparison with test results obtained with an identical damper but having a smaller clearance (cs = 0.141 mm) (San Andrés, L., 2012, “Damping and Inertia Coefficients for Two Open Ends Squeeze Film Dampers With a Central Groove: Measurements and Predictions,” ASME J. Eng. Gas Turbines Power, 134(10), p. 102506), show the prior damping and inertia coefficients are larger, ∼5.0 and ∼2.2 times larger than the current ones. These magnitudes agree modestly with theoretical ratios for damping and inertia coefficients scaling as (c/cs)3 = 5.7 and (c/cs) = 1.8, respectively. In spite of the large difference in depths between a groove and a film land, the magnitudes of dynamic pressures recorded at the groove are similar to those in the lands. That is, the groove profoundly affects the dynamic forced response of the test damper. A computational physics model replicates the experimental whirl motions and predicts force coefficients spanning the same range of whirl frequencies, orbit radii, and static eccentricities. The model predictions reproduce with great fidelity the experimental force coefficients. The good agreement relies on the specification of an effective groove depth derived from one experiment.

Effect of Oil Supply Pressure on the Force Coefficients of a Squeeze Film Damper Sealed With Piston Rings

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Sung-Hwa Jeung ◽  
Luis San Andrés ◽  
Sean Den ◽  
Bonjin Koo
Keyword(s):  
Added Mass ◽  
The Other ◽  
System Stability ◽  
Squeeze Film ◽  
Piston Rings ◽  
Film Pressure ◽  
Force Coefficients ◽  
Supply Pressure ◽  
Flow Demand ◽  
Load Tests

Squeeze film dampers (SFDs) aid to both reduce rotor dynamic displacements and to increase system stability. Dampers sealed with piston rings (PR), common in aircraft engines, are proven to boost damping generation, reduce lubricant flow demand, and prevent air ingestion. This paper presents the estimation of force coefficients in a short length SFD, PR sealed, and supplied with a light lubricant at two feed pressures, Pin-1 ∼ 0.69 barg and Pin-2 ∼ 2.76 barg, i.e., low and high. Two pairs of PRs are installed in the test SFD, one set has flow conductance CS1 = 0.56 LPM/bar, whereas the other pair has CS2 = 0.89 LPM/bar. The second set leaks more as it has a larger slit gap. Dynamic load tests show that both dampers, having seal flow conductances differing by 60%, produce damping and added mass coefficients of similar magnitude, differing by at most 20%. Other experiments quantify the effect of lubricant supply pressure, Pin-1 and Pin-2, on the dynamic film pressure and force coefficients of the PR-SFD. The damper configuration with CS1 and operating with the high Pin-2 shows ∼20% more damping and added mass coefficients compared with test results for the damper supplied with Pin-1. Film pressure measurements show that the air ingestion and oil vapor cavitation coexist for operation at the low Pin-1. Computational predictions accounting for the feed holes in the physical model agree with the experimental coefficients. On the other hand, predictions from classical formulas for an idealized damper geometry, fully sealed at its ends, largely overpredict the measured force coefficients.

Experimental Performance of an Open Ends, Centrally Grooved, Squeeze Film Damper Operating With Large Amplitude Orbital Motions

2014 ◽  
Author(s):  
Luis San Andrés ◽  
Sung-Hwa Jeung ◽  
Gary Bradley
Keyword(s):  
Large Amplitude ◽  
Mechanical Energy ◽  
Groove Depth ◽  
Forced Response ◽  
Dynamic Loads ◽  
Squeeze Film ◽  
Radial Clearance ◽  
Force Coefficients ◽  
Static Eccentricity ◽  
Physics Model

Aircraft engines customarily implement Squeeze Film Dampers (SFDs) to dissipate mechanical energy caused by rotor vibration and to isolate the rotor from its structural frame. The paper presents experimental results for the dynamic forced performance of an open ends SFD operating with large amplitude whirl motions, centered and off-centered. The test rig comprises of an elastically supported bearing with a damper section, 127 mm in diameter, having two parallel film lands separated by a central groove. Each film land is 25.4 mm long with radial clearance c = 0.251 mm. The central groove, 12.7 mm long, has a depth of 9.5 mm (38c). An ISO VG 2 lubricant flows into the groove via three 2.5 mm orifices, 120 degrees apart, and then passes through the film lands to exit at ambient condition. Two orthogonally placed shakers apply dynamic loads on the bearing to induce circular orbit motions with whirl frequency ranging from 10 Hz to 100 Hz. A static loader, 45° away from each shaker, pulls the bearing to a static eccentricity (es). Measurements of dynamic loads and the ensuing bearing displacements and accelerations, as well as the film and groove dynamic pressures, were obtained for eight orbit amplitudes (r = 0.08c to ∼0.71c) and under four static eccentricities (es = 0.0c to ∼0.76c). The experimental damping coefficients increase quickly as the bearing offset increases (es/c→0.76) while remaining impervious to the amplitude of whirl orbit (r/c→0.51). The inertia coefficients decrease rapidly as the orbit amplitude grows large, r>0.51c, but increase with the static eccentricity. A comparison with test results obtained with an identical damper but having a smaller clearance (cs = 0.141 mm) [1], show the prior damping and inertia coefficients are larger, ∼5.0 and ∼2.2 times larger than the current ones. These magnitudes agree modestly with theoretical ratios for damping and inertia coefficients scaling as (c/cs)3 = 5.7 and (c/cs) = 1.8, respectively. In spite of the large difference in depths between a groove and a film land, the magnitudes of dynamic pressures recorded at the groove are similar to those in the lands. That is, the groove profoundly affects the dynamic forced response of the test damper. A computational physics model replicates the experimental whirl motions and predicts force coefficients spanning the same range of whirl frequencies, orbit radii and static eccentricities. The model predictions reproduce with great fidelity the experimental force coefficients. The good agreement relies on the specification of an effective groove depth derived from one experiment.

A Novel Bulk-Flow Model for Improved Predictions of Force Coefficients in Grooved Oil Seals Operating Eccentrically

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
Luis San Andrés ◽  
Adolfo Delgado
Keyword(s):  
Test Data ◽  
Flow Model ◽  
Hydrodynamic Instability ◽  
Added Mass ◽  
Operating Conditions ◽  
Bulk Flow ◽  
Squeeze Film ◽  
Accurate Estimation ◽  
Force Coefficients ◽  
Process Gas

Oil seals in centrifugal compressors reduce leakage of the process gas into the support bearings and ambient. Under certain operating conditions of speed and pressure, oil seals lock, becoming a source of hydrodynamic instability due to excessively large cross coupled stiffness coefficients. It is a common practice to machine circumferential grooves, breaking the seal land, to isolate shear flow induced film pressures in contiguous lands, and hence reducing the seal cross coupled stiffnesses. Published tests results for oil seal rings shows that an inner land groove, shallow or deep, does not actually reduce the cross-stiffnesses as much as conventional models predict. In addition, the tested grooved oil seals evidenced large added mass coefficients while predictive models, based on classical lubrication theory, neglect fluid inertia effects. This paper introduces a bulk-flow model for groove oil seals operating eccentrically and its solution via the finite element (FE) method. The analysis relies on an effective groove depth, different from the physical depth, which delimits the upper boundary for the squeeze film flow. Predictions of rotordynamic force coefficients are compared to published experimental force coefficients for a smooth land seal and a seal with a single inner groove with depth equaling 15 times the land clearance. The test data represent operation at 10 krpm and 70 bar supply pressure, and four journal eccentricity ratios (e/c= 0, 0.3, 0.5, 0.7). Predictions from the current model agree with the test data for operation at the lowest eccentricities (e/c= 0.3) with discrepancies increasing at larger journal eccentricities. The new flow model is a significant improvement towards the accurate estimation of grooved seal cross-coupled stiffnesses and added mass coefficients; the latter was previously ignored or largely under predicted.

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