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Détail de l'auteur
Auteur Childs, Dara W.
Documents disponibles écrits par cet auteur
Affiner la rechercheDry-friction whip and whirl predictions for a rotor-stator model with rubbing contact at two locations / Childs, Dara W. in Transactions of the ASME . Journal of engineering for gas turbines and power, Vol. 134 N° 7 (Juillet 2012)
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 134 N° 7 (Juillet 2012) . - 11 p.
Titre : Dry-friction whip and whirl predictions for a rotor-stator model with rubbing contact at two locations Type de document : texte imprimé Auteurs : Childs, Dara W., Auteur ; Dhruv Kumar, Auteur Année de publication : 2012 Article en page(s) : 11 p. Note générale : Génie mécanique Langues : Anglais (eng) Mots-clés : Dry-friction whip Whirl phenomena Rigid rotor contacting Rubbing contact Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The present work investigates dry-friction whip and whirl phenomena for a rigid rotor contacting at two bearing locations. The idea originated with a paper by Clark et al. (2009, “Investigation of the NRG #40 Anemometer Slowdown,” American Wind Energy Association, Windpower 2009, Chicago, IL, pp. 1-16) on an anemometer undergoing dry-friction whip and whirl. The anemometer rotor was supported by two Teflon® bushings within an elastically supported housing. The dry-friction forces arose at the bushings. Prior models for dry friction whirl and whip have considered rub at one nonsupport location. The present analytical model consists of a rigid rotor connected to a rigid stator at two rubbing-contact locations. Analytical solutions are developed for the following normal reaction forces at the contact locations: (1) In phase, and (2) 180° out of phase. Analytical solutions are only possible for the same radius-to-clearance ratio (RCl) at the two rub locations and define regions where dry-friction whirl is possible in addition to indicating possible boundaries between whirl and whip. These solutions are similar to Black's (1968, “Interaction of a Whirling Rotor with a Vibrating Stator Across a Clearance Annulus,” J. Mech. Eng. Sci., 10(1), pp. 1-12) and Crandall's (1990, “From Whirl to Whip in Rotordynamics,” IFToMM Third Intl. Conf. on Rotordynamics, Lyon, France, pp. 19-26). A flexible-rotor/flexible-stator model with nonlinear connections at the bearings was developed to more correctly establish the range of possible solutions. The nonlinear connections at the rub surface are modeled using Hunt and Crossley's 1975 contact model with Coulomb friction (Hunt and Crossley, F., 1975, “Coefficient of Restitution Interpreted as Damping in Vibroimpact,” ASME J. Appl. Mech., 42, pp. 440). Dry friction simulations are performed for the following rotor center of gravity (C.G.) configurations with respect to the contact locations: (1) Centered, (2) [3/4]-span location, and (3) overhung, outside the contacts. Predictions from the in-phase analytical solutions and the nonlinear simulations agree to some extent when the rotor mass is centered and at the [3/4]-span location due to the fact that whirl-to-whip transitions occur near the pinned rotor-stator bounce frequency. For the overhung mass case, the nonlinear simulation predicts whip at different frequencies for the two contact locations. Neither analytical solution modes predicts this outcome. No 180 deg out-of-phase solutions could be obtained via time-transient simulations. Dry-friction whirling is normally characterized as supersynchronous precession with a precession frequency equal to the running speed omega times RCl. Simulation predictions for models with different RCl ratio mimic whirling. Specifically, with increasing rotor speed, the backward precessional (BP) frequency increases at each contact location. However, individual contact velocities show slipping at all conditions. Slipping is greater at one location than the other, netting a “whirl-like” motion. For the overhung model with different RCl ratios: in addition to whipping at different frequencies the two contacts also whirl at different frequencies corresponding to the separate RCl ratios at the respective contacts. Simulations predict a different running speed for the “jump up” in precession frequency associated with a transition from whirl-to-whip with increasing running speed than for the jump-down in precession frequency for whirl-to-whip in a speed-decreasing mode. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000134000007 [...] [article] Dry-friction whip and whirl predictions for a rotor-stator model with rubbing contact at two locations [texte imprimé] / Childs, Dara W., Auteur ; Dhruv Kumar, Auteur . - 2012 . - 11 p.
Génie mécanique
Langues : Anglais (eng)
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 134 N° 7 (Juillet 2012) . - 11 p.
Mots-clés : Dry-friction whip Whirl phenomena Rigid rotor contacting Rubbing contact Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The present work investigates dry-friction whip and whirl phenomena for a rigid rotor contacting at two bearing locations. The idea originated with a paper by Clark et al. (2009, “Investigation of the NRG #40 Anemometer Slowdown,” American Wind Energy Association, Windpower 2009, Chicago, IL, pp. 1-16) on an anemometer undergoing dry-friction whip and whirl. The anemometer rotor was supported by two Teflon® bushings within an elastically supported housing. The dry-friction forces arose at the bushings. Prior models for dry friction whirl and whip have considered rub at one nonsupport location. The present analytical model consists of a rigid rotor connected to a rigid stator at two rubbing-contact locations. Analytical solutions are developed for the following normal reaction forces at the contact locations: (1) In phase, and (2) 180° out of phase. Analytical solutions are only possible for the same radius-to-clearance ratio (RCl) at the two rub locations and define regions where dry-friction whirl is possible in addition to indicating possible boundaries between whirl and whip. These solutions are similar to Black's (1968, “Interaction of a Whirling Rotor with a Vibrating Stator Across a Clearance Annulus,” J. Mech. Eng. Sci., 10(1), pp. 1-12) and Crandall's (1990, “From Whirl to Whip in Rotordynamics,” IFToMM Third Intl. Conf. on Rotordynamics, Lyon, France, pp. 19-26). A flexible-rotor/flexible-stator model with nonlinear connections at the bearings was developed to more correctly establish the range of possible solutions. The nonlinear connections at the rub surface are modeled using Hunt and Crossley's 1975 contact model with Coulomb friction (Hunt and Crossley, F., 1975, “Coefficient of Restitution Interpreted as Damping in Vibroimpact,” ASME J. Appl. Mech., 42, pp. 440). Dry friction simulations are performed for the following rotor center of gravity (C.G.) configurations with respect to the contact locations: (1) Centered, (2) [3/4]-span location, and (3) overhung, outside the contacts. Predictions from the in-phase analytical solutions and the nonlinear simulations agree to some extent when the rotor mass is centered and at the [3/4]-span location due to the fact that whirl-to-whip transitions occur near the pinned rotor-stator bounce frequency. For the overhung mass case, the nonlinear simulation predicts whip at different frequencies for the two contact locations. Neither analytical solution modes predicts this outcome. No 180 deg out-of-phase solutions could be obtained via time-transient simulations. Dry-friction whirling is normally characterized as supersynchronous precession with a precession frequency equal to the running speed omega times RCl. Simulation predictions for models with different RCl ratio mimic whirling. Specifically, with increasing rotor speed, the backward precessional (BP) frequency increases at each contact location. However, individual contact velocities show slipping at all conditions. Slipping is greater at one location than the other, netting a “whirl-like” motion. For the overhung model with different RCl ratios: in addition to whipping at different frequencies the two contacts also whirl at different frequencies corresponding to the separate RCl ratios at the respective contacts. Simulations predict a different running speed for the “jump up” in precession frequency associated with a transition from whirl-to-whip with increasing running speed than for the jump-down in precession frequency for whirl-to-whip in a speed-decreasing mode. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000134000007 [...]
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 133 N° 9 (Septembre 2011) . - 10 p.
Titre : Friction factor behavior from flat-plate tests of smooth and hole-pattern roughened surfaces with supply pressures up to 84 bars Type de document : texte imprimé Auteurs : Childs, Dara W., Auteur ; Bassem Kheireddin, Auteur ; Stephen Phillips, Auteur Année de publication : 2012 Article en page(s) : 10 p. Note générale : Génie Mécanique Langues : Anglais (eng) Mots-clés : Compressors Flow Friction Plates (structures) Rotors Stators Surface roughness Test facilities Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : A flat-plate tester was used to measure the friction factor behavior for a hole-pattern roughened surface apposed to a smooth surface. The tests were executed to characterize the friction factor behavior of annular seals that use a roughened-surface stator and a smooth rotor. Friction factors were obtained from measurements of the mass flow rate and static pressure measurements along the smooth and roughened surfaces. In addition, dynamic pressure measurements were made at four axial locations at the bottom of individual holes and at facing locations in the smooth plate. The test facility is described, and a procedure for determining the friction factor is reviewed. Three clearances were investigated: 0.635 mm, 0.381 mm, and 0.254 mm. Tests were conducted with air at three different inlet pressures (84 bars, 70 bars, and 55 bars), producing a Reynolds numbers range from 50,000 to 700,000. Three surface configurations were tested, including smooth-on-smooth, smooth-on-hole, and hole-on-hole. The hole-pattern plates are identical with the exception of the hole depth. For the smooth-on-smooth and smooth-on-hole configurations, the friction factor remains largely constant or increases slightly with increasing Reynolds numbers. The friction factor increases as the clearance between the plates increases. The test program was initiated to investigate a friction-factor jump phenomenon cited by Ha et al. (1992, “Friction-Factor Characteristics for Narrow-Channels With Honeycomb Surfaces,” Trans. ASME, J. Tribol., 114, pp. 714–721) in test results from a flat-plate tester where, at elevated values of Reynolds numbers, the friction factor began to increase steadily with increasing Reynolds numbers. They tested apposed honeycomb surfaces. For the present tests, the phenomenon was also observed for tests of apposed roughened surfaces but was not observed for smooth-on-smooth or smooth-on-rough configurations. When the phenomenon was observed, dynamic pressure measurements showed a peak-pressure oscillation at the calculated Helmholtz frequency of the holes. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ00013 [...] [article] Friction factor behavior from flat-plate tests of smooth and hole-pattern roughened surfaces with supply pressures up to 84 bars [texte imprimé] / Childs, Dara W., Auteur ; Bassem Kheireddin, Auteur ; Stephen Phillips, Auteur . - 2012 . - 10 p.
Génie Mécanique
Langues : Anglais (eng)
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 133 N° 9 (Septembre 2011) . - 10 p.
Mots-clés : Compressors Flow Friction Plates (structures) Rotors Stators Surface roughness Test facilities Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : A flat-plate tester was used to measure the friction factor behavior for a hole-pattern roughened surface apposed to a smooth surface. The tests were executed to characterize the friction factor behavior of annular seals that use a roughened-surface stator and a smooth rotor. Friction factors were obtained from measurements of the mass flow rate and static pressure measurements along the smooth and roughened surfaces. In addition, dynamic pressure measurements were made at four axial locations at the bottom of individual holes and at facing locations in the smooth plate. The test facility is described, and a procedure for determining the friction factor is reviewed. Three clearances were investigated: 0.635 mm, 0.381 mm, and 0.254 mm. Tests were conducted with air at three different inlet pressures (84 bars, 70 bars, and 55 bars), producing a Reynolds numbers range from 50,000 to 700,000. Three surface configurations were tested, including smooth-on-smooth, smooth-on-hole, and hole-on-hole. The hole-pattern plates are identical with the exception of the hole depth. For the smooth-on-smooth and smooth-on-hole configurations, the friction factor remains largely constant or increases slightly with increasing Reynolds numbers. The friction factor increases as the clearance between the plates increases. The test program was initiated to investigate a friction-factor jump phenomenon cited by Ha et al. (1992, “Friction-Factor Characteristics for Narrow-Channels With Honeycomb Surfaces,” Trans. ASME, J. Tribol., 114, pp. 714–721) in test results from a flat-plate tester where, at elevated values of Reynolds numbers, the friction factor began to increase steadily with increasing Reynolds numbers. They tested apposed honeycomb surfaces. For the present tests, the phenomenon was also observed for tests of apposed roughened surfaces but was not observed for smooth-on-smooth or smooth-on-rough configurations. When the phenomenon was observed, dynamic pressure measurements showed a peak-pressure oscillation at the calculated Helmholtz frequency of the holes. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ00013 [...]
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 134 N° 5 (Mai 2012) . - 11. p.
Titre : Measured static and rotordynamic coefficient results for a rocker-pivot, tilting-pad bearing with 50 and 60% offsets Type de document : texte imprimé Auteurs : Chris D. Kulhanek, Auteur ; Childs, Dara W., Auteur Année de publication : 2012 Article en page(s) : 11. p. Note générale : Génie mécanique Langues : Anglais (eng) Mots-clés : Damping Dynamic testing Elasticity Machine bearings Rotors Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Static and rotordynamic coefficients are measured for a rocker-pivot, tilting-pad journal bearing (TPJB) with 50 and 60% offset pads in a load-between-pad (LBP) configuration. The bearing uses leading-edge-groove direct lubrication and has the following characteristics: 5-pads, 101.6 mm (4.0 in) nominal diameter,0.0814 -0.0837 mm (0.0032–0.0033 in) radial bearing clearance, 0.25 to 0.27 preload, and 60.325 mm (2.375 in) axial pad length. Tests were performed on a floating bearing test rig with unit loads from 0 to 3101 kPa (450 psi) and speeds from 7 to 16 krpm. Dynamic tests were conducted over a range of frequencies (20 to 320 Hz) to obtain complex dynamic stiffness coefficients as functions of excitation frequency. For most test conditions, the real dynamic stiffness functions were well fitted with a quadratic function with respect to frequency. This curve fit allowed for the stiffness frequency dependency to be captured by including an added mass matrix [M] to a conventional [K][C] model, yielding a frequency independent [K][C][M] model. The imaginary dynamic stiffness coefficients increased linearly with frequency, producing frequency-independent direct damping coefficients. Direct stiffness coefficients were larger for the 60% offset bearing at light unit loads. At high loads, the 50% offset configuration had a larger stiffness in the loaded direction, while the unloaded direct stiffness was approximately the same for both pivot offsets. Cross-coupled stiffness coefficients were positive and significantly smaller than direct stiffness coefficients. Negative direct added-mass coefficients were obtained for both offsets, especially in the unloaded direction. Cross-coupled added-mass coefficients are generally positive and of the same sign. Direct damping coefficients were mostly independent of load and speed, showing no appreciable difference between pivot offsets. Cross-coupled damping coefficients had the same sign and were much smaller than direct coefficients. Measured static eccentricities suggested cross coupling stiffness exists for both pivot offsets, agreeing with dynamic measurements. Static stiffness measurements showed good agreement with the loaded, direct dynamic stiffness coefficients. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000134000005 [...] [article] Measured static and rotordynamic coefficient results for a rocker-pivot, tilting-pad bearing with 50 and 60% offsets [texte imprimé] / Chris D. Kulhanek, Auteur ; Childs, Dara W., Auteur . - 2012 . - 11. p.
Génie mécanique
Langues : Anglais (eng)
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 134 N° 5 (Mai 2012) . - 11. p.
Mots-clés : Damping Dynamic testing Elasticity Machine bearings Rotors Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Static and rotordynamic coefficients are measured for a rocker-pivot, tilting-pad journal bearing (TPJB) with 50 and 60% offset pads in a load-between-pad (LBP) configuration. The bearing uses leading-edge-groove direct lubrication and has the following characteristics: 5-pads, 101.6 mm (4.0 in) nominal diameter,0.0814 -0.0837 mm (0.0032–0.0033 in) radial bearing clearance, 0.25 to 0.27 preload, and 60.325 mm (2.375 in) axial pad length. Tests were performed on a floating bearing test rig with unit loads from 0 to 3101 kPa (450 psi) and speeds from 7 to 16 krpm. Dynamic tests were conducted over a range of frequencies (20 to 320 Hz) to obtain complex dynamic stiffness coefficients as functions of excitation frequency. For most test conditions, the real dynamic stiffness functions were well fitted with a quadratic function with respect to frequency. This curve fit allowed for the stiffness frequency dependency to be captured by including an added mass matrix [M] to a conventional [K][C] model, yielding a frequency independent [K][C][M] model. The imaginary dynamic stiffness coefficients increased linearly with frequency, producing frequency-independent direct damping coefficients. Direct stiffness coefficients were larger for the 60% offset bearing at light unit loads. At high loads, the 50% offset configuration had a larger stiffness in the loaded direction, while the unloaded direct stiffness was approximately the same for both pivot offsets. Cross-coupled stiffness coefficients were positive and significantly smaller than direct stiffness coefficients. Negative direct added-mass coefficients were obtained for both offsets, especially in the unloaded direction. Cross-coupled added-mass coefficients are generally positive and of the same sign. Direct damping coefficients were mostly independent of load and speed, showing no appreciable difference between pivot offsets. Cross-coupled damping coefficients had the same sign and were much smaller than direct coefficients. Measured static eccentricities suggested cross coupling stiffness exists for both pivot offsets, agreeing with dynamic measurements. Static stiffness measurements showed good agreement with the loaded, direct dynamic stiffness coefficients. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000134000005 [...]
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 134 N° 7 (Juillet 2012) . - 09 p.
Titre : A new, iterative, synchronous-response algorithm for analyzing the Morton effect Type de document : texte imprimé Auteurs : Childs, Dara W., Auteur ; Rohit Saha, Auteur Année de publication : 2012 Article en page(s) : 09 p. Note générale : Génie mécanique Langues : Anglais (eng) Mots-clés : Morton effect problems Rotor synchronous-response Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Morton Effect problems involve the steady increase in rotor synchronous-response amplitudes due to differential heating across a fluid-film bearing that is induced by synchronous response. The present work presents a new computational algorithm for analyzing the Morton Effect. Previous approaches were based on Eigen or Nyquist analyses for stability studies and predicted an onset speed of instability. The present algorithm starts with a steady state elliptical orbit produced by the initial imbalance distribution, which is decomposed into a forward-precessing circular orbit and a backwards-precessing circular orbit. A separate (and numerically intensive) calculation based on the Reynolds equation plus the energy equation gives predictions for the temperature distributions induced by these separate orbits for a range of orbit radius-to-clearance ratios. Temperature distributions for the forward and backward orbits are calculated and added to produce the net temperature distribution due to the initial elliptic orbit. The temperature distribution is assumed to vary linearly across the bearing and produces a bent-shaft angle across the bearing following an analytical result due to Dimoragonas. This bent-shaft angle produces a synchronous rotor excitation in the form of equal and opposite moments acting at the bearing's ends. For a rotor with an overhung section, the bend also produces a thermally induced imbalance. The response is due to: (1) the initial mechanical imbalance, (2) the bent-shaft excitation, and (3) the thermally-induced imbalance are added to produce a new elliptic orbit, and the process is repeated until a converged orbit is produced. For the work reported, no formal stability analysis is carried out on the converged orbit. The algorithm predicts synchronous response across the rotor's speed range plus the speed where the response amplitudes becomes divergent by approaching the clearance. Predictions are presented for one example from the published literature, and elevated vibration levels are predicted well before the motion diverges. Synchronous-response amplitudes due to Morton Effect can be orders of magnitude greater than the response due only to mechanical imbalance, particularly near rotor critical speeds. For the example considered, bent-shaft-moment excitation produces significantly higher response levels than the mechanical imbalance induced by thermal bow. The impact of changes in: (1) bearing length-to-diameter ratio, (2) reduced lubricant viscosity, (3) bearing radius-to-clearance ratio and (4) overhung mass magnitude are investigated. Reducing lubricant viscosity and/or reducing the overhung mass are predicted to be the best remedies for Morton Effect problems. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000134000007 [...] [article] A new, iterative, synchronous-response algorithm for analyzing the Morton effect [texte imprimé] / Childs, Dara W., Auteur ; Rohit Saha, Auteur . - 2012 . - 09 p.
Génie mécanique
Langues : Anglais (eng)
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 134 N° 7 (Juillet 2012) . - 09 p.
Mots-clés : Morton effect problems Rotor synchronous-response Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Morton Effect problems involve the steady increase in rotor synchronous-response amplitudes due to differential heating across a fluid-film bearing that is induced by synchronous response. The present work presents a new computational algorithm for analyzing the Morton Effect. Previous approaches were based on Eigen or Nyquist analyses for stability studies and predicted an onset speed of instability. The present algorithm starts with a steady state elliptical orbit produced by the initial imbalance distribution, which is decomposed into a forward-precessing circular orbit and a backwards-precessing circular orbit. A separate (and numerically intensive) calculation based on the Reynolds equation plus the energy equation gives predictions for the temperature distributions induced by these separate orbits for a range of orbit radius-to-clearance ratios. Temperature distributions for the forward and backward orbits are calculated and added to produce the net temperature distribution due to the initial elliptic orbit. The temperature distribution is assumed to vary linearly across the bearing and produces a bent-shaft angle across the bearing following an analytical result due to Dimoragonas. This bent-shaft angle produces a synchronous rotor excitation in the form of equal and opposite moments acting at the bearing's ends. For a rotor with an overhung section, the bend also produces a thermally induced imbalance. The response is due to: (1) the initial mechanical imbalance, (2) the bent-shaft excitation, and (3) the thermally-induced imbalance are added to produce a new elliptic orbit, and the process is repeated until a converged orbit is produced. For the work reported, no formal stability analysis is carried out on the converged orbit. The algorithm predicts synchronous response across the rotor's speed range plus the speed where the response amplitudes becomes divergent by approaching the clearance. Predictions are presented for one example from the published literature, and elevated vibration levels are predicted well before the motion diverges. Synchronous-response amplitudes due to Morton Effect can be orders of magnitude greater than the response due only to mechanical imbalance, particularly near rotor critical speeds. For the example considered, bent-shaft-moment excitation produces significantly higher response levels than the mechanical imbalance induced by thermal bow. The impact of changes in: (1) bearing length-to-diameter ratio, (2) reduced lubricant viscosity, (3) bearing radius-to-clearance ratio and (4) overhung mass magnitude are investigated. Reducing lubricant viscosity and/or reducing the overhung mass are predicted to be the best remedies for Morton Effect problems. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000134000007 [...]
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 132 N° 11 (Novembre 2010) . - 08 p.
Titre : Predicted rotordynamic behavior of a labyrinth seal as rotor surface speed approaches mach 1 Type de document : texte imprimé Auteurs : Manish R. Thorat, Auteur ; Childs, Dara W., Auteur Année de publication : 2011 Article en page(s) : 08 p. Note générale : Génie Mécanique Langues : Anglais (eng) Mots-clés : Aerodynamics Damping Elastic constants Flow instability Mach number Rotors Sealing materials Transfer functions Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Prior one-control-volume (1CV) models for rotor-fluid interaction in labyrinth seals produce synchronously reduced (at running speed), frequency-independent stiffness and damping coefficients. The 1CV model, consisting of a leakage equation, a continuity equation, and a circumferential-momentum equation (for each cavity), was stated to be invalid for rotor surface speeds approaching the speed of sound. However, the present results show that while the 1CV fluid-mechanic model continues to be valid, the calculated rotordynamic coefficients become strongly dependent on the rotor's precession frequency. A solution is developed for the reaction-force components for a range of precession frequencies, producing frequency-dependent stiffness and damping coefficients. They can be used to define a Laplace-domain transfer-function model for the reaction-force/rotor-motion components. Calculated results are presented for a simple Jeffcott rotor model acted on by a labyrinth seal. The model's undamped natural frequency is 7.6 krpm. The fluid properties, seal radius Rs, and running speed omega cause the rotor surface velocity Rsomega to equal the speed of sound c0 at omega=58 krpm. Calculated synchronous-response results due to imbalance coincide for the synchronously reduced and the frequency-dependent models. For an inlet preswirl ratio of 0.5, both models predict the same log-dec out to omega[approximate]14.5 krpm. The synchronously reduced model predicts an onset speed of instability (OSI) at 10 krpm, but a return to stability at 48 krpm, with subsequent increases in log-dec out to 70 krpm. The frequency-dependent model predicts an OSI of 10 krpm and no return to stability out to 70 krpm. The frequency-dependent models predict small changes in the rotor's damped natural frequencies. The synchronously reduced model predicts large changes. The stability-analysis results show that a frequency-dependent labyrinth seal model should be used if the rotor surface speed approaches a significant fraction of the speed of sound. For the present example, observable discrepancies arose when Rsomega=0.26c0. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ00013 [...] [article] Predicted rotordynamic behavior of a labyrinth seal as rotor surface speed approaches mach 1 [texte imprimé] / Manish R. Thorat, Auteur ; Childs, Dara W., Auteur . - 2011 . - 08 p.
Génie Mécanique
Langues : Anglais (eng)
in Transactions of the ASME . Journal of engineering for gas turbines and power > Vol. 132 N° 11 (Novembre 2010) . - 08 p.
Mots-clés : Aerodynamics Damping Elastic constants Flow instability Mach number Rotors Sealing materials Transfer functions Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Prior one-control-volume (1CV) models for rotor-fluid interaction in labyrinth seals produce synchronously reduced (at running speed), frequency-independent stiffness and damping coefficients. The 1CV model, consisting of a leakage equation, a continuity equation, and a circumferential-momentum equation (for each cavity), was stated to be invalid for rotor surface speeds approaching the speed of sound. However, the present results show that while the 1CV fluid-mechanic model continues to be valid, the calculated rotordynamic coefficients become strongly dependent on the rotor's precession frequency. A solution is developed for the reaction-force components for a range of precession frequencies, producing frequency-dependent stiffness and damping coefficients. They can be used to define a Laplace-domain transfer-function model for the reaction-force/rotor-motion components. Calculated results are presented for a simple Jeffcott rotor model acted on by a labyrinth seal. The model's undamped natural frequency is 7.6 krpm. The fluid properties, seal radius Rs, and running speed omega cause the rotor surface velocity Rsomega to equal the speed of sound c0 at omega=58 krpm. Calculated synchronous-response results due to imbalance coincide for the synchronously reduced and the frequency-dependent models. For an inlet preswirl ratio of 0.5, both models predict the same log-dec out to omega[approximate]14.5 krpm. The synchronously reduced model predicts an onset speed of instability (OSI) at 10 krpm, but a return to stability at 48 krpm, with subsequent increases in log-dec out to 70 krpm. The frequency-dependent model predicts an OSI of 10 krpm and no return to stability out to 70 krpm. The frequency-dependent models predict small changes in the rotor's damped natural frequencies. The synchronously reduced model predicts large changes. The stability-analysis results show that a frequency-dependent labyrinth seal model should be used if the rotor surface speed approaches a significant fraction of the speed of sound. For the present example, observable discrepancies arose when Rsomega=0.26c0. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ00013 [...]
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