Damper Seal Research Articles

민항기 door damper용 칼날형 댐핑 오리피스의 설계

In this paper a sharp-edged type damping orifice for an aircraft door damper were designed, where the dynamic viscosity of working fluid were assumed to change up to 400cSt. The discharge coefficient of the damping orifice were investigated by CFD analyses and experiments. In particular, the influences of orifice diameter, edge angle, flow direction and the Reynolds number were taken into consideration. Based on this, it has been deduced how high Coulomb friction forces of damper seals is to be allowed to meet the performance criterion with respect to the orifice size.

Investigations on the Rotordynamic Coefficients of Pocket Damper Seals Using the Multifrequency, One-Dimensional, Whirling Orbit Model and RANS Solutions

The numerical approach using the multifrequency one-dimensional whirling orbit model and Reynolds-averaged Navier-Stokes (RANS) solution was proposed for prediction of rotordynamic coefficients of pocket damper seal (PDS). By conducting the multiple frequencies one-dimensional whirling orbit for rotor center as the excitation signal, the unsteady RANS solutions combined with mesh deformation method were utilized to calculate the transient response forces on the PDS rotor surface. Unlike the single frequency whirling orbit models which require a separate computation for each frequency, the multifrequency whirling orbit model yields results for multiple frequencies and therefore requires only one computation for different frequencies. The rotor motion signal and response force signal were transformed to the frequency domain using the fast fourier transform, then the eight rotordynamic coefficients of the PDS were determined at fourteen different vibration frequencies 20–300 Hz. The numerical results of rotordynamic coefficients of the PDS were in good agreement with experimental data. The accuracy and availability of the proposed method was demonstrated. The effects of vibration frequencies and pressure ratios on the rotordynamic coefficients of PDS were also investigated using the presented numerical method. The multifrequency one-dimensional whirling orbit model is a promising method for prediction of the rotordynamic coefficients of the PDS.

Numerical investigations on the leakage flow characteristics of pocket damper labyrinth seals

A combined finite element analysis and computational fluid dynamics was developed to investigate the discharge characteristics and flow field of pocket damper seals at high pressure (up to 7.2 MPa) and high rotational speed (up to 50,200 r/min) flow condition. At first, the leakage flow rates of the published experimental pocket damper seals with the eight-bladed and eight-pocket were conducted at three different pressure drops and three different rotational speeds, and a comparison between finite element analysis and computational fluid dynamics results, experimental data and computational fluid dynamics results was presented. It showed that the utilized finite element analysis/computational fluid dynamics numerical method has sufficient precision (the maximum relative deviation is less than 1.3%) to predict the leakage flow rate through the pocket damper seals. The importance of taking rotor growth with rotational speed into consideration was made apparent by the rotor growth and leakage results. Then, six pressure ratios and seven rotational speeds were utilized to study the effects of these two factors on the leakage flow characteristics of the pocket damper seals. Numerical simulation results show that the leakage through a pocket damper seals increases with decreasing pressure ratio and asymptotically approach a maximum value corresponding to the choked flow condition when the pressure ratio is small enough (less than 0.26). The leakage flow rate with and without rotor growth included both decrease with the increasing rotational speed. In addition, it is quite necessary to take rotor growth with rotational speed into consideration when the rotational speed is very high (larger than 20,200 r/min) according to the numerical results and the engineering accuracy requirement. The influence of the pressure ratio on the structure of the leakage flow field in seal cavity is negligible. A significant influence of the rotational speed on the flow field in the active cavity has been observed. The circumferential partition wall can significantly decrease the circumferential flow in active cavity of the pocket damper seals.

Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio

The following paper presents and compares rotordynamic force coefficients for three types of non-contact annular gas seals, which include a labyrinth (LABY), honeycomb (HC), and a fully partitioned damper seal (FPDS). These three annular seals represent the typical seal types used in process gas centrifugal compressors at the balance piston location or center seal location to limit internal leakage and ensure a robust rotordynamic design. Tests were conducted on 170.6 mm (6.716 in) diameter seals for rotor speeds up to 15 kprm, inlet air pressure of 6.9 bar (100 psi), ambient back pressure, and with inlet gas preswirl. The three seals were designed to have the same nominal clearance and similar axial lengths. Testing was conducted on a controlled motion test rig possessing non-synchronous excitation capability up to 250 Hz. Three different test methods were employed to give confidence in the rotordynamic coefficients, which include static force deflection tests, mechanical impedance tests, and dynamic cavity pressure tests. Results from experiments compare force coefficients for all seal configurations while paying special attention to the crossover frequencies of the effective damping term. All seals possessed negative effective damping at lower excitation frequencies with inlet preswirl, where the straight-through FPDS possessed the lowest cross over frequency of 40 Hz at 15 krpm. The testing also revealed that the preswirl parameter had significantly more influence on effective damping levels and crossover frequencies when compared to rotor speed.

Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals

As oil fields deplete, in particular in deep sea reservoirs, pump and compression systems work under more strenuous conditions with gas in liquid and liquid in gas mixtures, mostly inhomogeneous. Off-design operation affects system overall efficiency and reliability, including penalties in leakage and rotordynamic performance of secondary flow components, namely seals. The paper details a bulk-flow model for annular damper seals operating with gas in liquid mixtures. The analysis encompasses all-liquid and all-gas seals, as well as seals lubricated with homogenous (bubbly) mixtures, and predicts the static and dynamic force response of mixture lubricated seals; namely: leakage, power loss, reaction forces, and rotordynamic force coefficients, etc., as a function of the mixture volume fraction (βS), supply and discharge pressures, rotor speed, whirl frequency, etc. A seal example with a nitrogen gas mixed with light oil is analyzed. The large pressure drop (70 bar) causes a large expansion of the gas within the seal even for (very) small gas volume fractions (βS). Predictions show leakage and power loss decrease as β→1; albeit at low βS (< 0.3) (re)laminarization of the flow and an apparent increase in mixture viscosity, produce a hump in power loss. Cross-coupled stiffnesses and direct damping coefficients decrease steadily with increases in the gas volume fraction; however, some anomalies are apparent when the flow turns laminar. Mixture lubricated seals show frequency-dependent force coefficients. The equivalent damping decreases above and below βS ∼ 0.10. The direct stiffness coefficients show atypical behavior: a low βS = 0.1 produces stiffness hardening as the excitation frequency increases. Recall that an all liquid seal has a dynamic stiffness softening as frequency increases due to the apparent fluid mass. The predictions call for an experimental program to quantify the static and dynamic forced performance of annular seals operating with (bubbly) mixtures and to validate the current predictive model results.

Improving the Design of a High Pressure Casing With the Help of Finite Element Analysis to Ensure the Rotor Dynamic Stability of a High Pressure Centrifugal Compressor Equipped With a Hole Pattern Seal

This paper focuses on the casing geometry of high pressure compressors. It is common practice to use damper seals, typically the hole pattern type, at the balance piston to ensure the stability of the compressor when compressing fluids with high density levels. Special attention must be paid to the clearance of the hole pattern seal, which must be kept convergent at all operating conditions because a clearance divergence can lead to a rotor dynamic instability of the compressor. Furthermore, the clearance must be kept as low as possible to reduce the leakage losses through the balance piston. Therefore, extensive fine element analyses are performed to determine the mechanical casing deflections in operation and hence, the correct clearance behavior of such damper seals. This paper discusses the history of the casing design during the last 10 years and compares the configurations with respect to the clearance distribution along the damper seal length. To validate the analytical predictions, leakage and stability measurements (using a magnetic shaker) are performed for these high pressure compressors during the full-load, full-pressure testing. This paper presents the stability measurements carried out on two compressors (390 bars and 655 bars discharge pressure) and compares the results.

Identification of Force Coefficients in a Squeeze Film Damper With a Mechanical Seal: Large Contact Force

Squeeze film dampers (SFDs) aid to reduce excessive vibration levels due to rotor imbalance and to raise stability thresholds in rotor-bearing systems. SFDs commonly include end seals to increase their damping capability with a lesser lubricant flow. Seals also aid to reduce the occurrence of air ingestion/entrapment that severely reduces the damper forced performance. However, most conventional end seals do not completely eliminate lubricant side leakage, which limits their effectiveness to prevent air ingestion. A novel end seal arrangement incorporates a spring loaded, contacting mechanical seal that effectively prevents lubricant side leakage and air ingestion. The mechanically sealed damper is intended for use in power engines for unmanned aircraft vehicles. The test damper journal is 2.54 cm in length and 12.7 cm in diameter, with a radial clearance of 0.127 mm. Prior literature reports dynamic load tests on the seal-SFD and measurements of orbital motions to characterize the mechanical parameters of both the mechanical seal and squeeze film damper section. The test data to date include damper operation for a single contact load (90 N) closing the mechanical seal. Presently, measurements of damper dynamic load performance are conducted with a larger contact force (260 N). A nonlinear parameter identification method in the frequency domain determines simultaneously the squeeze film damping and inertia coefficients and the seal dry-friction force. The test results show that the system equivalent viscous damping coefficients are twice as large as those obtained earlier with the smaller contact force. On the other hand, as expected, the squeeze film damper coefficients are nearly identical for both test configurations. Predicted squeeze film damping coefficients, from an improved model that includes the flow in the damper feed and discharge grooves, correlate well with the test data for small and moderate orbit radii. The experimental fluid added mass coefficients are in par with the actual mass of the bearing housing and accurately predicted.

An Approach to Calculate Leakage Flow, Stiffness and Damping of Annular Seals in Turbulent Flow Using Lubrication Theory, Part I: Analysis

This paper presents a general theoretical approach to evaluate the leakage flow, stiffness and damping of annular seals in turbulent flow regime including fluid inertia using turbulent lubrication theory. Fluid inertia effect has been incorporated by perturbing the pressure and fluid velocities following the procedure cited in the literature for laminar flow. Turbulent lubrication theory based on eddy viscosity model has been followed to develop turbulent Reynolds equation. Perturbation theory for small amplitude vibration of the journal center has been followed to determine stiffness and damping of the seal. Theoretical analysis presented is general and seal geometry effect can be incorporated through film shape description.

305 しゅう動面に四角穴パターンを有する平行ダンパシールの力学的特性(OS8-1 微細振動,オーガナイズドセッション:8 機械と車両のダイナミクスおよび諸問題)

305 しゅう動面に四角穴パターンを有する平行ダンパシールの力学的特性(OS8-1 微細振動,オーガナイズドセッション:8 機械と車両のダイナミクスおよび諸問題)

Rotordynamic Force Coefficients for a New Damper Seal Design

The objective of the following work was to determine frequency-dependent rotordynamic force coefficients for a new annular gas damper seal design. Both rotating and nonrotating experimental tests are presented for inlet pressures at 1000psig(69bar), a frequency excitation range of 20-300Hz, and rotor speeds up to 15,200rpm. Two different testing methods were used for determining coefficients: (1) dynamic pressure response method and (2) mechanical impedance method. The dynamic pressure method required the measurement of internal seal cavity pressure modulations in combination with the vibratory motion, whereas the mechanical impedance method used the measurement of external shaker forces, accelerations, and motion of the mechanical system. In addition to the new fully partitioned damper seal (FPDS) tests, the same experiments were conducted for a conventional pocket damper seal (PDS) design. Results of the frequency-dependent force coefficients and the internal seal dynamics for the two different gas damper seals are compared. The conclusions of the tests show that the FPDS design possesses significantly more positive direct damping and direct stiffness compared to the conventional PDS. The experiments also show the measurement of same-sign cross-coupled (cross-axis) stiffness coefficients for both seals, which indicate that the seals do not produce a destabilizing influence on rotor-bearing systems.

High-Pressure Pocket Damper Seals: Leakage Rates and Cavity Pressures

Abstract The turbomachinery component of interest in this paper, the pocket damper seal, has the dual purpose of limiting leakage and providing an additional source of damping at the seal location. The rotordynamic coefficients of these seals (primarily the direct stiffness and damping) are highly dependent on the leakage rates through the seals and the pressures in the seals’ cavities. This paper presents both numerical predictions and experimentally obtained results for the leakage and the cavity pressures of pocket damper seals operating at high pressures. The seals were tested with air, at pressures up to 1000psi(6.92MPa), as the working fluid. Earlier flow-prediction models were modified and used to obtain theoretical reference values for both mass flow rates and pressures. Leakage and static pressure measurements on straight-through and diverging-clearance configurations of eight-bladed and twelve-bladed seals were used for code validation and for calculation of seal discharge coefficients. Higher than expected leakage rates were measured in the case of the twelve-bladed seal, while the leakage rates for the eight-bladed seals were predicted with reasonable accuracy. Differences in the axial pitch lengths of the cavities and the blade profiles of the seals are used to explain the discrepancy in the case of the twelve-bladed seal. The analysis code used also predicted the static cavity pressures reasonably well. Tests conducted on a six-bladed pocket damper seal to further investigate the effect of blade profile supported the results of the eight-bladed and twelve-bladed seal tests and matched theoretical predictions with satisfactory accuracy.

The Influence of Same-Sign Cross-Coupled Stiffness on Rotordynamics

The following paper focuses on same-sign stiffness cross coupling from several viewpoints. First, the influence on rotordynamics is analyzed and discussed, followed by a discussion of how past experiments measuring cross-coupled forces on rotating components may have been in error due to an incomplete testing procedure. Finally, experimental tests are presented on a partitioned damper seal and the correct and complete experimental procedure for determining cross-coupled coefficients is described.

Rotordynamic Force Coefficients of Pocket Damper Seals

This paper presents measured frequency dependent stiffness and damping coefficients for 12-bladed and 8-bladed pocket damper seals (PDS) subdivided into four different seal configurations. Rotating experimental tests are presented for inlet pressures at 69 bar (1000 psi), a frequency excitation range of 20–300 Hz, and rotor speeds up to 20,200 rpm. The testing method used to determine direct and cross-coupled force coefficients was the mechanical impedance method, which required the measurement of external shaker forces, system accelerations, and motion in two orthogonal directions. In addition to the impedance measurements, dynamic pressure responses were measured for individual seal cavities of the eight-bladed PDS. Results of the frequency dependent force coefficients for the four PDS designs are compared. The conclusions of the tests show that the eight-bladed PDS possessed significantly more positive direct damping and negative direct stiffness than the 12-bladed seal. The results from the dynamic pressure response tests show that the diverging clearance design strongly influences the dynamic pressure phase and force density of the seal cavities. The tests also revealed the measurement of same-sign cross-coupled (cross-axis) stiffness coefficients for all seals, which indicate that the seals do not produce a destabilizing influence on rotor-bearing systems.

Test results of friction factor for round-hole roughness surfaces in closely spaced channel flow of water

For examining friction-factor characteristics of round-hole pattern surfaces which are usually applied on damper seals, flat plate test apparatus is designed and fabricated. The measurement method of leakage and pressure distribution along round-hole pattern specimen with different hole area is described and a method for determining the Fanning friction factor is discussed. Results show that the round-hole pattern surfaces provide a much larger friction factor than smooth surface, and the friction factor vs. clearance behavior yields that the friction factor generally decreases as the clearance increases unlike the results of Nava’s flat plate test. As the hole depth is decreased, the friction factor is increased, and maximum friction factor is obtained for 50% of hole area. Since the present experimental friction factor results show coincident characteristics with Moody’s friction factor model, empirical friction factors for round-hole pattern surfaces are obtained by using the Moody’s formula based on curve-fit of the experimental data. Results of Villasmil’s 2D CFD simulation support the present experimental test result.

A Study of the Effects of Inlet Preswirl on the Dynamic Coefficients of a Straight-Bore Honeycomb Gas Damper Seal

Honeycomb seals are frequently used as replacements for labyrinth seals in high-pressure centrifugal compressors to enhance rotordynamic stability. A concern exists that this enhanced stability will be lost if the honeycomb cavities become clogged. Static and dynamic tests were conducted on a honeycomb and a smooth seal (representing the honeycomb seal with completely clogged cells) at the same constant clearances using air with a supply pressure of 70 bars. The test matrix included three speeds, three pressure ratios, and three inlet preswirl conditions. The results show increased leakage, decreased synchronous stiffness, and decreased dynamic stability for the smooth seal with preswirled flow. The results strongly support the use of swirl brakes at the entrance of a honeycomb seal if clogging is a concern. Comparisons between test results and predictions from a two-control-volume theory by Kleynhans and Childs showed excellent agreement in general.

임의로 거칠게 한 표면의 점성 마찰특성

For measuring friction factor of artificially-roughened surfaces which are usually applied to damper seals, flat plate test apparatus is designed and fabricated. The measurements of leakage flow and pressure distribution through round-hole patterned specimen with different hole areas are described, and a method is discussed for determining the friction factor experimentally. Results show that the friction factor of the round-hole patterned surface is bigger than that of smooth surface, and increases as increasing the hole area. A empirical friction factor model for the round-hole patterned surface can be descrived by the Moody`s friction factor formula.

Experimental Study on Static and Dynamic Characteristics of Liquid Annular Convergent-Tapered Damper Seals With Honeycomb Roughness Pattern

Honeycomb damper seals with convergent-tapered clearance have been proposed to improve static and dynamic characteristics of liquid annular seals employed in pumps. Their characteristics are experimentally investigated and compared to those for a conventional straight (no taper) annular seal with smooth surface and a straight damper seal with identical honeycomb pattern in seal stator. Three convergent-tapered honeycomb damper seals are used in the test, and have different inlet clearance (maximum clearance) and almost the same outlet clearance (minimum clearance). Their outlet clearance is almost the same as the clearance of the straight smooth seal and is slightly smaller than the clearance of the straight damper seal. Experimental results show that the convergent-tapered damper seals as well as the straight damper seal have lower leakage flow rate and cross-coupled stiffness coefficients, and larger main damping coefficients than the straight smooth seal, resulting in larger effective damping coefficients. These results are mainly due to surface roughness in the seal stator such as a honeycomb pattern used in the present analysis. The convergent-tapered damper seals also have larger main stiffness coefficients than the straight smooth and damper seals, which is mainly due to the convergent-tapered clearance and yields larger radial reaction force for a small concentric whirling motion. Consequently, the convergent-tapered damper seals have better seal characteristics than the conventional straight smooth seal and the straight damper seal with the same roughness pattern from the viewpoints of decreasing the leakage and improving the rotor stability capacity.

A Bulk Flow Model for Off-Centered Honeycomb Gas Seals

A computational analysis for prediction of the static and dynamic forced performance of gas honeycomb seals at off-centered rotor conditions follows. The bulk-flow analysis, similar to the two-control volume flow model of Kleynhans and Childs (1997, “The Acoustic Influence of Cell Depth on the Rotordynamic Characteristics of Smooth-Rotor/Honeycomb-Stator Annular Gas Seals,” ASME J. Eng. Gas Turbines Power, 119, pp. 949–957), is brought without loss of generality into a single-control volume model, thus simplifying the computational process. The formulation accommodates the honeycomb effective cell depth, and existing software for annular pressure seals and is easily upgraded for damper seal analysis. An analytical perturbation method for derivation of zeroth- and first-order flow fields renders the seal equilibrium response and frequency-dependent dynamic force impedances, respectively. Numerical predictions for a centered straight-bore honeycomb gas seal shows good agreement with experimentally identified impedances, hence validating the model and confirming the paramount influence of excitation frequency on the rotordynamic force coefficients of honeycomb seals. The effect of rotor eccentricity on the static and dynamic forced response of a smooth annular seal and a honeycomb seal is evaluated for characteristic pressure differentials and rotor speeds. Leakage for the two seal types increases slightly as the rotor eccentricity increases. Rotor off-centering has a pronounced nonlinear effect on the predicted (and experimentally verified) dynamic force coefficients for smooth seals. However, in honeycomb gas seals, even large rotor center excursions do not sensibly affect the effective local film thickness, maintaining the flow azimuthal symmetry. The current model and predictions thus increase confidence in honeycomb seal design, operating performance, and reliability in actual applications.

Hybrid Brush Pocket Damper Seals for Turbomachinery

Pocket damper seals perform a dual function: both sealing the pressurized gas around a rotating shaft and providing large amounts of vibration damping. The annular cavity between the labyrinth seal teeth is subdivided into separate annular cavities around the circumference of the rotor by partitioning walls. Also, the upstream and downstream teeth have different radial clearances to the rotor. These seals have been shown to provide a remarkable amount of direct damping to attenuate vibration in turbomachinery, but they generally leak more than conventional labyrinth seals if both seals have the same minimum clearance. Conversely, brush seals allow less than half the leakage of labyrinth seals, but published test results show no significant amount of damping. They are considered to be a primary choice for the seals in new aircraft engine designs because of their low leakage. This paper will describe a recently invented hybrid brush/pocket damper seal that combines high damping with low leakage. Previous brush seal results were studied and calculations were made to select a brush seal to combine with the pocket damper design. The result is a hybrid seal with high damping and low leakage. A special design feature can also allow active vibration control as a bonus benefit. A computer code written for the original pocket damper seal was modified to include the brush element at the exit blade. Results from the computer code indicate that the hybrid seal can have less leakage than a six bladed (or 6 knives) labyrinth seal along with orders of magnitude more damping. Experimental evaluations of the damping and leakage performance of the hybrid seal are being conducted by the authors. The experimental work reported here tested the damping capability of the new hybrid brush seal by exciting the seal journal through an impedance head. A conventional six-bladed labyrinth seal of the same working dimensions was also tested. The brush hybrid pocket damper seal is found to leak less than the labyrinth seal while producing two to three times more damping than the original pocket damper seal (orders of magnitude more than the conventional labyrinth). [S0742-4795(00)01102-9]

Dynamic Force Coefficients of a Multiple-Blade, Multiple-Pocket Gas Damper Seal: Test Results and Predictions

Experimental rotordynamic force coefficients and leakage for a four-blade, two-four pocket gas damper seal are presented and compared to predictions based on a one control volume bulk-flow model. The test rig comprises a vertical shaft and a test seal housing and flexible structure suspended from a rigid centering frame. The experiments were conducted at increasing rotor speeds to 6000 rpm and inlet/exit pressure ratios from 1.0 to 3.0. The seal force coefficients are obtained from impact response measurements of the seal and flexible structure using a frequency domain parameter identification technique. Both measurements and predictions show the seal direct stiffness and damping coefficients are proportional to the inlet/exit pressure ratio and insensitive to rotor speed. The agreement between experimental results and analytical predictions is acceptable. Predicted cross-coupled stiffness coefficients are of small amplitude. However, the test results evidence cross-coupled stiffnesses without journal rotation due to a structural asymmetry induced by the external pressurization into the seal. [S0742-4787(00)04201-6]