Rotordynamic Coefficients Research Articles

Rotordynamic characteristic of water lubricated plain journal bearing under transient load

Transient startup operating parameter has a serious influence on the stability of water-lubricated bearings. In this study, CFD method was used to predict the rotordynamic coefficients of water lubricated bearings under transient loads. The stiffness and damping coefficients versus transient time for water-lubricated bearings are given. The stiffness and damping coefficients versus different transient loads for water-lubricated bearings are also given. The results show that the stiffness and damping coefficients will increase rapidly in a very short time. With the increase of impact time, both of them come to a steady value. With the increase of impact load, the direct stiffness and the direct damping coefficients of water lubricated bearing will increase. The value of cross-coupling stiffness and the cross-coupling damping coefficients will decrease.

Influence of Ambient Pressure on Measured Stiffness and Damping of Radial Gas Foil Bearings

Abstract As gas bearings have gained popularity, they have been used in a diverse range of applications. In many cases these bearings are used in various process gases at a range of pressures. Although numerous analytical tools exist to predict the behavior of gas bearings in these environments, these tools are challenging to validate. The current work describes a test loop that is capable of measuring rotordynamic force coefficients in a hermetically sealed environment. To the author's knowledge, this capability is the first of its kind. To illustrate operation of the test rig, an industrial 3-lobe gas-foil bearing was installed in the test fixture, and operated at pressures from 0.5 to 3.5 bara at speeds ranging from 25 to 65 krpm. The tests show that dynamic coefficients increase with speed and pressure; however, a slight reduction in coefficients were observed at the highest speed under ambient conditions. In addition, subambient pressure tests showed higher than ambient stiffness, but comparably low damping in comparison to the other test cases. It is believed that this is the first publication of gas foil bearing coefficients at subambient pressure.

Static and Rotordynamic Characteristics for Two Types of Novel Hole-Pattern Seals Operating in Supercritical CO2 Turbomachinery

Abstract In this paper, two novel hole-pattern seals were assessed for applications at the balance piston in a 14 MW supercritical CO2 turbine, focusing on the improvement of the seal leakage and rotordynamic performances. These two novel hole-pattern seals were derived from the conventional straight-through hole-pattern seal (HPS) with the same sealing clearance, diameter, axial length, hole diameter and depth, including a stepped hole-pattern damper seal (SHPS) and a grooved hole-pattern damper seal (GHPS). To enhance the seal net damping capability at high inlet preswirl condition, a straight swirl brake also was designed and employed at seal entrance for each type seal. A comprehensive assessment and comparison was conducted on the conventional HPS and the present two novel hole-pattern seals (SHPS and GHPS) with a static concentric rotor. The leakage flow rates, rotordynamic force coefficients, cavity pressure, and swirl velocity developments were analyzed for three hole-pattern seal designs with/without swirl brakes at two inlet preswirl ratios (0.1, 0.5), using a transient computational fluid dynamics (CFD)-based perturbation method based on the multiple-frequency elliptical-orbit rotor whirling model and the mesh deformation technique. To take into account of real gas effect with high accuracy, a table look-up procedure based on the National Institute of Standards and Technology (NIST) database was implemented, using an in-house code, for the fluid properties of CO2 in both supercritical and subcritical conditions. Results show that the present two novel hole-pattern seals have better sealing capability, especially for the GHPS seal which leaks less by a factor of 44%. In general, the GHPS seal possesses the lowest positive effective stiffness, highest effective damping, and the lowest crossover frequency of 60–70 Hz, especially at high inlet preswirl case. From a viewpoint of the rotor stability and unbalance sensitivity analysis, the GHPS seal without entrance swirl brake is a better seal design scheme for the balance piston seal in sCO2 turbine.

Numerical Investigations on Static and Rotordynamic Characteristics for Two Types of Liquid Hole-Pattern Seals With Tilted Cavities

Abstract Two types of liquid hole-pattern seals with axially oblique (A-HPS) or circumferentially oblique (C-HPS) hole cavities are designed. To evaluate the leakage and rotordynamic characteristics of the liquid hole-pattern seals, a 3D transient perturbation method is employed, which based on the multifrequency one-dimensional rotor whirling model and the mesh deformation technique. The accuracy and reliability of the proposed numerical approach is demonstrated based on the published experimental data of the leakage and rotordynamic force coefficients for a hole-pattern seal (HPS). Seal leakage and force coefficients are presented and compared for the A-HPS (axially oblique angle α=−30degto30deg), C-HPS (circumferentially oblique angle β=−30degto30deg), and HPS (α = 0, β = 0) at various rotational speeds (n = 0.05, 2.0, 4.0, and 6.0 krpm). Results reveal that the tilted hole cavity with positive α or β can reduce the seal effective clearance and strengthen the kinetic dissipation in hole cavities, yielding less leakage by 5–10%, especially at higher rotational speeds. The tilted hole cavity with a positive oblique angle (α=30deg,β=30deg) results in a moderate growth (by ∼6% for the A-HPS, by ∼15% for C-HPS) in the effective stiffness. Furthermore, the tilted hole cavity shows a very weak influence (<4.0%) on the effective damping, particularly for higher rotational speeds and vibration frequencies. Considering the decreasing leakage and nonworse rotordynamic characteristics, a tilted hole cavity with suitable positive oblique angles (10deg–30deg) is beneficial for the liquid hole-pattern seal.

Rotordynamic Characteristics of a Novel Labyrinth Seal With Swirl Brakes at Seal Entrance and Cavity

Abstract Annular gas seals used in turbomachinery are key elements to ensure a high machine efficiency, due to their effect on the leakage flow. As the most common type of annular gas seals, labyrinth seal is widely used as end and center balance drum, interstage, shaft end, and impeller eye seals. However, labyrinth seals have been confirmed to be a major source of destabilizing forces resulting in rotordynamic instability problems. The destabilizing cross-coupling forces, the major mechanism of inducing an instability vibration, are related to swirl velocity in labyrinth seal cavity. Installing swirl brake devices at the seal entrance is efficient to improve the seal stability by suppressing the inlet preswirl velocity. The swirl velocity downstream inlet swirl brakes can be reduced obviously, and it is usually less than the linear velocity of the rotor surface; then, the swirl velocity in seal cavity increases from inlet to outlet as a result of the viscous effect of rotating surface. In this article, to improve the rotordynamic performance of the labyrinth seal, a novel labyrinth seal (design 3), which possesses inlet swirl brakes at the seal entrance and swirl-reversal brakes in the fourth cavity, was designed. The effect of the swirl-reversal brakes on the seal rotordynamic characteristics was numerically investigated at two preswirl velocities and two rotational speeds. A detailed comparison was conducted for the rotordynamic characteristics of this novel labyrinth seal with those of a no-brake configuration (design 1) and an inlet swirl brakes configuration (design 2). The transient computational fluid dynamics (CFD) method based on the multifrequency elliptical whirling orbit model was used to predict frequency-dependent rotordynamic coefficients for these three seal configurations. The accuracy and availability of the present transient numerical method were demonstrated based on the experimental data. In general, for all three seal designs, the rotational speed and the inlet preswirl velocity have a significant effect on the cross-coupling stiffness and effective damping, but a weak influence on the direct stiffness and direct damping, where the effect of the inlet preswirl velocity is more remarkable. For the low inlet preswirl velocity w = 10 m/s case, compared to the original seal design 2, the magnitude of the negative cross-coupling stiffness shows an obvious increase (by ∼314% and ∼218% for the n = 6 krpm and n = 9 krpm case, respectively) for design 3. Compared to the seal design 1 with no swirl brakes, the effective damping increases by ∼33% for design 2 and by ∼130% for design 3 at the rotational speed of 6 krpm. At the high inlet preswirl velocity of w = 106 m/s, the labyrinth seals with no swirl brake (design 1) and with only inlet swirl brake (design 2) both produce the destabilizing positive cross-coupling stiffness and a crossover frequency (175 Hz for the seal design 1, 28 Hz for the seal design 2) below which the effective damping is negative for the n = 9 krpm case; however, design 3 still possesses the stabilizing negative cross-coupling stiffness (below 240 Hz) and the positive effective damping throughout whole frequency range for the n = 9 krpm case. The present novel labyrinth seal (design 3) produces the largest effective damping, followed by the seal design 2 and then the seal Design 1, for all rotational speed and inlet preswirl cases. Therefore, the novel labyrinth seal (design 3) possesses better rotordynamic characteristics than other two seals.

Research on the inlet preswirl effect of clearance flow in canned motor reactor coolant pump

For a pressurized water reactor power plant, the reactor coolant pump (RCP) is a kernel component. And for a canned motor RCP, the rotor system's properties determines its safety. The liquid coolant inside the canned motor RCP fills clearance between the metal shields of rotor and stator, forming a lengthy clearance flow. The influence of inlet preswirl on rotordynamic coefficients of clearance flow in canned motor RCP and their effects on the rotordynamic characteristics of the pump are numerically and experimentally investigated in this work. A quasi-steady state computational fluid dynamics (CFD) method has been used to investigate the influence of inlet preswirl. A vertical experiment rig has also been established for this purpose. Rotordynamic coefficients on different inlet preswirl ratios (IR) are obtained through CFD and experiment. Results show that the cross-coupled stiffness of the clearance flow would change significantly with inlet preswirl, but other rotordynamic coefficients would not change significantly with inlet preswirl. For the case of clearance flow between the stator and rotor cans, influence of inlet preswirl is not so significant as the IR is not large enough.

The Effect of Inflow Distortion on the Rotordynamic Characteristics of a 1400-MW Reactor Coolant Pump Annular Seal

The annular seal between stator and rotor substantively acts as a bearing that affects the rotordynamic characteristic of the turbomachinery rotor system. The rotor wake turbulence in a canned motor Reactor Coolant Pump (RCP) will lead to inflow pressure distortion at the annular seal entrance, thus further affecting the seal rotordynamic characteristics and threatening the stable operation of RCP. In order to obtain the seal rotordynamic coefficients, a transient numerical method applies the mesh deformation technique to simulate the multiple-frequency elliptical rotor whirling orbit model. The transient solutions were proposed to solve the unsteady reaction forces of annular seals at five excitation frequencies for each case. The inflow pressure distortion patterns were simplified as harmonic functions, including two important influence parameters that are impeller blades number m and pressure fluctuation ratio λ. The numerical results showed that with nonuniform time-averaged pressure distribution at the entrance of the annular seal in Case 2, the inflow distortion significantly affects the seal rotordynamic coefficients, while the rotational spinning speed in Case 3 can weaken the time-averaged nonuniformity and accordingly make a dent in the influence. Increasing impeller blades number m and pressure fluctuation λ both result in a sharp diminution of the negative stiffness Keff, as well as an obvious increase in the effective damping Ceff, which will strengthen rotor misalignment and system stability. In addition, the larger impeller blades number m and higher pressure fluctuation λ will make the effective damping Ceff more independent of the whirling frequency. These results provide theoretical guidance for the operation safety of RCP.

On the dynamic performance of a novel airfoil guider seal with the controlled circumferential flow: Numerical analysis and experimental validation

The positive circumferential flow is the major cause of seal instability. This paper proposes the airfoil guider seal (AGS) with specially designed airfoil guiders in cavities. The pressure difference between the suction and pressure surfaces induced by the airfoil guiders controls the circumferential flow. To validate the novel design, the flow field and flow-induced tangential force of the AGS are numerically calculated and compared with these of labyrinth seals (with/without swirl brakes), and pocket damper seal. The aerodynamic work and rotordynamic coefficients of the AGS are experimentally identified and compared with these of the labyrinth seal (with/without swirl brakes). With the airfoil guiders, the positive circumferential flow is effectively controlled. The AGS possesses the best stability among the studied seals.

Rotordynamic Characteristics of the Straight-Through Labyrinth Seal Based on the Applicability Analysis of Leakage Models Using Bulk-Flow Method

Abstract Labyrinth seals are widely applied in the turbomachinery to control the leakage flow through the clearance between stationary and rotating components. The fluid excitation induced by the labyrinth seal would deteriorate the stability of turbomachinery shaft. Developing an accurate and rapid prediction approach is crucial to the analysis of the fluid excitation rotordynamics of labyrinth seals. The objective of this study is to analyze the applicability of leakage models using Bulk-Flow method and investigate the factors affecting the rotordynamic characteristics of the labyrinth seal. An elliptical orbit for rotor whirling was assumed in the one-control-volume Bulk-Flow model considering an isentropic process to predict the frequency-dependent rotordynamic coefficients of the labyrinth seal. The optimal leakage model was determined by comprehensively analyzing the applicability of seventy-two leakage models. Employing the optimal leakage model in the Bulk-Flow method, the effects of sealing clearance, pressure ratio, preswirl ratio and rotational speed on the rotordynamic characteristics of the labyrinth seal were investigated. The conclusions show that the Bulk-Flow method has an average prediction error of around 10% for the leakage flow rate, cross-coupled stiffness and direct damping when equipped with the optimal leakage model. Increasing preswirl ratio has a significantly destabilizing effect on the rotor stability, while the influence of increasing rotational speed is strongly related to preswirl direction. The effective damping of the labyrinth seal is sensitive to the inlet pressure, but insensitive to the outlet pressure and sealing clearance. The crossover frequency is almost impervious to the inlet pressure, outlet pressure and sealing clearance.

An Algebraic Approach for Identification of Rotordynamic Parameters in Bearings with Linearized Force Coefficients

In this work, a novel methodology for the identification of stiffness and damping rotordynamic coefficients in a rotor-bearing system is proposed. The mathematical model for the identification process is based on the algebraic identification technique applied to a finite element (FE) model of a rotor-bearing system with multiple degree-of-freedom (DOF). This model considers the effects of rotational inertia, gyroscopic moments, shear deformations, external damping and linear forces attributable to stiffness and damping parameters of the supports. The proposed identifier only requires the system’s vibration response as input data. The performance of the proposed identifier is evaluated and analyzed for both schemes, constant and variable rotational speed of the rotor-bearing system, and numerical results are obtained. In the presented results, it can be observed that the proposed identifier accurately determines the stiffness and damping parameters of the bearings in less than 0.06 s. Moreover, the identification procedure rapidly converges to the estimated values in both tested conditions, constant and variable rotational speed.

Effects of Increased Tooth Clearance on the Performance of a Labyrinth Seal With Oil-Rich Bubbly Laminar Flow

Abstract In recent years, multiphase pumps have become more and more popular because of the capability to simplify the process, reduce the footprint, and lower the cost. To compensate for the axial thrust force, an annular seal is normally used as a balance piston seal, and the labyrinth seal is one of the choices. A typical labyrinth seal consists of a surface with teeth and a smooth surface. The teeth are either on the rotor or the stator. To protect the machine, one side (either the teeth or the smooth surface) is made of a material that can be safely sacrificed during a rub. After the rub, the teeth clearance is increased. This paper studies the impact of the increased teeth clearance on the performance of the labyrinth seal under oil-rich bubbly flow conditions. The test fluid is a mixture of silicone oil (PSF-5cSt) and air with inlet gas volume fraction (GVF) up to 9%. Tests are conducted with pressure drop (PD) = 34.5 bars, rotor speed ω = 5 krpm, and radial tooth clearance Cr = 0.102 mm and 0.178 mm. Test results show that, for all test conditions (before and after injecting air bubbles into the oil flow), increasing Cr from 0.102 mm to 0.178 mm increases the mass flow rate by about 40% but barely changes the test seal's rotordynamic coefficients, i.e., the increased tooth clearance would not change the pump vibration performance.

An Accelerating Sweep Frequency Excitation Method for the Rotordynamic Coefficients Identification of Annular Gas Seals Based on Computational Fluid Dynamics

Abstract This article proposes a rotordynamic identification method using the accelerating sweep frequency excitation method. The computational fluid dynamics transient solution combined with the moving grid method is utilized to obtain the transient flow field of the seal excited by the whirling rotor with an accelerating frequency. Rotordynamic coefficients at swept frequencies are obtained by analyzing the transient response force acting on the rotor. Rotordynamic coefficients of three published experimental seals including a labyrinth seal, a fully partitioned pocket damper seal and a honeycomb seal are identified to validate the proposed method. The results show that the predicted rotordynamic coefficients are all well agreement with the trend of experimental data. Compared with the existing numerical models based on transient solutions, the central processing unit consumption of the proposed method is substantially reduced by 98% when achieving the same frequency resolution. In addition, the impact of the exciting acceleration on the identification accuracy is also illustrated.

Measurement and simulation of the dynamic characteristics of plain and profiled annular seals

The performance and reliability of turbomachinery is often limited by shaft vibrations induced by fluid forces of (i) plain or profiled annular seals, (ii) pistons and (iii) journal bearings. The rotordynamic coefficients namely, stiffness, damping and inertia of plain annular seals are in general calculated using the bulk-flow approach developed by Childs. However, when applying the calculation method to profiled gaps, especially those with non-symmetrical profiles, large differences between simulation and experimental data are reported. Furthermore, reliable experimental data as well as large databases including experimental and simulation results from a single author are limited. To overcome this, the paper introduces a unique active magnetic bearing test rig, built and operated at the Technische Universität Darmstadt as well as the corresponding simulation method to determine the rotordynamic influence of annular seals. An extensive experimental study is carried out investigating plain, symmetrically profiled and non-symmetrically profiled annular seals within the relevant parameter range for turbulent flow in pumps. The results are compared to the simulation results showing a significant influence of profiled gaps on the dynamic characteristics in comparison to plain annular seals. Furthermore, it is shown that symmetric and non-symmetric profiles have a different influence on the dynamic quantities inertia, damping and stiffness.

Orbit Decomposition Method for Rotordynamic Coefficients Identification of Annular Seals

The elliptical orbit whirl model is widely used to identify the frequency-dependent rotordynamic coefficients of annular seals. The existing solution technique of an elliptical orbit whirl model is the transient computational fluid dynamics (CFD) method. Its computational time is very long. For rapid computation, this paper proposes the orbit decomposition method. The elliptical whirl orbit is decomposed into the forward and backward circular whirl orbits. Under small perturbation circumstances, the fluid-induced forces of the elliptical orbit model can be obtained by the linear superposition of the fluid-induced forces arising from the two decomposed circular orbit models. Due to that the fluid-induced forces of circular orbit, the model can be calculated with the steady CFD method, and the transient computations can be replaced with steady ones when calculating the elliptical orbit whirl model. The computational time is significantly reduced. To validate the present method, its rotordynamic results are compared with those of the transient CFD method and experimental data. Comparisons show that the present method can accurately calculate the rotordynamic coefficients. Elliptical orbit parameter analysis reveals that the present method is valid when the whirl amplitude is less than 20% of seal clearance. The effect of ellipticity on rotordynamic coefficients can be ignored.

Numerical calculation of the rotordynamic coefficients of a LOX-Turbopump Seal for the LUMEN LOX/LNG demonstrator rocket engine

In rocket engines seals are one of the key elements for a successful turbopump operation. They are not only responsible for reducing the leakage but they also have a huge influence on the rotordynamic behaviour. Depending on the design, the seal on a turbopump impeller can damp or excite rotordynamic instabilities. In this work we present transient 3D CFD simulations of straight gap seals, which are used to determine the rotordynamic coefficients. After that the numerical simulation is extended to characterize the influence of the impeller to housing clearance and swirl brakes on the rotor dynamic coefficients. We show that the inertia term cannot be neglected for the investigated nominal rotational speed of the rotor and high absolute whirl frequency ratios especially for the rotor housing clearance. Finally the coefficients calculated by numerical simulation are used in the equations of motion for a floating ring seal and the equations are solved numerically to do a sensitivity study of a floating ring to variations in axial force, rotor displacement and floating ring mass.

Influence of Manufacturing Variation on the Dynamic and Tribological Performance of Tilting Pad Journal Bearings

Abstract A robust approach to investigate a tilting pad journal bearing (TPJB) as a whole tolerance stack-up assembly is presented. Normal component variation within actual design tolerances is considered. The vector loop is expanded via Taylor series for sensitive analysis. The bearing shell and tilting pad machined radiuses for each pad are found to be the more influential dimensional characteristics on the assembled clearance and preload. A leading edge relief (LER) was used to avoid unloaded pads fluttering, while maintaining a satisfactory bearing assembled clearance in the loaded pads throughout the resultant preload variation. Pivot flexibility and preload loss due to pad wear in service life were included in the preload variation assessment. Surface response multivariate multiresponse models were built for a 4-pad TPJB under load between pad (LBP) and load on pad (LOP) configurations. Desirability functions rendered the maximum and minimum rotordynamic coefficient and tribological parameter responses across speed. The LOP configuration showed more variation in the direct rotordynamic coefficients, while the LBP configuration indicated more sensitive cross-coupled coefficients with very strong sign change in some cases. Among the tribological performance parameters, the eccentricity and pad maximum pressure were more affected, followed by the minimum film thickness, and weakly by the power loss, and oil film temperature. The dispersion of the tribology parameters under normal manufacturing variation is found of importance. Four, and seven extreme geometrical state cases were identified for the LBP, and LOP bearing configurations, respectively.

Test Results for the Static and Rotordynamic Characteristics of a Long (L/D = 0.75) Smooth Seal in Two-Phase (Mainly Gas) Conditions With a 62-Bar Inlet Pressure

Abstract Recent multiphase-pump developments encountered several rotordynamic issues with smooth balance-piston seals, creating a need to better understand the performance of annular seals under multiphase-flow operation. This paper presents measurements of static and dynamic characteristics of a long smooth seal (L/D = 0.75, D = 114.686 mm, and Cr = 0.200 mm) operating under pure- and mainly air condition in which air is mixed with silicone oil (PSF-5cSt). Tests are performed at a supply pressure of 62.1 bars-g, three rotation speeds (5, 10, and 15 krpm), three pressure ratios (PRs) (0.6, 0.5, 0.4), for a range of inlet liquid volume fraction (LVFi) from 0% to 8%. The results are then compared to: (1) the previous test reported by Zhang et al. (2017, “Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal with Two-Phase, Mainly-air Mixtures,” ASME J. Eng. Gas Turbines Power, 139(12), p. 122504) with similar testing condition but a different seal geometry (L/D = 0.65, D = 89.306 mm, and Cr = 0.188 mm) and (2) the predictions from a bulk-flow model developed by San Andrés (2012, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503). Results show a significant increase of direct dynamic stiffness KΩ as LVFi increases, especially at low PR. Test results reported by Zhang et al. (2017, “Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal with Two-Phase, Mainly-air Mixtures,” ASME J. Eng. Gas Turbines Power, 139(12), p. 122504) has an opposite tendency of KΩ as an impact of increasing LVFi values. Concerning cross-coupled dynamic stiffness kΩ and cross-coupled damping c, the results from Zhang et al. (2017) and the present results agree to the effects of changing speed, PR, and LVFi under pure- and mainly air conditions. As LVFi increases, direct damping C increases while test results reported by Zhang et al. (2017, “Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal with Two-Phase, Mainly-air Mixtures,” ASME J. Eng. Gas Turbines Power, 139(12), p. 122504) showed no significant increase. A steady-state computational fluid dynamics (CFD) result for a testing case suggests a significant difference happening inside the two seals (present seal and Zhang et al.'s (2017, “Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal with Two-Phase, Mainly-air Mixtures,” ASME J. Eng. Gas Turbines Power, 139(12), p. 122504) seal). A full investigation in CFD is needed. Except for the direct dynamic stiffness and the impact of changing LVFi on the cross-coupled dynamic stiffness, the bulk-flow model of San Andrés (2012, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) predicts decently the tendencies and magnitudes of the rotordynamic coefficients.

Rotordynamic force estimation of turbulent, annular seals using OpenFOAM®

3D CFD simulations are performed to resolve the leakage rates, axial pressure drops, and dynamic force coefficients of smooth annular seals. Simulations are performed for several, simple annular seal geometries subject to water and gas flows in order to verify and validate the open-source CFD library OpenFOAM for such applications. Rotordynamic force coefficients are determined using the whirling-rotor method. Accuracy of CFD-predicted results is assessed through direct comparison with published experimental data and bulk-flow model predictions. For the three water seals analyzed, the open-source, CFD-predicted dynamic coefficients closely agree with their experimental counterparts. The k- $$\omega $$ family of turbulence models is shown to outperform the k- $$\epsilon $$ models. For the gas seals, the open-source CFD solver is shown to be unsatisfactory in predicting damping coefficients and simulations are alternatively performed with ANSYS Fluent.

Experimental Study on the Performance of a See-Through Labyrinth Seal With Two-Phase, Mainly-Liquid Mixtures

Abstract With the increasing demand of the oil and gas industry, many pump companies are developing multiphase pumps, which can handle liquid–gas flow directly without separating the liquid from a mixed flow. The see-through labyrinth seal is one of the popular types of noncontact annular seals that act as a balancing piston seal to reduce the axial thrust of a high-performance centrifugal pump. The see-through labyrinth seal also generates reaction forces that can significantly impact the rotordynamic performance of the pump. Multiphase pumps are expected to operate from pure-liquid to pure-gas conditions. Zhang and Childs (2019) (Zhang, M., and Childs, D., 2019, “A Study on the Leakage and Rotordynamic Performance of a Long Labyrinth Seal Under Mainly-Air Conditions,” ASME J. Eng. Gas Turbines Power, 141(12), p. 121024) conducted a comprehensive experimental study on the performance (leakage and rotordynamic coefficients) of a see-through labyrinth seal under mainly gas conditions. This paper continues Zhang and Childs (2019) (Zhang, M., and Childs, D., 2019, “A Study on the Leakage and Rotordynamic Performance of a Long Labyrinth Seal Under Mainly-Air Conditions,” ASME J. Eng. Gas Turbines Power, 141(12), p. 121024) research and studies the performance of the see-through tooth-on-stator labyrinth seal under mainly liquid conditions. The test seal's inner diameter, length, and radial clearance are 89.256 mm, 66.68 mm, and 0.178 mm, respectively. The test fluid is a mixture of air and paper silicone oil (PSF-5cSt), and the inlet gas volume fraction (GVF) varies from zero to 12%. Tests are conducted at an exit pressure of 6.9 bars, an inlet temperature of 39.1 °C, three pressure drops (PDs) (27.6 bars, 34.5 bars, and 48.3 bars), and three rotating speeds ω (5 krpm, 10 krpm, and 15 krpm). The seal is always concentric with the rotor, and there is no intentional fluid prerotation at the seal inlet. The air presence in the oil flow significantly impacts the leakage as well as the dynamic forces of the test seal. The first air increment (increasing inlet GVF from 0% to 3%) slightly increases the leakage mass flow rate, while further air increments steadily decrease the leakage mass flow rate. For all test conditions, the leakage mass flow rate does not change as ω increases from 5 krpm to 10 krpm but decreases as ω is further increased to 15 krpm. The reduction in the leakage mass flow rate indicates that there is an increase in the friction factor, and there could be a highly possible flow regime change as ω increases from 10 krpm to 15 krpm. For ω ≤ 10 krpm, effective stiffness Keff increases as inlet GVF increases. Keff represents the test seal's total centering force on the pump rotor. The increase of Keff increases the seal's centering force and would increase the pump rotor's critical speeds. Ceff indicates the test seal's total damping force on the pump rotor. For ω ≤ 10 krpm, Ceff first decreases as inlet GVF increases from zero to 3%, and then remains unchanged as inlet GVF is further increased to 12%. For ω = 15 krpm, Keff first increases as inlet GVF increases from zero to 3% and then decreases as inlet GVF is further increased. As inlet GVF increases, Ceff steadily decreases for ω = 15 krpm.

Leakage and rotordynamic characteristics of labyrinth seal and hole-pattern damping seal with special-shaped 3D cavity

Purpose The purpose of this paper is to enhance sealing and rotordynamic performance of hole-pattern damping seal (HPDS) and labyrinth seal (LS) by structural innovation and geometrical optimization of special-shaped hole or annular-groove cavity. Design/methodology/approach The unsteady flow was transformed into steady one using moving reference frame method. The full period numerical models of LS and HPDS were established. The influence of special-shaped hole or annular-groove cavity at axial inclined angle on leakage rate and rotordynamic coefficient of these two seals at different whirl angular speed were investigated. Findings The results show that dynamic characteristics of straight-tooth LS are better than that of slanted-tooth LS. Compared to typical straight-hole damping seal, HPDS with windward oblique-hole when axial inclined angle ranges from 50 to 60° has superiority in both leakage and rotordynamic characteristics by considering smaller cross-coupled stiffness coefficient and whirl frequency ratio, larger direct damping coefficient and effective damping coefficient. Originality/value A novel HPDS with special-shaped three-dimensional hole cavity was proposed to enhance leakage and rotordynamic performance. The optimized geometrical structures of HPDS for excellent sealing and rotordynamic characteristics were obtained. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2020-0262/