Swirl Brakes Research Articles

Dynamic Characteristics of Labyrinth Seal and Rotor Stability Considering Swirl Brakes

Dynamic Characteristics of Labyrinth Seal and Rotor Stability Considering Swirl Brakes

Experimentally Measured Effectiveness of Different Shroud Swirl Brake Profiles in a Centrifugal Compressor

Abstract As centrifugal compressors are pushed to operate at higher pressures and higher power levels, destabilizing gas forces often increase the challenge of designing a stable rotordynamic system. While technical innovations like damper seals, swirl brakes, and damper bearings that help stabilize compressors are numerous, predicting the impact that these improvements will have on a specific system is somewhat of an art form. To this end, researchers are constantly improving the depth of knowledge on these features so that the impact of these improvements is well defined. In the current work, the authors experimentally measured the impact of different swirl brake/vane concepts on the flow characteristics of a centrifugal compressor shroud cavity. The eye seal configuration studied here is a tooth on rotor labyrinth eye seal. The different shroud swirl vane geometries considered consist of various castellated features, each having the intent to reduce swirl velocity in the shroud cavity prior to entering the seal. The purpose of the testing was to determine whether a significant reduction in swirl velocity entering or exiting the seal could be measured with the different antiswirl vane profiles over a conventional shroud cavity that was tested with the same setup. The metrics that determine the effectiveness of the swirl brake were based on upstream and downstream measurements of swirl velocity using pitot-probes at different depths in the shroud cavity, and measurements of seal exit angle and velocity using a traversing cobra-probe. The test data presented herein show definitively that the different swirl brake designs, including a slotted seal, a long vane, and a short vane, have a major impact on swirl velocities relative to the conventional shroud design. The most effective at reducing swirl entering/leaving the seal is the slotted seal, while the concepts employing shroud vanes were more effective at reducing swirl in the shroud cavity.

Swirl Brake Design for Improved Rotordynamic Vibration Stability Based on Computational Fluid Dynamics System Level Modeling

Abstract The accurate characterization of compressor rotordynamic coefficients during the design phase reduces the risk of subsynchronous vibration problems occurring in the field. Although rotordynamists extensively investigate discrete compressor components (such as seals and front shrouds) to tackle instability issues, integrated or system-level analysis of compressor rotordynamics is very sparse. In reality, the impeller, eye-labyrinth seal, and the front shroud heavily influence one another; and the collective dynamic behavior of the system differs from the sum of the dynamic behavior of isolated components. A computational fluid dynamics (CFD)-based approach is taken to evaluate the dynamic behavior of the system as a whole. The geometry and operating conditions in this work are based on the recent experimental study of Song et al. (2019, “Non-Axisymmetric Flows and Rotordynamic Forces in an Eccentric Shrouded Centrifugal Compressor—Part 1: Measurement,” ASME J. Eng. Gas Turbines Power, 141(11), p. 111014. 10.1115/1.4044874) on centrifugal compressor. The commercial CFD code cfx 19.0 is used to resolve Reynolds-averaged Navier–Stokes equations to quantify the eye-labyrinth seal and front cavity stiffness, damping, and added mass. The entire compressor stage is modeled to uncover the coupled behavior of the components and assess the stability of the whole system instead of just discrete components. In the current work, three CFD approaches, namely quasi-steady, transient static eccentricity, and transient mesh deformation techniques are studied and benchmarked against analytical and experimental results from the literature. Having established the efficacy of the proposed approach, four types of swirl brakes are proposed and analyzed for stability. The novel swirl brakes create negative swirls at the brake cavities and stabilize both the front shroud and the eye-labyrinth seal simultaneously.

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.

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.

Suppression of Cavitation Surge in Turbopump With Inducer by Reduced-Diameter Suction Pipe With Swirl Brake

Abstract Downsizing and high power density of turbopumps is achieved by increasing their rotational speed. Cavitation often becomes a problem while the influence of cavitation will be generally relieved by employing an inducer before the impeller. For general-use turbopumps with an inducer, instability-free operation, as well as high suction performance is required in a wide flow rate range including extremely low flow rate. However, the cavitation surge phenomenon with low frequency and large amplitude is often to be a serious problem even with inducer when operated at very low flow rates. In this study, a reduced-diameter suction pipe (RSP) equipped with swirl brake (SB) was proposed for a suppression device of the inlet backflow as well as of the cavitation surge through removing the swirling velocity component. The effectiveness of this device was investigated by computational fluid dynamics (CFD) and experiments. First, several geometries of RSP with SB were examined by CFD, and it was found that the extension of inlet backflow was stopped at this device provided that the swirl brake had a sufficient radial or axial length. Then, one of the proposed RSP with SB was manufactured, and the experimental evaluation of the effectiveness of this device was conducted. It seemed that RSP with SB could well prevent the extension of inlet backflow. The cavitation surge was completely suppressed even at extremely low flow rates. As a result, the suction performance was also improved at low flow rates.

Making Better Swirl Brakes Using Computational Fluid Dynamics: Performance Enhancement From Geometry Variation

Abstract A fluid with a large swirl (circumferential) velocity entering an annular pressure seal influences the seal cross-coupled dynamic stiffness coefficients, and hence it affects system stability. Typically comprising a large number of angled vanes around the seal circumference, a swirl brake (SB) is a mechanical element installed to reduce (even reverse) the swirl velocity entering an annular seal. SB design guidelines are not readily available, and existing configurations appear to reproduce a single source. By using a computational fluid dynamics (CFD) model, the paper details a process to engineer a SB upstream of a 16-tooth labyrinth seal (LS) with tip clearance Cr = 0.203 mm. The process begins with a known nominal SB* geometry and considers variations in vane length (LV* = 3.25 mm) and width (WV* = 1.02 mm), and stagger angle (θ* = 0 deg). The vane number NV* = 72 and vane height HV* = 2.01 mm remain unchanged. The SB–LS operates with air supplied at pressure PS = 70 bar, a pressure ratio PR = exit pressure Pa/PS = 0.5, and rotor speed Ω = 10.2 krpm (surface speed ΩR = 61 m/s). Just before the SB, the preswirl velocity ratio = average circumferential velocity U/shaft surface speed (ΩR) equals α = 0.5. For the given conditions, an increase in LV allows more space for the development of vortexes between two adjacent vanes. These are significant to the dissipation of fluid kinetic energy and thus control the reduction of α. A 42% increase in vane length (LV = 4.6 mm) produces a ∼43% drop in swirl ratio at the entrance of the LS (exit of the SB), from αE = 0.23 to 0.13. Based on the SB with LV = 4.6 mm, the stagger angle θ varies from 0 deg to 50 deg. The growth in angle amplifies a vortex at ∼70% of the vane height, while it weakens a vortex at 30% of HV. For θ = 40 deg, the influence of the two vortexes on the flow produces the smallest swirl ratio at the LS entrance, αE = −0.03. For a SB with LV = 4.6 mm and θ = 40 deg, the vane width WV varies from 0.51 mm to 1.52 mm (±50% of WV*). A reduction in WV provides more space for the strengthening of the vortex between adjacent vanes. Therefore, a SB with greater spacing of vanes also reduces the inlet circumferential velocity. For WV = 0.51 mm, αE further decreases to −0.07. Besides the design condition (α = 0.5), the engineered SB having LV = 4.6 mm, θ = 40 deg, and WV = 0.51 mm effectively reduces the circumferential velocity at the LS entrance for other inlet preswirl ratios equaling α = 0 and 1.3. Rather than relying on extensive experiments, the CFD analysis proves effective to quickly engineer a best SB configuration from the quantification of performance while varying the SB geometry and inlet swirl condition.

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.

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.

Swirl brakes optimization for rotordynamic performance improvement of labyrinth seals using computational fluid dynamics method

The various lengths, clearances, numbers, and rotation angles of swirl brakes are investigated to evaluate their effects on the static and rotordynamic performance of the labyrinth seal using Computational Fluid Dynamics. With increasing number, length and decreasing clearance, the efficiency of the straight swirl brake can be improved. The negative stagger angle could reverse the forward flow. The lean angle has relatively slight effects on the swirl brake. Increasing preswirl ratios tends to increase the circumferential velocity. Compared with the no-brake labyrinth seal, the labyrinth seal with swirl brakes shows lower cross-coupled stiffness and resultant larger effective damping. The effective damping with negative stagger angles is increased about 100%–200% compared to the straight swirl brakes.

On the Effect of Clearance on the Leakage and Cavity Pressures in an Interlocking Labyrinth Seal Operating With and Without Swirl Brakes: Experiments and Predictions

Abstract Gas labyrinth seals (LSs) improve turbomachinery operational efficiency and mechanical reliability by reducing secondary leakage. As interlocking labyrinth seals (ILSs) restrict more leakage than conventional see-through LSs, attention is due to their performance. An earlier paper (San Andrés et al. 2019, “Leakage and Cavity Pressures in an Interlocking Labyrinth Gas Seal: Measurements Vs. Predictions,” ASME J. Eng. Gas Turbines Power, 141(10), p. 101007.) details the performance of a particular ILS in an ad hoc test rig via measurements of mass flow (leakage) and cavity pressures while supplied with pressurized air at ambient temperature and operating with a rotor speed to a maximum of 10 krpm (surface speed 79 m/s). The test seal comprises two teeth on the rotor and three teeth on the stator to make a four cavity seal with radial clearance Cr = 0.2 mm. The experimental and numerical leakages for the ILS produce a modified flow factor (Φ¯) that is independent of the seal operating conditions, namely, inlet pressure, discharge pressure and rotor speed. The finding leads to an orifice-like loss coefficient cd = 0.36 and an effective clearance (cd × Cr) for the test seal, thus evidencing its effectiveness in reducing leakage. To complement the former research, this paper reports measurements of the leakage and cavity pressures for the same geometry interlocking labyrinth seals configured with two other clearances Cr = 0.3 mm and 0.13 mm. For the ILS with Cr = 0.3 mm, a first configuration is without a swirl brake (baseline), the second is with a swirl brake with 0 deg teeth pitch (axial ribs), and the third configuration is with a swirl brake with teeth angled at 40 deg in the direction of shaft rotation. For tests conducted without shaft rotation and with rotor spinning at 7.5 krpm (surface speed= 59 m/s), the inlet air pressure (Pin) ranges from 0.29 MPa to 0.98 MPa, while the exit pressure (Pout) is set to pressure ratios PR = (Pout/Pin) = 0.3, 0.5, 0.8. As to the ILS with Cr = 0.13 mm, it operates with an upstream swirl brake with axial ribs (0° teeth pitch) and w/o rotor speed. The supply pressure (Pin) varies from 0.59 MPa to 1.4 MPa and PR = 0.3, 0.5. The measurements and bulk-flow model predictions show that the seal mass leakage is proportional to the inlet pressure (Pin), increases as PR decreases and is not affected by either shaft speed or the swirl brake configuration. Seal cavity static pressures drop linearly for all inlet pressures (Pin) and PR = 0.5 and above, except under a choked flow condition at PR = 0.3. Processing of the test data to consolidate the numerous leakage measurements delivers a nearly invariant flow factor Φ¯ for each seal clearance and from this follows a unique orifice-like loss coefficient cd = 0.36 for the ILS with Cr = 0.3 mm and cd = 0.33 for the ILS with Cr = 0.13 mm. This finding is remarkable as the test results obtained for the ILS with Cr = 0.2 mm also deliver a similar loss coefficient (cd = 0.36). Finally, predictions of ILS leakage and cavity pressures are within 5% of the measurements for all test conditions. The test data and predictions are of significant value to better the selection and design of gas labyrinth seals in turbomachinery.

Effects of swirl brake axial arrangement on the leakage performance and rotor stability of labyrinth seals

Swirl brakes are usually introduced at the seal entrance in industry to improve the seal stability, especially for labyrinth seals suffering low seal clearance and high inlet preswirl ratio. However, how to arrange swirl brakes at the seal entrance to obtain better seal stability retains unknown, such as the axial distance between the brake trailing edge and the first tooth. To this end, the effects of the distance between brakes and the first tooth on the leakage flow rate and seal rotordynamic coefficients were numerically investigated at typical inlet preswirl ratios of 0.45 and 0.8, and the predicted results were in comparison with the results for no-brake configuration. The obtained results disclose that the brake configuration arranged against the first tooth produces lower circumferential velocity in the downstream of the brakes as a result of the larger counter-clockwise vortex moving from the brake trailing edge to the brake leading edge when compared to the brake configuration away from the first tooth. Besides, the dropped circumferential velocity in the downstream of brakes will cause larger pressure fluctuation in the circumferential direction and thus generates larger tangential force to restrain the rotor forward whirl. On the other hand, the effective damping is further increased when the distance between brakes and the first tooth decreases to zero, that is, the crossover frequency is even disappeared at the inlet preswirl ratio of 0.45 and successfully drops from 69.9 Hz to 32.7 Hz at the preswirl ratio of 0.8.

Spiral flow induced destabilizing force analysis and its reduction with a novel helix-comb gas seal

The circumferential spiral flow is the main cause of destabilizing tangential force. To control the circumferential spiral flow, the helix-comb seal is presented in this study. With the guiding role of helical teeth, the negative circumferential spiral flow can be generated. It can significantly suppress the circumferential flow within the seal gap. A 3D computational fluid dynamics method is employed to investigate the performance of helix-comb seal. From the parameter analysis, it is found that the minimum tangential force can be obtained with the helix angle of 15 deg. Tangential force decreases with the increasing helical tooth number and helical tooth height. Considering the rubbing between the helical teeth and rotor, helical tooth height should be slightly smaller than the height of its adjacent comb tooth. Comparisons are conducted between the labyrinth seal (with and without swirl brakes) and helix-comb seal. Results reveal that the three seals have a comparable sealing performance. Considering the destabilizing force, the helix-comb seal has the lowest tangential force and the best stability. The helix-comb seal provides a novel method to control self-excited vibration.

Experimental Study of the Static and Dynamic Characteristics of a Long (L/D = 0.75) Labyrinth Annular Seal Operating Under Two-Phase (Liquid/Gas) Conditions

AbstractA two-phase annular-seal stand at the Turbomachinery Laboratory of Texas A&M University is utilized to experimentally investigate a labyrinth seal operating under two-phase flow conditions (a mixture of silicone oil and air). A long labyrinth seal (length-to-diameter ratio L/D = 0.75, diameter D = 114.729 mm, and radial clearance Cr = 0.213 mm) is tested at a supply pressure of 62 bars-g with inlet gas volume fraction GVFi ranging from 90 to 100%. Tests were conducted at three pressure ratios PR (0.3, 0.4, 0.5), three rotating speeds (5, 10, 15 krpm), six GVFi (90%, 92%, 94%, 96%, 98%, and 100%), and three inlet-preswirl inserts, namely, zero, medium, and high. Specifically, the ratio between the fluid's circumferential velocity and the shaft surface's velocity are in ranges of 0.0–0.2, 0.5–1.6, and 0.5–2.7 for the zero, medium, and high preswirls respectively. The direct dynamic stiffness KΩ is negative. As GVFi decreases (more liquid), KΩ becomes more negative for the zero preswirl. The effect of changing GVFi on KΩ for the medium and high preswirls is not as clear as for the zero preswirl. For the zero preswirl, as GVFi decreases, the cross-coupled dynamic stiffness kΩ and direct damping C damping increase. However, the effective damping Ceff values converge to almost the same positive value for higher frequencies. Hence, there is no significant effect of change in GVFi for the zero preswirl. For the high preswirl, as GVFi decreases, kΩ decreases and C increases. As GVFi decreases, Ceff becomes less negative and eventually becomes positive for frequencies higher than Ωc. This result indicates that at certain frequencies, the presence of liquid can make the labyrinth seals with high preswirl more stable. For the seal tested, a compressor running at 15 krpm and PR (ratio of seal exit pressure and seal inlet pressure) = 0.5 with the first critical speed of 7500 rpm (125 Hz) would experience an increase in stability with presence of liquid in the flow stream for the medium and high preswirls. However, for the range of GVFi considered here, if swirl brakes are used in a compressor application to reduce the preswirl, there would be no impact of liquid presence on the stability of the compressor. Concerning static measurements, leakage rate m˙ increases with decreases in GVFi but remains unchanged with increasing preswirl.

Static and Rotordynamic Analysis of a Plain Annular (Liquid) Seal in the Laminar Regime With a Swirl Brake for Three Clearances

Tests are reported for a smooth seal with radial clearances 127 μm, 254 μm, 381 μm (1×, 2×, and 3×); length 45.72 mm, diameter 101.6 mm. An insert induced upstream preswirl. Swirl brakes (SBs), comprising 36 square cuts with axial depth 5.08 mm, radial height 6.35 mm, and circumferential width 6.35 mm each. Static and rotordynamic data were produced at ω = 2, 4, 6, 8 krpm, ΔP = 2.07, 4.14, 6.21, 8.27 bar, and eccentricity ratios ε0 = e0/Cr = 0.00, 0.27, 0.53, and 0.80. ISO VG 46 oil at a range of 46–49 °C was used, netting laminar flow (total Re ≤ 650). Dynamic measurements included components of the following vectors: (a) stator–rotor relative displacements, (b) acceleration, and (c) applied dynamic force in a stationary coordinate system. SBs were effective at the 3× clearance only. With the 3× seal, the cross-coupled stiffness coefficients have the same sign (not destabilizing). However, the seal has a negative direct stiffness K that could potentially “suck” the rotor into contact with the stator wall, along with dropping the pump rotor's natural frequency, further reducing its dynamic stability. Measurements were compared to predictions from a code by Zirkelback and San Andrés. Most predictions agree well with test data. Notable exceptions are the direct and cross-coupled stiffness coefficients for the 3× clearance. Predictions showed positive direct stiffness and opposite signs for the cross-coupled stiffness coefficients.

The Effects of Fluid Preswirl and Swirl Brakes Design on the Performance of Labyrinth Seals

In noncontact annular labyrinth seals used in turbomachinery, fluid prerotation in the direction of shaft rotation effectively increases fluid velocity in the circumferential direction and generates fluid forces with potential destabilizing effects to be exerted on the rotor. Swirl brakes are typically employed to reduce the fluid prerotation at the inlet of the seal. The inlet flow separates as it follows the swirl brakes, and the ratio between tangential component of the velocity at the seal, and the velocity of the rotor surface varies consequently. Effective swirl brakes can significantly suppress the destabilizing fluid forces as it is effectively reducing the tangential velocity. The literature shows that leakage rate can also be reduced by using swirl brakes with “negative-swirl.” In this study, a labyrinth seal with inlet swirl brakes is selected from the literature and considered the baseline design. The seal performance is evaluated using ANSYS-cfx. The design of experiments (DOEs) approach is used to investigate the effects of various design variables on the seal performance. The design space consists of the swirl brake's length, width, curvature at the ends, the tilt angle, as well as the number of swirl brakes in the circumferential direction. Simple random sampling method with Euclidean distances for the design matrix is used to generate the design points. Steady-state computational fluid dynamics simulations are then performed for each design point to analyze the performance of the swirl brakes. Quadratic polynomial fitting is used to evaluate the sensitivity of the average circumferential velocity with respect to the design variables, which gives a qualitative estimation for the performance of the swirl brakes. These results assist in creating a better understanding of which design variables are critical and more effective in reduction of the destabilizing forces acting on the rotor, and thus will support the swirl brake design for annular pressure seals.

Application of Swirl Brake in Pipeline Compressor System

According to level I and level II stability analysis of the API617 (7th edition), the stability of a natural gas pipeline compressor system is studied. It is found that the rotor bearing system is unstable. Therefore, adding swirl brake on the system is to improve the damping characteristics. The stability of the unit can meet the requirement of API617 dynamic design. The research demonstrates that the stability of compressor system is not solely determined by the rotor system, but the stator system is also an important factor. In particular, the swirl brake on the diaphragm can reduce the cross coupling stiffness generated by the seal, and it is significant to improve the stability of the compressor system.

Effect of energy equation in one control-volume bulk-flow model for the prediction of labyrinth seal dynamic coefficients

Abstract The influence of sealing components on the rotordynamic stability of turbomachinery has become a key topic because the oil and gas market is increasingly demanding high rotational speeds and high efficiency. This leads the turbomachinery manufacturers to design higher flexibility ratios and to reduce the clearance of the seals. Accurate prediction of the effective damping of seals is critical to avoid instability problems; in recent years, “negative-swirl” swirl brakes have been used to reverse the circumferential direction of the inlet flow, which changes the sign of the cross-coupled stiffness coefficients and generates stabilizing forces. Experimental tests for a teeth-on-stator labyrinth seal were performed by manufacturers with positive and negative pre-swirl values to investigate the pre-swirl effect on the cross-coupled stiffness coefficient. Those results are used as a benchmark in this paper. To analyse the rotor-fluid interaction in the seals, the bulk-flow numeric approach is more time efficient than computational fluid dynamics (CFD). Although the accuracy of the coefficients prediction in bulk-flow models is satisfactory for liquid phase application, the accuracy of the results strongly depends on the operating conditions in the case of the gas phase. In this paper, the authors propose an improvement in the state-of-the-art bulk-flow model by introducing the effect of the energy equation in the zeroth-order solution to better characterize real gas properties due to the enthalpy variation along the seal cavities. The consideration of the energy equation allows for a better estimation of the coefficients in the case of a negative pre-swirl ratio, therefore, it extend the prediction fidelity over a wide range of operating conditions. The numeric results are also compared to the state-of-the-art bulk-flow model, which highlights the improvement in the model.

A Study on Rotordynamic Characteristics of Swirl Brakes for Three Types of Seals

In order to understand swirl brakes mechanisms and their influence on rotordynamic characteristics for different types of seals, a three-dimensional flow numerical simulation was presented. Three typical seals including labyrinth seal, fully partitioned damper seal and hole-pattern seal were compared under three inlet conditions of no preswirl, preswirl and preswirl with swirl brakes. FAN boundary condition was used to provide inlet preswirl. A modified identification method of effective damping was proposed. Feasibility of the swirl brakes on improving performance of damper seals was discussed. The results show that the swirl brakes influence the seal stability characteristics with whirl frequency. For the labyrinth seal the swirl brakes reverse the sign of effective damping at low frequency and improve the seal stability performance in a wide frequency range. The swirl brakes also improve the damper seals’ stability performance by increasing the low frequency effective damping and reducing their crossover frequency. Further results indicate the swirl brakes affect the rotational direction of the maximum (minimum) pressure positions and enhance the stability of the seals by reducing tangential force in each cavity.

Including Housing–Casing Fluid in a Lateral Rotordynamics Analysis on Electric Submersible Pumps

Stability and synchronous-response predictions, which were presented by Childs et al. (2014, “A Lateral Rotordynamics Primer on Electric Submersible Pumps (ESPs) for Deep Subsea Applications,” 43th International Pump Users Symposium, Texas A&M University, College Station, TX, pp. 1–18), are re-evaluated to include the effect of the fluid between the pump housing and well casing. Conclusions are made based on these new findings. The same two-line rotor–housing model is used to model the pump's rotor and its housing. The model dimensions are based on direct measurements of an ESP. The pump rotor and pump housing are only connected together at each stage by reaction forces and moments from seals and bushings. The rotor model is pinned to the housing at the rotor's ends. The housing model is pinned to ground at its ends. Synchronous response predictions are presented for: (1) relative rotor–housing motion and (2) housing velocity-response amplitudes. When handling viscosity of water, the rotor–housing model is predicted to be stable at new (centered) 1× clearances but rapidly becomes unstable with enlarged clearances (2× and 3×), primarily due to rapidly dropping rotor–housing natural frequencies. The impact of introducing effective swirl brakes for the stages' wear ring seals was investigated for a pump running at 3600 rpm. Their predicted impact on stability and synchronous response was: (1) Onset speeds of instabilities (OSIs) were elevated well above running speed and (2) synchronous response amplitudes were reduced modestly. Housing-response amplitudes varied considerably with the choice of housing-termination locations. For a pump rotor length of Lr, varying the lengths of a centered housing over 1.5 Lr, 2 Lr, and 3 Lr changes the housing's natural frequency. This natural frequency can coincide with the running speed with proper termination conditions. If the running speed and natural frequency coincide, large housing vibration amplitudes associated with resonance would exceed most vibration regulations; however, relative rotor–stator response amplitudes were a small fraction of clearances for all the cases. When handling emulsions at markedly higher viscosities, with a pump speed of 3600 rpm and new centered clearances, the predicted OSI was below 300 rpm. The OSI rapidly increased as the seals were displaced eccentrically, quickly elevating the first rotor–stator natural frequency above 1800 rpm and the OSI above 3600 rpm. With the model stabilized at 0.2 eccentricity, the synchronous relative rotor–housing amplitudes were a small fraction of seal clearances. Swirl brakes were not predicted to be effective in elevating pump OSIs for high viscosity fluids with new clearances; however, they became effective as clearances were increased. An ESP housing can contact the well casing in many possible scenarios (axial locations, contact-area length or girth, etc.). A midspan, point radial contact was examined and modeled as a stiff-spring connection from the housing to ground. For both water and oil–water emulsions, a stiff housing-to-casing contact produced major elliptical housing motion (versus circular motion without contact). However, it had a comparably minor impact on relative rotor–housing response amplitudes or rotordynamic stability.