Labyrinth Teeth Research Articles

Numerical study of the characteristics of steam turbine shaft end combined seal

Abstract Effective sealing is a critical factor in ensuring the operational efficiency of steam turbines. This paper introduces a novel Spiral-Labyrinth Combined Seal (SLCS), which integrates spiral teeth with traditional labyrinth teeth to form a composite seal. By incorporating the pumping effect of spiral teeth into the Labyrinth Seal (LS) design, the leakage is significantly reduced. This study investigates the pressure drop, sealing effectiveness, and the reduction in seal length for SLCS with one to six spiral teeth under identical operating conditions and then compares these three factors with traditional LS. The results demonstrate that on the steam side, the leakage of all SLCS is lower than that of traditional LS, and the sealing performance of SLCS with three spiral teeth is the best. The pressure drop of the SLCS with three spiral teeth is 37.81% higher than that of LS at the same length, the steam leakage is 27.14% lower than that of LS, and the axial length of the R-SLCS is 9.3% shorter than that of the LS. These results validate the feasibility of the new SLCS design, providing valuable insights for optimizing steam turbine shaft-end seals.

Experimental and Numerical Investigations on the Leakage Flow Characteristics of Helical-Labyrinth-Brush Seals

Abstract The helical-labyrinth seal (HLS) can reduce rub between labyrinth teeth and rotor during the rotor vibration because the helical teeth on the stator and steps (or teeth) on the rotor are staggered in some positions. The helical-labyrinth seal with the bristle pack named as the helical-labyrinth-brush seal (HLBS) has excellent sealing performance, but the study on the leakage flow characteristics of the HLBS is not available. This paper, using computational fluid dynamic (CFD) analysis technology based on a porous medium model, investigates the leakage flow characteristics of two types of HLBSs (bristle pack installed upstream or downstream of helical-labyrinth tooth named as HLBS-U and HLBS-D, respectively) at various pressure ratios (1–1.3) and rotational speeds (0–10,000 r/min, surface speeds range from 0 to 209 m/s). The radial clearance cb between the rotor and the bristle pack ranges from 0 mm to 1.0 mm, and the radial clearance ct between the labyrinth teeth and the steps on the rotor is 1.6 mm. In parallel, the leakage flow characteristics of the HLBS-D with the constant cb of 1.0 mm are experimentally measured at the pressure ratio up to 1.3 and rotational speed up to 2000 r/min (surface speed 42 m/s). The CFD-derived leakage flow rate (represented as effective clearance) and static cavity pressure agree well with the experimental data in the whole range of test conditions. The shaft rotation eliminates the leakage hysteresis effect of the HLBS-D. Compared with the HLBS-D, the effective clearance of HLBS-U is less sensitive to rotational speed changes. The effective clearance of the HLBS-U is smaller than that of the HLBS-D in the case of cb = 0.5 mm and rotational speed n < 10,000 r/min, and the case of cb = 1.0 mm. However, for the case of cb = 0.5 mm and n = 10,000 r/min, and the case of cb ≤ 0.1 mm, the situation is opposite. The brush seal sections of the HLBS-U and the HLBS-D offer over 55% and 65% total static pressure drop in the case of cb = 1.0 mm, respectively. The brush seal sections of two HLBSs bear almost the same static pressure drop of the over 97% total static pressure drop as cb equals to 0.1 mm. What is more, the HLBS-U has lower turbulent kinetic energy upstream of the bristle pack than the HLBS-D does, which means that the intensity of the bristles flutter of the HLBS-U is lower.

Research on leakage performance and dynamic characteristics of a novel labyrinth seal with staggered helical teeth structure

A novel labyrinth seal with staggered helical teeth (SHT) structure is proposed in this paper. The influence of seal clearance, pressure drop, tooth number and eccentricity on the novel model is studied by numerical simulation. In addition, the differences of static characteristics between the labyrinth seal with SHT structure and labyrinth seal with ordinary helical teeth (OHT) structure are also compared and analyzed. The results show that the numerical model for the labyrinth teeth structure has better accuracy compared with the experimental results. The SHT structure has better leakage performance than OHT structure because the former can induce higher turbulent energy dissipation at the staggered position. Besides, the fluid-induced forces increase with the increase of eccentricity and pressure drop. The novel model and corresponding results can provide meaningful references for the research of ring seal structure.

Analysis of Hybrid Brush Seal Rotordynamic Coefficients According to Position of Brush and Clearance Using 3D CFD

The hybrid brush seal in which brush seal replaces one or two labyrinth teeth is preferred in turbomachinary to improve the efficiency and rotordynamic stability. Precise prediction method of leakage and rotordynamic coefficients is required in designing an effective hybrid brush seal. In this paper, the 3D CFD analysis method for rotordynamic coefficients is developed based on the seal dynamic analysis method using the relative coordinate system. The complex shape of the brush seal made of bristle wire pack is assumed to be a porous medium in this model. To evaluate the effect of the position of brush and clearance of brush seal on the rotordynamic coefficients, SSB and BSS type of hybrid brush seal are analyzed using FLUENT with cold clearance = 0 and 0.31mm. The rotordynamic coefficients(K, k, and C) obtained by the prposed analysis method are comparble to that of Pugachev

Effect of bristle pack position on the rotordynamic characteristics of brush-labyrinth seals at various operating conditions

Over the last few decades, the research on the effect of bristle pack position on the rotordynamic characteristics of the brush-labyrinth seals is not sufficient. To this end, two kinds of brush-labyrinth seals for the bristle pack element installed upstream of the labyrinth teeth named BSU and installed downstream of the labyrinth teeth called BSD were used to investigate the effect of bristle pack position on the rotordynamic characteristics of the brush-labyrinth seals. Using the numerical model combining the porous medium model and the whirling rotor method, the rotordynamic characteristics of the BSU and BSD at various operating conditions including four kinds of pressure ratios, five kinds of inlet preswirl speeds and four kinds of rotor spinning speeds were conducted. The obtained results show that the effects of operating conditions on rotordynamic coefficients for the different seal configurations are different. The direct stiffness, cross-coupled stiffness and direct damping of the BSU are lower than those of the BSD. The rotordynamic coefficients of the BSU are more insensitive to the operating conditions variation. From the perspective of the seal stability, the BSU is a better brush-labyrinth seal configuration at high pressure ratio, high positive preswirl or high rotor spinning speed conditions. While in the case of low pressure ratio, low positive preswirl or low rotor spinning speed conditions, the BSD is a better choice.

CFD를 활용한 브러쉬 실의 회전체동특성 해석

To improve the efficiency of steam turbines, this study examines the application of a brush seal with bristles having a minimum clearance in the rotor. To maximize the reduction of leakage, a hybrid seal was employed to replace one or two of the labyrinth teeth with bristles. In the design of the brush or hybrid seal, a dynamic coefficient prediction method was developed to evaluate vibration characteristics, in addition to the use of a leakage prediction method. In this paper, we propose a 3D computational fluid dynamics (CFD) analytical method based on dynamics analysis using the relative coordinate system, assuming that the bristle element is a porous medium and comparing the results with published experimental results. With the exception of K (direct stiffness), the dynamic characteristics of the brush seal with the bristles contacted with the rotor, k (cross coupled stiffness), C (damping), and c (cross coupled damping), were quite small and the vibration characteristics of the brush seal were relatively unaffected. Compared to the experimental results, 3D CFD analytical results of the hybrid seal revealed that K was 17.5% smaller, k was 3% larger, and C was 10% larger.

Leakage Degradation of Straight Labyrinth Seal Due to Wear of Round Tooth Tip and Acute Trapezoidal Rub-Groove

In this paper, labyrinth seal leakage is numerically quantified for an acute trapezoidal rub-groove accompanied with a rounded tooth, as a function of rub-groove sizes and tooth-groove axial positions. Analyses parameters include clearance, pressure ratio, number of teeth, and rotor speed. Labyrinth seals wear during engine transients. Radial incursion and axial movement of the rotor–stator pair cause the labyrinth teeth to rub against the unworn stator surface. The labyrinth teeth and/or stator wear depending on their material hardness. Wear damage in the form of material loss or deformation permanently increases seal clearance, and thus, leakage. This leakage is known to be dependent on the shape and geometry of the worn tooth and the stator rub groove. There are two types of reported tooth tip wear. These can be approximated as a mushroom shape and a round shape. The stator rub-groove shapes can be approximately simulated in five forms: rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this paper, the acute trapezoidal rub-groove shape is specifically chosen, since it is the most similar to the most commonly observed rub-groove form. The tooth tip is considered to be rounded, because the tooth tip wears smoothly and a round shape forms during rub-groove formation. To compare the unworn tooth, the flat stator is also analyzed as a reference case. All analyzed parameters for geometric dimensions (groove width, depth, wall angle, and tooth-groove axial position) and operating conditions (flow direction, clearance, pressure ratio, number of teeth, and rotor speed) are analyzed in their practical ranges. Computational fluid dynamics (CFD) analyses are carried out by employing a compressible turbulent flow solver in a 2D axisymmetrical coordinate system. CFD analyses show that the rounded tooth leaks more than an unworn sharp-edged tooth, due to the formation of a smooth and streamlined flow around the rounded geometry. This smooth flow yields less flow separation, flow disturbance, and less of vena contracta effect. The geometric dimensions of the acute trapezoidal rub-groove (width, depth, wall angle) significantly affect leakage. The effects of clearance, pressure ratio, number of teeth, and rotor speed on the leakage are also quantified. Analyses results are separately evaluated for each parameter.

Computational Fluid Dynamics Investigation of Labyrinth Seal Leakage Performance Depending on Mushroom-Shaped Tooth Wear

Conventional labyrinth seal applications in turbomachinery encounter a permanent teeth tip damage and wear during transitional operations. This is the dominant issue that causes unpredictable seal leakage performance degradation. Since the gap between the rotor and the stator changes depending on engine transitional operations, labyrinth teeth located on the rotor/stator wear against the stator/rotor. This wear is observed mostly in the form of the labyrinth teeth becoming a mushroom shape. It is known that, as a result of this tooth tip wear, leakage performance permanently decreases, which negatively affects the engine's overall efficiency. However, very limited information about leakage performance degradation caused by mushroom wear is available in open literature. This paper presents a study that numerically quantifies leakage values for various radii of mushroom-shaped labyrinth teeth by changing tooth-surface clearance, pressure ratio, number of teeth, and rotor speed. Analyzed parameters and their ranges are mushroom radius (R = 0–0.508 mm), clearance (cr = 0.254–2.032 mm), pressure ratio (Rp = 1.5–3.5), number of teeth (nt = 1–12), and rotor speed (n = 0–80 krpm). Computational fluid dynamics (CFD) analyses were carried out by employing compressible turbulent flow in 2D axisymmetrical coordinate system. CFD leakage results were also compared with well-known labyrinth seal semi-empirical correlations. Given a constant clearance, leakage increases with the size of the mushroom radius that forms on the tooth. This behavior is caused by less flow separation and flow disturbance, and the vena contracta effect for flow over the smoothly shaped mushroom tooth tip compared to the sharp-edged tooth tip. This leakage increase is higher when the tooth tip wear is considered as an addition to the unworn physical clearance, since the clearance dominates the leakage. The leakage affected by the number of teeth was also quantified with respect to the mushroom radius. The rotational effect was also studied as a secondary parameter.

Investigation into the flow behavior of multi-stage brush seals

Brush seals have found increasing use in turbomachinery within the past decade due to their superior sealing performance and flexible nature. The flow characteristics and fundamental behavior of single brush seal are known. However, the flow behavior of multi-stage brush seal combined with labyrinth teeth still requires further research. This article presents a detailed investigation into flow behavior of multi-stage brush seal combined with labyrinth teeth for turbine applications. The leakage rate through a two-stage brush seal combined with labyrinth teeth was measured using the rotating test rig at three operating clearances and four rotational speeds. The leakage rate through the seal was also numerically predicted using Reynolds-averaged Navier–Stokes solutions coupled with non-Darcian porous medium approach. To predict the seal leakage more accurately, the clearance reduction resulted from rotor centrifugal growth was considered in brush seal flow calculations. The effects of rotational speeds and sealing clearance on the leakage performance of the two-stage brush seal were evaluated based on the experimental and numerical results. In addition, to obtain an insight into flow mechanisms of multi-stage brush seals, the leakage rate, flow field, pressure distribution of bristle pack, and aerodynamic force acting on bristle pack of four different types of multi-stage brush seals were also analyzed and compared.

The degradation of abradable honeycomb labyrinth seal performance due to wear

This feature presents work undertaken to improve the understanding of the behaviour and performance of worn abradable honeycomb labyrinth seals. An experimental programme, backed up by CFD analysis, has investigated a range of clearances and pressure ratios plus the effect of tooth to groove location against a baseline using a flat metal stator. Significant increases in mass flow have been recorded when using a worn honeycomb stator. However, these increases can be reduced by running with the labyrinth teeth displaced with respect to the groove.

Influence of Rub-Grooves on Labyrinth Seal Leakage

An experimental investigation on the influence of stator rub-grooves on labyrinth seal leakage is presented in the present paper. In current labyrinth seal designs, abradable lands allow the rotor labyrinth teeth to rub grooves into the stator. These rub-grooves have a large influence on the seal leakage characteristic and impair the overall engine efficiency. To improve the understanding of rub-groove effects, discharge coefficients were determined using a plain nonrotating labyrinth seal model of scale 4:1 considering a wide variation of rub-groove geometries at different seal clearances. Three labyrinth seal types were covered in this investigation that are generally used in gas turbines, namely 1) straight-through labyrinth seals, 2) stepped labyrinth seals with forward facing steps, and 3) stepped labyrinth seals with backward facing steps. To attain a deeper insight into the flow mechanisms, water-channel visualizations were performed. The large data set generated in this study, provides the basis to analyze and quantify the influence of rub-grooves on the seal leakage for the three aforementioned labyrinth seal types. Current results were in agreement with previous studies on worn labyrinth seals for several seal geometries.

Computed Effect of Rub-Groove Size on Stepped Labyrinth Seal Performance

A numerical study was undertaken to explore the effects of the size of wear-in rub grooves that are typically cut into the abradable land of stepped labyrinth seals. The elliptic form of the 2-D axisymmetric Navier-Stokes equations for compressible turbulent flow were solved. The relationships among the friction coefficient, the leakage Reynolds number, the groove depth and width and the pre-rub radial clearance are examined. It was found that the standard k-, turbulence model and wall functions are effective for computing the friction coefficient and leakage for labyrinth seals with honeycomb land surfaces, both with and without the presence of rub grooves. The so-called rub grooves are the result of labyrinth teeth cutting wear grooves into the abradable surface of the land (stationary housing of the seal). Furthermore, it was found that the case of a small pre-rub tooth radial clearance, a wide rub groove and on intermediate step height is the most sensitive to the presence of a rub groove, with a leakage increase over the no-groove case of about 100 percent and 194 percent for the shallow and deep grooves, respectively. It was also found, for example, that tile leakage varied with pre-rub clearance and groove width, in order from lowest to highest leakage, as: (a)small clearance and narrow groove, (b)small clearance and wide groove, (c)large clearance and narrow groove and (d)large clearance and wide groove.

Measurement and Visualization of Leakage Effects of Rounded Teeth Tips and Rub-Grooves on Stepped Labyrinths

The effect of rounded labyrinth teeth tips and worn abradable lands has been found to give substantially increased leakage, which is well known to give reduced machine efficiency. Very little information concerning this exists, and some of the first measurements and visualization movies for stepped labyrinths are provided here to give an enhanced understanding of this phenomenon. A unique, very large-scale seal test facility was used. Glitter, and alternatively fluorescein dye, was employed as the flow tracer material. The flow visualization movies were digitally stored on the hard drive of a computer. Large decreases of leakage resistance due to the presence of worn teeth as well as rub-grooves were found. For the cases considered, the leakage resistance decrease for the large step height configurations were 85 percent, 55 percent, and 70 percent for the small, medium, and large pre-rub clearances, respectively. It was also found that the resistance varied with wear geometry, in order from highest to lowest resistance, as (a) ungrooved-unrounded-teeth, (b) ungrooved-rounded-teeth, (c) grooved-unrounded-teeth and (d) grooved-rounded-teeth. Further, a substantial tooth tip recirculation zone was visually observed only for the grooved-unrounded-teeth cases, and it was shown to be the mechanism by which the unrounded teeth give this configuration a higher resistance than do the rounded teeth.

Evaluation of Wear Ring Seals for High-Speed, High-Pressure Turbopumps

This paper describes a seal evaluation program for application as pump impeller wear rings for high-pressure, high-speed liquid rocket turbopumps. The principal design requirements were to control the leakage to values consistent with efficiency requirements and to eliminate potential explosions resulting from high velocity rubbing at the required clearances. Two approaches were taken: the first allowed rubbing of rotating labyrinth teeth on stationary, soft compatible inserts; the second utilized hydrostatic seals which maintained a fluid film between the stationary seal and the rotating wear ring.