Rotordynamic coefficients of a gas labyrinth seal are assumed to be frequency independent. However, this assumption loses its validity as rotor surface velocity approaches Mach 1. The solution procedure of 1CV model by Childs and Scharrer which assumes frequency independent force coefficients is modified to allow for calculating frequency dependent force coefficients. A comparative study of the impact of using frequency-dependent model and the original frequency-independent model on stability analysis is made. The results indicate that frequency dependency of force coefficients should be accounted for in stability analysis as rotor surface velocity approaches a significant fraction of Mach number. The bulk flow rotordynamic analysis model by Childs and Scharrer is modified to investigate the impact of leakage-flow models on predictions. A number of leakage models are incorporated in the one-control volume model, and a comparative study is made. Kinetic energy carryover factor of a leakage equation is one of the dominant factors in seal cross-force generation. A leakage equation based on a model proposed by Gamal which uses Hodkinson?s kinetic energy carryover factor is found to improve predictions of direct damping and cross-coupled stiffness. A test case is implemented to study the impact of variation of seal axial radial clearance on stability characteristics. The 1CV model by Childs and Scharrer and subsequent bulk flow models are based on the assumption of isothermal flow across the labyrinth seal. The 1CV model by Childs and Scharrer is modified to include energy equation, and the flow process is assumed to be adiabatic. However, predicted cross-coupled stiffness and direct damping coefficients using the new model do not compare well with the experimental results by Picardo as compared to the isothermal model. The impact of using real gas properties on static and rotordynamic characteristics of the seal is studied.

Rotordynamic coefficients of a gas labyrinth seal are assumed to be frequency independent. However, this assumption loses its validity as rotor surface velocity approaches Mach 1. The solution procedure of 1CV model by Childs and Scharrer which assumes frequency independent force coefficients is modified to allow for calculating frequency dependent force coefficients. A comparative study of the impact of using frequency-dependent model and the original frequency-independent model on stability analysis is made. The results indicate that frequency dependency of force coefficients should be accounted for in stability analysis as rotor surface velocity approaches a significant fraction of Mach number. The bulk flow rotordynamic analysis model by Childs and Scharrer is modified to investigate the impact of leakage-flow models on predictions. A number of leakage models are incorporated in the one-control volume model, and a comparative study is made. Kinetic energy carryover factor of a leakage equation is one of the dominant factors in seal cross-force generation. A leakage equation based on a model proposed by Gamal which uses Hodkinson?s kinetic energy carryover factor is found to improve predictions of direct damping and cross-coupled stiffness. A test case is implemented to study the impact of variation of seal axial radial clearance on stability characteristics. The 1CV model by Childs and Scharrer and subsequent bulk flow models are based on the assumption of isothermal flow across the labyrinth seal. The 1CV model by Childs and Scharrer is modified to include energy equation, and the flow process is assumed to be adiabatic. However, predicted cross-coupled stiffness and direct damping coefficients using the new model do not compare well with the experimental results by Picardo as compared to the isothermal model. The impact of using real gas properties on static and rotordynamic characteristics of the seal is studied.

Seals are basic mechanical devices commonly used in machinery to avoid undesired flow losses of working fluids. To understand the working of these seals specifically those placed between relatively moving parts is still one of the major engineering challenges for the scientific community. Particularly Annular seals are one of the most widely used in rotating machinery comprising turbines, compressors and pumps. They are mounted on the shaft that rotates within a stationary case. These seal designs make an impact on (i) machinery energy conversion efficiency and (ii) rotor dynamic stability due to the interaction between rotor and stator through fluid flow leakage. Among all annular seals straight through rectangular labyrinth seals are the most commonly used ones. Their designs have not changed much a lot since its inception by C.J. Parsons [1] back in 1901. These seals provide resistance to the fluid flow through tortuous path comprising of series of cavities and clearances. The sharp tooth converts the pressure energy to the kinetic which is dissipated through turbulence viscosity interaction in the cavity. To understand the accurate amount of leakage the flow is modeled using the discharge coefficient and for each tooth and the kinetic energy carry over coefficients. This research work is aimed at understanding the fluid flow though labyrinth seals with tooth mounted on the rotor. A matrix of fluid flow simulations has been carried out using commercially available CFD software Fluent® where all parameters effecting the flow field has been studied to understand their effect on the coefficients defining the seal losses. Also the rotor surface speed has been used varied in a step by step manner to understand the fluid flow behavior in high speed turbo-machinery. The carry over coefficient is found to be the function of all the geometric elements defining the labyrinth tooth configuration. A relation between the flow parameters and the carry over coefficient has also been established. The discharge coefficient of the first tooth has been found to be lower and varying in a different manner as compared to a tooth from a multiple cavity seal. Its dependence upon flow parameters and dimensionless geometric constants has been established. The discharge coefficient of the first teeth is found to be increasing with increasing tooth width. Further the compressibility factor has been defined to incorporate the deviation of the performance of seals with compressible fluid to that with the incompressible flow. Its dependence upon pressure ratio and shaft speed has also been established. Using all the above the mentioned relations it would be easy decide upon the tooth configuration for a given rotating machinery or understand the behavior of the seal currently in use.

A combination of 3-D and 2-D computational fluid dynamics (CFD) modeling as well as experimental testing of the labyrinth seal with hexagonal honeycomb cells on the stator wall was performed. For the 3-D and 2-D CFD models, the hexagonal honeycomb structure was modeled using the concept of the baffle (zero-thickness wall) and the simplified 2-D fin, respectively. The 3-D model showed that even a small axial change of the tooth (or honeycomb wall) location, or a small circumferential change of the honeycomb wall location significantly affected the flow patterns and leakage characteristics especially for small tooth tip clearance. Also, the local details of the flow field were investigated. The seven basic procedural steps to develop a 2-D axisymmetric honeycomb labyrinth seal leakage model were shown. Clearly demonstrated for varying test conditions was the 2-D model capability to predict the 3-D honeycomb labyrinth flow that had been measured at different operating conditions from that used in developing the 2-D model. Specifically, the 2-D model showed very close agreement with measurements. In addition, the 2-D model greatly reduced the computer resource requirement needed to obtain a solution of the 3-D honeycomb labyrinth seal leakage. The novel and advanced strategy to reduce the turbine ingress heating, and thus the coolant requirement, by injecting a “coolant isolation curtain” was developed numerically using a 3-D CFD model. The coolant isolation curtain was applied under the nozzle guide vane platform for the forward cavity of a turbine stage. Specifically, the isolation curtain serves to isolate the hot mainstream gas from the turbine outer region. The effect of the geometry change, the outer cavity axial gap clearance, the circumferential location of the injection curtain slot and the injection fluid angle on the ingress heating was investigated. Adding the chamfer to the baseline design gave a similar or higher maximum temperature T* max than did the baseline design without chamfer, but implementation of the injection curtain slot reduced substantially T* max of the outer region. In addition, a more desirable uniform adiabatic wall temperature distribution along the outer rotor and stator surfaces was observed due to the presence of the isolation curtain.

The flow field in an annular seal is simulated for synchronous circular whirl orbits with 60Hz whirl frequency and a clearance/radius ratio of 0.0154 using the Fluent Computational Fluid Dynamics (CFD) code. Fluent's Moving Reference Frame model (MRF) is used to render the flow quasi-steady by making transformations to a rotating frame. The computed flow fields for velocity, pressure and shear stress measurements are compared with the experimental data of Winslow, Thames and Cusano. The CFD predictions are found to be in good agreement with the experimental results. The present CFD methodology can be extended to other whirl frequencies and clearances. The dynamic wall pressure distributions in an annular seal for non-circular whirl orbits were obtained using CFD. The simulations were performed using a time dependant solver utilizing Fluent's Dynamic Mesh model and User Defined Functions (UDFs). The wall pressure distributions obtained from the simulations are compared with data of Cusano. The CFD simulations over predicted the pressure field when compared to experimental results however the general trends in pressure contours are similar. The flow fields for varying rotor eccentricities are also studied by performing coordinate transformations and rendering the flow quasi-steady at set eccentricities using Fluent's MRF model. The computed velocity and pressure fields are compared with the time dependant solution obtained using Fluent's Dynamic Mesh model and UDFs for the same eccentricity. Good agreement in the velocity fields is obtained; however the pressure fields require further investigation. 2D Labyrinth seal simulations were performed for comparisons with experimental LDA data from Johnson. The velocity fields match the experimental LDA data to a fair degree of extent; however, Fluent simulations under predicted the secondary recirculation zones in Labyrinth Backward Swirl (LBS) case.

The labyrinth seal is widely used in turbomachinery to minimize or control leakage between areas of different pressure. The present investigation numerically explored the effect of damage and wear of the labyrinth seal on the turbomachinery flow and temperature fields. Specifically, this work investigated: (1) the effect of rubgroove downstream wall angle on seal leakage, (2) the effect of tooth bending damage on the leakage, (3) the effect of tooth "€œmushrooming"€ damage on seal leakage, and (4) the effect of rub-groove axial position and wall angle on gas turbine ingress heating. To facilitate grid generation, an unstructured grid generator named OpenCFD was also developed. The grid generator is written in C++ and generates hybrid grids consisting primarily of Cartesian cells. This investigation of labyrinth seal damage and wear was conducted using the Reynolds averaged Navier-Stokes equations (RANS) to simulate the flows. The high- Reynolds k - Model and the standard wall function were used to model the turbulence. STAR-CD was used to solve the equations, and the grids were generated using the new code OpenCFD. It was found that the damage and wear of the labyrinth seal have a significant effect on the leakage and temperature field, as well as on the flow pattern. The leakage increases significantly faster than the operating clearance increase from the wear. Further, the specific seal configuration resulting from the damage and wear was found to be important. For example, for pure-bending cases, it was found that the bending curvature and the percentage of tooth length that is bent are important, and that the mushroom radius and tooth bending are important for the mushrooming damage cases. When an abradable labyrinth seal was applied to a very large gas turbine wheelspace cavity, it was found that the rub-groove axial position, and to a smaller degree, rub-groove wall angle, alter the magnitude and distribution of the fluid temperature.

The ability to quantify leakage flow and windage heating for labyrinth seals with honeycomb lands is critical in understanding gas turbine engine system performance and predicting its component lifes. Variety of labyrinth seal configurations (number of teeth, stepped or straight, honeycomb cell size) are in use in gas turbines, and for each configuration, there are many additional geometric factors that can impact a seal’s leakage and windage characteristics. To achieve high performance in modern gas turbine engines, the labyrinth seals operate at low clearances and high rotational speed which are generally deployed with honeycomb lands on the stator. During the transient operation of aircraft engines, the stator and rotor mechanical and thermal growths differ from one another and can often result in the rotor’s incursion into the stator honeycomb structure. The incursions create rub-grooves in the honeycomb lands that can subsequently enlarge as the engine undergoes various manoeuvres. However, the effects of different honeycomb cell size, rotation and presence of rub-groove have not been thoroughly investigated in previously published work. The objective of the present research is to numerically investigate the influence of the above three factors on seal leakage and windage heating. The present work focuses the development of a numerical methodology aimed at studying above effects. Specifically, a three-dimensional CFD model is developed utilizing commercial finite volume-based software incorporating the RNG k-ε turbulence model. Detail validation of the numerical model is performed by comparing the leakage and windage heating measurements of several rig tests. The turbulent Schmidt number is found to be an important parameter governing the leakage prediction. It depends on honeycomb cell size and clearance for honeycomb seals, and Reynolds number in the presence smooth lands. The present numerical model with the modified RNG k- turbulence model predicts seal leakage and windage heating within 3-10% with available experimental data. Using the validated numerical model, a broad parametric study is conducted by varying honeycomb cell size, radial clearance, pressure ratio and rotational speed for a four-tooth straight-through labyrinth seal with and without rub-grooves. They further indicate that presence of rub-grooves increases seal leakage and reduce windage heating, specifically at smaller clearance and for larger honeycomb cell size. Rotation significantly reduces leakage with smooth stator land and smaller honeycomb cells whereas the effect is minimal for larger (3.2mm) honeycomb cells. However, at very high rotational speed seal flow reduces in all seal configurations due to high temperature rise and Rayleigh line effects. At no rub condition and lower clearance, the larger honeycomb cells leak more flow due to high bypass flow through the honeycomb cells. This results into lower pocket swirl and higher windage. When the seal clearance increases the larger honeycomb cells offers more drag to the seal flow, therefore they leak less. At higher clearances the flow travels like a strong wall jet and isolates the pocket air from honeycomb cells. Hence, at open clearances labyrinth seals with any honeycomb cell size essentially produce the same pocket swirl and windage heating.

Windback seals work similarly to labyrinth seals except for the effect of helical groove. These seals are essentially a tooth on stator or tooth on rotor labyrinth seal where the grooves are a continuous helical cut like a thread. Windback seals are used in centrifugal gas compressor to keep oil out of the gas face seal area. These face seals cannot be contaminated by oil. A purge gas is applied to the seal to help force the oil back into the bearing area. The windback seal should be designed to prevent any oil contamination into the supply plenum and to reduce purge gas leakage. The CFD simulations have been performed with the effect of clearance, tooth width, cavity shape, shaft rotation, eccentricity, and tooth location on the seal leakage performance and the flow field inside the seal. The leakage flow rate increases with increasing the pressure differential, rotor speed, radial clearance, cavity size, and shaft diameter and with decreasing the tooth width. The eccentricity has a minimal effect for the windback seal. From oil simulations, the windback seal with 25% rotor eccentricity has some of the journal bearing action and drives back flow into the gas plenum. However the windback seal can be used to force the oil back into the bearing side before starting the compressor by applying a purge gas flow since the positive axial velocity inside the cavity is larger than the negative axial velocity. m A Rw cav & / ? is constant for varying shaft rotation since the leakage flow rate for the windback seal increases linearly as the the rotor speed increases. The leakage flow rate for the windback seal increases as the groove size increases due to the pumping action of the windback seal. A windback seal design based upon the numerical simulations that minimize gas leakage and help prevent gas face seal oil contamination was optimized. The windback seal has two leakage flow paths. Since the leakage flow rate under teeth of windback seals is the same as for a similar geometry labyrinth seal, the flow under the teeth can be predicted by two-dimensional labyrinth seal analysis. An empirical model for the leakage rate through the cavity has been developed which fits the data with a standard deviation of 0.12.

Adequate sealing in turbomachinery reduces secondary leakage and results in more efficient and stable systems. Labyrinth seals are most common, although brush seals are popular in specialized applications. The Hybrid Brush Seal (HBS) is a novel design that adds to the bristle brush matrix a number of cantilever pads that rest on the rotor surface. Upon shaft rotation the pads lift due to the generation of a hydrodynamic gas film while the brushes effectively seal an upstream pressure. Hence the HBS has no wear and no local thermal distortion effects. Measurements of leakage versus pressure differential are obtained in a three-teeth labyrinth, a conventional brush seal, and a hybrid brush seal for operation at high temperature (300ºC), with shaft surface speeds to 27 m/s, and at supply pressures to 3.5 bar. Flow measurements are presented in terms of a flow factor to remove dependency on the air temperature and supply pressure. The measurements demonstrate the HBS leaks less (~61%) than a standard brush seal and is significantly better (~38%) than a similarly sized labyrinth seal. Predictions of flow through a labyrinth seal predict well at supply pressures under 1.7 bar but overpredict by as much as 25% at high supply pressures. A porous medium fluid flow model predicts the flow through the HBS and brush seal. The model for the HBS and brush seal underpredicts the flow rate at low supply pressures but match well at high supply pressures. Measurements of the drag torque of the test seals show the HBS has a larger torque when pressurized compared to the brush seal and labyrinth seal. This indicates that the HBS experiences a larger degree of blow-down due to the pads decreasing the clearance. The mechanical parameters of the brush seal and HBS are found based upon the flexibility function from impact load tests. A combined structural and dry friction damping model represent well the measured flexibility. An equivalent damping is found based upon the energy dissipation. Based upon the damping ratio, the HBS has twice of the viscous damping as the brush seal at a supply pressure of 2.0 bar.

Annular seals are devices used in turbomachinery to avoid flow losses which reduce efficiency. The dynamic stability of the machine is also improved by the seal. Thus, it is an important subject to understand the flow behavior through the seal. Straight through triangular labyrinth seals are one of the most commonly used types of non-contacting annular seals. The energy dissipation through these seals is achieved by a series of teeth and cavities. As the flow passes above each tooth, a portion of its pressure energy is converted into kinetic energy. A portion of this kinetic energy is dissipated through turbulence-viscosity interaction in the cavity that follows. Moreover, some portion of the pressure energy is also lost through viscosity of the fluid. This research aims to understand the effects of flow parameters and seal geometry on these losses. This will make it possible to estimate the mass flow leakage through the seal. ANSYS Fluent is used to simulate the flow through the seal. The effect of seal geometry is studied by varying clearance, pitch, tooth height, tooth width and upstream side angle. It was found that, amongst other geometrical parameters, tooth clearance and pitch has a strong influence on carryover coefficient. Smaller values of c/s have better kinetic energy dissipation in the cavity. Carryover coefficient is also found to be a function of the Reynolds number and shaft speed. Discharge coefficient of the seal presents the overall efficiency while carryover coefficient only shows the cavity performance. Discharge coefficient is also found to be a strong function of tooth clearance, pitch, Reynolds number and shaft speed. Remaining parameters have smaller effects. It was observed that the discharge coefficient of first tooth is always lower than those of intermediate teeth. The compressibility effects are presented by using an expansion factor which is the ratio of compressible flow discharge coefficient to incompressible flow discharge coefficient. It was found that the expansion factor is fairly independent of geometrical parameters but a strong function of flow parameters. Considering the effects of seal geometry and flow parameters on carryover coefficient, discharge coefficient and expansion factor, the seal geometry is optimized to increase the kinetic energy dissipation and pressure head loss which in turn will reduce the mass flow leakage.