Abraham Meganathan

An Experimental and Numerical Study of Labyrinth Seal Flow

Leakage flow in 2-D constant rotor diameter stepped labyrinth seals is investigated by means of pressure and velocity field measurements and numerical simulation of 2-D and axisymmetric models. The basis of investigation is a generic stepped labyrinth seal currently used in industry in steam turbine generators. The performance of the baseline seal design was compared with new seal designs with specific features changed in order to examine their influence on leakage characteristics through such seals. Numerical modeling and experiments were performed over a range of seal pressure ratios from 1 to 10. A number of configurations were evaluated both experimentally and numerically. This paper discusses flow details associated with only one configuration as compared with the baseline. Results have been helpful in the understanding of seal flow leakage and total pressure loss mechanisms. Mechanisms of leakage reduction in labyrinth seals included turbulence induced viscous losses, chamber vortex generation, flow stagnation losses, and increased flow streamline curvature. Numerical results provided insight into the flow field details and were helpful in facilitating basic physical understandings used for improved seal designs.Copyright

Advanced Labyrinth Seals for Steam Turbine Generators

A new class of knives (C-Shaped) for reduced labyrinth seal discharge has been designed and assessed through two dimensional numerical modeling of the seal’s internal flow passages. Modeling procedures used for the analysis have been previously validated by comparison with static labyrinth seal experiments. The objectives of the new seal are to: 1) reduce flow leakage through the seal and 2) introduce structural flexibility in the knives so that design clearances could be maintained even after rub events during startup. The baseline chosen for comparative evaluation is an N2 packing used in GE steam turbines. The new seals have compliant C-shaped knives instead of the straight knives, found in an N2 packing. The best performing configuration has one tall ‘C’ shaped long knife and three ‘C’ shaped short knives in each stage. It was found that the best configuration at clearances similar to the baseline seal reduces flow leakage by 42%. Two dimensional numerical structural analyses showed that the new seal knife is more flexible than a straight knife. This is also intuitive by virtue of its geometric profile. A non-dimensional geometric parameter correlates with the degree of flexibility in the knife. These results indicate a potential for design of labyrinth seals that maintain lower design clearances throughout their life time by carefully selecting the knives’ geometric parameters and incorporating high performance composite materials. Then, the new design would result in significantly lower steam leakage.Copyright

An Experimental Study of Labyrinth Seal Flow

The leakage flow in a 2-D stationary stepped labyrinth seal is investigated by means of flow visualization, pressure field measurements, and Particle Image Velocimetry. The basis of investigation is a generic stepped labyrinth seal currently used by the industry in steam turbine generators. Geometric and flow parameters were varied in order to examine their influence on leakage through seals. Flow visualization results revealed inter-related mechanisms of energy loss in labyrinth seals to include turbulence induced viscous losses, chamber vortex generation, flow stagnation, and increased flow streamline curvature. A five times scale model was constructed and tested over a range of seal pressure ratios from 1:1 to 10:1. Model configurations included a baseline and six variants of the basic design that were conceptually devised to be superior and by varying step height and knife angle. Detail pressure and velocity measurements were carried out. Results show that with relatively minor changes in geometry, determined based on our understanding of the physics of the flow, leakage reductions of up to 17% were accomplished.Copyright

Cavity Flow Control Experiments and Simulations

In this work we describe a summary of flow control research studies on active, passive and adaptive methodologies designed to attenuate large scale flow unsteadiness and the resulting pressure fluctuations in cavity flows. Spectral analysis of high frequency dynamic pressure measurements is used to determine the control effectiveness. Various control techniques, depending on their geometry and or distribution, can be advantageous in attenuating both the peaks and the broad spectral bands generated by flow unsteadiness. Increased effectiveness is associated with redistribution of the shear-layer vorticity. Combination of experimental and numerical results assists in understanding the underlying flow physics and interaction processes involved.

Numerical modeling of an aspirated total temperature probe

Computations using a model of an aspirated total temperature probe are compared with some classical experimental data from NACA Lewis Laboratory and with a numerical solution using CFD ACE+. Convection and radiation to and from the probe surfaces, radiation from the hot gas surrounding the probe, and conduction in the probe materials are computed by the model. The model consistently predicted the recovery temperature to within about 5 degrees R (3 K) for a temperature range of 1500 to 2500 R (833 to 1389 K).

A Study of Steam Turbine Droplet Formation, Shedding and Blade Impact

Blade erosion is a serious problem in steam turbines and reduces the life of turbine, requiring periodic maintenance. A fundamental experiment is being performed to evaluate effects of certain control parameters on the droplet formation process and distribution of the shed droplets sizes. Three blade coatings include Epoxy, Teflon®, Viton®, three trailing edge geometries and a new and innovative trailing edge effect are to be experimentally studied. Measurements are to be made in a simulated stationary cascade utilizing real turbine components. The condensed liquid film and the large droplets formed on the three coated blades and one original finish blade will be optically measured. A typical cascade is modeled using Volume of Fluid with different surface properties used to model the formation and breakup of a thin water layer into droplets. Droplet distributions for the various conditions will be compared and the best surface finish and configurations will be identified. Most importantly, the reason for a successful configuration will be given, based on the measurement results.© 2008 ASME

An Experimental Study of Acoustic and Flow Characteristics of Hole Tones

Hole tones are generated when a jet passes through an aperture placed some distance downstream of a jet exit. Measurements have been made on hole tone generating geometries with axial length to jet exit diameter ratios (L c/D j) between 1 and 2.26 at Reynolds numbers ranging from 60000 to 230000. The major peak in the pressure spectra was well predicted by a simple relation characteristic of a feedback oscillator. Instantaneous and phase averaged flow fields are used to study and understand the physics of these self-sustained oscillations. Vortex rings generated near the leading edge (jet exit) impinge on the trailing edge (aperture) and generate a feedback mechanism with the axial length being the characteristic length scale.

Characteristics of Compressible Jets Passing Through Axisymmetric Cavities

Actuators capable of producing large amplitude oscillations at desired frequencies are needed in many flow control applications. In this study, turbulent jets were actuated using self-sustaining oscillations in axisymmetric cavities. Cavity length to depth ratio (Lc/Dc) was varied from 1 to 5 and data were collected at several intermediate steps. Tests were conducted at subsonic jet flow conditions, over a Reynolds number range of 40,000 to 225,000, based on jet exit diameter. Pressure signals and 2D PIV measurements from axisymmetric cavities with length to depth ratio 1.5 – 2.0 corresponding to very high amplitude oscillations are presented. Comparison with baseline jets showed that mixing characteristics of the actuated jets had improved significantly. Ensemble averaged flow structures at different phases of the oscillations indicate the mechanisms through which mixing enhancement is achieved.

LES Simulations of Three Dimensional Cavity Flow Field and Comparison with Experiments

Actuators capable of producing large amplitude oscillations at desired frequencies are needed in many flow control applications. Axisymmetric cavities are being studied as a potential source to excite natural jets. Experimental tests were conducted at subsonic jet flow conditions, over a Reynolds number range of 40,000 to 225,000, based on jet exit diameter and excited using axisymmetric cavities of length to depth ratios 1 4. 2D PIV measurements of the flow field within the cavity and the excite jet were made. Axisymmetric and 3D LES simulations have been carried out and compared with experimental results.