Ahmad Vakili

Control of cavity flow by upstream mass-injection

The purpose of this study was to use mass-injection upstream of a cavity to control the shear flow across the cavity for reducing or eliminating cavity flow oscillations. Results of an experimental effort performed at a nominal Mach number of 1.8 and at unit Reynolds number of 17 x 10 6 per foot for a cavity with length-todepth (LID) ratio of 2.54 are presented. Baseline measurements were performed for various mass-injection rates for two rectangular injection distributions. Significant attenuation of cavity oscillations was observed with upstream mass-injection. This was attributed to the thickening of the cavity shear layer, which altered its instability characteristics, such that its preferred vortex roll-up frequency was shifted outside of the natural frequencies of the cavity. As a result of the experimental investigation, it was concluded that mass-injection is effective in significantly reducing or eliminating cavity oscillations. Cavitys peak oscillation amplitude was reduced from about 174 dB (1.5 psi) without mass-injection to 147 dB (0.07 psi) at the blowing coefficient rate of 0.04.

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

An Experimental Investigation of 2-D Cylinders Affecting Supersonic Cavity Flow

Experiments were performed at nominal Mach Number 1.8 to better understand the mechanism of suppression of resonant tones caused by supersonic flow over an open cavity of L/D = 4.88 and L/W = 3.67. Much attention has been paid to the rod in crossflow because of its well-known ability to shed vortices at some Mach numbers and geometric conditions. A number of researchers have attributed the effectiveness of the rod to high frequency excitation due to spanwise coherent vortex shedding. This subsequently reduces the instabilities within the shear layer and the resonant tones within the cavity. To investigate this theory, other 2-D cylindrical models, designed specifically to inhibit vortex shedding, were experimentally compared with the rod. Several models, orientations and positions were tested, all at the same height from the cavity leading edge. Pressure spectra, particle image velocimetry and schlieren photography was carried out to measure and evaluate the effect of each model. The effects of cross sectional shape and horizontal position (within 4d of cavity leading edge) were found to be minimal. Models that are known to inhibit vortex shedding, such as a rod with a long splitter plate (tail), were just as effective as the rod. As designed, a supersonic airfoil model developed the thinnest wake and did not perform as well as the rod models of equivalent thickness. The cavity leading edge and trailing edge pressure sensors did not detect a vortex shedding frequency, in any of the tested configurations. In this study, vortex shedding in the wake of the 2-D cylinders was not detected or any other frequency dominated process affecting the measured suppression levels. Results indicate that the suppression of resonant tones is linked to wake effects. Each body’s wake size and relevant trajectory above the cavity trailing edge dominate the body’s effectiveness in cavity tone suppression.

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

High Angle of Attack Aerodynamics of Excitation of the Locked Leeside Vortex

Abstract : The control of flow over a multi-element wing has been investigated. At large incidence angles, the flow becomes fully separated. The wing geometry was considered critical to management of flow energy through organization of vorticity and controlling its shedding on the upper wing surface. A flat plate wing, with the planform of a F-15 wing, was reconfigured to accommodate the locked vortex. Spanwise blowing was used to initiate and/or increase the strength of the vortical flow. It was anticipated that in this manner an excited, stable, concentrated and strong vortex would be created on the wing. Using flow visualization as well as force measurements, it was found that the presence of the locked vortex favorably changed the flow pattern over the wing. Force measurements indicate moderate improvements on most configurations. One configuration showed more significant effect; but this should be considered preliminary.

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.