Full Annulus Research Articles

The Influence of Inlet Asymmetry on Steam Turbine Exhaust Hood Flows.

It has been widely recognized for some decades that it is essential to accurately represent the strong coupling between the last stage blades (LSB) and the diffuser inlet, in order to correctly capture the flow through the exhaust hoods of steam turbine low pressure cylinders. This applies to any form of simulation of the flow, i.e., numerical or experimental. The exhaust hood flow structure is highly three-dimensional and appropriate coupling will enable the important influence of this asymmetry to be transferred to the rotor. This, however, presents challenges as the calculation size grows rapidly when the full annulus is calculated. The size of the simulation means researchers are constantly searching for methods to reduce the computational effort without compromising solution accuracy. However, this can result in excessive computational demands in numerical simulations. Unsteady full-annulus CFD calculation will remain infeasible for routine design calculations for the foreseeable future. More computationally efficient methods for coupling the unsteady rotor flow to the hood flow are required that bring computational expense within realizable limits while still maintaining sufficient accuracy for meaningful design calculations. Research activity in this area is focused on developing new methods and techniques to improve accuracy and reduce computational expense. A novel approach for coupling the turbine last stage to the exhaust hood employing the nonlinear harmonic (NLH) method is presented in this paper. The generic, IP free, exhaust hood and last stage blade geometries from Burton et al. (2012. "A Generic Low Pressure Exhaust Diffuser for Steam Turbine Research,"Proceedings of the ASME Turbo Expo, Copenhagen, Denmark, Paper No. GT2012-68485) that are representative of modern designs, are used to demonstrate the effectiveness of the method. This is achieved by comparing results obtained with the NLH to those obtained with a more conventional mixing-plane approach. The results show that the circumferential asymmetry can be successfully transferred in both directions between the exhaust hood flow and that through the LSB, by using the NLH. This paper also suggests that for exhaust hoods of generous axial length, little change in Cp is observed when the circumferential asymmetry is captured. However, the predicted flow structure is significantly different, which will influence the design and placement of the exhaust hood internal "furniture."

A Reduced-Order Meshless Energy Model for the Vibrations of Mistuned Bladed Disks—Part II: Finite Element Benchmark Comparisons

This paper draws upon the theoretical basis and applicability of the three-dimensional (3-D) reduced-order spectral-based “meshless” energy technology presented in a companion paper (McGee et al., 2013, “A Reduced-Order Meshless Energy Model for the Vibrations of Mistuned Bladed Disks—Part I: Theoretical Basis,” ASME J. Turbomach., to be published) to predict free and forced responses of bladed disks comprised of randomly mistuned blades integrally attached to a flexible disk. The 3-D reduced-order spectral-based model employed is an alternative choice in the computational modeling landscape of bladed disks, such as conventionally-used finite element methods and component mode synthesis techniques, and even emerging element-free Hamiltonian–Galerkin, Petrov–Galerkin, boundary integral, and kernel-particle methods. This is because continuum-based modeling of a full disk annulus of mistuned blades is, at present, a steep task using these latter approaches for modal-type mistuning and/or rogue blade failure analysis. Hence, a considerably simplified and idealized bladed disk of 20 randomly mistuned blades mounted to a flexible disk was created and modeled not only to analyze its free and forced 3-D responses, but also to compare the predictive capability of the present reduced-order spectral-based “meshless” technology to general-purpose finite element procedures widely-used in industry practice. To benchmark future development of reduced-order technologies of turbomachinery mechanics analysts may use the present 3-D findings of the idealized 20-bladed disk as a new standard test model. Application of the 3-D reduced-order spectral-based “meshless” technology to an industry integrally-bladed rotor, having all of its blades modally mistuned, is also offered, where reasonably sufficient upper-bounds on the exact free and forced 3-D responses are predicted. These predictions expound new solutions of 3-D vibration effects of modal mistuning strength and pattern, interblade mechanical coupling, and localized modes on the free and forced response amplitudes.

Transcatheter valve used in a bailout technique during complicated open mitral valve surgery

Here, we describe the case of a 62-year old woman who required aortic and mitral valve replacement plus coronary artery bypass grafting. Transoesophageal echocardiogram revealed stenosis of the aortic valve (Ao valve area, 0.9 cm(2); PG, 45 mmHg; MG, 25 mmHg) and a diseased calcified mitral valve with stenosis and regurgitation (mitral valve area, 1.1 cm(2); MG, 10 mmHg; RV, 25 ml; ERO, 12 mm(2)). The mitral annulus calcifications were very deep into the left atrium and the left ventricle muscle, around the full annulus circumference. We decided to avoid complete deep mitral annulus decalcification. The left atrium was surgically exposed, and we deployed a 26-mm Edwards SAPIEN XT endovalve through the left atriotomy. To prevent paravalvular leakage, we then used a pericardial patch to close the gap between the endovalve and the calcified mitral annulus. The postoperative echocardiogram showed perfect anchoring of the endovalve in the mitral annulus without any paravalvular leakage.

Computational Fluid Dynamics Modeling Three-Dimensional Unsteady Turbulent Flow and Excitation Force in Partial Admission Air Turbine

Air turbines are widely used to convert kinetic energy into power output in power engineering. The unsteady performance of air turbines with partial admission not only influences the aerodynamic performance and thermodynamic efficiency of turbine but also generates strong excitation force on blades to impair the turbine safely operating. Based on three-dimensional viscous compressible Navier-stokes equations, the present study employs RNG (Renormalization group)k-εturbulence model with finite volume discretization on air turbine with partial admission. Numerical models of four different admission rates with full annulus are built and analyzed via CFD (computational fluid dynamics) modeling unsteady flows. Results indicate that the unsteady time-averaged isentropic efficiency is lower than the steady isentropic efficiency, and this difference rises as unsteady isentropic efficiency fluctuates stronger when the admission rate is reduced. The rotor axial and tangential forces with time are provided for all four admission rates. The low frequency excitation forces generated by partial admission are extraordinarily higher than the high frequency excitation forces by stator wakes.

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

Unsteady Forces of Rotor Blades in Full and Partial Admission Turbines

A numerical and experimental study of partial admission in a low reaction two-stage axial air test turbine is performed in this paper. In order to model one part load configuration, corresponding to zero flow in one of the admission arcs, the inlet was blocked at one segmental arc, at the leading edge of the first stage guide vanes. Due to the unsymmetrical geometry, the full annulus of the turbine was modeled numerically. The computational domain contained the shroud and disk cavities. The full admission turbine configuration was also modeled for reference comparisons. Computed unsteady forces of the first stage rotor blades showed cyclic change both in magnitude and direction while moving around the circumference. Unsteady forces of first stage rotor blades were plotted in the frequency domain using Fourier analysis. The largest amplitudes caused by partial admission were at first and second multiples of rotational frequency due to the existence of single blockage and change in the force direction. Unsteady forces of rotating blades in a partial admission turbine could cause unexpected failures in operation; therefore, knowledge about the frequency content of the unsteady force vector and the related amplitudes is vital to the design process of partial admission turbine blades. The pressure plots showed that the nonuniformity in the static pressure field decreases considerably downstream of the second stage’s stator row, while the nonuniformity in the dynamic pressure field is still large. The numerical results between the first stage’s stator and rotor rows showed that the leakage flow leaves the blade path down into the disk cavity in the admitted sector and re-enters downstream of the blocked channel. This process compensates for the sudden pressure drop downstream of the blockage but reduces the momentum of the main flow.

Simulation of rotating stall in a whole stage of an axial compressor

Aerodynamic instabilities that naturally occur in compression systems, such as surge and rotating stall, largely reduce the life duration and performance of system components. The prediction of the compressor operating range is thus a key parameter for the design of gas turbines. This paper investigates the ability of an unsteady flow solver to simulate the rotating stall phenomenon in the full annulus of an axial compressor stage. A comparison with experimental data indicates that the simulation correctly estimates the stability limit. However the rotating stall flow patterns are different. While measurements show only one full span rotating stall cell (40 Hz), the simulation shows first a part span stall with 10 cells (790 Hz) that evolves then towards a full span stall with three cells (170 Hz). A spectral analysis based on numerical results underlines the role of rotor–stator interactions in the development of rotating stall. The effects of downstream volumes and inlet distortions are also discussed, showing the necessity to consider the whole geometry to correctly predict the rotating stall frequency.

Online compressor washing: A numerical survey of influencing parameters

Online compressor washing is an advanced method to recover power losses caused by compressor blade fouling without incurring the availability penalty of having to shut down the gas turbine engine. Liquid is sprayed into the compressor at full or near full load to wash off particulates accumulated on the compressor surfaces. In particular, the cleaning of the first stage is vital to reinstate the mass flow of the engine, and a uniform fluid distribution is desirable in order to cover the full annulus. To achieve this, washing systems are generally developed empirically. Owing to the variety of intake duct geometries and gas turbine engines, the design of washing systems is generally related to individual power plants. To illustrate the trends of the main influencing parameters, a numerical investigation has been undertaken, based on an application case of a washing system installed in a heavy-duty gas turbine. The parameters studied using computational fluid dynamics (CFD) were airflow reduction, injection location and direction, droplet mass, and injection velocity. The effectiveness of the washing system was evaluated from the fluid distribution at the compressor inlet plane. It has been shown that, depending on the spray nozzle location, different optimum droplet sizes and injection velocities are required. Consequently, the application of different nozzle types is advisable. The operating condition of the engine has a significant effect on the fluid distribution at the compressor inlet and therefore changes in engine mass flow have to be considered when deciding on a washing scheme.

Design and Test of an Ultralow Solidity Flow-Controlled Compressor Stator

A full annulus fluidic flow-controlled compressor stator ring was designed and tested in the third stage of a four-stage low-speed research compressor. The solidity of the flow-controlled stator was near unity and significantly below design practice with a commensurately high diffusion factor. The design intent was to reduce the vane count by 30% and load the stator to the point of stall at the design point, then employ flow control to restore attached boundary layers and regain design-point stage matching. The flow control applied, which maintained attached flow, was 1% of the compressor mass flow and was introduced via discrete steady jets on the suction side of the stator. The design method used steady Computational Fluid Dynamics (CFD) with the flow control jets simulated to drive stator exit angles, velocities, and blockage to match the baseline machine. The experiment verified the pretest predictions and demonstrated degraded compressor performance without flow control and restoration of the pumping characteristics of the baseline high solidity compressor when flow control was applied. An assessment of the engine cycle impact of the flow-controlled compressor shows a 2.1 point stage efficiency reduction for the increased loading. Extrapolation of the data and analysis to a high-speed compressor shows a more modest 0.5 point stage efficiency trade.

UV finite brane decay

The decay of an unstable D-brane via closed string emission and open string pair production is considered in subcritical string theory with a spacelike linear dilaton. The decay rate is given by the imaginary part of the annulus, which has ambiguities corresponding to the choices of incoming closed and open string vacua. An exact expression for the full annulus diagram is computed with a natural choice of incoming vacua. It is found that the ultraviolet divergences present in critical string theory in both of these processes are absent for any nonzero spacelike dilaton. Implications for the vexing issue of the tachyon dust are discussed.

Assessing Viscous Body Forces for Unsteady Calculations

A strategy has been developed to model the three-dimensional unsteady flows through turbomachines subject to nonaxisymmetric flow/geometrical conditions such as low order distortions with relatively long length-scale unsteadiness, by modeling the viscous effects as local source terms for a coarse computational mesh, but not calculating them directly. In general full annulus multi-row calculations are required for such flows, but currently the computational resources are devoted to resolving detailed viscous flow very close to the walls, which in some cases is not the center of concern. By avoiding resolving detailed viscous effects the model can accelerate the calculation by at least two orders of magnitude. The method has been illustrated to be able to resolve disturbances down to the blade passing frequency and give good estimates of overall unsteady blade forces due to blade row interactions. Obviously, the correct modeling of the viscous body force as source terms in the governing equations is the key for accuracy of such calculations. Different ways of constructing/approximating the viscous body force term are discussed and their adequacy in unsteady flow calculations is assessed. It is found that in general the viscous force is relatively small compared to the total blade force, even smaller the unsteady fluctuation of the viscous force and a simple drag coefficient model is quite adequate to model both time mean and dynamic viscous effects. However, for the cases when separations are present variations in the drag coefficient may become large and more detailed modeling may be required.

Turbine Tip and Shroud Heat Transfer and Loading—Part A: Parameter Effects Including Reynolds Number, Pressure Ratio, and Gas-to-Metal Temperature Ratio

Turbine tip and shroud flow and heat transfer are some of the most complex, yet important, issues in turbine design. Most of the work performed to date has been performed in linear cascades and has investigated such items as the effect of tip geometries and turbulence on tip and shroud pressure and heat transfer. There have been very few full annulus or rotating measurements in the literature. Experimental measurements have been made on a single stage high pressure turbine at the US Air Force Turbine Research Facility (TRF) to aid in the understanding of this phenomena. The TRF is a full scale, rotating rig that operates at matched flow conditions to the true turbine environment. Heat flux measurements were acquired with both Pyrex insert strip and button gages, while the pressure measurements were taken with surface-mounted Kulite® pressure transducers. This paper presents one of the first full rotating, simultaneous pressure and heat transfer measurements to be taken in the turbine tip shroud region. These measurements provide some of the details needed for accurately quantifying the true flow condition in this complex flow regime. Comparisons between the present data and the existing 2-D cascade data were made. This investigation quantified the effects of Reynolds number, inlet temperature, turbine pressure ratio and inlet flow temperature profiles. This provides a benchmark data set for validation of numerical codes.

Effect of swirl on the flow characteristics in the outer annulus of a prototype reverse-flow gas turbine combustor

Flow characteristics in the outer annulus and its effects on the flow splits through different liner holes have been established by conducting experiments on a full annulus prototype reverse-flow gas turbine combustor for different intensities of swirl imposed on the inlet flow. Measurements have been taken with a direction-sensitive pressure probe and a standard hot wire probe. It is seen that imposition of swirl improves the uniformity of outlet flow in θ-plane. It is observed that the flow split through the turn-section cooling holes and primary holes remains nearly constant up to S=0.55, while the split through the dilution holes decreases. With increase in the swirl intensity (S=0.84) the flow split through the turn-section and dilution holes increases while flow split through the primary holes reduces. The turbulence intensity level at the outlet is not affected by the imposed inlet swirl.

Feasibility Study for Performance Prediction of Circumferentially Compound-Stage Model Turbines

Aconcept for reducing the testing timeof turbine-stage families is proposedand its feasibility is investigatedusing computational  uid dynamics (CFD). A compound stage is used, whose blading around the circumference varies sectorwise, each sector containing a sequence of equal blades that are different from the blades in the neighboring sectors. The example chosen involves three blade types in the stator and rotor, realizing temporarily nine different stage conŽ gurations during a rotor revolution. The application of fast-response  ow-measuring techniques is required for such conŽ gurations because globalmeasurements of mass  ow and torque are not relevant. The CFD study in a generic subsonic research turbine stage and the subsequent performance analysis are aimed at showing that the time-dependent short-lived  owŽ elds in the compound stage are representative of  owŽ elds obtained in stages with equivalent but circumferentially uniform bladings. A two-dimensional unsteady Euler solver is used to predict the full annulus time-dependent  owŽ eld within the compoundand uniform-stage. The time-average of the unsteady results are fed into loss correlations leading to performance maps for the nine stage conŽ gurations in the compound stage. The comparison of the computed compound stage results with the uniform stage suggests that this concept for assessing the performance of subsonic turbine stages is basically feasible. This opens up the prospect for time-resolving  uid  ow measuring systems to be successfully applied in such conŽ guration for rapid stage prototyping.

Full-Annulus Simulations of Airfoil Clocking in a 1-1/2 Stage Axial Compressor

Axial compressors have inherently unsteady flow fields because of relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid (potential) interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of stators or rotors can change these interactions, leading to different unsteady forcing functions on airfoils and different compressor efficiencies. In addition, as the Mach number increases the interaction between blade rows can be intensified due to potential effects.It has been shown, both experimentally and computationally, that airfoil clocking can be used to improve the efficiency and reduce the unsteadiness in multiple-stage axial turbomachines with equal blade counts in alternate blade rows. While previous investigations have provided an improved understanding of the physics associated with airfoil clocking, more research is needed to determine if airfoil clocking is viable for use in modern gas-turbine compressors. This paper presents the results of a combined experimental/computational research effort to study the physics of airfoil clocking in a high-speed axial compressor. Computational simulations have been performed for eight different clocking positions of the stator airfoils in a 1-1/2 stage high-speed compressor. To accurately model the experimental compressor, full-annulus simulations were conducted using 34 IGV, 35 rotor and 34 stator airfoils. It is common practice to modify blade counts to reduce the computational work required to perform turbomachinery simulations, and this approximation has been made in all computational clocking studies performed to date. A simulation was also performed in the present study with 1 inlet guide vane, 1 rotor airfoil, and 1 stator airfoil to model blade rows with 34 airfoils each in order to examine the effects of this approximation. Time-averaged and unsteady data (including performance and boundary layer quantities) were examined. The predicted results indicate that simulating the full annulus gives better qualitative agreement with the experimental data, as well as more accurately modeling the interaction between adjacent blade rows.Copyright © 1999 by ASME

The Spectrum of a Hyperbolic Evolution Operator

We study the time T-map U which maps initial data ( u 0, u 1) to the solution and its derivative ( u( T), u t ( T)) of the homogeneous wave equation in a domain with time T-periodic boundary. The spectrum of U and the type of the spectrum are completely analyzed. The spectrum is the unit circle if a certain rotation number is irrational. It is a full annulus if this rotation number is rational and a weak additional assumption is satisfied.

Response pooling between chromatic and luminance systems

Two experiments were designed to examine interactions of the responses of the chromatic and luminance systems to suprathreshold stimuli. We measured simple reaction times (RT) to eight photometrically matched (1 cd/m 2) wavelengths between 448 and 658 nm. These chromatic test stimuli were incrementally presented on either a spatially coextensive 1.2 or a larger 2° steady white background. Sectors of the outer annulus region (between 1.2 and 2°) could be removed to allow systematic variation of the extent of the spatial contour between test and background fields. When the white background was spatially coincident with the test field, RTs showed trichromatic saturation-like wavelength dependence, with a maximum RT at 572 nm. As white sectors were added to the annulus (introducing spatial transients between test and background fields), RTs became less and less wavelength-dependent, and were nearly wavelength-independent when a full annulus was used. The data were analysed in terms of a two-system RT model for processing chromaticity and luminance.

An Analysis of Self-Energized Hydrostatic Shaft Seals

An analysis of a self-energized hydrostatic shaft seal operating with incompressible fluid in the laminar regime is presented. Dimensionless results are optimized for maximum stiffness. Comparisons are made between full annulus and sectored annulus recess configurations. It is shown that translational and torsional stiffnesses are critically dependent upon orifice size. A sample problem is included to demonstrate use of equations, design curves and criteria developed.

Turbine-Blade Vibration and Strength

Abstract In the design of turbines, the strength of the blades is a matter of great importance since blading is the heart of the turbine. The authors discuss the factors which affect the strength of turbine blades and show what recent progress has been made and what studies are under way toward improved blade designs. The discussion is limited to the full-admission type of blade row which is subjected to steam flow through its full annulus, the most common type of blade row. A discussion of the problems of designing partial-admission rows was given by R. P. Kroon in a paper presented before the Society in 1939.