Loss Reduction Mechanism Research Articles

Experimental and Numerical Investigation of Optimized Blade Tip Shapes—Part II: Tip Flow Analysis and Loss Mechanisms

The leakage flows within the gap between the tips of unshrouded rotor blades and the stationary casing of high-speed turbines are the source of significant aerodynamic losses and thermal stresses. In the pursuit for higher component performance and reliability, shaping the tip geometry offers a considerable potential to modulate the rotor tip flows and to weaken the heat transfer onto the blade and casing. Nevertheless, a critical shortage of combined experimental and numerical studies addressing the flow and loss generation mechanisms of advanced tip profiles persists in the open literature. A comprehensive study is presented in this two-part paper that investigates the influence of blade tip geometry on the aerothermodynamics of a high-speed turbine. An experimental and numerical campaign has been performed on a high-pressure turbine stage adopting three different blade tip profiles. The aerothermal performance of two optimized tip geometries (one with a full three-dimensional contoured shape and the other featuring a multicavity squealer-like tip) is compared against that of a regular squealer geometry. In the second part of this paper, we report a detailed analysis on the aerodynamics of the turbine as a function of the blade tip geometry. Reynolds-averaged Navier-Stokes (RANS) simulations, adopting the Spalart–Allmaras turbulence model and experimental boundary conditions, were run on high-density unstructured meshes using the numecafine/open solver. The simulations were validated against time-averaged and time-resolved experimental data collected in an instrumented turbine stage specifically setup for the simultaneous testing of multiple blade tips at scaled engine-representative conditions. The tip flow physics is explored to explain variations in turbine performance as a function of the tip geometry. Denton's mixing loss model is applied to the predicted tip gap aerodynamic field to identify and quantify the loss reduction mechanisms of the alternative tip designs. An advanced method based on the local triple decomposition of relative motion is used to track the location, size and intensity of the vortical flow structures arising from the interaction between the tip leakage flow and the main gas path. Ultimately, the comparison between the unconventional tip profiles and the baseline squealer tip highlights distinct aerodynamic features in the associated gap flow field. The flow analysis provides guidelines for the designer to assess the impact of specific tip design strategies on the turbine aerodynamics and rotor heat transfer.

Heat transfer and pressure loss characteristics in a swirl cooling tube with dimples on the tube inner surface

This paper presents a numerical study of heat transfer and pressure loss characteristics in a swirl cooling tube with five tangential inlet jets and with dimples on the tube inner surface. The swirl tube has a length of 20 times the tube diameter, and the Reynolds number based on the tube diameter ranges from 10,000 to 40,000. Polyhedral meshes as well as the SST k-ω turbulence model were chosen for the numerical calculations. A swirl cooling system with a smooth tube was investigated as baseline, and comparatively with dimples on the tube inner surface. The results showed different swirling flow and heat transfer patterns in the swirl tubes. Dimples on the tube inner surface can reduce the wetted-area averaged heat transfer coefficient on the tube inner surface. However, the total heat transferred in the dimpled swirl cooling tubes can be increased by up to 7.2% due to the increased heat transfer area and the interactions between the swirling flow and the dimpled wall. On the other hand, the pressure loss in the dimpled tube can be appreciably reduced by up to 17.6% compared with that of the smooth swirl tube. Detailed flow interactions between the jets, the tube wall and the dimples are illustrated to explain the heat transfer change and pressure loss reduction mechanisms.

A trusted model using improved-AODV in MANETS with packet loss reduction mechanism

A trusted model using improved-AODV in MANETS with packet loss reduction mechanism

Driving an inductive piezoelectric transducer with class E inverter

Piezoelectric transducers are conventionally driven at their resonance frequency, where they show resistive characteristics. However, the resonance frequency does not take advantage of the loss reduction mechanism, which occurs between the resonance and anti-resonance frequency. The resonance drive leads to more heat generation and lower efficiency. In this paper, an innovative driving scheme of a Langevin piezoelectric transducer under its inductive frequency range is proposed, which takes advantage of the maximum efficiency frequency between the resonance and anti-resonance. In this approach, first, a constant vibration velocity measurement system is used to find the optimum driving frequency, which is defined as the point where the real input electric power is the lowest for a given output mechanical vibration level. The transducer has an inductive behavior at the optimum frequency. Next in this approach an equivalent circuit of the transducer based on Butterworth-Van Dyke (BVD) model is established, whose parameters are used to design Class E inverter driving circuits. Using MATLAB, two Class E inverters are precisely designed to drive the transducer at the resonance frequency (resistive) and transducer at the optimum frequency (inductive), impedance converter is added when driving at the optimum frequency. The required power for the optimum frequency driving method is reduced by 39% compared with the resonance frequency driving method, and smaller increase in temperature is also revealed according to the experiments.

Effects of periodic wakes on boundary layer development on an ultra-high-lift low pressure turbine airfoil

The laminar-turbulent transition process in the boundary layer is of significant practical interest because the behavior of this boundary layer largely determines the overall efficiency of a low pressure turbine. This article presents complementary experimental and computational studies of the boundary layer development on an ultra-high-lift low pressure turbine airfoil under periodically unsteady incoming flow conditions. Particular emphasis is placed on the influence of the periodic wake on the laminar-turbulent transition process on the blade suction surface. The measurements were distinctive in that a closely spaced array of hot-film sensors allowed a very detailed examination of the suction surface boundary layer behavior. Measurements were made in a low-speed linear cascade facility at a freestream turbulence intensity level of 1.5%, a reduced frequency of 1.28, a flow coefficient of 0.70, and Reynolds numbers of 50,000 and 100,000, based on the cascade inlet velocity and the airfoil axial chord length. Experimental data were supplemented with numerical predictions from a commercially available Computational Fluid Dynamics code. The wake had a significant influence on the boundary layer of the ultra-high-lift low pressure turbine blade. Both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the ultra-high-lift low pressure turbine airfoil. The upstream unsteady wake segments convecting through the blade passage behaved as a negative jet, with the highest turbulence occurring above the suction surface around the wake center. Transition of the unsteady boundary layer on the blade suction surface was initiated by the wake turbulence. The incoming wakes promoted transition onset upstream, which led to a periodic suppression of the separation bubble. The loss reduction was a compromise between the positive effect of the separation reduction and the negative effect of the larger turbulent-wetted area after reattachment due to the earlier boundary layer transition caused by the unsteady wakes. It appeared that the successful application of ultra-high-lift low pressure turbine blades required additional loss reduction mechanisms other than “simple” wake-blade interaction.

AN ULTRA-LOW LOSS SPLIT RING RESONATOR BY SUPPRESSING THE ELECTRIC DIPOLE MOMENT APPROACH

We propose an efiective way to realize the ultra-low loss in a split ring resonator (SRR) by suppressing the electric dipole moment approach. To tremendously reduce the loss, the loss mechanism of the SRR is theoretically analyzed in detail. The nonuniform current distribution on the SRR loop results in the residual electric dipole moment and thus brings the high radiation losses. Three difierent SRR conflgurations that the lumped capacitor, the distributed capacitor and the dielectric medium are incorporated into the SRR metamaterial are conceived, by which the uniform current distribution can be observed. This leads to in a flnite bandwidth deviated from the resonance frequency where the SRR's loss performance dramatically improves owing to suppression of the residual electric dipole moment. The proposed the loss reduction mechanism has potential applications in negative and zero index metamaterials, especially at THz frequencies and in the optical regime.

역삼투 담수시스템용 에너지회수장치의 손실극복 메커니즘 설계

Novel mechanisms for Energy Recovery Devices are proposed to diminish the pressure loss in the high-pressure reverse-osmosis system. In the beginning, the state-of-the-art in the design of Energy Recovery Devices is reviewed and the features of each model are investigated. The direct-coupled axial piston pump(APP) and axial piston motor(APM) showed 39% energy recovery at operating pressure of reverse osmosis desalination systems, 60 bar. Meanwhile, the developed PM2D model, in which APM pistons are arranged parallel to those of APP, is more compact and showed higher efficiency in a preliminary test. Loss-reduction mechanisms employing rod piston and double raw valve port are additionally proposed to enhance the efficiency and durability of the device.

Experimental analysis of a new zero-voltage switching synchronous rectifier buck converter

High-efficiency converters with excellent low ripple regulated output load must feature the outstanding switching loss reduction mechanism. The rapid advancement of low-loss converter design has posed stringent challenges to designers for efficient power delivery and tight voltage regulation. In this work, the new synchronous rectifier buck converter (SRBC) circuit is proposed. It has been found that the SRBC can operate effectively at 1 MHz switching frequency with low switching and gate drive losses operating in zero-voltage switching condition.

Measurements of Secondary Losses in a Turbine Cascade With the Implementation of Nonaxisymmetric Endwall Contouring

An approach to endwall contouring has been developed with the goal of reducing secondary losses in highly loaded axial flow turbines. The present paper describes an experimental assessment of the performance of the contouring approach implemented in a low-speed linear cascade test facility. The study examines the secondary flows of a cascade composed of Pratt & Whitney PAKB airfoils. This airfoil has been used extensively in low-pressure turbine research, and the present work adds intrapassage pressure and velocity measurements to the existing database. The cascade was tested at design incidence and at an inlet Reynolds number of 126,000 based on inlet midspan velocity and axial chord. Quantitative results include seven-hole pneumatic probe pressure measurements downstream of the cascade to assess blade row losses and detailed seven-hole probe measurements within the blade passage to track the progression of flow structures. Qualitative results take the form of oil surface flow visualization on the endwall and blade suction surface. The application of endwall contouring resulted in lower secondary losses and a reduction in secondary kinetic energy associated with pitchwise flow near the endwall and spanwise flow up the suction surface within the blade passage. The mechanism of loss reduction is discussed in regard to the reduction in secondary kinetic energy.

Loss Reduction Mechanism for Coupled Cavity Waveguides in One-Dimensional Photonic Crystals

This paper presents a loss reduction mechanism for coupled cavity waveguides (CCWs) relying on 1-D photonic crystals microcavities. The mechanism is based on structural modification in the vicinity of the cavity defects. With appropriate modifications, the tangential wavenumber components of the leaky guided modes, which can couple to cladding modes, can be decreased, resulting in the reduction of propagation losses due to diffraction. We numerically demonstrate the usefulness of this mechanism through the design of low loss CCWs. The diffraction losses in the CCWs, where air holes adjacent to the defects are changed in size, decrease dramatically compared with CCWs with air holes of constant size. This loss reduction mechanism can be applied even to the CCWs with an increased number of intercavity air holes, while the group velocity can be further decreased.

Study on Secondary Flow within Low Aspect Ratio Steam Turbine Cascade (2nd Report, Flow Control Technology)

In order to develop the performance improving technology for low aspect ratio cascade of the steam turbine, mechanism of the secondary loss generation has been investigated using the smoke visualization and detailed internal flow measurement in phase I. Next in phase 2, flow control technologies such as the blade fillet, endwall convergence and negative curving are introduced. This paper presents the loss reduction mechanism of these flow control technologies by applying the computational fluid dynamics (CFD) and the test results of air model turbine to verify the effects of endwall convergence and negative curving in rotating flow field. In conclusion, the technologies greatly contribute to improving the performance of the actual steam turbine with low aspect ratio cascade.

Effects of annealing on electron-beam evaporated 193nm Al2O3/MgF2 HR mirrors

193nm Al2O3/MgF2 HR mirrors were designed and deposited by electron-beam evaporation, and then annealed in air at different temperatures from 250℃ to 400℃. The center wavelength shifted to shorter wavelength after annealing. And the reflectance of HR region increased but the transmittance changed negligibly little with the increasing of the annealing temperature. The total integrated scattering was measured to distinguish the scattering loss and the absorption loss. The results indicated that the total optical loss in the HR region was absorption dominant rather than scattering dominant. To know clearly the loss-reduction mechanism of annealing, the corresponding single-layers of Al2O3 were deposited and annealed in the same way. The change of optical constants was analyzed to explain the reduction of the total optical loss. And the reflectance of the HR mirrors reached was higher than 98%.

Reduction in the diffraction loss of an arrayed-waveguide grating by use of an interference fringe between slab and arrayed waveguides

We propose a novel method for reducing the diffraction loss in an arrayed-waveguide grating. Our method introduces an interference fringe at the interface between slab and arrayed waveguides to relax the mode mismatch between them. We achieved a loss reduction of more than 0.42 dB with this method. Moreover, we provide an explanation of the loss-reduction mechanism of our method in terms of the point of imaging property of light.

Unsteady boundary layer studies on ultra-high-lift low-pressure turbine blades

The unsteady boundary layer development on the suction surface of two ultra-high-lift low-pressure (LP) turbine blades, known as T106C and U2, was investigated to further understand the loss reduction mechanism, which is integral to optimize the blade design in unsteady flow. The T106C profile is a mid-loaded ultra-high-lift LP turbine blade. Owing to the strong local adverse pressure gradient, the laminar boundary layer on the suction surface separates shortly after the suction side peak velocity. The turbulence within the incoming wakes cannot induce transition around the separation point because of the low receptivity of the laminar boundary layer. This allows the wake's negative jet to induce roll-up vortex, which reduces the benefits of the wake-induced transition. The wake-turbulence induced transition, which occurs downstream of the separation point, helps the separated boundary layer to reattach earlier. Owing to its mid-loaded nature, a large portion of the suction surface is covered by the reattached turbulent flow, which also contributes to the high profile loss. Therefore, LP turbine blades designed to make best use of unsteady flow effects should be aft-loaded. The ultra-high-lift blade U2 is an aft-loaded profile. Furthermore, the mild local adverse pressure gradient after the suction side peak velocity allows the laminar boundary layer to further develop before separation and results in a local Reθ of about 250 at separation. Therefore, the turbulence in the passing wake is able to induce transition close to the separation point. The earlier wake-induced transition and calmed region significantly reduce the size of the separation bubble. Furthermore, the earlier occurrence of wake-turbulence induced transition prevents the wake's negative jet from generating roll-up vortex, which also leads to lower losses.

Aerodynamic Design and Testing of Three Low Solidity Steam Turbine Nozzle Cascades

Three sets of low solidity steam turbine nozzle cascades were designed and tested. The objective was to reduce cost through a reduction in parts count while maintaining or improving performance. The primary application is for steam turbine high pressure sections where Mach numbers are subsonic and high levels of unguided turning can be tolerated. The base line design A has a ratio of pitch to axial chord of 1.2. This is the pitch diameter section of a 50% reaction stage that has been verified by multistage testing on steam to have a high level of efficiency. Designs B and C have ratios of pitch to axial chord of 1.5 and 1.8, respectively. All three designs satisfy the same inlet and exit vector diagrams. Analytical surface Mach number distributions and boundary layer transition predictions are presented. Extensive cascade test measurements were carried out for a broad incidence range from −60to+35deg. At each incidence, four outlet Mach numbers were tested, ranging from 0.2 to 0.8, with the corresponding Reynolds number variation from 1.8×105 to 9.0×105. Experimental results of loss coefficient and blade surface Mach number are presented and compared for the three cascades. The experimental results have demonstrated low losses over the tested Mach number range for a wide range of incidence from −45to15deg. Designs B and C have lower profile losses than design A. The associated flow physics is interpreted using the results of wake profile, blade surface Mach number distribution, and blade surface oil flow visualization, with the emphasis placed on the loss mechanisms for different flow conditions and the loss reduction mechanism with lower solidity. The effect of the higher profile loading of the lower solidity designs on increased end wall losses induced by increased secondary flow, especially on low aspect ratio designs, is the subject of ongoing studies.

Combined Effects of Surface Trips and Unsteady Wakes on the Boundary Layer Development of an Ultra-High-Lift LP Turbine Blade

An experimental investigation of the combined effects of upstream unsteady wakes and surface trips on the boundary layer development on an ultra-high-lift low-pressure turbine blade, known as T106C, is described. Due to the large adverse pressure gradient, the incoming wakes are not strong enough to periodically suppress the large separation bubble on the smooth suction surface of the T106C blade. Therefore, the profile loss is not reduced as much as might be possible. The first part of this paper concerns the parametric study of the effect of surface trips on the profile losses to optimize the surface trip parameters. The parametric study included the effects of size, type, and location of the surface trips under unsteady flow conditions. The surface trips were straight cylindrical wires, straight rectangular steps, wavy rectangular steps, or wavy cylindrical wires. The second part studies the boundary layer development on the suction surface of the T106C linear cascade blade with and without the recommended surface trips to investigate the loss reduction mechanism. It is found that the selected surface trip does not induce transition immediately, but hastens the transition process in the separated shear layer underneath the wakes and between them. In this way, the combined effects of the surface trip and unsteady wakes further reduce the profile losses. This passive flow control method can be used over a relatively wide range of Reynolds numbers.

A new general approach to optical waveguide path design

This paper introduces a new general approach to waveguide path design. We propose an alternative approach to the usual concatenation of offset line segments and arc of circles that is geometrically less constrictive and more versatile. We also propose an adapted pure bend loss reduction mechanism that relies on a continuous widening of the waveguide together with the reduction of transition loss through curvature adaptation. The numerical results presented here show that this method can improve the loss figures and ease the burden of waveguide path design. Moreover, because of its continuous nature, the proposed approach is intrinsically less dependent on wavelength than the usual concatenation technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of chlorination on optical absorption in sol-gel-derived optical fibers

Gel chlorination was investigated to determine the effect on optical absorption in sol-gel-derived optical fibers. The chlorination effect was examined in non-doped and Ge-doped fibers. In the latter, a pronounced saturation appeared. The loss reduction mechanism and saturation phenomenon are discussed in relation to the thermodynamic equilibrium in the chlorination reaction of the impurity metal oxides, including the glass components.

Laser annealing of Corning 7059 glass waveguides

Corning 7059 glass thin film waveguides prepared by rf sputtering and vapor deposition were annealed with a CO(2) laser at various powers and scan velocities. Some reduction in optical loss was observed after laser annealing of the films. The results show a reduction in optical loss with the number of laser scans. Optical profilometer measurements indicated no change in surface roughness after laser annealing. RBS analysis of laser annealed samples show no change in elemental composition. We discuss possible optical loss reduction mechanisms and present data on the marking of thin films when annealing is done at higher CO(2) laser powers.

A novel electromagnetic field shaper for pipe-type cable systems

A novel electromagnetic field shaper designed to reduce the electrical losses and increase the current carrying capacity of pipe-type cable systems was conceived in 1983. The salient feature of this field is that, in addition to significantly reducing the eddy current losses in the pipe or cable enclosure, it also reduces the losses in the cables due to their proximity to the metallic enclosure. The field shaper effectively minimizes the ratio of the AC over the DC resistance of the cables, and consequently maximizes the ampacity of the system, at a relatively low increase in the initial cost of the system. The authors introduce the concept of the electromagnetic field shaper and present a general analysis of the loss reduction mechanisms associated with its design features and parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>