bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of Bayesian Belief Nets for Modeling Uncertainty in Structural Dynamics
Authors: Calanni Giorgio , Volovoi Vitali , Ruzzene Massimo , Vining Charles , Cento Peter
Published: March 2009

The quality of modeling and performance prediction for any structural system is affected and limited by an inherent presence of various sources of uncertainty. To date, uncertainty has been usually quantified by means of uncertainty propagation techniques (e.g. Monte Carlo simulations), where a statistical realization of the system’s input parameters is propagated through a (usually) numerical model to construct the statistics of the system’s outputs. This approach works well for sensitivity studies, but some limitations arise when data is available at the output level (as in the case of experiments) or at some intermediate stage within the analysis. The primary objective of this paper is to investigate the feasibility of using Bayesian Belief Networks (BBN) to model multi-directional uncertainty propagation in a process where experimental data can be introduced as evidence. The problem under consideration has the objective of estimating the modal parameters of a structural system with uncertain parameters. The estimation is based on a model of the system, but it is assumed that a limited set of experimental data may be available on input or output parameters. The procedure is first applied to the simple case of a beam structure, for which a number of natural frequencies are evaluated in the presence of uncertainty. Next, it is extended to the estimation of modal quantities of a turbine engine bladed disk sector, which provides the motivation for these investigations.
Large Eddy Simulation of Leading Edge Film Cooling: Part I — Computational Domain and Effect of Coolant Pipe Inlet Condition
Authors: Rozati Ali , Tafti Danesh K.
Published: March 2009

A numerical investigation is conducted to study compound angle leading edge film cooling with Large Eddy Simulation. The leading edge has two rows of coolant holes located at ±15° of the stagnation line. Coolant jets are injected into the flow field at 30° (span-wise) and 90° (stream-wise). Mainstream Reynolds number is 100,000 based on the free stream velocity and cylinder diameter. Jet to mainstream velocity and density ratios are 0.4 and 1.0, respectively. It is found that during startup the stagnation line at the leading edge is not stationary but moves on a timescale much larger than the characteristic turbulent scales generated by the jet-mainstream interaction. To alleviate the long time integration necessitated by this feature, only half the domain is calculated (fixed stagnation) by showing that there is very little correlation in the flow structures generated by the jet-mainstream interaction on either side of stagnation. A comparison is made between a laminar uniform profile at the coolant pipe inlet with a time-dependent turbulent profile extracted from an auxiliary turbulent pipe flow calculation. The former over-predicts the span-wise averaged effectiveness, while the latter promotes better mixing in the outer region of jet-mainstream interaction and lowers the adiabatic effectiveness showing good agreement with measurements. In both cases, a characteristic low frequency interaction between the jet and the mainstream is identified at a non-dimensional frequency between 0.79 and 0.95 based on jet diameter and velocity. Even in the absence of any free-stream and jet turbulence, a turbulent boundary layer is established within a diameter downstream of the jet due to the strong lateral entrainment downstream of injection. The entrainment is primarily driven by an asymmetric counterrotating vortex pair in the immediate wake of the coolant jet. The driving mechanism for the formation of these vortices is a low pressure zone in the wake which entrains mainstream flow laterally into this region.
RANS Prediction of Transonic Compressive Rotor Performance Near Stall
Authors: Grosvenor Allan D.
Published: March 2009

A series of test cases have been studied to draw conclusions on the numerical prediction of boundary layer development under adverse pressure gradient, including strong shockwave / boundary layer interaction phenomena that influence the performance of a transonic compressive rotor. A density based RANS technique has been selected, employing the Spalart-Allmaras turbulence model. The study concentrates on the predicted effect of endwall boundary layer development and its interaction with the passage shock system. Particular attention is given to predicting the near-stall condition.
Modeling Approach for Lean Blowout Phenomenon
Authors: Kutsenko Yu. G. , Onegin S. F. , Gomzikov L. Y. , Belokon’ A. , Zakharov V.
Published: March 2009

Modern concepts for reducing thermal NO emissions require the use of very lean fuel/air mixtures. Therefore a problem of lean quench should be solved during design process of gas turbine combustor and it’s operational development. Since maintenance of flame stability for wide range of gas turbine engine operational modes is essential, therefore there is a great demand for models which are able to predict lean blow out limits of turbulent, premixed and partially premixed, aerodynamically stabilized flames. In this paper a model describing flame destabilization process is presented. This model takes into account various physical processes, which lead to flame destabilization. The model is based on equation for reaction progress variable. An expression of source term of this equation contains turbulent flame speed, which is calculated with the use of Zimont’s formula modification, proposed by authors. The results of simulation were compared with test results for our lean premixed combustor. Fuel mass flow rate of pilot zone was decreased during test until heat release of pilot flame front became insufficient and couldn’t support a combustion process in a lean premixed zone. Our simulation with modified model allows to get prediction of lean blowout limit.
Experimental Investigation on the Solid Particle Erosion in the Control Stage Nozzles of Steam Turbine
Authors: Wang Shunsen , Liu Guanwei , Mao Jingru , Feng Zhenping
Published: March 2009

This study is concerned with experiments for the relation of solid particle erosion (SPE) and the nozzle profiles. The exfoliated scale from boiler tubing results in hard particles that erode steam turbine components, especially on the control stage nozzles of high parameters turbine. To characterize SPE, solid particle trajectory is measured using particle image velocimetry (PIV) and its relation with the erosion rate of the nozzle surfaces is correlated. In addition, erosion characteristic of nozzle material is investigated by experiments and results reveal that the erosion rate is directly proportional to the impacting velocity of particles with the power of 2.31 and the maximum erosion rate is taken place at the impact angle of 20–25 degree. Furthermore, 0.5% increase in the erosion rate for every one degree of steam temperature rise is observed in the range of 839K∼883K. The erosion rate of front-loaded nozzle A is 2∼3 times higher than that of conventional design nozzle B. The life cycle of nozzle is determined by the erosion of outlet edge, and the life of nozzle B is about 5 times as long as the life of nozzle A. Based on the relation of erosion rate and nozzle profile characteristic, it can be inferred that a aft-loaded nozzle with a contoured endwall substituting a planar endwall may outperform over other nozzle profiles in anti-SPE, prolonging the life cycle of the nozzle.
Application of the v2-f Turbulence Model to Turbomachinery Configurations
Authors: Tartinville B. , Lorrain E. , Hirsch Ch.
Published: March 2009

Enhancing the representation of turbulence processes is a critical issue for CFD codes devoted to turbomachinery industry. Though more complex methods are available, Reynolds-Averaged Navier-Stokes models are widely used in the daily design process. Therefore, there is a constant need for improving eddy-viscosity based turbulence models. The present work aims at investigating the capabilities of the v2-f turbulence model for turbomachinery applications. Three types of flow features that are of importance for such applications are investigated in details: heat transfer, secondary flows, and laminar to turbulent transition. From a series of test cases, it appears that the v2-f turbulence model is specially adapted for heat transfer applications and shows potentialities in predicting laminar to turbulent transition.
Numerical Investigation of Performance Characteristics in a Propane-Fueled Mesoscale Catalytic Reactor
Authors: Karagiannidis Symeon , Mantzaras John , Jansohn Peter , Boulouchos Konstantinos
Published: March 2009

Catalytic reactors are well-suited for hydrocarbon combustion in gas-turbine-based mesoscale (up to 1 kW electric) power generation applications. A platinum-coated, propane-fueled catalytic monolithic reactor has been investigated for portable gas-turbine-based power generation, its design being part of a recent Swiss initiative. Detailed parametric numerical studies were conducted for two reactor types, with ceramic and metallic monolithic structures, and combustion efficiencies were assessed at different inlet velocities and equivalence ratios. A 2-D elliptic numerical code was employed, with full treatment of all relevant heat transfer mechanisms inside the reactor channel. An operating scenario was chosen with a moderate mixture compression at p = 2.5 bar, fuel-to-air equivalence ratios up to φ = 0.40 and preheat temperature TIN = 750 K (achievable in recuperated mesoscale gas turbine devices). Predicted gas-phase ignition delay times have initially affirmed the operational safety for the chosen operating parameters. Power output curves and combustion efficiencies have been calculated for different inlet velocities, equivalence ratios, and solid thermal conductivities. It was shown that, for a given reactor length, metallic materials were more favorable regarding maximum power output and combustion efficiency. Reactors with high solid thermal conductivity displayed increased fuel conversion at high inlet velocities, due to reduced light-off distances and the larger surfaces exposed to high temperatures compared to reactors with low wall thermal conductivities. Metallic reactors benefited from more robust construction (thicker solid walls) when operated under fuel-leaner conditions, while ceramic reactors profited only by increased equivalence ratios. Finally, metallic reactors yielded more uniform outlet gas properties over a wider operational range compared to ceramic ones.
Tip Leakage Flow and Heat Transfer Predictions for Turbine Blades
Authors: Yang Dianliang , Feng Zhenping
Published: March 2009

A numerical study was performed to simulate the tip leakage flow and heat transfer on the first stage rotor blade in GE-E3 engines. Calculations were performed for both flat and squealer blade tips by means of solving Reynolds-averaged N-S equations in conjunction with the k-ω two-equation turbulence model. For squealer tip blades, several cases were considered with five different tip gap-blade span ratios of 0.4%, 0.7%, 1.0%, 1.3%, and 1.6%, and four different groove depth-blade span ratios of 1%, 2%, 3%, and 4%. A linear experimental cascade was calculated first to validate this turbulence model, and the results show good agreement with that of the experiment. With the results of several cases discussed and compared, flow structure and heat transfer characteristics in the tip gap are illustrated. The effects of the tip gap and groove depth on flow and heat transfer are significant. The leakage flow is weakened by deepening the tip groove. However, when the groove depth is larger than 3% of the blade span, no further leakage flow reduction will occur. The average heat transfer rate on the tip surface reduces as groove depth increases. An increase in tip gap height will increase the leakage flow, and thus lead to an increase in average heat transfer rate on the tip surface.
Numerical Study on the Response of Tip Leakage Flow Unsteadiness to Micro Tip Injection in a Low-Speed Isolated Compressor Rotor
Authors: Geng Shaojuan , Zhang Hongwu , Chen Jingyi , Huang Weiguang
Published: March 2009

A numerical study on the unsteady tip leakage flow with discrete micro tip injection from casing shroud in a low-speed isolated axial compressor rotor is presented. The main target is to clarify the flow mechanism of how the stall control measures act on the tip leakage flow typified by its self-induced unsteady flow characteristics. At operating condition near stall point, a series of calculations have been carried out for different axial position of injector and different injected mass flow rate. The computation results of flow field near rotor tip region show that under the influence of injected flow, the transient pressure distribution fluctuates along blade chord on both pressure and suction sides with respect to the relative position of injector and rotor. The pressure difference across the pressure and suction sides of compressor blade changes correspondingly, thus introduces a forced flow unsteadiness interacting with the unsteady tip leakage flow. When the injection is relatively strong and able to meet the tip leakage flow at its origination, the self-induced unsteadiness of tip leakage flow can be suppressed completely. In most cases, both frequency components of the self-induced unsteadiness and forced-induced unsteadiness are co-existing. The corresponding transient flow contours show that a local high pressure spot appears near blade pressure side, which moves downstream and shifts the tip leakage flow trajectory with less or without touching the neighboring pressure surface of the blade. Based on this understanding of discrete tip injection as force-induced flow unsteadiness, the numerical results are also analyzed to optimize the effect of injection in changing the route of tip leakage flow trajectories and therefore the chance of stability improvement of the compressor rotor.
The Influence of Sweep on Axial Flow Turbine Aerodynamics in the Endwall Region
Authors: Pullan Graham , Harvey Neil W.
Published: March 2009

Sweep, when the stacking axis of the blade is not perpendicular to the axi-symmetric streamsurface in the meridional view, is often an unavoidable feature of turbine design. In a previously reported study, the authors demonstrated that sweep leads to an inevitable increase in mid-span profile loss. In this paper, the influence on the flowfield close to the endwalls is investigated. Experimental data from two linear cascades, one unswept, the other swept at 45 degrees but having the same overall turning and mid-span pressure distribution, are presented. It is shown that sweep causes the blade to become more rear-loaded at the hub and fore-loaded at the casing. This is further shown to reduce the penetration of the secondary flow at the hub, and to produce a highly unusual secondary flow structure, with low endwall over-turning, at the casing. A computational study is then presented in which the development of the secondary flows of both blades is studied. The differences in the endwall flowfields are found to be caused by a combination of the effect of sweep on both the endwall blade loading distribution, and on the bulk movements of the primary irrotational flow.
The Influence of Shroud and Cavity Geometry on Turbine Performance — An Experimental and Computational Study: Part I — Shroud Geometry
Authors: Rosic Budimir , Denton John D. , Curtis Eric M.
Published: March 2009

Imperfections in the turbine annulus geometry, caused by the presence of the shroud and associated cavity have a significant influence on the aerodynamics of the main passage flow path. In this paper the datum shroud geometry, representative of steam turbine industrial practice, was systematically varied and numerically tested. The study was carried out using a three-dimensional multi-block solver, which modelled the flow in a 1.5 stage turbine. The following geometry parameters were varied: - Inlet and exit cavity length, - Shroud overhang upstream of the rotor leading edge and downstream of the trailing edge, - Shroud thickness for fixed casing geometry and shroud cavity depth, and - Shroud cavity depth for the fixed shroud thickness. The aim of this study was to investigate the influence of the above geometric modifications on mainstream aerodynamics, and to obtain a map of the possible turbine efficiency changes caused by different shroud geometries. The paper then focuses on the influence of different leakage flow fractions on the mainstream aerodynamics. This work highlighted the main mechanisms through which leakage flow affects the mainstream flow and how the two interact for different geometrical variations and leakage flow mass fractions.
The Influence of Shroud and Cavity Geometry on Turbine Performance — An Experimental and Computational Study: Part II — Exit Cavity Geometry
Authors: Rosic Budimir , Denton John D. , Curtis Eric M. , Peterson Ashley T.
Published: March 2009

The geometry of the exit shroud cavity where the rotor shroud leakage flow re-enters the main passage flow is very important due to the dominant influence of the leakage flow on the aerodynamics of low aspect ratio turbines. The work presented in this paper investigates, both experimentally and numerically, possibilities for the control of shroud leakage flow by modifications to the exit shroud cavity. The processes through which the leakage flow affects the mainstream aerodynamics identified in the first part of this study were used to develop promising strategies for reducing the influence of shroud leakage flow. The experimental program of this study was conducted on a three-stage model air turbine, which was extensively supported by CFD analysis. Three different concepts for shroud leakage flow control in the exit cavity were analysed and tested: a) profiled exit cavity downstream end-wall, b) axial deflector, and c) radial deflector concept. Reductions in aerodynamic losses associated with shroud leakage were achieved by controlling the position and direction at which the leakage jet re-enters the mainstream when it leaves the exit shroud cavity. Suggestions are made for an optimum shroud and cavity geometry.
Scaling Thermo-Acoustic Characteristics of LP and LPP Swirl Flames
Authors: Russ Michael , Meyer Axel , Bu¨chner Horst
Published: March 2009

One of the main objectives of combustion research in field of gas turbine application during the last decades was and still is the reduction of pollutant emissions. The most promising technology to reduce these pollutants turned out to be Lean Premixed (LP) and Lean Premixed Pre-Vaporized (LPP) combustion. However, serious problems concerning combustion-driven instabilities occurred with the implementation of the LP/LPP-concept. Today, prediction and systematic suppression of self-sustained combustion instabilities is an issue still unsolved, due to incomplete understanding of the physical feedback mechanism and the lack of models for dynamic flame response, i.e. frequency dependent characteristics of LP/LPP swirl flames. In that context, the purpose of the current paper is the establishment of a physical model to describe frequency dependent flame dynamics concerning burning velocity of steady-state premixed flames. Derived from that basic understanding, scaling laws for the prediction of unstable operation conditions will be established in dependence on main operation parameters such as thermal load, mixture temperature, air equivalence ratio and especially of fuel and operating pressure. Therefore, a new swirl-burner has been designed, offering the feasibility to choose the type of fuel, to adjust the swirl number for main and pilot burner and the burner exit geometry steplessly and to vary preheating temperatures, air equivalence ratios and thermal loads in a range of industrial relevance for gas turbine applications. To establish a periodical modulation of the mixture mass flow of the main L(P)P flame at the burner outlet sinusoidally in-time with well-defined frequencies and amplitudes, a pulsating unit was used. Using a mixing/ pre-vaporizing unit to create a time-independent and spatial homogeneous mixture of natural gas/ kerosene vapor and combustion air at the burner outlet, flame transfer functions of LP- and LPP swirl flames depending on main operating parameters were determined. The discussed results then lead to stability map for a given combustion system depending on the main operation parameters based on the knowledge of only one fully-described parameter combination leading to an instable condition. Based on this scaling procedure and confirmed by further experimental work the prediction of stability limits depending especially on the type of fuel, the swirl number and the operating pressure will be possible.
The Effect of a Casing Step on the Over-Tip Aerothermodynamics of a Transonic HP Turbine Stage
Authors: Thomas Gregory A. , Atkins Nicholas R. , Thorpe Steven J. , Ainsworth Roger W. , Harvey Neil W.
Published: March 2009

The casing end-wall of a shroudless high-pressure turbine is subject to severe thermal loading. A backwards facing step positioned upstream of the leading edge of the rotor has been shown to reduce the heat flux in this region. The testing has been conducted at fully engine scaled Mach number, Reynolds number and gas-to-wall temperature ratio. Measurements of local heat-flux and static pressure were made using thin-film and semi-conductor gauges respectively. Normalised heat-flux, static pressure, adiabatic wall temperature and heat-transfer coefficient distributions are compared with those from a previously tested plain annulus baseline geometry. The introduction of the casing step has lowered the casing end-wall heat-load. This reduction is associated primarily with a decrease in adiabatic wall temperature on the casing end-wall. Analysis of the time-resolved static pressure distribution, combined with a Computational Fluid Dynamics simulation, has shown that this reduction in adiabatic wall temperature is the result of work extraction from the fluid entrained behind the casing step. The variation of the blade tip loading with upstream guide vane phase has also been reduced.
Model of an Oxygen Transport Membrane for Coal Fired Power Cycles With CO2 Capture
Authors: Sander Frank , Foeste Sebastian , Span Roland
Published: March 2009

Greenhouse gas emissions from power generation will increase in future if the demand for electrical energy does not subside. Therefore capture and storage of carbon dioxide (CO2) will become important technologies for lowering the rate of increase of global CO2 emissions, or even reducing them. A promising technology for coal fired power cycles is the integrated gasification combined cycle (IGCC), where CO2 is separated from the syngas coming from the gasifier before the syngas is combusted in a more or less conventional gas turbine. But oxygen is required for the gasification process to achieve a high carbon conversion rate. The energy demand for the cryogenic air separation unit (ASU) lowers the net power output of the IGCC cycle. An alternative way of producing the oxygen could eliminate this disadvantage of the IGCC cycle. Oxygen transport membranes (also known as mixed conducting membranes – MCM) show a high potential for such applications in power cycles. In this paper results of an investigation on an IGCC cycle with CO2 capture and an integrated oxygen transport membrane (OTM) reactor are reported. The operating conditions of the membrane reactor have been analyzed; the feed inlet temperature and the pressure differences between permeate and retentate sides of the membrane reactor have been varied. The impact on the overall IGCC cycle has been discussed. The most optimistic assumptions give an overall net efficiency close to the case without CO2 capture. In this case the net efficiency is reduced by only 3 percentage points compared to an IGCC process without CO2 capture. But these assumptions lead to very challenging conditions for the membrane reactor. A pressure difference of 14.5 bar is assumed. Less severe operating conditions for the OTM reactor, which seem closer to realization, show less promising results. For sweep stream pressures of 10 and 15 bar the net efficiency ranges from 36% to 39%. This is in the range of an IGCC process with cryogenic ASU which achieves a net efficiency of 37% to 38%. It can be concluded that the integration of an OTM reactor into the IGCC cycle is an option with good prospects if the membrane is capable of bearing the challenging operating conditions. Calculations of investment costs have not been investigated in the frame of this work. Both the total capital costs and the durability are very important aspects for the membrane technology to be realized in power cycles such as IGCC.
Three-Dimensional Viscous Flutter Analysis of Standard Configuration 10
Authors: Petrie-Repar Paul J. , McGhee Andrew , Jacobs Peter A.
Published: March 2009

The results of a three-dimensional (3D) viscous flutter analysis for a compressor stage, Standard Configuration 10, are presented. The unsteady flow simulations were performed by a 3D linearized Navier-Stokes flow solver using the Spalart and Allmaras turbulence model. Significant flow blockage due to corner separation at the hub on the suction surface was predicted by the steady-state 3D viscous simulation at a design condition. Corner separation was not predicted by 3D inviscid or two-dimensional (2D) viscous simulations. The corner separation was found to have a destabilizing effect and changed the nature of the unsteady flow. In fact, the 3D viscous simulations predicted negative aerodynamic damping for almost half of the inter-blade phase angles, while the 2D and 3D inviscid simulations predicted stable positive aerodynamic damping for all inter-blade phase angles. An off-design flow condition was also examined and significant differences between the 2D and 3D viscous simulations were found.
Gas Turbine Engine Off-Design Performance Simulation Using Syngas From Biomass Derived Gas as Fuel
Authors: Ferreira Sandro B. , do Nascimento Marco Antoˆnio R.
Published: March 2009

The use of syngas from gasified biomass as fuel for electric power generation based on gas turbine engines has been seriously studied over the past last two decades. Few experimental power plants have been built around the world. A small review of the use of syngas from gasified biomass and a cleaning system for gas turbine engines are presented. In this paper a computational program was presented and validated to simulate the design and off-design performance analysis of simple cycle gas turbine engines with one and two shafts. The aim was to assess the behavior and performance of the gas turbine engine without accounting for auxiliary syngas fuel compressor when the gasifier is atmospheric. It shows the behavior and performance at the off design condition of these two types of hypothetic gas turbine engines. The two engines were designed to use kerosene as fuel and at off-design conditions, and they were run using syngas from gasified biomass. The results show that the running line in the compressor characteristic moves towards the surge line and that the performance changes when the engine runs with the syngas.
An Integrated Experimental-Numerical Case Study for a University Course About Dynamic Design of Turbomachinery
Authors: Cenci G. , Pinelli M.
Published: March 2009

In the paper, the development of an integrated experimental-numerical case study for a university course of Fluid Dynamic Design of Turbomachinery (FDDT) is presented. Since 2004, a FDDT course has been held at the Engineering Department of the University of Ferrara (Italy). The basic idea of the FDDT course is to introduce the basic and advanced ideas beyond the design of turbomachinery supported by the use of integrated three-dimensional tools. Within the course, great effort has been devoted to practical experience, both numerical and experimental. In particular, the study of a simple but exhaustive geometry may represent a good exercise where students can practically and effectively train. For this reason, during the FDDT course, a centrifugal pump has been studied both experimentally and numerically as a test geometry. In the paper, the phases necessary to carry out this kind of project are presented and discussed.
Experimental and Theoretical Investigation of Twin-Entry Radial Inflow Gas Turbine With Unsymmetrical Volute Under Full and Partial Admission Conditions
Authors: Aghaali Habib , Hajilouy-Benisi Ali
Published: March 2009

In this paper the performance characteristics of turbocharger twin-entry radial inflow gas turbine with unsymmetrical volute and rotor tip diameter of 73.6 mm in steady state and under full and partial admission conditions are investigated. The employed method is based on one dimensional performance prediction which is developed for partial admission conditions. Furthermore this method is developed for unsymmetrical volute of the turbine considering flow specifications. Experimental investigation of the research carried out on special test facility under full and partial admission conditions for a wide range of speed. The comparison of experimental and modeling results shows good agreements. Interestingly, the turbine maximum efficiency occurs when the shroud side inlet mass flow is higher than that of hub side.
LES Investigation of Flow and Heat Transfer in a Channel With Dimples and Protrusions
Authors: Elyyan Mohammad A. , Tafti Danesh K.
Published: March 2009

LES calculations are conducted for flow in a channel with dimples and protrusions on opposite walls with both surfaces heated at three Reynolds numbers, ReH = 220, 940, and 9300 ranging from laminar, weakly turbulent to fully turbulent, respectively. Turbulence generated by the separated shear layer in the dimple and along the downstream rim of the dimple is primarily responsible for heat transfer augmentation on the dimple surface. On the other hand, augmentation on the protrusion surface is mostly driven by flow impingement and flow acceleration between protrusions, while the turbulence generated in the wake has a secondary effect. Heat transfer augmentation ratios of 0.99 at ReH = 220, 2.9 at ReH = 940, and 2.5 at ReH = 9300 are obtained. Both skin friction and form losses contribute to pressure drop in the channel, with form losses increasing from 45% to 80% with an increase in the Reynolds number. Friction coefficient augmentation ratios of 1.67, 4.82 and 6.37 are obtained at ReH = 220, 940, and 9300, respectively. Based on the geometry studied, it is found that dimples and protrusions may not be viable heat transfer augmentation surfaces when the flow is steady and laminar.
Transient Thermal Field Measurements in a High Aspect Ratio Channel Related to Transient Thermochromic Liquid Crystal Experiments
Authors: Jenkins Sean C. , Shevchuk Igor V. , von Wolfersdorf Jens , Weigand Bernhard
Published: March 2009

Measurements of transient fluid temperature distributions were made in a high aspect ratio (4:1) internally ribbed two-pass channel relating to the measurement of heat transfer using the transient thermochromic liquid crystal (TLC) technique. The temperature field was measured at several positions leading up to and around the 180° bend in a two-passage channel to account for variations in the bulk temperature used as a reference for the transient TLC technique. Results showed that the normalized distribution of the temperature field was time-invariant, an important result for the validation of heat transfer results using the transient TLC method. The normalized fluid temperature field was shown to be independent of the inlet temperature step and relatively independent of channel Reynolds number. Fluid temperature distributions were shown to be consistent over the length of the inlet channel, however, temperature field measurements made downstream of the bend exhibited a strong asymmetry. Finally, local temperature distributions were used to adjust the reference temperature used in calculating heat transfer coefficient distributions and to show the behavior of heat transfer due to 180° bends.
On the Coupling of Foil Bearing Supported Rotors: Part 1 — Analysis
Authors: Heshmat Hooshang , Walton James F. , Heshmat Crystal A.
Published: March 2009

The expanded application of high-speed rotor systems operating on compliant foil bearings will be greatly enhanced with the ability to adequately couple multiple shaft systems with differing bearing systems and dynamic performance. In this paper the results of an analytical tradeoff study assessing coupling dynamic characteristics and their impact on coupled rotor-bearing system dynamics are presented. This analysis effort was completed in an effort to establish the form of characteristics needed to couple foil bearing supported rotors to ball bearing supported rotors, other foil bearing supported rotors as well as coupling rigid and flexible rotors both supported on foil bearings. The conclusions from this study indicate that with appropriate coupling design, a wide array of foil bearing supported rotor systems may be successfully coupled.
The Reheat Concept: The Proven Pathway to Ultra-Low Emissions and High Efficiency and Flexibility
Authors: Gu¨the Felix , Hellat Jaan , Flohr Peter
Published: March 2009

Reheat combustion has proven now in over 80 units to be a robust, and highly flexible gas turbine concept for power generation. This paper covers three key topics to explain the intrinsic advantage of reheat combustion to achieve ultra-low emission levels. First, the fundamental kinetic and thermodynamic emission advantage of reheat combustion is discussed analyzing in detail the emission levels of the first and second combustor stages, optimal firing temperatures for minimal emission levels, as well as benchmarking against single-stage combustion concepts. Secondly, the generic operational and fuel flexibility of the reheat system is emphasized, which is based on the presence of two fundamentally different flame stabilization mechanisms, namely flame propagation in the first combustor stage and auto-ignition in the second combustor stage. Finally, the present fleet status is reported by highlighting the latest combustor hardware upgrade and its emission performance.
Evaluation of Plasma Sprayed Thermal Barrier Coatings Using NDE and SEM
Authors: Fahr Abbas , Mandache Catalin , Genest Marc , Chen Weijie , Wu Xijia , Thornton John
Published: March 2009

Thermal barrier coatings (TBC) are used to protect the hot section components of gas turbine engines from high temperatures. A TBC system consists of a ceramic topcoat and a metallic bond coat sprayed or deposited onto the metal substrate. TBC failure is often associated with oxidation of the metallic bond coat at elevated temperatures via formation of thermally grown oxides (TGO) that cause internal stresses leading to the final spallation of the TBC. The present study explores the application of eddy current and infrared thermal imaging techniques for the detection of TGO in thermally-exposed TBC with a view of finding the damage criteria and a suitable solution for nondestructive evaluation (NDE) of TBC. The eddy current technique is based on the induction of an electromagnetic field and is sensitive to minute changes in electrical or magnetic properties of the test piece while infrared thermal imaging is based on thermal diffusion process and measures small differences in surface temperature. The NDE results are validated through destructive testing and microscopic examination of the TBC samples in as-sprayed condition and after exposure to elevated temperatures.
The Application of CFD to Model Windage Power Loss From a Spiral Bevel Gear
Authors: Rapley Steve , Eastwick Carol , Simmons Kathy
Published: March 2009

In some aero-engine applications a spiral bevel gear is mounted in a bearing chamber. The bearings and gears create a highly rotating, turbulent and chaotic flow field within the chamber. In this paper a single-phase computational study of the flow around a spiral bevel gear is presented and the data is compared to available experimental data. Results are presented for clockwise and anticlockwise rotation for both unshrouded and shrouded configurations. Reasonable agreement of the computational data with the experimental data is obtained. Moment coefficients show the same trends in behaviour for clockwise and anticlockwise rotation although numerical agreement is not ideal with a difference of up to 27%, in the unshrouded case. For the shrouded configuration, static pressure profiles along the shroud are compared to experimental data, and these show good agreement although in the worst case there is 23% difference between computational and experimental values. The effect of different computational wall treatments is presented.
Welding Technologies Used in Assembling Recuperator of Microturbine System
Authors: Zhang D. J. , Wang Q. W. , Zeng M. , Wang S. Y.
Published: March 2009

Compact recuperator affects the thermal efficiency of the microturbine greatly and has been widely used in the microturbine system. Generally, a recuperator consists of a plurality of thin primary sheets, many relatively thick spacer bars and guide strips. The main steps of manufacturing a recuperator are joining a pair of sheets into a cell to form a high pressure air passage and sealing the cells together to form a high temperature gas passage. The key techniques are reducing the distortion of the sheets during jointing and keeping airproof performance of each passage. In this study, the special welding moulds and the corresponding assemble technologies for the split and annular recuperator are presented. Three problems during manufacturing a recuperator are solved: (1) Deformation of the cell during welding is effectively avoided by using the step-back welding method. (2) Modified primary sheet is presented. When making recuperators with this sheet, Resistance seam welding method can be adopted, and the number of spacer bars is significantly reduced. (3) With a special mould, an involute-curved-face cell is obtained synchronously with the welding process when making an annular recuperator. The welding methods introduced in this paper are devoted to recuperator manufacturing.
Computational and Analytical Study of Multiphase Rotary Separator Turbine Liquid Outlet
Authors: Chochua Gocha , Maier William C.
Published: March 2009

Gas-liquid separation is typically performed in settling tanks using gravitational force, or in cyclones generating higher centrifugal forces by swirl generators. However, in some applications such as offshore platforms or subsea compression stations, further compactness of the separating units is important. The Rotary Separator Turbine, a special type of cyclonic separator, generates even higher centrifugal forces, which allows the design of very compact separators. Due to the complexity of the flow fields in this type of machine, Computational Fluid Dynamic (CFD) analyses were used in the recent design of such a rotary separator. Reynolds Averaged Navier-Stokes equations are solved for multiphase turbulent flows in multiple frames of reference. This paper describes basic functionality of the RST and concentrates on one particular analysis of the lip discharge geometry. In addition to the numerical study, an analytical model is presented for this particular flow.
Dynamic Modelling and Controls of an Air Supply System for In-Flight Proton Exchange Membrane Fuel Cells (PEM-FC)
Authors: Barchewitz Lukas P. , Seume Joerg R.
Published: March 2009

To cover the increasing demand of on-board electrical power and for further reduction of emissions, the conventional auxiliary power unit (APU) may be replaced by a fuel cell system with an expected efficiency increase of 25% to 50% when compared to start-of-the-art GT-APU. The main components of an in-flight FC system are a compressor-turbine unit, a kerosene reformer, and the fuel cell. Polymer exchange membrane fuel cells (PEM-FC) may be favored because of their currently advanced level of development, their high power density and the available liquid water in the cathode-off gases which can be used as service water on-board. Transient requirements may have significant impact on system design and operating range and will therefore be addressed in this paper. During in-flight operation, air has to be compressed from the ambient to a pressure near standard conditions, which allows the application of state-of-the-art PEM-FC and ensures a constant power density independent from the operating altitude. A centrifugal compressor is chosen for pressurization of the system and is powered by a radial turbine, which allows autonomous operation at cruising altitude without external power. For off-design operation and transients, electric support from the PEM-FC is necessary, see [1]. The radial turbine itself is run by the hot exhaust gases from a post-combustor using the remaining energy in the cathode off-gases. A thorough trade-off between suitable compressor techniques for the air supply system was carried out in [1]. Turbomachinery revealed to be favourable for the PEM air supply system due to their low specific weight and high efficiency. The air supply system resembles the turbocharger for a combustion engine (Fig. 1), which represents a good starting point for a successful integration into the flight environment and further development due to known technology. Based on a turbomachinery design which satisfies the system requirements, the dynamic behavior of the air supply system is analyzed when coupled to the PEM fuel cell. The main focus is on the detection of sensitive system parameters causing system response delay or critical operating conditions. The present paper suggests system features, turbomachinery design parameters and controller types which achieve inherent stability and fast response of the air supply system throughout the entire flight envelope.
A Gas Turbine Cycle Selection Issue: Recuperated or ICR
Authors: Rodgers Colin , Stone Aubrey , White David
Published: March 2009

The intercooled recuperative gas turbine (ICR) potentially offers the advantages of higher specific power, and improved thermal efficiency compared to the recuperative gas turbine, such advantages are however contingent upon the additional parasitic encumbrances of the intercooler heat dissipation or recovery apparatus and pressure losses, plus flowpath ducting and complexity. The thermodynamic performances, relative sizing and relative costs of both an ICR and recuperative gas turbine engine, with a thermal efficiency goal approaching 40%, combined with low exhaust emission requirements were studied. The study encompassed primary candidate engine flowpath configurations comprising of single shaft, two shaft, and two spool designs, with both recuperation (R), and combined Intercooling and Recuperation (ICR). In conducting the study all engine flowpaths were sized for 300kW with a maximum turbine inlet temperature of 1837F (1000C), representative of conservative life limits for conventional un-cooled superalloy turbine rotors. Heat exchanger effectivenesses of the intercooler and recuperator were selected at 80 and 85%, as a compromise between cost, weight, and thermal efficiency considerations. The study confirmed that the simple recuperated cycle is capable of comparable peak thermal efficiency levels to the ICR provided that ICR intercooling parasitic losses are duly accounted, and furthermore has intrinsically lower manufacturing and development costs than the ICR. The cycle performance code used for the studies included prediction of engine exhaust emissions, part load characteristics, and compressor operating lines. The emissions assessment slightly favored the ICR as a consequence of its higher specific power. Assuming part load operation at variable speed and constant turbine exhaust temperature, the two spool ICR showed slightly better part load fuel economy than a recuperated engine.
A Probabilistic System Analysis of Intelligent Propulsion System Technologies
Authors: Tong Michael T.
Published: March 2009

NASA’s Intelligent Propulsion System Technology (Propulsion 21) project focuses on developing adaptive technologies that will enable commercial gas turbine engines to produce fewer emissions and less noise while increasing reliability. It features adaptive technologies that have included active tip-clearance control for turbine and compressor, active combustion control, turbine aero-thermal and flow control, and enabling technologies such as sensors which are reliable at high operating temperatures and are minimally intrusive. A probabilistic system analysis is performed to evaluate the impact of these technologies on aircraft CO2 (directly proportional to fuel burn) and LTO (landing and takeoff) NOx reductions. A 300-passenger aircraft, with two 396-kN thrust (85,000-pound) engines is chosen for the study. The results show that NASA’s Intelligent Propulsion System technologies have the potential to significantly reduce the CO2 and NOx emissions. The results are used to support informed decision-making on the development of the intelligent propulsion system technology portfolio for CO2 and NOx reductions.
Optimized Shroud Design for Axial Turbine Aerodynamic Performance
Authors: Porreca L. , Kalfas A. I. , Abhari R. S.
Published: March 2009

This paper presents a comprehensive study of the effect of shroud design in axial turbine aerodynamics. Experimental measurements and numerical simulations has been conducted on three different test cases with identical blade geometry and tip clearances but different shroud designs. The first and the second test cases are representative of a full shroud and a non-axisymmetric partial shroud geometry while the third test case however uses an optimized partial shroud. Partial shrouds are sometimes used in industrial application in order to benefit from the advantage of shrouded configuration as well as reducing mechanical stress on the blades. However, the optimal compromise between mechanical considerations and aerodynamic performances is still an open issue due to the resulting highly 3-dimensional unsteady flow field. Aerodynamic performance is measured in a low-speed axial turbine facility and shows that there are clear differences between the test cases. In addition, steady and time resolved measurements are performed together with computational analysis in order to improve understanding of the effect of the shroud geometry on the flow field and to quantify the sources of the resultant additional losses. The flow field analysis shows that the effect of the shroud geometry is significant from 60% blade height span to the tip. Tip leakage vortex in the first rotor is originated in the partial shroud test cases while the full shroud case present only a weak indigenous tip passage vortex. This results in a significant difference in the secondary flow development in the following second stator with associated losses that varies of about 1% in this row. The analysis shows that the modified partial shroud design has improved considerably the aerodynamic efficiency of about 0.6% by keeping almost unchanged the overall weight of this component and thus blade root stresses. The work therefore presents a comprehensive flow field analysis and the shows the impact of the shroud geometry in the aerodynamic performance.