bluuebunny/crossref_metadata_2025 huggingface dataset:
Distributed Reforming of Diesel, JP-8, and Other Heavy Hydrocarbon Fuels for Fuel Cell Applications: Challenges and Issues
Authors: Krause T. R. , Ahmed S. , Kumar R.
Published: September 2008

Reforming diesel, JP-8 (Jet Propellant 8 – standard U.S. military kerosene-based jet fuel), and other heavy hydrocarbon fuels is one option being investigated for providing H2 for distributed mobile and stationary fuel cell systems for military and civilian applications. Unlike natural gas, which is another hydrocarbon fuel being investigated, these fuels are high-boiling-point, multi-component liquids that contain high concentrations of refractory sulfur and aromatic compounds that can negatively impact the efficiency and operating lifetime of the fuel processor. Fuel injection, desulfurization, and carbon deposition are major issues that fuel processor developers must address when designing fuel processors for these fuels. The fuel injection system must prevent direct injection of liquid onto the catalyst surface and poor mixing of the fuel and oxidants (i.e., air and/or steam), both of which can result in excessively high temperatures that can damage or destroy the reforming catalyst. A highly efficient desulfurization process is required that can reduce the sulfur concentration to acceptable levels for both fuel processor and fuel cell catalysts without requiring large amounts of materials or complicated processes, and without generating excessive amounts of disposable waste. Highly active reforming catalysts and the proper choice of operating conditions (i.e., ratio of fuel to oxidant[s], temperature, residence time) are required for effective reforming of aromatic compounds to prevent carbon deposition on the reforming catalyst as well as in the fuel processor downstream of the reformer. In this paper, we will discuss how the chemical and physical properties of these fuels influence the design of the fuel processor focusing on the fuel injection system, the choice of desulfurization process, and the design and operation of the reformer. We will also discuss various approaches and design options for developing highly efficient fuel processors for reforming these fuels for both polymer electrolyte and solid oxide fuel cells.
Support Effects for Pt and Rh-Based Catalysts for Partial Oxidation of n-Tetradecane
Authors: Berry David A. , Shekhawat Dushyant , Gardner Todd H. , Salazar Maria , Haynes Daniel J. , Spivey James J.
Published: September 2008

Catalytic partial oxidation (CPOX) of liquid fuels is an attractive option for producing a hydrogen-rich gas stream for fuel cell applications. However, the high sulfur content along with aromatic compounds present in liquid fuels may deactivate reforming catalysts. Deactivation of these catalysts by carbon deposition and sulfur poisoning is a key technical challenge. The relationship between catalyst supports and deactivation have been studied here for three catalysts (Rh/Ce0.5Zr0.5O2, Pt/Ce0.5Zr0.5O2, and Pt/Al2O3) in a fixed bed catalytic reactor using a mixture of n-tetradecane, 1-methylnaphthalene, and dibenzothiophene to simulate logistic fuels. Carbon production during CPOX reforming was directly related to olefin formation. Olefins, which are known coke precursors, were observed on the Pt catalysts during CPOX of n-tetradecane with no sulfur (particularly from Pt/Al2O3), but not on Rh/Ce0.5Zr0.5O2. For the Rh/Ce0.5Zr0.5O2, yields of H2 and CO dropped to a stationary level after the introduction of sulfur-containing feed (1000 ppm sulfur) or aromatic-containing feed (5 wt%), however, the catalyst activity was restored after removing the sulfur or aromatics from the feed. For the Pt catalysts, H2 and CO yields dropped continuously over time in the presence of sulfur or aromatics in feed. The superior performance of Rh/Ce0.5Zr0.5O2 can be attributed to the higher oxygen-ion conductivity of the Ce0.5Zr0.5O2 support as well as the activity of the Rh sites.
Load Transient Mitigation for Solid Oxide Fuel Cells
Authors: Wang Caisheng , Nehrir M. Hashem
Published: September 2008

A load transient mitigation scheme is proposed in this paper for stand-alone solid oxide fuel cell (SOFC)-battery power systems. The scheme is based on low-pass filtering of load transients and fuel cell current control. The fuel cell is controlled in such a way to provide the steady-state load, while the battery will supply the transient load. The technique can be used to improve the output power quality of the overall system as well as the SOFC durability by mitigating the stresses on SOFC caused by the load transients. Simulation studies have been carried out to verify the proposed technique. Simulation results show the effectiveness of the proposed technique, which prevents the load transient to affect the fuel cell performance.
PEM Fuel Cells With Multiwalled Carbon Nanotubes as Catalyst Support Material
Authors: Reddy A. Leela Mohana , Shaijumon M. M. , Rajalakshmi N. , Ramaprabhu S.
Published: September 2008

Multi-walled carbon nanotubes (MWNTs) have been synthesized by the pyrolysis of acetylene using hydrogen decrepitated Mischmetal (Mm) based AB3 alloy hydride catalyst. MWNTs have been characterized by SEM, TEM, Raman and XRD studies. Pt-supported MWNTs (Pt/MWNTs) have been prepared by chemical reduction method using functionalized MWNTs. Composites of Pt/MWNTs and Pt/C have been used as electrocatalysts for oxygen reduction reaction in Proton Exchange Membrane Fuel Cell (PEMFC). Cathode catalyst with 50% Pt/MWNTs and 50% Pt/C gives the best performance because of the better dispersion and good accessibility of MWNTs support and the Pt electrocatalysts in the mixture for the oxygen reduction reaction in PEMFC. The paper emphasizes that Pt/C and Pt/MWNTs composites have good potential as catalyst support material in PEMFC.
Numerical Simulation of an Axisymmetric Ethanol Reforming Reactor for Hydrogen Fuel Cell Applications
Authors: Buck Gregory A. , Obara Hiroyuki
Published: September 2008

Hydrogen fuel cell technology is currently capable of providing adequate power for a wide range of stationary and mobile applications. Nonetheless, the sustainability of this technology rests upon the production of hydrogen from renewable resources. Among the techniques under current study, the chemical reforming of alcohols and other bio-hydrocarbon fuels, appears to offer great promise. In the so called autothermal reforming process, a suitable combination of total and partial oxidation supports hydrogen production from ethanol with no external addition of energy required. Furthermore, the autothermal reforming process conducted in a well insulated reactor, produces temperatures that promote additional hydrogen production through the endothermic steam reforming and the water-gas shift reactions, which may be catalyzed or uncatalyzed, with the added benefit of lowered carbon monoxide concentrations. In this study, an adiabatic ethanol reforming reactor was simulated assuming the reactants to be air (21% O2 and 79% N2) and ethanol (C2H5OH) and the products to be H2O, CO2, CO and H2, with all constituents taken to be in the gaseous state. The air was introduced uniformly through a ring around the side of the reactor and the gaseous ethanol was injected into the center of one end, with products withdrawn from the center of the opposite end, to create an axisymmetric flow field. The gas flows within the reactor were assumed to be turbulent, and the chemical kinetics of a simple four reaction system was assumed to be controlled by turbulent mixing processes. Air and fuel flow rates into the reactor were varied to obtain six different levels of oxidation (air-fuel ratios) while maintaining the same total gaseous mass flow out of the reactor. The numerical results for the reacting flow show that hydrogen production is maximized when the air-fuel ratio on a mass basis is held at approximately 2.8. These findings are in qualitative agreement with observations from previous experimental studies.
Development of Low Temperature Solid Oxide Fuel Cells
Authors: Zhu Bin , Liu Xiangrong , Zhu Zhigang , Ljungberg Rikard
Published: September 2008

Based on innovative ceria-based composite (CBC) material advantages we have made strong efforts to make technical developments on scaling up material production, fabrication technologies on large cells and stack operated at low temperatures (300 to 600°C). Next generation materials for solid oxide fuel cells (SOFCs) have been developed based on abundant natural resources of the industrial grade mixed rare-earth carbonates named as LCP. Here we show the LCP-based materials used as functional electrolytes to achieve excellent fuel cell performances, 300–800 mWcm2 for low temperatures, exhibiting a great availability for industrialization and commercialization.
Development of New High AN2 Last LP Stage Turbine and Exhaust Systems: A Cost Effective Solution for the 21st Century
Authors: Haller Brian
Published: September 2008

A new high AN2 last LP stage turbine has been developed to provide leading performance for turbomachines in the 21st Century. It required a multi-disciplinary design approach involving aerodynamics, materials, mechanics (stress and vibration) and manufacturing technologies. The objective of the design was to achieve around 5% gain in last stage total-to-static efficiency, relative to the current competitive datum, for a very compact machine with few parts count ie reduced cost. It is shown how the optimum aerodynamic design was achieved for the stage. The paper presents details of the novel approaches used for the design including preliminary optimization, blading design and results from multistage 3D viscous predictions. The new stage has been tested in a Warm Air Turbine test rig at full scale engine representative conditions. The “loop was closed” by comparing the detailed test measurements with throughflow and 3D viscous analyses. This gives high confidence in the new approaches used. Additionally, the development of new compact high performance axial-radial and axial exhaust systems, which were designed to operate downstream of the new last LP stage, are described. To achieve the optimum performance, the last LP stage has to designed as a coupled system with the exhaust diffusers. The new LP technologies have already been scaled and cloned to a new engine design and sold by the Company.
Numerical Simulations of Heat Transfer and Fluid Flow for a Rotating High-Pressure Turbine
Authors: Mumic F. , Ljungkruna L. , Sunden B.
Published: September 2008

In this work, a numerical study has been performed to simulate the heat transfer and fluid flow in a transonic high-pressure turbine stator vane passage. Four turbulence models (the Spalart-Allmaras model, the low-Reynolds-number realizable k-ε model, the shear-stress transport (SST) k-ω model and the v2-f model) are used in order to assess the capability of the models to predict the heat transfer and pressure distributions. The simulations are performed using the FLUENT commercial software package, but also two other codes, the in-house code VolSol and the commercial code CFX are used for comparison with FLUENT results. The results of the three-dimensional simulations are compared with experimental heat transfer and aerodynamic results available for the so-called MT1 turbine stage. It is observed that the predictions of the vane pressure field agree well with experimental data, and that the pressure distribution along the profile is not strongly affected by choice of turbulence model. It is also shown that the v2-f model yields the best agreement with the measurements. None of the tested models are able to predict transition correctly.
Measurements and Computations of Compressible Flow Through a Turbine Cascade With Surface Roughness
Authors: Yuan Lan Qin , Kind Richard J.
Published: September 2008

Experiments and computations have been carried out for a high-pressure turbine cascade having bands of roughness on the blade surfaces, for cascade-exit Mach numbers ranging from 0.4 to 1.13 Tests were carried out with three different relative roughness heights as well as with smooth surfaces. The results comprise profile-loss coefficients and deviation angle. Corresponding flow computations were done using the Fluent 6.0 CFD code with the Spalart-Almaras turbulence model and wall functions to model the near-surface flow. Good agreement was found between the experimental and computational results, especially with regard to trends with Mach number, roughness height and roughness-band configuration. This indicates that current computational methods, which use essentially the same approach to model surface-roughness effects as validated for incompressible flow, can give good predictions of compressible flow over rough surfaces, including details of the viscous flow field. The experimental data constitutes a valuable resource for future efforts to improve prediction capabilities for flows involving rough surfaces.
Investigation of Secondary Flow Behavior in a Radial Turbine Nozzle
Authors: Putra Mohammed Alexin , Joos Franz
Published: September 2008

Fundamental investigation of secondary flow phenomena in a radial turbine nozzle are presented. L2F measurements have been used for validation of numerical CFD calculations. Having a good agreement by using the Reynolds stress turbulence model (RSM) the numerical results have been used further to analyse the structure of secondary vortices. Contour plots of the flow angle with typical isoline pattern as well as the vorticity have been evaluated. It is shown that the channel of the radial nozzle similar secondary vorticity systems generates as known from the axial turbine nozzles. The formation and the development of the horse-shoe vortex and the corner vortex are discussed. The well known passage vortex of the axial turbines could not been found because of the small curvature of the streamlines. Instead of these an additional single vortex can be observed, called the “inflow” vortex caused by the unsymmetrical flow into the radial cascade from the upstream scroll.
Optimization of A-Weighted Discrete Noise Using Unequally Spaced Blades
Authors: Wu Xian-Jun , Cao Jian-Hua
Published: September 2008

A fan with equally spaced blades causes noticeable discrete noise. An efficient way to cut the discrete noise down is using the unequally spaced blades. An optimization method to minimize the A-weighted sound level was given in this paper by designing the circumferential angles of the blades. The compositive attenuation properties made by the A-weighted function and a Sc parameter, and the limited human audible range of frequency all contribute to the noise reducing. Simulation was given and the efficacy of the optimization model was proved.
Experimental and Numerical Investigation of a Subsonic Compressor With Bend Skewed Slot Casing Treatment
Authors: Lu Xingen , Chu Wuli , Zhu Junqiang , Wu Yanhui
Published: September 2008

Based on the test results of discrete axial and blade angle slot casing treatment, a new type of casing treatment was designed for a subsonic axial flow compressor rotor by optimising various geometry parameters. To obtain a wide operating range and to minimize penalties in terms of isentropic efficiency, seven compressor configurations incorporating casing treatments of 0%, 16.6%, 33.3%, 50%, 66.6%, 83.3% and 100% rotor exposure were experimentally investigated. The results showed that significant improvements in stall margin are possible in all exposures and insignificant isentropic efficiency sacrifices are recorded in some exposures. Nearly 21.43% stall margin improvement in terms of the corrected mass flow rate was achieved with 33.3% rotor blade tip axial chord exposure. The compressor build with 16.6% rotor exposure was the best configuration in terms of maximum isentropic efficiency gain. The second issue of the paper was to offer a contribution to the understanding of the physical mechanism by which bend skewed slot casing treatment improve stall margin under subsonic conditions. By applying a concept similar to “Domain Scaling” approach (as often used in multistage turbomachinery Flow-fields) to the interface between the rotor blade passage and end-wall treatments, a time-dependent 3-dimentional numerical simulation was performed for the subsonic axial-flow compressor rotor with bend skewed slot casing treatment. The numerical results agreed well with experimental results. Detailed analyses of the coupled flow through bend skewed slot casing treatment and rotor blade passage under subsonic conditions led to some preliminary conclusions as to the flow physics involved in the stall margin improvements afforded by the use of bend skewed slot casing treatment.
Numerical Simulation of Erosion in Modified Nozzles of Axial Flow Turbine
Authors: Mazur Zdzislaw , Campos-Amezcua Rafael , Campos-Amezcua Alfonso
Published: September 2008

Solid particle erosion in a turbine nozzle (turbine control stage) has been investigated by means of CFD. Literature attempt to couple fluid mechanics and erosion modeling and improvements in the hydrodynamics models together with improvements in the erosion models are reviewed. The solid particle bearing steam flow through the nozzle was investigated using a 3D numerical model and the finite volume code Fluent V6.0.12, looking for a reduction of the erosion process. The flow simulation was carried out for the original and modified (nozzle) designs with changes of the angle of particle impact on the nozzle surface. Numerical predictions have been carried out using the Renormalization Group (RNG) k-ε turbulence mode. To account for the influence of turbulent fluid fluctuations on particle motion, the stochastic tracking Discrete Random Walk model is used, which includes the effect of instantaneous turbulent velocity fluctuations on the particle trajectories. The removal of wall material due to erosion is calculated using the Finnie model developed for ductile materials. The numerical predictions showed a 50 percent reduction of the erosion rate for the modified (nozzle) design due to changes of the particles trajectories and impingement angle (angle of particle impact). The results obtained show that numerical simulation can be used in a predictive manner to solve a real practical design problem.
Short Length-Scale Rotating Stall Inception in a Transonic Axial Compressor: Criteria and Mechanisms
Authors: Hah Chunill , Bergner Jo¨rg , Schiffer Heinz-Peter
Published: September 2008

The current paper reports on investigations aimed at advancing the understanding of the flow mechanism that leads to the onset of short-length scale rotating stall in a transonic axial compressor. Experimental data show large oscillation of the tip clearance vortex as the rotor operates near the stall condition. Inception of spike-type rotating stall is also measured in the current transonic compressor with high response pressure transducers. Computational studies of a single passage and the full annulus were carried out to identify flow mechanisms behind the spike-type stall inception in the current transonic compressor rotor. Steady and unsteady single passage flow simulations were performed, first to get insight into the interaction between the tip clearance vortex and the passage shock. The conventional Reynolds-averaged Navier-Stokes method with a standard turbulence closure scheme does not accurately reproduce tip clearance vortex oscillation and the measured unsteady pressure field. Consequently, a Large Eddy Simulation (LES) was carried out to capture more relevant physics in the computational simulation of the rotating stall inception. The unsteady random behavior of the tip clearance vortex and it’s interaction with the passage shock seem to be critical ingredients in the development of spike-type rotating stall in a transonic compressor. The Large Eddy Simulation was further extended to the full annulus to identify flow mechanisms behind the measured spike-type rotating stall inception. The current study shows that the spike-type rotating stall develops after the passage shock is fully detached from the blade passages. Interaction between the tip clearance vortex and the passage shock creates a low momentum area near the pressure side of the blade. As the mass flow rate decreases, this low momentum area moves further upstream and reversed tip clearance flow is initiated at the trailing edge plane. Eventually, the low momentum area near the pressure side reaches the leading edge and forward spillage of the tip clearance flow occurs. The flows in the affected blade passage or passages then stall. As the stalled blade passages are formed behind the passage shock, the stalled area rotates counter to the blade rotation just like the classical Emmon’s type rotating stall. Both the measurements and the computations show that the rotating stall cell covers one to two blade passage lengths and rotates at roughly 50% of the rotor speed.
An Improved Calculation Model of Axial Flow Fan’s Vortex Noise
Authors: Wu Xian-Jun , Cao Jian-Hua
Published: September 2008

Improvements were made on T. Fukano’s model, and the swept angle’s influence on the broadband noise is considered. It was found that the vortex noise’s sound power varies as the fifth power of the cosine value of swept angle of the blade edge. A model used Gauss distributing formula to calculate the spectrum content of broadband noise is also brought out. The model is more suitable for calculating of A-weighted sound level because the spectrum content is calculated. Experiments were done for validation and good agreement was observed.
Interaction of Tip Clearance Flow and Three-Dimensional Separations in Axial Compressors
Authors: Gbadebo Semiu A. , Cumpsty Nicholas A. , Hynes Tom P.
Published: September 2008

This paper considers the interaction of tip clearance flow with three-dimensional (3D) separations in the corner region of a compressor cascade. Three-dimensional numerical computations were carried out using ten levels of tip clearance, ranging from zero to 2.18% of blade chord. The 3D separations on the blade suction surface were largely removed by the clearance flow for clearance about 0.58% of chord. For this cascade, experimental results at zero and 1.7% chord tip clearance were used to assess the validity of the numerical predictions. The removal mechanism was associated with the suppression of the leading edge horseshoe vortex and the interaction of tip clearance flow with the endwall boundary layer, which develops into a secondary flow as it is drifted towards the blade suction surface. Such interaction leads to the formation of a new 3D separation line on the endwall. The separation line forms the base of a separated stream surface which rolls up into the clearance vortex.
Mechanism of the Interaction Between Casing Treatment and Tip Leakage Flow in a Subsonic Axial Compressor
Authors: Lu Xingen , Chu Wuli , Zhu Junqiang , Wu Yanhui
Published: September 2008

The use of slots and grooves in the shroud over the tips of compressor blades, known as casing treatment, is known as a powerful method to control tip leakage flow through the clearance gap and enhance the flow stability in compressors. This paper present a detailed steady and unsteady numerical studies of the coupled flow through rotor blade passages and two different types of casing treatment for a modern subsonic axial-flow compressor rotor. Particular attention was given to examining the interaction between the tip leakage flow and the casing treatment. In order to validate the multi block model applied in the rotor blade end-wall region, the computational results for the modern subsonic compressor rotor both with and without casing treatment were correlated with available experimental test data for estimation of the global performance. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases with casing treatment and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by the casing treatment is a result of the tip clearance flow manipulation. The repositioning of the tip clearance vortex further towards the trailing edge of the blade passage and delaying the movement of incoming/tip clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.
Clearance Effects on the Evolution of the Flow in the Vaneless Diffuser of a Centrifugal Compressor at Part Load Condition
Authors: Schleer Matthias , Abhari Reza S.
Published: September 2008

This work reports on flow measurements taken within the vaneless diffuser of an scaled-up model of a small-scale, highly loaded unshrouded compressor with large relative tip clearance. The aims are to describe and to analyze the influence of the clearance flow on the flow structure at impeller exit in part load operation. The kind of compressor described herein are widely used in distributed power applications and automotive turbo-charging. They demand further enhancement of the operation range as well as a high head rise and an improved efficiency. Therefore, the understanding of flow features and their interaction is crucial. The interaction and mixing of the flow pattern flow downstream of the impeller is shown using spatially- and temporally resolved 3D-velocity data. The measurements have been obtained by using a 3D Laser Doppler Anemometry system (LDA) throughout the vaneless parallel wall diffuser. This unique data set provides insight into the development of the flow within the diffuser and allows conclusions on the mixing and migration of the 3-dimensional pattern. The flow structure in part load condition is strongly affected by the flow across the large relative tip gap. Due to the large relative tip clearance a low momentum zone is formed as an additional pattern at the shroud. This clearance flow is highly vortical and interacts with the channel wake structure but remains stable throughout the vaneless diffuser. At the pressure side hub corner a jet structure is formed which interacts rapidly with the blade wake. This flow behavior doesn’t comply with the classical jet wake pattern. It is proposed that in centrifugal compressor with large relative tip clearance a modified flow model which includes tip leakage is more appropriate to describe the flow structure at part load condition.
Unsteady CFD Simulation of an Entire Gas Turbine High-Spool
Authors: Schlu¨ter Jo¨rg , Apte Sourabh , Kalitzin Georgi , Pitsch Heinz , van der Weide Edwin , Alonso Juan
Published: September 2008

We present an approach for the numerical prediction of the full engine performance and the assessment of multi-component behavior. In this approach, multiple components of a gas turbine are solved with different CFD solvers, each optimized for its assigned task. The solvers are executed simultaneously and during the computation exchange flow data at the interfaces. This enables efficient and accurate simulation of the entire gas turbine. We have developed a coupling software that enables multi-code/multi-physics simulations to perform seamlessly. In this paper, we will present the approach of multi-code simulations, describe the necessary interface and boundary conditions and demonstrate the value of integrated multi-code simulations for the assessment for multi-component effects. Finally, we will show and demonstrate the possibility of full engine simulations using CFD.
Numerical Investigation of a Transonic Centrifugal Compressor
Authors: Marconcini Michele , Rubechini Filippo , Arnone Andrea , Ibaraki Seiichi
Published: September 2008

A three-dimensional Navier-Stokes solver is used to investigate the flow field of a high pressure ratio centrifugal compressor for turbocharger applications. Such a compressor consists of a double-splitter impeller followed by a vane diffuser. The inlet flow to the open shrouded impeller is transonic, thus giving rise to interactions between shock waves and boundary layers and between shock waves and tip leakage vortices. These interactions generate complex flow structures which are convected and distorted through the impeller blades. Detailed Laser Doppler Velocimetry (LDV) flow measurements are available at various cross sections inside the impeller blades highlighting the presence of low velocity flow regions near the shroud. Particular attention is focused on understanding the physical mechanisms which govern the flow phenomena in the near shroud region. To this end numerical investigations are performed using different tip clearance modelizations and various turbulence models, and their impact on the computed flow field is discussed.
Active Flow Control for Increasing the Surge Margin of an Axial Flow Compressor
Authors: Kefalakis M. , Papailiou K. D.
Published: September 2008

Steady and pulsating jets were used targeting to increase the surge margin of an axial flow compressor. In this experimental study, a rotating valve was built for generating pulsating jets and a number of pulsating jet actuators were installed on the compressor casing. Approximately 10% surge margin increase was obtained for a jet mass flow rate of less than 1% of the compressor mass flow rate. The active control mechanism is analyzed and the influence of each parameter is studied in an effort to quantify their relative influence and increase our understanding of the phenomenon.
Detailed Analysis of Experimental Investigations on Boundary Layer Transition in Wake Disturbed Flow
Authors: Cattanei Andrea , Zunino Pietro , Schro¨der Thomas , Stoffel Bernd , Matyschok Berthold
Published: September 2008

In the framework of a co-operation between the University of Genoa and the Darmstadt University of Technology measurement data of a former investigation at Darmstadt, comprising measurements with surface-mounted hot-film sensors on the boundary layer transition in wake disturbed flow, were transferred to Genoa, then re-evaluated and in great detail analyzed, much further than the original data evaluation. In these experimental investigations at Darmstadt, the boundary layer transition with and without transitional separation bubbles was studied on a circular cylinder in cross flow. The comparison of hot-wire traverses with the surface-mounted hot-film distributions clearly indicated that the surface-mounted hot-film technique is a very suitable measurement technique to obtain reliable information on transition and separation phenomena with both high spatial and temporal resolution. The new data evaluation techniques applied to these data at Genoa further enhanced the insight into the details of the boundary layer transition and separation process. The surface-mounted hot-film data were evaluated by means of time-space diagrams for the first three statistical moments (mean, RMS and skewness), with which the origin and the extent of unsteady separation bubbles clearly could be seen. The results obtained from these data analyses on the one hand yield a considerable enhancement of the understanding of the periodically unsteady boundary layer transition process and on the other hand they form the basis for the application of surface-mounted hot-film sensors in more complex flow situations like e.g. in cold flow multistage turbine or compressor test rigs or even in the hostile environment of real aero engine compressors or turbines.
The Impact of Gas Modeling in the Numerical Analysis of a Multistage Gas Turbine
Authors: Rubechini Filippo , Marconcini Michele , Arnone Andrea , Maritano Massimiliano , Cecchi Stefano
Published: September 2008

In this work a numerical investigation of a four stage heavy-duty gas turbine is presented. Fully three-dimensional, multistage, Navier-Stokes analyses are carried out to predict the overall turbine performance. Coolant injections, cavity purge flows and leakage flows are included in the turbine modeling by means of suitable wall boundary conditions. The main objective is the evaluation of the impact of gas modeling on the prediction of the stage and turbine performance parameters. To this end, four different gas models were used: three models are based on the perfect gas assumption with different values of constant cp, and the fourth is a real gas model which accounts for thermodynamic gas properties variations with temperature and mean fuel/air ratio distribution in the through-flow direction. For the real gas computations, a numerical model is used which is based on the use of gas property tables, and exploits a local fitting of gas data to compute thermodynamic properties. Experimental measurements are available for comparison purposes in terms of static pressure values at inlet/outlet of each row and total temperature at the turbine exit.
Simulation of Surge Inception and Performance of Axial Multistage Compressor
Authors: Tauveron Nicolas , Ferrand Pascal , Leboeuf Francis
Published: September 2008

This work concerns transient multistage turbomachine modeling. The main application is the simulation of off-design regimes and unstable situations (essentially surge). Two specific approaches are developed: the first is a simple and fast model, based on a simplification of actuator disk model (0D). The second approach is based on 1D axisymmetric Navier-Stokes equations at the scale of the row. The models are tested on open literature cases of the gas turbine aircraft community. The numerical results compare favorably with these data from a qualitative point of view. The description of deep surge occurrence and the prediction of quantitative elements of compressor performance during surge are satisfactory provided few steady-state parameters are correctly determined. Otherwise the fully deterministic approach gives approximate, but acceptable results for a 0D or 1D model.
Multi-Disciplinary Design Tool for Axial Flow Turbines
Authors: Kenny S. C. , Gauthier J. E. D. , Huang X.
Published: September 2008

A preliminary design tool has been created to aid in the design of axial flow turbines. The design tool outputs all of the required geometry and flow conditions for the preliminary design of a single stage axial turbine. Inherent to the tool is its ability to produce performance estimates, both aerodynamically and structurally. The aerodynamic analysis is largely empirical based and makes use of the most up-to-date correlations available in the literature. The tool has been created to obtain a fast estimate of performance and a fast screening of various design variables. The design tool is also required in order to produce a geometrical input for more advanced computational fluid dynamic (CFD) and finite element method (FEM) analyses. A test case has been conducted through the design and development of two single stage turbines for a 1-MW gas turbine engine. The results of the design tool were then compared to those results obtained from extensive CFD and FEM analyses to validate the accuracy of the tool. Overall, the results showed excellent agreement both aerodynamically and structurally.
Modern Compressor Aerodynamic Blading Process Using Multi-Objective Optimization
Authors: Keskin Akin , Dutta Amit Kumar , Bestle Dieter
Published: September 2008

Aerodynamic design of an axial compressor is a challenging design task requiring a compromise between contradicting requirements like wide operating range, high efficiency, low number of stages and high surge margin. Therefore, the design process is typically subdivided into a sequence of subproblems where the blading design is a key process. According to flow conditions, which result from throughflow calculations on axis-symmetric stream surfaces, 2-dimensional blade profiles have to be designed, which then may be stacked along a radial stacking line in order to find the 3D-blade geometry. The design of the blade sections is rather time consuming due to many iterations with different programs. Usually a geometry generation tool is used to describe the blade sections which are then evaluated by a blade-to-blade CFD solver. The quality of a single blade section is typically characterized by the overall loss at design flow conditions and the working range determined by an amount of loss increase due to incidence variation. The aerodynamic performance of the final airfoils and thus of the whole compressor depends significantly on the design of the individual blade sections. In this investigation an automated multi-objective optimization strategy is developed to find best blade section geometries with respect to loss and working range. The multi-objective optimization approach provides Pareto-optimal compromise solutions at reasonable computational costs outperforming a given Rolls-Royce datum design which has been ‘optimized’ manually by a human design engineer.
Exploiting the Capabilities of Experiment and Numerical Methods
Authors: Herring Steven J.
Published: September 2008

The last 20 years has seen the development of reliable, user-friendly, RANS CFD codes which can model the steady state features of complex flows. This coupled with the diminishing cost of computing power has made the solution of large problems economical. As result of this there has been a significant reduction in the amount of testing activity undertaken. Nevertheless, high quality experiments remain essential, and the real need is to effectively exploit the strengths of the two approaches. Whilst improvements in theoretical aerodynamic methods may be leading to a decline in test activity as a whole, paradoxically, it also means that when tests are carried out there is a need for the data to be of an ever increasing quality and quantity. There are two key commercial drivers acting on test activities which are: to achieve reductions in overall test costs and to reduce the test cycle time as a part of the broader goal of reducing time to market. The paper discusses the strengths and weaknesses of experimental and numerical methods, with reference to the limitations on resolution and intrusive effect of instrumentation, and the limitations of the physical models within numerical methods. From this conclusions are drawn regarding how the two approaches complement each other. The levels of accuracy and repeatability now required in turbomachinery testing are discussed with reference to work carried out at Cranfield University. The need for a continuous improvement in both capabilities and cost effectiveness of test facilities is identified.
Flow Control of Annular Compressor Cascade by Synthetic Jets
Authors: Zheng Xinqian , Hou Anping , Li Qiushi , Zhou Sheng , Lu Yajun
Published: September 2008

An experimental investigation conducted in a stationary annular cascade wind tunnel demonstrated that unsteady flow control using synthetic (zero mass-flux) jets can effectively reduce flow separation from suction side of the blade in axial compressor cascade. The synthetic jets driven by a high-power speaker were introduced through the casing radially into the flow-field just adjacent to the leading edge of compressor cascade. The experimental results revealed that the aerodynamic performance of compressor cascade could be improved amazingly by synthetic jets and the maximum relative reduction of loss coefficient was up to 27.5%. The optimal analysis of the excitation frequency, excitation location was systematically investigated at different incidences. In order to obtain detail information on flow-field structure, DPIV technique was adopted. The experimental results showed that the intensity of wake vortices became much weaker and streamlines became smoother and more uniform by synthetic jets.
Unsteady Aerodynamic Blade Excitation at the Stability Limit and During Rotating Stall in an Axial Compressor
Authors: Mailach Ronald , Vogeler Konrad
Published: September 2008

The stable operating range of axial compressors is limited by the onset of rotating stall and surge. These flow conditions endanger the reliability of operation and have definitely to be avoided in compressors of gas turbines. However, there is still a need to improve the physical understanding of these flow phenomena to prevent them while utilizing the maximum available working potential of the compressor. This paper discusses detailed experimental investigations of the rotating stall onset with the main emphasis on the aerodynamic blade excitation in the Dresden four-stage Low-Speed Research Compressor. The stall inception, which is triggered by modal waves, as well as the main flow features during rotating stall operation are discussed. To investigate the unsteady pressure distributions, both the rotor and the stator blades of the first stage were equipped with piezoresistive pressure transducers. Based on these measurements the unsteady blade pressure forces are calculated. Time-resolved results at the stability limit as well as during rotating stall are presented. For all operating conditions rotor-stator-interactions play an important role on the blade force excitation. Furthermore the role of the inertia driven momentum exchange at the stall cell boundaries on the aerodynamic blade force excitation is pointed out.
Numerical Investigation of the Effect of Roughness and Passing Wakes on LP Turbine Blades Performance
Authors: Pacciani Roberto , Spano Ennio
Published: September 2008

A quasi-three-dimensional, blade-to-blade, time-accurate, viscous solver has been used to study the effects of blade surface roughness elements, and incoming bar wakes on blade loading and losses of modern LP blade cascades at low Reynolds number. The T106 and VK1-T2 cascades, recently tested experimentally at the Von Karman Institute, were selected as test cases. A simplified geometrical model was adopted for the roughness element, in order to incorporate it in quasi-3D analyses using a single grid structure. Good agreement with experimental data in terms of blade loading distributions and losses was found using a low-Reynolds number formulation of Menter’s SST, k-ω model. The blade performance improvements, expected in the presence of roughness elements or incoming wakes, were correctly predicted by the calculations. The basic flow mechanisms leading to such improvements was recognized in the suction side separation reduction, which was satisfactorily reproduced in the numerical analisys.
About the Numerical Simulation of Rotating Stall Mechanisms in Axial Compressors
Authors: Gourdain N. , Burguburu S. , Michon G. J. , Ouayahya N. , Leboeuf F. , Plot S.
Published: September 2008

This paper deals with the first instability which occurs in compressors, close to the maximum of pressure rise, called rotating stall. A numerical simulation of these flow phenomena is performed and a comparison with experimental data is made. The configuration used for the simulation is an axial single-stage and low speed compressor (compressor CME2, LEMFI). The whole stage is modeled with a full 3D approach and tip clearance is taken into account. The numerical simulation shows that at least two different mechanisms are involved in the stall inception. The first one leads to a rotating stall with 10 cells and the second one leads to a configuration with only 3 cells. Unsteady signals from the computation are analyzed thanks to a time-frequency spectral analysis. An original model is proposed, in order to predict the spatial and the temporal modes which are the results of the interaction between stall cells and the compressor stage. A comparison with measurements shows that the computed stall inception point corresponds to the experimental limit of stability. The performance of the compressor during rotating stall is also well predicted by the simulation.
Experimental Investigations of Clocking in a One and a Half Stage Transonic Turbine Using Laser-Doppler-Velocimetry and a Fast Response Aerodynamic Pressure Probe
Authors: Schennach O. , Woisetschla¨ger J. , Fuchs A. , Go¨ttlich E. , Marn A. , Pecnik R.
Published: September 2008

The current paper presents experimental clocking investigations of the flow field in midspan in a high-pressure transonic turbine with a downstream vane row (1.5 stage machine). Laser-Doppler-Velocimetry measurements were carried out in order to record rotor phase resolved velocity, flow angle and turbulence distributions upstream and downstream of the second vane row at several different vane-vane positions. Additionally, a fast response aerodynamic pressure probe was used to get the total pressure distribution downstream of the second vane row for the same positions. Altogether, the measurements were performed for ten different 1st vane to 2nd vane positions (clocking positions) for measurements downstream of the 2nd vane row and two different clocking positions for measurements upstream of the 2nd vane row. The paper shows that different clocking positions have a significant influence on the flow field downstream of the 2nd vane row. Furthermore different measurement lines upstream of the 2nd vane row indicate that clocking has nearly no influence on the flow field close to the rotor exit.
Turbine Blade Duty Re-Design by Controlling Lean and Sweep Using an Innovative Iterative Inverse Design Method
Authors: Pa´scoa Jose´ C. , Mendes Anto´nio C.
Published: September 2008

Inverse methods able to work in the transonic regime have for a long time been the weak part in the inverse method universe, albeit most of current gas turbines work in this regime. The present iterative inverse method is based on a Finite Volume time-marching scheme that is able to accurately compute the flowfield inside turbomachinery passages. In our design method we prescribe the acrodynamic load and blade thickness. Imposition of these variables precludes any existence and uniqueness problems and enables us to incorporate information regarding thermal and mechanical stresses in the first design stages. The method herein presented starts with an analysis of the flow in an initial geometry that is afterward adjusted by a modification in camber line. A new time-lagged formulation for the camber line generator will be presented. In order to design in three-dimensions a flexible stacking line generator will be introduced, as a mean to independently control sweep and lean for the blades. The results presented illustrate how an annular turbine cascade can be re-designed, with the present method, for better blade performance.
Multilevel Hierarchical Decomposition: Identification Procedures for Edge of Design Space Feasibility
Authors: Nagendra Somanath , Midgley Jeff , Staubach Joseph B.
Published: September 2008

In high performance machines, multiple active MDO constraints dictate the edge of feasibility, i.e. boundary of the design space. It is essential to have an accurate description of the boundary in terms of design variables. Given a sample of data, the recognition of a design feature (e.g. design shape) is not usually familiar to the design domain experts but must be extracted based on data-driven procedures. The “edge of feasibility” could be evaluated as a continuous or piece wise continuous function of active constraints. In this work, the focus is on a class of quasiseparable optimization problems. The subsystems for these problems involve local design shape variables and global system variables, but no variables from other subsystems. The system in this particular case is the engine component (i.e. HPT) and the subsystem is the turbine disk. The system is hierarchically decomposed to the system and subsystem components respectively. The HPT flowpath and its defined thermodynamic and geometric parameters define the system. The subsystem is the HPT turbine disk and its associated geometric shape variables. A system level DOE determines the design space of the HPT system. The optimized subsystem turbine disk is the solution to the DOE of the system and feasible disk designs are the shapes that can withstand the design loads and stresses. The focus of the paper is to develop a methodology that would systematically utilize minimum weight optimum shape designs across the design space and predict new designs close to being optimal in performance for a specified range of design conditions. The shape of minimum weight disks are identified as a solution of a system of inverse response surface equations that can determine disk shapes with good confidence. The methodology is developed using synthetic turbine disk problems with known regions of feasibility and infeasibility. The edge of feasibility is determined and the functional dependence on the design variables estimated.