bluuebunny/crossref_metadata_2025 huggingface dataset:
Actuation Timing Strategies for a Portable Powered Ankle Foot Orthosis
Authors: Morris Emily A. , Shorter K. Alex , Li Yifan , Hsiao-Wecksler Elizabeth T. , Kogler Geza F. , Bretl Timothy , Durfee William K.
Published: May 2012

Ankle-foot orthoses (AFOs) are used to assist persons with lower-limb neuromuscular impairments. We have developed the portable powered AFO (PPAFO). This device uses a bidirectional pneumatic actuator powered by a CO2 bottle to provide dorsiflexor and plantarflexor torque assistance. The PPAFO operates tether-free, allowing for use outside of the laboratory. This system has been tested on one impaired and multiple healthy subjects. Timing of the assistance provided by the PPAFO has been determined by: 1) direct event detection using sensor feedback with threshold triggers, and 2) state estimation in which gait events are estimated using a cross-correlation based algorithm. Direct event detection, while simple to implement, can be unreliable for subjects with certain gait impairments. State estimation, while more complicated to implement, provides access to state information that cannot be directly measured by the AFO, which allows for greater flexibility in assistance timing. Current hardware limitations and future work are also discussed.
Modeling the Negative Feedback Mechanism in the Enzyme Carboxyltransferase
Authors: Cai Xiaoyu , de Queiroz Marcio , Meades Glen , Waldrop Grover
Published: May 2012

The enzyme acetyl-CoA carboxylase catalyzes the first committed step in fatty acid synthesis in all organisms. The E. coli form of the carboxyltransferase subunit was recently found to regulate its own activity and expression by binding its own mRNA. By binding acetyl-CoA or the mRNA encoding its own subunits, Carboxyltransferase is able to sense the metabolic state of the cell and attenuate its own translation and enzymatic activity using a negative feedback mechanism. In this paper, this network of interactions is modeled mathematically using mass action kinetics. Numerical simulations of the model show agreement with experimental results.
Model Based Fault Detection of Freeway Traffic Sensors
Authors: Dervisoglu Gunes , Horowitz Roberto
Published: May 2012

This paper presents a model based fault detection and exclusion scheme that implements a decision logic to automatically identify faulty or mislocated freeway traffic sensors in the presence of unknown on-ramp and off-ramp flows. The algorithm is deployed within the framework of a suite of software tools, named TOPl, which models traffic flow via a macroscopic model, calibrates the model based on available data and runs simulations to evaluate various operational strategies such as ramp metering, demand management, incident management, etc. TOPl has been used to model various freeways in California, such as Interstate 80, Interstate 210, Interstate 880 and Interstate 680. Two main difficulties with data collection on California freeways were found to be missing ramp flow and faulty mainline data, which decrease the accuracy of the model and increase the time and effort invested in model calibration. The former of these difficulties has been previously addressed by an iterative learning algorithm that estimates the missing ramp flows and the latter is tackled by the method presented in this work.
Iterative Learning Based Driver Input Synthesis for Evaluating Transient Diesel Soot Emissions
Authors: Ahlawat Rahul , Fathy Hosam K. , Stein Jeffrey L.
Published: May 2012

This paper presents the synthesis of driver inputs for evaluating diesel soot emissions using iterative learning control. Transient soot emissions from diesel engine vehicles are extremely sensitive to driver aggressiveness. Using closed-loop tracking controllers to follow a vehicle over a prescribed drive cycle usually do not account for the fact that drivers potentially adapt their driving styles to a given powertrain design. This work develops an algorithm producing driver input traces that significantly reduces the soot emissions for a given drive cycle, thus providing a consistent basis for evaluating the influence of powertrain design changes on soot emissions. Possible improvements are first explored using conventional optimal techniques and results are obtained using linear programming. It is then shown that a first-order PD-type iterative learning control based algorithm can deliver good performance, substantially reducing the total soot emissions at a fraction of the computational cost.
A Lyapunov Approach for SOSM Based Velocity Estimation and Its Application to Improve Bilateral Teleoperation Performance
Authors: Chawda Vinay , O’Malley Marcia K.
Published: May 2012

In many mechatronic applications, velocity estimation is required for implementation of closed loop control. Proportional-Integral control based differentiation has been proposed to estimate velocity in bilateral teleoperation. We propose a Second Order Sliding Mode (SOSM) based velocity estimation scheme for this application, since the SOSM approach is robust to small disturbances near the origin. Simulation results demonstrate the superior performance of the SOSM based velocity estimation over the PI-control approach for bilateral teleoperation in viscous environments. Additionally, a novel Lyapunov function based approach to stability analysis of the SOSM based differentiator is presented.
Active Tremor Estimation and Suppression in Human Elbow Joint
Authors: Taheri Behzad , Case David , Richer Edmond
Published: May 2012

A new control algorithm was developed for tremor estimation and suppression in a second order linear model of the human elbow joint. An adaptive method developed to estimate a simple harmonic disturbance was generalized for tremorous motion with spectral composition similar to the clinical reports for action tremor. Numerical simulations showed the ability of proposed controller to reduce tremor amplitude without generating significant resistance against voluntary motion of the arm. The designed algorithm can be used in an upper-limb orthosis to suppress debilitating tremorous motion of the arm.
Linear Parameter Varying Control of a Robot Manipulator for Aortic Valve Implantation
Authors: Ramezanifar A. , Salimi A. , Mohammadpour J. , Kilicarslan A. , Grigoriadis K. , Tsekos N. V.
Published: May 2012

In this paper, we propose a linear parameter varying (LPV) control design approach for trajectory tracking in a robotic system, intended to be involved in an image-guided teleoperated cardiac surgery. The robot is eventually aimed to guide a 3 degree-of-freedom medical tool (a catheter) inside the left ventricle (LV) and achieve the implantation of a prosthetic aortic valve. The successful delivery of the valve from the apical entrance to the aortic annulus strongly depends on the precise navigation of the catheter such that its probable collision with the LV’s changing environment is avoided. The LPV control strategy is utilized here due to its ability to capture the nonlinearities of the designed robot manipulator and adapt in real-time based on the varying end effector’s angle. The simulation studies demonstrate promising results achieved for a guaranteed safe navigation through LV.
State of Charge Estimation Error due to Parameter Mismatch in a Generalized Explicit Lithium Ion Battery Model
Authors: Lin Xinfan , Stefanopoulou Anna , Laskowsky Patricia , Freudenberg Jim , Li Yonghua , Anderson R. Dyche
Published: May 2012

Model-based state of charge (SOC) estimation with output feedback of the voltage error is steadily augmenting more traditional coulomb counting or voltage inversion techniques in hybrid electric vehicle applications. In this paper, the state (SOC) estimation error in the presence of model parameter mismatch is calculated for a general lithium ion battery model with linear diffusion or impedance-based state dynamics and nonlinear output voltage equations. The estimation error due to initial conditions and inputs is derived for linearized battery models and also verified by nonlinear simulations. It is shown that in some cases of parameter mismatch, the state, e.g. SOC, estimation error will be significant while the voltage estimation error is negligible.
Synthesis of Digital PID Controllers for Discrete-Time Systems With Guaranteed Non-Overshooting Transient Response
Authors: Mohsenizadeh Navid , Darbha Swaroop , Bhattacharyya Shankar P.
Published: May 2012

In this paper, we present a new method of synthesizing digital PID controllers for discrete-time, Linear Time Invariant (LTI) Systems satisfying a class of transient response specifications. The problem of synthesizing a controller to achieve desirable transient specifications, such as requiring the transient response to be within an allowable range of overshoot, can be carried out as a problem of guaranteeing the impulse response of an appropriate closed loop error transfer function to be non-negative. An earlier result by the authors provides necessary and sufficient conditions for the impulse response of a discrete-time transfer function to be non-negative in terms of the requirement of a sequence of polynomials to be sign-invariant on the interval [1, ∞). An application of this result to the error transfer function yields a sequence of polynomials which are required to be sign-invariant on [1, ∞) but whose coefficients are polynomial functions of the controller gains k1, k2 and k3.
A Controller Framework for Autonomous Drifting: Design, Stability, and Experimental Validation
Authors: Hindiyeh Rami Y. , Gerdes J. Christian
Published: May 2012

This paper presents a controller framework for autonomous drifting of a rear wheel drive vehicle. The controller uses a successive loop structure, where yaw rate is treated as a synthetic input to control the vehicle’s sideslip dynamics, and yaw rate is in turn controlled through coordination of steering and rear drive force. Relative to prior designs, the drift controller presented in this work enables a straightforward, physically insightful stability analysis where local closed-loop stability of the desired high sideslip “drift equilibrium” is demonstrated. When implemented on a by-wire testbed, the new controller achieves experimental performance that matches or exceeds prior designs, generating sustained and robust autonomous drifts.
Selection Criteria of Spring Stiffness for Nozzle Type Check Valves in Compressor Station Applications
Authors: Botros K. K.
Published: May 2012

Nozzle type check valves are often employed in compressor stations in three locations: compressor outlet, station discharge and station by-pass. The fundamental design concept of these valves is based on creating a converging diverging flow through the valve internal geometry such that a minimum area is achieved at a location corresponding to the back of the check valve disc at fully open position. This will ensure maximum hydrodynamic force coefficient which allows the valve to be fully open with minimum flow. Spring forces and stiffness determine the performance of this type of check valves and impact the overall operation and integrity of compressor station. This paper examines the effects of various spring characteristics and stiffness in relation to the compressor and station flow characteristics. The results show that when the spring forces are higher than the maximum hydrodynamic force at minimum flow, the disc will not be at fully open position, which will give rise to disc fluttering and potential for cyclic high velocity impact between components of the internal valve assembly. This could lead to self destruction of the check valve and subsequent risk of damage to the compressor unit itself. The paper also points to the fact that the spring selection criteria for a unit check valve are different than that for station and bypass check valves. An example of a case study with actual field data from a high pressure ratio compressor station employing this type of check valves is presented to illustrate the associated dynamic phenomena and fluid-structure interaction within the internal assembly of the check valve.
Sound Deadening on Fans
Authors: Karstadt Sascha , Matyschok Berthold , Pelz Peter F.
Published: May 2012

In addition to the objective of increasing the efficiency of fans, acoustic efficiency gains more and more importance in order to reduce fan noise exposure. Whereas previously, research in this field was focused basically on blade design, nowadays the peripheral devices of fans are more often the object of study, since technical understanding of noise development is comparatively incomplete. Therefore, fundamental studies are essential to gain insight in the patterns of noise development and relatively easy opportunities to reduce noise level. In order to combine good aerodynamic properties with a silent fan, this experimental research investigates the acoustic and aerodynamic characteristics of an axial turbomachine and possibilities of reducing the emitted sound. Therefore a sound absorber ring is built directly around the rotor of a fan in order to absorb the sound very close to the origin. The fan assembly is installed in a test rig according to ISO 5136, which defines a standard for determining sound power radiated into a duct by fans. Acoustic signals are recorded with two microphones in the test duct, one on the pressure side, the second on the suction side, each close to anechoic terminations at the ends. The aerodynamic characteristics are determined with a calibrated inlet nozzle and static pressure measurements over the fan stage. To confirm the expectation that a significant part of the emitted sound power is tip clearance noise, which changes with the operating point, the volume flow of the fan is varied over its entire operating range. In this study, five different porous materials are tested for their ability of deadening the sound of the fan. In order to measure the influence of the construction which contains the materials, the perforated casing ring with a sound-reflecting termination and a plane ring with the same tip clearance are measured additionally as a reference. The noise exposure is analyzed over the complete frequency spectrum in order to determine the absorbing frequencies of the materials.
A Dual Flow High Mach Number Centrifugal Compressor for a Small Gas Turbine APU
Authors: Rodgers Colin , Brown Dan
Published: May 2012

Small gas turbine auxiliary power units (APU’s) of conventional load compressor type wherein the gas generator or core module directly drives a separate centrifugal load compressor are installed in aircraft and helicopters to supply both compressor air for main engine starting and air conditioning combined with shaft power to drive an electric generator. This paper describes the test development of a dual flow centrifugal compressor (DFC) where the impeller flow was split into two streams, the inner (hub) stream supplying compressed air to the gas generator core module, and the outer (DFB) bleed stream delivering a compressed air to the aircraft pneumatic power system. DFC development rig testing revealed that the hub or core stream satisfied compressor design requirements but that the DFB stream flowpath demonstrated unstable characteristics with decreasing efficiency as test speeds were increased. At the time of the development program in the early 1990’s convergence difficulties were encountered with CFD attempts to corroborate the test results, and thus pinpoint plausible explanations, as a consequence a renewed upgraded 2010 CFD analysis of the dual flow compressor is presented herein confirming the test performance characteristics of both flow streams and the fundamental reason for poor DFB performance as excessive diffusion at high relative Mach numbers.
Fouling Mechanisms in Axial Compressors
Authors: Kurz Rainer , Brun Klaus
Published: May 2012

Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Fouling is caused by the adherence of particles to airfoils and annulus surfaces. Particles that cause fouling are typically smaller than 2 to 10 microns. Smoke, oil mists, carbon, and sea salts are common examples. Fouling can be controlled by appropriate air filtration systems, and can often be reversed to some degree by detergent washing of components. The adherence of particles is impacted by oil or water mists. The result is a build-up of material that causes increased surface roughness and to some degree changes the shape of the airfoil (if the material build up forms thicker layers of deposits). Fouling mechanisms are evaluated based on observed data, and a discussion on fouling susceptibility is provided. A particular emphasis will be on the capabilities of modern air filtration systems.
Comparative Performance of a Thermal Barrier Coating System Utilizing Platinum Aluminide Bond Coat on Alloys CMSX-4® and MAR M® 002DS
Authors: Tawancy H. M. , Al-Hadhrami Luai M.
Published: May 2012

It is known that the relative performance of thermal barrier coatings is largely dependent upon the oxidation properties of the bond coat utilized in the system. Also, the oxidation properties of diffusion-type bond coats (aluminides and their modifications) are functions of the superalloy substrate used in blade applications. Therefore, the performance of a given coating system utilizing a diffusion-type bond coat can significantly vary from one superalloy to another. Toward the objective of developing coating systems with more universal applicability, it is essential to understand the mechanisms by which the superalloy substrate can influence the coating performance. In this study, we examined the relative performance of yttria-stabilized zirconia/platinum aluminide coating system on alloys CMSX-4 and MAR M 002DS representing single-crystal and directionally-solidified alloy systems respectively using thermal exposure tests at 1150°C with a 24-hour cycling period to room temperature. Changes in coating microstructure were characterized by various electron-optical techniques. Experiment showed that the coating system on alloy MAR M 002DS had outperformed that on alloy CMSX-4, which could be related to the high thermal stability of the bond coat on alloy MAR M 002DS. From a detailed microstructural characterization, this difference in behavior could be explained at least partially in terms of variation in chemical composition of the two alloys, which was also reflected on the exact failure mechanism of the coating system.
Removals for Cause: A 35-Year Assessment of LM2500 Engine Removals by the United States Navy
Authors: Driscoll Matthew J. , McFetridge Eric M. , Patterson Jeffrey S. , See Craig A.
Published: May 2012

The United States (US) Navy has operated the General Electric LM2500 gas turbine on all its surface combatants for the past 35 years. The LM2500 is utilized as the propulsion engine aboard the US Navy’s newest surface combatants including the FFG 7, CG 47 and DDG 51 Class ships. The US Navy owns and operates 400 LM2500 engines. An on-condition maintenance philosophy is employed whereby engines are run-to-failure rather than removed from service upon achieving some operating milestone. This paper assesses the reasons for the removal of the US Navy’s LM2500s over their entire service life with a focus on how fleet maintenance capabilities have impacted and affected the cause for engine replacements over time.
Study on Vibration Characteristics of Single Crystal Blade and Directionally Solidified Blade
Authors: Kaneko Yasutomo
Published: May 2012

In modern gas turbines, SC (Single Crystal) and DS (Directionally Solidified) nickel alloys are applied which, compared to CC (Conventionally Casting) alloys, hold a higher cyclic life and a significantly improved creep rupture strength. Because SC and DS alloys feature a significant directionally dependence of material properties, the vibration analysis of the SC and DS blade has to be carried out, taking account of the anisotropy of material properties. In the vibration analysis by FEA (Finite Element Analysis), the DS blade has to be modeled approximately as transverse orthotropic material, while the SC blade can be modeled exactly as orthotropic material in lattice directions. In order to design the SC and DS blade with high reliability, it is necessary to establish the analysis model and to clarify the influence of the anisotropy of the material properties on the vibration characteristics of the blade. In this paper, first, the effect of the anisotropy of elastic constants on the vibration characteristics of the SC and DS blade is investigated. Second, the validity of the assumption of the transverse isotropy for the DS blade, which is applied in the current blade design, is examined by Monte Carlo simulation. Finally, the frequency deviation of the SC and DS blade is analyzed by the sensitivity analysis method, and is compared to that of the CC blade.
Aeroelasticity at Reversed Flow Conditions: Part 2—Application to Compressor Surge
Authors: Schoenenborn Harald , Breuer Thomas
Published: May 2012

The prediction of blade loads during surge is still a challenging task. In literature the blade loading during surge is often referred to as “surge load”, which suggests that there is a single source of blade loading. In the second part of the paper it is shown that the “surge load” in reality may consist of two physically different mechanisms: the pressure shock when the pressure breaks down and aeroelastic excitation (flutter) during the blow-down phase in certain cases. This leads to a new understanding of blade loading during surge. The front block of a multistage compressor is investigated. For some points of the backflow characteristic the quasi steady-state flow conditions are calculated using a RANS-solver. The flow enters at the last blade row, goes backwards through the compressor and leaves the compressor in front of the inlet guide vane. The results show a very complex flow field characterized by large recirculation regions on the suction sides of the airfoils and stagnation regions close to the trailing edges of the airfoils. Based on these steady solutions unsteady calculations are performed with a linearized aeroelasticity code. It can be shown that some of the rotor stages are aerodynamically unstable in the first torsional mode. Thus, in addition to the pressure shock the blades may be excited by flutter during the surge blow-down phase. In spite of the short blow-down phase typical for aero-engine high pressure compressors, this may lead to very high blade stresses due to high aeroelastic excitation at these special flow conditions. The analytical results compare very well with the observations during rig testing. The correct nodal diameter of the blade vibration is reproduced and the growth rate of the blade vibration is predicted quite well, as a comparison with tip-timing measurements shows. A new flutter region in the compressor map was detected experimentally and analytically.
The ORegen™ Waste Heat Recovery Cycle: Reducing the CO2 Footprint by Means of Overall Cycle Efficiency Improvement
Authors: Del Turco Paolo , Asti Antonio , Scotti Del Greco Alberto , Bacci Alessandro , Landi Giacomo , Seghi Giacomo
Published: May 2012

The growing concern for the role of man-made CO2 emissions with respect to global warming combined with the large increase in energy demand spurred by developing nations and a growing global population that is foreseen over the next 15 years have recently turned attention to potential CO2-neutral energy supply solutions. Waste heat recovery cycles applied to fossil fueled plants offer a local zero-emission solution to producing additional electric energy, thereby increasing the overall plant efficiency with a considerable reduction in the emission of CO2 per unit of energy produced. GE Oil & Gas with GE Global Research Europe has developed a new and attractive solution for recovering waste heat energy from a variety of thermal sources ranging from reciprocating combustion engines to gas turbines. This new recovery cycle is called ORegen™. The ORegen™ recovery cycle is a rankine cycle, with superheating, that recovers waste heat and converts it into electric energy by means of a double closed loop system. The ORegen™ system represents one of the very few viable solutions for recovering heat from sources (such as mechanical drive gas turbines) whose load may vary dramatically over time or where the equipment is located at a site where water is not readily available. For the temperature range of interest, a thorough comparison between many working fluids was performed, leading to the conclusion that the substance that delivers the highest efficiency is Cyclopentane. A high-efficiency Rankine cycle based on such a working fluid places a particularly high demand on the expansion ratio, which influences some of the basic architectural choices of the expander machine. This article introduces the ORegen™ recovery cycle and describes the process used in GE Oil & Gas to design the family of double supersonic stage turboexpanders, covering the power range of 2–17MW. Examples of the application of the ORegen™ cycle to gas turbine are also provided to demonstrate attractive opportunities to increase the overall plant efficiency.
Film-Cooled Trailing Edge Measurements: 3D Velocity and Scalar Field
Authors: Benson Michael , Laskowski Gregory , Elkins Chris , Eaton John K.
Published: May 2012

Aircraft turbine blade trailing edges commonly are cooled by blowing air through pressure-side cutback slots. The surface effectiveness is governed by the rate of mixing of the coolant with the mainstream, which is typically much faster than predicted by CFD models. 3D velocity and coolant concentration fields were measured in and around a cutback slot using a simple uncambered airfoil with a realistic trailing edge cooling geometry at a Reynolds number of 110,000 based on airfoil chord length, which is lower than practical engines but still in the turbulent regime. The results were obtained using magnetic resonance imaging (MRI) techniques in a water flow apparatus. Magnetic resonance concentration (MRC) scans measured the concentration distribution with a spatial resolution of 0.5 mm3 (compared to a slot height of 5 mm) and an uncertainty near 5%. Magnetic resonance velocimetry (MRV) was used to acquire 3D, three-component mean velocity measurements with a resolution of 1.0 mm3. Coupled concentration and velocity measurements were used to identify flow structures contributing to the rapid mixing, including longitudinal vortices and separation bubbles. Velocity measurements at several locations were compared with an unsteady RANS model. Concentration measurements extrapolated to the surface provided film cooling effectiveness and showed that the longitudinal vortices decreased effectiveness near the lands and reduced the average film cooling effectiveness.
The Role of Streamwise Vorticity in Flows Over Turbomachine Blade Suction Surfaces
Authors: Gostelow J. P. , McMullan W. A. , Walker G. J. , Mahallati A.
Published: May 2012

Streamwise streaks and vortices are frequently encountered in low Reynolds number flows over blading. Observations have shown that, in addition to flows over concave pressure surfaces, convex suction surfaces are also influenced by streamwise vortices. These observations are based on surface flow visualization studies and computational work with highly resolved Large Eddy Simulation. Fine scale organized streaks exist in the laminar regions of turbine and compressor blading and are predictable. For a turbine blade with a blunt leading edge, at Reynolds numbers typical of aircraft cruise conditions, the streamwise vorticity may persist, on a time-average basis, to influence the entire suction surface. Time resolution is required to capture the flow complexity that is fundamental for an understanding of the physical behavior of the laminar boundary layer and its separation and transition. Progress has been made in modeling and predicting transition and laminar separation and the new findings of interesting vortical behavior need to be incorporated. In the leading edge region spanwise vorticity may promote early transition and bubble closure; further downstream streamwise vorticity may become established. The physics of this streamwise vorticity imposes severe requirements on the temporal and spatial resolution of both experimental and computational methods. A narrow spanwise computational strip does not allow the streamwise vorticity to settle into an organized pattern; if it is to become organized, an adequate spanwise domain is required.
Aerodynamic Optimisation of a Centrifugal Compressor Return Channel and U-Turn With Genetic Algorithms
Authors: Hildebrandt A.
Published: May 2012

This paper presents the numerical analysis and aerodynamic optimisation of a return channel system and U-turn for multi stage single shaft centrifugal compressor machinery. An optimisation program is used, based on an OpenSource 3D viscous Navier Stokes Solver and an OpenSource evolutionary optimisation algorithm. The coupling between the Navier Stokes flow solver and the evolutionary algorithm is direct, without any meta-model such as ANN (Artificial Neural Network) or a Kriging model. Two different U-turn geometries — based on two different flow inlet profiles — are optimised, analysed and finally coupled to a return channel blade geometry, which is optimised itself. The optimisation of the return channel blade has been set up in two different ways, once with parallel meridional hub and shroud walls and once with a non-parallel optimised meridional shroud contour line. Results show a reduction of total pressure loss by 3% for the optimised blade shapes compared to a standardised blade shape of constant change of tangential momentum. A significant reduction in pressure loss coefficient from 0.825 to 0.627 could be achieved by the optimisation of the meridional contour lines of the return channel. Furthermore, the effect of non-sufficient cross -sectional area at the return channel trailing edge on the overall performance is presented.
Investigation of Turbulent Models for the Flow Field From a Typical Strut-Based Scramjet Combustor
Authors: Luo Shi-bin , Huang Wei , Qin Hui , Wang Zhen-guo , Liu Jun , Xia Zhi-xun , Lei Jing , Pourkashanian Mohamed , Ma Lin , Ingham Derek B. , Luo Wen-lei
Published: May 2012

The two-dimensional coupled implicit RANS equations and three turbulent models have been employed to numerically simulate the nonreacting and reacting flow fields of a typical strut-based scramjet combustor, and the numerical results have been compared with the experimental data. At the same time, three different grid scales have been used to test the grid independence in the numerical simulations, namely the small scale (81,590 nodes), the moderate scale (98,510 nodes) and the large scale (147,470 nodes). The obtained results show that the RNG k-ε model is more suitable to numerically simulate the flow field in the scramjet combustor than the realizable k-ε model and the SST k-ω model, and the numerical results obtained by the moderate and large grid scales show reasonably better agreement with the experimental data. The quasi-diamond wave system is formed in both the nonreacting and reacting flow fields. In the reacting flow field, there are two clear strong shear layers generated between the fuel injection and the supersonic freestream, and at the intersection point between the shear layer and the reflected shock wave, the reaction zone is broader than anywhere else. In the corner formed between the upper surface of the strut and the shear layer, an expansion wave is clearly generated, and another also exists in the symmetrical corner.
Modeling of Soot and NOx in a Full Scale Turbine Engine Combustor With Detailed Chemistry
Authors: Blacha T. , Di Domenico M. , Rachner M. , Gerlinger P. , Aigner M.
Published: May 2012

A numerical simulation of spray atomization, combustion and soot formation in a full scale turbine engine combustor is presented and discussed in this work. Turbulence has been captured by a two equation turbulence model, chemistry by a detailed kinetic mechanism and turbulence chemistry interactions have been accounted for by an APDF-approach (assumed probability density function approach). For the kerosene fuel a surrogate has been used, consisting of n-decane, isooctane and toluene. The injection of the liquid fuel spray has been included by coupling the gas field CFD code with a spray code. Apart from the n-decane and isooctane reaction paths, the chemical kinetic reaction mechanism accounts for species as heavy as toluene. Polycyclic aromatic hydrocarbons heavier than toluene are represented by a sectional approach, while soot is calculated via soot volume fraction and particle number density. Main flow features are investigated and good agreement with the experimental measurements can be reported.
Experimental and Numerical Study of NOX Formation From the Lean Premixed Combustion of CH4 Mixed With CO2 and N2
Authors: Fackler K. Boyd , Karalus Megan F. , Novosselov Igor V. , Kramlich John C. , Malte Phillip C.
Published: May 2012

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NOX) from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet stirred reactor (JSR). In this study, CH4 is mixed with varying levels CO2 and N2. NOX measurements are taken at a nominal combustion temperature of 1800 K, atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for NOX emissions as a function of fuel dilution: (1) more NOX is produced per kg of CH4 consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N2 dilution increases NOX production more effectively than equivalent CO2 dilution. Chemical kinetic modelling suggests that NOX production is less effective for the mixture diluted with CO2 due to both a decrease in N2 concentration and the ability of CO2 to deplete the radicals taking part in NOX formation chemistry. In order to gain insight on flame structure within the JSR, three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH4 combustion. A global CH4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH4 and CO oxidation quite well. The CFD model illustrates the flow-field, temperature variation, and flame structure within the JSR. A 3-element chemical reactor network (CRN), including detailed chemistry, is constructed using insight from detailed spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NOX emissions for all fuel blends. The experimental and modelling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.
A Method to Evaluate the Impact of Power Demand on HPT Blade Creep Life
Authors: Mohamed W. , Eshati S. , Pilidis P. , Ogaji S. , Laskaridis P. , Nasir A.
Published: May 2012

Peak load operation requires gas turbines to operate at high firing temperature with consequence reduction in the useful lives of components. This paper studies the quantitative relationship between gas turbine power setting and the hot gas-path components’ life consumption. A 165MW gas turbine engine is modelled and investigated in this study. A comparative lifing model, which performs stress and thermal analyses, estimates the minimum creep life of components using the parametric Larson Miller method. This lifing model was integrated with in-house performance simulation software to simulate the engine performances at design point and off-design conditions. The results showed that the combined effect of the operating environment and the power demand could have significant impact on blade creep life. Predicting this impact will aid gas turbine users in the decision making processes associated with gas turbine operation.
The Effects of Radially Distorted Incident Flow on the Performance of Axial-Flow Fans With Forward-Skewed Blades
Authors: Yang B. , Gu Ch. G.
Published: May 2012

In this study, the experiments and simulations have been carried out to evaluate the effects of radially distorted incident flow on the performance of axial-flow fans equipped with forward-skewed blades. These fans were manufactured with different forward-skewed angle, such as 3°, 6° and 12° respectively. Based on the experimental and numerical results, it is found that under the uniform inlet condition the fan with the 12° forward-skewed angle manifests the most excellent performance, and its peak efficiency is increased by 6.27% and 3.41% respectively in comparison with those of two other fans. However, when a hub-covered screen is mounted and then the uniform inlet condition is consequently destroyed, the peak efficiency of all three fans is decreased by more than 2.8%. Under the third inlet condition, as the main stream is compelled towards blade hub by a tip-covered screen, the effect of forward-skewed blades on the fan performance is reinforced and fan performance is improved. In order to get further understanding, contours of the loss coefficient and the axial velocity at exit have also been studied both at the design point and the lower mass rate point. All results exhibit that there are beneficial effects of forward-skewed blades on fan performance and the performance is quite sensitive to radially distorted incident flow. Furthermore, rotating stall has been observed by use of three dynamic pressure sensors. The instantaneous results reveal that when the fans are operated with different kinds of incident flow, pressure fluctuations with time have different modes as the fans are sliding down into rotating stall.
Effects of Nonuniform Reactant Stoichiometry on Combustion Instability
Authors: Kim Kyu Tae , Kashinath Karthik , Hochgreb Simone
Published: May 2012

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to acoustic forcing in a laboratory-scale stratified burner. The double-swirler, double-channel annular burner was specially designed to generate acoustic velocity oscillations and radial fuel stratification at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the flame response are not considered. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a hot wire anemometer and photomultiplier tubes with narrowband OH*/CH* interference filters. Time-averaged CH* chemiluminescence intensities were measured using an intensified CCD camera. Results show that flame stabilization mechanisms vary depending on stratification ratio for a constant global equivalence ratio. For a uniformly premixed condition, an enveloped M-shaped flame is observed. For stratified conditions, however, a dihedral V-flame and a detached flame are developed for outer stream and inner stream fuel enrichment cases, respectively. Flame transfer function (FTF) measurement results indicate that a V-shaped flame tends to damp incident flow oscillations, while a detached flame acts as a strong amplifier relative to the uniformly premixed condition. The phase difference of FTF increases in the presence of stratification. More importantly, the dynamic characteristics obtained from the forced stratified flame measurements are well correlated with unsteady flame behavior under limit-cycle pressure oscillations. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, which has not been well explored to date.
Design and Validation of a New Test Rig for Brush Seal Testing Under Engine Relevant Conditions
Authors: Pfefferle D. , Dullenkopf K. , Bauer H.-J.
Published: May 2012

Brush seals play an increasing role in turbomachinery due to their improved behavior towards leakage and their capability to compensate for gap variations caused by thermal expansion and rotor excursions. The flexible bristles of brush seals are able to endure short-term reductions in gap width without severe damage. Consequently the necessary gap between the rotor and brush seal can virtually be reduced to zero, leading to a considerable reduction in air leakage of up to 80 percent. However the reduced gap height increases the probability of rubbing between the bristle package and the rotor surface. The friction forces generated can cause an unwanted heat load on the rotor, bristles and leakage air. In addition, the surfaces involved are exposed to abrasion effects. Especially in the thin and lightweight rotor structures of aircraft engines, the additional heat impact can lead to a problematic level of material stress. To study these effects and to give reliable quantitative design rules, a versatile test rig for brush seals was designed and built. The simulation of seal behavior under relevant engine conditions is the main emphasis of this rig, including high pressure drop, leakage flow and high surface speed. The key feature is the possibility to vary the axis symmetric radial gap width during the test rig operation by up to a 0.5 mm overlap. The so caused rubbing induces a transient rotor temperature rise which is measured via a set of 12 thermocouples embedded in the rotor. These temperature readings can be used to calculate the brush seal heat impact on the rotor structure. Preliminary results with moderate differential pressure and rotor speed proved the functionality of the test rig and confirmed the global approach of the project.
Numerical Investigations on the Leakage Flow Characteristics of Pocket Damper Seals
Authors: Li Zhigang , Li Jun , Feng Zhenping , Yang Jiandao , Yang Rui , Shi Liqun
Published: May 2012

The effects of pressure ratios, rotational speeds and sealing clearances on the leakage flow characteristics of pocket damper seals (PDS) were numerically investigated using Reynolds-Averaged Navier-Stokes (RANS) solutions. The leakage flow rate of the experimental PDS with the eight-bladed and eight-pocket was conducted at three different pressure drops and three different rotational speeds. The numerical results were in agreement with the experimental data. Six pressure ratios, four rotational speeds and four sealing clearances were utilized to study the effects of theses factors on the leakage flow characteristics of the PDS. Numerical results show that the leakage rate of the PDS increases with decreased pressure ratio. The leakage rate decreases with the increasing rotational speed, and this phenomenon is more pronounced at higher rotational speed. At the highest rotational speed 20200rpm, the flow coefficient is up to 4.4% less than that of the non-rotating case. The leakage rate increases linearly with sealing clearance increasing. The comparison of the leakage flow rate shows that the PDS leaks slightly less than that of the labyrinth seal at the same pressure ratio, rotational speed and sealing clearance, especially at the higher rotational speed case. Furthermore, the circumferential partition wall can significantly decrease the circumferential flow in the PDS cavity. At the highest rotational speed with 20200rpm, the swirl ratio in the active and inactive cavity of the PDS is reduced by 94.5% and 46% compared to the labyrinth seal, respectively.
Application of a CFD-Based Film Cooling Model to a Gas Turbine Vane Cascade With Cylindrical and Shaped Hole Endwall Film Cooling
Authors: auf dem Kampe Tilman , Vo¨lker Stefan
Published: May 2012

This paper presents the application of a CFD-based film cooling model to a gas turbine vane cascade test rig. The experimental investigations feature aerodynamic and endwall film cooling measurements on a first stage gas turbine vane in a linear cascade. An extended version of a previously developed cylindrical hole film cooling model has been employed, which now includes modeling of shaped hole cooling flows. The computational domain extends approximately one axial chord length upstream of the leading edge and downstream of the trailing edge of the vane. Adjacent solid parts are included by means of a conjugate heat transfer analysis to account for conduction effects. A hybrid mesh with resolved boundary layers and high spatial mesh resolution in the near-wall region is being used. This meshing approach ensures that the near-wall mesh resolution requirements of the film cooling model are satisfied, while maintaining a manageable total node count. Results obtained using the film cooling model are compared to surface distributions of film cooling effectiveness from the experimental cascade. Due to the moderate node count (≈ 3.5 × 106), CFD calculations including film cooling flows can be performed at comparatively low computational cost. The film cooling model, which previously had been validated against flat plate measurement data and applied to single cooling hole configurations only, is therefore shown to be a viable tool for the thermal design of gas turbine components with film cooling.
Effects of Ribs on Internal Blade-Tip Cooling
Authors: Salameh Tareq , Sunden Bengt
Published: May 2012

This work concerns an experimental study of pressure drop and heat transfer for turbulent flow inside a U-duct with relevance for tip cooling of gas turbine blades. The U-duct models the internal blade cooling flow passages. Both friction factors and convective heat transfer coefficients were measured along the bend (turn) part of the U-duct for three different rib configuration cases, namely (a) single rib at three different rib positions, i.e., inlet, middle and outlet, (b) two ribs with three different configurations, i.e., at the inlet and middle, at the middle and outlet as well as at the inlet and outlet, and (c) three ribs. The rib height-to-hydraulic diameter ratio, e/Dh, was 0.1 and the pitch ratios were 10 and 20. The Reynolds number was varied from 8,000 to 20,000. The test rig has been built in such a way that various experimental setups can be handled as the bend (turn) part of the U-duct can easily be removed and the rib configurations can be changed. The surface temperature was measured by using a high-resolution measurement technique based on narrow band thermochromic liquid crystals (TLC R35C5W) and a CCD camera placed facing the bend (turn) part of the U-duct. The calibration of the TLC is based on the hue-based color decomposition system using an in-house designed calibration box. Both the friction factor and heat transfer coefficient were affected by the position and configuration of the ribs along the bend wall. The highest friction factor was found for two ribs placed at the middle and outlet positions of the bend wall, respectively. The highest heat transfer coefficient was found for two ribs placed at the inlet and middle positions of the bend wall, respectively. The uncertainties in the experiments were estimated to be 3% and 6% for the Nusselt number and friction factor, respectively.