bluuebunny/crossref_metadata_2025 huggingface dataset:
Increasing Flashback Resistance in Lean Premixed Swirl-Stabilized Hydrogen Combustion by Axial Air Injection
Authors: Reichel Thoralf G. , Terhaar Steffen , Paschereit Oliver
Published: September 2014

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a non-swirling jet on the central axis of the radial swirl generator which influences the vortex breakdown position. In the present work changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by the means of the Qualitative Light Sheet (QLS) technique.

The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note, that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.
Effects of Unsteady Wakes on Flow Through High Pressure Turbine Passages With and Without Tip Gaps
Authors: Volino Ralph J. , Galvin Christopher D. , Brownell Cody J.
Published: September 2014

Experiments were conducted in a linear high pressure turbine cascade with wakes generated by moving upstream rods. The cascade included an adjustable top endwall that could be raised and lowered above the airfoils to change the tip gap. Conditions were considered with no tip gap, and gaps of 1.5% and 3.8% of axial chord. For each of these, cases were documented both with and without upstream wakes. The pressure distributions on the airfoils were acquired at the midspan and near the tip for each case. The total pressure loss was measured in the endwall region. Velocity fields were acquired in two planes normal to the flow direction using particle image velocimetry (PIV). For the case with no tip gap, the passage vortex and other vortices were clearly visible in the velocity fields. For the cases with a tip gap, the tip leakage vortex was the dominant flow feature, and it became stronger as the gap size increased. The other vortices were still present, but were moved by the tip leakage vortex. For the cases with unsteady wakes, the PIV data were ensemble-averaged based on phase within the wake passing cycle, to show the motion and change in strength of the vortices in response to the wake passing. The regions of high total pressure loss can be explained in terms of the secondary velocity field.
Multi-Point Optimization of Lean and Sweep Angles for Stator and Rotor Blades of an Axial Turbine
Authors: Asgarshamsi Abolhassan , Hajilouy-Benisi Ali , Assempour Ahmad , Pourfarzaneh Hossein
Published: September 2014

In this research, numerical optimization of the rear part of a gas turbine, consisting of a single stage axial turbine is carried out. Automated aerodynamic shape optimization is performed by coupling a CFD flow simulation code with the Genetic Algorithm. An effective multi-point optimization method to improve efficiency and/or pressure ratio of the axial turbine is performed. Some variations of optimization parameters such as lean and sweep angels of stator and rotor blades are accomplished. Furthermore, during the optimization process, three-dimensional and turbulent flow field is numerically investigated using a compressible Navier-Stokes solver. The gas turbine experimental investigations are used for choosing optimization points, specifying the boundary conditions and validation of the simulations. Two operating speeds of the gas turbine are selected for the optimization. Using verified numerical simulation models and the genetic algorithm, the effects of stator and rotor lean and sweep angles on the turbine performance are studied. The present optimization method successfully obtains a result that improves 1.31% and 1.17% in the turbine stage total-to-total efficiency in its design and off-design speeds, respectively. The modified turbine mass parameter at choke condition increases 2.40%.
Bayesian Belief Network for Robust Engine Design and Architecture Selection
Authors: Kestner Brian K. , Perullo Christopher A. , Sands Jonathan S. , Mavris Dimitri N.
Published: September 2014

Designing propulsion system architectures to meet next generation requirements requires many tradeoffs be made. These trades are often between performance, risk, and cost. For example, the core of an engine is the most expensive and highest risk area of a propulsion system design. However, a new core design provides the greatest flexibility in meeting future performance requirements. The decision to upgrade or redesign the core must be justified by comparison with other lower risk options. Furthermore, for turboshaft applications, the choice of compressor, whether axial or centrifugal, is a major decision and trade with the choice being heavily driven by both current and projected weight and performance requirements. This problem is confounded by uncertainty in potential benefits of technologies or future performance of components. To address these issues this research proposes the use of a Bayesian belief network (BBN) to extend the more traditional robust engine design process. This is done by leveraging forward and backward inference to identify engine upgrade paths that are robust to uncertainty in requirements performance. Prior beliefs on the different scenarios and technology uncertainty can be used to quantify risk. Forward inference can be used to compare different scenarios.

The problem will be demonstrated using a two-spool turboshaft architecture modeled using the Numerical Propulsion System Simulation (NPSS) program. Upgrade options will include off the shelf, derivative engine (fixed core) with no technologies, derivative engine with new technologies, a new engine with no technologies, and a new engine with new technologies. The robust design process with a BBN will be used to identify which engine cycle and upgrade scenario is needed to meet performance requirements while minimizing cost and risk. To demonstrate how the choice of upgrade and cycle change with changes in requirements, studies are performed at different horsepower, ESFC, and power density requirements.
Flow Field and Flame Dynamics of Swirling Methane and Hydrogen Flames at Dry and Steam-Diluted Conditions
Authors: Terhaar Steffen , Krüger Oliver , Paschereit Christian Oliver
Published: September 2014

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown and exhibits a complex flow field including zones of recirculating fluid and regions of high shear. Often, self-excited helical flow instabilities are found in these flows that may influence the combustion process in beneficial and adverse ways. In the present study we investigate the occurrence and shape of self-excited hydrodynamic instabilities and the related heat-release fluctuations over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry and simultaneous OH*-chemiluminescence imaging to resolve the flow velocities and heat release distribution, respectively. The results reveal four different flame shapes: A detached annular flame, a long trumpet shaped flame, a typical V-flame, and a very short flame anchored near the combustor inlet. The flame shapes were found to closely correlate with the reactivity of the mixture. Highly steam-diluted or very lean flames cause a detachment, whereas hydrogen fuel leads to very short flames. The detached flames feature a helical instability, which in terms of frequency and shape is similar to the isothermal case. A complete suppression of the helical structure is found for the V-flame. Both, the trumpet shaped flame and the very short flame feature helical instabilities of different frequencies and appearances. The phase-averaged OH*-chemiluminescence images show that the helical instabilities cause large scale-heat release fluctuations. The helical structure of the fluctuations is verified using a tomographic reconstruction technique.
Aerodynamic Design of S-Shaped Diffusers Using Ball Spine Inverse Design Method
Authors: Madadi Ali , Nili-Ahmadabadi Mahdi , Kermani Mohammad Jafar
Published: September 2014

Recently, an inverse design algorithm called ball-spine algorithm (BSA) is introduced for the design of 2-D ducts. In this approach, the walls are considered as a set of virtual balls that can freely move along the straight directions called spines. In the present work the method is developed for quasi 3-D design of S-shaped ducts with a predefined width. To do so, the upper and lower lines of the S-duct symmetric section are modified under the BSA and then, the 3-D S-duct geometry is obtained based on elliptic cross sectional profiles. The target pressure distributions along the upper and lower lines are prescribed so that the separation does not occur. Finally, the flow through the designed S-duct is numerically analyzed using a viscous flow solver with the SST turbulence model to validate the designed S-duct performance.
Response of Non-Axisymmetric Premixed, Swirl Flames to Helical Disturbances
Authors: Acharya Vishal , Lieuwen Timothy
Published: September 2014

Flow oscillations associated with hydrodynamic instabilities comprise a key element of the feedback loop during self-excited combustion driven oscillations. This work is motivated in particular by the question of how to scale thermoacoustic stability results from single nozzle or sector combustors to full scale systems. Specifically, this paper considers the response of non-axisymmetric flames to helical flow disturbances of the form u^i′∝expimθ where m denotes the helical mode number. This work closely follows prior studies of the response of axisymmetric flames to helical disturbances. In that case, helical modes induce strong flame wrinkling, but only the axisymmetric, m = 0 mode, leads to fluctuations in overall flame surface area and, therefore, heat release. All other helical modes induce local area/heat release fluctuations with azimuthal phase variations that cancel each other out when integrated over all azimuthal angles. However, in the case of mean flame non-axisymmetries, the azimuthal deviations on the mean flame surface inhibit such cancellations and the asymmetric helical modes (m ≠ 0) cause a finite global flame response. In this paper, a theoretical framework for non-axisymmetric flames is developed and used to illustrate how the flame shape influences which helical modes lead to net flame surface area fluctuations. Example results are presented which illustrate the contributions made by these asymmetric helical modes to the global flame response and how this varies with different control parameters such as degree of asymmetry in the mean flame shape or Strouhal number. Thus, significantly different sensitivities may be observed in single and multi-nozzle flames in otherwise identical hardware in flows with strong helical disturbances, if there are significant deviations in time averaged flame shape between the two, particularly if one of the cases is nearly axisymmetric.
On the Numerical Simulation of Turbine Blade Tangling After a Shaft Failure
Authors: Gonzalez Aleixo , Pachidis Vassilios
Published: September 2014

Blade tangling is a mechanical interaction between rotors and stators that usually takes place in the expansion side of the engine. It is typically caused by the axial dislocation of the turbine that occurs after certain shaft failures. The impacts generated by this contact and the destruction of the blading in a controlled manner can be used as a mechanism to avoid hazardous effects after a shaft fracture. This paper aims to analyse blade tangling and assess its capacity to limit the turbine over-speed. A FEA model was set up to derive correlations that could characterise this event. These were used to calculate the braking torque and the rotor damage in a test engine. Effects of geometry are also discussed in this paper.
Heat Transfer and Pressure Loss Characteristics of a Narrow Internal Cooling Passage at Low Reynolds Number
Authors: Siw Sin Chien , Miller Nicholas , Chyu Minking K. , Alvin Mary Anne
Published: September 2014

This paper describes a detailed experimental investigation of a narrow rectangular channel based on the double-wall cooling concept that can be applicable to a gas turbine airfoil. The channel has dimensions of 63.5 mm by 12.7 mm, corresponding to an aspect ratio of 5:1. A single pin-fin element, arranged in 9 rows is fitted into the channel. The pin diameter, D, is 12.7 mm, and the ratio of pin-height-to-diameter, H/D is 1. The pins are arranged based on the typical inter-pin spacing of 2.5D in both spanwise and streamwise directions. The Reynolds number, based on the hydraulic diameter of the pin fin and the mean bulk velocity, ranges from 6,000 to 15,000. The experiments employ a hybrid technique based on transient liquid crystal imaging to obtain the distributions of the local heat transfer coefficient over all of the participating surfaces, including the endwalls and all the pin elements. Commercially available CFD software, ANSYS CFX, is used to qualitatively correlate the experimental results and to provide detailed insights of the flow field created by the array.The heat transfer on both the endwall and pin-fin surfaces revealed similar pattern compared to the typical circular pin-fin array, which were conducted at higher Reynolds number. The total heat transfer enhancement of current pin-fin array is approximately five times higher than that of fully developed smooth channel with low pressure loss, which resulted in much higher thermal performance compared to other pin-fin array as reported in the literature.
Direct Measurement of Heat Transfer Coefficient Augmentation at Multiple Density Ratios
Authors: Boyd Emily J. , McClintic John W. , Chavez Kyle F. , Bogard David G.
Published: September 2014

Knowing the heat transfer coefficient augmentation is imperative to predicting film cooling performance on turbine components. In the past, heat transfer coefficient augmentation was generally measured at unit density ratio to keep measurements simple and uncertainty low. Some researchers have measured heat transfer coefficient augmentation while taking density ratio effects into account, but none have made direct temperature measurements of the wall and adiabatic wall to calculate hf/h0 at higher density ratios. This work presents results from measuring the heat transfer coefficient augmentation downstream of shaped holes with a 7° forward and lateral expansion at DR = 1.0, 1.2, and 1.5 on a flat plate using a constant heat flux surface. The results showed that the heat transfer coefficient augmentation was low while the jets were attached to the surface and increased when the jets started to separate. At DR = 1.0, hf/h0 was higher for a given blowing ratio than at DR = 1.2 and DR = 1.5. However, when velocity ratios are matched, better correspondence was found at the different density ratios. Surface contours of hf/h0 showed that the heat transfer was initially increased along the centerline of the jet, but was reduced along the centerline at distances farther downstream. The decrease along the centerline may be due to counter-rotating vortices sweeping warm air next to the heat flux plate toward the center of the jet, where they sweep upward and thicken the thermal boundary layer. This warming of the core of the coolant jet over the heated surface was confirmed with thermal field measurements.
Investigation of the Combination of Bypass System and OPC Logic for ORC System Stability Under Load Reduction Disturbance
Authors: Zhao Pan , Wang Mingkun , Wang Jiangfeng , Dai Yiping
Published: September 2014

Organic Rankine cycle (ORC) is recognized as a promising technology for converting the energy from low grade heat sources into electricity. From a general perspective, the ORC system mainly operates in island mode and also has a small scale power capacity. When this type of system sustains the disturbance of load reduction, the overpressure protection in drum boiler and overspeed protection in axial turbine are the two big challenges for ORC system’s stability. In the present study, the configuration type by combining the bypass system and overspeed protection control (OPC) logic is proposed in ORC system for solving these problems. Firstly, the ORC unit model consisting of the drum boiler, boiler control system, axial turbine, bypass system, turbine control system and OPC logic is established. Then, the drawback and improvements of current OPC logic in China are carried out. Finally, the simulation of this configuration type of ORC system is investigated under load reduction disturbance. The results show that the proposed configuration of ORC system has excellent performance in purpose of power system stability. The axial turbine in the simulation model operates in constant pressure mode.
Experimental and Numerical Investigation of Non-Impregnated Aramid Fibers and Winding for Combined Fan Case
Authors: Krundaeva Anastasia N. , Shmotin Yury N. , Didenko Roman A. , Karelin Dmitry V.
Published: September 2014

The layer of non-impregnated aramid fibers is widely used in the containment systems of aircraft gas turbine engines. Such systems are found to be especially cost-effective and light weight for mitigating engine debris during a fan blade-out event. This is mostly because non-impregnated aramid fibers have a high strength per unit weight. Moreover, it is inexpensive to manufacture such a containment system compared to traditional metallic systems. To properly utilize this advantage, it is necessary to have a finite element (FE) analysis modeling methodology for daily design tasks. Non-impregnated aramid fibers winding for fan case modeling for engine containment systems is a difficult task.

This research implied both experimental and modeling techniques, and data characterizing the behavior of fabric materials for engine containment systems. This research was aimed at addressing engine containment modeling issues. Thus this work has resulted in the following major accomplishments:

• Experimental Characterization of Non-impregnated Aramid Fibers: the fabric material model originally was created during this phase. The independent laboratory tests conducted at NPO Saturn form the basis of this model. These material models are general enough to be used as the constitutive model for both static and dynamic/explicit FE analyses.

• Static Ring Tests: Static tests of containment wraps subjected to loads through a blunt nose impactor were performed at NPO Saturn. Ballistic tests of containment wraps subjected to a high-velocity projectile were performed at NPO Saturn. These tests provided the test cases (the benchmark results) to validate the developed FE methodology.

• FE Material Model Development: The material models were used by the research team in the FE simulation of static and ballistic tests. The static test results have been validated by NPO Saturn using the ANSYS FE program. The ballistic test results were validated by NPO Saturn using the LS-DYNA FE program.

• Engine Fan Blade-Out (FBO) Simulation: The knowledge gained from previous tasks was used by NPO Saturn for the the numerical simulation of real engine FBO events involving the existing production engine models and compared to the test results (employing thelayer of non-impregnated aramid fibers containment).

• Combined Fan and Metal Case Comparison: The relative comparison between the non-impregnated aramid fibers and the metal materials in engine FBO containment systems has been carried out in order to ascertain that the non-impregnated aramid fibers case is more advantageous.
Numerical and Experimental Findings of a Highly-Loaded Aspirated Cascade
Authors: Power Bronwyn , Xu Liping , Wellborn Steven
Published: September 2014

Comparisons between numerical predictions and experimental tests of a type of highly loaded aspirated compressor cascade are presented. The cascade profile features low solidity and low profile loss with applied blade and end-wall suction control. The cascade mid-span profile design was generated using multi-objective CFD optimization. Two successive levels of modeling were used to obtain the airfoil numerical predictions: the Euler/coupled-boundary layer solver, MISES, and RANS based CFD, executed in FLUENT and with mesh generation conducted in ICEM. A low-speed aspirated cascade rig was designed and built to allow for testing of the aspirated blade design at conditions approaching engine representative Reynolds numbers and inlet turbulence intensity levels. Both numerical and experimental results validated the aspirated cascade design concept. The results also showed evidence of a ‘soft’ failure mode for the aspirated blade when a reduced or zero suction level is applied.
Experimental and Numerical Investigation of Fuel-Air Mixing in a Radial Swirler Slot of a Dry Low Emission Gas Turbine Combustor
Authors: Agbonzikilo Festus Eghe , Stewart Jill , Sadasivuni Suresh Kumar , Owen Ieuan , Riley Mike , Sanderson Victoria
Published: September 2014

This paper presents the results of an investigation in which the fuel/air mixing process in a single slot within the radial swirler of a dry low emission (DLE) combustion system is explored using air/air mixing. Experimental studies have been carried out on an atmospheric test facility in which the test domain is a large-scale representation of a swirler slot from a Siemens DLE SGT-400 combustion system. Hot air with a temperature of 300°C is supplied to the slot, while the injected fuel gas is represented using air jets with temperatures of about 25°C. Temperature has been used as a scalar to measure the mixing of the jets with the cross-flow. The mixture temperatures were measured using thermocouples while Pitot probes were used to obtain local velocity measurements. The experimental data have been used to validate a computational fluid dynamics (CFD) mixing model.

Numerical simulations were carried out using CFD software ANSYS-CFX. Due to the complex three-dimensional flow structure inside the swirler slot, different RANS turbulence models were tested. The shear stress transport (SST) turbulence model was observed to give best agreement with the experimental data. The momentum flux ratio between the main air flow and the injected fuel jet, and the aerodynamics inside the slot, were both identified by this study as major factors in determining the mixing characteristics. It has been shown that mixing in the swirler can be significantly improved by exploiting the aerodynamic characteristics of the flow inside the slot. The validated CFD model provides a tool which will be used in future studies to explore fuel/air mixing at engine conditions.
Heat Transfer to an Actively Cooled Shroud With Blade Rotation
Authors: Tamunobere Onieluan , Drewes Christopher , Acharya Sumanta
Published: September 2014

In this paper, an experimental study of the shroud heat transfer behavior and the effectiveness of shroud cooling under the conditions of rotation is undertaken in a single stage turbine at low rotation speeds. The shroud consists of a periodic distribution of cooling holes that are 1 mm in diameter (D). The holes are angled at 45 degrees in a repeating pattern consisting of 5 unique hole pitches around the shroud circumference. Measurements of the normalized Nusselt number and film cooling effectiveness are done using liquid crystal thermography. These measurements are reported for the no coolant case, nominal blowing ratios of 1.0, 1.5, 2.0, 2.5 and 3.0, and rotation speeds of 300, 400, 500, 600 and 700 RPM. The results with no coolant injection show that the high Nu/Nu0 region migrates upstream toward the shroud leading edge with increasing rotation. The cooling results show that increasing the blowing ratio increases the area-averaged film cooling effectiveness in the shroud hole region for all rotation speeds studied. Furthermore, increasing the blade rotation speed increases the area-averaged Nusselt numbers and decreases the area-averaged film cooling effectiveness in the shroud hole region for all blowing ratios studied. As in the no-coolant case, with increasing rotation speeds, the high Nu/Nu0 region migrates upstream toward the shroud leading edge and disrupts the cooling effectiveness in this region. Finally, the results show that decreasing the shroud coolant hole spacing changes the lateral heat transfer profile from a periodic sinusoidal distribution for a shroud hole spacing of P/D = 10.4 to a more even distribution for a smaller P/D = 4.8.
Part Load Performance and Operating Strategies of a Natural Gas–Biomass Dual Fuelled Microturbine for CHP Generation
Authors: Camporeale Sergio Mario , Fortunato Bernardo , Torresi Marco , Turi Flavia , Pantaleo Antonio Marco , Pellerano Achille
Published: September 2014

The focus of this paper is on the part load performance of a small scale (100 kWe) combined heat and power (CHP) plant fired by natural gas and solid biomass to serve a residential energy demand. The plant is based on a modified regenerative micro gas turbine (MGT), where compressed air exiting from recuperator is externally heated by the hot gases produced in a biomass furnace; then the air is conveyed to combustion chamber where a conventional internal combustion with natural gas takes place, reaching the maximum cycle temperature allowed by the turbine blades. The hot gas expands in the turbine and then feeds the recuperator, while the biomass combustion flue gases are used for pre-heating the combustion air that feeds the furnace. The part load efficiency is examined considering a single shaft layout of the gas turbine and variable speed regulation. In this layout, the turbine shaft is connected to a high speed electric generator and a frequency converter is used to adjust the frequency of the produced electric power. The results show that the variable rotational speed operation allows high the part load efficiency, mainly due to maximum cycle temperature that can be kept about constant.

Different biomass/natural gas energy input ratios are also modelled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD) and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average and mild climate conditions.
Experimental Comparison of Axial Turbine Performance Under Steady and Pulsating Flows
Authors: St. George A. , Driscoll R. , Gutmark E. , Munday D.
Published: September 2014

The performance of an axial turbine is studied under close-coupled, out-of-phase, multiple-admission pulsed air flow to approximate turbine behavior under pulsed detonation inflow. The operating range has been mapped for four frequencies and compared using multiple averaging approaches and five formulations of efficiency. Steady performance data for full and partial admission are presented as a basis for comparison to the pulsed flow cases. While time-averaged methods are found to be unsuitable, mass-averaged, work-averaged, and integrated instantaneous methods yield physically meaningful values and comparable trends for all frequencies. Peak work-averaged efficiency for pulsed flow cases is within 5% of the peak steady, full admission values for all frequencies, in contrast to the roughly 15–20% performance deficit experienced under steady, 50% partial admission conditions. Turbine efficiency is found to be a strong function of corrected flow rate and mass-averaged rotor incidence angle, but only weakly dependent on frequency.
Pareto-Based Multi-Objective Optimization of Recuperated S-CO2 Brayton Cycles
Authors: Mohagheghi Mahmood , Kapat Jayanta , Nagaiah Narasimha
Published: September 2014

In this paper, two configurations of the S-CO2 Brayton cycles (i.e., the single-recuperated and recompression cycles) are thermodynamically modeled and optimized through a multi-objective approach. Two semi-conflicting objectives, i.e., cycle efficiency (ηc) and cycle specific power (Φsp) are maximized simultaneously to achieve Pareto optimal fronts. The objective of maximum cycle efficiency is to have a smaller and less expensive solar field, and a lower fuel cost in case of a hybrid scheme. On the other hand, the objective of maximum specific power provides a smaller power block, and a lower capital cost associated with recuperators and coolers. The multi-objective optimization is carried out by means of a genetic algorithm which is a robust method for multidimensional, nonlinear system optimization. The optimization process is comprehensive, i.e., all the decision variables including the inlet temperatures and pressures of turbines and compressors, the pinch point temperature differences, and the mass flow fraction of the main compressor are optimized simultaneously. The presented Pareto optimal fronts provide two optimum trade-off curves enabling decision makers to choose their desired compromise between the objectives, and to avoid naive solution points obtained from a single-objective optimization approach. Moreover, the comparison of the Pareto optimal fronts associated with the studied configurations reveals the optimum operational region of the recompression configuration where it presents superior performance over the single-recuperated cycle.
Application of a Fuzzy Logic Controller for Speed Control on a Small-Scale Turbojet Engine
Authors: Üşenmez Serdar , Ekinci Sinan , Uzol Oğuz , Yavrucuk İlkay
Published: September 2014

Having a small-scale turbojet engine operate at a desired speed with minimum steady state error, while maintaining good transient response is crucial in many applications, such as UAVs, and requires precise control of the fuel flow.

In this paper, first the mathematical model of a Small-Scale Turbojet Engine (SSTE) is obtained using system identification tests, and then based on this model, a classical PI controller is designed. Afterwards, to improve on the transient response and steady state performance of this classical controller, a Fuzzy Logic Controller (FLC) is designed. The design process for the FLC employs logical deduction based on knowledge of the engine behavior and iterative tuning in the light of software- and hardware-in-the-loop simulations.

The classical and fuzzy logic controllers are both implemented on an in-house, embedded Electronic Control Unit (ECU) running in real time. This ECU is an integrated device carrying a microcontroller based board, a fuel pump, fuel line valves, speed sensor and exhaust gas temperature sensor inputs, and starter motor and glow plug driver outputs. It mainly functions by receiving a speed reference value via its serial communication interface. Based on this reference, a voltage is calculated and applied to the fuel pump in order to regulate the fuel flow into the engine, thereby bringing the engine speed to the desired value. Pre-defined procedures for starting and stopping the engine are also automatically performed by the ECU. Further, it connects to a computer running an in-house comprehensive Graphical User Interface (GUI) software for operating, monitoring, configuration and diagnostics purposes.

The designed controllers are used to drive a generic SSTE. Reference inputs consisting of step, ramp and chirp profiles are applied to the controllers. The engine response using both controllers are recorded and inspected. The results show that the FLC exhibits a comparable performance to the classical controller, with possible opportunities to improve this performance.
The Effect of Free-Stream Turbulence on Deposition for Nozzle Guide Vanes
Authors: Whitaker Steven M. , Prenter Robin , Bons Jeffrey P.
Published: September 2014

An evaluation of the effect of free-stream turbulence intensity on the rate of deposit accumulation for nozzle guide vanes was performed using the TuRFR accelerated deposition facility. The TuRFR allowed flows up to 1350 K at inlet Mach numbers of 0.1 to be seeded with coal fly ash particulate in order to rapidly evaluate deposit formation on CFM56 nozzle guide vanes. Hot film and PIV measurements were taken to assess the free-stream turbulence with and without the presence of a grid upstream of the NGVs. It was determined that baseline turbulence levels were approximately half that of the flow exiting typical gas turbine combustors and were reduced by approximately 30 percent with the grid installed. Deposition tests indicated that the rate of deposition increases as the free-stream turbulence is increased, and that this increase depends upon the particle size distribution. For ash with a mass median diameter of 4.63 μm, the increase in capture efficiency was approximately a factor of 1.7, while for ash with a larger median diameter of 6.48 μm, the capture efficiency increased by a factor of 2.4. The increase in capture efficiency is due to the increased diffusion of particles to the vane surface via turbulent diffusion. Based on these results, smaller particles appear to be less susceptible to this mechanism of particle delivery. Overall, the experiments indicate that the reduction of turbulence intensity upstream of nozzle guide vanes may lead to reduced deposit accumulation, and, consequently, increased service life. A CFD analysis was performed at turbulence levels equivalent to the experiments to assess the ability of built-in particle tracking models to capture the physics of turbulent diffusion. Impact efficiencies were shown to increase from 21 percent to 73 percent as the free-stream turbulence was increased from 5.8 to 8.4 percent. An analysis incorporating the mass of the particles into the impact efficiency resulted in an increase of the mass-based impact efficiency from 17 percent to 27 percent with increasing turbulence. Relating these impact efficiencies directly to capture efficiencies, the predicted increase in capture efficiency with higher turbulence is much less than that observed in the experiments. In addition, the variation in impact efficiencies between the two ash sizes was much smaller than the capture efficiency difference from experiments. This indicates that the particle tracking models are not capturing all of the relevant physics associated with turbulent diffusion of airborne particles.
A Model-Based Anomaly Detection Approach for Analyzing Streaming Aircraft Engine Measurement Data
Authors: Simon Donald L. , Rinehart Aidan W.
Published: September 2014

This paper presents a model-based anomaly detection architecture designed for analyzing streaming transient aircraft engine measurement data. The technique calculates and monitors residuals between sensed engine outputs and model predicted outputs for anomaly detection purposes. Pivotal to the performance of this technique is the ability to construct a model that accurately reflects the nominal operating performance of the engine. The dynamic model applied in the architecture is a piecewise linear design comprising steady-state trim points and dynamic state space matrices. A simple curve-fitting technique for updating the model trim point information based on steady-state information extracted from available nominal engine measurement data is presented. Results from the application of the model-based approach for processing actual engine test data are shown. These include both nominal fault-free test case data and seeded fault test case data. The results indicate that the updates applied to improve the model trim point information also improve anomaly detection performance. Recommendations for follow-on enhancements to the technique are also presented and discussed.
Bleed Slot Benefits on Turbocharger Centrifugal Compressor Stability
Authors: Gancedo Matthieu , Guillou Erwann , Gutmark Ephraim
Published: September 2014

Bleed slots located in the inducer region of centrifugal compressors have been demonstrated to extend the surge margin with minimal negative impact on performance. This paper describes the investigation of the effect of a bleed slot on map width enhancement of a turbocharger centrifugal compressor used for heavy duty diesel engine application. The goal is to evaluate the overall pressure instabilities on the compressor map and to study the dynamic phenomena occurring at low mass flow rates to better understand the benefits of the bleed slot on the compressor stability and surge line. The obstruction of the bleed slot permitted to compare the compressor behavior with the recirculation feature and without it. The pressure instability levels were measured along the accessible compressor map for the two cases using a high speed response pressure transducer at the compressor outlet. In addition, static and dynamic pressure measurements were conducted within the diffuser using respectively pressure taps and high speed response pressure transducers. The compressor with open bleed slot proved to have lower instability levels at low mass flow rates when not experiencing deep surge. Frequency responses at low mass flow rates showed that the implementation of the bleed slot suppresses broad band frequencies below the rpm frequency, which improves the overall stability. These frequencies are associated with rotating instabilities (RIs) with changing propagation speed depending on the rotational speed. At lower speeds, RIs are propagated with the wheel rotation whereas at higher speeds, they tend to propagate with the speed associated with their characteristic frequency.
Study of a Supercritical CO2 Turbine With TIT of 1350 K for Brayton Cycle With 100 MW Class Output: Aerodynamic Analysis of Stage 1 Vane
Authors: Schmitt J. , Willis R. , Amos D. , Kapat J. , Custer C.
Published: September 2014

This study seeks to design the aerodynamic features a first stage vane for a 100 MW class supercritical CO2 Brayton cycle turbomachine. For a turbine inlet temperature of 1350 K, the recuperated configuration is found to provide the highest cycle efficiency, and the corresponding cycle parameters are then used to design the turbine stages. A 6-stage turbine is selected and the first stage is designed following a one-dimensional mean line approach. Initial mean line turbomachine parameters (work coefficient and flow coefficient) are selected to provide high thermodynamic efficiency and simple radial equilibrium equation principles. Turning loss correlations are utilized to define and optimize hub and casing velocity triangle parameters. Typical turbomachinery characteristic parameters are used to compare the carbon dioxide turbine with typical air combustion turbines. Detailed aerodynamic analysis is performed on a complete three-dimensional model of the vane flow field using a commercial computational fluid dynamics code, STAR-CCM+. Actual properties of the working fluid are input to the model from the REFPROP database provided by the US National Institute of Standards and Technology (NIST). The detailed flow field is computed, from which aerodynamic loss coefficients are calculated. The computer model confirms that the design is successful in turning supercritical carbon dioxide at the prescribed angle and pressure. However, results of the real fluid simulation show that aerodynamic losses caused the stage efficiency to be about 4% below the design target.
Study on Mistuning Identification of Vehicle Turbocharger Turbine BLISK
Authors: Hattori Hiroaki
Published: September 2014

Blade vibration is one of the most critical items to be solved in turbocharger design. Turbocharger has much broader operating range than other turbo machinery and some unavoidable resonance may remain in operating range. In such a critical situation, predicting resonant vibration level is desirable for evaluating criticality. Additionally, knowing what amount of margin should be taken for the predicted nominal vibratory stress (known as “Magnification Factor”) is also important for the structural design. Blade to blade vibration response have significant scatter in real rotor. This kind of phenomena is called as “mistuned bladed disk effect” and discussed by many researchers in aero-engine field. Some research studies have appeared also in the automotive turbocharger field but its feature is not well understood especially on the item of mistuning identification and the cause of mistuning. Considering above situation, studies on mistuned vibration were performed for turbine BLISK of automotive turbocharger. Three type of mistuning (frequency mistuning, geometric mistuning, and mistuning of macroscopic property of directionally solidified material) were assumed and these effect on above items (mistuning identification and the cause of mistuning) were investigated.

This paper consists of three parts. At first, frequency mistune model and geometry mistune models were prepared. To build a frequency mistuning model, FMMID proposed by Feiner and Griffin [6] is applied. After the basic function test by virtual BLISK model with known mistuning, FMMID was applied to the actual BLISK. Natural frequency mistuning of each blade was identified by FMMID based on the modal measurement result. Obtained frequency mistuning is reflected to the (geometry tuned) FE model by changing the Young’s modulus of each blade in corresponding rate. The 3D measurement was also performed to the same BLISK and dimensional information from this measurement was reflected to the geometrically mistuned (material property tuned) FE model.

In the next step, vibration analysis (eigenvalue and frequency response) was performed and these results were compared to the measurement result. Vibration measurement in operating condition was performed at the resonant point of mode3 and nozzle count excitation frequency by utilizing the NSMS (Non-intrusive stress measurement system). Analytical result of the frequency mistuning model shows a good agreement with the experimental, while the analytical results of the geometrically mistuned model did not match to the experimental result.

At the last part, cause of discrepancy between the analytical result of the geometry mistuning model and the measured result was investigated from the view point of the effect of the anisotropy of elastic constants on the vibration characteristics of the DS (Directionally Solidified) blades.
Simulation of Liquid Fuel Atomization by a Complex High-Shear Swirling Injector
Authors: Ham Frank , Kim Dokyun , Bose Sanjeeb , Le Hung , Herrmann Marcus
Published: September 2014

The research and development described in this paper arises from the need for modeling a realistic fuel atomization process in a complex combustor/augmentor fuel injector. In the atomization process, it is important to understand the primary breakup mechanism and to predict the resulting fuel droplets. However, the mechanism of atomization and the resulting spray formation processes in realistic complex fuel injectors are not well understood because experimental access to the atomization region is typically severely limited. A significant portion of the atomization process occurs in spatial regions adjacent to solid walls that block experimental access into the injector so that experimental studies are limited to either far field measurements of complex injectors, after most of the atomization has occurred, or to simple injector geometries such as a circular cross-section pipes injecting into crossflow channels.
Nonlinear Dynamic Study on Effects of Rub-Impact Caused by Oil-Rupture in a Multi-Shafts Turbine With a Squeeze Film Damper
Authors: Shiau T. N. , Wang C. R. , Liu D. S. , Hsu W. C. , Young T. H.
Published: September 2014

An investigation is carried out the analysis of nonlinear dynamic behavior on effects of rub-impact caused by oil-rupture in a multi-shafts turbine system with a squeeze film damper. Main components of a multi-shafts turbine system includes an outer shaft, an inner shaft, an impeller shaft, ball bearings and a squeeze film damper. In the squeeze film damper, oil forces can be derived from the short bearing approximation and cavitated film assumption. The system equations of motion are formulated by the global assumed mode method (GAMM) and Lagrange’s approach. The nonlinear behavior of a multi-shafts turbine system which includes the trajectories in time domain, frequency spectra, Poincaré maps, and bifurcation diagrams are investigated.

Numerical results show that large vibration amplitude is observed in steady state at rotating speed ratio adjacent to the first natural frequency when there is no squeeze film damper. The nonlinear dynamic behavior of a multi-shafts turbine system goes in its way into aperiodic motion due to oil-rupture and it is unlike the usual way (1T = >2T = >4T = >8T etc) as compared to one shaft rotor system. The typical routes of bifurcation to aperiodic motion are observed in a multi-shafts turbine rotor system and they suddenly turn into aperiodic motion from the periodic motion without any transition. Consequently, the increasing of geometric or oil parameters such as clearance or lubricant viscosity will improve the performance of SFD bearing.