Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2011;134(2):021501-021501-9. doi:10.1115/1.4004375.

This paper describes simulation of a small stationary gas turbine combustor of a reverse flow, semi-silo type for power generation. The premixed coherent flame model (PCFM) is applied for partially premixed methane/air with an imposed downstream flame area density (FAD) to avoid flashback and incomplete combustion. Physical models are validated against the measurements of outlet temperature, product gas composition, and NO emission at the low operating pressure. Parametric study is performed to investigate the effect of load and pilot/total (P/T) fuel ratio on mixing characteristics and the resulting temperature distribution and pollutant emissions. As the P/T fuel ratio increases, the high temperature region over 1900 K enhances reaction of the mixture from the main nozzle in the primary mixing zone. For low P/T ratios, the pilot stream dilutes the mixture, on the contrary, to suppress reaction with an increasing height of the lifted flame. The NO is associated with the unmixedness as well as the mean temperature level and tends to increase with increasing load and P/T ratio. The high operating pressure does not affect overall velocity and temperature distribution, while it tends to increase NO and liner temperature under the given boundary conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):021502-021502-8. doi:10.1115/1.4004183.

The flame transfer function (FTF) of a premixed swirl burner was identified from a time series generated with computational fluid dynamics simulations of compressible, turbulent, reacting flow at nonadiabatic conditions. Results were validated against experimental data. For large eddy simulation (LES), the dynamically thickened flame combustion model with one step kinetics was used. For unsteady simulation in a Reynolds-averaged Navier–Stokes framework (URANS), the Turbulent Flame Closure model was employed. The FTF identified from LES shows quantitative agreement with experiment for amplitude and phase, especially for frequencies below 200 Hz. At higher frequencies, the gain of the FTF is underpredicted. URANS results show good qualitative agreement, capturing the main features of the flame response. However, the maximum amplitude and the phase lag of the FTF are underpredicted. Using a low-order network model of the test rig, the impact of the discrepancies in predicted FTFs on frequencies and growth rates of the lowest order eigenmodes were assessed. Small differences in predicted FTFs were found to have a significant impact on stability limits. Stability behavior in agreement with experimental data was achieved only with the LES-based flame transfer function.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):021503-021503-7. doi:10.1115/1.4004373.

Jets in crossflow are widely used in the industry for homogenization or cooling tasks. Recently, pulsating jets have been investigated as a mean to increase the scalar mixing efficiency of such configurations, whether for a single jet or for an array of jets. To avoid the disadvantages of mechanically actuated flows (costs, maintenance), a new injector based on a fluidics oscillator has been designed. Four injectors have been implemented in a generical jet in crossflow configuration and the mixing efficiency of the setup was compared with the one of the same setup equiped with standard non oscillating jets. With help of high-speed concentration measurement technique, the scalar mixing quality of both setups was measured at three positions downstream of the injection plane. In all the cases tested, the fluidics injectors present a better temporal homogenization, characterized by the Danckwerts unmixedness criterion, than the standard jets. For a defined mixing quality, a decrease of the mixing length by approximately 50% can be achieved with the fluidics injectors. Furthermore, the new injectors exhibit a mixing quality which is less sensitive to variations of the jet to crossflow momentum. The flapping motion of the fluidics injectors induces a wider azimuthal spreading of the fluidics jets immediately downstream of the injection location. This increases the macro- and micro-mixing phenomea which lead then to the high gains in mixing quality. It is thus demonstrated that fluidics oscillators present a strong potential to improve the passive scalar homogenization of jet in crossflow configurations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):021504-021504-15. doi:10.1115/1.4004388.

Accurate chemistry models are required to predict the combustion behavior of different fuels, such as synthetic gaseous fuels and liquid jet fuels. A detailed reaction mechanism contains chemistry for all the molecular components in the fuel or its surrogates. Validation studies that compare model predictions with the data from fundamental combustion experiments under well-defined conditions are least affected by the effect of transport on chemistry. Therefore they are the most reliable means for determining a reaction mechanism’s predictive capabilities. Following extensive validation studies and analysis of detailed reaction mechanisms for a wide range of hydrocarbon components reported in our previously published work (Puduppakkam , 2010, “Validation Studies of a Master Kinetic Mechanism for Diesel and Gasoline Surrogate Fuels,” SAE Technical Paper No. 2010-01-0545; Naik , 2010, “Validated F-T Fuel Surrogate Model for Simulation of Jet-Engine Combustion,” Proc. ASME Turbo Expo, Paper No. GT2010-23709; Naik , 2010, “Applying Detailed Kinetics to Realistic Engine Simulation: The Surrogate Blend Optimizer and Mechanism Reduction Strategies,” SAE J. Engines 3 (1), pp. 241–259; Naik , 2010, “Modeling the Detailed Chemical Kinetics of Mutual Sensitization in the Oxidation of a Model Fuel for Gasoline and Nitric Oxide,” SAE J. Fuels Lubr. 3 (1), pp. 556–566; and Puduppakkam , 2009, “Combustion and Emissions Modeling of an HCCI Engine Using Model Fuels,” SAE Technical Paper No. 2009-01-0669), we identified some common issues in the predictive nature of the mechanisms that are associated with inadequacies of the core (C0 -C4 ) mechanism, such as inaccurate predictions of laminar flame speeds and autoignition delay times for several fuels. This core mechanism is shared by all of the mechanisms for the larger hydrocarbon components. Unlike the reaction paths for larger hydrocarbon fuels; however, reaction paths for the core chemistry do not follow prescribed reaction rate-rules. In this work, we revisit our core reaction mechanism for saturated fuels, with the goal of improving predictions for the widest range of fundamental experiments. To evaluate and validate the mechanism improvements, we performed a broad set of simulations of fundamental experiments. These experiments include measurements of ignition delay, flame speed and extinction strain rate, as well as species composition in stirred reactors, flames and flow reactors. The range of conditions covers low to high temperatures, very lean to very rich fuel-air ratios, and low to high pressures. Our core reaction mechanism contains thermochemical parameters derived from a wide variety of sources, including experimental measurements, ab initio calculations, estimation methods and systematic optimization studies. Each technique has its uncertainties and potential inaccuracies. Using a systematic approach that includes sensitivity analysis, reaction-path analysis, consideration of recent literature studies, and an attention to data consistency, we have identified key updates required for the core mechanism. These updates resulted in accurate predictions for various saturated fuels when compared to the data over a broad range of conditions. All reaction rate constants and species thermodynamics and transport parameters remain within known uncertainties and within physically reasonable bounds. Unlike most mechanisms in the literature, the mechanism developed in this work is self-consistent and contains chemistry of all saturated fuels.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2011;134(2):021601-021601-8. doi:10.1115/1.4004397.

This paper is intended to serve as a template for incorporating technical management majors into a traditional engineering design course. In 2002, the Secretary of the Air Force encouraged the United States Air Force (USAF) Academy to initiate a new interdisciplinary academic major related to systems engineering. This direction was given in an effort to help meet the Air Force’s growing need for “systems” minded officers to manage the development and acquisition of its ever more complex weapons systems. The curriculum for the new systems engineering management (SEM) major is related to the “engineering of large, complex systems and the integration of the many subsystems that comprise the larger system” and differs in the level of technical content from the traditional engineering major. The program allows emphasis in specific cadet—selected engineering tracks with additional course work in human systems, operations research, and program management. Specifically, this paper documents how individual SEM majors have been integrated into aeronautical engineering design teams within a senior level capstone course to complete the preliminary design of a gas turbine engine. As the Aeronautical Engineering (AE) cadets performed the detailed engine design, the SEM cadets were responsible for tracking performance, cost, schedule, and technical risk. Internal and external student assessments indicate that this integration has been successful at exposing both the AE majors and the SEM majors to the benefits of “systems thinking” by giving all the opportunity to employ SE tools in the context of a realistic aircraft engine design project.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):021602-021602-8. doi:10.1115/1.4004147.

A specific actuator able to modulate the air feed of a gas a burner at a given frequency and amplitude is presented. The Combustion Department at the Institute for Thermal Turbomachinery and Machine Dynamics at the Graz University of Technology has experience on the study of combustion instabilities in gas turbines using a flow excitor. The stability of an industrial burner is tested at elevated pressure and temperature conditions in the frame of the NEWAC project. For practical matters of operation among which the possibility to induce progressively a perturbation when the flame conditions are all set, the need was expressed to design, construct and validate a flexible actuator able to set an air flow modulation at a given frequency and at a desired amplitude level, with the possibility during operation to let these two factors vary in a given range independently from each other. This device should operate within the 0–1 kHz range and 0%–20% amplitude range at steady-state, during transients, or follow a specific time sequence. It should be robust and sustain elevated pressures. The objective is to bring a perturbation in the flow to which the combustor will respond, or not. For elevated levels of pulsation, it can simulate the presence of vortex-driven combustion instabilities. It can also act as a real-time actuator able to respond in frequency and in phase to actively damp a “natural” combustion instability. Other issues are a better and quicker mixing due to the enhanced turbulence level, and pushing forward the blow out limits at lean conditions with controlled injection dynamics. The basic construction is the one of a siren, with an elevated pressure side where the air is throttled, and a low pressure outlet where the resulting sonic jet is sheared by a rotating wheel. A mechanism allows to let vary the surface of interaction between the wheel and the jet. Two electromotors driven by Labview set both frequency and amplitude levels. This contribution describes the actuator’s principles, design, operation range and the results of the characterization campaign.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2011;134(2):021801-021801-15. doi:10.1115/1.4004374.

This study examines the performance of a solid oxide fuel cell- (SOFC-) based integrated gasification power plant concept at the utility scale (>100 MW). The primary system concept evaluated was a pressurized ∼150 MW SOFC hybrid power system integrated with an entrained-flow, dry-fed, oxygen-blown, slagging coal gasifier and a combined cycle in the form of a gas turbine and an organic Rankine cycle (ORC) power generator. The analyzed concepts include carbon capture via oxy-combustion followed by water knockout and gas compression to pipeline-ready CO2 sequestration conditions. The results of the study indicate that hybrid SOFC systems could achieve electric efficiencies approaching 66% [lower heating value (LHV)] when operating fueled by coal-derived clean syngas and without carbon dioxide capture. The system concept integrates SOFCs with the low-pressure turbine spool of a 50 MW Pratt & Whitney FT8-3 TwinPak gas turbine set and a scaled-up, water-cooled 20 MW version of the Pratt & Whitney (P&W) PureCycle ORC product line (approximately 260 kW). It was also found that a system efficiency performance of about 48% (LHV) is obtained when the system includes entrained-flow gasifier and carbon capture using oxygen combustion. In order to integrate the P&W FT8 into the SOFC system, the high-pressure turbine spool is removed which substantially lowers the FT8 capital cost and increases the expected life of the gas turbine engine. The impact of integrating an ORC bottoming cycle was found to be significant and can add as much as 8 percentage points of efficiency to the system. For sake of comparison, the performance of a higher temperature P&W ORC power system was also investigated. Use of a steam power cycle, in lieu of an ORC, could increase net plant efficiency by another 4%, however, operating costs are potentially much lower with ORCs than steam power cycles. Additionally, the use of cathode gas recycle is strongly relevant to efficiency performance when integrating with bottoming cycles. A parameter sensitivity analysis of the system revealed that SOFC power density is strongly influenced by design cell voltage, fuel utilization, and amount of anode recycle. To maximize the power output of the modified FT8, SOFC fuel utilization should be lower than 70%. Cathode side design parameters, such as pressure drop and temperature rise were observed to only mildly affect efficiency and power density.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Industrial & Cogeneration

J. Eng. Gas Turbines Power. 2011;134(2):022001-022001-9. doi:10.1115/1.4004044.

The output and efficiency of gas turbines are reduced significantly during the summer, especially in areas where the daytime temperature reaches as high as 50°C. Gas turbine inlet fogging and overspray has been considered a simple and cost-effective method to increase the power output. One of the most important issues related to inlet fogging is to determine the most effective location of the fogging device by determining (a) how many water droplets actually evaporate effectively to cool down the inlet air instead of colliding on the wall or coalescing and draining out (i.e., fogging efficiency), and (b) quantifying the amount of nonevaporated droplets that may reach the compressor bellmouth to ascertain the erosion risk for compressor airfoils if wet compression is to be avoided. When the silencer is installed, there is an additional consideration for placing the fogging device upstream or downstream of the silencer baffles. Placing arbitrarily the device upstream of the silencer can cause the silencer to intercept water droplets on the silencer baffles and lose cooling effectiveness. This paper employs computational fluid dynamics (CFD) to investigate the water droplet transport and cooling effectiveness with different spray locations such as before and after the silencer baffles. Analysis on the droplet history (trajectory and size) is employed to interpret the mechanism of droplet dynamics under influence of acceleration, diffusion, and body forces when the flow passes through the baffles and duct bent. The results show that, for the configuration of the investigated duct, installing the fogging system upstream of the silencer is about 3 percentage points better in evaporation effectiveness than placing it downstream of the silencer, irrespective of whether the silencer consists of a single row of baffles or two rows of staggered baffles. The evaporation effectiveness of the staggered silencer is about 0.8 percentage points higher than the single silencer. The pressure drop of the staggered silencer is 6.5% higher than the single silencer.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):022002-022002-11. doi:10.1115/1.4004163.

The inlet fogging effects on the stable range of a NASA transonic compressor stage, Stage 35, are numerically simulated and analyzed in this paper. The 3D two-phase flow fields in the compressor stage are investigated under different operating flow conditions with varying levels of the injected water flow rates and the fogging droplets sizes. The special attention is given to the stall and the choking operating points to investigate changes in the stable operating range of the compressor stage as a result of different wet compression conditions. The preliminary results indicate that the inlet fogging has different effects on either the stall and/or the choking range. The change in the stable range of this transonic compressor stage depends on the fogging flow rate and droplets diameters.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2011;134(2):022401-022401-9. doi:10.1115/1.4004185.

To optimize both production and maintenance, from both a technical and an economical point of view, it would be advisable to predict the future health condition of a system and of its components, starting from field measurements taken in the past. For this purpose, this paper presents a methodology, based on the Monte Carlo statistical method, which aims to determine the future operating state of a gas turbine. The methodology allows the system future availability to be estimated, to support a prognostic process based on past historical data trends. One of the most innovative features is that the prognostic methodology can be applied to both global and local performance parameters, as, for instance, machine specific fuel consumption or local temperatures. First, the theoretical background for developing the prognostic methodology is outlined. Then, the procedure for implementing the methodology is developed and a simulation model is set up. Finally, different degradation-over-time scenarios for a gas turbine are simulated and a sensitivity analysis on methodology response is carried out, to assess the capability and the reliability of the prognostic methodology. The methodology proves robust and reliable, with a prediction error lower than 2%, for the availability associated with the next future data trend.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):022402-022402-9. doi:10.1115/1.4004372.

Energy required to transport the fluid is an important parameter to be analyzed and minimized in pipeline applications. However, the pipeline system requirements and equipment could impose different constraints for operating pipelines in the best manner possible. One of the critical parameters that it is looked at closely, is the machines’ efficiency to avoid unfavorable operating conditions and to save energy costs. However, a compression-transport system includes more than one machine and more than one station working together at different conditions. Therefore, a detailed analysis of the entire compression system should be conducted to obtain a real power usage optimization. This paper presents a case study that is focused on analyzing natural gas transport system flow maximization while optimizing the usage of the available compression power. Various operating scenarios and machine spare philosophies are considered to identify the most suitable conditions for an optimum operation of the entire system. Modeling of pipeline networks has increased in the past decade due to the use of powerful computational tools that provide good quality representation of the real pipeline conditions. Therefore, a computational pipeline model was developed and used to simulate the gas transmission system. All the compressors’ performance maps and their driver data such as heat rate curves for the fuel consumption, site data, and running speed correction curves for the power were loaded in the model for each machine. The pipeline system covers 218 miles of hilly terrain with two looped pipelines of 38″ and 36″ in diameter. The entire system includes three compressor stations along its path with different configurations and equipment. For the optimization, various factors such as good efficiency over a wide range of operating conditions, maximum flexibility of configuration, fuel consumption and high power available were analyzed. The flow rate was maximized by using instantaneous maximum compression capacity at each station while maintaining fixed boundary conditions. This paper presents typical parameters that affect the energy usage in natural gas pipeline applications and discusses a case study that covers an entire pipeline. A modeling approach and basic considerations are presented as well as the results obtained for the optimization.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2011;134(2):022501-022501-7. doi:10.1115/1.4004143.

Engineered design of modern efficient turbomachinery based on accurate model predictions is of importance as operating speed and rate power increase. Industrial applications use hydrodynamic fluid film bearings as rotor support elements due to their advantages over rolling element bearings in operating speed, system stability (rotordynamic and thermal), and maintenance life. Recently, microturbomachinery (< 250 kW) implement gas foil bearings (GFBs) as its rotor supports due to its compact design without lubricant supply systems and enhanced stability characteristics. To meet the needs from manufacturers, the turbomachinery development procedure includes a rotordynamic design and a gas foil journal bearing (GFJB) analysis in general. The present research focuses on the role of gas foil thrust bearings (GFJBs) supporting axial load (static and dynamic) in an oil-free turbo blower with a 75 kW (100 HP) rate power at 30,000 rpm. The turbo blower provides a compressed air with a pressure ratio of 1.6 at a mass flow rate of 0.92 kg/s, using a centrifugal impeller installed at the rotor end. Two GFJBs with a diameter of 66mm and a length of 50 mm and one pair of GFTB with an outer diameter of 144 mm and an inner diameter of 74 mm support the rotor with an axial length of 493 mm and a weight of 12.7 kg. A finite element rotordynamic model prediction using predicted linearized GFJB force coefficients designs the rotor-GFB system with stability at the rotor speed of 30,000 rpm. Model predictions of the GFTB show axial load carrying performance. Experimental tests on the designed turbo blower; however, demonstrate unexpected large amplitudes of subsynchronous rotor lateral motions. Post-inspection reveals minor rubbing signs on the GFJB top foils and significant wear on the GFTB top foil. Therefore, GFTB is redesigned to have the larger outer diameter of 166 mm for the enhanced load capacity, i.e., 145%, increase in its loading area. The modification improves the rotor-GFB system performance with dominant synchronous motions up to the rate speed of 30,000 rpm. In addition, the paper studies the effect of GFTB tilting angles on the system performance. Insertion of shims between the GFTB brackets changes the bearing tilting angles. Model predictions show the decrease in the thrust load capacity by as large as 86% by increase in the tilting angle to 0.0006 rad (0.03438 deg). Experimental test data verify the computational model predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):022502-022502-8. doi:10.1115/1.4004145.

In a traditional turbine-generator set, rotor shaft designers and blade designers have their own models and design process which neglects the coupled effect. Since longer blade systems have recently been employed (Saito 1998, “Development of a 3000 rpm 43-in. last stage blade with high efficiency and reliability,” International Joint Power Generation Conference, pp. 89–96.) for advanced turbine sets to get higher output and efficiency, additional consideration is required concerning rotor bending vibrations coupled with a one-nodal (k = 1) blade system. Rotor-blade coupled bending conditions generally include two types so that the parallel and tilting modes of the shaft vibrations are respectively coupled with in-plane and out-of-plane modes of blade vibrations with a one-nodal diameter (k = 1). This paper proposes a method to calculate the natural frequency of a shaft blade coupled system. According to this modeling technique, a certain blade mode is reduced to a single mass system, which is connected to the displacement and angle motions of the shaft. The former motion is modeled by the m-k system to be equivalent to the blade on the rotating coordinate. The latter motion is commonly modeled in discrete form using the beam FEM on an inertia coordinate. Eigenvalues of the hybrid system covering both coordinates provide the natural frequency of the coupled system. In order to solve the eigenfrequencies of the coupled system, a tracking solver method based on sliding mode control concept is used. An eight-blade system attached to a cantilever bar is used for an example to calculate a coupled vibration with a one-nodal diameter between the blade and shaft.

Topics: Vibration , Blades , Equations
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):022503-022503-8. doi:10.1115/1.4004130.

As oil fields deplete, in particular in deep sea reservoirs, pump and compression systems work under more strenuous conditions with gas in liquid and liquid in gas mixtures, mostly inhomogeneous. Off-design operation affects system overall efficiency and reliability, including penalties in leakage and rotordynamic performance of secondary flow components, namely seals. The paper details a bulk-flow model for annular damper seals operating with gas in liquid mixtures. The analysis encompasses all-liquid and all-gas seals, as well as seals lubricated with homogenous (bubbly) mixtures, and predicts the static and dynamic force response of mixture lubricated seals; namely: leakage, power loss, reaction forces, and rotordynamic force coefficients, etc., as a function of the mixture volume fraction (βS ), supply and discharge pressures, rotor speed, whirl frequency, etc. A seal example with a nitrogen gas mixed with light oil is analyzed. The large pressure drop (70 bar) causes a large expansion of the gas within the seal even for (very) small gas volume fractions (βS ). Predictions show leakage and power loss decrease as β1; albeit at low βS (< 0.3) (re)laminarization of the flow and an apparent increase in mixture viscosity, produce a hump in power loss. Cross-coupled stiffnesses and direct damping coefficients decrease steadily with increases in the gas volume fraction; however, some anomalies are apparent when the flow turns laminar. Mixture lubricated seals show frequency-dependent force coefficients. The equivalent damping decreases above and below βS  ∼ 0.10. The direct stiffness coefficients show atypical behavior: a low βS  = 0.1 produces stiffness hardening as the excitation frequency increases. Recall that an all liquid seal has a dynamic stiffness softening as frequency increases due to the apparent fluid mass. The predictions call for an experimental program to quantify the static and dynamic forced performance of annular seals operating with (bubbly) mixtures and to validate the current predictive model results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):022504-022504-11. doi:10.1115/1.4004214.

This work is aimed at a theoretical study of the dynamic behavior of a rotor-tilting pad journal bearing (TPJB) system under different lubrication regimes, namely, thermohydrodynamic (THD), elastohydrodynamic (EHD), and hybrid lubrication regime. The rotor modeled corresponds to an industrial compressor. Special emphasis is put on analyzing the stability map of the rotor when the different lubrication regimes are included into the TPJB modeling. Results show that, for the studied rotor, the inclusion of a THD model is more relevant when compared to an EHD model, as it implies a reduction on the instability onset speed for the rotor. Also, results show the feasibility of extending the stable operating range of the rotor by implementing a hybrid lubrication regime.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2011;134(2):022801-022801-11. doi:10.1115/1.4003971.

Concern about the depletion of petroleum reserves, rising prices of conventional fuels, security of supply and global warming have driven research toward the development of renewable fuels for use in diesel engines. These fuels have different physical and chemical properties that affect the diesel combustion process. This paper compares between the autoignition, combustion, performance and emissions of soy-bean derived biodiesel, Jet propellant (JP-8) and ultra low sulfur diesel (ULSD) in a high speed single-cylinder research diesel engine equipped with a common rail injection system. Tests were conducted at steady state conditions at different injection pressures ranging from 600 bar to 1200 bar. The ‘rate of heat release’ traces are analyzed to determine the effect of fuel properties on the ignition delay, premixed combustion fraction and mixing and diffusion controlled combustion fractions. Biodiesel produced the largest diffusion controlled combustion fraction at all injection pressures compared to ULSD and JP-8. At 600 bar injection pressure, the diffusion controlled combustion fraction for biodiesel was 53% whereas both JP-8 and ULSD produced 39%. In addition, the effect of fuel properties on engine performance, fuel economy, and engine-out emissions is determined. On an average JP-8 produced 3% higher thermal efficiency than ULSD. Special attention is given to the oxides of nitrogen (NOx ) emissions and particulate matter characteristics. On an average biodiesel produced 37% less NOx emissions compared to ULSD and JP-8.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):022802-022802-9. doi:10.1115/1.4003973.

JP-8 is being closely watched as a suitable fuel for the “One fuel policy” by US Army. Some of the main targets of Army in the battle are the fuel economy and smoke/soot emissions. Soot emissions can be reduced in two ways, by increasing the injection pressure or by increasing swirl. An investigation was conducted to find out the more effective way to reduce soot emissions and to evaluate the influence of the swirl motion on JP-8 fuel combustion, performance and emissions in a single cylinder diesel engine. Increasing swirl increased heat losses and produced lower temperatures during injection process. Increasing swirl improved the premixed combustion fraction and produced higher peak temperatures and in turn increased NOx emissions. Increasing swirl also increased the nano-particle emissions.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Gas Turbines Power. 2011;134(2):024501-024501-5. doi:10.1115/1.4004401.

Identifying thermal characteristics of gas foil bearings (GFBs) provides an insight for successful implementation into high speed oil-free turbomachinery. The paper presents temperature measurements of a bump type GFB floating on a hollow shaft for various operating conditions. Two angular ball bearings support the hollow shaft at one end (right), and the other end (left) is free. Test GFB has the outer diameter of 100 mm and the axial length of 45 mm, and the hollow shaft has the outer and inner diameters of 60 mm and 40 mm, respectively. An electric motor drives the hollow shaft using a spline coupling connection. A mechanical loading device provides static loads on test GFB upward via a metal wire, and a strain gauge type load cell placed in the middle of the wire indicates the applied loads. During experiments for shaft speeds of 5 krpm, 10 krpm, and 15 krpm and with static loads of 58.9 N (6 kgf ), 78.5 N (8 kgf ), and 98.1 N (10 kgf ), twelve thermocouples measure the outer surface temperatures of test GFB at four angular locations of 45 deg, 135 deg, 215 deg, and 315 deg, with an origin at the top foil free end, and three axial locations of bearing centerline and both side edges at each angle. Two infrared thermometers measure the outer surface temperature of the hollow shaft at free and supported ends close to test GFB. Test results show that GFB temperatures increase as the shaft speed increases and as the static load increases, with higher temperatures in the loaded zone (135 deg and 215 deg) than those in the unloaded zone (45 deg and 315 deg). In general, the recorded temperatures are highest at 225 deg where a highest hydrodynamic pressure is expected to build up. Measured temperatures at the bearing centerline are higher than those at the side edges, as expected. In addition, large thermal gradients are recorded in the hollow shaft along the axial direction with higher temperatures at the supported end. The axial thermal gradient of the shaft is thought to cause higher temperatures at the bearing right edge facing the ball bearing support than those at the left edge. The paper presents test data along with detailed test GFB/shaft geometries and material properties.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;134(2):024502-024502-5. doi:10.1115/1.4004394.

An energy-based fatigue lifing procedure for the determination of full-life and critical-life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce fifteen fracture curves at four operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.

Commentary by Dr. Valentin Fuster

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