Research Papers: Gas Turbines: Ceramics

J. Eng. Gas Turbines Power. 2008;131(2):021301-021301-6. doi:10.1115/1.2969091.

Foreign object damage behavior of an oxide/oxide (N720/AS) ceramic matrix composite was determined at ambient temperature using impact velocities ranging from 100 m/s to 400 m/s by 1.59 mm diameter steel-ball projectiles. Two different support configurations of target specimens were used: fully supported and partially supported. The degree of post-impact strength degradation increased with increasing impact velocity and was greater in a partially supported configuration than in a fully supported one. For the fully supported configuration, frontal contact stress played a major role in generating composite damage, while for the partially supported case, both frontal contact and backside flexure stresses were the combined sources of damage generation. The oxide/oxide composite was able to survive high energy (1.3J) impacts without complete structural failure. The degree of relative post-impact strength degradation of the oxide/oxide composite was similar to that of an advanced SiC/SiC composite observed from a previous study, regardless of the type of specimen support. Like the SiC/SiC composite, impact-damage tolerance was greater in the oxide/oxide than in monolithic silicon nitride ceramics for impact velocities >300m/s.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2008;131(2):021501-021501-8. doi:10.1115/1.2981181.

Conventional ignition systems of aeroengines are an integral part of the combustion chamber’s structure. Due to this hardware-related constraint, the ignition spark has to be generated in the quench zone of the combustion chamber, which is far from the optimum regarding thermo- and aerodynamics. An improved ignitability of the fuel-air mixture can be found in the central zone of the combustor, where higher local equivalence ratios prevail and where mixing is favorable for a smooth ignition. It would be a major advancement in aeroengine design to position the ignition kernel in these zones. A laser system is able to ignite the fuel-air mixture at almost any location inside of the combustion chamber. Commercial laser systems are under development, which can replace conventional spark plugs in internal combustion engines and gas turbines. This study was conducted to evaluate the applicability of laser ignition in liquid-fueled aeroengines. Ignition tests were performed with premixed natural gas and kerosene to evaluate the different approaches of laser and spark plug ignition. The experiments were carried out on a generic test rig with a well-investigated swirler, allowing sufficient operational flexibility for parametric testing. The possibility of the free choice of the laser’s focal point is the main advantage of laser-induced ignition. Placing the ignition kernel at the spray cone’s shear layer or at favorable locations in the recirculation zone could significantly increase the ignitability of the system. Consequently, the laser ignition of atomized kerosene was successfully tested down to a global equivalence ratio of 0.23. Furthermore, the laser outperformed the spark plug at ignition locations below axial distances of 50 mm from the spray nozzle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):021502-021502-10. doi:10.1115/1.2969088.

In the design process, new burners are generally tested in combustion test rigs. With these experiments, computational fluid dynamics, and finite element calculations, the burners’ performance in the full-scale engine is sought to be predicted. Especially, information about the thermoacoustic behavior and the emissions is very important. As the thermoacoustics strongly depend on the acoustic boundary conditions of the system, it is obvious that test rig conditions should match or be close to those of the full-scale engine. This is, however, generally not the case. Hence, if the combustion process in the test rig is stable at certain operating conditions, it may show unfavorable dynamics at the same conditions in the engine. In previous works, the authors introduced an active control scheme, which is able to mimic almost arbitrary acoustic boundary conditions. Thus, the test rig properties can be tuned to correspond to those of the full-scale engine. The acoustic boundary conditions were manipulated using woofers. In the present study, proportional valves are investigated regarding their capabilities of being used in the control scheme. It is found that the test rig impedance can be tuned equally well. In contrast to the woofers, however, the valves could be used in industrial applications, as they are more robust and exhibit more control authority. Additionally, the control scheme is further developed and used to tune the test rig at discrete frequencies. This exhibits certain advantages compared with the case of control over a broad frequency band.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):021503-021503-7. doi:10.1115/1.2836613.

Reheat combustion has been proven now in over 80units to be a robust and highly flexible gas turbine concept for power generation. This paper covers three key topics to explain the intrinsic advantage of reheat combustion to achieve ultralow emission levels. First, the fundamental kinetic and thermodynamic emission advantage of reheat combustion is discussed, analyzing in detail the emission levels of the first and second combustor stages, optimal firing temperatures for minimal emission levels, as well as benchmarking against single-stage combustion concepts. Second, the generic operational and fuel flexibility of the reheat system is emphasized, which is based on the presence of two fundamentally different flame stabilization mechanisms, namely, flame propagation in the first combustor stage and autoignition in the second combustor stage. This is shown using simple reasoning on generic kinetic models. Finally, the present fleet status is reported by highlighting the latest combustor hardware upgrade and its emission performance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):021504-021504-8. doi:10.1115/1.2969093.

Lean premixed natural gas/air flames produced by an industrial gas turbine burner were analyzed using laser diagnostic methods. For this purpose, the burner was equipped with an optical combustion chamber and operated with preheated air at various thermal powers P, equivalence ratios Φ, and pressures up to p=6bars. For the visualization of the flame emissions OH chemiluminescence imaging was applied. Absolute flow velocities were measured using particle image velocimetry (PIV), and the reaction zones as well as regions of burnt gas were characterized by planar laser-induced fluorescence (PLIF) of OH. Using these techniques, the combustion behavior was characterized in detail. The mean flow field could be divided into different regimes: the inflow, a central and an outer recirculation zone, and the outgoing exhaust flow. Single-shot PIV images demonstrated that the instantaneous flow field was composed of small and medium sized vortices, mainly located along the shear layers. The chemiluminescence images reflected the regions of heat release. From the PLIF images it was seen that the primary reactions are located in the shear layers between the inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2008;131(2):021601-021601-9. doi:10.1115/1.2978994.

The ultimate proof of the soundness and viability of a novel technology is a full-scale demonstration test in which actual components are run successfully over the entire operating envelope. Consequently, the collection of accurate and meaningful test data is of utmost importance to the success of the test. An analysis of such data will validate the original design concepts and will lead to paths of further improvement for the next generations thereof. Statistical fundamentals to determine the accuracy and precision of measured data are amply documented and readily available in well-established standards. The yardstick that should be used for the “meaningfulness” of the measured test data is the satisfaction of the fundamental laws of conservation. While it is known that the “true” values of the sensor data when inserted into the governing equations for the tested component will result in perfect balances, “actual” measured values will always result in “imbalances.” Therefore, reconciliation of the individual measurements with the governing conservation equations is a must prior to the actual analysis of the data. Reconciliation in this context is an estimation of the true values of the sensor data from the actual sensor data by using statistical concepts. This paper describes the development of a data reconciliation concept that is universally applicable to any process or power plant system where sensor data are used. The usefulness and power of the technique are demonstrated by its application to a single-shaft combined cycle with both gas turbine and steam turbine driving a common generator. In the absence of a reliable and accurate measuring system to individually determine gas and steam turbine shaft outputs, data reconciliation is vital to an accurate analysis of the data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):021602-021602-12. doi:10.1115/1.2969092.

The present paper proposes a concept for a water-cooled high temperature unsteady total pressure probe intended for measurements in the hot sections of industrial gas turbines or aero-engines. This concept is based on the use of a conventional miniature piezoresistive pressure sensor, which is located at the probe tip to achieve a bandwidth of at least 40 kHz. Due to extremely harsh conditions and the intention to immerse the probe continuously into the hot gas stream, the probe and sensor must be heavily cooled. The short term objective of this design is to gain the capability of performing measurements at the temperature conditions typically found at high pressure turbine exit (1100–1400 K) and in the long term at combustor exit (2000 K or higher).

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2008;131(2):021701-021701-7. doi:10.1115/1.2982147.

This paper presents the results of an evaluation of advanced combined cycle gas turbine plants with precombustion capture of CO2 from natural gas. In particular, the designs are carried out with the objectives of high efficiency, low capital cost, and low emissions of carbon dioxide to the atmosphere. The novel cycles introduced in this paper are comprised of a high-pressure syngas generation island, in which an air-blown partial oxidation reformer is used to generate syngas from natural gas, and a power island, in which a CO2-lean syngas is burnt in a large frame machine. In order to reduce the efficiency penalty of natural gas reforming, a significant effort is spent evaluating and optimizing alternatives to recover the heat released during the process. CO2 is removed from the shifted syngas using either CO2 absorbing solvents or a CO2 membrane. CO2 separation membranes, in particular, have the potential for considerable cost or energy savings compared with conventional solvent-based separation and benefit from the high-pressure level of the syngas generation island. A feasibility analysis and a cycle performance evaluation are carried out for large frame gas turbines such as the 9FB. Both short-term and long-term solutions have been investigated. An analysis of the cost of CO2 avoided is presented, including an evaluation of the cost of modifying the combined cycle due to CO2 separation. The paper describes a power plant reaching the performance targets of 50% net cycle efficiency and 80% CO2 capture, as well as the cost target of 30$ per ton of CO2 avoided (2006 Q1 basis). This paper indicates a development path to this power plant that minimizes technical risks by incremental implementation of new technology.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2008;131(2):022101-022101-7. doi:10.1115/1.2979747.

Thermal barrier coatings (TBCs) have been recently introduced to hot section components, such as transition pieces and the first two stages of turbine blades and vanes of advanced F, G, and H class land-based turbine engines. The TBC is typically applied on metallic-coated components. The metallic bond coat provides oxidation and/or corrosion protection. It is generally believed that the primary failure mode of TBCs is delamination and fracture of the top ceramic coating parallel to the bond coat in the proximity of the thermally grown oxide (TGO) between coatings. One of the concerns associated with the use of a TBC as a prime reliant coating is its long-term stability. The effect of long-term operation at typical land based turbine operating temperatures of below 1010°C (1850°F) of the failure mode of TBCs is unknown. Long-term isothermal tests were conducted on the thermal barrier-coated specimens at three temperatures, 1010°C (1850°F), 1038°C (1900°F), and 1066°C (1950°F), to determine the effects of long term exposure on the TBC failure location (mode). Following the isothermal testing, the samples were destructively examined to characterize the degradation of the TBC and determine the extent of TGO cracking, TGO growth, bond coat oxidation, and TBC failure location after long term exposure for up to 18,000h. Optical microscopy and a scanning electron microscope (SEM) attached with an energy dispersive spectroscopy (EDS) system were used to study the degradation of the TBC and bond coatings. The results showed that long term isothermal exposure leads to a change in the TBC failure mode from the delamination of the TBC at the TGOTBC interface to the internal oxidation of the bond coat and bond coat delamination. In this paper, the effect of long-term exposure on the delamination of TBC and the bond coat failure mode is discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2008;131(2):022301-022301-6. doi:10.1115/1.2982151.

High speed permanent magnet (PM) machines are used in microturbine applications due to their compactness, robust construction, and high efficiency characteristics. These machines are integrated with the turbines and rotate at the same speeds. This paper discusses in detail the losses in high speed PM machines. A typical PM machine designed for microturbine application is presented with its detailed loss calculations. Various loss verification methods are also discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2008;131(2):022501-022501-7. doi:10.1115/1.2966392.

As gas foil journal bearings become more prevalent in production machines, such as small gas turbine propulsion systems and microturbines, system level performance issues must be identified and quantified in order to provide for successful design practices. Several examples of system level design parameters that are not fully understood in foil bearing systems are thermal management schemes, alignment requirements, balance requirements, thrust load balancing, and others. In order to address some of these deficiencies and begin to develop guidelines, this paper presents a preliminary experimental investigation of the misalignment tolerance of gas foil journal bearing systems. Using a notional gas foil bearing supported rotor and a laser-based shaft alignment system, increasing levels of misalignment are imparted to the bearing supports while monitoring temperature at the bearing edges. The amount of misalignment that induces bearing failure is identified and compared with other conventional bearing types such as cylindrical roller bearings and angular contact ball bearings. Additionally, the dynamic response of the rotor indicates that the gas foil bearing force coefficients may be affected by misalignment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022502-022502-8. doi:10.1115/1.2979004.

The present work describes the detailed design and operational capabilities of a general purpose test facility developed to evaluate the dynamics and performance of gas lubricated journal bearings. The component level test facility was developed to serve as an initial tollgate test platform for certifying gas lubricated journal bearings into aircraft engine applications. A rotating test rig was engineered to test 70–120 mm diameter bearings at 40,000–80,000 rpm and 1200°F. The test rig described in this paper possesses design elements that enable the simultaneous application of dynamic and static load profiles of up to 1000 lb while monitoring and measuring the bearing torque. This capability allows for the characterization of several critical metrics such as bearing lift off speed characteristics, load capacity, and frequency dependent rotordynamic force coefficients. This paper discusses the functionality of the test facility and presents sample test measurements from several experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022503-022503-11. doi:10.1115/1.2967476.

The following work presents a new type of hybrid journal bearing developed for enabling oil-free operation of high performance turbomachinery. The new design integrates compliant hydrostatic-hydrodynamic partitioned bearing pads with two flexibly mounted integral wire mesh dampers. The primary aim of the new bearing configuration was to maximize the load-carrying capacity and effective damping levels while maintaining adequate compliance to misalignment and variations in rotor geometry. The concept of operation is discussed along with the description of the bearing design. Several experiments using room temperature air as the working fluid were performed that demonstrate proof of concept, which include lift-off tests, bearing load tests, and rotordynamic characterization tests. The experiments demonstrate stable operation to 40,000 rpm (2.8×106 DN) of a 2.750 in. (70 mm) diameter bearing. In addition to the experimental results, an analytical model is presented for the compliant bearing system. The aeroelastic theory couples the steady state numerical solution of the compressible Reynolds flow equation with a flexible structure possessing translational and rotational compliance. This was achieved by formulating a fluid-structure force balance for each partitioned bearing pad while maintaining a global mass flow balance through the hydrostatic restrictors and bearing lands. Example numerical results for pad pressure profile, film thickness, torque, and leakage are shown.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022504-022504-12. doi:10.1115/1.2980058.

The fatigue crack growth behavior of Grainex Mar-M 247 is evaluated for NASA’s turbine seal test facility. The facility is used to test air-to-air seals primarily for use in advanced jet engine applications. Because of extreme seal test conditions of temperature, pressure, and surface speeds, surface cracks may develop over time in the disk bolt holes. An inspection interval is developed to preclude catastrophic disk failure by using experimental fatigue crack growth data. By combining current fatigue crack growth results with previous fatigue strain-life experimental work, an inspection interval is determined for the test disk. The fatigue crack growth life of NASA disk bolt holes is found to be 367cycles at a crack depth of 0.501mm using a factor of 2 on life at maximum operating conditions. Combining this result with previous fatigue strain-life experimental work gives a total fatigue life of 1032cycles at a crack depth of 0.501mm. Eddy-current inspections are suggested starting at 665cycles since eddy current detection thresholds are currently at 0.381mm. Inspection intervals are recommended every 50cycles when operated at maximum operating conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022505-022505-11. doi:10.1115/1.2940356.

In this paper, a forced response prediction method for the analysis of constrained and unconstrained structures coupled through frictional contacts is presented. This type of frictional contact problem arises in vibration damping of turbine blades, in which dampers and blades constitute the unconstrained and constrained structures, respectively. The model of the unconstrained/free structure includes six rigid body modes and several elastic modes, the number of which depends on the excitation frequency. In other words, the motion of the free structure is not artificially constrained. When modeling the contact surfaces between the constrained and free structure, discrete contact points along with contact stiffnesses are distributed on the friction interfaces. At each contact point, contact stiffness is determined and employed in order to take into account the effects of higher frequency modes that are omitted in the dynamic analysis. Depending on the normal force acting on the contact interfaces, quasistatic contact analysis is initially employed to determine the contact area as well as the initial preload or gap at each contact point due to the normal load. A friction model is employed to determine the three-dimensional nonlinear contact forces, and the relationship between the contact forces and the relative motion is utilized by the harmonic balance method. As the relative motion is expressed as a modal superposition, the unknown variables, and thus the resulting nonlinear algebraic equations in the harmonic balance method, are in proportion to the number of modes employed. Therefore the number of contact points used is irrelevant. The developed method is applied to a bladed-disk system with wedge dampers where the dampers constitute the unconstrained structure, and the effects of normal load on the rigid body motion of the damper are investigated. It is shown that the effect of rotational motion is significant, particularly for the in-phase vibration modes. Moreover, the effect of partial slip in the forced response analysis and the effect of the number of harmonics employed by the harmonic balance method are examined. Finally, the prediction for a test case is compared with the test data to verify the developed method.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022506-022506-10. doi:10.1115/1.2968868.

The problem of determining the maximum forced response vibration amplification that can be produced just by the addition of a small mistuning to a perfectly cyclical bladed disk still remains not completely clear. In this paper we apply a recently introduced perturbation methodology, the asymptotic mistuning model (AMM), to determine which are the key ingredients of this amplification process and to evaluate the maximum mistuning amplification factor that a given modal family with a particular distribution of tuned frequencies can exhibit. A more accurate upper bound for the maximum forced response amplification of a mistuned bladed disk is obtained from this description, and the results of the AMM are validated numerically using a simple mass-spring model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022507-022507-10. doi:10.1115/1.2982159.

Auxiliary bearings are used to prevent rotor/stator contact in active magnetic bearing systems. They are sacrificial components providing a physical limit on the rotor displacement. During rotor/auxiliary bearing contact significant forces normal to the contact zone may occur. Furthermore, rotor slip and rub can lead to localized frictional heating. Linear control strategies may also become ineffective or induce instability due to changes in rotordynamic characteristics during contact periods. This work considers the concept of using actively controlled auxiliary bearings in magnetic bearing systems. Auxiliary bearing controller design is focused on attenuating bearing vibration resulting from contact and reducing the contact forces. Controller optimization is based on the H norm with appropriate weighting functions applied to the error and control signals. The controller is assessed using a simulated rotor/magnetic bearing system. Comparison of the performance of an actively controlled auxiliary bearing is made with that of a resiliently mounted auxiliary bearing. Rotor drop tests, repeated contact tests, and sudden rotor unbalance resulting in trapped contact modes are considered.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022508-022508-11. doi:10.1115/1.2968869.

Forming the first part of a two-part paper, the experimental approach to acquire resonant vibration data is presented here. Part II deals with the estimation of damping. During the design process of turbomachinery components, mechanical integrity has to be guaranteed with respect to high cycle fatigue of blades subject to forced response or flutter. This requires the determination of stress levels within the blade, which in turn depend on the forcing function and damping. The vast majority of experimental research in this field has been performed on axial configurations for both compressors and turbines. This experimental study aims to gain insight into forced response vibration at resonance for a radial compressor. For this purpose, a research impeller was instrumented with dynamic strain gauges and operated under resonant conditions. Modal properties were analyzed using finite element method and verified using an optical method termed electronic-speckle-pattern-correlation-interferometry. During the experiment, unsteady forces acting on the blades were generated by grid installations upstream of the impeller, which created a distorted inlet flow pattern. The associated flow properties were measured using an aerodynamic probe. The resultant pressure fluctuations on the blade surface and the corresponding frequency content were assessed using unsteady computational fluid dynamics. The response of the blades was measured for three resonant crossings, which could be distinguished by the excitation order and the natural frequency of the blades. Measurements were undertaken for a number of inlet pressure settings starting at near vacuum and then increasing. The overall results showed that the installed distortion screens generated harmonics in addition to the fundamental frequency. The resonant response of the first and the second blade mode showed that the underlying dynamics support a single-degree-of-freedom model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022509-022509-9. doi:10.1115/1.2968870.

Forming the second part of a two-part paper, the estimation of damping is presented here. Part I discusses the experimental approach and the results on blade resonant response measurements. In the study of forced response, damping is a crucial parameter, which is measured to quantify the ability of a vibrating system to dissipate vibratory energy in response to a given excitation source. The blading of turbomachinery components is particularly prone to forced response excitation, which is one of the major causes of high cycle fatigue failure during operation. In turbocharging applications, forced response cannot be avoided due to a number of factors, i.e., change in speed, inlet bends, or obstructions in the flow field. This study aims to quantify the damping parameter for the lightly damped blades of a centrifugal compressor. The impeller geometry is typical of turbocharging applications. As a first step, the nonrotating impeller was excited using piezos, and the transfer function was derived for a number of pressure settings. Both circle-fit and curve-fit procedures were used to derive material damping. In the second step, measurements were taken in the test facility where forced response conditions were generated using distortion screens upstream of the impeller. The main blade strain response was measured by sweeping through a number of resonant points. A curve-fit procedure was applied to estimate the critical damping ratio. The contributions of material and aerodynamic dampings were derived from a linear curve-fit applied to the damping data as a function of inlet pressure. Overall, it will be shown that aerodynamic damping dominates the dissipation process for applications with an inlet pressure of 1 bar. Damping was found to depend on the throttle setting of the compressor, and where applicable computational fluid dynamics results were used to point toward the possible causes of this effect.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022510-022510-9. doi:10.1115/1.2969094.

An effective method has been developed to calculate the sensitivity of the resonance peak frequency and forced response level to variation of parameters of nonlinear friction contact interfaces and excitation. The method allows determination of the sensitivity characteristics simultaneously with the resonance peak frequency and response level calculated as a function of any parameter of interest and without significant computational expense. Capabilities of the method are demonstrated on examples of analysis of large-scale finite element models of realistic bladed disks with major types of the nonlinear contact interfaces: (i) a blisk with underplatform dampers, (ii) a bladed disk with friction damping at blade fir-tree roots, and (iii) a high-pressure bladed disk with shroud contacts. The numerical investigations show high efficiency of the method proposed.

Topics: Resonance , Friction , Disks
Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2008;131(2):022801-022801-6. doi:10.1115/1.2978995.

This paper describes experimental research aimed at developing an on-board smoke sensor for diesel engines. The sensor element was similar to a conventional spark plug. Electrical heating of the insulator was used to prevent carbon fouling from the diesel soot. The sensing element created sparks within the exhaust pipe and changes in smoke levels were detected through analysis of the voltage levels of the sparks. The system was tested in a heavy duty diesel engine equipped with exhaust gas recirculation (EGR) and compared with reference measurements of the filter smoke number (FSN). The experiments showed good sensitivity to step changes in smoke levels (accomplished by varying EGR levels) at smoke levels below 0.5 FSN. However, the sensor suffered from temperature induced signal drift and was unstable under some circumstances. The use of a spark plug with a smaller electrode tip diameter improved the signal stability. It is proposed that measurement and control of the electrode temperature will be necessary to control the signal drift.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022802-022802-8. doi:10.1115/1.3019006.

The influence of inlet liquid fuel temperature on direct-injection diesel engines can be noticeable and significant. The work in this paper investigates the effects of inlet fuel temperature on fuel-injection in-cylinder combustion, and output performance and emissions of medium-speed diesel engines. An enhanced understanding and simplified modeling of the variations in the main fuel-injection parameters affected by inlet fuel temperature are developed. The study indicates that the main injection parameters affected include the injection timing at the injector end relative to the injection-pump actuation timing, the fuel-injection rate, the fuel-injection duration, and the injection spray atomization. The primary fuel temperature effects on the injection parameters are from the fuel bulk modulus of elasticity and the density with the fuel viscosity less significant as the injector-nozzle flow is usually in a turbulent region. The developed models are able to predict the changes in the injection parameters versus the inlet fuel temperature. As the inlet fuel temperature increases, the nozzle fuel-injection-start timing is predicted to be relatively retarded, the injection rate is reduced, and the needle-lift duration is prolonged from the baseline. The variation trends of the engine outputs and emissions versus fuel temperature are analyzed by considering its consequent effect on in-cylinder combustion processes. It is predicted that raising fuel temperature would result in an increase in each of CO, HC, PM, and smoke emissions, and in a decrease in NOx, and may adversely affect the fuel efficiency for a general type of diesel engine at a full-load condition. The experimental results of the outputs and emissions from testing a medium-speed four-stroke diesel engine agreed with the trends analytically predicted. The understanding and models can be applied to compression-ignition direct-injection liquid fuel engines in general.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022803-022803-7. doi:10.1115/1.3019331.

Compression ignition engine technologies have been advanced in the past decade to provide superior fuel economy and high performance. These technologies offer increased opportunities for optimizing engine calibration. Current engine calibration methods rely on deriving static tabular relationships between a set of steady-state operating points and the corresponding values of the controllable variables. While the engine is running, these values are being interpolated for each engine operating point to coordinate optimal performance criteria, e.g., fuel economy, emissions, and acceleration. These methods, however, are not efficient in capturing transient engine operation designated by common driving habits, e.g., stop-and-go driving, rapid acceleration, and braking. An alternative approach was developed recently, which makes the engine an autonomous intelligent system, namely, one capable of learning its optimal calibration for both steady-state and transient operating points in real time. Through this approach, while the engine is running the vehicle, it progressively perceives the driver’s driving style and eventually learns to operate in a manner that optimizes specified performance criteria. The major challenge to this approach is problem dimensionality when more than one controllable variable is considered. In this paper, we address this problem by proposing a decentralized learning control scheme. The scheme is evaluated through simulation of a diesel engine model, which learns the values of injection timing and variable geometry turbocharging vane position that optimize fuel economy and pollutant emissions over a segment of the FTP-75 driving cycle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022804-022804-14. doi:10.1115/1.2938394.

In “multijet” common rail (CR) diesel injection systems, when two consecutive injection current pulses approach each other, a merging of the two injections into a single one can occur. Such an “injection fusion” causes an undesired excessive amount of injected fuel, worsening both fuel consumption and particulate emissions. In order to avoid this phenomenon, lower limits to the dwell-time values are introduced in the control unit maps by a conservatively overestimated threshold, which reduces the flexibility of multiple-injection management. The injection fusion occurrence is mainly related to the time delay between the electrical signal to the solenoid and the nozzle opening and closure. The dwell-time fusion threshold was found to strongly decrease particularly with the nozzle closure delay. A functional dependence of the nozzle opening and closure delays on the solenoid energizing time and nominal rail pressure was experimentally assessed, and the injection temporal duration was correlated to the energizing time and rail pressure. A multijet CR injection-system mathematical model that was previously developed, including thermodynamics of liquids, fluid dynamics, mechanics of subsystems, and electromagnetism equations, was applied to better understand the cause and effect relationships for nozzle opening and closure delays. In particular, numerical results on the time histories of delivery- and control-chamber pressures, pilot- and needle-valve lifts, and mass flow rates through Z and A holes were obtained and analyzed to highlight the dependence of nozzle opening and closure delays on injector geometric features, physical variables, and valve dynamics. The nozzle closure delay was shown to strongly depend on the needle dynamics. Parametric numerical tests were carried out to identify configurations useful for minimizing the nozzle closure delay. Based on the results of these tests, a modified version of a commercial electroinjector was built, so as to achieve effectively lower nozzle closure delays and very close sequential injections without any fusion between them.

Commentary by Dr. Valentin Fuster

Research Papers: Nuclear Power

J. Eng. Gas Turbines Power. 2008;131(2):022901-022901-6. doi:10.1115/1.3032392.

To explore the mechanism of differential pressure fluctuation inducing cross flow between subchannels in the tight-lattice rod bundle, an evaluation method is presented, which permits the prediction in detail of the unsteady differential pressure fluctuation behavior between subchannels. The instantaneous fluctuation of differential pressure between two subchannels in gas-liquid slug flow regime is deemed as a result of the intermittent nature of slug flow in each subchannel. The method is based on the detailed numerical simulation result of two-phase flow that pressure drop occurs mainly in the liquid slug region and it is, however, negligibly small in the bubble region. The instantaneous fluctuation of differential pressure between two subchannels is associated with pressure gradient in the liquid slug for each channel. In addition to a hydrostatic gradient, acceleration and frictional gradients are taken into account to predict pressure gradient in the liquid slug. This method used in conjunction with the numerical simulation code works satisfactorily to reproduce numerical simulation results for instantaneous fluctuation of differential pressure between two modeled subchannels. It is shown that the static head, acceleration, and frictional pressure drops in the liquid slug are main contributions to the fluctuation of differential pressure between subchannels.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;131(2):022902-022902-7. doi:10.1115/1.3032417.

The concept of an innovative plate-type steam generator (SG) for a fast reactor, fabricated using hot isostatic pressing (HIP), was presented. The heat-transfer plate, which is assembled with rectangular tubes and fabricated using HIP, is surrounded by a leak-detection layer. The optimum form for the detection layer was determined by crack extension analysis. The concept can be applied in both pool-type and loop-type liquid-metal cooled fast reactors (LMFRs). In this report, the fabrication technique is described as applied to a loop-type LMFR. Optimum HIP conditions of 1423 K (1150°C), 1200kgf/cm2(117MPa), and 3 h for modified 9Cr–1Mo steel were determined from HIP tests, tensile tests, and structural inspection. Nickel-type solder (BNi-5) and gold-type solder (BAu-4) were examined as potential joining materials to laminate the heat-transfer plates. Tensile test comparisons showed that BAu-4 was superior, so it was used. No problems were observed in the fabrication of a partial model of a SG (HIP of the rectangular tubes, brazing the plates, welding the header, etc.)

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2009;131(2):022903-022903-6. doi:10.1115/1.3032437.

The paper describes the application of computational fluid dynamics (CFD) in simulating density wave oscillations in triangular and square pitch rod bundles. The FLUENT code is used for this purpose, addressing typical conditions proposed for supercritical water reactor (SCWR) conceptual design. The RELAP5 code and an in-house 1D linear stability code are also adopted to compare the results for instability thresholds obtained by different techniques. Transient analyses are performed both by the CFD code and RELAP5 , with increasing heating rates and constant pressure drop across the channel, up to the occurrence of unstable behavior. The obtained results confirm that the density wave mechanism is similar in rod bundle and in axisymmetric configurations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2009;131(2):022904-022904-7. doi:10.1115/1.3032461.

Between 1973 and 1990 four units of the Russian nuclear power plants type WWER-440/230 were operated in Greifswald (former East Germany). Material probes from the pressure vessels were gained in the frame of the ongoing decommissioning procedure. The investigations of this material started with material from the circumferential core weld of unit 1. First, this paper presents results of the reactor pressure vessel (RPV) fluence calculations depending on different loading schemes and on the axial weld position based on the Monte Carlo code TRAMO . The results show that the use of the dummy assemblies reduces the flux by a factor of 2–5 depending on the azimuthal position. The circumferential core weld (SN0.1.4) received a fluence of 2.4×1019neutrons/cm2 at the inner surface; it decreases to 0.8×1019neutrons/cm2 at the outer surface. The material investigations were done using a trepan from the circumferential core weld. The reference temperature T0 was calculated with the measured fracture toughness values, KJc, at brittle failure of the specimen. The KJc values show a remarkable scatter. The highest T0 was about 50°C at a distance of 22 mm from the inner surface of the weld. The Charpy transition temperature TT41J estimated with results of subsized specimens after the recovery annealing was confirmed by the testing of standard Charpy V-notch specimens. The VERLIFE procedure prepared for the integrity assessment of WWER RPV was applied on the measured results. The VERLIFE lower bound curve indexed with the Structural Integrity Assessment Procedures for European Industry (SINTAP) reference temperature, RTT0SINTAP, envelops the KJc values. Therefore for a conservative integrity assessment the fracture toughness curve indexed with a RT representing the brittle fraction of a data set of measured KJc values has to be applied.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2009;131(2):022905-022905-8. doi:10.1115/1.3043816.

Different scenarios of small break loss of coolant accident for pressurized water reactors (PWRs) lead to the reflux-condenser mode in which steam enters the hot leg from the reactor pressure vessel (RPV) and condenses in the steam generator. A limitation of the condensate backflow toward the RPV by the steam flowing in counter current could affect the core cooling and must be prevented. The simulation of counter-current flow limitation conditions, which is dominated by 3D effects, requires the use of a computational fluid dynamics (CFD) approach. These numerical methods are not yet mature, so dedicated experimental data are needed for validation purposes. In order to investigate the two-phase flow behavior in a complex reactor-typical geometry and to supply suitable data for CFD code validation, the “hot leg model” was built at Forschungszentrum Dresden-Rossendorf (FZD). This setup is devoted to optical measurement techniques, and therefore, a flat test-section design was chosen with a width of 50 mm. The test section outlines represent the hot leg of a German Konvoi PWR at a scale of 1:3 (i.e., 250 mm channel height). The test section is mounted between two separators, one simulating the RPV and the other is connected to the steam generator inlet chamber. The hot leg model is operated under pressure equilibrium in the pressure vessel of the TOPFLOW facility of FZD. The air/water experiments presented in this article focus on the flow structure observed in the region of the riser and of the steam generator inlet chamber at room temperature and pressures up to 3 bar. The performed high-speed observations show the evolution of the stratified interface and the distribution of the two-phase mixture (droplets and bubbles). The counter-current flow limitation was quantified using the variation in the water levels measured in the separators. A confrontation with the images indicates that the initiation of flooding coincides with the reversal of the flow in the horizontal part of the hot leg. Afterward, bigger waves are generated, which develop to slugs. Furthermore, the flooding points obtained from the experiments were compared with empirical correlations available in literature. A good overall agreement was obtained, while the zero penetration was found at lower values of the gaseous Wallis parameter compared with previous work. This deviation can be attributed to the rectangular cross section of the hot leg model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2009;131(2):022906-022906-10. doi:10.1115/1.3043821.

Dryout powers have been evaluated at selected inlet-flow conditions for two proposed designs of Canada deuterium uranium, CANDU® (a registered trademark of Atomic Energy of Canada Limited (AECL)) bundles and compared with those of the 37-element and CANDU Flexible, CANFLEX® (a registered trademark of AECL and Korea Atomic Energy Research Institutes (KAERI)) bundles. These proposed designs consist of a large center element (18 mm for one design and 20 mm for the other) and three rings of elements of 11.5 mm in outer diameter. The critical heat flux for each bundle design has been predicted using the correlation derived with Freon data obtained from the corresponding full-scale bundle test. An improvement in dryout power has been shown for the proposed design having a 20 mm center element with a radial power profile corresponding to the natural-uranium fuel as compared with other bundles, particularly the natural-uranium 37-element bundle, with a symmetric-cosine axial power profile. The dryout power improvement is further enhanced for the upstream-skewed axial power profile.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2009;131(2):022907-022907-7. doi:10.1115/1.3043822.

Sodium-water reaction (SWR) is a design basis accident of a sodium-cooled fast reactor (SFR). A breach of the heat transfer tube in a steam generator results in contact of liquid sodium with water. Typical phenomenon is that the pressurized water blows off, vaporizes, and mixes with the liquid sodium. It is necessary to quantify the SWR phenomena in the safety evaluation of the SFR system. In this paper, a new computer program has been developed and the SWR in a counterflow diffusion flame is studied by a numerical simulation and an experiment. The experiment is designed based on the numerical simulation so that the stable reaction flame is maintained for a long time and physical and chemical quantities are measured. From the comparison of the analysis and the experiment, there exist discrepancies that may be caused by the assumptions of the chemical reaction. Hence, a new experiment is proposed to enhance the measurement accuracy and to investigate the reason of the disagreement. The authors propose a depressurized experiment and show the preliminary result of the experiment. It is found that a stable chemical reaction flame is formed. With the depressurization, it is expected that the flame location can be controlled and the reaction region becomes thicker because of decrease in the reactant gas density.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Gas Turbines Power. 2008;131(2):024501-024501-2. doi:10.1115/1.2978996.

Moore’s law relates how the integration of semiconductors has progressed in time. This research shows that the exponential trend shown in the electronics manufacturing industry can have applications elsewhere. This study shows that the internal combustion engine followed the same trend for over 70 years. Though not the most used engine variable, engine power density shows the same trends for engines as transistor density does for microchips. This now mature technology has ended its period of rapid growth. However, the present day engine trends can show how Moore’s law can be extended to include the slower growth of long established technologies. Because exponential growth cannot go on forever, the extension Moore’s law requires that the logistic function be used. The new function also allows one to predict a theoretical value for maximum power density.

Commentary by Dr. Valentin Fuster

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