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RESEARCH PAPERS: Advanced Energy Systems

J. Eng. Gas Turbines Power. 1998;120(1):1-8. doi:10.1115/1.2818077.

This paper presents a general design approach involving automatic, intelligent process simulation procedures. The aim is to derive a general set of design principles and methodologies that can be developed into computer-assisted procedures. This first part deals with numerical, quantitative calculations, i.e., with what commonly goes under the name of “Numerical Process Simulation.” It is argued that the existing design methods can result in computer codes or packages that perform exactly (and deterministically) the numerical operations an engineer would perform. It is also shown that modularity in these codes is dictated by the necessity of automatically implementing numerical procedures that depend on the structure of the process under examination, rather than by user’s convenience and ease of maintenance. An example of a modular, structure-oriented code (CAMEL) is given and discussed in detail, while numerical applications are discussed elsewhere [4]. The second part deals with the more complex qualitative approach to process design, i.e., with the possibility of implementing automatic “expert” procedures that perform the same conceptual tasks as human process engineers. It is shown that by means of Artificial Intelligence techniques it is possible to mimic (to an extent) the “thinking patterns” of a human expert, and to produce process schemes that are both acceptable and realistic. A general process synthesis package (COLOMBO) is described and some of its applications discussed. The main goal of the two parts of the paper is to show that the very complex activity of process design can be executed automatically, not only in principle, but in actual applications, and that both qualitative synthesis and quantitative calculations are possible with the present state of the art of our computational facilities.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):9-16. doi:10.1115/1.2818094.

This paper presents an explicit mapping of the conceptual activities that constitute a “process design task” into a series of well-posed, complete, and general formal simulation procedures. Part I of this series of two papers dealt with numerical procedures for process simulation and showed that structure independence and modularity are two prerequisites for a general-purpose simulator. Part II approaches the problem from a completely different point of view and considers the question: Is it possible to derive a general set of design guidelines that can be implemented into a knowledge-based system and result in an automatic, computer-assisted process design procedure? This problem is different in character from that tackled in part I. First, it is by its own nature qualitative, i.e., it requires conceptual rather than numerical answers. Second, it is formulated at a higher level (in Artificial Intelligence terms, at a metalevel). Its solution is clearly in the domain of the logic of process design and, therefore, embeds (contains) all possible quantitative numerical schemes and does not depend on any of them for either its position or its solution. If the answer to this question is affirmative, the resulting code would be a sort of “Expert Assistant” to the engineer in the sense that it would suggest what process can be best suited for the particular application under consideration. The study proceeds by trying to detect conceptual similarities in different design procedures, to construct a suitable knowledge base, and to implement a general macro-procedure that could automatically assist the engineer in the largest possible number of process design operations. The contention here is that the most recent developments of AI-based methods make it possible to extract from human experts all the essential knowledge that pertains to “engineering design,” with the final goal of transferring this body of knowledge—in a form suitable to machine communication—to an “Expert System for Process Design,” which can then be applied (interactively or on a stand-alone basis) with a high degree of confidence to a variety of particular process simulations. A prototype version of an Expert System Assistant is briefly discussed, and an application is analyzed in detail. The code is called COLOMBO and is available as a research tool from the author. Finally, Part II builds on Part I of this series of papers. In particular, it is assumed that a general, modular, numerical Process Simulation Package exists and that it is capable of executing the quantitative mass and energy balance operations described in Part I.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Gas Turbines: Combustion and Fuels

J. Eng. Gas Turbines Power. 1998;120(1):17-23. doi:10.1115/1.2818073.

An investigation has been conducted to develop appropriate technologies for a low-NOx , liquid-fueled combustor. The combustor incorporates an effervescent atomizer used to inject fuel into a premixing duct. Only a fraction of the combustion air is used in the premixing process. This fuel-rich mixture is introduced into the remaining combustion air by a rapid jet-shear-layer mixing process involving radial fuel–air jets impinging on axial air jets in the primary combustion zone. Computational modeling was used as a tool to facilitate a parametric analysis appropriate to the design of an optimum low-NOx combustor. A number of combustor configurations were studied to assess the key combustor technologies and to validate the three-dimensional modeling code. The results from the experimental testing and computational analysis indicate a low-NOx potential for the jet-shear-layer combustor. Key features found to affect NOx emissions are the primary combustion zone fuel–air ratio, the number of axial and radial jets, the aspect ratio and radial location of the axial air jets, and the radial jet inlet hole diameter. Each of these key parameters exhibits a low-NOx point from which an optimized combustor was developed. Also demonstrated was the feasibility of utilizing an effervescent atomizer for combustor application. Further developments in the jet-shear-layer mixing scheme and effervescent atomizer design promise even lower NOx with high combustion efficiency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):24-33. doi:10.1115/1.2818084.

This paper presents an experimental/computational study of cold flow in the combustor–diffuser system of industrial gas turbines employing can-annular combustors and impingement-cooled transition pieces. The primary objectives were to determine flow interactions between the prediffuser and dump chamber, to evaluate circumferential flow nonuniformities around transition pieces and combustors, and to identify the pressure loss mechanisms. Flow experiments were conducted in an approximately one-third geometric scale, 360-deg annular test model simulating practical details of the prototype including the support struts, transition pieces, impingement sleeves, and can-annular combustors. Wall static pressures and velocity profiles were measured at selected locations in the test model. A three-dimensional computational fluid dynamic analysis employing a multidomain procedure was performed to supplement the flow measurements. The complex geometric features of the test model were included in the analysis. The measured data correlated well with the computations. The results revealed strong interactions between the prediffuser and dump chamber flows. The prediffuser exit flow was distorted, indicating that the uniform exit conditions typically assumed in the diffuser design were violated. The pressure varied circumferentially around the combustor casing and impingement sleeve. The circumferential flow nonuniformities increased toward the inlet of the turbine expander. A venturi effect causing flow to accelerate and decelerate in the dump chamber was also identified. This venturi effect could adversely affect impingement cooling of the transition piece in the prototype. The dump chamber contained several recirculation regions contributing to the losses. Approximately 1.2 dynamic head at the prediffuser inlet was lost in the combustor–diffuser, much of it in the dump chamber where the fluid passed though narrow pathways. A realistic test model and three-dimensional analysis used in this study provided new insight into the flow characteristics of practical combustor–diffuser systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):34-40. doi:10.1115/1.2818085.

During the development of the BR710 jet engine, audible combustor instabilities (termed “rumble”) occurred. Amplitudes measured with test cell microphones were up to 130 dB at around 100 Hz. Disturbances of this amplitude are clearly undesirable, even if only present during start-up, and a research program was initiated to eliminate the problem. Presented here is the methodical and structured approach used to identify, understand, and remove the instability. Some reference is made to theory, which was used for guidance, but the focus of the work is on the research done to find the cause of the problem and to correct it. The investigation followed two separate, but parallel, paths—one looking in detail at individual components of the engine to identify possible involvement in the instability and the other looking at the pressure signals from various parts of a complete engine to help pinpoint the source of the disturbance. The main cause of the BR710 combustor rumble was found to be a self-excited aerodynamic instability arising from the design of the fuel injector head. In the end, minor modifications lead to spray pattern changes, which greatly reduced the combustor noise. As a result of this work, new recommendations are made for reducing the risk of combustion instabilities in jet engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):41-47. doi:10.1115/1.2818086.

This paper describes the development of an ultra-low NOx gas turbine combustor for cogeneration systems. The combustor, called a double swirler staged combustor, utilizes three-staged premixed combustion for low NOx emission. The unique feature of the combustor is its tertiary premix nozzles located downstream of the double swirler premixing nozzles around the combustor liner. Engine output is controlled by simply varying the fuel gas flow, and therefore employs no complex variable geometries for airflow control. Atmospheric combustion tests have demonstrated the superior performance of the combustor. NOx level is maintained at less than 3 ppm (O2 = 15 percent) over the range of engine output between 50 and 100 percent. Assuming the general relationship that NOx emission is proportional to the square root of operating pressure, the NOx level is estimated at less than 9 ppm (O2 = 15 percent) at the actual pressure of 0.91 MPa (abs.). Atmospheric tests have also shown high combustion efficiency; more than 99.9 percent over the range of engine output between 60 and 100 percent. Emissions of CO and UHC are maintained at 0 and 1 ppm (O2 = 15 percent), respectively, at the full engine load.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):48-59. doi:10.1115/1.2818087.

The influence of the structure of perfectly premixed flames on NOx formation is investigated theoretically. Since a network of reaction kinetics modules and model flames is used for this purpose, the results obtained are independent of specific burner geometries. Calculations are presented for a mixture temperature of 630 K, an adiabatic flame temperature of 1840 K, and 1 and 15 bars combustor pressure. In particular, the following effects are studied separately from each other: • molecular diffusion of temperature and species; • flame strain; • local quench in highly strained flames and subsequent reignition; • turbulent diffusion (no preferential diffusion); • small scale mixing (stirring) in the flame front. Either no relevant influence or an increase in NOx production over that of the one-dimensional laminar flame is found. As a consequence, besides the improvement of mixing quality, a future target for the development of low-NOx burners is to avoid excessive turbulent stirring in the flame front. Turbulent flames that exhibit locally and instantaneously near laminar structures (“flamelets”) appear to be optimal. Using the same methodology, the scope of the investigation is extended to lean-lean staging, since a higher NOx -abatement potential can be expected in principle. As long as the chemical reactions of the second stage take place in the boundary between the fresh mixture of the second stage and the combustion products from upstream, no advantage can be expected from lean-lean staging. Only if the primary burner exhibits much poorer mixing than the second stage can lean-lean staging be beneficial. In contrast, if full mixing between the two stages prior to afterburning can be achieved (lean-mix-lean technique), the combustor outlet temperature can in principle be increased somewhat without NO penalty. However, the complexity of such a system with a larger flame tube area to be cooled will increase the reaction zone temperatures, so that the full advantage cannot be realized in an engine. Of greater technical relevance is the potential of a lean-mixlean combustion system within an improved thermodynamic cycle. A reheat process with sequential combustion is perfectly suited for this purpose, since, first, the required low inlet temperature of the second stage is automatically generated after partial expansion in the high pressure turbine, second, the efficiency of the thermodynamic cycle has its maximum and, third, high exhaust temperatures are generated, which can drive a powerful Rankine cycle. The higher thermodynamic efficiency of this technique leads to an additional drop in NOx emissions per power produced.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):60-68. doi:10.1115/1.2818088.

Spatially locked vortices in the cavities of a combustor aid in stabilizing the flames. On the other hand, these stationary vortices also restrict the entrainment of the main air into the cavity. For obtaining good performance characteristics in a trapped-vortex combustor, a sufficient amount of fuel and air must be injected directly into the cavity. This paper describes a numerical investigation performed to understand better the entrainment and residence-time characteristics of cavity flows for different cavity and spindle sizes. A third-order-accurate time-dependent Computational Fluid Dynamics with Chemistry (CFDC) code was used for simulating the dynamic flows associated with forebody-spindle-disk geometry. It was found from the nonreacting flow simulations that the drag coefficient decreases with cavity length and that an optimum size exists for achieving a minimum value. These observations support the earlier experimental findings of Little and Whipkey (1979). At the optimum disk location, the vortices inside the cavity and behind the disk are spatially locked. It was also found that for cavity sizes slightly larger than the optimum, even though the vortices are spatially locked, the drag coefficient increases significantly. Entrainment of the main flow was observed to be greater into the smaller-than-optimum cavities. The reacting-flow calculations indicate that the dynamic vortices developed inside the cavity with the injection of fuel and air do not shed, even though the cavity size was determined based on cold-flow conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):69-76. doi:10.1115/1.2818089.

This paper addresses optical-based techniques for measuring soot particulate loading in the exhaust stream of gas turbine engines. The multi-angle scattering and multi-wavelength extinction of light beams by ensembles of submicrometer soot particles was investigated as a diagnostic means of inferring particle field characteristics. This is, the particle size distribution function and particle number density were deduced using an innovative downhill simplex inversion algorithm for fitting the deconvolved Mie-based scattering/extinction integral to the measured scattering/extinction signals. In this work, the particle size distribution was characterized by the widely accepted two-parameter log-normal distribution function, which is fully defined with the specification of the mean particle diameter and the standard deviation of the distribution. The accuracy and precision of the algorithm were evaluated for soot particle applications by applying the technique to noise-perturbed synthetic data in which the signal noise component is obtained by Monte Carlo sampling of Gaussian distributed experimental errors of 4, 6, and 10 percent. The algorithm was shown to yield results having an inaccuracy of less than 10 percent for the highest noise levels and an imprecision equal to or less than the experimental error. Multi-wavelength extinction experiments with a laboratory bench-top burner yielded a mean particle diameter of 0.039 μm and indicated that molecular absorption by organic vapor-phase molecules in the ultraviolet region should not significantly influence the measurements. A field demonstration test was conducted on one of the JT-12D engines of a Sabre Liner jet aircraft. This experiment yielded mean diameters of 0.040 μm and 0.036 μm and standard deviations of 0.032 μm and 0.001 μm for scattering and extinction methods, respectively. The total particulate mass flow rate at idle was estimated to be 0.54 kg/h.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):77-83. doi:10.1115/1.2818090.

Numerical calculations of the two-phase flow in an experimentally well-investigated research combustor are presented. The comparison between measurements and calculations demonstrates the capabilities of the state-of-the-art Euler/Lagrange method for calculating two-phase flows, when applied to a complex reacting liquid-fueled combustor. The governing equations for gaseous and liquid phase are presented, with special emphasis on the control of the coupling process between the two phases. The relaxation method employed, together with a convergence history, shows a suitable way to achieve a fast and accurate solution for the strongly coupled two-phase flow under investigation. Furthermore, methods are presented to simulate the stochastic behavior of the atomization process caused by an air-blast atomizer. In addition to the numerical methods, experimental techniques are presented that deliver detailed information about droplet starting conditions.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Gas Turbines: Controls and Diagnostics

J. Eng. Gas Turbines Power. 1998;120(1):84-88. doi:10.1115/1.2818091.

SatCon Technology Corporation has completed design, fabrication, and the first round of test of a 373 kW (500 hp), two-spool, intercooled gas turbine engine with integral induction type alternators. This turbine alternator is the prime mover for a World Sports Car class hybrid electric vehicle under development by Chrysler Corporation. The complete hybrid electric vehicle propulsion system features the 373 kW (500 hp) turbine alternator unit, a 373 kW (500 hp) 3.25 kW-h (4.36 hp-h) flywheel, a 559 kW (750 hp) traction motor, and the propulsion system control system. This paper presents and discusses the major attributes of the control system associated with the turbine alternator unit. Also discussed is the role and operational requirements of the turbine alternator unit as part of the complete hybrid electric vehicle propulsion system.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Gas Turbines: Electric Utilities

J. Eng. Gas Turbines Power. 1998;120(1):89-96. doi:10.1115/1.2818092.

A lumped parameter model to predict the high frequency pressure oscillations observed in a water brake dynamometer is presented. It explains how the measured low frequency variations of the torque are a consequence of the variation in amplitude of the high frequency flow oscillations. Based on this model, geometrical modifications were defined, aiming to suppress the oscillations while maintaining mechanical integrity of the device. An experimental verification demonstrated the validity of the model and showed a very stable operation of the modified dynamometer even at very low torque.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 1998;120(1):97-104. doi:10.1115/1.2818093.

From a cost point of view SNECMA has found DS columnar grain manufacturing technology to be highly attractive compared to single crystal. CM 186 LC alloy exhibits enhanced mechanical and environmental properties and temperature capability compared to MAR M 200 Hf alloy; these properties are close to first-generation single crystal alloys up to 982°C (1800°F). The alloy is shown to be amenable to various coating and brazing high-temperature processes. The longer term creeprupture/phase stability data base on the alloy has now been extended out to 8300 hours at 1038°C (1900°F). Castings for engine test have been produced using CM 186 LC alloy.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 1998;120(1):105-109. doi:10.1115/1.2818059.

The continual increase in the use of magnetic bearings in various capacities, including high-speed aerospace applications such as jet engine prototypes, dictates the need to quantify power losses in this type of bearing. The goal of this study is to present experimentally measured power losses during the high-speed operation of a pair of magnetic bearings. A large-scale test rotor has been designed and built to obtain unambiguous power loss measurements while varying a variety of test parameters. The test apparatus consists of a shaft supported in two radial magnetic bearings and driven by two electric motors also mounted on the shaft. The power losses of the spinning rotor are determined from the time rate of change of the kinetic energy of the rotor as its angular speed decays during free rotation. Measured results for the first set of magnetic bearings, a pair of eight-pole planar radial bearings, are presented here. Data from three different parameter studies including the effect of the bias flux density, the effect of the bearing pole configuration, and the effect of the motor stator on the power loss are presented. Rundown plots of the test with the bearings in the paired pole (NNSS) versus the alternating (NSNS) pole configuration show only small differences, with losses only slightly higher when the poles are in the alternating pole (NSNS) configuration. Loss data were also taken with the motor stators axially removed from the motor rotors for comparison with the case where the motor stators are kept in place. No measurable difference was observed between the two cases, indicating negligible windage and residual magnetic effects. Throughout most of the speed range, the dominant loss mechanism appears to be eddy currents.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):110-114. doi:10.1115/1.2818060.

The continual increase in the use of magnetic bearings in various capacities, including high-speed aerospace applications such as jet engine prototypes, dictates the need to quantify power losses in this type of bearing. The goal of the present study is to develop and experimentally verify general power loss equations for the high-speed operation of magnetic bearings. Experimental data from a large-scale test rotor have been presented in Part 1 of this study. Analytical/empirical predictions are presented here for the test bearings, a pair of eight-pole planar radial bearings, for comparison to the experimental results from Part 1. Expressions for the four loss components, eddy current, alternating hysteresis, rotating hysteresis, and windage, are also presented. Analytical/empirical predictions for the test bearings at three different bias flux levels demonstrate good correlation with corresponding experimental data. Throughout most of the speed range the dominant loss mechanism appears to be eddy currents.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):115-119. doi:10.1115/1.2818061.

In 1966 it was shown that the maximum factor by which the amplitude of forced vibration of blades can increase due to mistuning is, with certain assumptions, 1/2(1 + N), where N is the number of blades in the row. This report gives a further investigation of the circumstances when this factor can be obtained. These are small damping, and a relationship must hold between the mistuning distribution γ(s) and the interblade coupling function c(r), where r is the model number. The mistuning distribution must be symmetric about the blade on which maximum amplitude is to occur, s = 0. The coupling function must be symmetric about r = R, where R is the mode number of the excitation. If the coupling is purely mechanical, additional conditions apply. The coupling function c(r) must consist of a number of identical symmetric substrips. A 1976 result for mechanical coupling is amended.

Topics: Vibration , Blades , Damping
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):120-125. doi:10.1115/1.2818062.

The quest for higher performance engines in conjunction with the requirement for lower life cycle costs has resulted in stage configurations that are more susceptible to high cycle fatigue. One solution is the use of innovative approaches that introduce additional mechanical damping. The present paper describes an approach that may be used to assess the benefits of friction dampers located within internal cavities of a hollow structure. The friction dampers used in this application are often relatively thin devices that, if unconstrained, have natural frequencies in the same range as the natural frequencies of the hollow airfoil. Consequently, the analytical approach that is developed is distinct in that it has to take into account the dynamic response of the damper and how it changes as the amplitude of the vibration increases. In this paper, results from the analytical model are compared with independently generated results from a time integration solution of a three mass test problem. Results from the analytical model are compared with experimental data in a companion paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):126-130. doi:10.1115/1.2818063.

The use of hollow airfoils in turbomachinery applications, in particular fans and turbines, is an essential element in reducing the overall engine weight. However, state-of-the-art airfoil geometries are of low aspect ratio and exhibit unique characteristics associated with plate like modes. These modes are characterized by a chordwise form of bending and high modal density within the engine operating speed range. These features combined with the mistuning effects resulting from manufacturing tolerances make accurate frequency and forced response predictions difficult and increase the potential for High Cycle Fatigue (HCF) durability problems. The present paper summarizes the results of an experimental test program on internal damping of hollow bladelike specimens. Friction damping is provided via sheet metal devices configured to fit within a hollow cavity with various levels of preload. The results of the investigation indicate that such devices can provide significant levels of damping, provided the damper location and preload is optimized for the modes of concern. The transition of this concept to actual engine hardware would require further optimization with regard to wear effects and loss of preload particularly in applications where the preload is independent of rotational speed. Excellent agreement was achieved between the experimental results and the analytical predictions using a microslip friction damping model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):131-139. doi:10.1115/1.2818064.

Friction dampers are widely used to improve the performance of rotating blades. This paper is concerned with the steady state response and stability analysis of rotating composite plates in the presence of non linear friction damping. Direct Integration Method (DIM) and Harmonic Balance Method (HBM) are used to determine the steady state response due to periodic lateral external forces. In addition, an alternate procedure, Hybrid Method (HM) is proposed for this analysis to substantiate the results from DIM and HBM. The analysis shows that the steady state response is a function of friction damping magnitude as well as its location besides the excitation frequency and the rotational speed. A stability analysis of the composite blades is also made by including periodic in-plane excitation using Floquet-Liapunov theory.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):140-148. doi:10.1115/1.2818065.

There has been much research work carried out on various aspects of individual squeeze-film dampers (SFDs) but very little on the interplay between a damper and the rotating assembly of which it forms a part. In this paper, a flexible rotor-bearing assembly in a configuration, typical of a small centrifugal pump and incorporating an SFD, is investigated theoretically and experimentally from the points of view of forced vibration control and stability control. It is found that change in rotor unbalance, SFD static eccentricity ratio, and SFD supply pressure can cause significant movement of system resonances and vibration resulting from excessive damping. The provision of an SFD also delays the onset of instability and because of its nonlinearity, the SFD contributes more damping than can a linear damper when the vibration amplitude becomes large as instability develops. It is shown that this instability is curbed at some limit cycle, whose frequency is a system natural frequency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):149-154. doi:10.1115/1.2818068.

The existence of a “breathing” crack can not only cause variations in harmonic components but also produce extra transient components into vibration signals of a rotor system. In the present paper, the composition of cracked rotor vibration signals is studied first by some analytical methods. Based on this study, an approach of signal processing is suggested to detect transient components of the vibration signals and thereby the crack.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Gas Turbines: Vehicular

J. Eng. Gas Turbines Power. 1998;120(1):155-161. doi:10.1115/1.2818069.

This paper, which is the first of two in a series, provides an overview of a viscoplastic constitutive model that accounts for time-dependent material deformation (e.g., creep, stress relaxation, etc.) in monolithic ceramics. Using continuum principles of engineering mechanics, the complete theory is derived from a scalar dissipative potential function first proposed by Robinson (1978), and later utilized by Duffy (1988). Derivations based on a flow potential function provide an assurance that the inelastic boundary value problem is well posed, and solutions obtained are unique. The specific formulation used here for the threshold function (a component of the flow potential function) was originally proposed by Willam and Warnke (1975) in order to formulate constitutive equations for time-independent classical plasticity behavior observed in cement and unreinforced concrete. Here constitutive equations formulated for the flow law (strain rate) and evolutionary law employ stress invariants to define the functional dependence on the Cauchy stress and a tensorial state variable. This particular formulation of the viscoplastic model exhibits a sensitivity to hydrostatic stress, and allows different behavior in tension and compression.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):162-171. doi:10.1115/1.2818070.

The desirable properties of ceramics at high temperatures have generated interest in their use for structural applications such as in advanced turbine systems. Design lives for such systems can exceed 10,000 hours. The long life requirement necessitates subjecting the components to relatively low stresses. The combination of high temperatures and low stresses typically places failure for monolithic ceramics in the creep regime. The objective of this paper is to present a design methodology for predicting the lifetimes of structural components subjected to creep rupture conditions. This methodology utilizes commercially available finite element packages and takes into of a account the time-varying creep strain distributions (stress relaxation). The creep life of a component is discretized into short time, steps, during which the stress and strain distributions are assumed constant. The damage is calculated for each time step based on a modified Monkman–Grant creep rupture criterion. Failure is assumed to occur when the normalized accumulated damage at any point in the component is greater than or equal to unity. The corresponding time will be the creep rupture life for that component. Examples are chosen to demonstrate the CARES/CREEP (Ceramics Analysis and Reliability Evaluation of Structures/CREEP) integrated design program, which is written for the ANSYS finite element package. Depending on the component size and loading conditions, it was found that in real structures one of two competing failure modes (creep or slow crack growth) will dominate. Applications to benchmark problems and engine components are included.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):172-178. doi:10.1115/1.2818071.

The development of turbine components for the automotive 100 kW ceramic gas turbine has entered the final stage of the seven-year project and is making satisfactory progress toward the goals. We have attained the interim targets of the aerodynamic performances and have been carrying out tests to further improve efficiency. As for ceramic parts, we have changed the material of the turbine rotor to a new one that is excellent in long-sustained and high-temperature strength properties, and have confirmed substantial strength at high temperature through hot-spin tests. After evaluating blade-vibration stress through analyses and experiments, we completed an endurance evaluation at 1200°C (1473 K) TIT (Turbine Inlet Gas Temperature) and a rated speed of 100,000 rpm. We are now carrying out endurance tests at 1350°C (1623 K) TIT. For ceramic stationary parts, we already finished the evaluations at 1200°C TIT and are also conducting an endurance test at 1350°C TIT. Using these parts in a full-assembly test, together with other elements, we confirmed that they cause no functional problem in tests performed at 1200°C TIT level up to the rated speed (100,000 rpm), and are evaluating their performances.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):179-185. doi:10.1115/1.2818072.

The four-year European Gas Turbine Program “AGATA” was started in January 1993 with the objective of developing three critical components aimed at a 60 kW turbogenerator in an hybrid electric vehicle: a catalytic combustor, a radial turbine wheel and a static heat exchanger. The AGATA partners represent car manufacturers as well as companies and research institutes in the turbine, catalyst, and ceramic material fields in both France and Sweden. This paper outlines the main results of the AGATA project for the first three-year period. Experimental verification of the components started during the third year of the program. A high-pressure/temperature test rig for the combustor and the heat exchanger tests has been built and is now being commissioned. A high-temperature turbine spin rig will be ready late 1995. The turbine wheel design is completed and ceramic Si3 N4 spin disks have been manufactured by injection molding and Hot Isostatic Pressing (HIP). A straight blade design has been selected and FEM calculations have indicated that stress levels that occur during a cold start are below 300 MPa. The catalytic combustor final design for full-scale testing has been defined. Due to the high operating temperature, 1350°C, catalyst pilot tests have included aging, activity, and strength tests. Based on these tests, substrate and active materials have been selected. Initial full-scale tests including LDV measurements in the premix duct will start late 1995. The heat exchanger design has also been defined. This is based on a high-efficiency plate recuperator design. One critical item is the ceramic thermoplastic extrusion manufacturing method for the extremely thin exchanger plates another is the bonding technique: ceramic to ceramic and ceramic to metal. Significant progress on these two items has been achieved. The manufacturing of quarter scale prototypes is now in process.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):186-190. doi:10.1115/1.2818074.

The ongoing Japanese Ceramic Gas Turbine (CGT) project, as a part of the New Sunshine Project funded by the Ministry of International Trade and Industry (MITI), aims to achieve higher efficiency, lower pollutant emission, and multifuel capability for small to medium sized gas turbine engines to be used in cogeneration systems. The final target of this project is to achieve a thermal efficiency over 42 percent at a turbine inlet temperature (TIT) of 1350°C. Under this project, Kawasaki Heavy Industries (KHI) is developing the CGT302 (a regenerative twin-spool CGT). The CGT302 has several unique features: simple-shaped ceramic components, KHI’s original binding system for turbine nozzle segments, stress-free structure using ceramic springs and rings, etc. In addition to these features, a high turbine tip speed and a metal plate fin recuperator were adopted. At the end of the fiscal year 1994, an intermediate appraisal was carried out, and the CGT302 was recognized to have successfully achieved its target. The CGT302 endurance test at the intermediate stage required 20 hours’ operation of the basic ceramic engine. The actual testing accomplished 40 hours at over 1200°C TIT, which included 30 hours of operation without disassembling. The target thermal efficiency of 30 percent at 1200°C has almost been reached, 29.2 percent having been achieved. In 1995 the CGT302 successfully recorded 33.1 percent at 1190°C of TIT with no trouble. We will introduce the current status of R&D of the CGT302 and its unique features in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):191-198. doi:10.1115/1.2818075.

The solution for the stress state present in the vicinity of transverse matrix cracks within a composite laminate is typically obtained by assuming a regular crack spacing geometry for the problem and applying a shear-lag analysis. In order to explore the validity of this underlying assumption, the probability density function for the location of the next transverse matrix crack within a crack bounded region is examined. The regular crack spacing assumption is shown to be reasonable from an engineering point of view. Continuing with this assumption, a generalized shear-lag model for multilayer, off-axis laminates subjected to full in-plane loads is developed. This model is used to quantitatively evaluate the effective elastic properties of the damaged material. The results are applicable to materials such as ceramic matrix or polymer matrix unidirectional fiber systems where damage in the form of transverse matrix cracks arises.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Internal Combustion Engines

J. Eng. Gas Turbines Power. 1998;120(1):199-208. doi:10.1115/1.2818076.

A numerical study of the interactions between hydrodynamic/boundary lubrication, oil transport, and radial dynamics of a piston ring using a mass-conserving (cavitation) algorithm is presented. The scheme outlined in this investigation facilitates the calculation of the volume of oil accumulating at the leading and trailing edges of the piston ring as it scrapes against the line. The calculation of this oil accumulation is important in the estimation of lubricating oil consumption in engines. The numerical procedure employed in this study is capable of depicting the transition between the various modes of piston ring lubrication (hydrodynamic, mixed, and boundary) over an engine cycle, including the detachment of oil film from the ring and its subsequent re-attachment. Additionally, the effects of (a) liner lubricant availability and (b) ring face profiles on the oil accumulation are also discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):209-216. doi:10.1115/1.2818078.

A multizone quasi-dimensional model that illustrates the intake, compression, combustion, expansion, and exhaust processes has been developed for a single-cylinder four-stroke spark-ignition engine. The model takes into consideration mass and energy conservation in the engine cylinder, intake and exhaust plenums, and crank-case plenum. The model calculates instantaneous variations in gas thermodynamic states, gas properties, heat release rates, in-cylinder turbulence, piston ring motion, blowby, nitric oxide, and carbon monoxide formation. The cycle simulation accounts for the induced gas velocities due to flame propagation in the turbulence model (k–ε type), which is applied separately to each gas zone. This allows for the natural evolution of the averaged mean and turbulent velocities in burned and unburned gas regions. The present model predictions of thermal efficiency, indicated mean effective pressure, peak values of gas pressure, ignition delay, concentrations of nitric oxide, carbon monoxide, and carbon dioxide are proven to be in agreement with experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):217-224. doi:10.1115/1.2818079.

An experimental study of the spray structure from air-shrouded dual-stream injectors with different air mixing mechanisms was carried out extensively to understand the spray characteristics of dual-stream port injectors for applications to four-valve gasoline engines. The injectors were tested under both steady and transient conditions at different injection pressures and air shrouding pressure differentials. The global spray structure was visualized using the planar laser Mie scattering technique and spray atomization processes were characterized by the phase-Doppler anemometry method. The experimental results showed that spray atomization characteristics are improved markedly by the air-shrouding technique and also strongly dominated by the air-mixing mechanisms. When the air flows into the injector tip mainly from the radial direction, two streams of the spray are forced to merge together and as a result a single-stream spray is formed. When the radial velocity component of the air is reduced and the air is made to mix well with the fuel inside the injector tip, however, the two streams of the spray are well separated over different injection conditions. Moreover, other spray parameters are also modified by the air shrouded into the injector, which must be optimized in order to achieve the best performance of the air-shrouded injector.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):225-231. doi:10.1115/1.2818080.

Changes in the physical and chemical processes during the ignition delay period of a gas-fueled diesel engine (dual-fuel engine) due to the increased admission of the gaseous fuels and diluents are examined. The extension to the chemical aspects of the ignition delay with the added gaseous fuels and the diluents into the cylinder charge is evaluated using detailed reaction kinetics for the oxidation of dual-fuel mixtures at an adiabatic constant volume process while employing n-heptane as a representative of the main components of the diesel fuel. In the examination of the physical aspects of the delay period, the relative contributions of changes in charge temperature, pressure, physical properties, pre-ignition energy release, heat transfer, and the residual gas effects due to the admission of the gaseous fuels are discussed and evaluated. It is shown that the introduction of gaseous fuels and diluents into the diesel engine can substantially affect both the physical and chemical processes within the ignition delay period. The major extension of the delay is due to the chemical factors, which strongly depend on the type of gaseous fuel used and its concentration in the cylinder charge.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):232-236. doi:10.1115/1.2818081.

The work presented in this paper compares the performance and emissions of the UBC “Squish-Jet” fast-burn combustion chamber with a baseline bowl-in-piston (BIP) chamber. It was found that the increased turbulence generated in the fastburn combustion chambers resulted in 5 to 10 percent faster burning of the air–fuel mixture compared to a conventional BIP chamber. The faster burning was particularly noticeable when operating with lean air–fuel mixtures. The study was conducted at a 1.7 mm clearance height and 10.2:1 compression ratio. Measurements were made over a range of air–fuel ratios from stoichiometric to the lean limit. At each operating point all engine performance parameters, and emissions of nitrogen oxides, unburned hydrocarbons, and carbon monoxide were recorded. At selected operating points a record of cylinder pressure was obtained and analyzed off-line to determine mass-burn rate in the combustion chamber. Two piston designs were tested at wide-open throttle conditions and 2000 rpm to determine the influence of piston geometry on the performance and emissions parameters. The UBC squish-jet combustion chamber design demonstrates significantly better performance parameters and lower emission levels than the conventional BIP design. Mass-burn fraction calculations showed a significant reduction in the time to burn the first 10 percent of the charge, which takes approximately half of the time to burn from 10 to 90 percent of the charge.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1998;120(1):237-243. doi:10.1115/1.2818082.

This work addresses primary atomization modeling, multidimensional spray prediction, and flow characteristics of compound nozzle gasoline injectors. Compound nozzles are designed to improve the gasoline spray quality by increasing turbulence at the injector exit. Under the typical operating conditions of 270-1015 kPa, spray atomization in the compound nozzle gasoline injectors is mainly due to primary atomization where the flow turbulence and the surface tension are the dominant factors. A primary atomization model has been developed to predict the mean droplet size far downstream by taking into account the effect of turbulent intensity at the injector exit. Two multidimensional spray codes, KIVA-2 and STAR-CD, originally developed for high-pressure diesel injection, are employed for the lower-pressure gasoline injection. A separate CFD analysis was performed on the complex internal flows of the compound nozzles to obtain the initial and boundary conditions for the spray codes. The TAB breakup model used in KIVA-2 adequately facilitates the atomization process in the gasoline injection.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Eng. Gas Turbines Power. 1998;120(1):244-246. doi:10.1115/1.2818083.

The stability analysis of a rotor system with electromagnetic control forces is investigated. The formulas for the electromagnetic forces between the stator and rotor of the electromagnetic actuator are derived with respect to the displacements of the journal center and coil currents. The minimum setting values of the controller, and the relationship between mode shapes at the positions of sensors and actuators of the system, are given by using a perturbation method and Routh–Hurwitz theory.

Commentary by Dr. Valentin Fuster

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