Micro gas turbine (MGT) technology is evolving toward a large variety of novel applications, such as weak gas electrification, inverted Brayton cycles, and fuel cell hybrid cycles; however, many of these systems show very different dynamic behaviors compared to conventional MGTs. In addition, some applications impose more stringent requirements on transient maneuvers, e.g., to limit temperature and pressure gradients in a fuel cell hybrid cycle. Besides providing operational safety, optimizing system dynamics to meet the variable power demand of modern energy markets is also of increasing significance. Numerical cycle simulation programs are crucial tools to analyze these dynamics without endangering the machines, and to meet the challenges of automatic control design. For these tasks, complete cycle simulations of transient maneuvers lasting several minutes need to be calculated. Moreover, sensitivity analysis and optimization of dynamic properties like automatic control systems require many simulation runs. To perform these calculations in an acceptable timeframe, simplified component models based on lumped volume or one-dimensional discretization schemes are necessary. The accuracy of these models can be further improved by parameter identification, as most novel applications are modifications of well-known MGT systems and rely on proven, characterized components. This paper introduces a modular in-house simulation tool written in fortran to simulate the dynamic behavior of conventional and novel gas turbine cycles. Thermodynamics, gas composition, heat transfer to the casing and surroundings, shaft rotation and control system dynamics as well as mass and heat storage are simulated together to account for their interactions. While the presented models preserve a high level of detail, they also enable calculation speeds up to five times faster than real-time. The simulation tool is explained in detail, including a description of all component models, coupling of the elements and the ODE solver. Finally, validation results of the simulator based on measurement data from the DLR Turbec T100 recuperated MGT test rig are presented, including cold start-up and shutdown maneuvers.

The Wankel rotary engine offers a greater power density than piston engines, but higher fuel consumption and higher hydrocarbon emissions, in large part due to poor gas sealing. This paper presents a modeling approach to evaluate the gas leakage of apex and corner seals in rotary engines. The apex seal is modeled as a deformable beam and its dynamics is coupled with the gas flows around the seal. It is shown that the main leakage mechanisms are: (1) corner seal clearance leakage, (2) leakage around the apex seal through the spark plug cavities, and (3) flank leakage at high speed. The side piece corner orifice and the trailing spark plug cavity also contribute to leakage, but to a lesser extent. Leakage through the seal–housing interface is negligible as the apex seal can conform to the distorted shape of the rotor housing.

Second ring fluttering and radial ring collapse are recognized as having significant influences on engine blowby and oil consumption. As the gas flow is coupled with the piston ring motion, understanding the ring dynamics is important for understanding not only the engine blowby mechanism, but also oil consumption mechanisms and how to control them. Only second ring flutter and collapse that occurs around the top dead center (TDC) firing conditions is examined in this paper based on a modern heavy-duty diesel engine. However, the principles described are equally applicable to all engines. First, the authors describe the fundamental mechanisms of how second ring fluttering and radial ring collapse occur. This is described by examining the forces that are acting on the second ring. Then, two cases are shown. One case shows second ring flutter and the other case shows stable second ring motion. The reasons for these two different cases are explained, including the effect of static twist and the end gaps of the rings. A sensitivity study was performed to evaluate the effect of changing the top and second ring end gaps on ring lift. It was shown how the gaps could affect the second ring flutter and ring collapse. It is concluded that the second ring will be more likely to flutter or collapse if it has a negative static twist, if the second ring end gap is large, and/or if the top ring end gap is small. If the second ring does not flutter, it may still be possible to design the ring pack such that there is not any reverse blowby. However, this must be carefully studied and controlled or the second land pressures will be too high, resulting in reverse blowby and/or top ring lifting.

In this paper, we investigate the real gas flows which occur within organic Rankine cycle (ORC) turbines. A new method for the design of nozzles operating with dense gases is discussed, and applied to the case of a high pressure ratio turbine vane. A Navier–Stokes method, which uses equations of states for a variety of working fluids typical of ORC turbines, is then applied to the turbine vanes to determine the vane performance. The results suggest that the choice of working fluid has a significant influence on the turbine efficiency.

A new effective method for comprehensive modeling of gas flow effects on vibration of nonlinear vibration of bladed disks has been developed for a case when the effect of the gas flow on the mode shapes is significant. The method separates completely the structural dynamics calculations from the significantly more computationally expensive computational fluid dynamics (CFD) calculations while providing the high accuracy of modeling for aerodynamic effects. A comprehensive analysis of the forced response using the new method has been performed for a realistic turbine bladed disk with root-disk joints, tip, and under-platform dampers. The full chain of aerodynamic and structural calculations are performed: (i) determination of boundary conditions for CFD, (ii) CFD analysis, (iii) calculation of the aerodynamic characteristics required by the new method, and (iv) nonlinear forced response analysis using the modal aerodynamic influence matrix (MAIM). The efficiency of the friction damping devices has been studied and compared for several resonance frequencies and engine orders. Advantages of the method for aerodynamic effect modeling have been demonstrated.

In this paper, computational fluid dynamics (CFD) gas flow simulations are carried out for the pebble bed reactor. In CFD calculations, geometry modeling and physical modeling are crucial to CFD results. The effects of the treatments of the interpebble contacts on gas flow fields and heat transfer are examined. A sensitivity analysis for the gap size is conducted with two spherical pebbles, in which the interpebble region is modeled by means of two types of interpebble gap and two kinds of direct contact. Both large eddy simulation and Reynolds-averaged Navier–Stokes models are employed to investigate the turbulent effects. It is found that the flow fields and relevant heat transfer are significantly dependent on the modeling of the interpebble region. The calculations indicate the complex flow structures present within the voids between the fuel pebbles.

A method has been developed for high-accuracy analysis of forced response levels for mistuned bladed disks vibrating in gas flow. Aerodynamic damping, the interaction of vibrating blades through gas flow, and the effects of structural and aerodynamic mistuning are included in the bladed disk model. The method is applicable to cases of high mechanical coupling of blade vibration through a flexible disk and, possibly shrouds, to cases with stiff disks and low mechanical coupling. The interaction of different families of bladed disk modes is included in the analysis providing the capability of analyzing bladed disks with pronounced frequency veering effects. The method allows the use of industrial-size sector models of bladed disks for analysis of forced response of a mistuned structure. The frequency response function matrix of a structurally mistuned bladed disk is derived with aerodynamic forces included. A new phenomenon of reducing bladed disk forced response by mistuning to levels that are several times lower than those of their tuned counterparts is revealed and explained.

An integrated experimental-numerical study of forced response for a mistuned bladed disk has been performed. A full chain for the predictive forced response analysis has been developed including data exchange between the computational fluid dynamics code and a code for the prediction of the nonlinear forced response for a bladed disk. The experimental measurements are performed at a full-scale single stage test rig with excitation by aerodynamic forces from gas flow. The numerical modeling approaches and the test rig setup are discussed. Comparison of experimentally measured and predicted values of blade resonance frequencies and response levels for a mistuned bladed disk without dampers is performed. A good correspondence between frequencies at which individual blades have maximum response levels is achieved. The effects of structural damping and underplatform damper parameters on amplitudes and resonance frequencies of the bladed disk are explored. It is shown that the underplatform damper significantly reduces scatters in values of the individual blade frequencies and maximum forced response levels.

Following a detailed study of two of the mechanical precipitators in the air preheaters of a thermoelectric power plant, a large amount of ash that was deposited on one of the inlet conduits was observed, obstructing the incoming gas flow. A comparison of the available data for the two most recent hopper cleaning operations revealed that, on the one hand, the amount of ash collected by the clogged precipitator (A) was significantly less than that collected by the other (B) and, on the other hand, the temperature of the ash in the former was noticeably lower than in the latter. Prior to the cleaning of the conduits, a certain amount of damage was caused to the boiler dome, which meant that subsequent cleaning required the use of a hydrolazer, where it was noted that inlet pressures were very high. All of this indicated that the cause of the clogging was not physical. This paper provides a comprehensive analytical analysis that explains what happened, as well as resolving the situation.

The efficiency of a cooled turbine stage has been discussed in the literature. All proposed definitions compare the actual power output with an ideal output, which has to be determined; but usually, one of two definitions has been used by turbine designers. In the first, the so-called Hartsel efficiency, the mainstream gas flow, and the various coolant flows to rotor and stator are assumed to expand separately and isentropically to the backpressure. In the second, it is assumed that these flows mix at constant (mainstream) gas pressure before expanding isentropically (sometimes, the rotor coolant flow is ignored in this definition). More recently, it has been suggested that a thermodynamically sounder definition is one in which the gas and coolant flows mix reversibly and adiabatically before isentropic expansion to the backpressure. In the current paper, these three efficiencies are compared, for a typical stage—the first cooled stage of a multistage industrial gas turbine. It is shown that all the efficiencies fall more or less linearly with increase of the fractional (total) coolant flow. It is also shown that the new definition of efficiency gives values considerably lower than the other two efficiencies, which are more widely used at present. Finally, the various irreversibilities associated with the flow through a cooled turbine are calculated. Although all these irreversibilities increase with the fractional coolant flow, it is shown that the “thermal” irreversibility associated with film cooling is higher than the other irreversibilities at large fractional coolant flow.

Blowback of engine oil suspended in combustion gases, when the gas flows from the piston second land back into the combustion chamber, is believed to contribute to oil consumption and hydrocarbon emissions in internal combustion engines. Oil accumulation in the region between top and second compression rings is a factor that influences this phenomenon. The effects of individual parameters, such as oil film thickness and viscosity, however, have still not been understood. The present study was aimed at constructing an experimental setup to study the effect of oil film thickness on oil accumulation in the second land of internal combustion engines. Due to the inherent difficulties of experimentation on production engines, a modeled piston-cylinder assembly was constructed. Total oil accumulation in the modeled second land after a single piston stroke was measured and compared to oil consumption in operating engines.

A multidimensional computational fluid dynamics (CFD) method has been used to improve the exhaust gas recirculation (EGR) distribution in the intake manifold. Since gas flow in the intake system is affected by intake pulsation caused by the gas exchange process, a pulsation flow simulation is used. A one-dimensional gas exchange calculation is combined with three-dimensional intake gas flow calculation to simulate pulsation flow. This pulsation flow simulation makes it possible to predict the EGR distribution. The gas flow in the intake system was analyzed in detail. It was found that a reverse flow region formed downstream of the throttle valve. The size and shape of the reverse flow region greatly depend on the engine operating conditions. With a conventional EGR system, it is difficult to distribute EGR uniformly under various engine operating conditions. A new EGR system that uses a spiral flow to mix the fresh air and EGR gas has been developed to obtain a uniform EGR distribution. As a result of adopting this system, a uniform EGR distribution is obtained regardless of the engine operating conditions. This spiral flow EGR system was applied to a low-emission vehicle (LEV) put on the Japanese market.

A multidimensional computational fluid dynamics (CFD) tool has been applied to analyze the exhaust system of a gasoline engine. Since gas flow in the exhaust manifold is affected by exhaust pulsations, prediction methods based on steady flow are not able to predict gas flow precisely enough. Therefore, a new multidimensional calculation method, called pulsation flow calculation, has been developed. A one-dimensional gas exchange simulation and a three-dimensional exhaust gas flow calculation are combined to simulate gas flow pulsations caused by the gas exchange process. Predicted gas flow in the exhaust manifold agreed with the experimental data. With the aim of reducing emissions, the pulsation flow calculation method has been applied to improve lambda feedback control using an oxygen sensor. The factors governing sensor sensitivity to the exhaust gas from each cylinder were clarified. The possibility of selecting the oxygen sensor location in the exhaust manifold on the basis of calculations was proved. The effect of an exhaust manifold with equal-length cylinder runners on achieving uniform sensor sensitivities was made clear. In addition, a new lambda feedback control method for an exhaust manifold with different-length cylinder runners is proposed.

Driven by energy conservation and environment protection, modern SI engines are required to have higher and higher power density. Consequently, engines and engine components are becoming tighter and lighter, and engines are more often operated at elevated speed and component temperature. Piston and piston ring designs thus face constant challenge to provide proper control of blow-by, oil consumption, friction, wear, and oil consumption. This paper is intended to give an overview of the problems associated with top ring performance in modern SI engines, the mechanisms behind these problems, and possible solutions. The analysis is based on engine test data and computer models. Major topics covered in this paper include (1) top ring flutter and collapse, and their influence on blow-by; (2) top ring reverse flutter and its influence on oil consumption; (3) contact between top ring and its groove, and its potential influence on ring/groove wear and micro-welding; (4) top ring/liner lubrication and its influence on ring face friction, wear, and scuffing.

A ring-dynamics and gas-flow model has been developed to study ring/groove contact, blowby, and the influence of ring static twist, keystone ring/groove configurations, and other piston and ring parameters. The model is developed for a ring pack with three rings. The dynamics of the top two rings and the gas pressures in the regions above the oil control ring are simulated. Distributions of oil film thickness and surface roughness on the groove and ring surfaces are assumed in the model to calculate the forces generated by the ring/groove contact. Ring static and dynamic twists are considered, as well as different keystone ring/groove configurations. Ring dynamics and gas flows are coupled in the formulation and an implicit scheme is implemented, enabling the model to resolve detailed events such as ring flutter. Studies on a spark ignition engine found that static twist or, more generally speaking, the relative angle between rings and their grooves, has great influence on ring/groove contact characteristics, ring stability, and blowby. Ring flutter is found to occur for the second ring with a negative static twist under normal operating conditions and for the top ring with a negative static twist under high-speed/low-load operating conditions. Studies on a diesel engine show that different keystone ring/groove configurations result in different twist behaviors of the ring that may affect the wear pattern of the keystone ring running surfaces.

A numerical investigation is presented for a two-dimensional simulation of the gas flow field and of the dynamic behavior of lignite particles inside Beater Wheel mills with classifier, installed in large coal-fired plants. A large number of representative particles are tracked using Lagrangian equations of motion, in combination with a stochastic model for particle turbulent dispersion. All the important mechanisms associated with the particle motion through the mill (particle-surface collisions and rebounding phenomena, fuel moisture evaporation and erosion wear of internal surfaces) are modeled. A special model is constructed to simulate the fragmentation of impacting particles and to calculate the size distribution of the final mill product. The models are regulated on the basis of available data from grinding mills of the Greek lignite power stations. The numerical code is capable of predicting the locations of significant erosion and to estimate the amount of particle mass that circulates through the mill via the classifying chamber. Mean impact velocity and impingement angle distributions along all the internal surfaces are also provided. The results indicate remarkable differences in the extent of the erosion caused at different locations of the mill. Also, the significant role of the leading blades arrangement inside the classifier on its classification performance and efficiency is elucidated.

A numerical computer simulation program was developed, aiding in finding optimum design parameters in the multibody-system piston, piston-rings, and cylinder with respect to optimum sealing, minimal friction, and minimum noise stimulation (impact impulse). In the simulation of piston secondary movement and piston ring motion, forces arising from the combustion process, subsonic/supersonic gas flow between the combustion chamber and the crank case, inertial forces and forces resulting from the hydrodynamic lubrication between cylinder liner and piston shaft and piston rings and between piston ring flanks and piston grooves are considered. In addition it is possible to account for effects of global, three-dimensional ring deformation as well as local piston deformation, roughness effects in lubricated contacts, and variable viscosity and variable oil supply. The governing differential equations for the pressure as well as the deformation are solved via finite element techniques, while initial value problems are solved by efficient implicit time integration schemes. The application of the developed computer code is presented in examples.

This paper discusses the determination and application of the isentropic exponent to the various thermodynamic processes found in a high-pressure natural gas transmission system. Increasing demands for more precise measurement of natural gas, coupled with the need for greater efficiency and accountability of transportation and processing operations, had led to our research and development efforts into the more precise measurement of gas flow, and the determination of gas thermodynamic properties including isentropic exponent. The isentropic exponent has many applications, some of which include: • the determination of the expansion factor ε, for calculation of flow using an orifice or venturi-type meter; • the volumetric efficiency in a reciprocating compressor; • the determination of the compression head for a centrifugal compressor; • the engine power required for the given conditions for a gas compressor; • the calculation of discharge temperatures for compressors; and • the direct measurement of gas density. As can be appreciated, the application of an incorrect value for the isentropic exponent represents an error in the parameter determined. For large volume gas flows, this can translate into a significant cost penalty.

This paper describes the specification, development, and testing of the FT8-2 Dry Low NO x control system, and how the lean burn process requires an integration of the control system and combustion hardware. The FT8-2 digital fuel control system was developed to achieve the precise multizone fuel metering of both gas and liquid fuels, the calculation of combustor air flow necessary to achieve Dry Low NO x and the traditional governing/limiting control loops necessary for safe, stable engine operation. The system design goals were accomplished by the concurrent development of software-based fuel metering algorithms and fuel metering hardware. The fuel metering hardware utilizes an all-electronic valve positioner, employing a combination of feedback and software to achieve closed-loop control of actual fuel flow. Extensive testing under actual gas flow conditions and closed-loop bench testing using a real time engine model and fuel system model was conducted to prove system operation and develop system transient response prior to installation on the test engine. The setup and results of the flow testing and closed-loop testing are described. The paper describes the control scheme used to apportion the gas fuel between combustion zones and how external conditions such as ambient temperature and fuel gas composition affect the apportionment. The paper concludes with a description of the control system installation in the engine test cell and a review of engine test results.

Fuel gas compressors installed in cogeneration systems must be highly reliable and efficient machines. The screw compressor can usually be designed to meet most of the gas flow rates and pressure conditions generally required for such installations. To an ever-increasing degree, alternative sources are being found for the fuel gas supply, such as coke-oven gas, blast-furnace gas, flare gas, landfill gas, and synthesis gas from coal gasification or from pyrolysis. A feature of the oil-free screw compressor when such gases are being considered is the isolation of the gas compression space from the bearing and gear lubrication system by using positive shaft seals. This ensures that the process gas cannot be contaminated by the lubricating oil, and that there is no risk of loss of lubricant viscosity by gas solution in the oil. This feature enables the compressed gas to contain relatively high levels of particulate contamination without danger of “sludge” formation, and also permits the injection of water or liquid solvents into the compression space, to reduce the temperature rise due to the heat of compression, or to “wash” any particulate matter through the compressor.