Critical Mode Operation Research Articles

Design of GaN-Based MHz Totem-Pole PFC Rectifier

The totem-pole bridgeless power factor correction (PFC) rectifier has recently been recognized as a promising front-end candidate for applications like servers and telecommunication power supplies. This paper begins with a discussion of the advantages of using emerging high-voltage gallium-nitride (GaN) devices in totem-pole PFC rectifiers rather than traditional PFC rectifiers. The critical-mode operation is used in the totem-pole PFC rectifier in order to achieve both high frequency and high efficiency. Then, several high-frequency issues and detailed design considerations are introduced, including extending zero-voltage-switching operation for the entire line-cycle, a variable on-time strategy for zero-crossing distortion suppression, and interleaving control for ripple current cancellation. The volume reduction of differential-mode electromagnetic interference filters is also presented, which benefits greatly from MHz high-frequency operation and multiphase interleaving. Finally, a dual-phase interleaved GaN-based MHz totem-pole PFC rectifier is demonstrated with 99% peak efficiency and 220 W/in $^{\mathrm {{3}}}$ power density.

Digital-Based Interleaving Control for GaN-Based MHz CRM Totem-Pole PFC

This paper presents a comparison between the performances of different interleaving control methods for gallium nitride devices-based MHz critical conduction mode (CRM) totem-pole power factor correction (PFC) circuit. Both closed-loop interleaving and open-loop interleaving are good for the 70-kHz CRM PFC; but for a MHz frequency CRM PFC with microcontroller (MCU) implementation, the open-loop interleaving outperforms the closed-loop interleaving with only a small and nonamplified phase error. After software optimization, the phase error of the open-loop interleaving is smaller than 3° at 1 MHz, when the control is implemented by a 60-MHz low-cost MCU. Significant ripple cancellation effect and differential-mode (DM) filter size reduction are achieved with good interleaving. For a 1.2-kW MHz totem-pole PFC, the DM filter size is reduced to one quarter when compared with the counterpart of a 100-kHz PFC. Last but not least, the stability of the open-loop interleaving is also analyzed indicating that the MHz CRM totem-pole PFC with voltage-mode control, open-loop interleaving, and turn-on instant synchronization can maintain critical mode operation with better stability compared with the 70-kHz CRM PFC.

Problem of consciousness in learning systems and environments

The article deals with the problems that arise when a human is involved into a complexly training systems and the environment. A theoretical analysis of the infl uence of consciousness on the training mechanisms within the framework of post non-classical conceptions of human psyche functioning was conceived. We argue that consciousness is not intended to refl ect the objective reality and acquire knowledge, and solves the problem of the construction of the subjective world, which is the sum of compromises between the subject and the dynamic environments of his experience. In contrast, consciousness selects and harmonizes the information useful for human life. The role of consciousness in training is largely contradictory and confusing. It is also the source of the new information. The information is distorted in order to create a world view, which tends to objectively contradictive to the outside observer and consistent for refl ective being. We stressed the important role of human’s multimodal sensory experience in the process of interaction between conscious and unconscious forms of knowledge creation and processing. We consider the qualitative difference between physical reality and the model presented in the subjective world. We postulated the existence in the mind of a human dynamic virtual model. According to this model, the natural world is delimited by person and opposed to it being a source of events that make up the content of his life. The border which was built in the consciousness divides constructed reality into the inner and outer worlds. These worlds have different significance for the subject, defining the nature of its activities. We analyzed models the structure of reality and reality as a world’s refl ection offered by Karl Popper, K. K. Kolin, R. Redfield, G. Roth. The attention was paid to the neurobiological model of subjective reality by V. Sergin, which proposed a hypothesis of automatic identification postulating a secondary to human sensory categories. These processes occur cyclically, and their frequency determines the speed of subjective time. The authors of this article propose a mechanism of generating a subject environment (the world of reality) according to which the environment is perceived by man. It is organized element of subjective reality constructed by the body in the process of analyzing the essential relationship of the body and physical reality. It is presented in the form of coordination with other autopoietic systems that allow for the integration of the body and the subject in a niche of individual existence. It is a complex structural evolutionary self-organizing system, resulting from the interaction and coordination of autopoietic systems of the body. Subjective reality is the result of a reduction in the physical reality organization of the human psyche. We show the infl uence of the mind on the mechanisms of human integration with artificial learning environments. It is concluded that in order to improve the effectiveness of complex training ergonomics systems must take into account the properties of conscious regulation of the subject, its dependence on the context. To protect against the destructive mechanisms of action of consciousness training we advise to restrict the user’s intervention in critical modes of operation created a complex training system.

Systematic analysis and design of dual-switch Flyback converter

This paper presents the circuit analysis and design considerations for the dual-switch Flyback converter. The operation modes are individually analyzed and the corresponding state equations are presented accordingly. Also, the design equations for key components are derived. Good agreement can be obtained among the Pspice simulation, the experimental results and the theoretical analysis. The analytic equations are valid not only for full load and continuous conduction-mode condition, but also for critical-mode operation, which meets the design specifications.

1D Model to Predict Ejector Performance at Critical and Sub-critical Operation in the Refrigeration System

Abstract A new 1D model using the real gas property is proposed to predict ejector performance at critical and sub-critical operational modes, while most previous 1D models usually used the ideal gas property and only predicted ejector performance at critical mode operation. Constant pressure mixing is assumed to occur inside the constant area section of the ejector at critical and sub-critical mode operation, and the effectiveness of the model is verified against experimental data. The results show that the proposed model accurately predicts ejector performance over all ranges of operation. The 1D model is a useful tool for predicting ejector performance within larger refrigeration cycle models.

A Stepping On-Time Adjustment Method for Interleaved Multichannel PFC Converters

A simple and effective stepping on-time adjustment (SOTA) method with status judgment for interleaved multichannel critical mode (CRM) boost-type power factor correction (PFC) converters is proposed. The boost-type PFC converter with CRM operation is commonly used for ac-dc conversion because of its circuit simplicity and high efficiency. The interleaved multichannel PFC converter becomes the trend for high-power applications. However, conventional interleaved methods are complicated and difficult to implement when more than two channels are required. Also, noise disturbances or on-time mismatching can easily lead to continuous conduction mode operation which will increase current distortion, reduce power conversion efficiency, or even damage the converter. The proposed SOTA method with status judgment can greatly simplify the control complexity of the interleaved multichannel operation. A prototype 600-W three-channel interleaved CRM boost PFC converter is built to verify the performance of the proposed SOTA method.

Control of resource-intensive computations under uncertainty. III. Dynamic concurrent resource allocation

Control problems for a heterogeneous system executing heterogeneous search tasks are considered. Data parallelization of tasks under objective uncertainty--for the unknown and rapidly changing flow of incoming requests--is studied. Critical modes of operation under big system work-loads are analyzed. The modified sliding scheduling method allows correcting the processing procedure in real time. A way of dynamic construction of a sequence of non-discriminating concurrent allocations of limited different-type resources between users with equal rights is proposed. A concept of normative maximal performance of the system processing one type of tasks in the single-user mode is introduced. Parametric guaranteed estimates are used to decompose tasks with respect to their type. Scheduling procedures according to which tasks that de facto require less computational costs receive the processing priority and are completed first of all are developed.

Relevance of aerodynamic modelling for load reduction control strategies of two-bladed wind turbines

A new load reduction concept is being developed for the two-bladed prototype of the Skywind 3.5MW wind turbine. Due to transport and installation advantages both offshore and in complex terrain two-bladed turbine designs are potentially more cost-effective than comparable three-bladed configurations. A disadvantage of two-bladed wind turbines is the increased fatigue loading, which is a result of asymmetrically distributed rotor forces. The innovative load reduction concept of the Skywind prototype consists of a combination of cyclic pitch control and tumbling rotor kinematics to mitigate periodic structural loading. Aerodynamic design tools must be able to model correctly the advanced dynamics of the rotor. In this paper the impact of the aerodynamic modelling approach is investigated for critical operational modes of a two-bladed wind turbine. Using a lifting line free wake vortex code (FVM) the physical limitations of the classical blade element momentum theory (BEM) can be evaluated. During regular operation vertical shear and yawed inflow are the main contributors to periodic blade load asymmetry. It is shown that the near wake interaction of the blades under such conditions is not fully captured by the correction models of BEM approach. The differing prediction of local induction causes a high fatigue load uncertainty especially for two-bladed turbines. The implementation of both cyclic pitch control and a tumbling rotor can mitigate the fatigue loading by increasing the aerodynamic and structural damping. The influence of the time and space variant vorticity distribution in the near wake is evaluated in detail for different cyclic pitch control functions and tumble dynamics respectively. It is demonstrated that dynamic inflow as well as wake blade interaction have a significant impact on the calculated blade forces and need to be accounted for by the aerodynamic modelling approach.Aeroelastic simulations are carried out using the high fidelity multi body simulation software SIMPACK. The aerodynamic loads are calculated using ECN's AeroModule and NREL's BEM code Aerodynl3.

A NEW 1D MODEL FOR SUPERSONIC EJECTOR PERFORMANCE EVALUATION AT CRITICAL AND SUB-CRITICAL OPERATIONAL MODES

A new 1D model is proposed to predict ejector performance at critical and sub-critical operational modes, while most previous 1D models have predicted ejector performance only at critical mode. Constant pressure mixing is assumed to occur inside the constant area section of the ejector at both operational modes. Experimental data from different ejector geometries and various operation conditions reported earlier are used to verify the effectiveness of the new model. The model has a good performance in predicting the mass flow rates and the entrainment ratio, a useful tool for predicting ejector performance within larger refrigeration cycle models.

The MYRRHA-FASTEF cores design for critical and sub-critical operational modes (EU FP7 Central Design Team project)

On April 2009, a three-year-project was launched within the 7th Framework Programme (FP) of the European Commission: the Central Design Team (CDT) for a FAst Spectrum Transmutation Experimental Facility (FASTEF). The main goal was to achieve an advanced level of design for an irradiation facility, cooled by lead-bismuth, operating in both critical and sub-critical modes. In continuity with the research studies on fast nuclear systems carried out in the 5–6th FPs, the CDT had the further ambitious objective to define a preliminary design of the MYRRHA reactor, planned to be built at the SCK•CEN research centre in Mol (Belgium). In addition to being a multi-purpose irradiation facility, MYRRHA should be able to demonstrate the Acceleration Driven System concept at ∼100MW power level and an efficient transmutation of minor actinides, as main contributors to high-level long-lived radioactive wastes.This paper describes the design of cores able to operate the MYRRHA-FASTEF plant in both critical and sub-critical modes. The design studies were performed by detailed neutronic analyses (with deterministic and Monte Carlo methods) and by accurate thermal-hydraulic evaluations (at the fuel assembly and pin sub-channel level), by taking also into account thermo-mechanical and safety constraints. Among the most significant core features, the fast flux level (Φ>0.75MeV∼1015cm−2s−1), the high flexibility for irradiation purposes and the limited overall dimension (impacting on the cost of the plant) can be noticed. The transmutation of minor actinides, enhanced by the high fast flux, reaches values of about 32kgHMTWh−1 in both operational modes.

A 1D model to predict ejector performance at critical and sub-critical operational regimes

This paper proposes a new 1D model to predict ejector performance at critical and sub-critical operational modes, while most previous 1D models have only predicted ejector performance at critical mode operation. Constant pressure mixing is assumed to occur inside the constant area section of the ejector at critical and sub-critical mode operation, and the effectiveness of the model is verified against four sets of experimental data that include different working fluids and geometries. The results show that the proposed model accurately predicts ejector performance over all ranges of operation, and is a useful tool for predicting ejector performance within larger refrigeration cycle models.

Pollutant emissions from engine starting with ethanol and butanol diesel blends

The starting is a special transient process of engine operation, which is included in the New European Driving Cycle of light duty vehicles (NEDC). This is a critical operation mode of diesel engines with respect to pollutant emissions and to stability of the combustion process. In this work, a turbocharged, direct injection (DI), diesel engine equipped with common rail injection system and EGR strategy was tested during the starting. The engine was tested at relatively cold and warm start. Pollutant emissions (HC, NOx, smoke opacity and particle size distributions), in-cylinder pressure and operating parameters such as rotation speed, relative fuel–air ratio and EGR valve position were registered during the tests. The engine was fueled with a pure low sulfur diesel fuel, and blended with ethanol and butanol. Fuel blends were prepared with the same oxygen content (~3.3% in mass). The results show that alcohol diesel blends lead to a positive effect on the reduction in the smoke opacity and particle concentration during warm engine start with similar nitrogen oxide (NOx) emissions. At cold start, blends tested produced combustion instabilities. This fact, jointly with inefficient operation of the diesel oxidation catalyst, produced an increase of all regulated pollutant emissions.

Hidraulinis slėginės nuotekų linijos modeliavimas

Vandens paskirstymo sistemų modeliavimas naudojant kompiuterį yra patikima, efektyvi ir saugi technologija, padedanti imituoti ir analizuoti sistemos veikimą esant plačiam hidraulinių sąlygų diapazonui. Straipsnyje aprašomas hidraulinių sąlygų identifikavimas realioje slėginėje nuotekų linijoje nuo siurblinės „X“ iki slėgio slopinimo kameros. Renovavus slėginę nuotekų liniją buvo pastebėtas avarijų dažnėjimas. Taikant modelį atlikti skaičiavimai ir identifikuotos hidraulinės sąlygos būdinguosiuose slėginės linijos taškuose. Identifikuotas ir aptartas hidraulinių smūgių poveikis vamzdynui.

Triple-Choking Model for Ejector

In this study, a 1D analysis has been presented for the prediction of ejector performance at critical mode operation. The new triple-choking model has been developed using the governing equations of the compressible fluids and thermodynamics properties based on the frictional adiabatic fluid study. A new approach has been introduced to consider the frictional effects on the mixing efficiencies by extending the 1D ejector theory. A very good agreement has been reported for the R141b and steam experimental data at critical mode operation. Furthermore, simulated results have been compared with some of the recent theoretical models. In addition, the influence of operation conditions on the ejector performance and the required cross-sectional area of the mixing chamber has been showed. Finally, the influence of the operation conditions (such as generator, condenser, and evaporator temperatures) and the size of ejector on the mixing efficiency have been studied.

The buckling of elongated rotating drill strings

There is a growing need for the development and implementation of new numerical methods for computer simulation of static and dynamic critical states of drill strings in connection with mastering deeper oil and gas resources. Current methods for analysis of extraordinary behavior of the drill strings, as a rule, concern their post-critical states related to essential change of their shape and contact interaction with borehole wall. In parallel with the post-critical regimes there also are critical modes of operation attended with incipiency of unstable equilibrium states and the DS buckling. Inasmuch as the critical states precede the post-critical ones, sometimes their prognosis can be more important. This paper considers the problem of quasi-static stability of elongated drill string tubes studied with allowance made for destabilizing effects of the longitudinal non-uniform preloading, action of torque, inertia forces of rotation and internal flows of mud. The constitutive equations are deduced, and the methods for their computer solving are proposed. As a result of the completed analysis the critical values of the characteristic parameters are found, and it is shown that the modes of incipient buckling possess complicated shapes associated with the tube spiral curving prevailing in the zone of bottom-hole-assembly.

Shock circle model for ejector performance evaluation

Abstract In this paper, a novel shock circle model for the prediction of ejector performance at the critical mode operation is proposed. By introducing the “shock circle” at the entrance of the constant area chamber, a 2D exponential expression for velocity distribution is adopted to approximate the viscosity flow near the ejector inner wall. The advantage of the “shock circle” analysis is that the calculation of ejector performance is independent of the flows in the constant area chamber and diffuser. Consequently, the calculation is even simpler than many 1D modeling methods and can predict the performance of critical mode operation ejectors much more accurately. The effectiveness of the method is validated by two experimental results reported earlier. The proposed modeling method using two coefficients is shown to produce entrainment ratio, efficiency and coefficient of performance (COP) accurately and much closer to experimental results than those of 1D analysis methods.

Improvement of gas-jet ejector discharge characteristics with heads, chevrons, and tubs

Properties of gas-jet ejectors with converging mixing chamber operated under critical and subcritical flow conditions are examined. Establishment of the critical operation mode in such ejectors started up with and without the secondary gas flow is considered. The mechanism underlying the emergence of flow pulsations at low rates of the low-head flow is described.

Detection of Critical Operating Conditions for Fuel Cell Applications via Distortion Analysis

This new fuel cell stack diagnostics technology utilizes a signal distortion analysis to determine the actual status of a fuel cell / fuel cell stack in terms of critical operation conditions. Oxygen and hydrogen starvation, cell floodings are detectable with this method. In this paper the results of a flooding situation and an air starvation case are discussed. Also a semi empirical model is described which enables the prediction of the cell behavior in such critical operation modes.

Detection of fuel cell critical status by stack voltage analysis

In comparison to state of the art fuel cell stack monitoring techniques, where for reliability and durability reasons either single cell or cell-block voltages are monitored separately, the new approach derives information about critical cell and stack status from the stack sum voltage only. The motivation for the development of such a technology is to establish a strongly simplified and low cost stack diagnosis unit. In comparison to the cell voltage monitoring (CVM) technology, where up to several hundreds voltage channels have to be measured separately or at least in pairs, the effort for wiring, contacting and instrumentation can be reduced dramatically. Critical cell operation occurs, if e.g. low air stoichiometry causes a sharp drop of voltage at a certain cell current. If in such case the current is superimposed by a small amplitude signal with specific frequency pattern, then the system response (i.e. stack voltage) will be distorted in the frequency domain. Particularly, this means that even if only one single cell is in a critical operation mode, it would cause distorted frequency fractions extractable from the entire stack voltage. Since this approach is related to the distortion of the frequency pattern only, noise and EMC issues are not significantly influencing the measuring quality. The instrumentation and processing effort is rather low and can be realized with a low cost DSP board. Measurement results for different critical stack operation modes, e.g. operation at low air or low hydrogen stoichiometry, with their correlation to the frequency distortion will be described and discussed.

Multi-component self-consistent nuclear energy system: protected plutonium production (P<SUP align=right>3</SUP>)

The research activity on Protected Plutonium Production (P3) has been performed in the framework of MC-SCNES that simultaneously achieves four requirements – energy production, fuel production, burning of radioactive wastes and safety by the combination of fission, spallation and fusion neutron sources. The increase of a fraction of 238Pu provides an essential protective measure to plutonium against the proliferation due to its high decay heat and spontaneous fission neutrons. It is discussed that 238Pu production by the transmutation of MA in both critical and sub-critical operation modes. The demonstration of P3 mechanisms in the reactor will provide a big possibility of new reactor markets in the world.