Critical Mode Operation Research Articles

A Resilience-Oriented Approach for Microgrid Energy Management with Hydrogen Integration during Extreme Events

This paper presents a resilience-oriented energy management approach (R-OEMA) designed to bolster the resilience of networked microgrids (NMGs) in the face of extreme events. The R-OEMA method strategically incorporates preventive scheduling techniques for hydrogen (H2) systems, renewable units, controllable distributed generators (DGs), and demand response programs (DRPs). It seeks to optimize the delicate balance between maximizing operating revenues and minimizing costs, catering to both normal and critical operational modes. The evaluation of the R-OEMA framework is conducted through numerical simulations on a test system comprising three microgrids (MGs). The simulations consider various disaster scenarios entailing the diverse durations of power outages. The results underscore the efficacy of the R-OEMA approach in augmenting NMG resilience and refining operational efficiency during extreme events. Specifically, the approach integrates hydrogen systems, demand response, and controllable DGs, orchestrating their collaborative operation with predictive insights. This ensures their preparedness for emergency operations in the event of disruptions, enabling the supply of critical loads to reach 82% in extreme disaster scenarios and 100% in milder scenarios. The proposed model is formulated as a mixed-integer linear programming (MILP) framework, seamlessly integrating predictive insights and pre-scheduling strategies. This novel approach contributes to advancing NMG resilience, as revealed by the outcomes of these simulations.

ON THE ISSUE OF ENSURING THE MICROCLIMATE CONDITIONS AND OPERATIONAL SAFETY OF HEAT SUPPLY SYSTEMS DURING EMERGENCY HEAT SUPPLY SHUTDOWNS

The issue of ensuring the microclimate conditions of the premises in the buildings and structures during cold season are of exceptional importance, as it is the basis for ensuring the safety of human life and health. Emergencies and significant violations of the thermal regime in premises and buildings worsen microclimate conditions for an indefinite period of time, until its parameters reach extreme values dangerous for human life. These issues are particularly relevant in the context of the growing number of emergency situations in heat supply systems in Ukraine, both due to the high level of wear and tear of their infrastructure in general, and wartime emergencies. Therefore, conducting research to ensure the safety of heat supply systems operation during emergency shutdowns by determining the regularities of temperature changes in rooms, reaching the critical temperature and operating time of heat supply systems, taking into acount the spatial location of rooms in buildings and their design features, is an urgent task. The purpose of the article. Study of the risk of reaching critical modes of heat supply systems operation in emergency situations, taking into acount the structural features of buildings and ensuring the necessary microclimate conditions. Conclusion. On the basis of the conducted research, the regularities of the structural features of buildings and structures in emergency situations in heat supply systems to ensure microclimate conditions in the premises have been determined. The limit conditions for achieving critical indoor microclimate conditions due to accidents in heat supply systems are determined, which allows predicting the limit conditions for eliminating the consequences of emergency situations and optimizing the activities of repair and rescue services, which is especially important in connection with the failure of outdated heat supply systems and combat operations on the territory of the country.

Determination of the Heat-Controlled Accumulator Volume for the Two-Phase Thermal Control Systems of Spacecraft

<div>For spacecraft with high power consumption, it is reasonable to build the thermal control system based on a two-phase mechanically pumped loop. The heat-controlled accumulator is a key element of the two-phase mechanically pumped loop, which allows for the control of pressure in the loop and maintains the required level of coolant boiling temperature or cavitation margin at the pump inlet. There can be two critical modes of loop operation where the ability to control pressure will be lost. The first critical mode occurs when the accumulator fills with liquid at high heat loads. The second critical mode occurs when the accumulator is at low heat loads and partial loss of coolant, for example, due to the leak caused by micrometeorite breakdown. Both modes are caused by insufficient accumulator volume or working fluid charge. To analyze the loop characteristics in critical modes, experiments were conducted on a test bench with ammonia coolant, and a mathematical simulation of a two-phase mechanically pumped loop was performed. The results show that the loop can operate in critical modes in a certain range of heat loads. The conducted studies allow for the design of a heat-controlled accumulator with the minimum required volume, expand the performance range of a two-phase mechanically pumped loop, and increase the reliability of its operation in orbit during long-term missions.</div>

ANALYSIS OF NONLINEAR RIGIDITY CHARACTERISTICS OF AIR BLOWER COMPRESSOR ELASTIC SUPPORTS

Vibrations of rotor systems are largely determined by elastic characteristics of supports. In the general case, the dependence of displacements on force for such supports is nonlinear. This applies to bearing supports of various types. In addition to bearings, elastic elements are also used in supports of rotor systems. They are designed to change the overall stiffness of the supports. The paper analyzes the radial stiffness of flexible ring dampers used in rotor supports. For this, two main approaches are proposed. The first is a conventional simulation of this problem of contact mechanics using the finite element method. The second is an analytical method that can be used as an alternative to expensive numerical calculations. This method is based on the principle of minimum additional energy. A special closed-form variational formulation is developed using the Euler-Bernoulli beam approximation for an elastic ring and a simplified model of normal contact on the ring flanges. It was shown that the surface tolerances of parts have a significant effect on the radial response of a flexible ring, which can become nonlinear. Tight fit of the ring on both sides makes it much stiffer, and non-tight fit leads to free movement of the rotor and much weaker damping of its movement. Both methods gave results that are consistent for the considered cases. By varying their design parameters, it is possible to adjust the rotor systems from critical modes of operation. Keywords: rotor system, elastic support, rigidity, finite element analysis, stress-strain state, contact interaction, principle of minimum additional energy

Artificial neural network based shape optimization of supersonic ejectors in the critical flow regime

Supersonic ejectors are widely used in energy, refrigeration, and aerospace propulsion applications. A supersonic ejector is a passive gasdynamic device consisting of a converging–diverging nozzle located in a varying area mixing duct where the secondary fluid is compressed by mixing with high momentum primary fluid. The entrainment ratio (ER, ratio of mass flow rates of the secondary to primary flow) is dependent on the mixing duct geometry, necessitating shape optimization. Complex gasdynamic interactions in the ejector are not fully understood, thus making physics-based or CFD modeling, a challenging exercise. In this paper, experimental data for ejector performance with varying geometries and operating parameters, viz. area ratio, primary flow Mach number, mixing duct length to height ratio (L/D), and stagnation pressure ratio is presented. The mixing length parameter is quantified using data derived from high-speed schlieren images. Improvement in ER to the extent of 20% is achieved by changing L/D to 15 for the AR of 3.33, with further increase in L/D resulting in a reduction in ER. Subsequently, computationally inexpensive model using ANN is developed to predict the ejector performance using the experimental database. The model predicts ER with an accuracy of 7%. Finally, the ANN model is used to optimize the ejector geometry for critical mode operation, which provides a 30% enhancement in entrainment ratio compared to the baseline geometry.

Hydrodynamic Forces of DP Jack-Up Leg when Operating in Vicinity of Seabed

_ Leg placement and removal are the two most critical operational modes for dynamically positioned jack-ups when working close to an offshore asset. Any positional deviation may lead to collision and damage to the asset. The industry operates with a weak link between the dynamic positioning (DP) system and the jacking system. Current DP systems operate without any sensors identifying the hydrodynamic force variations on the legs and spudcans, which vary between different leg and spudcan designs. When the spudcan is near to the sea bottom, the hydrodynamic force must be reported to avoid large positional deviations driven by the DP system. This article promotes a mechanism to measure these forces using Computational Fluid Dynamics (CFD) analysis to analyze the jack-up behavior, when the spudcan assembly is operating close to the sea bottom. Introduction A jack-up’s dynamic positioning (DP) control system requires minimum 23–30 minutes for the mathematical model to learn the vessel’s hydrodynamic behavior and response to the environment. Although when moving between locations, DP jack-up vessels provide time for the DP model to learn the hydrodynamic behavior, the spudcan that holds the vessel position and headings does not allow the mathematical model to learn. The residual current remains constant until the spudcan is in the seabed. As a result, the DP mathematical model-building process does not help the DP system to estimate the additional forces in the form of residual current. Soon after the spudcan detaches from the seabed, the vessel drift occurs because the vessel thrusters’ response need a rapid response of thrust and azimuth (directions). The DP system manufacturers currently use a sensorless approach to account for the hydrodynamic forces on the legs and spudcans to build a factor into the mathematical model. The jack-up DP system addresses two simultaneous forces on the legs. The leg element in the air is subject to aerodynamic effects and the leg and spudcan elements in the water are subject to hydrodynamic effects. DP systems currently use drag coefficients (Cd) to compute drag forces, however the hydrodynamic force variations during the complete lowering and raising processes are never completely considered. This weak link in the overall operation leads to positional error and is generally unrecognized by the vessel operators. The risk falls to DP officer and the jacking master to handle. The DP and jacking simultaneous operations mode (SIMOPS) may easily last between 15 and 90 minutes, depending on jacking speed, operational water depth, and field procedures, on approach to the asset. The area of operation is close to the asset, which increases the risk of collision with the asset. Most of the studies on jack-up vessels focus on impact force acting on the leg during touchdown or penetrations, such as Elkadi et al. (2014) and Kreuzer et al. (2014).

Based on the Matrix of Silicon Nanocrystals a Modeling Study of Thermo-hydraulic Properties of Micro-channel Heat Transfer Elements

The construction of a heat exchange element based on a matrix of silicon whiskers for thermal stabilization systems of miniature heat sources with specific power up to 100 W/cm2 operating over a wide range of ambient temperatures is proposed. Based on the developed mathematical model of convective heat transfer in a microchannel compact heat exchanger with a developed heat exchange surface, numerical simulation of the hydrodynamics and heat transfer processes for various configurations of microchannel insertions was carried out. Fields of pressures, flow velocities, coolant temperatures and matrix from silicon single crystals have been obtained in a wide range of coolant flow rates, criteria dependencies for the Nusselt number and pressure losses of various geometric configurations of heat exchangers have been determined. Critical operation modes are investigated; optimization directions are proposed. According to the developed technology, prototypes for testing have been manufactured.

ОБ АКУСТИЧЕСКИХ ПАРАМЕТРАХ МЕТАЛЛА КОНСТРУКЦИИ ПРИ НАКОПЛЕНИИ ПОВРЕЖДЕНИЙ

In the work with the use of nondestructive control methods, the issues of determining acceptable durability or safe resource are considered. It is shown that the design monitoring procedure can be presented as follows: determines the original state zero zone, where the material of the product was subjected to low operational loads; further, with the use of nondestructive control methods, the acoustic parameter is measured (without disassembling the product)., such as the speed of elastic acoustic waves, nonlinear acoustic parameter, difference of velocities with twofrequency sensing, etc.; acoustic scanning of the product's metal is performed, in areas where there have been significant loads, signoff voltages, leading to an intensive accumulation of damage (the destruction of metal leading to cracks); defined zone “N” where the metal state parameter, for which the value is taken, characterizing the difference of the acoustic parameter (the speed of elastic waves, nonlinear acoustic parameter, the difference in velocity in twofrequency sensing) relative to the same parameter in the zone of zero exceeds the established level. The regularities established in the work linking the presence of plastic deformation with the difference in the delays (velocity) of elastic Rayleigh waves at different sounding frequencies at a fixed base between the emitter and the receiver of elastic waves, as well as the behavior of a nonlinear acoustic parameter during the safe resource time, suggest the possibility of using the observd fact as a principle for controlling the limiting state of the material due to plastic deformations on industrial structures. Based on the proposed approach, an engineering methodology for determining the technical condition of the material of the structures of production facilities is proposed, which allows to establish three stages of operation: the reliable operation mode; the controlled operation mode and the critical operation mode.

Characterization and Optimal Control of Totem-Pole PFC Converter With High Frequency GaN HEMTs and Low Frequency Si Diodes

A cost-effective totem-pole power factor correction converter with high-frequency gallium nitride high electron-mobility transistors and low-frequency Si diodes is investigated in this article. The reverse recovery characteristics of low-frequency diodes are evaluated experimentally. With the slow recovery characteristics, a negative current can flow through the low frequency diode, so that soft-switching and critical mode operation is achieved easily. However, if the duration of the negative current exceeds the reverse recovery time of the low-frequency diode, the diode will switch at high frequency, leading to severe losses and losing control of the inductor current. In order to avoid this issue, an adaptive off-time soft-switching strategy is adopted, and discontinuous conduction mode operation is suggested near the zero crossing of the grid voltage. Furthermore, since only the low-frequency grid current is sampled, this control strategy has the merits of low requirements on control resources and low cost. Experimental results of a 400W prototype are provided to verify the effectiveness of the proposed control strategy.

A New Diagnostic Technique for Reliable Decision-Making on Transformer FRA Data in Interturn Short-Circuit Condition

Interpreting results of a transformer frequency response analysis (FRA) is quite challenging. One of the common methods to summarize FRA data is to employ statistical indicators (SIs) over FRA spectra. However, SI-specific boundary conditions for various operational modes of transformers are left unexplored. The lack of such boundary conditions renders interpretation of SIs difficult and subjective. In this article, in an attempt to find data-driven boundary conditions, first the conventional measurement setup of FRA technique is modified to emulate interturn winding short-circuit. Then, the boundary conditions of various SIs for normal, suspicious, and critical operational modes of transformers under fault are obtained. Nevertheless, the price of moving subjective boundaries to their objective data-driven counterparts is paid in an intrinsic uncertainty introduced by the process of data collection per se. In order to capture and quantify this uncertainty, a novel solution inspired by bolstered error estimation used in pattern recognition is proposed. In particular, the proposed method allows reporting the level of confidence that an observed magnitude of SI belongs to a specific operational mode. Having this confidence level is also warranted from an operational perspective because it enables utility operators to enhance the decision-making process and estimate the severity of transformer faulty conditions.

Matrix Inductor With DC-Bias Effect Reduction Capability for GaN-Based DC-DC Boost Converter

In this brief, a 400-W Gallium Nitride device based DC-DC boost converter with a four phase matrix inductor is proposed. It is well-known that there is a significant influence of the DC bias on inductor core losses and the simplest solution is to employ multiphase structures. However, the size of the converter will also increase as the number of phases increases. Therefore, in order to achieve both high efficiency and high power density, a matrix inductor is proposed for four phases boost converter. The proposed matrix inductor retains the flux sharing advantages of conventional inductors with E-cores and thus greatly increases the power density and utilization rate, by integrating the inductors through flux cancellation. Therefore, both the size and the core loss are reduced when compared with four conventional inductors. Moreover, since the four phases are operated in parallel, the phase error, which is a critical problem in the interleaved structure, may not affect the proposed converter, removing the current balance issue. Critical operation mode is utilized to achieve zero-voltage switching (ZVS) for all switches. Finally, the proposed four phase DC-DC boost converter and matrix inductor is built and tested to verify its feasibility. The peak and CEC efficiency is tested to be 99.3% and 99.1%, respectively.

Analysis of acoustic signals in increments for functional diagnostics of multi-drive units

The current trend in the development of mechatronic systems is the use of multi-drive frame configurations. Such objects, as a rule, consist of a large number of interacting elements. The relative displacements of these elements generate vibrations that can critically affect the operation of precision mechatronic systems. This can lead to limiting operating conditions of the equipment, and in some cases, its failure. An effective means of preventing accidents, identifying critical modes of operation, diagnosing faults in equipment of electromechanical systems are the methods of functional diagnostics.Methods of functional diagnostics in real time require a large number of calculations, mathematical modeling of the object, processing a large amount of information, which leads to the mandatory use of computer technology. The main methods of processing the measurement results are the Fourier and Laplace transforms, classical methods for analyzing time series, multiple-scale wavelet analysis, etc. The disadvantage of the approaches used is computational complexity in the implementation of discrete transformations and the need for significant amounts of memory to store reference values. The considered methods are based on the analysis of the original signal, however, the increments of the signal of different orders may also be informative.The aim of the research is to develop methods for analyzing acoustic signals generated by working equipment of electromechanical complexes, in increments suitable for use in functional diagnostics systems when monitoring the operating modes of electric drives.In the process of research, an analysis of acoustic noise signals from a working multi-drive unit, obtained as a result of a series of experiments at two high-speed modes, was performed. To speed up the process of analyzing signals and eliminate their redundancy, a data aggregation method has been applied. The degree of permissible aggregation is determined using a multi-scale analysis. The developed method for analyzing aggregated acoustic signals in increments in phase space can be used for functional diagnostics of multi-drive units.

On the Scheduling of Energy-Aware Fault-Tolerant Mixed-Criticality Multicore Systems with Service Guarantee Exploration

Advancement of Cyber-Physical Systems has attracted attention to Mixed-Criticality Systems (MCSs), both in research and in industrial designs. As multicore platforms are becoming the dominant trend in MCSs, joint energy and reliability management is a crucial issue. In addition, providing guaranteed service level for low-criticality tasks in critical mode is of great importance. To address these problems, we propose “LETR-MC” scheme that simultaneously supports certification, energy management, fault-tolerance, and guaranteed service level in mixed-criticality multicore systems. In this paper, we exploit task-replication to not only satisfy reliability requirements, but also to improve the QoS of low-criticality tasks in overrun situation. Our proposed LETR-MC scheme determines the number of replicas, and reduces the execution time overlap between the primary tasks and replicas. Moreover, instead of ignoring low-criticality tasks or selectively executing them without any guaranteed service level in overrun mode, it mathematically explores the minimum achievable service guarantee for each low-criticality task in different execution modes, i.e., normal, fault-occurrence, overrun and critical operation modes. We develop novel unified demand bound functions (DBF), along with a DVFS method based on the proposed DBF analysis. Our experimental results show that LETR-MC provides up to 59 percent (24 percent on average) energy saving, and significantly improves the service levels of low-criticality tasks compared to the state-of-the-art schemes.

Critical flow venturi with a step-wall diffuser

The study investigates the critical flow venturi (CVN) with step-wall diffusers for constant volume gas flow rate in critical flow mode. An analytical method for the estimation of the pressure ratio required for the onset of critical flow from these CVN is given. Numerical simulation and experiments have both been employed to compare the onset of critical flow in two types of CVN: ISO 9300 [1] CVN and a CVN with a geometrically similar inlet and a step-wall diffuser with 4 cylindrical sections replacing the conical diffuser. Good agreement between the analytical, numerical and experimental data has been obtained. The CVN with step-wall diffusers have been demonstrated to provide stable flow rate in the critical flow operation mode, however somewhat higher pressure ratio is required to ensure the critical flow in this case.

Optimal Design of Planar Magnetic Components for a Two-Stage GaN-Based DC–DC Converter

This paper develops a 200-W wide-input-range (64-160-to-24-V) rail grade dc-dc converter based on gallium nitride devices. A two-stage configuration is proposed. The first regulated stage is a two-phase interleaved buck converter (>400 kHz), and the second unregulated stage is an LLC (2-MHz) dc transformer. In order to achieve high frequency and high efficiency, the critical-mode operation is applied for the buck converter, and the negative coupled inductors are used to reduce the frequency and the conduction losses. Then, a systematical methodology is proposed to optimize the planar-coupled inductors. For the unregulated LLC converter, it can always work at its most efficient point, and an analytical model is used to optimize the planar transformer. Finally, the proposed dc-dc converter, built in a quarter brick form factor, is demonstrated with a peak efficiency of 95.8% and a power density of 195 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

A GaN-Based DC–DC Module for Railway Applications: Design Consideration and High-Frequency Digital Control

This paper develops a 200 W two-stage rail grade dc–dc module based on gallium-nitride devices. It converts a wide input voltage (64–160 V) to a constant output voltage (24 V). Different devices and topologies are evaluated for the target application, based on which a two-stage configuration is proposed. The first stage is a two-phase regulated buck converter working under critical mode ( $>$ 400 kHz), which is followed by an unregulated 48-V/24-V LLC converter (2 MHz) for 3000-V dc isolation. The voltage regulation is discussed for the buck converter when the inductors are negatively coupled, and then the zero voltage switching (ZVS) extension is explored. However, due to the limitation of the digital controller, a tradeoff is made between the dynamic response and the footprint. The compromise solution is to use a small and slow microcontroller for high frequency control (including voltage regulation, critical mode operation, phase interleaving) and various demands (including soft start-up, external communication, and various protections). Finally, all the required functions are verified in a standalone quarter-brick prototype module. It can achieve 95.1% peak efficiency and 130 W/in $^3$ power density.

Time factor in critical modes of radio electronic system functioning

Introduction. At the present stage of the development of technical systems, it is important to predict their development, quality, and process characteristics. This also applies to radio electronic systems. Sufficiently unfavorable situations for the functioning of radio electronic systems arise in the context of information conflicts, when it becomes necessary to predict the consequences of information loss or the effect of deliberate interference on the capabilities of radio electronic systems. In critical situations the time factor plays a significant role. On the one hand, information that comes with a delay and loses its relevance. From another point of view, it is necessary to carry out the assigned task within the prescribed time. Thus, the task of determining time indicators of the functioning of radio electronic systems in critical regimes remains urgent and requires further research. Problem statement. Take into account that the effectiveness of the radio electronic systems functioning can be estimated by the performing the task probability. In the process of information flow this probability increases and asymptotically come close to one. In critical modes of operation due to the uncertainty and unpredictability of emerging situations the processing time of information increases. Thus the available information which is perceived by the system is different from the information that is provided by the regular mode. It is necessary to find out the influence of time on processing information on the regularity of the change in the probability of accomplishing the task. It is necessary to estimate the radio electronic operation time, which is necessary to perform the task. It is necessary to set restrictions on the processing time of information and to clarify the requirements for the main parameters that characterize the operation of radio electronic systems in critical modes. From a practical point of view it is necessary to compare real and potential indicators of radio electronic systems and to provide for the possibility of predicting the consequences of their functioning critical modes. Results. On the basis of the informational approach to the analysis of processes in radio electronic systems, the main relations regarding the necessary time of their work and the restrictions on the information delay time are established. Typical situations of critical operating modes of radio electronic systems and the effect of false or lost information on their qualitative characteristics are analyzed. Indicators of a systems operation stable mode are defined. System information analysis of processes using typical forecasting models and the method of analogies is carried out. The effectiveness of the radio electronic systems depends on the amount and value of information that comes to them in the process of functioning. Taking into account the peculiarities of the work of the radio electronic systems In critical operating modes, we will consider useful information and misleading or lost information. Analysis of the work of systems with feedback in the conditions of information delay, allowed to set limits on the permissible delay time, determine the efficiency and the required operating time of the systems. For systems with information losses in the information conflict context in conditions of information capacities increasing and delaying the reaction of one of the parties to the conflict the following indicators are established: real and potential systems efficiency; limitations on the possible efficiency of systems; admissible delay time of reaction by one of the parties to the conflict; necessary time for the performance of the assigned functional task. Conclusions. The obtained results show that in critical modes of the functioning of radio electronic systems, the factor influences both the radio electronic systems performance indicators and their potential capabilities. For information systems that extract information, it became possible to take into account the time that is necessary for processing information, use a typical predictive model for the regularity of increasing the information amount and determine the efficiency and necessary time of the system operating. These results can be used in the process of the radio electronic systems modernization and at the initial stage of radio electronic systems designing.

Performance analysis and optimization of an ejector refrigeration system using alternative working fluids under critical and subcritical operation modes

Ejector systems are receiving considerable attention due to their simplicity, lower maintenance requirements, use of low grade heat, longer lifespan and low cost. In this paper an improved model to predict the performance of an ejector refrigeration system under both the critical and subcritical modes of operation was developed and validated. The model predicts ejector performance more precisely compared to studies following the same modeling approach in the literature. Using the developed model, performances with environmentally benign refrigerants, including R1233zd(E), HFO1336mzz(Z), R1234ze(Z), R600, RE245fa2, and RE245fa2 as alternatives to R141b and R245fa were investigated. Furthermore, for ejector area ratios between 4.45 and 12.98, evaporator temperatures between 0 °C and 16 °C and condenser temperatures between 20 and 40 °C, the optimal performance of the ejector system was determined. Results show that for each refrigerant, higher area ratios give higher coefficients of performance, but require higher generator temperatures for better critical condensing temperatures. R600 showed the best performance followed by R1234Ze(Z) and R1233Zd(E) for the entire range of parameters considered. Results further show that there is an optimum generator temperature at each area ratio that maximizes performance. The optimal generator temperature increases as the area ratio and the condensing temperature increase. An alternative and more convenient approach to optimize ejector performance has been suggested in this work.

Primary pump coast-down characteristics analysis in lead cooled fast reactor under loss of flow transient

The primary pump coast-down characteristics research is very important to ensure reactor safety during power failure events in the lead cooled fast reactor (LFR) design. The primary pump coast-down Flow Halving Time (FHT) effects on a 10 MWth lead-bismuth cooled research reactor with critical and sub-critical operation modes were analyzed under loss of flow transient in this paper. Comparison of different coast-down FHT effects on critical and sub-critical reactors was conducted. Moreover, other relevant parameters those may affect the coast-down FHT requirements (e.g. clad temperature limitation and temperature trigger threshold) were analyzed as well. The results showed that: Increasing the primary pump coast-down FHT can decrease the clad peak temperature in lead cooled fast reactor under LOF transient. Both unprotected and protected loss of flow (ULOF and PLOF) transients in critical reactor and ULOF transient in sub-critical reactor were insensitive to coast-down FHT, while PLOF transient in sub-critical reactor was found to be sensitive to the coast-down FHT. The coast-down FHT effects on sub-critical reactor was bigger than critical reactor in the early phase of LOF transient. The primary pump coast-down FHT had little effects on the critical reactor safety under LOF transients. The clad peak temperature of the sub-critical reactor under ULOF transient was the only one that exceeded the temperature limitation among these four transients. Raising the clad temperature limit of the sub-critical reactor contributes little to reactor safety under ULOF transient. Measures should be taken to avoid the happening of serious accident, such as ULOF transient in sub-critical reactor. Scram signal setup (e.g. core outlet temperature trigger threshold) would not enhance the safety of lead cooled critical and sub-critical reactors and would not change the coast-down FHT requirements under LOF transient.

Ejector performance prediction at critical and subcritical operational modes

Traditional ejector models are focusing on the ejector performance predictions at critical mode under design conditions. In reality, ejector systems cannot be operated under these conditions perfectly. Thus, the study of ejector performance at subcritical mode under off-design conditions is important. In this paper, novel models for ejector performance predictions at critical point and breakdown point are developed based on constant-pressure mixing and constant-pressure disturbing assumptions. Then, the two models are integrated as the model to predict ejector performance at critical and subcritical operational modes. In order to determine the ejector component efficiencies in the models, a novel concept, the effect of the change (EOC) of efficiency, is introduced to identify the efficiencies which affect ejector performance significantly. Then, the identified efficiencies are determined by sparsity-enhanced optimization method. The predicted results obtained by our model are much more accurate than those obtained by existing methods.