in this paper, an electromechanically coupled mathematic model of multi-roller driving system for belt conveyor is set up, and the computing equations for dynamic displacement and dynamic tension of the conveyor are a...in this paper, an electromechanically coupled mathematic model of multi-roller driving system for belt conveyor is set up, and the computing equations for dynamic displacement and dynamic tension of the conveyor are also formulated when the hoister is used for straining. Based on the belt conveyor of main inclined shaft in Chengzhuang coal mine, the driving torque, driving power and starting-speed characteristic of each electric motor are studied and measured when multi-roller variable-frequency drive (power distribution 2∶1) is used. The optimal control and the optimal starting-acceleration of the multi-roller variable-frequency drive are determined by a large number of industrial experiments and theoretical calculations.展开更多
Startups form an information network that reflects their growth trajectories through information flow channels established by shared investors.However,traditional static network metrics overlook temporal dynamics and ...Startups form an information network that reflects their growth trajectories through information flow channels established by shared investors.However,traditional static network metrics overlook temporal dynamics and rely on single indicators to assess startups’roles in predicting future success,failing to comprehensively capture topological variations and structural diversity.To address these limitations,we construct a temporal information network using 14547 investment records from 1013 global blockchain startups between 2004 and 2020,sourced from Crunchbase.We propose two dynamic methods to characterize the information flow:temporal random walk(sTRW)for modeling information flow trajectories and temporal betweenness centrality(tTBET)for identifying key information hubs.These methods enhance walk coverage while ensuring random stability,allowing for more effective identification of influential startups.By integrating sTRW and tTBET,we develop a comprehensive metric to evaluate a startup’s influence within the network.In experiments assessing startups’potential for future success—where successful startups are defined as those that have undergone M&A or IPO—incorporating this metric improves accuracy,recall,and F1 score by 0.035,0.035,and 0.042,respectively.Our findings indicate that information flow from key startups to others diminishes as the network distance increases.Additionally,successful startups generally exhibit higher information inflows than outflows,suggesting that actively seeking investment-related information contributes to startup growth.Our research provides valuable insights for formulating startup development strategies and offers practical guidance for market regulators.展开更多
A startup company is not just a simple company;it is an engine for developing the country that creates jobs,creates economic growth,drains the brain,and attracts finance institutions.The economic advancement of a nati...A startup company is not just a simple company;it is an engine for developing the country that creates jobs,creates economic growth,drains the brain,and attracts finance institutions.The economic advancement of a nation can often be gauged by the maturity and robustness of its startup ecosystem,alongside the level of governmental investment and support provided to startups.This research aims to examine the definition and conceptualization of the term“startup”,identify established criteria for classifying a startup,and determine whether explicit legal provisions exist that differentiate startups from newly established companies with a focus on Europe.To achieve these objectives,the research includes analysis methods,focusing on secondary data and official institutional data.This approach facilitated a thorough examination of existing definitions,legal frameworks,and institutional practices regarding the identification and registration of startups across various European nations.The findings reveal a significant discrepancy within Europe concerning legislative clarity on startups.Although many European countries exhibit a well-developed startup ecosystem,not many countries possess formal legislative frameworks that explicitly define a startup and delineate its distinguishing characteristics from those of other newly founded businesses.Notably,according to the Global Startup Index 2024,countries such as Sweden,Germany,France,and the Netherlands rank highest in terms of favourable conditions for startup development.Even of high rankings and developed ecosystems,Germany and Sweden have lack of legal framework and definitions of startups,instead relying primarily on individual programs and ad-hoc initiatives to support entrepreneurial ventures like France and the Netherlands do.The research identified that some European countries,Lithuania,Estonia,Spain,Latvia,and France,have adopted legal regulations that formally define and make differences from startups to other companies,outlining clear criteria and benefits associated with startup status.In contrast,analysis of the former Yugoslavian countries,specifically Slovenia,Croatia,Bosnia and Herzegovina,Macedonia,and Montenegro,except Serbia,indicates a conspicuous absence of clear definitions,formal criteria,or legal frameworks distinguishing startups from newly established businesses.These nations lack provisions within their legislative frameworks explicitly addressing the identification and registration of startup companies,resulting in ambiguity and inconsistency in classification and support measures.Results from this research underscore the necessity for clearer,standardized definitions and robust legal frameworks to effectively support and foster startup ecosystems.It provides a foundation for policymakers and stakeholders within the startup ecosystem to enhance clarity in legislation,facilitate targeted support,and promote sustainable economic growth through innovative entrepreneurial activities.展开更多
Centrifugal pumps always work under steady conditions,and many researches focus on the steady operation.But transient conditions,such as sudden startup and shutdown,are inevitable.The researches on the inner flow of c...Centrifugal pumps always work under steady conditions,and many researches focus on the steady operation.But transient conditions,such as sudden startup and shutdown,are inevitable.The researches on the inner flow of centrifugal pumps under transient conditions have been done,and they show that the transient operation is different from the steady operation.In order to research the evolution of unsteady flow in a centrifugal pump under transient conditions,and to investigate the mechanism of transient effects by analyzing the unsteady flow in a centrifugal pump,the external characteristic experiment and the internal flow numerical calculation of the centrifugal pump with an open impeller during startup is presented.The relationships of the rotation speed,capacity and head between start-time are obtained by the external characteristics experiment.The numerical calculations under startup process are carried out by using the k-e model and N-S equation.The distribution of velocity and pressure in the inner channel of the tested pump was obtained by choosing fourteen start-time points and twelve geometrical points in the impeller channel during startup.The calculation results show that the velocity and the pressure increase linearly with the start-time before rotation＇s speed gets steady,then changes almost horizontally after rotation speed becomes steady,then fluctuates until being steady.The internal flow characteristics are in good agreement with the external characteristic experimental results and numerical calculation.The simulation methods and results make the basis for the diagnosis and optimization of under flow in the centrifugal pump during transient operation.展开更多
The transient behavior of centrifugal pumps during transient operating periods, such as startup and stopping, has drawn more and more attention recently because of urgent needs in engineering. Up to now, almost all th...The transient behavior of centrifugal pumps during transient operating periods, such as startup and stopping, has drawn more and more attention recently because of urgent needs in engineering. Up to now, almost all the existing studies on this behavior are limited to using water as working fluid. The study on the transient behavior related to solid-liquid two-phase flow has not been seen yet. In order to explore the transient characteristics of a high specific-speed centrifugal pump during startup period delivering the pure water and solid-liquid two-phase flow, the transient flows inside the pump are numerically simulated using the dynamic mesh method. The variable rotational speed and flow rate with time obtained from experiment are best fitted as the function of time, and are written into computational fluid dynamics （CFD） code-FLUENT by using a user defined function. The predicted heads are compared with experimental results when pumping pure water. The results show that the difference in the transient performance during startup period is very obvious between water and solid-liquid two-phase flow during the later stage of startup process. Moreover, the time for the solid-liquid two-phase flow to achieve a stable condition is longer than that for water. The solid-liquid two-phase flow results in a higher impeller shaft power, a larger dynamic reaction force, a more violent fluctuation in pressure and a reduced stable pressure rise comparing with water. The research may be useful to tmderstanding on the transient behavior of a centrifugal pump under a solid-liquid two-phase flow during startup period.展开更多
To achieve stable and efficient nitrogen and phosphorus removal and to investigate the characteristics of the A/A SBR enriched with denitrifying phosphorus removal bacteria(DPB),the whole course of startup was studied...To achieve stable and efficient nitrogen and phosphorus removal and to investigate the characteristics of the A/A SBR enriched with denitrifying phosphorus removal bacteria(DPB),the whole course of startup was studied with two reactors operated in different mode.The reactor I was operated under anaerobic/settling/anoxic/settling mode,and the reactor II was operated under anaerobic/anoxic/settling mode.Differences between the two reactors in removal efficiency of COD,nitrogen and phosphorus were examined.The results indicated that efficient performance could be achieved in both reactors with different startup operation mode,while the phosphorus removal efficiency was improved sooner in reactor I than in reactor II,which suggested that reactor I would supply a more favorable condition for DPB proliferation.Meanwhile,it was observed that the amount of organic substrates consumption had a linear correlation to that of phosphorus release in anaerobic phase when DPB was accumulated in the A/A SBR denitrifying phosphorus removal system.展开更多
A suitable model for high-temperature heat pipe startup is a prerequisite to realizing the numerical simula-tion for the heat pipe cooled reactor startup from the cold state.It is required that this model not only des...A suitable model for high-temperature heat pipe startup is a prerequisite to realizing the numerical simula-tion for the heat pipe cooled reactor startup from the cold state.It is required that this model not only describes the transient behavior during the startup period,but also reduces the computing resources of the heat pipe cooled reactor simulation in the simplest way.In this study,a simplified model that integrates the two-zone and network models is proposed.In this model,vapor flow in the vapor space,evaporation,and condensation in the vapor–liquid interface are decoupled with heat conduction to achieve a fast calculation of the transient characteristics of the heat pipe.An experimental system for a high-temperature heat pipe was developed to validate the proposed model.A potassium heat pipe was utilized as the experimental material.Startup experiments were performed with differ-ent heating powers.Compared with the experimental results,the accuracy of the proposed model was verified.Moreover,the proposed model can predict the vapor flow,pressure drop,and temperature drop in the vapor space.As indicated by the analysis results,the essential requirements for successful startup are also determined.The heat pipe cannot achieve a successful startup until the heating power satisfies these requirements.All the discussions indicate the capability of the proposed model for the simulation of a high-temperature heat pipe startup from the frozen state;hence,can act as a basic tool for the heat pipe cooled reactor simulation.展开更多
Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present s...Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present study evaluated the effects of beam startup characteristics on the dose delivery accuracy for each segment at low MUs for step-and-shoot IMRT with an Elekta Precise accelerator at the highest dose rates. We used a two-dimensional semi-conductor detector for the dose measurements. The field size of each segment was assumed to be 20 ×20 cm2 and each segment was set to deliver 1 - 10 MUs. Our results show a variation in dose delivery accuracy between segments for the same IMRT beam, which can be attributed to the beam startup characteristics. This variability is attributed to the changes in the transient changes in the temperatures of the electron gun filament and the magnetron. That is, the transient increase in the temperature of the filament leads to increasing doses with time and that of the magnetron leads to decreasing doses with time during the first few MUs.展开更多
Owing to the inherent instability of the natural circulation system,flow instability can easily occur during the operation of a natural circulation lead-cooled fast reactor,especially during the startup phase.A compre...Owing to the inherent instability of the natural circulation system,flow instability can easily occur during the operation of a natural circulation lead-cooled fast reactor,especially during the startup phase.A comprehensive startup scheme for SNCLFR-100,including primary and secondary circuits,is proposed in this paper.It references existing more mature startup schemes in various reactor types.It additionally considers the restriction conditions on the power increase in other schemes and the characteristics of lead-based coolant.On this basis,the multi-scale coupling code ATHLET-OpenFOAM was used to study the flow instability in the startup phase under different power-step amplitudes and power duration times.The results showed that obvious flow instability phenomena were found in the different startup schemes,such as the short-term backflow phenomenon of the core at the initial time of the startup.Moreover,an obvious increase in the flow rate and temperature to the peak value at the later stage of a continuous power rise was observed,as well as continuous oscillations before reaching a steady state.It was determined that the scheme with smaller power-step amplitude and a longer power duration time requires more time to start the reactor.Nevertheless,it will be more conducive to the safe and stable startup of the reactor.展开更多
The goal of the Sino-United Spherical Tokamak (SUNIST) at Tsinghua University is to extend the understanding of toroidal plasma physics at a low aspect ratio (R/a≈1.3) and to demonstrate a maintainable target plasma ...The goal of the Sino-United Spherical Tokamak (SUNIST) at Tsinghua University is to extend the understanding of toroidal plasma physics at a low aspect ratio (R/a≈1.3) and to demonstrate a maintainable target plasma by non-inductive startup. The SUNIST device is designed to operate with up to 13 kA of ohmic heating field current, and to 0.15 T of toroidal field at 10 kA of discharge current. All of the poloidal fields can provide 30 mVs of Volt-seconds transformer. Experimental results of plasma startup show that SUNIST has remarkable characteristics of high ramp rate (dIp/dt≈50 MA/s ), high normalized current In of about 2.8 (In = Ip/αBt), and high-efficiency (Ip/Irod ≈0.4) production of plasma current while operating at a low toroidal field. Major disruption phenomena have not been observed from magnetic diagnostics of all testing shots. Initial discharges with 52 kA of plasma current (exceeding the designed value of 50 kA), 2 ms of pulse length and 50 MA/s of ramp rate have been achieved easily with pre-ionized filament.展开更多
Preionization has been widely employed to create initial plasma and help the toroidal plasma current formation.This research focuses on implementing a simple,economical and practical electron cyclotron resonance(ECR)p...Preionization has been widely employed to create initial plasma and help the toroidal plasma current formation.This research focuses on implementing a simple,economical and practical electron cyclotron resonance(ECR)preionization technique on the newly constructed EXL-50 spherical tokamak,and evaluating the effectiveness on improving the plasma current startup.Two types ECR microwave preionization experiments for the plasma initialization without the central solenoid are reported:(1)2.45 GHz microwave preionization and current startup with2.45 GHz ECR source;(2)2.45 GHz microwave preionization and current startup with 28 GHz ECR source.Application of the 2.45 GHz ECR microwave preionization to the experiments has contributed to(1)getting rid of the plasma breakdown delay;(2)the significant improvement of the discharge quality:the discharge is much longer and more stable while the driven plasma current is larger,compared to the discharge without preionization.展开更多
Serious startup drift of the Ring Laser Gyroscope(RLG)is observed during cold startup process,which will dramatically degrade the performances of the corresponding Inertial Navigation System(INS).In this paper,correla...Serious startup drift of the Ring Laser Gyroscope(RLG)is observed during cold startup process,which will dramatically degrade the performances of the corresponding Inertial Navigation System(INS).In this paper,correlation analysis method,which analyzes the relationship between the startup drift of the RLG and the temperature change,is used to determine the significant temperature-related terms during gyroscope startup.Based on the significant temperature-related terms and the startup time length,a startup drift compensation model for RLG based on monotonicity-constrained Radial Basis Function(RBF)neural network is proposed and validated.Compared with the raw RLG data without compensation,the standard deviation of the RLG output with the proposed constrained RBF network model is decreased by more than 46%,and the peak-to-peak value is decreased by more than 35%.Compared with the traditional multiple regression model,the standard deviation and peak-to-peak value of the RLG output are decreased by more than 10%and 6%,respectively.Compared with the common RBF network model,the standard deviation and peak-to-peak value of the RLG output are decreased by more than 8%and 3%,respectively.Navigation experiments also validate the effectiveness of the compensation model.展开更多
There are many motors in operation or on standby in nuclear power plants,and the startup of group motors will have a great impact on the voltage of the emergency bus.At present,there is no special or inexpensive softw...There are many motors in operation or on standby in nuclear power plants,and the startup of group motors will have a great impact on the voltage of the emergency bus.At present,there is no special or inexpensive software to solve this problem,and the experience of engineers is not accurate enough.Therefore,this paper developed a method and system for the startup calculation of group motors in nuclear power plants and proposed an automatic generation method of circuit topology in nuclear power plants.Each component in the topology was given its unique number,and the component class could be constructed according to its type and upper and lower connections.The subordination and topology relationship of switches,buses,and motors could be quickly generated by the program according to the component class,and the simplified direct power flow algorithm was used to calculate the power flow for the startup of group motors according to the above relationship.Then,whether the bus voltage is in the safe range and whether the voltage exceeds the limit during the startup of the group motor could be judged.The practical example was used to verify the effectiveness of the method.Compared with other professional software,the method has high efficiency and low cost.展开更多
An Hα line-emission detection system was developed on the joint texas experimental tokamak （J-TEXT）, which is used to determine the Hα emission level during the gas breakdown and hereafter to control the startup o...An Hα line-emission detection system was developed on the joint texas experimental tokamak （J-TEXT）, which is used to determine the Hα emission level during the gas breakdown and hereafter to control the startup of the plasma current. The detector consists of an Hα interference filter, a focusing lens, a photodiode and a preamplifier. In the J-TEXT operation, the Hα emission is taken as a monitor signal which is highly sensitive to the generation of a plasma. Furthermore, the power supply control system using the above signal as an input is capable of determining whether and when to fire the Ohmic heating capacitor banks, which are applied to drive the plasma current ramp-up. The experimental results confirm that the Hα emission criterion is acceptable for controlling the plasma current promotion in the J-TEXT tokamak.展开更多
Transient process of ECR startup in SUNIST is analyzed and simulated in a one- dimensional model. Contributions from the generation, drift, diffusion and loss of electrons are taken into account in the model and estim...Transient process of ECR startup in SUNIST is analyzed and simulated in a one- dimensional model. Contributions from the generation, drift, diffusion and loss of electrons are taken into account in the model and estimated from experimental parameters, e.g., toroidal field, vertical field and gas filling pressure. Typical discharges are simulated with the conditions close to the experimental ones. Experimental and simulated results are comparable qualitatively in amplitudes and semi-quantitatively in time. From the results it is confirmed that the transient process of ECR startup ill SUNIST is dominated, as preliminarily deduced from experimental observations, by a pair of exclusive factors, namely the absorption, due to gas ionization, and the reflection of microwave, caused by the shift of over dense plasmas due to outward E x B drift. In addition to these two factors, electron loss along the field line is also very important in determining the character of discharges.展开更多
Wind turbines are usually designed and operated with fixed startup speed. It could perform startup and shutdown operations repeatedly when the wind fluctuates around the startup speed. The excessive stress induced by ...Wind turbines are usually designed and operated with fixed startup speed. It could perform startup and shutdown operations repeatedly when the wind fluctuates around the startup speed. The excessive stress induced by frequent startup and shutdown could enhance the likelihood of component failure and hence negatively impact the availability of a wind turbine. Startup speed with dead band is proposed in this article to prevent the wind turbine from frequent startup. 22 years wind data from the Cheung Chau wind station in Hong Kong are analyzed to evaluate the reduction in the number of startup and potential loss of wind power production using the proposed approach. Numerical simulation suggests that the number of startup could be reduced by half with trivial reduction in potential wind power generation in most of investigated sites once an appropriate dead band is adopted.展开更多
The stronglink with muhi-try function based on MEMS technology and the PC startup in authentication system have been designed and fabricated. The generation principle and structure of UQS code are introduced, which co...The stronglink with muhi-try function based on MEMS technology and the PC startup in authentication system have been designed and fabricated. The generation principle and structure of UQS code are introduced, which consists of two groups of metal counter-meshing gears, two pawl/ratchet mechanisms, two driving micromotors and two resetting micromotors. The energy-coupling element is a photoelectric sensor with a circular and notched plate. It is fabricated using the UV-LiGA process and precision mechanical engineering. The PC startup authentication system is controlled by BIOS program, which is written into the chip according with special format. The program in BIOS output signals controls the running of stronglink to finish the process of authentication. The device can run more than 10000 times before a stop. The driving voltage is 12 V, and the normal decoding time is 3 s.展开更多
文摘in this paper, an electromechanically coupled mathematic model of multi-roller driving system for belt conveyor is set up, and the computing equations for dynamic displacement and dynamic tension of the conveyor are also formulated when the hoister is used for straining. Based on the belt conveyor of main inclined shaft in Chengzhuang coal mine, the driving torque, driving power and starting-speed characteristic of each electric motor are studied and measured when multi-roller variable-frequency drive (power distribution 2∶1) is used. The optimal control and the optimal starting-acceleration of the multi-roller variable-frequency drive are determined by a large number of industrial experiments and theoretical calculations.
基金the funding from the National Natural Science Foundation of China(Grant Nos.42001236,71991481,and 71991480)Young Elite Scientist Sponsor-ship Program by Bast(Grant No.BYESS2023413)。
文摘Startups form an information network that reflects their growth trajectories through information flow channels established by shared investors.However,traditional static network metrics overlook temporal dynamics and rely on single indicators to assess startups’roles in predicting future success,failing to comprehensively capture topological variations and structural diversity.To address these limitations,we construct a temporal information network using 14547 investment records from 1013 global blockchain startups between 2004 and 2020,sourced from Crunchbase.We propose two dynamic methods to characterize the information flow:temporal random walk(sTRW)for modeling information flow trajectories and temporal betweenness centrality(tTBET)for identifying key information hubs.These methods enhance walk coverage while ensuring random stability,allowing for more effective identification of influential startups.By integrating sTRW and tTBET,we develop a comprehensive metric to evaluate a startup’s influence within the network.In experiments assessing startups’potential for future success—where successful startups are defined as those that have undergone M&A or IPO—incorporating this metric improves accuracy,recall,and F1 score by 0.035,0.035,and 0.042,respectively.Our findings indicate that information flow from key startups to others diminishes as the network distance increases.Additionally,successful startups generally exhibit higher information inflows than outflows,suggesting that actively seeking investment-related information contributes to startup growth.Our research provides valuable insights for formulating startup development strategies and offers practical guidance for market regulators.
文摘A startup company is not just a simple company;it is an engine for developing the country that creates jobs,creates economic growth,drains the brain,and attracts finance institutions.The economic advancement of a nation can often be gauged by the maturity and robustness of its startup ecosystem,alongside the level of governmental investment and support provided to startups.This research aims to examine the definition and conceptualization of the term“startup”,identify established criteria for classifying a startup,and determine whether explicit legal provisions exist that differentiate startups from newly established companies with a focus on Europe.To achieve these objectives,the research includes analysis methods,focusing on secondary data and official institutional data.This approach facilitated a thorough examination of existing definitions,legal frameworks,and institutional practices regarding the identification and registration of startups across various European nations.The findings reveal a significant discrepancy within Europe concerning legislative clarity on startups.Although many European countries exhibit a well-developed startup ecosystem,not many countries possess formal legislative frameworks that explicitly define a startup and delineate its distinguishing characteristics from those of other newly founded businesses.Notably,according to the Global Startup Index 2024,countries such as Sweden,Germany,France,and the Netherlands rank highest in terms of favourable conditions for startup development.Even of high rankings and developed ecosystems,Germany and Sweden have lack of legal framework and definitions of startups,instead relying primarily on individual programs and ad-hoc initiatives to support entrepreneurial ventures like France and the Netherlands do.The research identified that some European countries,Lithuania,Estonia,Spain,Latvia,and France,have adopted legal regulations that formally define and make differences from startups to other companies,outlining clear criteria and benefits associated with startup status.In contrast,analysis of the former Yugoslavian countries,specifically Slovenia,Croatia,Bosnia and Herzegovina,Macedonia,and Montenegro,except Serbia,indicates a conspicuous absence of clear definitions,formal criteria,or legal frameworks distinguishing startups from newly established businesses.These nations lack provisions within their legislative frameworks explicitly addressing the identification and registration of startup companies,resulting in ambiguity and inconsistency in classification and support measures.Results from this research underscore the necessity for clearer,standardized definitions and robust legal frameworks to effectively support and foster startup ecosystems.It provides a foundation for policymakers and stakeholders within the startup ecosystem to enhance clarity in legislation,facilitate targeted support,and promote sustainable economic growth through innovative entrepreneurial activities.
基金supported by National Natural Science Foundation of China (Grant No. 50879080, Grant No. 50609025)Zhejiang Provincial Natural Science Foundation of China (Grant No. Y1100013,Grant No. R1100530)
文摘Centrifugal pumps always work under steady conditions,and many researches focus on the steady operation.But transient conditions,such as sudden startup and shutdown,are inevitable.The researches on the inner flow of centrifugal pumps under transient conditions have been done,and they show that the transient operation is different from the steady operation.In order to research the evolution of unsteady flow in a centrifugal pump under transient conditions,and to investigate the mechanism of transient effects by analyzing the unsteady flow in a centrifugal pump,the external characteristic experiment and the internal flow numerical calculation of the centrifugal pump with an open impeller during startup is presented.The relationships of the rotation speed,capacity and head between start-time are obtained by the external characteristics experiment.The numerical calculations under startup process are carried out by using the k-e model and N-S equation.The distribution of velocity and pressure in the inner channel of the tested pump was obtained by choosing fourteen start-time points and twelve geometrical points in the impeller channel during startup.The calculation results show that the velocity and the pressure increase linearly with the start-time before rotation＇s speed gets steady,then changes almost horizontally after rotation speed becomes steady,then fluctuates until being steady.The internal flow characteristics are in good agreement with the external characteristic experimental results and numerical calculation.The simulation methods and results make the basis for the diagnosis and optimization of under flow in the centrifugal pump during transient operation.
基金supported by National Natural Science Foundation of China(Grant Nos.51076144,51276172)Zhejiang Provincial Natural Science Foundation of China(Grant Nos.R1100530,LY12E06002)National Basic Research Program of China(973 Program,Grant No.2009CB724303)
文摘The transient behavior of centrifugal pumps during transient operating periods, such as startup and stopping, has drawn more and more attention recently because of urgent needs in engineering. Up to now, almost all the existing studies on this behavior are limited to using water as working fluid. The study on the transient behavior related to solid-liquid two-phase flow has not been seen yet. In order to explore the transient characteristics of a high specific-speed centrifugal pump during startup period delivering the pure water and solid-liquid two-phase flow, the transient flows inside the pump are numerically simulated using the dynamic mesh method. The variable rotational speed and flow rate with time obtained from experiment are best fitted as the function of time, and are written into computational fluid dynamics （CFD） code-FLUENT by using a user defined function. The predicted heads are compared with experimental results when pumping pure water. The results show that the difference in the transient performance during startup period is very obvious between water and solid-liquid two-phase flow during the later stage of startup process. Moreover, the time for the solid-liquid two-phase flow to achieve a stable condition is longer than that for water. The solid-liquid two-phase flow results in a higher impeller shaft power, a larger dynamic reaction force, a more violent fluctuation in pressure and a reduced stable pressure rise comparing with water. The research may be useful to tmderstanding on the transient behavior of a centrifugal pump under a solid-liquid two-phase flow during startup period.
基金Sponsored by the National Natural Science Foundation of China(Grant No.50778052)Development Program for Outstanding Young Teachers in Harbin Institute of Technology(Grant No.NQQQ92324547)
文摘To achieve stable and efficient nitrogen and phosphorus removal and to investigate the characteristics of the A/A SBR enriched with denitrifying phosphorus removal bacteria(DPB),the whole course of startup was studied with two reactors operated in different mode.The reactor I was operated under anaerobic/settling/anoxic/settling mode,and the reactor II was operated under anaerobic/anoxic/settling mode.Differences between the two reactors in removal efficiency of COD,nitrogen and phosphorus were examined.The results indicated that efficient performance could be achieved in both reactors with different startup operation mode,while the phosphorus removal efficiency was improved sooner in reactor I than in reactor II,which suggested that reactor I would supply a more favorable condition for DPB proliferation.Meanwhile,it was observed that the amount of organic substrates consumption had a linear correlation to that of phosphorus release in anaerobic phase when DPB was accumulated in the A/A SBR denitrifying phosphorus removal system.
基金This work was supported by the National Key Research and Development Project of China(No.2020YFB1901700)Science Challenge Project(No.TZ2018001)+1 种基金the National Natural Science Foundation of China(Nos.11775126 and 11775127)the Tsinghua University Initiative Scientific Research Program.
文摘A suitable model for high-temperature heat pipe startup is a prerequisite to realizing the numerical simula-tion for the heat pipe cooled reactor startup from the cold state.It is required that this model not only describes the transient behavior during the startup period,but also reduces the computing resources of the heat pipe cooled reactor simulation in the simplest way.In this study,a simplified model that integrates the two-zone and network models is proposed.In this model,vapor flow in the vapor space,evaporation,and condensation in the vapor–liquid interface are decoupled with heat conduction to achieve a fast calculation of the transient characteristics of the heat pipe.An experimental system for a high-temperature heat pipe was developed to validate the proposed model.A potassium heat pipe was utilized as the experimental material.Startup experiments were performed with differ-ent heating powers.Compared with the experimental results,the accuracy of the proposed model was verified.Moreover,the proposed model can predict the vapor flow,pressure drop,and temperature drop in the vapor space.As indicated by the analysis results,the essential requirements for successful startup are also determined.The heat pipe cannot achieve a successful startup until the heating power satisfies these requirements.All the discussions indicate the capability of the proposed model for the simulation of a high-temperature heat pipe startup from the frozen state;hence,can act as a basic tool for the heat pipe cooled reactor simulation.
文摘Intensity modulated radiation therapy (IMRT) is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many segments each having low monitor units (MUs). The present study evaluated the effects of beam startup characteristics on the dose delivery accuracy for each segment at low MUs for step-and-shoot IMRT with an Elekta Precise accelerator at the highest dose rates. We used a two-dimensional semi-conductor detector for the dose measurements. The field size of each segment was assumed to be 20 ×20 cm2 and each segment was set to deliver 1 - 10 MUs. Our results show a variation in dose delivery accuracy between segments for the same IMRT beam, which can be attributed to the beam startup characteristics. This variability is attributed to the changes in the transient changes in the temperatures of the electron gun filament and the magnetron. That is, the transient increase in the temperature of the filament leads to increasing doses with time and that of the magnetron leads to decreasing doses with time during the first few MUs.
文摘Owing to the inherent instability of the natural circulation system,flow instability can easily occur during the operation of a natural circulation lead-cooled fast reactor,especially during the startup phase.A comprehensive startup scheme for SNCLFR-100,including primary and secondary circuits,is proposed in this paper.It references existing more mature startup schemes in various reactor types.It additionally considers the restriction conditions on the power increase in other schemes and the characteristics of lead-based coolant.On this basis,the multi-scale coupling code ATHLET-OpenFOAM was used to study the flow instability in the startup phase under different power-step amplitudes and power duration times.The results showed that obvious flow instability phenomena were found in the different startup schemes,such as the short-term backflow phenomenon of the core at the initial time of the startup.Moreover,an obvious increase in the flow rate and temperature to the peak value at the later stage of a continuous power rise was observed,as well as continuous oscillations before reaching a steady state.It was determined that the scheme with smaller power-step amplitude and a longer power duration time requires more time to start the reactor.Nevertheless,it will be more conducive to the safe and stable startup of the reactor.
基金Improving Tsinghua University of China to Top-ranking Foundation
文摘The goal of the Sino-United Spherical Tokamak (SUNIST) at Tsinghua University is to extend the understanding of toroidal plasma physics at a low aspect ratio (R/a≈1.3) and to demonstrate a maintainable target plasma by non-inductive startup. The SUNIST device is designed to operate with up to 13 kA of ohmic heating field current, and to 0.15 T of toroidal field at 10 kA of discharge current. All of the poloidal fields can provide 30 mVs of Volt-seconds transformer. Experimental results of plasma startup show that SUNIST has remarkable characteristics of high ramp rate (dIp/dt≈50 MA/s ), high normalized current In of about 2.8 (In = Ip/αBt), and high-efficiency (Ip/Irod ≈0.4) production of plasma current while operating at a low toroidal field. Major disruption phenomena have not been observed from magnetic diagnostics of all testing shots. Initial discharges with 52 kA of plasma current (exceeding the designed value of 50 kA), 2 ms of pulse length and 50 MA/s of ramp rate have been achieved easily with pre-ionized filament.
基金funded by the compact fusion project in the ENN group。
文摘Preionization has been widely employed to create initial plasma and help the toroidal plasma current formation.This research focuses on implementing a simple,economical and practical electron cyclotron resonance(ECR)preionization technique on the newly constructed EXL-50 spherical tokamak,and evaluating the effectiveness on improving the plasma current startup.Two types ECR microwave preionization experiments for the plasma initialization without the central solenoid are reported:(1)2.45 GHz microwave preionization and current startup with2.45 GHz ECR source;(2)2.45 GHz microwave preionization and current startup with 28 GHz ECR source.Application of the 2.45 GHz ECR microwave preionization to the experiments has contributed to(1)getting rid of the plasma breakdown delay;(2)the significant improvement of the discharge quality:the discharge is much longer and more stable while the driven plasma current is larger,compared to the discharge without preionization.
基金supported in part by the National Natural Science Foundation of China(No.61203199)。
文摘Serious startup drift of the Ring Laser Gyroscope(RLG)is observed during cold startup process,which will dramatically degrade the performances of the corresponding Inertial Navigation System(INS).In this paper,correlation analysis method,which analyzes the relationship between the startup drift of the RLG and the temperature change,is used to determine the significant temperature-related terms during gyroscope startup.Based on the significant temperature-related terms and the startup time length,a startup drift compensation model for RLG based on monotonicity-constrained Radial Basis Function(RBF)neural network is proposed and validated.Compared with the raw RLG data without compensation,the standard deviation of the RLG output with the proposed constrained RBF network model is decreased by more than 46%,and the peak-to-peak value is decreased by more than 35%.Compared with the traditional multiple regression model,the standard deviation and peak-to-peak value of the RLG output are decreased by more than 10%and 6%,respectively.Compared with the common RBF network model,the standard deviation and peak-to-peak value of the RLG output are decreased by more than 8%and 3%,respectively.Navigation experiments also validate the effectiveness of the compensation model.
基金Key Project of National Natural Science Foundation of China(52237008)Beijing Municipal Education Commission Research Program Funding Project(KM202111232022)。
文摘There are many motors in operation or on standby in nuclear power plants,and the startup of group motors will have a great impact on the voltage of the emergency bus.At present,there is no special or inexpensive software to solve this problem,and the experience of engineers is not accurate enough.Therefore,this paper developed a method and system for the startup calculation of group motors in nuclear power plants and proposed an automatic generation method of circuit topology in nuclear power plants.Each component in the topology was given its unique number,and the component class could be constructed according to its type and upper and lower connections.The subordination and topology relationship of switches,buses,and motors could be quickly generated by the program according to the component class,and the simplified direct power flow algorithm was used to calculate the power flow for the startup of group motors according to the above relationship.Then,whether the bus voltage is in the safe range and whether the voltage exceeds the limit during the startup of the group motor could be judged.The practical example was used to verify the effectiveness of the method.Compared with other professional software,the method has high efficiency and low cost.
基金supported by the National 973 Program of China (No. 2007ID105)
文摘An Hα line-emission detection system was developed on the joint texas experimental tokamak （J-TEXT）, which is used to determine the Hα emission level during the gas breakdown and hereafter to control the startup of the plasma current. The detector consists of an Hα interference filter, a focusing lens, a photodiode and a preamplifier. In the J-TEXT operation, the Hα emission is taken as a monitor signal which is highly sensitive to the generation of a plasma. Furthermore, the power supply control system using the above signal as an input is capable of determining whether and when to fire the Ohmic heating capacitor banks, which are applied to drive the plasma current ramp-up. The experimental results confirm that the Hα emission criterion is acceptable for controlling the plasma current promotion in the J-TEXT tokamak.
基金support.ed by the Major State Basic Research Developlnent Program from MOST of China (No. 2008CB717804, 2009GB105002)National Nat.ural Science Foundation of China (Nos. 10535020, 10775086, 10990214 and 11005066)in part by the JSPS-CAS Core University program ill the field of 'Plasma and Nuclear Fsion'
文摘Transient process of ECR startup in SUNIST is analyzed and simulated in a one- dimensional model. Contributions from the generation, drift, diffusion and loss of electrons are taken into account in the model and estimated from experimental parameters, e.g., toroidal field, vertical field and gas filling pressure. Typical discharges are simulated with the conditions close to the experimental ones. Experimental and simulated results are comparable qualitatively in amplitudes and semi-quantitatively in time. From the results it is confirmed that the transient process of ECR startup ill SUNIST is dominated, as preliminarily deduced from experimental observations, by a pair of exclusive factors, namely the absorption, due to gas ionization, and the reflection of microwave, caused by the shift of over dense plasmas due to outward E x B drift. In addition to these two factors, electron loss along the field line is also very important in determining the character of discharges.
文摘Wind turbines are usually designed and operated with fixed startup speed. It could perform startup and shutdown operations repeatedly when the wind fluctuates around the startup speed. The excessive stress induced by frequent startup and shutdown could enhance the likelihood of component failure and hence negatively impact the availability of a wind turbine. Startup speed with dead band is proposed in this article to prevent the wind turbine from frequent startup. 22 years wind data from the Cheung Chau wind station in Hong Kong are analyzed to evaluate the reduction in the number of startup and potential loss of wind power production using the proposed approach. Numerical simulation suggests that the number of startup could be reduced by half with trivial reduction in potential wind power generation in most of investigated sites once an appropriate dead band is adopted.
基金Sponsored by the National High Technology Research and Development Program (863 ) of China (Grant No.2003AA404210, 2005AA404250,2003AA404210, 2006AA01Z443)
文摘The stronglink with muhi-try function based on MEMS technology and the PC startup in authentication system have been designed and fabricated. The generation principle and structure of UQS code are introduced, which consists of two groups of metal counter-meshing gears, two pawl/ratchet mechanisms, two driving micromotors and two resetting micromotors. The energy-coupling element is a photoelectric sensor with a circular and notched plate. It is fabricated using the UV-LiGA process and precision mechanical engineering. The PC startup authentication system is controlled by BIOS program, which is written into the chip according with special format. The program in BIOS output signals controls the running of stronglink to finish the process of authentication. The device can run more than 10000 times before a stop. The driving voltage is 12 V, and the normal decoding time is 3 s.
