Active damped LCL-filter-based inverters have been widely used for grid-connected distributed generation(DG) systems. In weak grids, however, the phase-locked loop(PLL) dynamics may detrimentally affect the stability ...Active damped LCL-filter-based inverters have been widely used for grid-connected distributed generation(DG) systems. In weak grids, however, the phase-locked loop(PLL) dynamics may detrimentally affect the stability of grid-connected inverters due to interaction between the PLL and the controller. In order to solve the problem, the impact of PLL dynamics on small-signal stability is investigated for the active damped LCL-filtered grid-connected inverters with capacitor voltage feedback. The system closed-loop transfer function is established based on the Norton equivalent model by taking the PLL dynamics into account. Using an established model, the system stability boundary is identified from the viewpoint of PLL bandwidth and current regulator gain. The accuracy of the ranges of stability for the PLL bandwidth and current regulator gain is verified by both simulation and experimental results.展开更多
As one of the key units of space CCD camera,the temperature range and stability of CCD components affect the image's indexes.Reasonable thermal design and robust thermal control devices are needed.One kind of temp...As one of the key units of space CCD camera,the temperature range and stability of CCD components affect the image's indexes.Reasonable thermal design and robust thermal control devices are needed.One kind of temperature control loop heat pipe(TCLHP) is designed,which highly meets the thermal control requirements of CCD components.In order to study the dynamic behaviors of heat and mass transfer of TCLHP,particularly in the orbital flight case,a transient numerical model is developed by using the well-established empirical correlations for flow models within three dimensional thermal modeling.The temperature control principle and details of mathematical model are presented.The model is used to study operating state,flow and heat characteristics based upon the analyses of variations of temperature,pressure and quality under different operating modes and external heat flux variations.The results indicate that TCLHP can satisfy the thermal control requirements of CCD components well,and always ensure good temperature stability and uniformity.By comparison between flight data and simulated results,it is found that the model is to be accurate to within 1℃.The model can be better used for predicting and understanding the transient performance of TCLHP.展开更多
The mechanically stirred internal loop airlift reactors equipped with or without static mixers are devised for intensification of gas-liquid mass transfer rate. The influences of superficial gas velocity, agitation o...The mechanically stirred internal loop airlift reactors equipped with or without static mixers are devised for intensification of gas-liquid mass transfer rate. The influences of superficial gas velocity, agitation or static mixers on gas hold-up, mixing time, liquid circulating velocity and volumetric mass transfer coefficient have been investigated with tap water and carboxymethyl cellulose (CMC) aqueous solution. The experimental results indicate that mechanical agitation is more efficacious than static mixer in highly viscous media for improving mass transfer in airlift reactors. The empirical correlation of volumetric mass transfer coefficient with apparent viscosity, and energy consumption for mechanical agitation and aeration is developed.展开更多
Cryogenic loop heat pipes are highly efficient heat transfer devices at cryogenic temperature range,which have promising application prospects in satellites,spacecrafts,electronics,and so on.Cooling down process is a ...Cryogenic loop heat pipes are highly efficient heat transfer devices at cryogenic temperature range,which have promising application prospects in satellites,spacecrafts,electronics,and so on.Cooling down process is a most critical process for a CLHP before startup.At present,secondary loop is a major way for a CLHP to fulfil cooling down and most studies are concentrated on heat transfer characteristics during normal operation.However,few investigations have been carried out on the cooling down process.In this paper,the cooling down process of a nitrogen-charged CLHP assisted with a secondary loop was experimentally investigated.A simple qualitative approach to estimate the cooling down time was proposed according to the law of conservation of energy.The two flow paths of the working fluid in the CLHP during the cooling down process were described.Experimental studies on the cooling down process with various secondary heat loads and working fluid inventory were presented in detail.With the increase of secondary heat load,the elapsed time of Stage III decreased significantly due to the larger mass flow rate in Path I.In addition,the effect of the working fluid inventory on the cooling down time was generally small in the range from 2.99 MPa to 3.80 MPa.However,with 2.80 MPa working fluid inventory,it required much longer cooling down time,which was because of the lack of liquid in the CLHP with low working fluid inventory.Moreover,the influence of gravity on the temperature variation of the components during the experiments was analyzed.This work is beneficial for better understanding of the cooling down process and optimizing of CLHPs.展开更多
In recent years,the problem of heat dissipation in airborne directed energy weapons has attracted considerable research interest.Spray cooling can be applied to cool airborne directed energy weapons,owing to its sever...In recent years,the problem of heat dissipation in airborne directed energy weapons has attracted considerable research interest.Spray cooling can be applied to cool airborne directed energy weapons,owing to its several advantages such as a large heat transfer coefficient,absence of boiling hysteresis and uniform surface temperature.To examine the potential of an airborne spray cooling system,the typical high heat flux dissipation methods were compared,and the state of the art research on spray cooling was reviewed.This review was focused on studies related to the spray cooling hydrodynamic mechanism,experimental studies of closed loop spray cooling,numerical simulation studies about spray cooling and the identification of the factors influencing spray cooling systems,and investigations related to the multiple nozzle spray cooling technology and heat transfer correlation predictions.Overall,there is a need for further research to investigate the failure phenomenon after the critical state,matching operation of the total system and microscopic characteristics of airborne specific parameters.展开更多
基金supported by Science Foundation for Distinguished Young Scholars of Hebei Province(No.E2016203133)Hundred Excellent Innovation Talents Support Program of Hebei Province(No.SLRC2017059)
文摘Active damped LCL-filter-based inverters have been widely used for grid-connected distributed generation(DG) systems. In weak grids, however, the phase-locked loop(PLL) dynamics may detrimentally affect the stability of grid-connected inverters due to interaction between the PLL and the controller. In order to solve the problem, the impact of PLL dynamics on small-signal stability is investigated for the active damped LCL-filtered grid-connected inverters with capacitor voltage feedback. The system closed-loop transfer function is established based on the Norton equivalent model by taking the PLL dynamics into account. Using an established model, the system stability boundary is identified from the viewpoint of PLL bandwidth and current regulator gain. The accuracy of the ranges of stability for the PLL bandwidth and current regulator gain is verified by both simulation and experimental results.
文摘As one of the key units of space CCD camera,the temperature range and stability of CCD components affect the image's indexes.Reasonable thermal design and robust thermal control devices are needed.One kind of temperature control loop heat pipe(TCLHP) is designed,which highly meets the thermal control requirements of CCD components.In order to study the dynamic behaviors of heat and mass transfer of TCLHP,particularly in the orbital flight case,a transient numerical model is developed by using the well-established empirical correlations for flow models within three dimensional thermal modeling.The temperature control principle and details of mathematical model are presented.The model is used to study operating state,flow and heat characteristics based upon the analyses of variations of temperature,pressure and quality under different operating modes and external heat flux variations.The results indicate that TCLHP can satisfy the thermal control requirements of CCD components well,and always ensure good temperature stability and uniformity.By comparison between flight data and simulated results,it is found that the model is to be accurate to within 1℃.The model can be better used for predicting and understanding the transient performance of TCLHP.
文摘The mechanically stirred internal loop airlift reactors equipped with or without static mixers are devised for intensification of gas-liquid mass transfer rate. The influences of superficial gas velocity, agitation or static mixers on gas hold-up, mixing time, liquid circulating velocity and volumetric mass transfer coefficient have been investigated with tap water and carboxymethyl cellulose (CMC) aqueous solution. The experimental results indicate that mechanical agitation is more efficacious than static mixer in highly viscous media for improving mass transfer in airlift reactors. The empirical correlation of volumetric mass transfer coefficient with apparent viscosity, and energy consumption for mechanical agitation and aeration is developed.
基金supported by the National Natural Science Foundation of China(No.51606207)Youth Innovation Promotion Association,CAS,China(No.2018036)partially supported by Beijing Municipal Natural Science Foundation(No.3202033)。
文摘Cryogenic loop heat pipes are highly efficient heat transfer devices at cryogenic temperature range,which have promising application prospects in satellites,spacecrafts,electronics,and so on.Cooling down process is a most critical process for a CLHP before startup.At present,secondary loop is a major way for a CLHP to fulfil cooling down and most studies are concentrated on heat transfer characteristics during normal operation.However,few investigations have been carried out on the cooling down process.In this paper,the cooling down process of a nitrogen-charged CLHP assisted with a secondary loop was experimentally investigated.A simple qualitative approach to estimate the cooling down time was proposed according to the law of conservation of energy.The two flow paths of the working fluid in the CLHP during the cooling down process were described.Experimental studies on the cooling down process with various secondary heat loads and working fluid inventory were presented in detail.With the increase of secondary heat load,the elapsed time of Stage III decreased significantly due to the larger mass flow rate in Path I.In addition,the effect of the working fluid inventory on the cooling down time was generally small in the range from 2.99 MPa to 3.80 MPa.However,with 2.80 MPa working fluid inventory,it required much longer cooling down time,which was because of the lack of liquid in the CLHP with low working fluid inventory.Moreover,the influence of gravity on the temperature variation of the components during the experiments was analyzed.This work is beneficial for better understanding of the cooling down process and optimizing of CLHPs.
基金supported by the National Natural Science Foundation of China(Grant No.51806096)China Postdoctoral Science Foundation(No.2019M661812)+1 种基金Natural Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.18KJB560007)the Research Fund of Key Laboratory of Aircraft Environment Control and Life Support,MIIT,Nanjing University of Aeronautics and Astronautics(Grant No.KLAECLS-E-201902)。
文摘In recent years,the problem of heat dissipation in airborne directed energy weapons has attracted considerable research interest.Spray cooling can be applied to cool airborne directed energy weapons,owing to its several advantages such as a large heat transfer coefficient,absence of boiling hysteresis and uniform surface temperature.To examine the potential of an airborne spray cooling system,the typical high heat flux dissipation methods were compared,and the state of the art research on spray cooling was reviewed.This review was focused on studies related to the spray cooling hydrodynamic mechanism,experimental studies of closed loop spray cooling,numerical simulation studies about spray cooling and the identification of the factors influencing spray cooling systems,and investigations related to the multiple nozzle spray cooling technology and heat transfer correlation predictions.Overall,there is a need for further research to investigate the failure phenomenon after the critical state,matching operation of the total system and microscopic characteristics of airborne specific parameters.
