This paper presents a general analytical model of flexible isolation system for application to the installation of high-speed machines and lightweight structures. Piezoelectric stack actuators are employed in the mode...This paper presents a general analytical model of flexible isolation system for application to the installation of high-speed machines and lightweight structures. Piezoelectric stack actuators are employed in the model to achieve vibration control of flexible structures, and dynamic characteristics are also investigated. Mobility technique is used to derive the governing equations of the system. The power flow transmitted into the foundation is solved and considered as a cost function to achieve optimal control of vibration isolation. Some numerical simulations revealed that the analytical model is effective as piezoelectric stack actuators can achieve substantial vibration attenuation by selecting proper value of the input voltage.展开更多
The main objective of this study is to enhance the strength of CNTs （carbon nanotubes） which reinforced AI matrix composites by introducing an appropriate amount of Copper（Cu） into the composite material. AI-Cu/MW...The main objective of this study is to enhance the strength of CNTs （carbon nanotubes） which reinforced AI matrix composites by introducing an appropriate amount of Copper（Cu） into the composite material. AI-Cu/MWCNTs （multi-walled carbon nanotubes） nanocomposites were produced via compaction, sintering and hot extrusion process of AI-Cu/MWCNTs powders, which were fabricated by a conventional ball mill process with AI powders and Cu-MWCNTs composite powders which were synthesized by molecular level mixing technique. Also the change of mechanical properties with different content ratio of Cu/MWCNT composite powders in A1 matrix is analyzed. It is found that the addition of the proper Cu/MWCNTs powders which are well distributed in AI matrix leads to high mechanical stiffness. The 2 wt% Cu/MWCNTs reinforced A1 composites which exhibited 3.2 times higher tensile strength and 4.4 times higher yield strength than pure AI.展开更多
Idealized cycles of refrigerating machines with adiabatic and isothermal compression of refrigerant vapor were investigated. Energetic characteristics of cycles： specific mass and volume cooling capacity q0 and qv, w...Idealized cycles of refrigerating machines with adiabatic and isothermal compression of refrigerant vapor were investigated. Energetic characteristics of cycles： specific mass and volume cooling capacity q0 and qv, work of compression 1, refrigerating coefficient of performance e and power N for drive of compressor were compared. These characteristics were calculated for eight refrigerants at temperature of their condensation 30 ℃ and temperatures of boiling -15℃ and -30 ℃. The calculations show that the use of isothermal compression of refrigerant vapor ensures economy of energy during refrigerating machine operation.展开更多
We propose a novel series transformer based diode-bridge-type solid state fault current limiter （SSFCL）. To control the fault current, a series RLC branch is connected to the secondary side of an isolation series tr...We propose a novel series transformer based diode-bridge-type solid state fault current limiter （SSFCL）. To control the fault current, a series RLC branch is connected to the secondary side of an isolation series transformer. Based on this RLC branch, two current limiting modes are created. In the first mode, R and C are bypassed via a paralleled power electronic switch （insulated-gate bipolar transistor, IGBT） and L remains connected to the secondary side of the transformer as a DC reactor. In the second mode, the series reactor impedance is not enough to limit the fault current. In this case, the fault current can be con- trolled by selecting a proper on-off duration of the parallel IGBT, across the series damping resistor （R） and capacitor, which inserts high impedance into the line to limit the fault current. Then, by controlling the magnitude of the DC reactor current, the fault current is reduced and the voltage of the point of common coupling （PCC） is kept at an acceptable level. In addition, in the new SSFCL, the series RC branch, connected in parallel with the IGBT, serves as a snubber circuit for decreasing the transient recovery voltage （TRV） of the IGBT during on-off states. Therefore, the power quality indices can be improved. The measure- ment results of a built prototype are presented to support the simulation and theoretical studies. The proposed SSFCL can limit the fault current without any delay and successfully smooth the fault current waveform.展开更多
基金Project (No. 2004035223) supported by the Postdoctoral ScienceFoundation of China
文摘This paper presents a general analytical model of flexible isolation system for application to the installation of high-speed machines and lightweight structures. Piezoelectric stack actuators are employed in the model to achieve vibration control of flexible structures, and dynamic characteristics are also investigated. Mobility technique is used to derive the governing equations of the system. The power flow transmitted into the foundation is solved and considered as a cost function to achieve optimal control of vibration isolation. Some numerical simulations revealed that the analytical model is effective as piezoelectric stack actuators can achieve substantial vibration attenuation by selecting proper value of the input voltage.
文摘The main objective of this study is to enhance the strength of CNTs （carbon nanotubes） which reinforced AI matrix composites by introducing an appropriate amount of Copper（Cu） into the composite material. AI-Cu/MWCNTs （multi-walled carbon nanotubes） nanocomposites were produced via compaction, sintering and hot extrusion process of AI-Cu/MWCNTs powders, which were fabricated by a conventional ball mill process with AI powders and Cu-MWCNTs composite powders which were synthesized by molecular level mixing technique. Also the change of mechanical properties with different content ratio of Cu/MWCNT composite powders in A1 matrix is analyzed. It is found that the addition of the proper Cu/MWCNTs powders which are well distributed in AI matrix leads to high mechanical stiffness. The 2 wt% Cu/MWCNTs reinforced A1 composites which exhibited 3.2 times higher tensile strength and 4.4 times higher yield strength than pure AI.
文摘Idealized cycles of refrigerating machines with adiabatic and isothermal compression of refrigerant vapor were investigated. Energetic characteristics of cycles： specific mass and volume cooling capacity q0 and qv, work of compression 1, refrigerating coefficient of performance e and power N for drive of compressor were compared. These characteristics were calculated for eight refrigerants at temperature of their condensation 30 ℃ and temperatures of boiling -15℃ and -30 ℃. The calculations show that the use of isothermal compression of refrigerant vapor ensures economy of energy during refrigerating machine operation.
文摘We propose a novel series transformer based diode-bridge-type solid state fault current limiter （SSFCL）. To control the fault current, a series RLC branch is connected to the secondary side of an isolation series transformer. Based on this RLC branch, two current limiting modes are created. In the first mode, R and C are bypassed via a paralleled power electronic switch （insulated-gate bipolar transistor, IGBT） and L remains connected to the secondary side of the transformer as a DC reactor. In the second mode, the series reactor impedance is not enough to limit the fault current. In this case, the fault current can be con- trolled by selecting a proper on-off duration of the parallel IGBT, across the series damping resistor （R） and capacitor, which inserts high impedance into the line to limit the fault current. Then, by controlling the magnitude of the DC reactor current, the fault current is reduced and the voltage of the point of common coupling （PCC） is kept at an acceptable level. In addition, in the new SSFCL, the series RC branch, connected in parallel with the IGBT, serves as a snubber circuit for decreasing the transient recovery voltage （TRV） of the IGBT during on-off states. Therefore, the power quality indices can be improved. The measure- ment results of a built prototype are presented to support the simulation and theoretical studies. The proposed SSFCL can limit the fault current without any delay and successfully smooth the fault current waveform.
