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Search Results: 1 - 10 of 123438 matches for " Dong Zhang "
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Performance Assessment of Islanding Detection for Mul-ti-inverter Grid-connected Photovoltaic Systems  [PDF]
Xing Zhang, Dong Xie
Energy and Power Engineering (EPE) , 2013, DOI: 10.4236/epe.2013.54B287
Abstract:

Islanding detection is an essential function for safety and reliability in grid-connected Distributed Generation Systems (DGS). Passive and active islanding detection methods have been analyzed in literature considering DGS with only one inverter connected to the utility. With the big scale application of photovoltaic (PV) power systems, islanding detection technology of multi-inverter DGS has been paid more attention. This paper analyzes the performance of diverse islanding detection methods in multiple inverters grid-connected PV systems. Non-Detection Zones (NDZ) of multi-inverter systems in a load parameter space are used as analytical tool. The paper provides guidance for the islanding detection design in multiple grid-connected inverters.

Impact Factor Dynamic Forecasting Model for Management Science Journals Based on Grey System Theory  [PDF]
Xiao Dong, Shiqiang Zhang
Open Journal of Social Sciences (JSS) , 2015, DOI: 10.4236/jss.2015.33005
Abstract:

A forecast method of the impact factor trend was given based on grey system theory. Using this method, combined with the top 20 management science journals, the grey system GM (1, 1) model was constructed. The model evaluates and predicts the average impact factor trend of the top 20 management science journals.

The Relationship between Family’s Social Economic Status and Psychological Suzhi of Middle School Students: The Intermediary Effect of Parenting Style  [PDF]
Zesong Dong, Dajun Zhang
Open Journal of Social Sciences (JSS) , 2015, DOI: 10.4236/jss.2015.312010
Abstract: This research focused on the relationship among family’s social economic status (SES), parenting style and psychological suzhi of middle school students. The results indicated that SES has significant positive correlation with middle school students’ psychological suzhi; parenting style significantly predicts psychological suzhi of middle school students; among them, mother’s emotional warmth and father’s emotional warmth significantly positively predict psychological suzhi; mother’s rejection and father’s over protection significantly negatively predict psychological suzhi. SES not only has independent prediction effect on middle school students’ psychological suzhi, but also positively predicts psychological suzhi through father’s emotional warmth and mother’s emotional warmth by intermediary role.
Does VC Spur Regional Innovation?  [PDF]
Shun Zhang, Lining Dong
Open Journal of Social Sciences (JSS) , 2016, DOI: 10.4236/jss.2016.44017
Abstract: Studies of domestic and foreign scholars found that technological innovation is one of the key driving forces for economic growth. Venture capital (VC) had a new investment mechanism accompanied by the emergence technological innovation, and invested in a unique way and in a special operation mode, which determined the effective promotion of technological innovation. In this paper, the effect of VC on technological innovation was examined in two perspectives—“capital- increase effect” and “innovation propensity-increase effect”. Chinese provincial unbalanced panel data from 2009 to 2013 were gathered to verify the two effects above. And we also explored how VC affected regional innovation and its mechanism. The results showed that VC had a positive role in regional innovation promoting, and “capital-increase effect” can be verified. VC was to produce more innovations while reducing blind innovation input so as to improve innovation efficiency. The “innovation propensity-increase effect” can be verified.
Hyperaccreting Neutron-Star Disks, Magnetized Disks and Gamma-Ray Bursts
Dong Zhang
Physics , 2009,
Abstract: This thesis focuses on the study of the hyperaccreting neutron-star disks and magnetized accretion flows. It is usually proposed that hyperaccreting disks surrounding stellar-mass black holes with a huge accretion rate are central engines of gamma-ray bursts (GRBs). However, hyperaccretion disks around neutron stars may exist in some GRB formation scenarios. We study the structure and neutrino emission of a hyperaccretion disk around a neutron star. We consider a steady-state hyperaccretion disk, and as a reasonable approximation, divide it into two regions, the inner and outer disks. The outer disk is similar to that of a black hole. The inner disk has a self-similar structure, such as the entropy-conservation or the advection structure, depending on the energy transfer and emission in the disk. We see that the neutron star disk can cool more efficiently, produce much higher neutrino luminosity and neutrino annihilation luminosity than a black hole disk. The neutrino emission from the neutron star surface boundary layer could increase the neutrino annihilation luminosity. Moreover, we study the effects of a global magnetic field on viscously-rotating and vertically-integrated accretion disks around compact objects using a self-similar treatment, and use two methods to study magnetized flows with convection. We also give a review on GRB progenitors, central engines and an outlook in this thesis.
Impact of Primordial Ultracompact Minihaloes on the Intergalactic Medium and First Structure Formation
Dong Zhang
Physics , 2010, DOI: 10.1111/j.1365-2966.2011.19602.x
Abstract: (Abridged) Ultracompact Minihaloes (UCMHs), which formed by dark matter accretion onto primordial black holes (PBHs) or initial dark matter overdensity produced by the primordial density perturbation, provide a new type of compact dark matter structure to ionize and heat the IGM after matter-radiation equality z_eq, which is much earlier than the formation of the first cosmological dark halo structure and later first stars. We show that dark matter annihilation density contributed by UCMHs can totally dominated over the homogenous dark matter annihilation background even for a tiny UCMH abundance, and provide a new gamma-ray background in the early Universe. The IGM ionization fraction x_ion and gas temperature T_m can be increased from the recombination residual and adiabatically cooling in the absence of energy injection to the highest value of x_ ion 0.1 and T_m ~ 5000 K at z>10 for the upper bound UCMH abundance constrained by the CMB optical depth. A small fraction of UCMHs are seeded by PBHs. The X-ray emission from gas accretion onto PBHs may totally dominated over dark matter annihilation and become the main cosmic ionization source, but the constraints of gas accretion rate and X-ray absorption by the baryon accumulation within the UCMHs and accretion feedback show that X-ray emission can only be a promising source much later than UCMH annihilation at z
Game Theoretic Analysis of Joint Rate and Power Allocation in Cognitive Radio Networks  [PDF]
Dong LI, Xianhua DAI, Han ZHANG
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2009, DOI: 10.4236/ijcns.2009.21001
Abstract: Spectrum sharing is an essential enabling functionality to allow the coexistence between primary user (PU) and cognitive users (CUs) in the same frequency band. In this paper, we consider joint rate and power allocation in cognitive radio networks by using game theory. The optimum rates and powers are obtained by iteratively maximizing each CU’s utility function, which is designed to guarantee the protection of primary user (PU) as well as the quality of service (QoS) of CUs. In addition, transmission rates of some CUs should be adjusted if corresponding actual signal-to-interference-plus-noise ratio (SINR) falls below the target SINR. Based on the modified transmission rate for each CU, distributed power allocation is introduced to further reduce the total power consumption. Simulation results are provided to demonstrate that the proposed algorithm achieves a significant gain in power saving.
Joint Adaptive Modulation and Power Allocation in Cognitive Radio Networks  [PDF]
Dong LI, Xianhua DAI, Han ZHANG
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2008, DOI: 10.4236/ijcns.2008.13027
Abstract: Link adaptation is an important issue in the design of cognitive radio networks, which aims at making efficient use of system resources. In this paper, we propose and investigate a joint adaptive modulation and power allocation algorithm in cognitive radio networks. Specifically, the modulation scheme and transmit power are adjusted adaptively according to channel conditions, interference limit and target signal-to-interference-plus-noise ratio (SINR). As such the total power consumption of cognitive users (CUs) is minimized while keeping both the target SINR of CUs and interference to primary user (PU) at an acceptable level. Simulation results are provided to show that the proposed algorithm achieves a significant gain in power saving.
EPr Solution to a System of Matrix Equations  [PDF]
Changzhou Dong, Yuping Zhang, Jianmin Song
Advances in Linear Algebra & Matrix Theory (ALAMT) , 2013, DOI: 10.4236/alamt.2013.34010
Abstract:

A square complex matrix \"\"is called \"\"if it can be written in the form \"\"with \"\"being fixed unitary and \"\" being arbitrary matrix in \"\". We give necessary and sufficient conditions for the existence of the \"\"solution to the system of complex matrix equation \"\"and present an expression of the \"\"solution to the system when the solvability conditions are satisfied. In addition, the solution to an optimal approximation problem is obtained. Furthermore, the least square \"\" solution with least norm to this system mentioned above is considered. The representation of such solution is also derived.

Preparation and Properties of Two-Component and Double-Crosslinking Waterborne Polyurethane-Acrylic Dispersions  [PDF]
Nan Gao, Zhuo Zhang, Qingzhi Dong
Open Journal of Organic Polymer Materials (OJOPM) , 2013, DOI: 10.4236/ojopm.2013.32005
Abstract:

In this paper, isophorone diisocyanate (IPDI), polyethylene glycol (PEG), dimethylolpropionic acid (DMPA) and internal crosslinking agent trimethylolpropane (TMP) were used to prepare waterborne polyurethane. And then double-crosslinked polyurethane-acrylic composite aqueous dispersion was prepared in which polyacrylate was adopted to modify waterborne polyurethane and some special external crosslinking agents were added including silicone and trifunctional aziridine. The influence of the amounts of internal and external crosslinking agents, emulsifier, initiator on the particle size, particle size distribution, viscosity, molecular weight, as well as water adsorption ratio were studied.

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