共查询到20条相似文献,搜索用时 31 毫秒
1.
The control of flow in microscale is one of the most important problems in microfluidic devices, which in particular, are used as micro heat exchangers. The use of electric field is one of the efficient methods of control of dielectric liquid flow in microscale. The electric field influences liquid flow by the EHD force which affects liquid behaviour in terms of the flow rate and pressure.The EHD force consists of three components: the first is the electrostatic force due to free charges present in the liquid, the next one is the force due to the gradient of permittivity of material, and the third one is caused by the change in the electric field intensity.The EHD force is used also in many commercial devices, for example EHD pumps or dielectrophoretic separators. An own approach to apply the EHD force to control the liquid flow rate is presented in this paper. Authors paid a close attention to the dielectrophoresis effect. Dielectric liquid in a non-uniform electric field tends to drift/migrate towards the region of high electric field intensity. With decreasing the electrode dimensions, the dielectrophoresis force becomes relatively stronger. For the dimensions under 400 μm the dielectrophoresis phenomenon can be used for control and actuation of the liquid flow in microchannels. The originally developed design of such flow controller is presented in this paper. The experimental investigations covered flow rate measurement of 2-propanol in microchannel flow controller with application of AC field. It was showed that the dielectrophoresis phenomenon could effectively control the flow. The results for distilled water are also comparatively discussed in the paper. 相似文献
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《Journal of Electrostatics》2006,64(12):850-859
The objective of the present work was to use a commercial Computational Fluid Dynamics (CFD) code to simulate the electrohydrodynamic (EHD) atomization process. Although the physics of the atomization and cone formation has been discussed in numerous publications, a comprehensive theory has not been presented. Some of the previous approaches are discussed below. A CFD model can give a unique capability to describe and simulate the liquid cone formation and atomization. The approach in this work was to simultaneously solve the coupled (EHD) and electrostatic equations. The heat conduction equation, solved by the CFD solver, has been modified to solve the electrostatic field equations. From the electrostatic field, the electric body forces have been determined and included in the Navier–Stokes equations. The model does not include any current. The key liquid property for the coupling is the permittivity. The predicted velocity fields for heptane and ethanol and the operating window of heptane were found to be consistent with published results. The model does not include a droplet break-up model. If the jet is cylindrical, the droplet size can be calculated from the jet diameter. The droplet size of ethanol was predicted and compared well with experiments. 相似文献
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Holmberg DG Chen GS Lin HT Wo AM 《The Journal of the Acoustical Society of America》2003,114(2):782-791
A heat-driven thermoacoustic refrigerator has been designed and tested. A detailed thermal model of the device is presented. Energy balances within the system are discussed using external, heat exchanger, and stack control volumes in order to clarify the relationships of work and heat fluxes below and above onset. Thermal modeling is discussed as a tool for performance analysis as well as for determining system heat losses and finding input heat flows required by a thermoacoustic code. A method of using the control volume balance equations to find stack work and device efficiencies is presented. Experimental measurements are compared to DELTAE thermoacoustic modeling predictions. Modeling results show that viscous losses within the system have a significant impact on the device performance as well as on the ability of DELTAE to accurately predict performance. Modeling has led to an understanding of system performance and highlighted loss sources that are areas for improvement in a redesign. 相似文献
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Abhishek Dhar 《物理学进展》2013,62(5):457-537
Recent results on theoretical studies of heat conduction in low-dimensional systems are presented. These studies are on simple, yet non-trivial, models. Most of these are classical systems, but some quantum-mechanical work is also reported. Much of the work has been on lattice models corresponding to phononic systems, and some on hard-particle and hard-disc systems. A recently developed approach, using generalized Langevin equations and phonon Green's functions, is explained and several applications to harmonic systems are given. For interacting systems, various analytic approaches based on the Green–Kubo formula are described, and their predictions are compared with the latest results from simulation. These results indicate that for momentum-conserving systems, transport is anomalous in one and two dimensions, and the thermal conductivity κ diverges with system size L as κ ~ L α. For one-dimensional interacting systems there is strong numerical evidence for a universal exponent α = 1/3, but there is no exact proof for this so far. A brief discussion of some of the experiments on heat conduction in nanowires and nanotubes is also given. 相似文献
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《Heat Recovery Systems and CHP》1992,12(1):49-55
A heat transformer can upgrade heat to a higher temperature. A two-stage heat transformer has a greater temperature upgrading potential than a single-stage heat transformer, e.g. heat can be upgraded from a level of about 130–140°C to temperatures of about 200°C. A practical method to select suitable hydrides to be used in a two-stage heat transformer is presented. The example discussed shows that the selected alloys result in a reasonable operation of the two-stage heat transformer. Three different evaluation criteria viz. coefficient of performance, alloy output and temperature output, are introduced to compare the operational characteristics of heat transformers with different alloys; the influence of some metal hydride properties on the operational characteristics is also discussed. 相似文献
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《Heat Recovery Systems and CHP》1989,9(1):67-100
The purpose of this paper is to present a review of the recent heat pipe research and development efforts in the Americas. After discussing the research and development of high performance monogroove, tapered artery, trapezoidally grooved, dual-slot, double wall artery, ceramic and other specially designed heat pipes, the transient modeling and testing efforts are reviewed for a variety of heat pipes that also includes variable conductance heat pipes and thermosyphons. The application of heat pipes to terrestrial systems is discussed for the heat recovery systems, Rankine and solar systems, fusion reactor core cooling and the thermal control of electronic equipment. The application of heat pipes to space systems includes the thermal control of space station and satellites, and the radiator design of large space power systems. Although many advances have been made in the past few years in the development of high heat transfer performance heat pipes, the heat pipe modeling efforts are not in step with the technological requirements. 相似文献
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针对微槽内饱和沸腾汽泡建立了简化模型,并利用COMSOL Mu ltiphysics软件对电场中汽泡动力学特性进行了数值模拟,分析了微槽道内EHD(electrohydro dynam ics)强化沸腾传热机理。实验以去离子水为工质,研究了外加直流电场下两种规格的矩形微细槽道内饱和沸腾传热强化特性,电压在0~28kV内,EHD技术对微细槽道内的饱和沸腾传热有明显的强化效果。 相似文献
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《Heat Recovery Systems and CHP》1990,10(3):177-185
A new absorption heat pump, using a mixture of three elements (ammonia-water-potassium hydroxide), is presented in this paper. The principal aim of the proposed machine is to solve practical problems which represent the most critical points of a traditional heat pump using ammonia-water system, i.e. high pressure and rectification of ammonia. For this new system, the general working scheme, the thermodynamic design data, its performance as a heat pump machine and a comparison with the ammonia-water absorption heat pump are presented. The research activity concerning this work has been supported by ENEA (Italy). 相似文献
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《Nuclear Physics B》1986,275(1):1-17
The problem of critical slowing down in simulations of statistical mechanics systems is discussed in the context of hybrid stochastic differential equations. By modifying the kinetic energy part of the system's molecular dynamics lagrangian, new algorithms are presented which evolve the system's infra-red modes as rapidly as their ultraviolet ones. These improved (“accelerated”) algorithms should yield accurate measurements of the critical behavior of models of magnets, glasses, etc. which have not been practical in the past. 相似文献
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This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results. 相似文献
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In this work, the results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a spike-plate type electrostatic precipitator (ESP) are presented. The EHD secondary flow was measured using 2- and 3-dimensional particle image velocimetry (PIV) method under the negative DC voltage. The PIV measurements were carried out in several cross-sectional planes along and across the ESP duct. The results show a complex and turbulent flow structure in the ESP. The EHD secondary flow significantly depends on applied voltage and measuring cross-sectional plane position in respect to the spike tip. The partial collection efficiency of the ESP was measured for negative and positive DC voltage. The particle concentration with and without discharge was measured at the ESP exit using an optical aerosol spectrometer. 相似文献
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Discrete variational principle and first integrals for Lagrange--Maxwell mechanico-electrical systems
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This paper presents a discrete variational principle and a method to
build first-integrals for finite dimensional Lagrange--Maxwell
mechanico-electrical systems with nonconservative forces and a
dissipation function. The discrete variational principle and the
corresponding Euler--Lagrange equations are derived from a discrete
action associated to these systems. The first-integrals are obtained
by introducing the infinitesimal transformation with respect to the
generalized coordinates and electric quantities of the systems. This
work also extends discrete Noether symmetries to mechanico-electrical
dynamical systems. A practical example is presented to illustrate the
results. 相似文献
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V. F. Vdovin 《Radiophysics and Quantum Electronics》2005,48(10-11):779-791
We present the theoretical fundamentals and features of development of cryogenic cooling systems for receivers in the range 0.1–1 THz. The results of development of cryogenic systems for sustaining the temperatures in the range from 150 to 0.3 K are considered. The systems are based on a wide class of cryogenic devices employing various principles and thermodynamic cycles. The described developments are based on the unity of the thermal and radiophysical complexes of the cooled receiver and the cryosystem. The discussed cryosystems are specifically used to cool receivers with the mixers based on Schottky-barrier diodes and superconductor-insulator-superconductor structures, as well as on various bolometers. The problems of heat insulation against the surrounding medium and heat transfer from the receiver to the cryogenic liquid, the features of the input/output of signals in a wide frequency range and of mechanical vacuum-tight thermo-decoupled inputs to the cryostat, and the control systems for cryoelectronic complexes are considered in detail. The presented results can be used for both laboratory experiments and practical applications in radio astronomy, atmosphere spectroscopy, and other fields. 相似文献
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We give a brief review of the past development of model studies on one-dimensional heat conduction. Particularly, we describe
recent achievements on the study of heat conduction in one-dimensional gas models including the hard-point gas model and billiard
gas channel. For a one-dimensional gas of elastically colliding particles of unequal masses, heat conduction is anomalous
due to momentum conservation, and the divergence exponent of heat conductivity is estimated as α≈0.33 in k ∼ L
α
. Moreover, in billiard gas models, it is found that exponent instability is not necessary for normal heat conduction. The
connection between heat conductivity and diffusion is investigated. Some new progress is reported. A recently proposed model
with a quantized degree of freedom to study the heat transport in quasi-one dimensional systems is illustrated in which three
distinct temperature regimes of heat conductivity are manifested. The establishment of local thermal equilibrium (LTE) in
homogeneous and heterogeneous systems is also discussed. Finally, we give a summary with an outlook for further study about
the problem of heat conduction. 相似文献
19.
A methodology was developed to analyze relation-based systems evolving in time by using the fundamental concepts of thermodynamics. The behavior of such systems can be tracked from the scattering matrix which is actually a network of directed vectors (or pathways) connecting subsequent values, which characterize an event, such as the index values in stock markets. A system behaves in a rigid (elastic) way to an external effect and resists permanent deformation, or it behaves in a viscous (or soft) way and deforms in an irreversible way. It was shown in the past that a formula derived using the slope of paths gives a measure about the extent of viscoelastic behavior of relation-based systems Gündüz (2009) [5] Gündüz and Gündüz (2010) [6]. In this research the ‘work’ associated with ‘elastic’ component, and ‘heat’ associated with ‘viscous’ component were discussed and elaborated. In a simple two subsequent pathway system in a scattering diagram the first vector represents ‘the cause’ and the second ‘the effect’. By using work and heat energy relations that involve force and also storage and loss modulus terms, respectively, one can calculate the energy involved in relation-based systems. The modulus values can be found from the parallel and vertical components of the second vector with respect to the first vector. Once work-like and heat-like terms were determined the internal energy is also easily found from their summation. The parallel and vertical components can also be used to calculate the magnitude of torque and torque energy in the system. Three cases, (i) the behavior of the NASDAQ-100 index, (ii) a social revolt, and (iii) the structure of a melody were analyzed for their ‘work-like’, ‘heat-like’, and ‘torque-like’ energies in the course of their evolution. NASDAQ-100 exhibits highly dissipative behavior, and its work terms are very small but heat terms are of large magnitude. Its internal energy highly fluctuates in time. In the social revolt studied work and heat terms are of comparable magnitude. The melody depicts highly organized structure, and usually has larger work terms than heat terms, but at some intervals heat terms burst out and attain very large magnitudes. Torque terms reach high values when the system is recovering from a minimum value. 相似文献
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《Heat Recovery Systems and CHP》1993,13(4):353-361
Approximately 50% of residential energy use in the U.S. is for space heating and cooling. The most popular type of system installed in U.S. houses is a central air-circulating system combining a natural-gas-fired direct-air-heating furnace and an electric-driven vapor compression cooling system. Natural-gas-driven heat pump technology offers considerable benefits over these existing technologies including a reduction in energy use, improved environmental impact, and reduced investment in electric power plant construction. However, over the past 30 years, more than 100 major natural-gas-driven heat pump development projects have been undertaken without a currently successful product. These past projects have focused on engine-driven vapor compression refrigeration cycles and liquid sorption refrigeration cycles. New solid sorption technology offers significant advantages over these engine and liquid sorption technologies. The fundamental advantages of solid sorption refrigeration technology in natural gas residential heat pumps are presented and the societal benefits over existing residential systems discussed. 相似文献