Showing papers by "Tsinghua University published in 1994"
TL;DR: In this paper, the authors investigated the single-phase forced-flow convection of water or methanol flowing through microchannels with rectangular cross-section and found that the fully developed turbulent convection regime was initiated at about Re = 1000-1500.
418 citations
382 citations
TL;DR: In this article, the forced-flow convection of water through rectangular microchannels having hydraulic diameters of 0.133-0.367mm and aspect ratios of H/W = 0.333-1 was investigated experimentally.
Abstract: The forced-flow convection of water through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and aspect ratios of H/W = 0.333-1 was investigated experimentally. The flow friction was measured to analyze the heat transfer regimes and to explore the physical aspects of the connective heat transfer. The experimental measurements indicated that the upper bound of the laminar heat transfer regime occurred at a Reynolds number of 200-700, and fully turbulent connective heat transfer was reached at Reynolds numbers of 400-1,500. The transition Reynolds number diminished with the reduction of the microchannel dimension, and the transition range was observed to become smaller in magnitude. For the laminar heat transfer regime, the Nusselt number was found to be proportional to Re0.62while the turbulent heat transfer case exhibited a typical relationship between Nu and Re, but with a different empirical coefficient, CH,t. The geometric parameters were found to be important variables that could...
261 citations
TL;DR: In this paper, a three-city household air pollution study covering particulates (PM10), nitrogen dioxide (NO2), and carbon monoxide (CO) was conducted in and near households spanning the most important current steps in each city's energy ladder.
203 citations
TL;DR: In this article, the energy levels and binding energies of an off-center donor in a spherical quantum dot are calculated by a linear variational method, and the results clearly show the quantum size effect.
Abstract: The energy levels and binding energies of an off-center donor in a GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As spherical quantum dot are calculated by a linear variational method. The results clearly show the quantum size effect. The location effects of a donor ion on splitting, level ordering, and binding of the donor states are studied. It is found that the alteration of the position of a single Coulomb center can largely change the single-electron spectrum in a quantum dot with a larger radius.
166 citations
TL;DR: In this paper, the authors examined the phenomenon of perosvskite-type ceramics deforming under electric fields and fracture in a multilayer actuator and found that, under a given electric field, cracking is suppressed in a multi-layer actuator if the ceramic layers are sufficiently thin.
Abstract: Many perosvskite-type ceramics deform appreciably under electric fields; they make good actuators which deliver motions upon receiving electrical signals. High electric fields are usually applied to induce large strains. Fracture has been observed in the actuators under electrical loading. In this theoretical study, the phenomenon is examined on the basis of electrostriction and fracture mechanics. Attention is focused on a crack emanating from an internal electrode or a conducting damage path. At the edge of the conducting path, the electric field is intense and nonuniform, inducing incompatible electrostrictive strains. Consequently, a stress field is set up in the ceramic, localized around the edge of the conducting path. The condition for the stress to extend a crack is estimated by two models, using either quadratic or step-like electrostriction law. It is found that, under a given electric field, cracking is suppressed in a multilayer actuator if the ceramic layers are sufficiently thin.
161 citations
13 Apr 1994
TL;DR: A method for Chinese syllable recognition based upon the unique C/V structure of Chinese syllables is described and Experimental results show that recognition rate is above 90% when applying this method to a real-time recognition system.
Abstract: All Chinese words are single syllables with C/V (Consonant/Vowel) structure. In this paper, a method for Chinese syllables recognition based upon the unique C/V structure of Chinese syllables is described. All the 1254 Chinese toned syllables (excluding some rare syllables) are decomposed into 155 sub-syllable units, 117 for consonants and 38 for vowels. In this way, the problem of performance degradation caused by insufficient training data is avoided, and the robustness of recognition is improved. Experimental results show that recognition rate is above 90% when applying this method to a real-time recognition system. >
160 citations
TL;DR: In this article, a nonaxisymmetric analysis of piston ring lubrication has been established, where a piston ring with an arbitrary free shape is fitted into the cylinder bore, and the determination of ring deflection and contact load has been modeled mathematically as a linear complementary problem.
Abstract: The assumption of axisymmetry, employed by most of studies on piston ring lubrication, probably gives a too idealistic model for the real situation. A theoretical model for a nonaxisymmetrical analysis of piston ring lubrication has been established in the present study. When a piston ring with an arbitrary free shape is fitted into the cylinder bore, the determination of ring deflection and contact load has been modeled mathematically as a Linear Complementary Problem (LCP). By combining LCP solution with lubrication analysis, the film thickness and contact load distribution over the circumference are obtained, leading to a more realistic simulation for piston ring lubrication
159 citations
TL;DR: Li, J, Polka, H-M and Gmehling as discussed by the authors developed a gE model for electrolyte systems based on results from statistical thermodynamics and took into account the interactions between all species present in the electrolyte solution.
137 citations
TL;DR: In this article, a kinetic model for ED was presented and the model predicts the decrease of the forming rate as an exponential function of time, and the effects of applied electric field, viscosity, dielectric constant, amount of suspending agent, and zeta potential of the solid particles in Al(OC[sub 3]H[sub 7]) [sub 3]-C[sub 2]H [sub 7]OH-CH] sol on ED processing are discussed.
Abstract: Electrophoretic deposition (ED) was used to form uniform-structure green bodies with a thickness of about 3 mm of SiC whisker-tetragonal zirconia polycrystals (TZP) mixtures. A kinetic model for ED is presented. The model predicts the decrease of the forming rate as an exponential function of time. Diagrams of colloidal groups and electrochemical reactions of charged solid particles on the deposited electrode are proposed. The effects of applied electric field, viscosity, dielectric constant, amount of suspending agent, and zeta potential of the solid particles in Al(OC[sub 3]H[sub 7])[sub 3]-C[sub 3]H[sub 7]OH-CH[sub 3]OH sol on ED processing are discussed.
112 citations
TL;DR: Mixed trialkylphosphine oxide (TRPO) was chosen as the extractant for the removal of uranium, neptunium, plutonium, and americium from highly active waste (HAW) in China.
Abstract: Mixed trialkylphosphine oxide (TRPO) (alkyl is C[sub 6]-C[sub 8]) was chosen as the extractant for the removal of uranium, neptunium, plutonium, and americium from highly active waste (HAW) in China. Composition and properties of the extractant and process chemistry are based on 30 vol% TRPO-kerosene as solvent. Hexa- and tetravalent actinides are highly extractable in 30 vol% TRPO extraction from acidic HAW, and trivalent americium (curium) can be extracted effectively from HAW with a nitric acid concentration of [approximately]1 mol/[ell]. Actinides extracted can be stripped successively by 5.5 mol/[ell] HNO[sub 3], 0.6 mol/[ell] H[sub 2]C[sub 2]O[sub 4], and 5% Na[sub 2]CO[sub 3] into americium + rare earth, neptunium + plutonium, and uranium fractions, respectively. The loading capacity of TRPO solvent is higher than that of bifunctional organophosphorus extractants, and the radiolytic stability of TRPO is higher than that of tributyl phosphate (TBP) and bis(2-ethyl hexyl)phosphoric acid. The extraction and stripping rate of TRPO is high enough to be compatible with the centrifugal contactors. Optimized process parameters of multistage countercurrent extraction and stripping and results of experimental verification are established. In both a batch experiment with simulated nuclear power plant (NPP) spent-fuel Purex HAW and a continuous experiment with real NPPmore » spent-fuel Purex HAW, 99.9% recovery of actinides was achieved. The modification of the solvent system with TBP to fit the conditions in the chemical pretreatment of defense HAW is considered.« less
TL;DR: The proposed fuzzy model following controller is applied to the speed control of an induction motor drive and the performance and effectiveness are demonstrated by some simulated and experimental results.
Abstract: Since the dynamic response trajectory of a traditional fuzzy controller can not be quantitatively controlled, a fuzzy model following controller is proposed in this paper. In the proposed controller, an output feedback linear model following controller (LMFC) is first designed according to the roughly estimated plant model to let its response follow the output generated by a reference model. Then a model following error driven control signal is synthesized such that good model following characteristics can be preserved at various operating conditions. The proposed controller is applied to the speed control of an induction motor drive. Dynamic signal analysis of the model following behavior is made and the procedure for constructing the control algorithms is described in detail. The performance of the drive and the effectiveness of the proposed controller are demonstrated by some simulated and experimental results. >
TL;DR: The accuracy of object feature measurement is proposed as a criterion for judging the quality of segmentation results and assessing the performance of applied algorithms.
TL;DR: In this article, the definitions of both material and physical symmetries which are described in terms of point groups are revisited and the reality of every compact point group in the description of a material or a physical symmetry is justified in four aspects: (i) point groups characterized by a finite set of tensors, (ii) Hilbert's theorem for integrity bases, (iii) correlation between integrity bases and function bases, and (iv) physical reality.
Abstract: We reconsider the definitions of both material symmetries and physical symmetries which are described in terms of point groups, i.e. subgroups of the full orthogonal group, because these two concepts are often confused and the classical descriptions of physical symmetry for inelastic behaviour of materials are impracticable. All two- and three-dimensional point groups are classified into two types: compact and non-compact. The reality of every compact point group in the description of a material or a physical symmetry is justified in four aspects, that is: (i) point groups characterized by a finite set of tensors, (ii) Hilbert's theorem for integrity bases, (iii) correlation between integrity bases and function bases (generalization of Wineman and Pipkin's theorem), and (iv) physical reality. The unreality of any non-compact point group in the description of a material or a physical symmetry is proposed as a new principle of continuum physics. As applications, the complete sets of all classes of two-and three-dimensional point groups which describe physical symmetries for linear physical properties (such as thermal expansion, piezoelectricity, elasticity, etc.) and for more general mechanical constitutive laws are given.
TL;DR: The precise formulation of the equivalence theorem for longitudinal weak boson scatterings is given both in the Higgs theory and in the realistic SU(2) electroweak theory to all orders in the perturbation for an arbitrary Higgs boson mass.
Abstract: Based on a systematic analysis of the renormalization schemes in the general ${R}_{\ensuremath{\xi}}$ gauge, the precise formulation of the equivalence theorem for longitudinal weak boson scatterings is given both in the ${\mathrm{SU}(2)}_{L}$ Higgs theory and in the realistic SU(2) \ifmmode\times\else\texttimes\fi{} U(1) electroweak theory to all orders in the perturbation for an arbitrary Higgs boson mass ${m}_{H}$. It is shown that there is generally a renormalization-scheme- and $\ensuremath{\xi}$ dependent modification factor ${C}_{\mathrm{mod}}$ and a simple formula for ${C}_{\mathrm{mod}}$ is obtained. Furthermore, a convenient particular renormalization scheme is proposed in which ${C}_{\mathrm{mod}}$ is exactly unity. Results of ${C}_{\mathrm{mod}}$ in other currently used schemes are also discussed especially on their $\ensuremath{\xi}$ and ${m}_{H}$ dependence through explicit one-loop calculations. It is shown that in some currently used schemes the deviation of ${C}_{\mathrm{mod}}$ from unity and the $\ensuremath{\xi}$ dependence of ${C}_{\mathrm{mod}}$ are significant even in the large-${m}_{H}$ limit. Therefore care should be taken when applying the equivalence theorem.
TL;DR: In this paper, the effects of dislocation confinement on fracture behavior in laminates consisting of alternating submicron ductile and brittle layers are studied, and an equilibrium number of dislocations exists at a given load level.
Abstract: The effects of dislocation confinement on fracture behavior in laminates consisting of alternating submicron ductile and brittle layers are studied. When the ductile layer thickness is below the micron level, dislocations must be treated individually. Dislocations emitted from the crack tip have two effects: they blunt the crack and thereby reduce the tensile stress at the crack tip ; and pile up against an interface and send a back stress to the crack tip to hinder further dislocation emission. Consequently, an equilibrium number of dislocations exists at a given load level. We estimate this number by considering the stability conditions for dislocations threading in the ductile layer, and dislocation pile-up is treated as an equivalent superdislocation. Furthermore, the competition between further dislocation emission and cleavage at the blunted crack tip is considered. Our result shows that because of the confinement, as the applied load increases, the tensile stress at the blunted crack tip also increases. Cleavage occurs when the tensile stress at the crack tip reaches the theoretical strength. Given a sufficiently thin constraining layer, cleavage can even occur in ductile metals such as copper and aluminum. The implications of this model for several material systems are discussed.
TL;DR: In this article, the possibility and mechanism of anaerobic acidification, ultraviolet radiation, and ozonation as pretreatment methods for improving the biotreatability of coke-plant wastewater have been investigated, a new process has been proposed and experiments in lab-scale apparatus and pilot plant have been conducted.
TL;DR: Simulation results show that transient stability enhancement and voltage regulation of a power system under a large sudden fault can be achieved.
TL;DR: In this paper, the NH3-ion-beam-assisted deposition with bombardment energies of 200-800 eV at room temperature was investigated and it was found that the structure of the carbon nitride thin films varied with the bombardment energy.
Abstract: Carbon nitride thin films have been prepared by NH3-ion-beam-assisted deposition with bombardment energies of 200-800 eV at room temperature. These films have been characterized by transmission electron microscopy, Auger electron spectroscopy and x-ray photoelectron spectroscopy for chemical analysis. It was found that the structure of the films varied with the bombardment energy. In the case of 400 eV bombardment, the tiny crystallites immersed on an amorphous matrix were identified to be beta -C3N4. X-ray photoelectron spectroscopy indicated that some carbon atoms and nitrogen atoms form unpolarized covalent bonds in these films.
TL;DR: In this article, a unique phosphor tracer technique was developed that measured the solids tracer concentration with time at different radial and axial positions within the cocurrent downflow circulating fluidized bed with a 140 mm inner diameter.
TL;DR: The algorithm can handle pathological cases and identify all possible correct solutions efficiently and has efficient techniques for constructing candidate wireframes, handling broken lines in the input data, generating face loops, determining cutting edges and vertices, building body loops, and assembling body loops into candidate objects.
Abstract: An efficient algorithm for reconstructing all polyhedral 3D solid models from 2D orthographic projections is addressed. The algorithm can handle pathological cases and identify all possible correct solutions efficiently. The algorithm has efficient techniques for constructing candidate wireframes, handling broken lines in the input data, generating face loops, determining cutting edges and vertices, building body loops, and assembling body loops into candidate objects. Several symmetric and complex orthographic projections are given to show the completeness of the algorithm.
26 Jun 1994
TL;DR: The simulation results show that the proposed method, only using dynamic information acquired by ultrasonic sensors, can perform robot navigation in complex and uncertain environments by efficiently weighting reactive behaviors, such as obstacle avoidance, edge following, and moving to the target.
Abstract: This paper presents a method for fuzzy logic-based 'perception-action' behavior control of a mobile robot in uncertain environments. A key problem in 'perception-action' behavior control is to coordinate and integrate more reactive behaviors when the mobile robot executes tasks in complex environments. The main idea of the paper is to formulate 'perception-action' behaviors and to coordinate their conflicts and competitions by fuzzy sets and fuzzy rules. An advantage of this method is that the coordination of more reactive behaviors is very robust (nearly independent of dynamic environments). The simulation results show that the proposed method, only using dynamic information acquired by ultrasonic sensors, can perform robot navigation in complex and uncertain environments by efficiently weighting reactive behaviors, such as obstacle avoidance, edge following, and moving to the target. >
TL;DR: A general proof of the equivalence theorem in electroweak theories with the symmetry breaking sector described by the chiral Lagrangian is given in the R ξ gauge by means of the Ward-Takahashi identities as discussed by the authors.
TL;DR: The results showed that melittin adsorbed on the lipid layer surface contains less alpha-helix than its counterpart inserted into the lipid Layer, and a possible molecular mechanism underlyingmelittin insertion is proposed.
Abstract: Two model systems were used to study the conformational changes of melittin upon insertion into phospholipid. The first model system is phospholipid monolayers with different surface pressure. The second one is small unilamellar vesicles at above or below their phase transition temperature. The conformational changes of melittin upon insertion were elucidated by employing a combination of monolayer technique, fluorescence technique, CD, and FTIR. The results showed that melittin adsorbed on the lipid layer surface contains less alpha-helix than its counterpart inserted into the lipid layer. As the penetration depth of melittin increases, more ordered structures (alpha-helix) appear. A possible molecular mechanism underlying melittin insertion is proposed.
TL;DR: The free-energy diagram can explain the formation and the thermal stability of the above solid phases and the growth kinetics of the hcp and fcc MC phases were discussed in terms of the structural compatibility between the newly formed phase and the matrix in the ion-irradiation process.
Abstract: In the Ni-Ta system, several amorphous alloys were synthesized by room-temperature 200-keV xenon-ion irradiation of multilayered films with various compositions. The metallic-glass-forming range was determined experimentally to extend from 25 to 75 at. % Ni. In addition, a hcp metastable crystalline (MC) phase was observed in the Ni-rich ${\mathrm{Ni}}_{80}$${\mathrm{Ta}}_{20}$ and ${\mathrm{Ni}}_{75}$${\mathrm{Ta}}_{25}$ multilayers at different irradiation stages, and a fcc MC phase was formed in the Ta-rich ${\mathrm{Ta}}_{65}$${\mathrm{Ni}}_{25}$ and ${\mathrm{Ta}}_{75}$${\mathrm{Ni}}_{35}$ multilayers before and after the films were amorphized, respectively. Interesting evolution behavior of the above nonequilibrium solid phases upon thermal annealing was also observed. A free-energy diagram of the system was constructed based on the model of Miedema, de Boer, and de Boer and the method of Alonso and Simozar. The calculation included the free-energy curves of all the competing phases, especially the MC phases, which were considered as compoundlike, and the enthalpies were calculated based on well-identified structural parameters. The calculated energy sequence of the phases was verified at certain compositions by the phase appearance order in the respective as-deposited multilayers upon steady-state thermal annealing, thus confirming the relevance of the diagram in its general outline. The free-energy diagram can explain the formation and the thermal stability of the above solid phases. The growth kinetics of the hcp and fcc MC phases were also discussed in terms of the structural compatibility between the newly formed phase and the matrix in the ion-irradiation process.
TL;DR: In this paper, a simple formulation of two triangular membrane elements is implemented similar to that of the generalized conforming plate bending element, and the triangle has nine degrees of freedom, namely, six translations and three rigid rotations at the vertexes.
TL;DR: By introducing vertex rigid rotation as a freedom, two new quadrilateral membrane elements with a simple form of formulation are derived from the concept of the generalized conforming finite element as mentioned in this paper, and they pass the patch test for any geometry, therefore assuring convergence to an exact solution with consistent mesh refinement.
TL;DR: In this paper, temperature fields and mean heat transfer coefficients were obtained on a finned tube and the diameter of the fin was varied in order to find out about the effect of fin height.
TL;DR: Simulation results show that the proposed method can be applied to efficient robot navigation in complex and unknown environments by weighting different varieties of reactive behavior, such as avoiding obstacles, following edges, moving towards a target, and so forth.
TL;DR: In this article, a visual-aided finite-element method was employed to analyze the electromagnetic and thermal fields of induction motors during starting, and the starting performance, including the distributions of magnetic flux density, current density, loss density and temperature rise, were presented dynamically by computer animation.
Abstract: In order to improve the starting performance of squirrel-cage induction motors, the so-called "skin effect" becomes very serious, which causes a highly uneven distribution of rotor-bar currents. The corresponding conduction losses of the upper and lower portions of rotor bars create high thermal stress, which may damage the rotor bars, particularly at the junction of rotor bars and end-rings. Hence, it is important to predict the starting and operating characteristics of induction motors more accurately for the purpose of ensuring the reliability of motor operation. This paper employs a visual-aided finite-element method to analyze the electromagnetic and thermal fields of induction motors during starting. By using visual and interactive pre- and post-processing techniques, the analysis can be applied to squirrel-cage induction motors with any physical dimensions, material properties, and system parameters. The starting performance, including the distributions of magnetic flux density, current density, loss density and temperature rise, are presented dynamically by computer animation, which are convenient and powerful for the evaluation and design optimization of induction motors. The theoretical predictions are verified by direct comparison with experimental results. >