Showing papers by "Tsinghua University published in 1995"

Cocurrent downflow circulating fluidized bed (downer) reactors — A state of the art review

Abstract: Depuis une dizaine d'annees, les reacteurs a lit fluidise circulant a cocourant descendant ont ete proposes pour faire concurrence aux reacteurs a lit fluidise circulant ascendant. Dans cet article, on resume les resultats publies des etudes sur les reacteurs de type descendant, et des directions sont recommandees pour la recherche. On a trouve que les reacteurs descendants offraient des avantages particuliers par rapport aux reacteurs a lit fluidise circulant ascendant et qu'ils etaient potentiellement interessants dans de nombreux procedes industriels, en raison principalement d'une structure d'ecoulement du gaz et des solides plus uniforme que dans le cas des reacteurs ascendants.

Observations of damage morphologies in nacre during deformation and fracture

Abstract: The deformation, fracture and toughening mechanisms of nacre from a kind of fresh-water bivalve mollusc (Cristaria plicata) were studied by SEM, TEM and microindentation tests. Experimental results revealed a strong anisotropy of the damage behaviour reflecting the microstructural character of nacre. The fractured surface parallel to the cross-sectional surface of nacre was much more tortuous than that parallel to the platelet surface. The crack line on the cross-sectional surface was step-like, while that on the platelet surface was polygonal. Sliding of aragonite layer combined with the plastic deformation of organic matrix is the main plastic deformation mechanism of nacre. Three main toughening mechanisms have been found acting in concert: crack deflection, fibre pull-out and organic matrix bridging.

Theory and experiment of an aperture-coupled hemispherical dielectric resonator antenna

Abstract: A hemispherical dielectric resonator (DR) antenna using aperture coupling for excitation is studied both theoretically and experimentally. The reciprocity method is used to formulate the problem. The exact magnetic field Green's function due to the equivalent magnetic current in the slot is derived rigorously, and it is presented in a form which can be evaluated very efficiently. Moment method is used to solve the magnetic current from which the input impedance of the antenna configuration is obtained. The equivalent radius of the slot is used so that simple formulae developed for the cylindrical dipole can be applied directly. The effects of the slot's length, the slot's position, and the slot's width on the broadside TE/sub 111/ mode input impedance are studied, and reasonable agreement between theory and experiment is obtained. >

Crack Growth in Ferroelectric Ceramics Driven by Cyclic Polarization Switching

Abstract: Ferroelectric ceramics are susceptible to stable crack growth under cyclic electric field of high magnitude. The stresses originate from mismatch strains induced by the electric field around inhomogeneities, such as processing flaws, terminated electrodes and the cracks themselves. The phenomenon is studied using a lead lanthanum zirconate-titanate ceramic. A flaw is indented on the surface of a ceramic sample. Subject to a cyclic electric field of magnitude exceeding the coercive field, cracks eminate from the flaw, in the direction perpendicular to the voltage drop. Tests relevant to the phenomenon are also performed. Polarization and strain as functions of applied electric field are measured at various temperatures and applied stresses. Discharge is demonstrated through an air gap between two ceramic plates subjected to a voltage drop. Models are presented that explain the cyclic nature of the crack growth.

Detailed investigation on a negative photochromic spiropyran

Abstract: The negative photochromic properties of a spiropyran are investigated in various solvents and in solutions of different acidity The results indicate that the polarity of the solvent has a prominent effect on λ max of the coloured merocyanine form (MC), whereas its effects on λ max of the closed form (SP) and on the coloration rates after visible bleaching are very weak From the effect of acid concentration on the coloration rates the p K a of the cis intermediate Y in the coloration process is determined to be about 11 lower than that of the protonated merocyanine (MCH + ) The structures and configurations of MC, SP, MCH + and Y are assigned via their 1 H NMR spectra

Micro-fluid Flow In Microchannel

Abstract: General dynamic analysis for fluid flow in microchannel was presented. A microfluid flow measuring system is set up for experimental study, then the microfluid flow behaviour in microchannel with different cross sections and characteristic size is investigated under the pressure difference ranged from 0-200Kpa. The Silicon nozzle/diffuser elements are fabricated and the flow directing property of them is investigated under 0-10 Kpa . The experiment results agree well with the expected.

New methods for measuring mechanical properties of thin films in micromachining: Beam pull-in voltage (VPI) method and long beam deflection (LBD) method

Abstract: New stress-measurement methods for determining both tensile and compressive stress and Young's modulus in surface micromachining are presented. The investigation is concentrated on the development of two techniques: (1) beam pull-in voltage (VPI) and (2) long beam deflection (LBD). The VPI method is based on the pulling down of the upper electrode (beam) when the voltage between two electrodes exceeds a critical level. Both tensile and compressive stress and Young's modulus of a thin film can be derived using this method. the LBD method, on the other hand, converts the axial strain in the beam into a transverse deflection w(x) after the structure is released to be free standing; this deflection is large enough to be measured easily. These techniques have been analysed and tested experimentally. A comparison with other known stress-measurement techniques shows good agreement using polysilicon films. Both techniques have been shown to be quite promising for simple and accurate on-chip thin-film stress measurements.

Anomalous alloying behavior induced by ion irradiation in a system with a large positive heat of mixing

Abstract: Amorphization was achieved by room temperature 200-keV xenon ion mixing in two isolated composition ranges, i.e., ${\mathrm{Y}}_{22}$--${\mathrm{Y}}_{27}$ and ${\mathrm{Y}}_{72}$--${\mathrm{Y}}_{83}$, in the Y-Mo system, which has a positive heat of mixing exceeding +26 kJ/mol. In addition, three metastable crystalline (MX) phases, i.e., a Y-rich fcc (fcc-I), a Mo-rich hcp (hcp-I), and another fcc (fcc-II) phase, were observed at different irradiation stages. Kinetically, the fcc-I MX phase was formed through a single-step transition of hcp\ensuremath{\rightarrow}fcc, while the hcp-I and fcc-II phases were formed sequentially via a two-step transition of bcc\ensuremath{\rightarrow}hcp\ensuremath{\rightarrow}fcc. To give further insight into the singularity of the amorphization behavior of the system and the MX phase formation, a Gibbs-free-energy diagram of the system was constructed by calculating the free energies of the amorphous phase, the solid solutions, three MX phases, and of the as-deposited Y-Mo multilayers, for which the interfacial free energies were added. The established diagram can explain well the irradiation-induced amorphization behavior and the MX phase formation in the Y-Mo system.

Correlations between solvatochromism, Lewis acid-base equilibrium and photochromism of an indoline spiropyran

Abstract: The solvatochromism, Lewis acid-base equilibrium and photochromism of an indoline spiropyran in neat and mixed organic solvents were studied by UV-visible spectroscopy. The variation of the transition energy E T of the coloured form (PC) in different solvents was used as an empirical parameter to measure the polarity of the solvents and as a reference process to predict the shift of the Lewis acid-base equilibrium between PC and the colourless form (SP) and the thermal decolouration rates of the PC form in neat and mixed organic solvents. From the linearity of the plot of In K e or In k dc vs. E T , the Lewis acid-base equilibrium, photochromism and solvatochromism of the spiropyran were found to be similar solvent-dependent processes. The changes in K e and k dc were interpreted in terms of the differences in solvation of PC in solvents of different polarity. The relatively small influence of solvent polarity on the colouration rate k op serves as evidence supporting the view that the rate-determining ring-opening process is a concerted electrocyclic reaction.

A dynamic on-line parameter identification and full-scale system experimental verification for large synchronous machines

Abstract: An on-line parameter identification and full-scale experimental verification for large synchronous machines (>50 MVA) is presented in this paper. A step change of excitation is imposed to a generator when the machine is in normal operation. The transient voltages, currents and the power angle are recorded. Based on the large disturbance equations and using the measured power angle as an observation argument in an identification algorithm, the synchronous machine electrical parameters (x/sub d/, x/sub d/', x/sub d/", T/sub do/', T/sub do/", x/sub q/, x/sub q/", T/sub qo/") and mechanical parameters (H,D) are obtained. In addition, the system parameters (equivalent infinite bus voltage V/sub bus/ and line reactance x/sub e/) are identified as well. The proposed method has been repetitively applied to turbogenerators and hydrogenerators with capacities up to 300 MVA. In particular, a field test has been conducted on a system with a capacity of 15000 MVA. The experimental results from all of the full-scale tests are consistent and the effectiveness of the proposed on-line identification method is verified. The plant experiences indicate that by adopting the identified parameters, the stability margin of the generator can be improved by up to 5%, resulting in 30-50 MVA more power generation. >

Equivalence theorem and probing the electroweak symmetry breaking sector

Abstract: We examine the Lorentz noninvariance ambiguity in longitudinal weak-boson scatterings and the precise conditions for the validity of the equivalence theorem (ET). {ital Safe} Lorentz frames for applying the ET are defined, and the intrinsic connection between the longitudinal weak-boson scatterings and probing the symmetry-breaking sector is analyzed. A universal precise formulation of the ET is presented for both the standard model and the chiral Lagrangian formulated electroweak theories. It is shown that in electroweak theories with a strongly interacting symmetry-breaking sector, the longitudinal weak-boson scattering amplitude {ital under} {ital proper} {ital conditions} can be replaced by the corresponding Goldstone-boson scattering amplitude in which all the internal weak-boson lines and fermion loops are ignored.

Synthesis of linear alkylbenzene in a liquid–solid circulating fluidized bed reactor

Abstract: The alkylation reaction of benzene with 1-dodecene was investigated in a liquid–solid circulating fluidized bed reactor (LSCFBR) using HY zeolite as catalyst. The conversion of 1-dodecene, the apparent reaction rate constant, the activity of catalyst and the product distribution in the LSCFBR were analysed and compared with the values obtained using a stirred tank batch reactor (STBR).

Phosphoryl amino acids: Common origin for nucleic acids and protein

Abstract: A series of compounds (DAP-AA) composed of an amino acid (AA) and a dialkyl phosphoryl group (DAP) is the basic elements of life chemistry. Self-catalysis of DAP-AA gives the self-assembly oligopeptides, even in aqueous medium at 38°C. The oligo-nucleotides could also be assembled from nucleosides' phosphorylation by DAP-AA. DAP-AA acts as the energy source as well as the phosphoryl donor for the synthesis of nuclic Acids and protein. A general expression for the self assembly system is proposed.

Lateral and Axial Mixing of the Dispersed Particles in CFB

Abstract: The lateral and axial mixing of dispersed particles in a circulating fluidized bed were measured by using the phosphor tracer technique. The measured residence time distribution was satisfactorily described by a proposed two dimensional plug flow dispersion flow. Compared with that in the cocurrent downflow circulating fluidized bed (CDCFB), the dispersion Peclet numbers for lateral and axial mixing of dispersed particles is about the same value with that in CDCFB (downer). Correlations of the Peclet numbers under the operating conditions are presented.

Prefiltering-based ESPRIT for estimating sinusoidal parameters in non-Gaussian ARMA noise

Abstract: Addresses the ESPRIT algorithm for estimating sinusoidal parameters in non-Gaussian ARMA noise. The authors show that after prefiltering the output data via the estimated AR polynomial of the noise model, a new ESPRIT based on some extensions of ESPRIT can be used to estimate sinusoidal parameters. >

Immobilization of microbial cells using polyvinyl alcohol (PVA) — polyacrylamide gels

Abstract: A novel cell immobilization technique is developed in which polyvinyl alcohol is crosslinked with boric acid with addition of an acrylamide as a polymerizing agent The presence of the polymerizing agent overcomes the problem of swelling of PVA gels in aqueous solution The new immobilization method was used to entrap a phenol-degrading microorganism, a species of Pseudomonas Phenol was successfully degraded in a shake-flask test of the beads demonstrating that cell viability was maintained following the immobilization procedure

Dynamics of the freezing process in relaxor ferroelectrics.

Abstract: The dynamics of the freezing process in relaxor ferroelectrics are investigated by Monte Carlo computer simulation. The concepts of ``frozen dipole'' and ``slow dipole'' are proposed according to the flipping frequency during the observation time of simulation. The deviation from Curie-Weiss behavior and the frequency dispersion in relaxor ferroelectrics have been proved to be caused by the increasing proportion of slow dipoles. The simulation shows that the external field reduces the dielectric constant and increases the temperature of dielectric maximum, which agrees with the experiment results. The polarization behavior with various electrical and thermal histories and starting from different initial states is also simulated and a systematic explanation based on the bifurcation tree of components in phase space is given. It is proposed that the relaxor characteristics are caused by a gradual freezing process of polar microregions, which arises from the inhomogeneous crystal structure of this kind of material.

Joint wavelet-transform correlator for image feature extraction

Abstract: We describe a joint wavelet-transform correlator in which the wavelet function is combined with the input image as the input joint image to realize the wavelet transform of the objective image. The Haar wavelet and the Roberts filter are chosen as the wavelet functions to extract the features of the objective image. The relationship of the Haar wavelet and the Roberts filter is analyzed mathematically based on admissible condition of the wavelet. Computer simulations are provided to verify the theory and to illustrate the performance of this correlator.