螺旋管内单相液体紊流脉动流动传热

以水为工质对螺旋管内单相液体充分发展紊流脉动传热特性进行了实验研究 ,发现了紊流脉动传热系数与流量脉动相位间的反相关系 ,引进了两个新的准则数 :脉动频率数W0 和脉动振幅率Ap,对紊流脉动传热规律进行了分析讨论 ,并根据紊流脉动传热参数的瞬态变化规律 ,对脉动过程中的管内二次流特征和变化作了详细讨论和分析 .     

Evaluation of interfacial properties in adhesive joints of aluminum alloys using angle-beam ultrasonic spectroscopy

This paper reports an ultrasonic angle-beam technique for the evaluation of interfacial properties of adhesive joints. The technique is based on measurement of the frequency response of obliquely incident ultrasonic signals from an adherend-adhesive interface. A theoretical model was developed for analysis of the interaction between the obliquely incident ultrasonic waves and multilayered adhesive joints. A special ultrasonic goniometer using only one ultrasonic transducer was built to measure the reflected signals. Samples with controlled interfacial properties were prepared for experimental determination of these properties. The experimental results were in good agreement with those predicted from the model. It was found that some of the minima loci in the reflected frequency spectra are independent of either the thickness or the elastic properties of the interfacial layer. This enables us to develop a simple and stable reconstruction procedure to determine the thickness and interfacial properties from experimental data.     

Analysis of pulsatile flow and its role on particle removal from surfaces

The use of pulsatile flow for energy efficient particle removal from surfaces is evaluated through modeling calculations. The governing equation for pulsatile flow in a channel between parallel plates with an oscillatory pressure input is solved and wall shear stress, identified as a measure for particle removal, calculated for fixed power input. It is observed that as the frequency of oscillation is increased the average wall shear stress with an oscillatory pressure input is higher than the corresponding steady state value only above a critical frequency. Similar results are obtained for pulsatile flow in a pipe. Explanation for this observation is presented based on how velocity profile changes as a function of frequency and consequently its effect on wall shear stress versus power dissipated. Based on these observations we propose that there is a critical frequency above which an oscillatory pressure input will be energy efficient for particle removal.     

A SIMPLE EXPERIMENTAL TECHNIQUE TO MEASURE GAS PHASE DISPERSION IN BUBBLE COLUMNS

A new technique to measure gas phase dispersion in bubble columns is presented. This technique is fairly simple to implement, accurate and inexpensive as compared to conventional methods used in the literature. Nitrogen is used as a tracer and a step change is effected by switching from air to the tracer, nitrogen. The gas phase concentration of oxygen is monitored by means of a polarographic oxygen sensor. The system studied is air-water and the experimental results compare well with the literature data. The technique presents a simple way to measure gas phase axial dispersion in bubble columns and can also be applied to other types of reactors     

A SIMPLE EXPERIMENTAL TECHNIQUE TO MEASURE GAS PHASE DISPERSION IN BUBBLE COLUMNS

A new technique to measure gas phase dispersion in bubble columns is presented. This technique is fairly simple to implement, accurate and inexpensive as compared to conventional methods used in the literature. Nitrogen is used as a tracer and a step change is effected by switching from air to the tracer, nitrogen. The gas phase concentration of oxygen is monitored by means of a polarographic oxygen sensor. The system studied is air-water and the experimental results compare well with the literature data. The technique presents a simple way to measure gas phase axial dispersion in bubble columns and can also be applied to other types of reactors     

Understanding surfaces and buried interfaces of polymer materials at the molecular level using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies on polymer surfaces/interfaces using sum frequency generation (SFG) vibrational spectroscopy. SFG theory, technique, and some experimental details have been presented. The review is focused on the SFG studies on buried interfaces involving polymer materials, such as polymer–water interfaces and polymer–polymer interfaces. Molecular interactions between polymer surfaces and adhesion promoters as well as biological molecules such as proteins and peptides have also been elucidated using SFG. This review demonstrates that SFG is a powerful technique to characterize molecular level structural information of complicated polymer surfaces and interfaces in situ. Copyright © 2006 Society of Chemical Industry     

Local measurement of oxygen transfer around a single bubble by planar laser-induced fluorescence

The aim of this work is to develop a non-intrusive experimental technique to measure oxygen concentration in a liquid phase. This technique relies on the planar laser-induced fluorescence (PLIF). In its classical version, PLIF consists in viewing the presence of a fluorescent dye in a liquid phase by means of a laser sheet. Digital image analysis gives the relationship between local grey levels and the dye concentration. The principle of the oxygen concentration measurement rests on the fact that the oxygen molecules inhibit this fluorescence, in proportion to their concentration. After calibration, analysis of the grey levels gives an image of the 2D field of oxygen concentration.     

Inverse phase suspension polymerization of acrylamide in a batch oscillatory baffled reactor

We report, for the first time, our experimental investigation of inverse phase suspension polymerization of acrylamide in a batch oscillatory baffled reactor. In such a reactor, the oscillatory motion is achieved by moving a set of orifice baffles up and down the column at the top of the reactor. The effects of both operational and design parameters on the mean particle size and size distribution of polymer beads were investigated, including oscillation amplitude, oscillation frequency, baffle spacing, baffle free area, and monomer addition time. The experimental results indicated that the mean particle size and size distribution of the polymer beads depended predominantly on the product of oscillation frequency and amplitude, i.e., the oscillation velocity. The size distributions are narrow and of essentially a Gaussian distribution. The level of fines produced is consistently less than 1% for all 100 experiments performed. We demonstrated that the mean particle size and size distribution in an oscillatory baffled reactor can be controlled precisely by simply selecting the appropriate oscillation velocity. The effect of the baffle spacing on the mean particle size is much less compared with that of the baffle free area. The monomer injection time has a noticeable influence on the mean particle size, but the rate of change is relatively small. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1669–1676, 2000     

The Auger Parameter of Adsorbed Xenon as a Probe of Surface Polarizability

The change in Auger parameter, α (α = Auger kinetic energy minus photoelectron kinetic energy from core levels) between gas phase and adsorbed phase xenon provides an unambiguous measure of final state electronic relaxation in the surface toward the xenon ion. For many surfaces this relaxation is purely electronic polarization in the surface so that the Auger parameter provides a measure of the surface polarizability. Moreover, the Auger parameter for adsorbed xenon can be related to the relaxation energy in core level photoemission with high accuracy. Measurement of the xenon Auger parameter and its sensitivity to surface dielectric properties is illustrated by experiments on a series of oxygen treated Mo(100) surfaces.     

A measuring system for the time variation of size and charge of a single spherical particle and its applications

A single charged particle is trapped in a simple quadrupole electrode assembly and oscillated by means of a controlled electric field. A laser Doppler velocimeter (LDV) of the fringe mode is used for detecting the amplitude and phase lag of the particle oscillation with respect to the driving ac field. A unique method for the LDV signal processing is presented that takes the advantage of the sinusoidal oscillation of the particle at a known frequency. Superimposed to the ac drive, a dc drive is added for highly accurate measurements of particle size and charge. This enables us also to discriminate the polarity of charge without Bragg cells.In this paper, the basic principle of the method of the size and the charge measurements is explained and the accuracy of the measurements is demonstrated experimentally. The errors in the size and charge measurements, respectively, are less than 1% and 3% with a confidence coefficient of 99%. Since this apparatus repeats the measurements every 0.3 s for a single particle, the errors can be reduced to 0.1% and 0.3% when the measured values over a period of 30 s, or over 100 data are averaged.As some areas of its applications, experimental data are presented on the Rayleigh instability of evaporating charged droplets. It is shown that there are three types of Rayleigh fission. One of the types seemed to show occurrence of air breakdown around a micron sized spherical particle, which has not been reported so far. However experiments in highly insulating gas (SF₆) revealed that it was not the case but a type of Rayleigh fission. Nevertheless the experimental results gave important information on the charge limit of spherical solid particle due to electric breakdown in air at normal room conditions.Some cares to ensure the advantages of the present method are presented, and possible improvements of the apparatus are also suggested for future use.     

Measurement of Spatially Resolved Impedance Spectroscopy with Local Perturbation

The characteristics of the measured total impedance spectra of polymer electrolyte membrane fuel cells are often not representative of the performance of the cell. This is due in part mainly to two reasons; total impedance measurements are not representative of local phenomena and perturbation of the cell around the steady‐state polarization can result in oscillations of the gas concentrations downstream the flow channels that can affect the trend of the local spectrum. In this study, we overcome these two challenges by measuring the spectra of a segmented cell using local excitation of the segments. With these capabilities, we experimentally investigate the explanation given in works found in literature in regards to the oxygen oscillation in the channel and its effect on the spectra of locally perturbed segments. We further investigate the characteristics of the low frequency arc, measuring for the first time a loop in the spectrum around the transition frequency between the high and low frequency arcs. The characteristics of the spectra are investigated by varying the flow properties with a focus on the effect of air stoichiometry.     

The organized flow structure of an oscillating bubble plume

The detailed experimental investigation of an oscillating bubble plume created in a quasi-two-dimensional bubble column is reported for low void fraction and millimeter bubbles. The plume exhibits periodic oscillations and generates large-scale coherent structures in the liquid. The local and transient hydrodynamics of this bubbly flow was investigated with particle-image velocimetry (PIV) in the liquid phase and via optical fiber probe and the shadowgraph technique in the gas phase. First, long time averaging is performed (over a large number of plume oscillation periods). Thus, horizontal profiles of gas fraction and of horizontal and vertical components of mean and root mean square velocities in both phases can be examined. Then fluctuations of liquid velocities are studied, in terms of probability density function (indicating organized and random fluctuations), and in terms of phase-averaged components. Finally, proper orthogonal decomposition is applied to PIV data to extract coherent structure contributions more efficiently.     

Population balance modelling of droplet coalescence and break-up in an oscillatory baffled reactor

This paper presents new population balance analysis to describe simultaneous coalescence and break-up in the formation of methylmethacrylate droplets in a batch oscillatory baffled reactor. It is concluded that the droplet data can very well be described by a model in which coalescence is taken to be shear induced, selection for break-up proceeds at a rate proportional to droplet volume and approximately four equally sized break-up fragments are produced per break-up event. It is shown that the experimental droplet size distribution data are self-similar in form and exhibit asymptotic behaviour characteristics also seen in the model. The coalescence rate is found to vary as the square of the oscillation frequency and the selection rate to vary with the oscillation frequency to the power five. As a result the asymptotic mean droplet volume is inversely proportional to the oscillation frequency.     

Liquid-liquid equilibria in polymer blends–molecular weight effects

Flory's equation-of-state theory has been successfully used by several scientists to predict the qualitative nature of the liquid-liquid phase equilibria in polymer blends. This theory in its most simple form requires characteristic PVT parameters for each component plus an interaction energy parameter for the mixture. The melt titration technique was used to measure the miscibility of a series of polystyrene fractions (Mw = 4000 to 1,800,000) in low density polyethylene. Because the PVT properties of both of these polymers are well known, the experimental results could be compared with the predicted equilibrium compositions from Flory's equation-of-state theory using the interaction energy as an adjustable parameter. This comparison led to the conclusion that the interaction energy parameter may have some molecular weight dependence.     

The effect of bond formation on the tack of polymers

The tack of polymers to be used as adhesives is measured by a two-stage process of bond formation and bond separation. Bond formation is governed by the contact time, the contact force, the roughness of the surfaces, surface and interfacial tensions, and the mechanical or viscoelastic properties of the adhesive and substrate. This paper presents experimental studies of the contact formation of various model polymers on steel surfaces with well-defined and different degrees of roughness. The tack was measured with an instrument of the probe tack type, which determines the adhesive (interfacial) fracture energy per unit of interface as a measure of the tack and by means of which the most important parameters during bond formation and separation, such as the contact time, contact force, rate of separation, and temperature, can be adjusted and measured over sufficiently wide ranges. In the typical time interval for the contact time, the polymers are found in the plateau range of their viscoelastic spectrum. This means that entanglements strongly affect their bonding behaviour. Good agreement was found between the experimental results presented in this study and a model of contact formation on rough surfaces, published recently by Creton and Leibler [1], especially concerning the dependence of the adhesive fracture energy on the contact force and the contact time for smooth and rough substrate surfaces. The influence of the surface roughness becomes significant at low contact forces, where full contact is not yet developed on a rough substrate surface, and for polymers with comparatively high moduli. The fracture energy increases with the contact time and shows the same time dependence as the reciprocal modulus.     

Electrochemical impedance spectroscopy as a tool to measure cathodic disbondment on coated steel surfaces: Capabilities and limitations

The disbondment of protective organic coatings under excessive cathodic protection potentials is a widely reported coating failure mechanism. Traditional methods of evaluating cathodic disbondment are based on ex situ visual inspection of coated metal surfaces after being exposed to standard cathodic disbondment testing conditions for a long period of time. Although electrochemical impedance spectroscopy (EIS) has been employed as an effective means of evaluating various anti-corrosion properties of organic coatings; its application for assessing the cathodic disbondment resistance of coatings has not been sufficiently exploited. This paper reports an experimental study aimed at developing EIS into a tool for in situ measurement and monitoring of cathodic disbondment of coatings. A clear correlation between EIS parameters and the disbonded coating areas has been confirmed upon short term exposure of epoxy-coated steel electrodes to cathodic disbondment conditions; however the degree of this correlation was found to decrease with the extension of exposure duration. This observation suggests that EIS loses its sensitivity with the propagation of coating disbondment, and that in order to achieve quantitative determination of the coating cathodic disbondment localized EIS measurements are required to measure the parameters related to local disbonded areas.     

Deformation in adhesive joints using moiré interferometry

A wholefield optical method, moiré interferometry, is shown to be capable of measuring deformations in adhesive joints with very high sensitivity. A high frequency phase grating attached to the specimen is interrogated with an optically generated reference grating to reveal in-plane isothetics with a sensitivity of 0.83 μm/fringe. This technique thus enables local deformations within the adhesive bond to be investigated, as well as giving an overall deformation contour map.     

Frequency and phase estimation in time series with quasi periodic components

In this article, we consider frequency and phase estimation in a noisy oscillation with potentially non‐constant phase increments resulting from an underlying non‐constant frequency. A maximum periodogram method on segments is used to estimate the time‐varying frequency and a subsequent least squares approach to estimate the phase. A key problem addressed in this article is the question how to set up a meaningful concept of asymptotic statistics for this model. This problem is solved by a special infill asymptotics concept. We use this concept to prove consistency and asymptotic normality of the estimates. Furthermore, the phase estimate is compared to the Hilbert transform in a simulation.     

Effects of composition and phase relations on mechanical properties and crystallization of silicate glasses

Crystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter-relations of these properties as a function of glass composition have received little investigation. Soda-lime-silica glasses with Na₂O-MgO-CaO-Al₂O₃-SiO₂ compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X-Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro-Vicker's indentation, and single-edge-notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower-melting soda-lime-silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda-lime-silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda-lime-silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.     

Experimental study of oscillation behaviors in confined impinging jets reactor under excitation

Dynamic behaviors in a three‐dimensional confined impinging jets reactor (CIJR) under excitation were experimentally studied by a flow visualization technique at 75 ≤ Re ≤ 150. The effects of inlet Reynolds numbers (Re), excitation frequencies and excitation amplitudes on the oscillation behaviors in CIJR have been investigated by a particle image velocimetry (PIV) and a high‐speed camera. Results indicate that the excitation in the inflow of the opposed jets can induce periodic oscillation of the impingement plane along the axis, whose oscillation frequency is equal to the excitation frequency. At Re ≤ 100, the induced axial oscillation can further cause a deflective oscillation with a frequency nearly equal to the excitation, and the scale of the vortex in the impingement plane is well regulated by the excitation frequency. At Re = 150, the excitation of amplitude less than 20% has insignificant effect on the deflective oscillation existing in CIJR. A semiempirical formula has been proposed to predict the oscillation amplitude of the impingement plane in CIJR under excitation. © 2014 American Institute of Chemical Engineers AIChE J, 61: 333–341, 2015