New concept: Low-pressure,wide-angle atomiser

Paper summarises results obtained in feasibility study of a novel pneumatic atomiser for generation of a spray of fine droplets. The new feature is pulsation of supplied air so that it issues in anti-phase from two nozzles acting on the water jet. The pulsation is generated using an integral no-moving-part fluidic oscillator. Tests involved recording by high-speed camera and laser-light scattering measurements of droplet size. Despite the low pressure (<10 kPa) of supplied air, the mean diameter of droplets was small – mere 158 μm – and the spray fan angle full 90 deg.     

正反馈式流体振荡器内部流动特性的数值模拟

为探索正反馈式流体振荡器内部流动特性,建立了正反馈式流体振荡器的三维数值模型。采用数值模拟的方法对流体振荡器的内部流动特性进行了研究,通过监测特征点的压力变化和特征面的质量流率变化,对振荡器的内部流动过程进行量化并分析其内部的流动过程和原理。结果表明,在柯恩达效应和涡流的共同作用下,该正反馈式流体振荡器内部流场呈周期性振荡,振荡腔和反馈通道中的流体流动过程具有良好的周期性和稳定性,且频率与振荡器振荡频率一致。     

Shape oscillations and path transition of bubbles rising in a model bubble column

We investigate experimentally the occurrence of shape oscillations accompanied by path transition of periodically produced air bubbles rising in water. Within the period of bubble formation, the induced velocity is measured to examine bubble-liquid and bubble-bubble interactions. The flow is produced in a small-scale bubble column with square-shaped cross section. A capillary aerator produces bubbles of size 3.4 mm at a frequency of 5 Hz. Measuring techniques employed are high-speed imaging to capture bubble shape oscillations and path geometry, and laser-Doppler anemometry (LDA) to measure the velocity in the liquid near the rising bubbles. The experimentally obtained bubble shape data are expanded in Legendre polynomials. The results show the occurrence of oscillations by the periodicity of the expansion coefficients in space. Significant shape oscillations accompanied by path transition are observed as the second-mode oscillation frequency converges to the frequency of the initial shape oscillations. The mean velocity field in the water obtained by LDA agrees well with potential theory. An analysis of the decay of the induced flow shows that there is no interaction between the flow fields of two succeeding 3.4 mm bubbles in the rectilinear path when the bubble production frequency is lower than 7.4 Hz.     

On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation

Microbubble generation by a novel fluidic oscillator driven approach is analyzed, with a view to identifying the key design elements and their differences from standard approaches to airlift loop bioreactor design. The microbubble generation mechanism has been shown to achieve high mass transfer rates by the decrease of the bubble diameter, by hydrodynamic stabilization that avoids coalescence increasing the bubble diameter, and by longer residence times offsetting slower convection. The fluidic oscillator approach also decreases the friction losses in pipe networks and in nozzles/diffusers due to boundary layer disruption, so there is actually an energetic consumption savings in using this approach over steady flow. These dual advantages make the microbubble generation approach a promising component of a novel airlift loop bioreactor whose design is presented here. The equipment, control system for flow and temperature, and the optimization of the nozzle bank for the gas distribution system are presented.     

Theoretical investigation of electronic structure and field emission properties of carbon nanotube–ZnO nanocontacts

A density functional theory study is performed to understand electronic structures and field emission properties of carbon nanotube–ZnO nanocontacts. The carbon nanotube–ZnO nanocontacts have high energetic stabilities and small energy gaps. The energy gaps of the nanocontacts exhibit an oscillatory behavior as a function of the length of carbon nanotubes. The ionization potentials of all carbon nanotube–ZnO nanocontacts are smaller than 7.155 eV of the ZnO nanocage. The ionization potentials of carbon nanotube–ZnO nanocontacts with more 2 carbon layers exhibit odd–even oscillation in the absence and presence of an electric field. The carbon nanotube–ZnO nanocontact with 4 carbon layers has a smallest ionization potential of 3.625 eV under 0.2 eV/Å external electric field. These results indicate that the field-emission properties of simplex ZnO and carbon nanotube materials can be enhanced significantly by the formation of carbon nanotube–ZnO nanocontacts.     

A method for liquid velocity measurement by using two interdigital transducers

A technique using a liquid delay line oscillator is presented for measuring the liquid velocity. The key element of the delay line oscillator is two interdigital transducers operating at a liquid-solid interface, which are used for the radiation or the detection of the longitudinal wave in the liquid. The oscillation frequency of the device changes with the liquid velocity. The validity of the operation principle is verified experimentally for the water velocity change.     

Observed electrochemical oscillations during the oxidation of aqueous sulfur dioxide on a sulfur modified platinum electrode

The electrochemical oxidation of aqueous sulfur dioxide has been investigated on a sulfur modified polycrystalline platinum electrode. It has been found that time and potential-based current oscillations occur on a modified electrode in the oxidation region >0.7 V vs. SHE. The oscillation is specific to catalytic oxidation on sulfur modified platinum and is not observed on an unmodified electrode. Using i–R compensation techniques, the oscillation has been found to stem from electrochemical origins; the interfacial potential being an essential variable. The influence of other controllable variables including holding potential, sulfur dioxide concentration and sulfuric acid concentration have also been investigated for their effects on the oscillation. The electrochemical oscillation is explained here in part by the intermediate formation of dithionate. Two possible mechanisms, including a dual pathway mechanism, are explored.     

Effect of oscillation amplitude on velocity distributions in an oscillatory baffled column (OBC)

This paper presents a numerical study of the effect of oscillation amplitude in oscillatory baffled column (OBC) using computational fluid dynamics. The numerical work was carried out for single phase liquid flow for an unsteady 3-D model using commercial software, Fluent (2006). This work was concentrated on the effect of oscillation amplitude. Three amplitudes of 5, 10 and 15 mm with constant frequency of 1 Hz are applied. Vortex and cycle average velocities at different points are analyzed. The studies show the maximum velocity for 5 mm, 10 mm and 15 mm in an OBC are 0.11 m/s, 0.25 m/s and 0.40 m/s respectively in the first cycle of oscillation. At a constant frequency, greater oscillation amplitude displaces the liquid to a further distance and builds a larger vortex. Vortex length was 1.5 times bigger when oscillation amplitude changes from 5 mm to 10 mm and 2 times when the amplitude is triple from 5 mm. The detailed validation is presented somewhere else; this research is focused on the effect of oscillation.     

Flow-injection amperometric glucose biosensors based on graphene/Nafion hybrid electrodes

In this research, we demonstrated the fabrication of flow-injection amperometric glucose biosensors based on RGO/Nafion hybrids. The nanohybridization of the reduced graphene oxide (RGO) by Nafion provided the fast electron transfer (ET) for the sensitive amperometric biosensor platforms. The ET rate (k_s) and the charge transfer resistance (R_CT) of GOx-RGO/Nafion hybrids were evaluated to verify the accelerated ET. Moreover, hybrid biosensors revealed a quasi-reversible and surface controlled process, as confirmed by the low peak-to-peak (ΔE_p) and linear relations between I_p and scan rate (ν). Hybrid biosensors showed the fast response time of ∼3 s, the sensitivity of 3.8 μA mM⁻¹ cm⁻², the limit of detection of 170 μM, and the linear detection range of 2–20 mM for the flow-injection amperometric detection of glucose. Furthermore, interference effect of oxidizable species such as ascorbic acid (AA) and uric acid (UA) on the performance of hybrid biosensors was prevented at the operating potential of −0.20 V even under the flow injection mode. Therefore, the fast, sensitive, and stable amperometric responses of hybrid biosensors in the flow injection system make it highly suitable for automatically monitoring glucose.     

Internal bias field relaxation in poled Mn-doped Pb(Mn1/3Sb2/3)O3–Pb(Zr,Ti)O3 ceramics

The frequency, electric field cycling and temperature dependences of the polarization–electric field (P–E), strain–electric field (S–E) loops in poled Mn-doped 0.05Pb(Mn_1/3Sb_2/3)O₃–0.50PbZrO₃–0.45PbTiO₃ ceramics have been investigated. The P–E and S–E loops are strongly asymmetric corresponding to the presence of an internal bias field E_i after poling and aging, indicating that the domain walls are strongly pinned by preferentially oriented defect dipoles formed by the acceptor dopant ions (Mn²⁺/Mn³⁺) and O²⁻ vacancies. Whereas, the loops exhibit a tendency of changing from asymmetric shapes to normal symmetric ones with increasing electric field amplitude or decreasing frequency. Repeated electric field cycling as well as high temperature results in a similar effect. Meanwhile, the E_i reduces consequently, providing evidence of domain depinning or internal bias field relaxation. It is suggested that the reorientation of the defect dipoles and depinning of domain walls arising from high temperature or electric field cycling are responsible for this extrinsic internal bias field relaxation process.     

Measuring local viscoelastic properties of complex materials with tapping mode atomic force microscopy

Tapping mode atomic force microscopy is a technique to measure the topography and properties of surfaces involving a micro-cantilever with a tip at one end that is excited into an oscillation near its resonance frequency. The phase lag between the excitation signal and the observed oscillation is sensitive to local mechanical properties under certain experimental conditions. We have found that by using silicon as an internal standard reference surface we can unambiguously relate the phase lag to local viscoelastic properties of a polymeric material. A model describing this relation has been built, validated with experimental data and finally inverted such that it can be used to determine local properties. This allows us to measure high frequency local viscoelastic properties on length scales as small as several nanometers. This technique works well for relatively compliant polymer surfaces with a shear modulus less than about 1 GPa.     

PDA experiments and CFD simulation of a lab-scale oxidation ditch with surface aerators

Though hydrodynamic characteristic is an important parameter for the optimization of reactors, very little research deals with the hydrodynamics of oxidation ditches. This paper focuses on the hydrodynamics of an oxidation ditch from the point of view of both experiments and simulations. The three-dimensional flow field in the oxidation ditch aerated with surface aerators like inverse umbrella is simulated with computational fluid dynamics (CFD). The two-fluid model and the standard k–? model are used for the turbulent solid–liquid two-phase flow. The distributions of liquid velocity and volume fraction of solid phase are obtained. Experiments are performed in a lab-scale oxidation ditch with particle dynamic analyser (PDA). The comparison between simulations and PDA experiments confirms that the results and the methods employed in this paper are reliable. The liquid and solid have similar flow velocity, while the vertical velocity of solid phase is slightly lower than that of liquid. With the increase of aerator speed, the velocity increases and solid phase disperses more evenly. The flow in the oxidation ditch has noticeable three-dimensional features, especially between the circular channel and the straight channel. And solid phase is ready to settle in these regions. All these results form the basis for the optimization and deep research of oxidation ditches.     

Glass Microspheres for Optics

Preparation and application of glass spheres as resonators are discussed. Surface-tension mold (StM) and localized-laser heating (LLH) techniques have been developed to fabricate glass super spheres and true spheres, respectively. These prepared glass spherical particles have been examined to evaluate their suitability for whispering gallery mode (WGM) oscillators or laser cavities. Super spherical glass acts as a WGM oscillator using its equatorial plane. An equatorial circle roundness > 0.99 provides a high Q-value to lead laser oscillation. LLH demonstrates its excellent capability to prepare “true spheres” on a transparent substrate under gravity. A tellurite glass sphere prepared by the LLH technique exhibits a few-mW-order threshold for lasing at the 1.06-μm region. An added-on terrace and an installed bubble in the sphere serve as an input/output gate and as a “non-WGM” coupling input gate for laser action, respectively. Glass is showed to have strong potential with respect to both preparation and utilization of spherical shapes for optics.     

On the new possibility of applying oscillating liquidmembrane systems for molecular recognition substances responsible for taste

It was previously suggested that liquid membrane oscillator systems might be used for the manufacture of tastesensors. The influence of substances responsible for taste belonging to bitterness, sweetness, sourness and saltiness on oscillation patterns of a liquid membrane oscillator with cationic surfactant benzyldimethyltetradecylammonium chloride was examined. It was concluded that independently of the nature of the membrane solvent present in the liquid membrane phase, the oscillation characteristics are sensitive to the investigated taste substances in a specific way.     

悬浮载体对不同曝气器氧转移特性的影响研究

以中试装置为研究对象,对比曝气器形式、通气量和悬浮载体填充率对曝气设备氧转移特性的影响。结果表明,微孔曝气器标准氧总转移系数(KLas)高于大孔曝气器;微孔与大孔曝气器的KLas随通气量的增加而提高,悬浮载体的增加有助于提高微孔曝气器的KLas,也可显著提高大孔曝气器的KLas。随着通气量增加或载体填充率的增大,微孔曝气器比标准氧转移效率(SSOTE)增大,优化通气量和填充率分别为≤40 m^3/h和≤40%;大孔曝气器的SSOTE显著增加,优化填充率为40%。随着通气量增加,微孔曝气器理论动力效率(Ep)保持不变或降低,且其通气量应≤32 m^3/h;而大孔曝气器Ep显著增加;二者优化填充率均为<40%。     

Unsteady auto-ignition of hydrogen in a perfectly stirred reactor with oscillating residence times

The ignition characteristics of a hydrogen–air mixture in a perfectly stirred reactor (PSR) with an oscillating residence time were investigated numerically. An unsteady numerical algorithm was developed and solved using a stiff-equation solver in order to investigate the unsteady auto-ignition behavior of the fuel/air mixture. The amplitude, frequency, and phase of the residence time oscillations were varied, and the effects on the simulated ignition behavior were recorded. Under small amplitude oscillations of the residence time, once ignited, the temperature in the reactor varied following the phase of the oscillations. Under larger amplitude variations, periodic ignition, and extinction events were observed. A critical frequency was observed, where the ignition delay time became significantly large than at the other frequencies. The existence of this critical frequency was found to depend on the phase of the residence time oscillation, and only occurred when the phase was such that the residence time decreased from the initial conditions. Ignition did not occur for frequencies of the oscillation in the residence time beyond 2.84 kHz, regardless of the phase. The physics of ignition delay for the case where the oscillatory residence time decreased initially could be clarified by investigating the time variation of characteristic chemical times of important reactions to ignition.At low frequencies of the residence time oscillation, similar behavior to that of the steady state was observed. However, the ignition delay time was found to be significantly different at high frequencies, especially for larger amplitude fluctuations in the residence time. Combustion of the fuel/air mixture could be sustained at shorter residence times under the oscillating residence time conditions than under the steady-state conditions. The reaction could not be sustained at high frequencies, and a pulsed-mode flame was observed, where the period of the ignition and extinction events was the same as the period of the oscillations in the residence time.The concentration of free radicals was found to increase with time prior to ignition, and the H radical concentration saturated at a maximum at the ignition time, indicating that the H radical concentration is a good indicator of ignition time under oscillating residence times.     

Particle fluctuations and dispersion in three-phase fluidized beds with viscous and low surface tension media

Particle fluctuations and dispersion were investigated in a three-phase (gas–liquid–solid) fluidized bed with an inside diameter of 0.102 m and height of 2.5 m. Effects of gas and liquid velocities, particle size (0.5–3.0 mm), viscosity (1.0–38×10⁻³ Pa s) and surface tension (52–72×10⁻³ N/m) of continuous liquid media on the fluctuating frequency and dispersion coefficient of fluidized particles were examined, by adopting the relaxation method base on the stochastic model. The fluctuations and dispersion of fluidized solid particles were successfully analyzed by means of the pressure drop variation with time, which was chosen as a state variable, based on the stochastic model. The fluctuating frequency and dispersion coefficient of particles increased with increasing gas velocity, due to the increase of bubbling phenomena and bed porosity in which particles could move, fluctuate and travel. The frequency and dispersion coefficient of particles showed local maximum values with a variation of liquid velocity. The two values of fluctuating frequency and dispersion coefficient of particles increased with increase in particle size, but decreased with increase in liquid viscosity due to the restricted movement and motion of particles in the viscous liquid medium. Both fluctuating frequency and dispersion coefficient of particles increased with decrease in surface tension of liquid phase, due to the increase of bubbling phenomena with decrease in σ_L. The values of obtained particle dispersion coefficient were well correlated in terms of dimensionless groups as well as operating variables.     

Current Oscillation of Piezoelectric–Ceramic Vibrators Driven by a Constant High Electric Field

Current oscillation will occur when a piezoelectric ceramic vibrator is driven by a high electric field. The mechanism of current oscillation is discussed, based on the results of the present experiment. Ring-shaped lead zirconate titanate (PZT) piezoelectric vibrators are driven by a 5.65 V electric field with a fixed frequency of slightly less than resonance, to form an oscillating current. The sample current, temperature, and phase between the driven electric field and sample current are recorded. The resonance frequency of PZT rings shifts in the direction of low frequency under a high electric field due to the heat produced by dissipated power, and then shifts back due to the effects of aging, which results in current oscillation. The oscillation phenomenon is explained according to the production of strong internal stresses due to abrupt change in temperature, and then, aging process through the rearrangement of domains to relieve various stresses.     

Experimental study of oscillation of axisymmetric turbulent opposed jets with modulated airflow

Oscillation behaviors of axisymmetric opposed jets with modulated airflow were experimentally studied. The oscillation frequency, the oscillation amplitude, and the movement velocity of the impingement plane at various nozzle separations, excitation frequencies, and exit turbulence intensities have been investigated by a hot‐wire anemometer and flow visualization technique combined with a high‐speed camera. Results show that the oscillation frequency of the impingement plane is nearly equal to the excitation frequency, whereas the oscillation amplitude decreases with the increase of the excitation frequency. The full‐scale amplitude oscillation occurs at low excitation frequencies and 2 ≤ L/D ≤ 8 (where L is the nozzle separation and D is the diameter of the nozzle exit). With the increase of the exit turbulence intensity caused by a turbulence generating plate, the oscillation amplitude decreases remarkably. Flow regimes of axisymmetric opposed jets with excitations are analyzed and discussed based on the experimental results. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4828–4838, 2013     

机械振荡和超声波对摆式反应器气液传质性能的影响

采用氢氧化钠水溶液吸收二氧化碳的方法,考察了机械振荡的摆频和超声波振荡对摆式反应器的气液传质性能的影响.结果表明,在无超声波作用条件下,当摆频小于117min-1时,加大摆频可显著提高反应器的总容积传质系数;当摆频大于117 min-1时,增大摆频对反应器的容积传质系数影响较小.在反应器不摆动和摆频较低的条件下,超声波振荡可提高反应器容积传质系数3～4倍,但在摆频较高的条件下,超声波振荡对反应器总容积传质系数的影响不明显.