Dynamic Behavior of Bubble Interface During Boiling

A brief review with discussions is conducted for some pertinent works, done and ongoing in the Laboratory of Phase-Change and Interfacial Phenomena at Tsinghua University, on interfacial behavior of vapor bubbles and interfacial transport phenomena during liquid nucleation boiling. From a sequence of experimental investigations, some new phenomena, particularly, the visually observed interfacial transport phenomena or processes including jet-like flows, bubble interaction and spatial scale effect, were described in this article. The interfacial effects and transport phenomena associated with surface tension gradients caused by temperature and concentration variations were theoretically analyzed to reveal the marked influence on bubble interfacial shape and dynamic behavior, the bubble dynamics including nucleation, bubble motion and coalescence. Several theoretical models and methods were proposed to describe the dynamic characteristics and explain the physics of interfacial phenomena/processes. The spe     

Bubble generation in micro-volumes of “nanofluids”

We report experimental investigation of a transparent flat mini evaporator heated by laser beam. The influence of non-absorbing and absorbing nanoparticles immersed in pure water, and heat absorbing fluid on the heat transfer intensification was analyzed. Nanoparticles may initiate vaporization and boiling of fluid at low heat input. Providing specific task and conditions the nanoparticles may be used in passive or active modes. Passive mode assumes that nanoparticles do not generate thermal energy and improve bubble nucleation conditions due to additional nucleation of the fluid, thus decreasing boiling/vaporization temperature thresholds. Active mode assumes that nanoparticles act as converters of optical energy into thermal one.     

Bubble小波在高温熔体图像分割中的应用

在基于CCD图像传感器构成的高温测量系统中，为了计算高温目标的温度场，必须将高温目标图像与其背景图像准确分割。把通常用于图像边缘检测的小波模极大值取阈值法直接应用于目标图像的分割，设计一种基于Bubble小波变换的高温熔体图像分割算法，实验结果表明能快速准确地分割出目标图像，有很强的实用性。     

Experimental Investigation of Flow Instabilities in a Laminar Separation Bubble

The present paper reports the results of a detailed experimental study aimed at investigating the dynamics of a laminar separation bubble, from the origin of separation up to the breakdown to turbulence of the large scale co- herent structures generated as a consequence of the Kelvin-Helmholtz instability process. Measurements have been performed along a fiat plate installed within a double contoured test section, designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles, which induces the formation of a laminar separation bubble at low Reynolds number condition. Measurements have been carried out by means of comple- mentary techniques： hot-wire （HW） anemometry, Laser Doppler Velocirnetry （LDV） and Particle Image Veloci- metry （PIV）. The high accuracy 2-dimensional LDV results allow investigating reverse flow magnitude and both Reynolds normal and shear stress distributions along the separated flow region, while the high frequency response of the HW anemometer allows analyzing the amplification process of flow oscillations induced by instability mechanisms. PIV results complement the flow field analysis providing information on the generation and evolu- tion of the large scale coherent structures shed as a consequence of the separated shear layer roll-up, through in- stantaneous velocity vector maps. The simultaneous analysis of the data obtained by means of the different meas- uring techniques allows an in depth view of the instability mechanisms involved in the transition/reattachrnent processes of the separated shear layer.     

Numerical Simulation of Sub-cooled Cavitating Flow by Using Bubble Size Distribution

A new cavitating model by using bubble size distribution based on mass of bubbles is proposed. Liquid phase is treated with Eulerian framework as a mixture containing minute cavitating bubbles. Vapor phase consists of various sizes of minute vapor bubbles, which is distributed to classes based on their mass. The change of bubble number density for each class was solved by considering the change of bubble mass due to phase change as well as generation of new bubbles due to heterogeneous nucleation. In this method the mass of bubbles is treated as an independent variable, in other word, a new coordinate, and dependant variables are solved in Eulerian framework for spatial coordinates and bubble-mass coordinate. The present method is applied to a cavitating flow in a convergent-divergent nozzle, and the two-phase flow with bubble size distribution and phase change was successfully predicted.     

Heat Transfer and Hydrodynamic Investigations of a Baffled Slurry Bubble Column

Heat transfer and hydrodynamic investigations have been conducted in a 0.108 m internal diameter bubble column at ambient conditions. The column is equipped with seven 19mm diameter tubes arranged in an equilateral triangular pitch of 36.5 mm. A Monsanto synthetic heat transfer fluid, Therminol-66 having a viscosity of 39.8 cP at 303 K, is used as a liquid medium. Magnetite powders, average diameters 27.7 and 36.6 μm, in five concentrations up to 50 weight percent in the slurry, are used. As a gas phase, industrial grade nitrogen of purity 99.6 percent is employed.Gas holdup in different operating modes and regimes have been measured for the two- and three-phase systems over a superficial gas velocity range up to 0.20 m/s in the semi-batch mode. Heat transfer coefficients are measured at different tube locations in the bundle at different radial and vertical locations over a range of operating conditions. All these data are compared with the existing literature correlations and models. New correlations are proposed.     

High-Speed Visualization of Bubble Behaviors for Pool Boiling of R-141b

A visualization study on the behavior of bubbles has been carried out for pool boiling of R141b on a horizontal transparent heater at pressure 0.1 MPa. The behaviors of bubbles were recorded by a high-speed camera placed beneath the heater surface. The departure diameter, departure time of bubbles and nucleation site density at different heat flux were obtained. The visualization results show that bubble departure diameter and departure time decrease , while the nucleation site density increases as the heat flux increases. It is also observed that there is no liquid recruited into the microlayer in the experiment. Based on the experimental results, boiling curve for R141b was predicted by using the dynamic microlayer model. As a result, the agreement between the predictive result based on the dynamic microlayer model and the experiment data for boiling curve of R141b is good at high heat flux.     

Electrical conductivity in Li–Si–P–O–N oxynitride thin-films

Nitrogen-incorporated lithium silicophosphate (LiSiPON) thin-film electrolytes, which contain two glass-forming elements, are fabricated by sputtering from a (1−x)Li₃PO₄·xLi₂SiO₃ target in a nitrogen reactive plasma. The results of impedance measurements show that the activation energy for conduction decreases as the Si content increases, which leads to an increase in the ionic conductivity of the films. It is suggested that these improvements in the electrical properties of the films are due to the combined effect of the mixed former and nitrogen incorporation. It appears that the decomposition potential of the electrolyte film in contact with Pt is about 5.5 V.     

Electrochemical behavior of Mg–Li,Mg–Li–Al and Mg–Li–Al–Ce in sodium chloride solution

Mg–Li, Mg–Li–Al and Mg–Li–Al–Ce alloys were prepared and their electrochemical behavior in 0.7 M NaCl solutions was investigated by means of potentiodynamic polarization, potentiostatic current–time and electrochemical impedance spectroscopy measurements as well as by scanning electron microscopy examination. The effect of gallium oxide as an electrolyte additive on the potentiostatic discharge performance of these magnesium alloys was studied. The discharge activities and utilization efficiencies of these alloys increase in the order: Mg–Li < Mg–Li–Al < Mg–Li–Al–Ce, both in the absence and presence of Ga₂O₃. These alloys are more active than commercial magnesium alloy AZ31. The addition of Ga₂O₃ into NaCl electrolyte solution improved the discharging currents of the alloys by more than 4%, and enhanced the utilization efficiencies of the alloys by more than 6%. It also shortened the transition time for the discharge current to reach to a steady value. Electrochemical impedance spectroscopy measurements showed that the polarization resistance of the alloys decreases in the following order: Mg–Li > Mg–Li–Al > Mg–Li–Al–Ce. Mg–Li–Al–Ce exhibited the best performance in term of activity, utilization efficiency and activation time.     

Microstructure and electrochemical properties of Zr–Ti–V–Ni–Mn–LM alloys for application in Ni–MH secondary battery

A series of multi-component Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4; y=0.0,0.02,0.05,0.1,0.2,0.3, LM; lantanum-rich-mischmetal) alloys are prepared and their crystal structure and P–C–T curves are analyzed. The alloys have been modified by adding LM and their gaseous and electrochemical hydrogenation properties are studied to find out the effect of LM elements. Also, the second phase and initial activation performance are investigated. The Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3,0.4; y=0.0,0.02,0.05,0.1,0.2,0.3) alloys have C14 Laves phase hexagonal structure, so the volume expansion ratio of lattice parameters with LM has increased. As the amount of LM in alloy has increased, correspondingly the second phase is also increased. The second phase is LM, Ti and V-rich. The second phase improve the activation of La-rich misch-metal, and also the concentration of elements Ti, V〉LM〉 matrix in alloys.The addition of LM in Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4) alloys have increased the activation rate and hydrogen storage capacity significantly, but the plateau pressure and the discharge capacity have been decreased due to the formation of second phase. For more Zr in electrode alloys, the activation of rate becomes slow.     

Features of Boiling on an Artificial Surface—Bubble Formation, Wall Temperature Fluctuation and Nucleation Site Interaction

The artificial surfaces are applied to study the pool boiling features, including the bubble behaviors, the surface temperature fluctuation, the heat transfer characteristics and nucleate site interaction. Three sets of experiments are carried out to investigate the influences of cavity shape, cavity size, cavity spacing on the boiling phenomena. Experimental results reveal that bubbling from the cylindrical as well as reentrant cavity is generally stable. The influence of cavity diameter on the bubble behaviors and the temperature fluctuation seems very weak while the effect of cavity depth cannot be neglected. As for the two cavity conditions, the bubble behaviors show the different features depending on the dimensionless cavity spacing. Three significant factors (thermal interaction, hydraulic interaction, bubble coalescence) control the nucleation site interaction, and the competition and dominance of the factors yield four interaction regimes.     

Preparation and performance of novel Li–Ti–Si–P–O–N thin-film electrolyte for thin-film lithium batteries

Novel Li–Ti–Si–P–O–N thin-film electrolyte was successfully fabricated by RF magnetron sputtering from a Li–Ti–Si–P–O target in N₂ atmosphere at various temperatures. XRD, SEM, EDX, XPS, and EIS were employed to characterize their structure, morphology, composition and electrochemical performances. The films were smooth, dense, uniform, without cracks or voids, and possessed an amorphous structure. Their room temperature lithium-ion conductivities were measured to be from 3.6 × 10⁻⁷ S cm⁻¹ to 9.2 × 10⁻⁶ S cm⁻¹, and the temperature dependence of the ionic conductivities fits the Arrhenius relation. This kind of electrolyte possessed good properties is a promising candidate material for solid-state thin-film lithium batteries.     

Li–B–O–N electrolytes for all-solid-state thin film batteries

We report for the first time new Li⁺ ion conducting thin film solid electrolytes based on Li–B–O–N system. Substitution of oxygen in xLi₂O–B₂O₃ (x = 1, 3, 5) by nitrogen was successfully achieved by reactive sputtering under nitrogen plasma. FTIR and XPS analyses indicated that N atom was incorporated in the Li–B–O matrix film, and an increase in the composition of Li₂O together with N-substitution caused the structural conversion from ring-type borate to open structured one, where more free space and ionic bonding characteristic are offered for higher mobility of Li⁺ ions. A high ionic conductivity of ca. 2.3 × 10⁻⁶ S cm⁻¹ at room temperature was obtained from the thin film electrolyte of Li_3.09BO_2.53N_0.52 glass that was prepared using 3Li₂O–B₂O₃ target. Electrochemical analyses suggest the high Li⁺ ion conductivity is induced by the reduced activation energy through the control of the composition and the structure.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

Hydrogen absorption characteristics of mechanically alloyed Ti–Zr–Ni and Ti–V–Ni powders

A first investigation into the production of amorphous and nanostructured Ti-based alloys with nominal compositions Ti_41.5Zr_41.5Ni₁₇, Ti₆₁Zr₂₂Ni₁₇, Ti_41.5V_41.5Ni₁₇ and Ti₆₁V₂₂Ni₁₇ by mechanical alloying (MA) technique is presented. This technique was adopted to produce alloys' powders with high fresh surface area that were active for hydrogen storage. Hydrogen absorption characteristics and structure changes in the alloys after hydrogenation were investigated. Gas phase hydrogenation of the Ti–Zr–Ni alloys, at 573 K and an initial hydrogen pressure of 2 MPa, exhibited good hydriding properties and started at a maximal rate without induction period with a hydrogenation capacity up to 1.2 wt%. However, hydriding of Ti–V–Ni alloys at the same conditions exhibited slower rates. The Ti₆₁V₂₂Ni₁₇ composition showed high hydrogen absorption capacity of 1.8 wt% and exceeded 4 wt% at 345 K. In addition, the Ti–V–Ni alloys showed structure stability after hydrogenation and retained the amorphous structure.     

Polymer Ni–MH battery based on PEO–PVA–KOH polymer electrolyte

An alkaline polymer electrolyte film has been prepared by a solvent-casting method. Poly(vinyl alcohol), PVA is added to improve the ionic conductivity of the electrolyte. The ionic conductivity increases from 10⁻⁷ to 10⁻² S cm⁻¹ at room temperature when the weight percent ratio of poly(ethylene oxide), PEO to PVA is increased from 10:0 to 5:5. The activation energy of the ionic conductivity for the PEO–PVA–KOH polymer electrolyte is 3–8 kJ mol⁻¹. The properties of the electrolyte film are characterized by a wide variety of techniques and it is found that the film exhibits good mechanical stability and high ionic conductivity at room temperature. The application of such electrolyte films to nickel–metal-hydride (Ni–MH) batteries is examined and the electrochemical characteristics of a polymer Ni–MH battery are obtained.     

Crystallographic and electrochemical characteristics of melt-spun Ti–Zr–Ni–Y alloys

Ti₄₅Zr₃₀Ni₂₅Y_x (x = 1, 3, 5 and 7) alloys were prepared by melt-spinning at wheel velocity of 20 m s⁻¹. The effect of additive Y on phase structure and electrochemical performance of melt-spun alloys was investigated. Ti₄₅Zr₃₀Ni₂₅Y_x melt-spun alloys were composed of I-phase and amorphous phase. The amorphous phase increased with increasing x value, indicating amorphous forming ability improved with increasing Y content. The maximum discharge capacity and high-rate dischargeability decreased with increasing x value, which may be ascribed to the decrease of nickel content. Cycling stability first increased with increasing x from 1 to 3, and then decreased when x increased to 7, which was resulted from the combined effect of the decrease of nickel content and the increase of amorphous phase.     

Alkaline composite PEO–PVA–glass-fibre-mat polymer electrolyte for Zn–air battery

An alkaline composite PEO–PVA–glass-fibre-mat polymer electrolyte with high ionic conductivity (10⁻² S cm⁻¹) at room temperature has been prepared and applied to solid-state primary Zn–air batteries. The electrolyte shows excellent mechanical strength. The electrochemical characteristics of the batteries were experimentally investigated by means of ac impedance spectroscopy and galvanostatic discharge. The results indicate that the PEO–PVA–glass-fibre-mat composite polymer electrolyte is a promising candidate for application in alkaline primary Zn–air batteries.     

Synthesis and electrochemical properties of layered Li–Ni–Mn–O compounds

An attempt to prepare solid solutions in the system of LiNiO₂, LiMnO₂ and Li₂MnO₃ was performed by heating metal acetates. The solid solutions between end members LiNiO₂ and Li₂MnO₃ can be successfully prepared in the overall compositional ranges. Both the structure and capacity were compared based on Rietveld analysis and electrochemical investigation on solid solutions between LiNiO₂ and Li₂MnO₃. The result showed that the cationic disorder as well as capacity was closely related to the ratio of Li, Mn and Ni in formula. The investigation of chronopotentiogram and ex situ XRD on the solid solutions indicated that the complex phase transitions in LiNiO₂ during delithiation were strongly suppressed with low Mn content (Mn/(Mn+Ni) ratio was 0.1 or 0.2) and completely suppressed with the ratio more than 0.5.