Altering Bubble Dynamics via In Situ Vapor Extraction

Spatiotemporal cooling of electronics using latent energy might be achieved by closely spaced, rapid departure of small bubbles. One means to achieve small diameters during boiling is to provide an additional upward force during bubble formation, such as that from vapor extraction. Experiments were conducted of bubble extraction using constant flow rates of both air and vapor that ranged from 30 to 90 mm³/s. Extraction was achieved with a hydrophobic porous membrane sealed to a tube in which a vacuum was drawn. The gap between the extraction and supply surface was varied from 0.5 to 3.25 mm. Only individual bubbles that ruptured at the top surface while still attached to the supply surface were considered. Bubble departure diameters are approximately 80% of the gap height. As with unconfined bubbles in pool boiling, the bubble frequency varies inversely with departure diameter. Correlations for bubble rupture, bubble departure, and bubble frequency are presented as a function of gap height. Using the three distinct regimes identified in the experimental study, namely, growth only, growth with extraction, and extraction only, an effective bubble diameter model and an appropriate static force balance were developed. These were used to predict bubble departure frequencies and diameters, respectively, under confined extraction conditions.     

Analysis of Micro Vapor Bubble Growing Process in Open Capillary Microgrooves

The growing process of the individual microbubble in an open rectangular capillary microgroove was theoretically analyzed in this study. Several correlations of bubble growth rate for pool boiling were proved not available for microgroove boiling. A theoretical model based on thermal equilibrium and force balance was developed in this article. The growing process of the individual microbubble was divided into three stages: initial growing stage, normal and axial confined ellipsoidal growing stage, and axial subcylindrical growing stage. Growth period and volume increment of the micro vapor bubble were analyzed. The calculation results indicate that the growth of the micro vapor bubble is confined by the geometric structure of the microgroove. Comparison of the results between calculation and experiment shows that the correlation is available to predict the bubble growth rate for boiling in microgrooves.     

Numerical Simulation on Collapse of Vapor Bubble Using Particle Method

In this paper, the collapse of a void bubble filled with vapor content is numerically investigated using a novel moving particle semi-implicit with meshless advection by flow-directional local grid (MPS-MAFL) method. The interfacial velocity, collapse time, bubble shape variation, peak pressure, rebound bubble radius, and other interesting parameters were obtained and are discussed profoundly. The vapor bubble undergoes several cycles of oscillation with reduced amplitude during the whole collapse process, which is similar to cavitation bubble collapse. The computational results show that the bubble collapse time is linearly proportional to the initial bubble size, which agrees with the Rayleigh equation. The minimum rebound bubble radius ratio is less affected by initial bubble size for a large bubble. Comparison work was also conducted against experimental data by Board and Kimpton. The comparison revealed that the MPS method supplied with an adiabatic compression assumption for vapor content is more suitable to evaluate the collapse behaviors of a low-pressure vapor bubble. This work is helpful for further application of the moving particle semi-implicit with meshless advection using flow-directional local grid (MPS-MAFL) method to solving complicated bubble dynamics.     

Approximate Solution for the Heat-Transfer-Controlled Growth of a Steadily Translating Vapor Bubble

The existing analytical solution for the problem of the heat-transfer-controlled growth of a spherical vapor bubble moving with a constant velocity under the assumptions of a thin thermal boundary layer and potential flow results in a complicated integral equation for the bubble radius and is too unwieldy to be used in multiphase flow models. The goal of this work is to suggest for this problem an approximate solution that gives correct asymptotic behavior and yields a simpler expression for the bubble growth rate. Comparison with the exact solution showed that this way a good approximation can be obtained.     

珠江流域水汽输送特征分析

基于NCEP/NCAR逐日再分析资料及同期逐日降水数据,分析了珠江流域1981~2010年多年平均状况下水汽含量、水汽输送通量和水汽输送通量散度特征,并与流域降水分布特征做了对比分析。结果表明,多年平均水汽含量与年降水量空间分布较为一致,均从西北向东南逐渐增大,而降水时空分布更为复杂;水汽输送季节变化特征具有过渡性和周期性,与水汽输送通量散度变化特征相一致,与降水的季节变化相对应。     

Lattice Boltzmann Simulation of Growth and Deformation for a Rising Vapor Bubble Through Superheated Liquid

Combining with a lattice Boltzmann thermal model, a lattice Boltzmann multiphase model with a large density ratio can be extended to describe phase change with mass and heat transferring through the interface. Based on the Stefan boundary condition, the phase change is considered as a change of phase order parameter and is disposed as a source term of the Cahn-Hilliard equation. This hybrid model is applied to simulate the motion and growth of a rising vapor bubble through a uniformly superheated liquid. Meanwhile, the parametric effect on the bubble growth, deformation and rising in the different surface tension forces and kinetic viscosities are also presented.     

An Investigation on Transport Properties Near the Wall in Porous Media by Fractal Models

A comprehensive investigation on the wall effects on the transport properties, permeability, thermal conductivity, and thermal dispersion conductivity is performed, based on the fractal models for these properties and the porosity variations near the wall in porous media. The results show that the fractal models for transport properties of porous media can provide good agreement with the conventional models in the region near the wall in porous media. This indicates that the fractal models for transport properties of porous media also hold in the region near the wall in porous media if the wall effects are taken into account.     

Visualization of Heat Transport during Natural Convection Within Porous Triangular Cavities via Heatline Approach

In this article, natural convection in a porous triangular cavity has been analyzed. Bejan's heatlines concept has been used for visualization of heat transfer. Penalty finite-element method with biquadratic elements is used to solve the nondimensional governing equations for the triangular cavity involving hot inclined walls and cold top wall. The numerical solutions are studied in terms of isotherms, streamlines, heatlines, and local and average Nusselt numbers for a wide range of parameters Da (10^?5–10^?3), Pr (0.015–1000), and Ra (Ra = 10³–5 × 10⁵). For low Darcy number (Da = 10^?5), the heat transfer occurs due to conduction as the heatlines are smooth and orthogonal to the isotherms. As the Rayleigh number increases, conduction dominant mode changes into convection dominant mode for Da = 10^?3, and the critical Rayleigh number corresponding to the on-set of convection is obtained. Distribution of heatlines illustrate that most of the heat transport for a low Darcy number (Da = 10^?5) occurs from the top region of hot inclined walls to the cold top wall, whereas heat transfer is more from the bottom region of hot inclined walls to the cold top wall for a high Darcy number (Da = 10^?3). Interesting features of streamlines and heatlines are discussed for lower and higher Prandtl numbers. Heat transfer analysis is obtained in terms of local and average Nusselt numbers (Nu _l , Nu _t ) and the local and average Nusselt numbers are found to be correlated with heatline patterns within the cavity.     

Analysis of the Moisture and Heat Transport in a Wet Porous Sand Layer with Water Sucking Ability

The moisture content and heat transfer in a wet porous sand layer was influenced by the weather conditions while the layer was exposed outdoors. The changes in water content and temperature in the wet porous bed with water supplied due to the weather conditions differed from the case without water supplied via the bottom in the tests. A one‐dimensional mathematical model describing the heat and mass transfer in the unsaturated porous layer was used to analyze the change of the water content, temperature and rate of water evaporation or vapor condensation in the wet porous layer. As the ambient temperature, relative humidity and solar irradiation changed periodically and the gravity and capillary affected the water transport greatly in the wet porous media, variations of water content and temperature occurred cyclically in the wet porous layer that was exposed to outdoor conditions. In the wet porous bed, the rate of the water evaporation or vapor condensation was closely related to the temperature, gradient of the temperature along the depth and the rate of temperature variation over time. The particle size and porosity associated with the permeability had great impact on the water content and its variation range in the wet porous media with water sucking ability while the weather conditions changed periodically. The simulations of the water content and temperature variation in the sand bed agreed with the test data. All these results can be used to analyze the behavior of heat and moisture in the unsaturated porous layer under weather conditions.     

Marangoni Convection Heat and Mass Transport of Power-Law Fluid in Porous Medium with Heat Generation and Chemical Reaction

This article presents a study for Marangoni convection of power-law fluid in a porous medium driven by a power-law temperature and a power-law concentration with heat generation and first-order chemical reaction. It is assumed that the surface tension varies linearly with both the temperature and the concentration. The effects of power-law viscosity on temperature and concentration fields are taken into account bya modified Fourier law and Darcy's Law for power-law fluid. An approximate analytical solution is obtained using a homotopy analytical method, which is verified by numerical ones with good agreement. The transport characteristics of velocity, temperature, and concentration fields are analyzed in detail.     

Heatlines for Visualization of Heat Transport for Natural Convection within Porous Trapezoidal Enclosures with Various Wall Heating

A detailed study on natural convection heat transfer within porous trapezoidal enclosures has been carried out for two different cases. The effect of linearly heated side walls on flow pattern is investigated in case 1; whereas, the effect of linearly heated left wall and cold right wall is studied in case 2. In both cases, the bottom wall of the cavity is uniformly heated and the upper wall is adiabatic. The results are analyzed for a wide range of parameters such as Rayleigh number, Ra(10³ ≤ Ra ≤ 10⁶), Prandtl number, Pr(0.015 ≤Pr ≤ 988.24), and Darcy number, Da(10^?5 ≤ Da ≤ 10^?3).     

Numerical Simulation of Heat and Momentum Transport at the Coupled Interface between a Rectangular Channel and Porous Media

Porous media have been extensively used for the enhancement of heat transfer,due to their large specific surface area.In many cases,the flow and heat transfer i...     

Bubble generation in quiescent and co-flowing liquids

A numerical simulation has been accomplished to analyze the problem of dynamic bubble formation from a submerged orifice in an immiscible Newtonian liquid under the condition of constant gas inflow. We have considered two cases for the surrounding liquid, namely the liquid in a quiescent condition and the liquid as a co-flowing stream with the gas. The full cycle, from formation to detachment of the bubbles and the corresponding bubble dynamics, was simulated numerically by using a coupled level-set and volume-of-fluid (CLSVOF) method. The role of the liquid to gas mean velocity ratio, the Bond number and the Weber number in the bubble formation process was studied and the order of magnitude of forces involved in bubble dynamics are presented. Our simulation results show that the minimum radius of the neck decreases with a power law behavior and the power law exponent in a co-flowing liquid is less than 1/2 as predicted by the Rayleigh–Plesset theory for quiescent inviscid liquids. Single periodic and double periodic bubbling (with pairing and coalescence) regimes are observed in the present investigation. It is identified that a moderate co-flowing liquid may inhibit the bubble coalescence. The volume of the bubble and the bubble formation time decrease with increasing liquid to gas mean velocity ratios. For small Bond numbers, significant differences pertaining to bubble dynamics are observed between the co-flowing liquid and the quiescent liquid. Furthermore, the generation and breakup of the Worthington jet after bubble pinch-off and formation of tiny drops inside the detached bubbles are observed.     

Bubble growth with chemical reactions in microchannels

This work investigates the nucleation and growth of CO₂ bubbles due to chemical reactions of sulfuric acid and sodium bicarbonate in three types of microchannels: one with uniform cross-section, one converging, and another one diverging. The Y-shaped test section, composed of main and two front microchannels, was made of P-type 〈1 0 0〉 orientation SOI (silicon on insulator) wafer. Bubble nucleation and growth in microchannels under various conditions were observed using a high-speed digital camera. The theoretical model for bubble dynamics with a chemical reaction is reviewed or developed. In the present study, no bubble was nucleated at the given inlet concentration and in the range of flow rate in the converging microchannel while the nucleation and growth of bubbles were observed in the diverging and uniform cross-section microchannels. Bubbles are nucleated at the channel wall and the equivalent bubble radius increases linearly during the initial period of the bubble growth. The bubble growth behavior for a particular case, without relative motion between the bubble and liquid, shows that the mass diffusion controls the bubble growth; consequently, the bubble radius grows as a square root of the time and agrees very well with the model in the literature. On the other hand, for other cases the bubbles stay almost at the nucleation site while growing with a constant gas product generation rate resulting in the instant bubble radius following the one-third power of the time.     

黏性液体中锐孔处气泡的形成

考察一定流量气体,通过锐孔在静止黏性液体中连续溢出气泡的过程。应用动力学平衡半经验关系式,综合考虑气泡受力,分析气泡形成过程,给出合理假设,预测气泡直径。分析表面张力、气体流速、锐孔直径及液相物性对气泡脱离尺寸的影响,找到影响气泡脱离尺寸的主要因素。计算预报值与实验结果符合良好。     

Bubble induced heat transfer in gas fluidized beds

The influence of gas bubbles on heat transfer in gas fluidized beds has been investigated. A platinum wire has been used as a heat-transfer probe and the aggregative gas fluidized bed has been simplified by generating a single continuous stream of gas bubbles into an incipiently fluidized bed. It has been found that in the case of aggregative gas fluidized beds of small particles operating below the radiative temperature level, transient conduction into the emulsion phase is responsible for at least 90% of heat transfer and that the remainder is contributed by the superimposed gas convection. A theoretical model of the bubble induced heat transfer has been developed. Finally, experimental justification for the concept of the property boundary layer introduced in [2] is presented.     

Double‐Diffusive Convective Transport in a Nanofluid‐Saturated Porous Layer with Cross Diffusion and Variation of Viscosity and Conductivity

The onset of double‐diffusive nanofluid convection in a fluid‐saturated horizontal porous layer is studied with thermal conductivity and viscosity dependent on the nanoparticle volume fraction. The Darcy model has been used for the porous medium, while the nanofluid incorporates the effects of Brownian motion along with thermophoresis. The nanofluid is assumed to be diluted and this enables the porous medium to be treated as a weakly heterogeneous medium with variation in the vertical direction of conductivity and viscosity. In addition, the thermal energy equation includes regular diffusion and cross diffusion terms. The linear stability analysis is based on the normal mode technique, while for nonlinear analysis, minimal representation of the truncated Fourier series representation involving only two terms has been used. It is found that for the stationary mode the Soret parameter, Dufour parameter, viscosity ratio, and conductivity ratio have a stabilizing effect, while the solutal Rayleigh number destabilizes the system. For the oscillatory mode, the Soret parameter, Dufour parameter, and viscosity ratio have a stabilizing effect while the solutal Rayleigh number and conductivity ratio destabilize the system. For steady finite amplitude motions, the heat and mass transport decreases with an increase in the values of the Dufour parameter and solutal Rayleigh number. The Soret parameter enhances the solute concentration Nusselt number while it retards the thermal Nusselt number and concentration Nusselt number. The viscosity ratio and conductivity ratio enhances the heat and mass transports. We also study the effect of time on transient Nusselt numbers which is found to be oscillatory when time is small. However, when time becomes very large, all three transient Nusselt values approach a steady value. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(7): 628–652, 2014; Published online 11 November 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21102     

润滑油气泡对增压器轴承的危害

通过具体事例介绍大功率柴油机由于润滑系统安装不当，造成润滑油中产生过多气泡层膜，分析其对增压器轴承的危害和改进方法。