核态池沸腾中气泡生长和脱离的动力学特征：气泡的脱离直径与？…

在前人工作的基础上提出了表征核态池沸腾中气泡脱离和生长过程的特征时间和特征尺度，并进而得到了气泡生长时间和气泡脱离直径的通用关系式，应用传热学类比方法建立了计算气泡脱离直径的一般公式。本文的研究配前人的实验结果甚为相符。     

核态池沸腾中气泡生长和脱离的动力学特征_气泡动力学研究回顾

系统地总结和分析了近半个世纪以来气泡动力学的研究进展情况,指出了气泡生长脱离研究中在的不足及今后的发展方向,以促进气泡动力学研究的深入发展。     

静止液体中浸没垂直导管口气泡形成实验

利用高速摄像技术研究气流通过浸没垂直导管口在液体中形成气泡的机理及其行为规律,分析导管内径、气体流量、导管口浸没深度和导管外径对气泡脱离直径的影响。结果表明:在导管内径分别为7、10和14 mm,气体流量在0~450 mL/s的条件下,气泡脱离直径随导管内径和气体流量的增加而增大;在浸没深度为0.05~1 m的条件下,导管口浸没深度对气泡脱离直径的影响很小可以忽略;当气体流量在100 mL/s以上,导管内径为10 mm、导管外径为14~26 mm时,随着导管外径的增加,气泡脱离直径减小。     

微小通道内气泡逸出行为的VOF模拟

针对微小型直接甲醇燃料电池阳极流场,采用VOF(volume of fluid)方法模拟了液体通流微小通道内壁面逸出气泡的动态行为,讨论了液体物性、气体流速、逸出气孔直径对气泡形成、生长及脱离等过程以及流动阻力的影响.结果表明:随着甲醉溶液浓度的升高,单个气泡的脱离体积、脱离时间和流动阻力系数均减小;气体流速增加,气泡...     

近壁面液膜层中微孔注气气泡动力学研究

为了降低近壁面沸腾过程中气泡动力学分析的不确定性,利用光谱共焦传感器构建了7 mm的液膜层,对近壁面液膜层中单孔注气气泡动力学进行了系统研究。实验测试段为270 mm×6 mm×12 mm(长×宽×高)的矩形通道,注气小孔的直径为0.49 mm,气体流量为0.30～27.00 mL/min,液体流量为72.00～324.00 mL/min。研究结果表明:在流动和静止液膜中,不同气体流量下气泡的接触线直径均先增大后减小;受到液体水平曳力的影响,相同气体流量下,气泡在水平流动液膜中的脱离频率比静止液膜中大,脱离体积比静止液膜中小,并且气泡会沿流动方向倾斜,气泡前进接触角与后退接触角的差值随着液体流量的增加而增加;近壁面液膜层中注气气泡动力学的实验研究精确地测量了气泡接触线直径以及接触角等形状参数,为相似工况下沸腾气泡的受力分析研究提供重要参考。     

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。     

微反应器流道内单气泡逸出动力学模拟

采用VOF方法,对不同流道结构下液体通流微小通道内壁面逸出气泡的形成、生长及脱离运动进行了数值模拟,讨论了槽道高宽比对气泡动力学行为的影响。结果表明:流道截面积不变时,气泡的脱离体积、脱离时间随槽道高宽比的减小呈现先增大,后减小的趋势,当高宽比为2时,气泡的脱离体积、脱离时间、槽道容积含气率和流动阻力因子均达到最大值。     

电场作用下冷气泡行为试验研究

气泡的生成、成长、脱离和上升等行为均是决定电场强化传热机理的主要过程,为获得电场作用下气泡生长的动态过程,研究了电场作用下冷态氮气泡的行为特性,利用高速摄像机拍摄了不同电场强度下的气泡生长试验图像,并对气泡脱离形态、周期和速度及加速度进行对比分析。试验结果表明,冷态氮气泡沿着场强方向拉长,呈圆柱体形状脱离壁面,气泡的脱离周期、速度与场强成正比,而加速度与场强成反比。     

非均匀润湿性表面的气泡动力学特性

为探究不同润湿性表面气泡动力学特性,本文采用FLUENT软件中VOF模型对四种不同的微结构下气泡的成长、脱离进行了数值模拟,分析了气泡脱离频率、生长速度、体积、气泡最高点等,获得了不同接触角匹配特性及微结构对气泡动力学特性的影响规律。数据分析表明,对于具有微结构的非均匀润湿表面,凹穴出口外部的表面润湿特性对气泡脱离起主导作用,凹穴结构本身的几何结构和润湿性对气泡脱离的影响相对较弱。     

单气泡沿过热曲面运动的格子Boltzmann模拟

基于一般化的插值补充格子Boltzmann方法和直角坐标系下的复合格子Boltzmann相变模型,构建了贴体坐标系下的复合LBM相变模型。利用Laplace定律及气泡在过热液体中的生长过程对该模型进行验证,所得计算结果与Laplace定律、Mikic解析解吻合良好。采用所构建的模型研究了不同Ja数下单气泡沿过热曲面运动的运动特性.结果表明：气泡上升过程经历了沿壁面滑移和脱离壁面两个阶段,且Ja数越大气泡生长越快,上升速度变化也越快。曲面导致的气泡形变会使得气泡速度变化产生波动,波动频率和幅度与Ja数成正相关。气泡并非一直生长变大,其在脱离壁面后,由于受到周围过冷液体冷凝开始变小;气泡对温度场的扰动程度也与Ja数成正相关,同时温度场的剧烈变化也反过来影响气泡的生长。     

A model to predict the effect of contact angle on the bubble departure diameter during heterogeneous boiling

Over the past eighty years, bubble release during heterogeneous boiling has been the subject of numerous investigations. However, current understanding of factors that influence this process is still incomplete. In this paper, a model is developed to describe the effect of contact angle on the bubble departure from an upward-facing horizontal surface. Based on the concept of macro- and micro-contact angles, this model gives an explicit theoretical relation between the bubble departure diameter and the contact angle. Agreement with previous experimental data confirms the predictive ability of the present model.     

Bubble dynamics in microchannels. Part II: two parallel microchannels

The present work investigates experimentally the bubble dynamics in two parallel trapezoidal microchannels with a hydraulic diameter of 47.7 μm for both channels. The fabrication process of the two parallel microchannels employs a silicon bulk micromachining and anodic bounding process. The results of this study demonstrate that the bubble growth and departure is generally similar to that in a single microchannel, i.e., bubbles, in general, grow linearly with time and their departure is governed by surface tension and drag due to bulk two-phase flow. For the two low mass flow rates, the growth of bubble in slug flow is also investigated. It is found that the bubble grows in the axial direction both forward and backward with its length increases exponentially due to evaporation of the thin liquid film between the bubble and heating wall. However, the coefficient of exponent is much smaller than that caused by evaporation due to the limitation effect of liquid pressure around the bubble.     

Bubble Dynamics and Heat Transfer during Pool and Flow Boiling

A large number of studies of bubble growth rate and departure diameter have been reported in the literature. Because of uncertainty in defining the shape of an evolving interface, empirical constants are invariably used to match the model predictions with data. This is especially true when force balance is made on a vapor bubble to determine the departure diameter. In this paper, the results of an alternate approach based on a complete numerical simulation of the process are given. Single and multiple bubbles are considered for both pool and flow boiling. The simulations are based on the solution of the conservation equations of mass, momentum, and energy for both phases. Interface shape is captured through a level set function. A comparison of bubble shape during evolution, bubble diameter at departure, and bubble growth period is made with data from well-controlled experiments. Among other variables, the effect of magnitude of gravity and contact angle is explicitly investigated.     

Numerical and experimental investigation of heat transfer on heating surface during subcooled boiling flow of liquid nitrogen

Closure correlations describing bubble nucleation and departure on the heating surface is indispensable when modeling subcooled boiling flow using a two-fluid model. Due to the small contact angle and surface tension, nucleation and departure of nitrogen vapor bubble has different characteristics to those of high-boiling liquids. For the purpose of accurate two-fluid model formulation, these factors have to be taken into consideration. In this study, some closure correlations of the bubble departure diameter, active site density and bubble waiting time were tested in the frame of the two-fluid model and the CFX code. Benchmark experiments were then performed to evaluate the correlations. Comparison of the numerical results against the experimental data demonstrates that the surface tension is crucial to modeling the bubble departure diameter and the active site density. The bubble waiting time correlation formulated according to bubble growth is expected to be used as a criterion of judging the transition from subcooled to saturated boiling.     

Numerical Analysis of Bubble Growth and Departure from a Microcavity

A numerical approach is presented for analysis of bubble growth and departure from a microcavity during nucleate boiling. The level-set formulation for tracking the phase interfaces is modified to include the effect of phase change on the liquid–vapor interface and to treat the no-slip and contact angle conditions on the immersed (or irregularly shaped) solid surface of the microcavity. Also, the formulation is coupled with a simple and efficient model for predicting the evaporative heat flux from the liquid microlayer on an immersed solid surface. The effects of cavity size and geometry on the bubble growth and departure in nucleate boiling are investigated.     

A numerical investigation of bubble growth on and departure from a superheated wall by lattice Boltzmann method

The bubble growth on and departure from a superheated wall has been simulated by an improved hybrid lattice Boltzmann method. The Briant’s treatment of partial wetting boundary was introduced and the new hybrid model was validated by the single bubble growth on and departure from the superheated wall. The results showed that parametric dependencies of the bubble departure diameter were in good agreement with the experimental correlation from some recent literatures. This new model was also employed to simulate twin-bubble growth, coalescence on and departure from a horizontal superheated wall.     

Development of bubble departure and lift-off diameter models in low heat flux and low flow velocity conditions

The forces acting on a bubble were analyzed to determine the criteria for bubble departure and lift-off from a heated wall. Mechanistic models for the bubble departure and lift-off diameters were developed from the analysis results to predict the evaporation heat flux under low heat flux and low flow velocity conditions. The developed model for the departure diameter depends strongly on the contact angle, but the model for the lift-off diameter was found to be a function of the lift-off number, which indicates the ratio between the shear lift force and the growth force. Wall boiling experiments were performed to measure bubble departure and lift-off diameters using digital image processing. From the experimental data, the developed models were validated, and the results showed good agreement between the experimental data and the predicted diameters.     

Dynamics of bubble formation and detachment from an immersed micro-orifice on a plate

Visualization experiments were carried out in the present study to investigate formation and detachment of the bubbles developing from an immersed micro-orifice on a plate in a stagnant and isothermal liquid. A sub-fitting method was proposed to describe the bubble edges and to extrude the bubble characteristics. Taking the microscale effect into consideration, the dynamic behavior of bubbles emerging from various orifices with 0.5, 0.12 and 0.054 mm in diameter was analyzed and compared. The experimental results showed that the bubble waiting time, departure time and departure volume decreased with decreasing orifice diameter. Based on the analysis on actual gas flow rate at the orifice, the evolution of bubble formation process was described by three or four stages for different orifices. The bubble formation under the condition of 0.5 mm orifice mainly experienced the nucleation stage, while the steady growth stage was the dominator for the micro-orifices with 0.12 and 0.054 mm in diameter. The rupture scene and evolution of bubble contact ring at the detachment moment were found to significantly vary with the orifice diameter. The inrush of several trailing bubbles into the detached leading bubble was observed for the orifices with 0.12 and 0.054 mm in diameter, resulting in a significant fluctuation in the interface.     

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.     

Bubble growth,departure and the following flow pattern evolution during flow boiling in a mini-tube

In the present study, the bubble growth, departure and the following flow pattern evolution during flow boiling in the mini-tube were visualized and quantitatively investigated, along with the simultaneous measurement of the local heat transfer coefficient around a specified nucleation site. Liquid nitrogen was employed as the working fluid and the test section was a segment of vertically upward quartz glass tube with the inner diameter range of 1.3–1.5 mm, which was coated by a layer of transparent ITO film as the heater on the outer surface. The growth rates of bubbles had similar and constant growth rate in two periods of time, i.e., before and after the bubbles departing from the nucleation site, which indicated the bubble growth was primarily governed by the inertial force. The bubble departure diameter and bubble period were investigated and the corresponding correlation was obtained based on the experimental data, which showed that the tube size of the mini-tube had no notable effect on the bubble departure and the trend of the bubble departure was similar to that in macro-tubes. Whereas the following flow pattern evolution was apparently confined due to the size effect, which presented desirable heat transfer performance in mini-tubes. The heat transfer coefficients for different flow patterns along the mini-tube were obtained in terms of bubbly, slug, annular flow and the flow regimes of flow reversal and post dryout. It was found that the dominant heat transfer mechanism was the liquid film evaporation which offered desirable heat transfer capability. The heat transfer performance would be deteriorated in the post dryout regime, while flow reversal could somewhat enhance the heat transfer upstream of the nucleation site. Boiling curves around the specified nucleation site were recorded and analyzed based on the recorded flow patterns.