Experimental study on single bubble growth under subcooled, saturated, and superheated nucleate pool boiling

Nucleate pool boiling experiments with constant wall temperature were performed using pure R113 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5,000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. This study will provide good experimental data with precise constant wall temperature boundary condition for such works.     

Elongated bubble behavior during flow boiling in a microchannel

Elongated bubble flow is a unique flow pattern that occurs during flow boiling in a microchannel. The incepted bubbles quickly grow to the channel size and form long elongated bubbles due to the confinement of the microchannel. The flow regimes and heat transfer coefficients in the microchannel are closely related to the elongated bubble behavior. Numerical and experimental investigations were performed to study the behavior of elongated bubbles and their effect on flow boiling. Elongated bubble growth in a 100-μm-diameter microchannel was calculated by using a two-zone model, while flow visualization of elongated bubble behavior was obtained with a high-speed CCD camera. Comparisons between the calculated results and experimental observations showed that the growth of the elongated bubble was governed by the evaporation of the thin liquid film that formed between the vapor core and the heated channel wall.     

基于VOF的蒸汽泡溃灭过程数值研究

针对蒸汽泡在外界压力作用下溃灭的问题,基于Open FOAM平台对单个三维纯蒸汽泡在外界压力驱动下溃灭及回弹的动态变化过程进行了数值模拟研究;提出了一种运用流体体积(VOF)方法追踪相间的运动界面,并考虑表面张力、粘度和汽液两相的可压缩性,采用汽液相变模型模拟蒸发和冷凝等相变过程的计算方法,通过蒸汽泡与气体泡的结果对照得到了蒸汽泡溃灭的一系列特征。研究结果表明,该计算方法能有效模拟蒸汽泡的溃灭过程,捕捉到高温高压现象。在第一次溃灭过程中,蒸汽泡、气体泡半径变化与解析解保持一致,但蒸汽泡溃灭达到的最小半径则远小于气体泡,蒸汽泡中心点处压强远大于气体泡;在回弹阶段,蒸汽泡只回弹到了初始半径的25%左右,而气体泡回弹半径几乎与初始半径相同。     

Numerical study on the splitting of a vapour bubble in the process of EDM

Electrical discharge machining (EDM) is a powerful technique for machining of hard and brittle materials. In this process, because of electrical discharge, a vapour bubble is generated in the dielectric liquid between the tool and the workpiece. The growth and collapse phases of the vapour bubble have significant effect on the hydrodynamic behaviour of the dielectric liquid domain between the tool and the workpiece and cause molten material to escape from the crater. Previous numerical studies on the dynamics of an electrical discharge-generated vapour bubble have simulated the growth and collapse of the bubble until it has taken the shape of an hour-glass. This is necking phenomenon which is followed by splitting of the bubble into two parts. In this paper dynamics of an electrical discharge-generated vapour bubble between the tool and the workpiece after its splitting are investigated by using the boundary integral equation method. Development of a liquid jet on the boundary of the each of the upper and lower parts of the bubble and the impingement of the liquid jets on the nearby rigid surfaces are sought. This paper consists of two parts. In part one, the vapour bubble is initially located between the tool and the workpiece. Consequently the dynamic behaviour of the two parts of the bubble in the absence of the buoyancy forces are symmetric with respect to a horizontal axis through the mid-point between the tool and the workpiece. In part two, the elrecrical discharge-generated vapour bubble is initially located in the vicinity of the workpiece. Therefore during the necking phenomenon the upper part of the bubble is smaller than its lower part. Consequently the dynamic behaviour of the two parts of the bubble after its splitting are significantly different.     

Experimental study on bubble size distribution of subcooled flow boiling in cylinder head of internal combustion engines

Subcooled flow boiling is becoming an efficient and widely used heat transfer approach in internal combustion engine cooling systems. Bubble evolution behaviors are crucial for understanding the mechanism of subcooled flow boiling. In this study, a diesel engine test platform equipped with endoscopic high-speed photography system was built to investigate the characteristics of boiling bubble. Under various inlet liquid subcoolings and flow rates, the mean bubble diameters and bubble size distributions were measured and analyzed based on the image processing techniques. Most of the bubbles approximated the spherical and ellipsoidal shapes. The bubble size was distributed between 1 and 2.5 mm, and the bubble diameters increased with the decrease of the inlet subcooling and flow rate. The flow rate had a more significant effect on the formation of large boiling bubbles.

     

Experimental study on bubble motion in a rectangular bubble column using high-speed video observations

A two-phase flow in a rectangular bubble column of 100×20 mm cross-section and 1.5 mm height was studied using a high-speed video system. Series of images were taken at different elevations at a frequency of 500 Hz. The images were processed using a bubble recognition algorithm. In such a way, an individual bubble in the gas swarm could be tracked. The time-averaged velocity profiles and the turbulent diffusion coefficients were derived as a function of the superficial gas velocity.The lateral displacement of bubbles travelling over a certain vertical distance was transformed into a probability density distribution in order to measure the turbulent diffusion coefficient of the gaseous phase. The shape of the distributions obtained was found to fit well to the Gaussian standard distribution. The dispersion coefficients were observed to grow proportionally to the square root of the vertical distance. The diffusion coefficients were calculated from the proportionality factor and were compared with some correlations published in the literature. The experiments were performed for superficial gas velocities ranging from 1 to 6 mm/s. The bubbles were generated either by a porous sparger or a set of capillaries placed at the bottom of the column. The measurements were taken at different heights between 1 and 1.2 m where the bubble cloud was occupying the entire cross-section.     

Numerical study of coupling damage between contact fatigue and sliding wear

Journal of Mechanical Science and Technology - For mechanical elements subjected to the coupling damage from both contact fatigue and sliding wear, the traditional single failure criterion has...     

Numerical analysis of bubble motion under a pressure pulsing condition

Journal of Mechanical Science and Technology - The bubble motion in a compressible liquid under a pressure pulsing condition was studied numerically by solving the conservation equations of mass...     

Temperatures reached during sliding

Temperatures reached by thermocouples embedded in wax specimens have been measured, and the measured temperatures agree in some respects with those predicted by the classical equations.     

Numerical simulation of bubble resonance in an acoustic field

The resonant motion of a compressible bubble in an acoustic field is numerically investigated by using a level-set method, which is extended to examine compressible two-phase flows of air and weakly compressible liquid water. Computations are performed for bubble motion on a solid surface that is exposed to a periodic pressure wave. Numerical results show that the resonant bubble motion is significantly magnified by the surrounding compressible liquid when a specific height is reached. The effects of liquid height, contact angle, and bubble size on the oscillation amplitudes of bubble radius and pressure are quantified.     

Numerical investigation on the influence of surface tension and viscous force on the bubble dynamics with a CLSVOF method

We numerically investigated the rising of bubbles in a quiescent liquid layer. The numerical simulation is performed by solving the incompressible, multiphase Navier-Stokes equations via computational code in axisymmetric coordinates using a Coupled level-set and volume-of-fluid (CLSVOF) method. The numerical results show that the CLSVOF method with a novel algebraic relation between F and ? for axisymmetric two-phase flows not only can predict the bubble surface accurately, but also overcome the deficiency in preserving volume conservation. The effects of the Reynolds number Re and the Bond number Bo on the bubble deformation and its motion are investigated. The results show that with the increasing of Re (10 < Re < 150), the bubble shape transfers from oblate ellipsoidal cap to toroidal when Bo = 116. With the increasing of Bo (10 < Bo < 700), the bubble shape transfers from oblate ellipsoidal to toroidal when Re = 30. Although the toroidal bubble shapes are reached in these two cases, the transition modes are different. For the case Bo = 116, the bubble front is pierced by an upward jet from the rear of the bubble. While for the case Re = 30, the rear of the bubble is pierced by a downward jet from the front part.     

孔-槽沸腾强化结构的犁切-挤压试验研究

基于犁切-挤压加工方法,在2mm厚的铜板表面进行交错犁切-挤压加工,以加工具有孔-槽特征的沸腾强化结构。试验发现犁切-挤压的深度和进给量是影响结构成形的主要加工参数,以0.8mm和1.2mm为一次犁切-挤压的深度和进给量,并以1.68mm为二次犁切-挤压进给量时,加工出具有孔-槽特征的沸腾强化结构。     

Electrochemical noise measurements on stainless steel during corrosion-wear in sliding contacts

In this study, electrochemical noise measurements performed during corrosion-wear sliding tests with the working electrode coupled to a microelectrode, are presented. A microelectrode was used to record during fretting experiments current variations resulting from a modification of the working electrode induced by sliding. The tribocorrosion system investigated consists of an AISI 304 stainless steel plate sliding in a reciprocating mode against a corundum ball, both immersed in electrolytes of different pH. A detailed discussion of the in situ electrochemical noise measurements demonstrates that they are useful to identify and/or to unravel materials modification processes taking place during corrosion-wear sliding tests on passivating materials.     

六自由度并联机器人线性化反馈RBF神经滑模控制研究

采用分散控制的策略和线性化反馈RBF神经滑模的控制算法,实现了对六自由度并联机器人高精度自适应的轨迹跟踪控制。所设计的控制器实现了控制实时性和自适应性,有效抑制了滑动模态的抖振,仿真结果和实际实验均验证了此方案的实效性。     

Experimental study of heating surface angle effects on single bubble growth

Nucleate pool boiling experiments were performed using pure R11 for various surface angles under constant heat flux conditions during saturated pool boiling. A 1-mm-diameter circular heater with an artificial cavity in the center that was fabricated using a MEMS technique and a high-speed controller were used to maintain the constant heat flux. Bubble growth images were taken at 5000 frames per second using a high-speed CCD camera. The bubble geometry was obtained from the captured bubble images. The effects of the surface angle on the bubble growth behavior were analyzed for the initial and thermal growth regions using dimensional scales. The parameters that affected the bubble growth behavior were the bubble radius, bubble growth rate, sliding velocity, bubble shape, and advancing and receding contact angles. These phenomena require further analysis for various surface angles and the obtained constant heat flux data provide a good foundation for such future work.     

A study on pressure drop characteristics of refrigerants in horizontal flow boiling

An experimental investigation on the flow pattern and pressure drop was carried out for both an adiabatic and a diabatic two-phase flow in a horizontal tube with pure refrigerants R134a and R123 and their mixtures as test fluids. The observed flow patterns were compared to the flow pattern map of Kattan et al., which predicted well the present data over the entire regions of mass velocity in this study. The measured frictional pressure drop in the adiabatic experiments increased with an increase in vapor quality and mass velocity. These data were compared to various correlations proposed in the past for the frictional pressure drop. The Chisholm correlation underpredicted the present data both for pure fluids and their mixtures in the entire mass velocity range of 150 to 600 kg/m²s covered in the measurements, while the Friedel correlation was found to overpredict the present data in the stratified and stratified-wavy flow region, and to underpredict in the annular flow region.     

Direct observation of the interface during sliding tribo-corrosion

A new test apparatus has been developed to allow the sliding interface between the inert and “active” surfaces during tribo-corrosion to be viewed directly. This apparatus allows the nominal contact area to be viewed and will, it is hoped, eventually allow the real contact area and the role of wear debris to be clearly revealed. Initial experiments reciprocating glass plates against AISI 316 balls in 0.1 M Na₂SO₄ have shown that the corrosion current does not linearly depend on the wear scar area and that the electrochemical contribution to the total material loss increases with increasing sliding distance.     

轮轨滑动摩擦生热分析

利用ANSYS有限元软件建立轮轨滑动摩擦热-结构耦合简化模型,针对特定载荷、摩擦因数和相对滑动速度工况进行了轮轨的温度场和应力场研究.结果表明:轮轨处在滑动摩擦状态时,需考虑材料参数随温度变化的影响;热影响区分布于轮轨表面很薄层,并在此薄层产生很高的热应力;随着轴重力、摩擦因数和相对滑动速度的增大,轮轨的热效应越明显.轮轨滑动过程的热效应问题研究将有助于揭示接触过程中的表面磨损机理.     

Numerical study on the dynamics of an electrical discharge generated vapor bubble in EDM with different shapes of the tool and the workpiece

In this paper, the effect of the shapes of the tool and the workpiece on the dynamic behavior of an electrical discharge generated vapor bubble in the process of EDM (Electrical Discharge Machining) is investigated. The boundary integral equation method is employed for numerical simulation of the problem. Results show that in the case of the concave tool and workpiece the lifetime of the electrical discharge generated vapor bubble is longer than the case of the flat tool and workpiece. By increasing the concavity of the shapes of the tool and the workpiece, the lifetime of the bubble increases. Results also show that in the case of the convex tool and workpiece, the lifetime of the electrical discharge generated vapor bubble is shorter than the lifetime of the bubble in the case of the flat tool and workpiece. In this case also, by increasing the convexity of the shapes of the tool and the workpiece, the lifetime of the bubble decreases. Numerical results show that in the case of the concave tool and workpiece the rate of pressure drop on the surface of the workpiece is higher than the rate of pressure drop on the workpiece in the cases of the flat and convex tool and workpiece. The high rate of pressure drop on the surface of the workpiece leads to the ejection of more molten material from the crater and consequently leads to more material removal rate.