大长宽比矩形微通道内的气泡生长与相变传热分析

研究了液体工质FC-72在大长宽比矩形截面微通道内流动沸腾过程中的单个气泡生长情况。选取的微通道水力直径为571、762和1 454μm,通道截面的长宽比分别为20、20和10。实验中运用透明加热技术,在对微通道表面加热的同时实现了通道透明可视化。观测获取了完整气泡生长周期内的三个生长阶段,探讨了气泡形状变化的主导因素。此外,气泡生长过程中几何形状的演变与通道截面长宽比密切相关。讨论了气泡当量半径、气泡长宽比等随热流密度、面积质量流量以及微通道尺寸和形状等的变化趋势。此外,还探讨了基于气泡体积增长得到的蒸发热流密度及其影响因素,对气泡生长过程中的热量传递进行了初步的分析。研究表明,蒸发热流密度随着气泡的生长逐渐升高,当气泡开始快速指数增长后蒸发热流密度高于微通道壁面的热流密度。另外,蒸发热流密度随着面积质量流量的增加而有所降低。     

Multiphase mass transport in mini/micro-channels microreactor

This paper describes a computational study of two-phase gas/liquid flow in mini/micro-scale reaction channels at low Reynolds numbers. The direct fluorination of toluene is used as a sample process. We consider two different configurations, a falling film and membrane microreactor. The detailed mathematical model of the processes in these configurations is based on mass and momentum conservation equations, which are solved numerically using the finite element method.

Gas-phase mass transport in both reactor configurations is analysed by means of the mathematical model. For fully developed gas flow a correlation for the gas-phase mass transport is developed in terms of the Sherwood and the relative Reynolds number. It is shown that the flow pattern in this regime and entrance effects strongly influence mass transport from the bulk flow to the reaction plane. The velocity profile for the falling film reactor yields higher Sherwood numbers compared to the membrane reactor. The latter has the advantage over the falling film reactor that the gas and liquid phases are decoupled and operating conditions and channel design can be freely chosen.     

Impact response of three-dimensional stitched sandwich composite

The paper aims at evaluating the damage resistance of sandwich structures composed of stitched foam core and glass facesheets subjected to low-velocity impact. To obtain a suitable baseline comparison, the equivalent set of properties was measured for an equivalent unstitched sandwich.Based on the force and energy histories, parameters have been introduced as following: load at incipient damage, maximum load, penetration depth at maximum load, total energy absorbed during impact and impact damage area. The impact resistance of the sandwich structure is greatly improved by the presence of the stitches. Skin/core delamination is limited and initial energy is used to degrade core’s stitches. Moreover the global behavior under impact is influenced by the stitching geometrical parameters.     

Homogenization of the core layer in stitched sandwich structures

After highlighting the improvement of the mechanical performances involved by transverse reinforcement implementation in previous several studies, the mechanical behavior of stitched sandwich structures is analytically approached in this paper. The final purpose of this work is a modeling of the elastic performances of these structures. To predict the in-plane behavior, the classical theory of sandwiches is adapted and used by treating the foam core strengthened by stitches as a homogenized volume. This approach leads to the creation of an orthotropic equivalent core material. Its elastic properties depend on each component and their volume participation. The comparison between simulated and experimental values is quite good. The main interest of the multi-scale approach concerns a predictive tool. Indeed, it becomes realistic to obtain the elastic properties of stitched sandwich according to the geometrical parameters of the stitches and the mechanical properties of the components.     

Ageing of trisiloxane solutions

Surfactants are commonly employed in numerous industrial applications and everyday products due to their enhancing wetting abilities. Trisiloxane surfactants in particular have proved to be matchless in reducing the surface tension of the liquid and consequently in enhanced wetting on even highly hydrophobic surfaces. In many practical applications trisiloxanes are frequently used as water-based solution. For convenience and reduced cost they are often pre-mixed with water and left for long periods of time in aqueous environment. We provide evidence that ageing of trisiloxane surfactants in water solutions may lead to loss of their interfacial activity. A mechanism for the observed behaviour is suggested and the phenomenon quantified. Along with a detailed explanation of the problem, we shed light on the dynamics of the process and provide some limitations of pre-mixed solutions.     

Effects of heat flux,vapour quality,channel hydraulic diameter on flow boiling heat transfer in variable aspect ratio micro-channels using transparent heating

Experiments on flow boiling heat transfer in high aspect ratio micro-channels with FC-72 were carried out. Three channels with different hydraulic diameters (571, 762 and 1454 μm) and aspect ratios (20, 20 and 10) were selected. The tested mass fluxes were 11.2, 22.4 and 44.8 kg m^?2 s^?1 and heat fluxes ranging from 0–18.6 kW m^?2. In the present study, boiling curves with obvious temperature overshoots are presented. Average heat transfer coefficient and local heat transfer coefficient along stream-wise direction are measured as a function of heat flux and vapour quality respectively. Slug-annular flow and annular flow are the main flow regimes. Convective boiling is found to be the dominant heat transfer mechanism. Local heat transfer coefficient increases with decreasing hydraulic diameter. Moreover, the effect of hydraulic diameter is more significant when mass flux is higher. The unique channel geometry is considered as the decisive reason of the flow regimes as well as heat transfer mechanisms.     

Prediction of out-of-plane behavior of stitched sandwich structure

This article provides an improvement of the technology of through-the-thickness reinforcement of thermosetting composite sandwiches by the insertion of stitching yarns.The aim of this work is to propose a model that would lead to the representation of the tri-dimensional elastic and ultimate performances of these 3D structures under out-of-plane shearing and compressive stress. To reach that goal, an analytical approach is proposed without the contribution of the numerical finite element technique. From experimental observations, this complex structure can be considered as a simple lattice made up of the stitches treated as bars composed of the UD impregnated yarns. The transverse compression and the out-of-plane shear module can be determined from the geometrical parameters of the structure. Moreover, the ultimate stress can be approximated once the cinematic of the failure of the structure has been experimentally identified for each stress case. Finally, the buckling of stitches ends up in the ruin of the material in each case. Thus, a classical theory of buckling can be used and a particular work has been done on the representation of the anchorage point of the stitches on the skin. The final model which is endowed with a good sensitivity enables us to predict the ultimate values in both loading cases. Finally, the variation of stitching angle allows to virtually optimize the material used according to the external loading.     

Leidenfrost Droplets on Microstructured Surfaces

The lifetime of a droplet deposited on a hot plate decreases when the temperature of the plate increases, but above the critical Leidenfrost temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapor between the droplet and the substrate, which plays a double role: First, it thermally insulates the droplet from the plate, and second, it allows the droplet to “levitate.” The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water droplets ranging in size from 8 to 24 μl and the behavior of the droplets was recorded using high-speed digital photography. The Leidenfrost point was found to vary by up to 120°C for pillar spacings from 10 to 100 μm. If the droplet is placed on the boundary between structured sections, the droplet becomes asymmetric and may move or spin. An axisymmetric computational fluid dynamics (CFD) model is also presented that shows qualitative agreement with experimental observations.     

Effect of Hydrothermal Waves on Evaporation Distribution During Drop Evaporation

The objective of this study is to clarify physical mechanisms involved in the evaporation of small (a few microliters) sessile drops. We aim to understand the relation between local thermal information at the solid–liquid interface and overall evaporation. An infrared (IR) camera and a charge-coupled device (CCD) camera were used to determine the temperature and heat flux distribution at the solid–liquid interface and the profile of the evaporating drop, respectively. The temperature distribution at the solid–liquid interface was determined using a multilayer substrate consisting of a silicon wafer coated with a thin thermal insulator that is partially transparent to IR. The liquids used were water and FC-72. The evaporation rate of water drops was found to occur mostly at the contact line. However, the heat transfer distribution at the liquid–solid interface was relatively uniform, indicating the heat transferred from the wall must be transported within the drop to the contact line. The mechanisms by which this occurs have yet to be determined. In contrast, the evaporation rate of FC-72 drops where hydrothermal waves were present was found to be proportional to the liquid–vapor interface area rather than the circumference of the drop, indicating a more uniform distribution of evaporation.