Capillary Dynamics of Water/Ethanol Mixtures

Surface tension and contact angle play important roles in capillary dynamics during washing commission of spray-dried detergent powder where interaction between surface chemistry and liquid is concerned. An experimental study for the dynamics of water/ethanol mixtures in both hydrophilic and hydrophobic capillary tubes has been investigated. The combination of water/ethanol mixtures and trimethylchlorosilane coating on capillaries provides a variety of liquid surface tension and contact angle for the penetrating system. Significant differences of penetrating speed on hydrophilic and hydrophobic tubes indicate that dynamic contact angle dominates the hydrophobic surface while liquid surface tension plays a more important role on the hydrophilic surface. The speed gradient of different liquids on hydrophilic surface is greater than the hydrophobic surface, mainly due to the domination between hydrogen bonding structures and water polarity while liquid moves on surfaces covered with different chemistry, physically absorbed water on silica capillaries or silanol group covered capillaries

Experimental, Theoretical and Numerical Evaluation of Wicking Models for Liquid Imbibition in Dry Porous Wicks

Nowadays commercial wicks are utilized by consumer product companies in several important commercial applications including Tiki® Brand torches, the passive lubricants of machine gears, propellant management device, and fragrance dispersion units. Spontaneous imbibition of a liquid into porous wicks, also called wicking, is modeled using the single-phase Darcy’s law after assuming a sharp flow-front marked by full saturation behind the front. An analytical expression for the height of the wicking flow-front as a function of time is tested through comprehensive experiments using different wicks and an oil as the wicking liquid. We proposed a model based on sharp liquid-front where a good match with the experimental data was achieved. However, the proposed model based on the sharp liquid-front fails to account for partial saturation in the wicks. As a result, we applied the Richards equation to predict partial liquid saturations in wicks where the equation is solved numerically in 2-D using COMSOL and analytically in 1-D using Mathematica for glass-fiber wicks after treating them as transversely-isotropic porous media. As a novel contribution, the relative permeability and capillary pressure are determined directly from pore-scale simulations in wick microstructure using the state-of-the-art software GeoDict. The saturation along the wick length is determined experimentally through a new liquid-N2 based freezing technique. After including the gravity effect, good agreements between the numerical/analytical predictions and experimental results are achieved in saturation distributions. We also validated the Richards equation-based model while predicting absorbed liquid-mass into the wick as a function of time. A series of wicking experiments with wicks procured from our industrial partners were conducted where the use of a dyed liquid revealed essentially three types of macroscopic (visual) fronts—sharp, semi-sharp, and diffuse. The particulate wicks (i.e. the wicks formed by sintering polymer beads) invariably formed sharp fronts, while the fibrous wicks (i.e. wicks formed from fibers) formed either semi-sharp or diffuse fronts. The porosity was also found to play a role—the lower-porosity fibrous wicks displayed semi-sharp fronts, while the higher-porosity fibrous wicks caused the fronts to be diffuse. A study of SEM (Scanning Electron Microscopy) micrographs revealed that the latter behavior was caused by clustering of fibers thus leading to the formation of an inhomogeneous porous medium (perhaps promoting finger formation on micro-fronts). The experiments also revealed that the visually-observed fronts, for most parts, achieved a good match with the fronts estimated through the sharp-front mass gain formula. (Such a match was found to be lacking in the fibrous wicks displaying diffuse fronts.) We also investigated two parameters of interest to the users of wicks: 1) steady-state (SS) height reached by the visual front at very large times, 2) the liquid supply rate when the front is near the top. The parameters estimated using our sharp-front model matched well with the experimentally-observed ones. Finally, we conducted a CFD simulation using FLUENT where the flow of wicking liquid through a 2D microstructure made of ellipses of varying aspect ratio was modeled. A series of microstructures were created by varying the ellipse aspect ratio from 1:1 (20*20 µm) to 1:64 (20*1280 µm), with lower values representing particulate porous media and the higher values representing fibrous porous media. To study the effect of porosity, two values of 50% and 70% were considered. The flow simulation in particulate porous media produced somewhat even micro-fronts that indicate a flat visual (macroscopic) front. On the other hand, simulations in fibrous porous media produced highly uneven micro-fronts that point to a semi-sharp or diffuse visual fronts. Increasing the porosity results in clustering of solid phase and leads to further increase in the unevenness of micro-fronts, thus pointing to purely diffuse visual fronts. The evolution of saturation plots along the flow direction, obtained using a grid superimposed on fluid distribution pictures, was also studied and the predictions matched our previous experimental and numerical observations, i.e., particulate media create sharp fronts while fibrous media create diffuse fronts

Computational Analysis of Fluid Flow, Heat Transfer, and Phase Change in Capillary Channels

The fluid mechanics and heat transfer associated with capillary-driven flows are of great interest for modeling transport phenomena in micro/miniature devices. Currently, a deeper understanding of this area is necessary for the design of more effective products. The primary objective of this dissertation is to develop a novel computational fluid dynamics model to study the dynamics of meniscus formation, capillary flow, heat transfer, and phase change between vertical parallel plates. To do so, an arbitrary Lagrangian-Eulerian (ALE) approach is employed to predict and reconstruct the shape of the meniscus with no need to employ implicit interface tracking schemes. The developed model is validated by comparing the equilibrium capillary height and meniscus shape with those predicted by available theoretical models. The model was used to predict the capillary flow of water in hydrophilic (silver) and hydrophobic (Teflon) vertical channels with wall spacings ranging from 0.5 mm to 3 mm. It is shown that the computational model accurately predicts the capillary flow regardless of the channel width, whereas the theoretical models fail at relatively large wall spacings. The model captures several important hydrodynamic phenomena that cannot be accounted for in the theoretical models, including the presence of developing flow in the entrance region, time-dependent formation of the meniscus, and the inertial effects of the liquid in the reservoir. In the next step, the previously developed ALE model was extended to directly track the formation and evolution of the evaporating meniscus during spontaneous liquid penetration within a capillary channel. The two-dimensional time-dependent conservation equations for mass, momentum, and energy were solved in a finite-volume framework implemented on a moving and deforming grid. The sharp interface tracking method developed here enables direct access to the flow variables and transport fluxes at the meniscus with no need for averaging techniques. The model was validated by comparing the predicted dynamic response of the capillary height subject to interfacial evaporation against theoretical results. The effects of wall spacing and liquid superheat on the capillary flow, and the evaporation rate were studied. It was found that thermal diffusion adjacent to the meniscus has a critical effect on the evaporation rate, and neglecting it leads to significant overprediction of the evaporation rate. Results show that, in general, the inclusion of evaporation causes a reduction of the liquid column height compared to the non-evaporating case. It was also observed that the equilibrium capillary height is inversely proportional to the liquid superheat. Analyses of the transient regime show that evaporation tends to dampen the oscillatory flow regime compared to the non-evaporating meniscus case

Hydrodynamic characteristics and capillary-assisted heat transfer enhancement of non-condensing/condensing two-phase flows

Interfacial characteristics of two-phase flows were studied through visualization experiments and numerical simulation using computational fluid dynamics (CFD) based on the volume-of-fluid (VOF)-continuum surface force (CSF) method. An experimentally-validated analytical method was also presented for the geometrical correction of the optically-distorted objects in cylindrical tubes that is applicable to geometrical measurements (e.g., liquid-gas interfaces, solid particles, gas bubbles, void fraction) inside the tubes. The numerically-simulated two-phase flows agreed favorably with the visually-observed flows. The simulation of two-phase flows under reduced gravities indicated the important contribution of gravity on hydrodynamics of intermediate scale two-phase flows such as void fraction, pressure drop, slip ratio, and bubble velocity; the pressure drop of horizontal plug/bubble flows and Taylor bubble velocity of vertical slug flow is minimum around normal gravity. The computational model is then extended to account for convective and condensation heat transfer. The numerical results for a vertical slug flow show that a porous-tube-insert (PTI) promotes the internal liquid circulations in both axial and radial directions resulting in an enhanced convective heat transfer up to five times of that in bare tube. In addition, the PTI enhances the flow condensation heat transfer up to three times mainly due to the enforced ultra-thin liquid film near the tube wall and increased area for thin-film condensation

Understanding liquid movements in textiles for the development of liquid repellent strategies

The understanding of the liquid movements in textiles is important to the development of novel liquid repellent strategies based on the manipulation of liquid motion. In this thesis we focus on the two areas that have received little attention: (1) the liquid permeation across the thickness of a single-layer textile following the deposition of a static droplet and (2) the liquid movements following the impact of droplets on a single-layer textile. In the study of area (1) we report a time-resolved high resolution X-ray imaging of the motion of the liquid-vapour interface in the textile thickness direction. The imaging of the time-dependent position of the interface is made possible by the use of ultra-high viscosity liquids (dynamic viscosity 2.5·106 times higher than that of water). Imaging results suggested a three-stage permeation mechanism with each stage being associated with one type of capillary channels in the textile geometry. We also showed that the permeation dynamics cannot be described by the popular Washburn theory. In the study of area (2) we record the impact of droplets on textiles with high-speed imaging. We showed that the impact on textiles at short timescales involved no droplet shape deformation if the textile’s porosity was sufficiently high. We also showed that droplets could be captured by the textiles under some impact conditions. By balancing the dynamic and capillary pressures we showed that the droplet penetration was governed by a threshold pore size and the droplet diameter. Moreover, we identified 5 stages for the liquid spreading on the textile surfaces following the impact. Within the investigated range of impact velocity the surface chemistry of the textiles was unimportant in the determination of liquid repellency. We also investigated the transplanar liquid permeation across non-wettable textiles following the deposition of droplets. We showed that the permeation was governed by a critical pore size and the weight of the deposited liquid. We discussed the limitation of the Gillespie scaling, developed for the prediction of in-plane spreading area in papers, in the description of the in-plane capillary spreading dynamics in textiles