•Comprehensive numerical method on supercooled droplet impingement pertinent to aircraft icing.•Time-dependent solidification proportion and final frozen shape of droplets.•Concept of activity ...duration to investigate the effect of LWC.•Logarithmic correlation for predicting transient heat transfer through the wall.
In-flight icing usually occurs when supercooled droplets impact on the cold surface of an aircraft, which should be concerned for its adverse effects on aerodynamic performance. To fundamentally elucidate the detailed mechanism of aircraft icing, a mathematical model on the impingement dynamics and solidification of a supercooled water droplet was developed and further validated with previous experimental results. Considering the effects of surface tension, wall adhesion and contact line dynamics, the coupled volume of fluid and level-set method was used to track the air–water interface, with the solidification issue solved by the enthalpy-porosity method. The temporal evolutions of the water phase, flow velocity, temperature and heat flux distributions were tracked and analyzed after the phase transition of supercooled water occurred. High impact velocities and millimeter-sized droplets were considered in this study to make the results more applicable to in-flight icing. As concluded, the spreading ratios of the droplets mainly distribute in the range of 0.8 ± 0.1 corresponding to LWC = 1.0 g/m3. Besides, the transient heat transfer between the solid surface and droplets could be fitted by a logarithmic function after appropriate dimensionless processing, which was proportional to the 1.5th power of the liquid fraction. Contributions of this work could be an effort to understand the microphysical phenomenon in the aerodynamic icing process.
A computational study has been carried out to characterize the morphological and hydrodynamic behavior of water droplets during the impingement onto the hemispherical substrate. Volume of fluid (VOF) ...methodology is employed to carry out the simulations. Various essential and interesting stages, such as free falling, impact, cap formation, encapsulation, uncovering, oscillation, and detachment are encountered during entire impact cycle. The effects of various parameters are hemisphere-to-droplet diameter ratio (Dh/Do), contact angle (θ), Bond number (Bo), Ohnesorge number (Oh), and release height (h/Do) on deformation factor (ξ) of the droplet is delineated thoroughly. Droplet fails to detach from the target at higher Oh and greater Dh/Do. Based on this, a scatter regime plot has been represented to distinguish between two different hydrodynamic behavior of droplets. A correlation is developed to estimate the maximal deformation factors in terms of Dh/Do, Bo, Oh, θ, and h/Do and it performs extremely well within ±3.5% of the computational data.
•The contact angle hysteresis and dynamic angle deviation are unified to the surface friction.•Even the minimum i.e. atomic roughness is great enough to be effective on the contact line.•Ion ...microscopy reveals ubiquitous wrinkles along the contact line on seemingly smooth surface.•The convex nanobending at advancing contact line is reproduced in molecular simulation.•Rougher surface has greater scale of convex nanobending at advancing contact line.
Static contact angle hysteresis and dynamic angle variation are two fundamental phenomena about the contact angle deviating from the equilibrium state. Various factors including solid roughness and disjoining pressure have been considered as the origins of the phenomena. This work made a reduction to absurdity by employing absolutely smooth solid surfaces in large-scale molecular dynamics simulations. The results showed that the equilibrium angles were well established on the absolutely smooth surface just as regular solid surfaces, while the hysteresis and the dynamic deviation vanished. In contrast, the solids made of atoms, even with atomic roughness, could bring significant static hysteresis and dynamic deviation. The convex nanobending, which was recently experimentally confirmed as a feature structure at advancing contact lines, was reproduced on atomic solids while vanished on the absolutely smooth one. The results indicated that the angle deviations could be unified to originate from the friction on solids, either static or dynamic, and that even the atomic roughness could be effective. As comparison, 3D observations were made using state-of-the-art helium ion microscopy for the first time revealing the ubiquitous nanoscopic distortion along the contact line on atomically smooth surfaces.
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•TX surfactants with different hydrophilic chain length exhibit different impact behavior on the superhydrophobic surface.•Greater EO chain length of TX surfactant is conducive to the ...deposition of impacting droplets.•Rapid diffusion of surfactant to the interface in spreading stage is the key to deposition for droplets.•Superhydrophobic surface adsorbed by TX-100 turns into larger pinning effect and adhesion.
The behavior of high-speed droplets impacting on superhydrophobic surfaces plays an essential role in many daily applications. Adding surfactants is one excellent approach to improve droplet retention through enhancing the solid-liquid interface interaction. However, the regulating of the impact process has been rarely concerned about the effect of molecular structure on it. In our work, we study the impact behavior of nonionic surfactants Triton X (TX) on superhydrophobic surfaces by characterizing the diffusion rate of surfactant molecules and the receding contact angle and adhesion of surfactant solution on surfaces. We reveal that droplets of TX surfactant solution can deposit better on superhydrophobic surfaces as hydrophilic (ethylene oxide, EO) chain length increasing, which is mainly attributed by the faster diffusion speed towards the newly formed interface during the spreading phase, leading to better wettability transition and larger pinning area on the substrate. Our work promotes the understanding of the mechanism that controls the impact behavior of droplets of surfactant by adjusting the EO chain length, which provides an effective strategy to control droplet deposition on superhydrophobic surfaces.
•The characteristics of a three-dimensional CLPHP with different wettability are probed.•The flow reversal appears in CLPHP with hydrophobic surface.•Dynamic contact angle hysteresis and physical ...features of bubbles are compared in CLPHP with different wettability.•The pulsation amplitude of liquid plug is larger in CLPHP with hydrophilic surface.•CLPHP with hydrophilic surface can effectively raise the dry-out input heat load.
A three-dimensional closed-loop pulsating heat pipe (CLPHP), with different wettability and charged with deionized water is numerically investigated in present work. The thermal performance and bubble dynamics of CLPHP are obtained under the condition of various input heat loads. It's found that the performance of CLPHP is affected by both surface wettability and input heat load. Under lower input heat load, CLPHP with hydrophobic surface (including superhydrophobic surface) has lower thermal resistance than that with hydrophilic surface. Conversely, CLPHP with hydrophilic surface starts up earlier, and has better thermal performance under higher input heat load. Specially, compared with the CLPHP with superhydrophobic surface, the thermal resistance of CLPHP with superhydrophilic reduces by 10.8% under the input heat load of 20 W. Moreover, the reversal of flow direction is observed in CLPHP with hydrophobic surface, while the stable directional circulation is always maintained in CLPHP with hydrophilic surface. The results indicate that the difference between advancing and receding angles (dynamic contact angle hysteresis) leads to various capillary resistance. Furthermore, due to lower flow resistance and the effect of liquid film, CLPHP with hydrophilic surface can effectively raise the dry-out input heat load.
This study reports droplet-particle interaction of size ratio less than unity in the film boiling regime on a highly thermally conductive spherical particle surface. Specifically, the effects of ...impact Weber number (We) of subcooled state droplets comprising water (We=3.9–103.6) and isopropyl alcohol (IPA) (We=8.6–194.6) were studied using high speed imaging technique in the particle temperature range of 250–350°C. In general, non-wetting interaction behaviour was observed with two distinct outcomes – rebound and complete disintegration demarcated by a critical Weber number range instead of a single threshold value. Extent of surface wetting was characterised by the maximum droplet spread diameter parameter which was found to scale with impact Weber number in a power law form which agrees with the theoretical scaling argument. Additionally, an energy balance model was developed to compute this parameter which provided good agreement with the experimental measurements in the lower Weber number regime, however, higher deviations were noted near the transition regime. Also quantified from experiments was the droplet-particle contact time which exhibited a power law dependency on Weber number in the rebound regime, however, was noted to be almost independent of Weber number in the disintegration regime. Particle surface wettability was characterised by the experimentally measured dynamic contact angles which were found to vary in the range of 120–160o in low Weber number regime manifesting the hydrophobic nature of particle surface in film boiling regime. Also, all the parameters such as contact line velocity, particle temperature and droplet size apparently had relatively insignificant influence on the variation of dynamic contact angle. Temporal variation of non-dimensional spreading parameter exhibited a self-similar behaviour wherein all data collapsed on a single power law profile. It was further shown that the behaviour could also be described by a recovery type exponential profile through suitable non-dimensionalization and both profiles can be utilized to produce a spreading kinetics.
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•Droplet-particle interaction studied at different Weber numbers and temperatures.•Critical Weber number range noted from rebound to disintegration regime transition.•An energy balance model developed to determine maximum wetted area.•Maximum contact area increases with Weber number but contact time decreases.•Contact angle variation almost independent of contact line velocity and temperature.
•Pore-doublet scale simulation of the low salinity water flooding (LSWF) using computational fluid dynamics.•Effects of fluid/fluid interactions are considered in addition to the fluid/rock ...interactions.•Comprehensive simulations to investigate different degrees of wettability alterations and/or IFT variations.•Simulations of both tertiary and secondary injection scenarios are performed
Using our recently developed model, for the first time in the literature, the effect of fluid/fluid and rock/fluid interactions on the performance of Low Salinity Waterflooding (LSWF, as an Enhanced Oil Recovery process) at pore-doublet scale is investigated. The model is incorporated into OpenFOAM and both the Navier-Stokes equation for oil/water two-phase flow and the advection-diffusion equation for ion transport (at both fluid/fluid and rock/fluid interface) are solved via direct numerical simulation (DNS).
The model is validated against imbibition and drainage pore-doublet experiments reported in the literature, and then applied to investigate the sole effect of wettability alteration as well as its coupled effect with interfacial tension (IFT) variations on the redistribution and transport of phases within the model. Different degrees of contact angle alteration, different trends of IFT variations reported in the literature, as well as different injection scenarios (secondary or tertiary) are considered.
Simulation results show that in the case of high salinity water flooding (HSWF), IFT (within the range of 30 to 5 mN.m−1) has very limited effect on the residual oil saturation of the investigated model. For the case of HSWF in the oil-wet pore-doublet model (contact angle = 140°), significant amount of oil will be trapped in the small pores of the model. This residual oil cannot be produced in the absence of wettability alteration, no matter how much the IFT be reduced during LSWF (even as low as 0.1 mN.m−1). However in the absence of any IFT variation (IFT = 10 mN.m−1), the residual oil ganglia can be displaced with the sufficient degree of wettability alteration (contact angles below or equal to 30°). Nevertheless, in the presence of adequate contact angle reduction, fluid/fluid interactions (different IFT variation) can affect the outcome of the enhanced oil recovery. The LSWF performance boosts under appropriate degree of fluid/fluid and rock/fluid interactions as it leads to the coalescence of the detached or partially detached residual oil blobs and progress of chasing water front in both small and large pores of the doublets towards the downstream of the model. The results confirm the importance of fluid/fluid interactions such as IFT variation on such coalescence and dynamic LSWF, especially in the case of secondary LSWF.
Capillary dynamics is a ubiquitous everyday phenomenon. It has practical applications in diverse fields, including ink-jet printing, lab-on-a-chip, biotechnology, and coating. Understanding capillary ...dynamics requires essential knowledge on the molecular level of how fluid molecules interact with a solid substrate (the wall). Recent studies conducted with the surface force apparatus (SFA), atomic force microscope (AFM), and statistical mechanics simulation revealed that molecules/nanoparticles confined into the film/wall surfaces tend to self-layer into 2D layer/s and even 2D in-layer with increased confinement and fluid volume fraction. Here, the capillary rise dynamics of simple molecular fluids in cylindrical capillary is explained by the molecular self-layering model. The proposed model considers the role of the molecular shape on self-layering and its effect on the molecularly thin film viscosity in regards to the advancing (dynamic) contact angle. The model was tested to explain the capillary rise dynamics of fluids of spherical, cylindrical, and disk shape molecules in borosilicate glass capillaries. The good agreement between the capillary rise data and SFA data from the literature for simple fluid self-layering shows the validity of the present model. The present model provides new insights into the design of many applications where dynamic wetting is important because it reveals the significant impact of molecular self-layering close to the wall on dynamic wetting.
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•The novel role of liquid molecular self-layering in capillary dynamics is revealed;•For liquid with spherical shape molecules, the capillary dynamics can be predicted with the molecular diameter;•For liquids with non-spherical shape molecules, the capillary dynamics can be predicted based on the solvation force data.
To enhance the descent of droplets in the coke bed of a blast furnace, the sliding angle of the water droplet and the advancing and receding contact angles at the time of sliding were measured. Coke ...used as a reducing agent in a blast furnace was employed as a substrate. Because the shape of the coke surface varies with the gasification reaction with CO2, the coke substrate was treated with heat (1273 K) in a CO/CO2 atmosphere. Irregularities of approximately several micrometers were formed on the coke surface by the gasification reaction, and the sliding angle of the droplet decreased.
•A dynamic model is built for the oscillatory regime of liquid rise in capillaries.•Modifications are made on sub-models for the end pressure losses and for the receding dynamic contact ...angle.•Validity of the new model is proved by wide experimental results.•Dynamics of the oscillatory regime is discussed.•Occurrence criteria of oscillatory regime is obtained by a non-dimensional analysis.
The oscillatory regime for liquid rise in vertical capillaries has been observed but the analytical solution that can be applied for this regime is still in shortage. Some lack terms in the existing analytical solutions make them show deviations when predicting the oscillatory behavior of liquid rise. An improved dynamic model is built in this work for the oscillatory regime. Two main contributions are made, concerning the non-equal pressure losses at the entrance for liquid rise and fall, and the receding dynamic contact angle for the regions with negative capillary numbers. Experiments are performed to correlate the empirical parameters in the submodels and to validate the combined dynamic model. Good accuracies of the present model are obtained by comparing with the experimental data both in literature and from the present study. Inclusion of the non-equal pressure loss equations for the end effect makes the model well capture the high asymmetry of oscillations, while the two-stage dynamic contact angle models correct the local bouncing of the meniscus. The dynamics of liquid in the oscillatory regime is discussed by comparing the contribution of each pressure force. Occurrence criteria of the oscillatory regime is obtained through a non-dimensional analysis. It is shown that the oscillatory regime is affected not only by the combined parameter (ω), but also by the immersed height of tube (H0) and the pressure head loss coefficients (ξ).
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