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Super-hydrophobicity is one of the significant natural phenomena, which has inspired researchers to fabricate artificial smart materials using advanced manufacturing techniques. In ...this study, a super-hydrophobic aluminum surface was prepared by nanosecond laser texturing and FAS modification in sequence. The surface wettability turned from original hydrophilicity to super-hydrophilicity immediately after laser treatment. Then it changed to super-hydrophobicity showing a WCA of 157.6 ± 1.2° with a SA of 1.7 ± 0.7° when the laser-induced rough surface being coated with a layer of FAS molecules. The transforming mechanism was further explored from physical and chemical aspects based on the analyses of surface morphology and surface chemistry. Besides, the motion process of droplet impacting super-hydrophobic surface was systematically analyzed via the optimization of simulation calculation grid and the simulation method of volume of fluid (VOF). Based on this simulation method, the morphological changes, the inside pressure distribution and velocity of the droplet were further investigated. And the motion mechanism of the droplet on super-hydrophobic surface was clearly revealed in this paper. The simulation results and the images captured by high-speed camera were highly consistent, which indicated that the computational fluid dynamics (CFD) is an effective method to predict the droplet motion on super- hydrophobic surfaces. This paper can provide an explicit guidance for the selection of suitable methods for functional surfaces with different requirements in the industry.
•This model is applicable to narrow spaces.•Combining the evaporation–condensation model, a simulation is conducted on the flow field.•This model can effectively remove oxygen from condensate.•Alter ...the internal structure to improve deoxygenation efficiency.
To obtain a deaerator that can be applied to special requirements such as narrow spaces, based on the basic working principle of thermal deaeration, the structural design of a new type of bubbling thermal deaerator was first accomplished, and then its three-dimensional analysis model was established by combining the Volume of Fluid (VOF) multiphase flow model with the Lee evaporation and condensation model. Based on this model, numerical simulation of the internal flow field has been realized, and the effects of different steam feed rates and bubbling tube inlet position distributions on the temperature and oxygen content within the flow field have been compared. The results showed that: When the inlet feed water flow rate is determined, a lower steam inlet flow rate and a larger temperature difference are conducive to uniform heat transfer in the two-phase flow, while a higher steam inlet flow rate is conducive to accelerating the reduction of temperature and oxygen content gradient changes. In the three simulated design models, the oxygen removal effect was better when the steam inlet positions of the main and secondary bubbling tubes were uniformly distributed at the top and bottom, compared to when they were uniformly distributed around or underneath. In this configuration, both the oxygen content in the liquid phase outlet area and the deep oxygen removal area were the lowest.
Super-hydrophobic surfaces have been widely developed due to their unique properties, such as adhesion reduction and corrosion inhibition. It is of great significance to explore droplet dynamic ...process when impacting on super-hydrophobic surface, which can contribute to understanding the mechanism of super-hydrophobicity, and designing self-cleaning surfaces with ultralow adhesion and excellent corrosion resistance. Herein, three-dimensional (3D) numerical model was developed to study the droplet dynamic behaviors on super-hydrophobic NiTi surface. The volume of fluid (VOF) method was carried out to track the multi-phase flow interface and then predict droplet shape evolution during the spreading, retraction, and rebounding stages. In particular, the compression- and stretch-rebounding behaviors were investigated in detail. Based on the results of numerical simulation, the emphasis was placed on the relationship between droplet shape transformation and internal velocity/pressure distribution, revealing the mechanism of droplet morphological evolution during the impingement process. Then the experimental investigations were conducted on the planar and super-hydrophobic NiTi surfaces at different impact velocities to further verify the simulation models. It is indicated that the falling droplet would directly deposit on the planar surface, while it could fully rebound off the pillar-patterned super-hydrophobic surface. Moreover, the motion mechanism of droplet impacting on super-hydrophobic surface was elaborated, which can provide an essential instruction to effectively reduce liquid adhesion and improve anticorrosion ability for the NiTi material.
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In this paper, an HLLC-type contact preserving Riemann solver for incompressible two-phase flows using the artificial compressibility formulation is presented. This formulation is an improvement over ...the previous HLLC-VOF formulation (Bhat and Mandal, 2019). In this work, unlike the previous formulation, the variation of the volume fraction is taken into account when calculating the eigenvalues and eigenvectors. Hence, the equations for the intermediate states and the intermediate wave speed are closely-coupled with density variation during pseudo-time evolution of the solution. Additionally, an interface compression algorithm is used in tandem to ensure sharp interfaces. This modified Riemann solver (called HLLC-VOF-M) is found to be more robust and accurate compared to the older HLLC-VOF solver and the non-contact preserving HLL solver. Several test problems in two- and three-dimensions are solved to demonstrate and evaluate the efficacy of the solver on structured and unstructured meshes.
•Contact wave preserving Riemann solver for three-dimensional, incompressible two-phase fluid flows.•Artificial compressibility formulation closely-coupled with the volume of fluid method.•Sharp fluid interfaces are maintained using interface compression coupled with Riemann solver.•Viscous flux is computed using least-squares and surface tension is incorporated using the CSS model.•Robust formulation which is less sensitive to artificial compressibility parameter, based on several 2D and 3D test problems.
In this paper, an analytical investigation and 3D numerical simulation are presented for the breakup of floating non-Newtonian droplets in a non-Newtonian fluid. The considered geometry is a ...T-junction with unequal-width branches that can generate droplets with un-equal size. There is a very good agreement between the analytical solution and numerical simulation results obtained in this research. Various quantities such as branches flow rate ratio, branches velocity ratio, droplet’s length in each branch, the whole length of the droplet, vorticity and pressure have been investigated during the breakup process in this study. The results showed that the branches flow rate ratio and the branches velocity ratio were constant during the breakup process. It was also observed that the length of the droplet in each of the branches and the whole length of the droplet increased linearly during the breakup process. Also, the vorticity has its maximum at the breakup moment.
•DNS of droplets impinging, wetting, spreading and distribution on multiple particles.•Correlation of liquid partition coefficient with Weber number and solid volume fraction.•A sub-grid model for ...CFD simulation of liquid-injected fluidized beds.
Liquid partition coefficient, defined as the proportion of the liquid volume covered on particle surface to that in bulk phase, is a key parameter in the simulation of liquid-injected fluidized beds. To develop the sub-grid model for liquid partition coefficient as a function of local hydrodynamics, a two-dimensional Volume-of-Fluid (VOF) simulation was conducted to directly resolve droplets impinging, wetting and spreading on particles in a box. The model was first validated by the experiments in literature on the spreading diameter of a droplet impinging on a single particle. Then for a system of multiple droplets-particles impinging, we investigated the effects of Weber number, droplet-to-particle (DTP) ratio and particle volume fraction on several characteristic parameters, e.g., ratio of liquid area adhered to particles to the total liquid area, liquid film thickness, the percentage of wetted particle surface area and liquid holdup. All these parameters decrease with increasing Weber number when We = 10 ∼ 370, and then gradually stabilize. Higher particle volume fraction increases the collisional frequency of droplets and particles, and therefore the ratio of liquid area adhered to particles to the total liquid area. By contrast, DTP is not the dominant factor in liquid partition. Finally a correlation for liquid partition coefficient is proposed as a function of droplet Weber number and particle volume fraction. It may serve as a sub-grid model for CFD-DEM or multifluid models in simulation of large-scale liquid-injected fluidized beds.
•The wettability effect on minitube flow boiling with VOF has been investigated.•The 10 mm minitubes increase average heat flux up to 324,966.9 W/m2 with CA=5∘.•The 10 mm minitubes minimize total ...pressure drop Δp lower to 698.4 Pa with CA=150∘.•Gravity and acceleration pressure drops are minor to total pressure drops.•Under higher mass flux, the hydrophilic surface can enhance heat flux, and the hydrophobic surface reduces the pressure drop.
Flow boiling within minitubes holds immense promise for efficient heat dissipation owing to its substantial latent heat exchange capacity. The performance and efficacy of the pump, crucial beyond just necessitating high heat flux, are closely tied to maintaining a low-pressure drop. Surface treatment stands out as one of the techniques to augment thermal efficiency. While the significance of microscale surface properties, particularly wettability, in influencing flow boiling has been established, only a limited number of studies have explored its impact on pressure drop. This study rigorously investigates the wettability's influence on flow boiling within minitubes using the VOF approach. It reports key parameters within a 200 ms interval, including vapor volume fraction, average heat flux, and pressure drop. Employing a mass flux of 560 kg/m2s along a 10 mm length minitube, the study attains an impressive average heat flux of up to 324,966.9 W/m2 on a hydrophilic surface exhibiting a 5° contact angle. In contrast, a hydrophobic surface boasting a 150° contact angle achieves a significantly reduced total pressure drop (∆p) of 698.4 Pa, effectively preventing tube-clogging. Simultaneously accounting for heat flux and pressure considerations, it is conceivable that selecting a surface featuring a contact angle of 105° could represent an optimal choice for a 30 mm length minitube. The findings highlight that the hydrophilic surface encourages greater liquid presence, enhancing heat exchange efficiency considerably. Conversely, hydrophobic surfaces facilitate vapor layer stabilization, leading to reduced total pressure drop and minimized fluctuations. This work offers comprehensive insights into the fundamental relationship between wettability, flow boiling, and pressure drop, thereby providing valuable guidance for future miniube design strategies in cooling applications. Such insights hold immense potential to revolutionize various industrial and engineering sectors.
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