Drag reduction using superhydrophobic sanded Teflon surfaces

Drag reduction for liquid flow through micro-apertures

Steam condensation with non-condensable gas exists widely in petro-chemical, power plant and seawater desalination processes. Super-hydrophobic surfaces with high contact angle could be an promising condensing surface to promote DWC and enhance heat transfer. As pure steam condensing droplet completely occupies micro/nano-structures of super-hydrophobic surfaces, super-hydrophobic function becomes “invalid”. For stream-air mixture vapor, the non- condensable gas would occupy the micro/nano-structures instead of water. Droplets form Cassie or transition wetting mode, holding or partly holding super-hydrophobic feature. Inclination angle of the tube can influence the shape, size and dynamic characteristics of the condensing droplets, thus changing the heat transfer performance. Therefore, for getting deeply insights of the effect of the non-condensable gas on condensation heat transfer, it is necessary to study the condensing drops characteristics on super-hydrophobic tube for different inclined angles. According to dropwise condensation model, increasing contact angle hysteresis, departure droplet size, sweeping period would reduce the condensation heat transfer efficiency. In this paper, dropwise condensation is performed on super-hydrophobic copper tube, departure droplet size, sweeping period and the phenomenon of contact angle hysteresis are thoroughly studied and are compared with the case of hydrophobic surface or pure steam to explore the influence of inclined angle and surface wettability on droplet dynamics to achieve condensation heat transfer enhancement. The experimental result shows: (1) On the top of copper tube, for the condensation of steam with 10% non-condensable gas, the contact angle hysteresis on super-hydrophobic surface is smaller than that on hydrophobic surface. Compared with pure steam condensation on super-hydrophobic surface, the contact angle hysteresis of mixed steam is smaller because the non- condensable gas fills the micro/nano-structures. However, on the middle and bottom parts of the tube, the contact angle hysteresis on super-hydrophobic surface is greater than that on hydrophobic surface for the condensation of both pure steam and mixed steam. This phenomenon indicates that the contact angle hysteresis is determined by surface wettability instead of inclination angle. (2) The departure droplet size on super-hydrophobic surface increases with the inclination angle and the opposite trend is found on hydrophobic surface. With the increase of inclination angle, the support force provided by the tube would increase and change direction. So the wettability of the surface and departure droplet size would be changed. (3) The sweeping period of droplet on super-hydrophobic surface is longer because the micro/nano-structures increase the adhesion between the surface and the droplets. With the increase of the inclination angle, the sweeping period decreased slightly, shorter than the sweeping time when the tube is vertical. The sweeping period of vapor with non-condensable gas condensation is greater than that of pure steam. The presence of non-condensable gas increases thermal resistance, thus slowing down the drop growth rate and increasing the sweeping period. To sum up, on the top of copper tube, the contact angle hysteresis on super-hydrophobic surface is smaller than that on the hydrophobic surface. However, the totally opposite trend is found on the middle and bottom parts of the tube. The departure droplet size on super-hydrophobic surface increases with the inclination angle. The sweeping period of droplet on super-hydrophobic surface is longer. This work offers a new avenue to further enhance heat transfer of condensation with non-condensable gas.

Superhydrophobic (SH) surfaces, created from hydrophobic materials with micro- or nano- roughness, trap air pockets in the interstices of the roughness, leading, in fluid flow conditions, to shear-free regions with finite interfacial fluid velocity and reduced resistance to flow. Significant attention has been given to SH conditions on ordered, periodic surfaces. However, in practical terms, random surfaces are more applicable due to their relative ease of fabrication. We investigate SH behavior on a novel durable polymeric rough surface created through a scalable roll-coating process with varying micro-scale roughness through velocity and pressure drop measurements. We introduce a new method to construct the velocity profile over SH surfaces with significant roughness in microchannels. Slip length was measured as a function of differing roughness and interstitial air conditions, with roughness and air fraction parameters obtained through direct visualization. The slip length was matched to scaling laws with good agreement. Roughness at high air fractions led to a reduced pressure drop and higher velocities, demonstrating the effectiveness of the considered surface in terms of reduced resistance to flow. We conclude that the observed air fraction under flow conditions is the primary factor determining the response in fluid flow. Such behavior correlated well with the hydrophobic or superhydrophobic response, indicating significant potential for practical use in enhancing fluid flow efficiency.

Durable drag reduction and anti-corrosion for liquid flows inside lubricant-infused aluminum/copper capillaries

Sustainable drag reduction in Taylor-Couette flow using riblet superhydrophobic surfaces

In this paper, a review of different techniques used to measure the slip length on superhydrophobic surfaces with large slip length is presented. First, we present the theoretical models used to calculate the effective slip length on superhydrophobic surfaces in different configurations of liquid flow. Then, we present the different techniques used to measure the slip past these superhydrophobic surfaces: rheometry, particle image velocimetry, pressure drop, surface force apparatus and atomic force microscopy.

Slip boundary condition for lattice Boltzmann modeling of liquid flows

Drop impact on non-wetting surfaces has garnered significant interest due to its potential applications in water repellency, drag reduction, self-cleaning, and anti-icing. However, there are instances where a droplet fails to rebound from a superhydrophobic surface. It has been reported that the combined effect of gravito-capillary length and visco-capillary length determines the pinning–bouncing criteria. While the fluid properties, such as viscosity and weight, are often considered primary factors influencing droplet rebound, this study highlights the crucial role of surface characteristics, particularly the contact angle hysteresis, in determining post-impact behavior. We propose a modified criterion that predicts droplet bouncing and pinning on superhydrophobic surfaces by integrating both fluid properties and the contact angle hysteresis of the surface. The findings emphasize the importance of surface morphology in droplet dynamics, providing a more comprehensive understanding of droplet behavior on non-wetting surfaces.

Artificial intelligence approach to predict drag reduction in crude oil pipelines

In order to ensure the safety of civil aviation during the freezing weather, the application of superhydrophobic coatings (surfaces) on the aircraft is a useful and supplemental way to mitigate the accumulation of frost, ice or snow. In this work, superhydrophobic surface with hierarchical structure on SAE AMS 4037 aluminum alloy plate is prepared through chemical etching using hydrochloric acid and hydrogen peroxide with lower concentration successfully. Comparing with the bare aluminum substrate (untreated-Al), the superhydrophobic surface (treated-Al-11) possessed an excellent resistance to water, which water contact angle (CA) is as high as 163.6°, combined with minimum roll-off angle (RA) and contact angle hysteresis (CAH). The influences on wettability caused by different molar ratios of hydrochloric acid and hydrogen peroxide are studied by CA, RA, CAH and root mean square roughness (Rq). Hierarchical structure with irregular honeycomb-like protrusions and depressions of treated-Al-11 is found through SEM. Furthermore, treated-Al-11 is characterized by FTIR, EDS and XPS, which are demonstrated the presence of the aluminum stearate (AlSA) molecules on surface of treated-Al-11. Anti-icing tests are revealed that the weight of ice on the surface of treated-Al-11 with superhydrophobicity is significantly less than that of other Al plates at the same test time. The ice weight on treated-Al-11 is 1.7751 g compared to 1.9952 g on untreated-Al surface when test time is 120 min Based on the results of self-cleaning tests, the surface of treated-Al-11 is clean, however solution residues are remained on the untreated-Al surface.

We present a theoretical prediction for the drag reduction rate achieved by superhydrophobic surfaces in a turbulent channel flow. The predicted drag reduction rate is in good agreement with results obtained from direct numerical simulations at Reτ≃180 and 400. The present theory suggests that large drag reduction is possible also at Reynolds numbers of practical interest (Reτ∼105–106) by employing a hydrophobic surface, which induces a slip length on the order of ten wall units or more.

This paper describes a self-comparison measuring method for the slip length of fluid flow over superhydrophobic surfaces consisting of a pillar array based on the wetting transition. The wetting transition of the liquid resting on a superhydrophobic surface in the measuring course may lead to the disappearance of the slippage effect, which can be used to measure the slip length before the wetting transition. Based on this feature, the slip length formula for this measuring method is deduced. Then, to verify this method, a rheological experiment for 70 wt% glycerin solution flow over a superhydrophobic surface with carbon nanotube (CNT) forest was carried out on a commercial rheological system with plate-and-plate configuration. Results show that even on the CNT-coated surface that can induce little slip length, the experimental slip length is still on the same scale as the theoretical one. This measuring method can be used to measure the slip length of liquid flow over the superhydrophobic surface consisting of a pillar array with good flexibility.

Nature has developed materials, objects, and processes that function from the macroscale to the nanoscale. The emerging field of biomimetics allows one to mimic biology or nature to develop nanomaterials, nanodevices, and processes which provide desirable properties. Hierarchical structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in nature to provide properties of interest. There are a large number of objects including bacteria, plants, land and aquatic animals, and seashells with properties of commercial interest. Certain plant leaves, such as Lotus leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical roughness of their leaf surfaces. The self-cleaning phenomenon is widely known as the “Lotus effect”. These surfaces with high contact angle and low contact angle hysteresis with a self-cleaning effect also exhibit low adhesion and drag reduction for fluid flow. In this article, the theoretical mechanisms of the wetting of rough surfaces are presented followed by the characterization of natural leaf surfaces. The next logical step is to realize superhydrophobic surfaces based on understanding of the leaves. Next, a comprehensive review is presented on artificial superhydrophobic surfaces fabricated using various fabrication techniques and the influence of micro-, nano- and hierarchical structures on superhydrophobicity, self-cleaning, low adhesion, and drag reduction. An aquatic animal, such as a shark, is another model from nature for the reduction of drag in fluid flow. The artificial surfaces from the shark skin have been created, and the influence of structure on drag reduction efficiency is discussed. Furthermore, oleophobic surfaces can be used as a biomimetic coating that prevents contamination of the underwater parts of ships by biological and organic contaminants, including oil. The article discusses the wetting behavior of oil droplets on various superoleophobic surfaces.

Energy conservation and environmental protection have become pivotal components of the green economy. In recent years, underwater drag reduction technology has garnered significant interest. This study discusses a novel composite surface that combines divergent riblets with a superhydrophobic coating (D-rib&SHS) to enhance the drag reduction rate. Alongside this new surface, a riblet surface with a superhydrophobic coating (rib&SHS, without yaw angles) and a smooth surface are used as comparison groups. The turbulent boundary layer flow of these three surfaces is measured using a two-dimensional particle image velocimetry system. The results indicated that the maximum drag reduction rate of D-rib&SHS is approximately 27% higher than that of rib&SHS, and the drag reduction range is increased to Reθ≈4100 compared to rib&SHS (Reθ≈2200). Using correlation algorithms, it observed that the spacing between low-speed streaks over D-rib&SHS is larger than that of rib&SHS and the smooth surface. This finding suggested that the spacing between the hairpin vortex legs of D-rib&SHS is wider. The increased spacing between the hairpin vortex legs reduces the likelihood of vortex head formation between the two quasi-streamwise vortices, ultimately suppressing the auto-generation of hairpin vortices. Consequently, the development of hairpin vortex packets over D-rib&SHS is also inhibited. These phenomena observed over D-rib&SHS can be attributed to the combined effects of velocity slip on the superhydrophobic coating and the secondary flow over the divergent riblets near the wall. In addition, unlike divergent riblets that are not covered with a superhydrophobic coating, where the drag reduction effect is more pronounced with a yaw angle of 30° compared to 10°, D-rib&SHS with a 10° yaw angle demonstrated a superior drag reduction effect compared to D-rib&SHS with a 30° yaw angle. This innovative composite surface enhances the drag reduction effect and expands the drag reduction Reynolds number range, offering a new approach to mitigating drag in turbulent boundary layer flows.

In the pursuit of optimizing heat dissipation and improving performance in electronic devices, liquid-cooled heat sinks have garnered significant interest due to their potential to improve heat transfer efficiency. Presented study set out to assess the effectiveness of straight channel heat sinks as a thermal solution for high heat generating electronics components, while also investigating the impact of applying a hydrophobic coating on their performance. A series of tests were conducted at different heating powers (50W-250 W) and flow rates (200–950 ml/min), while also considering different orientations and directions of flow. The investigation focused on analysing critical aspects such as heat transfer performance, wall temperature variations, thermal resistance, and pressure drop characteristics of straight channel heat sinks with conventional and hydrophobic-coated surfaces. The results showed that the Nusselt number increased with higher Reynolds numbers and heating powers, indicating improved heat transfer efficiency. However, the hydrophobic coating on the heat sink surface led to a reduction in the Nusselt number due to the formation and retention of bubbles, hindering heat transfer. The maximum average reduction of 20.85% in Nusselt number was noticed at 100 W for the vertically positioned hydrophobic-coated heat sink with downward flow compared to conventional surface (CS) heat sink. Additionally, the findings revealed that pressure drop is influenced by Reynolds number and the presence of bubbles, with gravity playing a role in reducing pressure drop in certain orientations. A hydrophobic coating shows a small decrease in pressure drop, but bubble formation limits its effectiveness to some extent. Comparisons also indicated that the highest average decrease in pressure drop about 11.92% was observed for the same configuration, vertically positioned hydrophobic heat sink with downward flow, at 50 W. Moreover, the study's findings have broader implications beyond electronics. Heat sinks are widely employed in various industries, including automotive, aerospace, renewable energy systems, and industrial processes.