Potential Hydrodynamic Performance Enhancement of Hydrokinetic Turbine with Hydrophobic Coatings

Hydrokinetic energy is a promising technology to harness predictable renewable energy from free-flowing water,tides and ocean currents.Many studies have been conducted by researchers and engineers to find out ways to enhance the performance of the hydrokinetic turbine.The current paper reports the experimental study of using hydrophobic coating as an alternative way to improve the performance of hydrokinetic turbine.A hydrophobic coating can lower the friction drag of a surface that is in contact with liquid.For hydrokinetic turbine blade,reduction in friction drag may allow a blade section to have a better lift/drag ratio and have its efficiency improved.In this study,a formula to predict the pattern of drag reduction over a hydrophobic surface has been derived.Two hydrophobic coatings were applied on NACA 63418 hydrofoils and their performances were tested.It was found that NACA 63418 hydrofoil with the hydrophobic coatings had its drag reduced by an average of 3%−4.0%.When the coatings were applied on a 350 mm diameter three-bladed turbine,the maximum increment of rotational speed of the turbine was found to be 2.5%.The performance of the two coatings against marine fouling was also investigated.The weight of plate with and without the coatings increased by 10%and 100%,respectively.

Experimental investigation of flow past a circular cylinder with hydrophobic coating

A series of experiments are performed in a gravitational low-speed water tunnel to investigate the influence of the hydrophobic coating on the flow past a circular cylinder. The mean velocity and turbulence intensity profiles behind the cylinders are measured using the hot-film anemometer while the separation angles are obtained with the flow visualization technology. For the Reynolds number lower than 3 800, the hydrophobic coatings are in the Cassie state, the velocity deficit and the turbulence intensity in the wake of the hydrophobic cylinders are lower than those of the smooth cylinders which implies the drag reduction effect of the hydrophobic coatings. When the Reynolds number becomes higher than 6 600, the hydrophobic coatings turn into the Wenzel state. Through decomposing the velocity data in the turbulent wake into different scales based on the orthogonal wavelet transform, it is found that the total turbulence intensity in the wake of the hydrophobic cylinders becomes almost the same as in the wake of the smooth cylinders while the intensity of the large scales of vortex components in the wake of the hydrophobic cylinders is still lower. Furthermore, the separation angles show the same trend as a function of the Reynolds number but always take smaller values for the hydrophobic cylinders.

Inhibition of sulfate-reducing bacteria influenced corrosion on hydrophobic poly(dimethylsiloxane)coatings

Sulfate-reducing bacteria(SRB)has been pointed out as one of the causative agents of microbial induced corrosion in the marine environment.To address this problem,novel strategies are being experimented as against the earlier methods which have been banned due to their toxic effects on useful aquatic lives.Thus,the aim of this study was to investigate the effect of non-toxic perfluorodecyltrichlorosilane(PFDTS)on resistance of hydrophobic poly(dimethylsiloxane)/phosphoric acid-treated zinc oxide(PDMS/PA-treated ZnO)coatings to SRB-induced biofouling and corrosion.The surface features of the coatings before and after exposure to SRB/NaCl solution were analyzed by scanning electron microscopy(SEM).Wettability of the coatings before and after exposure was also measured.The interaction of SRB with the coatings was investigated by FTIR spectroscopy.The resistance performance of the modified coatings against SRB-induced corrosion was monitored by electrochemical impedance spectroscopy(EIS).The EIS measurements revealed that 0.20 g PFDTS-based coating displayed highest corrosion resistance with impedance modulus of 6.301×10^10 after 15 d of exposure to SRB/NaCl medium.The results were corroborated by surface and chemical interaction analyses,and thus,indicate that 0.20 g PFDTSmodified PDMS/PA-treated ZnO coating has potentials for excellent SRB-induced corrosion resistance and anti-biofouling performance.

Hydrophobic organic coating based water–solid TENG for water-flow energy collection and self-powered cathodic protection

Water-solid triboelectric nanogenerators(TENGs),as new energy collection devices,have attracted increasing attention in ocean energy harvesting and selfpowered sensing.Polyacrylic acid(PAA)coating,usually used on the surface of marine equipment,has the property of anti-aging and anti-wear but limits triboelectrical output when used with TENGs.In this paper,polyacrylic acid coating was modified with fluorinated polyacrylate resin(F-PAA)to increase its triboelectrical output,by 6 times,and also to increase its anti-corrosion property.In addition,the corrosion resistance property can be further enhanced by cathodic protection using the electrical output generated by the water-flow triboelectrical energy transfer process.Given their easy fabrication,water-flow energy harvesting,and corrosion resistance,PAA/F-PAA coating-based TENGs have promising applications in river and ocean energy collection and corrosion protection.

Non‐chromate conversion process for zinc coating with durable hydrophobicity and enhanced corrosion resistance

A Zn‐based coating with durable hydrophobicity and good corrosion resistance was formed on a mild steel substrate,which involves electroplating Zn from a non‐aqueous electrolyte,followed by passivation in an oleic acid(OA)solution.The electrodeposited Zn coatings were porous,which facilitated the formation of a chemical conversion layer of Zn oleate(ZO)during OA passivation.The Zn coating after passivation had a twolayer structure,which included an outer layer of ZO with a thickness of~26μm and an inner layer of Zn with a thickness of~6μm.The outer layer ZO is a type of metal soap with a smooth surface and durable hydrophobicity,such that water droplets can easily slip off its surface.Corrosion testing and electrochemical measurements in 3.5 wt.%NaCl aqueous solution indicate that the Zn coating after OA passivation exhibits outstanding anti‐corrosion properties compared with those exhibited by pure Zn coating.The corrosion products and mechanism of the two‐layer coating were explored.This study shows that smooth metal oleate coatings can provide hydrophobicity and corrosion resistance simultaneously to mild steel substrates.

Sol-gel preparation of a silica antireflective coating with enhanced hydrophobicity and optical stability in vacuum

A new silica antireflective coating with improved hydrophobicity and optical stability in a vacuum is obtained by a two-step route. Firstly, silica sols are prepared with a sol-gel process, in which tetraethyl orthosilicate is utilized as a precursor. And by introduction of fluorine containing glycol into the sols, the porosity of silica particles and surface polarity of the coatings are decreased. Afterward, coatings are constructed with low surface roughness by modification of PMBA-PMMA. The coatings retain transmission of up to 99.6%, and laser damage threshold of about 50 J/cm^2 at a wavelenth of 532 nm （1-on-1. 10 ns）

Review of ice-pavement adhesion study and development of hydrophobic surface in pavement deicing

Ice adhesion to materials is a significant concern in many fields. Hydrophobic surface has been used for anti-icing in fields of aircraft or transmission line, which prove to be efficacious and economical. However, such technique is seldom employed for road deicing, because of the texture and service environment of pavement. Instead, deicers such as rock salt are frequently used, which leads to serious corrosion problem of roads and bridges. In this paper, a number of studies that characterize mechanism of ice adhesion to common substrates, specifically to pavement, are reviewed. The most important researches undertaken on ice adhesion strength affecting factors are presented. An overview of studies carried out to find hydrophobie surface for asphalt and cement concrete pavement antiicing are presented. It was verified that the hydrophobicity had high correlation with icephobicity, and nano-engineered asphalt and cement concrete pavement surface exhibited favorable hydrophobicity, and also had good performance on weakening pavement-ice bonding. However, most ice-repelling pavements obtain hydrophobic surface via low surface energy coating, which could not exist on pavement for a long time under wheel abrasion. And the nano/micro structures on hydrophobic surfaces are also vulnerable and consumable. Thus, the long-term effect of hydrophobic surface still need to be improved, and durability of the hydrophobic surface should be the research and development priorities of ice-repelling pavement.

Protective Composite Coatings Formed on Mg Alloy Surface by PEO Using Organofluorine Materials

The results of the surface modification of magnesium alloys by plasma electrolytic oxidation（PEO） and subsequent treatment in suspension of the superdispersed polytetrafluoroethylene（SPTFE） or telomeric solution of tetrafluoroethylene（TFE） are presented. Composite coatings have been obtained by dipping with subsequent heat treatment. Electrochemical, tribological properties and wettability of protective composite coatings have been investigated. Composite coatings formed on PEO-layer by fourfold treatment of samples in SPTFE suspension possess best protective properties. The obtained coatings decrease the corrosion current density（5.4 × 10^（-11） A cm^（-2）） and wear（7.6 × 10^（-7）mm^3（Nm）^（-1））, and increase the polarisation resistance（1.7 × 10~9 cm^2） and impedance modulus（1.9 × 10~9 cm^2） by orders of magnitude in comparison with unprotected magnesium alloy and base PEO-coating. The highest value of contact angle（CA） has been obtained for coatings with triple application in telomeric solution. CA for such composite coatings attains（171 ± 2）？, as the result of multimodal roughness of the composite coating＇s surface.