Effect Of Superhydrophobic Surface Research Articles

Effect of superhydrophobic surfaces on rod bundle flow dynamics

Effect of superhydrophobic surfaces on rod bundle flow dynamics

A recent state of art review on heat transfer enhanced characteristics and material selection of SCTHX

The shell and coil tube heat exchangers (SCTHX) are widely used to transfer heat energy from one medium to another effecting medium, such as refrigeration and air conditioning system, chemical reactor, solar heater and steam power plant. The helical tubes are one of the passive heat transfer enhancement technique in heat exchanger, which are adopted in industries due to higher rate of heat transfer and simplicity in manufacturing. It has been widely employed that heat transfer coefficient in helical coils are higher compare to straight pipe by several researchers. The reason is behind that the formation of secondary flow of nano fluid. It is leading by dean vortex which is imposed to primary flow. Furthermore research on segmental and helical baffle geometry can improve the heat transfer rate in helical coil tube heat exchanger. This study is concerned with the enhancement of convective heat transfer interaction between fluid and solid surface of helical coil. The detailed descriptions of material selection, nano fluid flow physics and heat transfer enhancement inside helical coil tube are very less in open literature. A state of art review indicates that further research work can be carried out by modifying the geometry of SCTHX and enhance factors of fluid property. The heat transfer can also be enhanced by polymer additives added in nano fluids, effect of super hydrophobic surfaces and fluid bubbles generation from roughened heating surface. Finally new ideas are presented in this paper to improve the heat transfer rate and material selection for low cost manufacturing with minimum size.

Noise Reduction Effect of Superhydrophobic Surfaces with Streamwise Strip of Channel Flow

Superhydrophobic surface is a promising technology, but the effect of superhydrophobic surface on flow noise is still unclear. Therefore, we used alternating free-slip and no-slip boundary conditions to study the flow noise of superhydrophobic channel flows with streamwise strips. The numerical calculations of the flow and the sound field have been carried out by the methods of large eddy simulation (LES) and Lighthill analogy, respectively. Under a constant pressure gradient (CPG) condition, the average Reynolds number and the friction Reynolds number are approximately set to 4200 and 180, respectively. The influence on noise of different gas fractions (GF) and strip number in a spanwise period on channel flow have been studied. Our results show that the superhydrophobic surface has noise reduction effect in some cases. Under CPG conditions, the increase in GF increases the bulk velocity and weakens the noise reduction effect. Otherwise, the increase in strip number enhances the lateral energy exchange of the superhydrophobic surface, and results in more transverse vortices and attenuates the noise reduction effect. In our results, the best noise reduction effect is obtained as 10.7 dB under the scenario of the strip number is 4 and GF is 0.5. The best drag reduction effect is 32%, and the result is obtained under the scenario of GF is 0.8 and strip number is 1. In summary, the choice of GF and the number of strips is comprehensively considered to guarantee the performance of drag reduction and noise reduction in this work.

Deterioration of drag reduction effect of superhydrophobic surface by particle adhesion in turbulent channel flow

Deterioration of drag reduction effect of superhydrophobic surface by particle adhesion in turbulent channel flow

Flow past a rotating circular cylinder with superhydrophobic surfaces

Superhydrophobic surfaces (SHSs) are widely reported and applied to modify flow features. In this work, the two-dimensional flows past a rotating circular cylinder with SHSs are studied to explore its effect. The present numerical simulations take SHSs into account by alternating shear-free and no-slip boundary conditions with different gas fraction (GF) for non-dimensional rotation rates of $$0\le \alpha \le 6$$ at $${ Re}=100$$ . Numerical results indicate that SHSs can obviously modify the critical rotation rates of vortex shedding, frequency of vortex shedding, force acting on the cylinder as well as the instantaneous and mean velocities near the cylinder. Moreover, such variation exhibits diverse behaviors in different flow regimes. For instance, both drag enhancement and drag reduction are found in the presence of SHSs. For the rotation rate corresponding to a vortex shedding regime, the drag of cylinder is reduced by SHSs. On the contrary, SHSs enhance the drag of cylinder in the steady regime. SHSs play different roles in different flow regimes so that the first critical rotation rate decreases monotonically with the increase of GF, while the second and the third critical rotation rates increase monotonically with increasing GF. The effect of SHSs in such flow could have some potential application.

Effects of superhydrophobic surfaces on the flow around an NACA0012 hydrofoil at low Reynolds numbers

In the present study, the effects of superhydrophobic surface on the flow around an NACA0012 hydrofoil are experimentally investigated at low Reynolds number range of 0.2–1.0 $$\times 10^{4}$$ . The velocity fields were measured using two-dimensional digital particle image velocimetry in a water tunnel while varying the angle of attack from $$0^\circ$$ to $$20^\circ$$ . The spray-coating of hydrophobic nanoparticles was used to create superhydrophobic surfaces. Depending on the Reynolds number and angle of attack, we found that the effects of superhydrophobic surface show up differently, which is determined by the relative strength of turbulence caused by both the surface slip (influence of trapped air pockets) and roughness, compared to that of background (i.e., uncontrolled) flow. In general, the superhydrophobic surface imposes a little influence on the wake behind a hydrofoil when the angle of attack is very low (attached flow) or high (fully separated flow). At intermediate angles of attack (flow separates between the leading and trailing edges), however, it is measured that the flow over superhydrophobic surface has a stronger turbulence, and thus, the enhanced shear-layer instability forces the early vortex rollup in the wake and reduction of vortex formation length. Interestingly, there is a transitional range of angle of attack, in which this effect is reversed, and thus, the vortex rollup is slightly delayed. This trend can be explained based on the changes in the uncontrolled flow structures, and finally, we classify the effects of superhydrophobic surface in terms of Reynolds number and angle of attack, in the considered ranges of both.

Influence of slip on the three-dimensional instability of flow past an elongated superhydrophobic bluff body

The effects of superhydrophobic surfaces (SHSs), which consist of microgrates oriented transverse to the flow direction, on the onset of three-dimensional instability of flow past a bluff body were studied using Floquet analysis. The SHS was modelled on an air–water interface with a shear-free condition. The results showed that SHSs increased the vortex shedding frequency. Floquet analysis revealed that modes B$^{\prime }$ and S$^{\prime }$ were suppressed dramatically by the partial-slip condition compared with a regular no-slip body; however, mode A was less affected. Correspondingly, the critical spanwise wavelengths were not significantly affected by SHSs. A similar phenomenon was observed in flow past a circular cylinder coated by SHSs. The results also revealed that modes B$^{\prime }$ and S$^{\prime }$ were collapsed into mode A due to the increased width of the air–water region for flow past an elongated body. Furthermore, the critical Reynold numbers of different modes were diversely affected by gas fraction (GF) variations. The unstable modes with short wavelengths, such as modes B$^{\prime }$ and S$^{\prime }$, stabilized with increasing GF. Conversely, the opposite was seen for the unstable mode A with a longer wavelength. The exact critical Reynolds number depended on the geometric configuration, which should be between the critical values of the two extreme cases. The application of SHSs could modify the transition route from two- to three-dimensionality by alternating different unstable modes. As the wavelength of the unstable mode decreases, the inhibition of three-dimensional instability becomes more efficient by SHSs.

Drag reduction and heat transfer characteristics of water flow through the tubes with superhydrophobic surfaces

In the present study, the drag reduction and heat transfer characteristics of water flowing through the tubes with superhydrophobic surfaces were investigated. The tubes with inner superhydrophobic surfaces in diameters of 4.0, 8.0 and 12.0mm were fixed along the center axis of an outer tube to form the counter-current double-tube heat exchanger with a length of 1000.0mm. Hot and cold water were used as the working fluids flowing through the inner tube and annular space of the heat exchanger, respectively. The experiments were performed at the Reynolds numbers ranging from 3000 to 11,000. The results showed that the drag reduction ranged from 8.3% to 17.8% for superhydrophobic surfaces. The effect of superhydrophobic surface on drag reduction increased with the decrease of tube diameter, and it also decreased with the increase of the Reynolds number. The friction factors of superhydrophobic tubes were smaller than those of smooth tubes at the same Reynolds number. The Colebrook equation was not applicable to estimate the friction factors of superhydrophobic surfaces, and the modified equation was proposed to describe the experimental results. The heat transfer performance was suppressed by the air cavities on superhydrophobic surfaces. The Gnielinski equation also failed to predict the heat transfer coefficients of superhydrophobic surfaces, and the calculated results by the modified equations were consistent with the experimental results. The PECs of combined performance of the drag reduction and heat transfer for superhydrophobic tubes with inner diameter of 8.0mm was relatively larger than those of superhydrophobic tubes with inner diameters of 4.0 and 12.0mm.

Effects of Superhydrophobic and Superhydrophilic Surfaces on Heat Transfer and Oscillating Motion of an Oscillating Heat Pipe

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Effect of Super-Hydrophobic Surface on the Corrosion Performance of Copper and Copper Alloys

With copper and copper alloys widely used, copper corrosion has already caused huge economic losses. Super-hydrophobic surface treatment, a new kind of corrosion resistant technology, can effectively inhibit metal corrosion. This paper introduces the basic issues and the construction methods of copper and copper alloys super-hydrophobic surface. The effect of super-hydrophobic surface on the corrosion performance of copper and copper alloys is analyzed emphatically. Recent developments of super-hydrophobic surface and the existed technical barriers are also summarized in the paper.

Dual drag reduction mechanism of water-based dispersion with hydrophobic nanoparticles in core microchannel and experimental verification

Dual drag reduction mechanisms of water-based dispersion with nanoparticle is proposed. A contrastive study is take to verify the mechanism, in which the changes of surface microstructure and wettabilities of the core slices take place before and after treating by dispersion with hydrophobic nanoparticles and scouring by water. The results show that the surface of core slice which is treated by water-based dispersion with hydrophobic nanoparticles has strong hydrophilic property, and a compact nanoparticle adsorption layer forms on it. The nanoparticle adsorption layer still exists after scouring, but the core slice surface is changed into strong/super hydrophobic, reflecting that the surfactants which are adsorbed on the nanoparticles adsorption layer surface are gradually cleaned. The water-based dispersion with hydrophobic nanoparticles are mainly manifested as the chemical surfactant drag reduction effect during initial injection. With the injection continued, the mechanical drag reduction induced by the slip effect of super hydrophobic surface is reflected mainly. Core displacement results show that the water-phase effective permeability could increase about 84.3% on average. The results strongly confirm the dual drag reduction mechanism of the water-based dispersion with hydrophobic nanoparticles.