Search

The use of carbon nanotubes to create superhydrophobic coatings has been considered due to their ability to offer a relatively uniform nanostructure. However, carbon nanotubes (CNTs) may be considered delicate with a typical diameter of tens of nanometers for a multi-walled CNT; as-grown carbon nanotubes often require the addition of a thin-film hydrophobic coating to render them superhydrophobic. Furthermore, fine control over the diameter of the as-grown CNTs or the overall nanostructure is difficult. This work demonstrates the utility of using carbon infiltration to layer amorphous carbon on multi-walled nanotubes to improve structural integrity and achieve superhydrophobic behavior with tunable geometry. These carbon-infiltrated carbon nanotube (CICNT) surfaces exhibit an increased number of contact points between neighboring tubes, resulting in a composite structure with improved mechanical stability. Additionally, the native surface can be rendered superhydrophobic with a vacuum pyrolysis treatment, with contact angles as high as 160° and contact angle hysteresis on the order of 1°. The CICNT diameter, static contact angle, sliding angle, and contact angle hysteresis were examined for varying levels of carbon-infiltration to determine the effect of infiltration on superhydrophobicity. The same superhydrophobic behavior and tunable geometry were also observed with CICNTs grown directly on stainless steel without an additional catalyst layer. The ability to tune the geometry while maintaining superhydrophobic behavior offers significant potential in condensation heat transfer, anti-icing, microfluidics, anti-microbial surfaces, and other bio-applications where control over the nanostructure is beneficial. [ABSTRACT FROM AUTHOR]

Abstract: This paper reviews central receiver designs for concentrating solar power applications with high-temperature power cycles. Desired features include low-cost and durable materials that can withstand high concentration ratios (~1000 suns), heat-transfer fluids that can withstand temperatures >650°C, high solar absorptance, and low radiative and convective heat losses leading to a thermal efficiency >90%. Different receiver designs are categorized and evaluated in this paper: (1) gas receivers, (2) liquid receivers, and (3) solid particle receivers. For each design, the following information is provided: general principle and review of previous modeling and testing activities, expected outlet temperature and thermal efficiency, benefits, perceived challenges, and research needs. Emerging receiver designs that can enable higher thermal-to-electric efficiencies (50% or higher) using advanced power cycles such as supercritical CO2 closed-loop Brayton cycles include direct heating of CO2 in tubular receiver designs (external or cavity) that can withstand high internal fluid pressures (~20MPa) and temperatures (~700°C). Indirect heating of other fluids and materials that can be stored at high temperatures such as advanced molten salts, liquid metals, or solid particles are also being pursued, but challenges include stability, heat loss, and the need for high-temperature heat exchangers. [Copyright &y& Elsevier]

Abstract: Implementation of molten salt compounds as the heat transfer fluid and energy storage medium provides specific benefits to energy collection and conversion. Nitrate salts have been identified as a strong candidate for energy transfer and storage and have been demonstrated for use in these applications over time. As nitrate salts have solidification temperatures above ambient, concern for recovery from salt freezing events has instigated efforts to understand and predict this behavior. Accurate information of salt property behavior in the solid-phase is necessary for understanding recovery from a freeze event as well as for phase change thermal energy storage applications. Thermal and mechanical properties for three representative salts (solar salt, HITEC salt, and a Na–K–Li–Ca nitrate salt; spanning the range of liquidus temperatures from approximately 90–240°C), have been obtained. These properties include: specific heat, coefficient of thermal expansion, thermal conductivity, latent heat of fusion, compressive strength, tensile strength, Young’s modulus and Poisson’s ratio. Specific heat, thermal conductivity and latent heat of fusion were measured using differential scanning calorimetry. Temperature was not observed to have a significant effect on tensile strength using an indirect tensile test (Brazilian test). Peak stress and Young’s modulus (both from unconfined compressive strength testing) were shown to decrease while Poisson’s ratio increased with increasing temperature. [Copyright &y& Elsevier]

• Initial heat rate increases with wettability. • Heat flux increases with Reynolds number. • Significantly lower average maximum heat flux on superhydrophobic surfaces. • Temperature has negligible effect on heat transfer on superhydrophobic surfaces. Superhydrophobic (SH) surfaces possess desirable anti-fouling properties due to low wettability, but have also been shown to reduce heat transfer to subcooled water in impinging jet scenarios. In this work, superheated silicon substrates with varying wettability (hydrophilic or HPi, hydrophobic or HPo, SH) are quenched by an impinging water jet, where the substrate temperature is above the saturation temperature. Silicon wafers are either oxidized to create HPi surfaces, coated with Teflon to make the surface HPo, or plasma-etched and coated to create the necessary micro-texture for SH conditions. All wafers are integrated with an electric resistance heater and then heated to temperatures of 200–320 ∘ C before impingement with an axisymmetric room temperature water jet of varying specified flow rates yielding jet Reynolds numbers between 6000 and 18,000. High-speed visual data is collected, showing how the lamellar liquid contact region, limited by thermal breakup due to boiling, grows radially as the surface cools to temperatures below saturation. This data is correlated to temperature data recorded on the back side of the wafer using a thermal camera. Results of this study confirm previous conjecture that surface wettability can alter maximum heat flux, which is quantified here for the described scenario by up to 40%, and can also affect jet thin film spreading by up to 50%. Increasing initial surface temperature decreases thin film spreading rate on all surfaces, and increases heat transfer on all but the SH surfaces. Increasing Reynolds number yields an increase in heat flux, and affects both the thin film spreading rate as well as the maximum radius of the thin film region. [ABSTRACT FROM AUTHOR]

Thermal diffusion measurements on polymethylmethacrylate-coated Si substrates using heated atomic force microscopy tips were performed to determine the contact resistance between an organic thin film and Si. The measurement methodology presented demonstrates how the thermal contrast signal obtained during a force-displacement ramp is used to quantify the resistance to heat transfer through an internal interface. The results also delineate the interrogation thickness beyond which thermal diffusion in the organic thin film is not affected appreciably by the underlying substrate. [ABSTRACT FROM AUTHOR]

Thermal management systems for space equipment commonly use static solutions that do not adapt to environmental changes. Dynamic control of radiative surface properties is one way to respond to environmental changes and to increase the capabilities of spacecraft thermal management systems. This paper documents an investigation of the extent to which origami-inspired surfaces may be used to control the apparent absorptivity of a reflective material. Models relating the apparent absorptivity of a radiation shield to time-dependent surface temperatures are presented. Results show that the apparent absorptivity increases with increasing fold density and indicate that origami-inspired designs may be used to control the apparent radiative properties of surfaces in thermal management systems. [ABSTRACT FROM AUTHOR]

• Radiative fins with dynamic geometry can influence heat transfer rates in real time. • Self-irradiation decreases turn-down ratio but increases fin efficiency. • Fin efficiency is greatest for collapsed fins and smallest for extended fins. • A turn-down ratio of three or greater is possible with dynamic radiative fins. Efficient heat transfer is critical in the design and optimization of thermal control systems. Static radiative heat exchangers are often simple and reliable systems but typically cannot be adapted to environmental changes. Adaptable radiative heat exchangers can be adjusted in response to variations in the thermal environment or operating conditions and have the potential for increased efficiency and reduced cost. Dynamic control of a radiative heat exchanger is possible through geometric manipulation of a segmented, self-irradiating fin, consisting of rigid panels that are linked by thermal hinges in an accordion arrangement. In this paper, a numerical model is described to predict the temperature profile and efficiency of a radiative heat exchanger, accounting for conduction and self-irradiation. Governing equations are cast in terms of the conduction-radiation interaction parameter, surface emissivity, actuation angle, and the thermal conductance of the hinges linking the panels. Results indicate that a turn-down ratio (largest possible heat rate divided by smallest possible heat rate) of greater than three is possible for realistic panel geometries and materials. Self-irradiation decreases the turn-down ratio, and there is evidence that an optimal number of rigid panels exists for any combination of panel geometry and device temperature. The maximum efficiency occurs when the plates are in the collapsed position, but the heat rate is at a minimum in this configuration. Finally, the properties and geometry of the plates are shown to have a more significant effect on the turn-down ratio than the properties of the thermal hinges. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of drop impingement and thermal atomization on hydrophobic and superhydrophobic (SH) surfaces. Superhydrophobic surfaces having both microscale and nanoscale geometry are considered. Microscale SH surfaces are coated with a hydrophobic coating and exhibit micropillars and cavities which are classified using the surface solid fraction and center to center pitch. The solid fraction and pitch values explored in this study range from 0.05-1.0 and 8-60 μm respectively. Nanoscale textured surfaces are created by applying a blanket layer of carbon nanotubes. Both types of surfaces are further classified by a temperature jump length (λ T). All experiments were conducted at We = 85. Results of atomization as a function of time for the impingement event are provided for several surfaces of varying surface geometry, surface temperature, and temperature jump length. Nanoscale SH surfaces are shown to completely suppress atomization at all conditions explored. Results of the maximum atomization that occurred on a given surface are also shown as a function of the surface temperature. The surface temperature at which the maximum atomization occurs varies with surface geometry. Further, the time after impact when the maximum atomization occurs is also a function of the SH surface parameters. In general, the maximum atomization magnitude and the surface temperature at which maximum atomization occurs each decrease with increasing λ T. Further, the time when maximum atomization occurs increases with increasing λ T. • Drop impingement on superheated superhydrophobic surfaces is explored. • Time varied atomization magnitude is quantified using an image processing technique. • Test surfaces include micro and nanostructured superhydrophobic surfaces. • Atomization is completely suppressed on nanoscale superhydrophobic surfaces. • Atomization data is generalized using a temperature jump length. [ABSTRACT FROM AUTHOR]

Highlights • Superhydrophobic surfaces act as nucleation sites for mass transport. • Nucleating bubble behavior depends on the underlying micro-structure. • Large bubbles form and impede heat transfer. Abstract This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobic surfaces. All surfaces tested were submerged in a 20 mm deep pool of water and heated from below to maintain a constant surface temperature, while the side walls of the pool were insulated, and the top was covered. Three thermocouples positioned in the pool obtain the average pool temperature. A heat flux sensor is placed directly beneath the surface to measure the heat flux supplied to the pool. Free convection heat transfer coefficients are obtained for the sub-boiling temperature range of 40–90 °C. Six surface types are studied: smooth hydrophilic, smooth hydrophobic, superhydrophobic with rib/cavity structures, superhydrophobic with rib/cavity structures and additional sparsely spaced ribs to close off the cavities, circular posts, and circular holes. It is found that structured superhydrophobic surfaces provide cavities for nucleation to occur. More dissolved air effervesces from the water as the surface temperature increases due to an increased level of supersaturation and convection. The nucleation leads to large air bubble formations that reduce the overall convection coefficient when compared to the smooth surfaces. For the rib/cavity structured surfaces, the bubbles form in an anisotropic manner and are aligned with the surface structure. More bubbles are observed on the superhydrophobic surfaces where the cavities are bounded. Since water's ability to dissolve air is dependent on temperature, heat and mass transfer cannot be treated independently on any of the superhydrophobic surfaces studied here. [ABSTRACT FROM AUTHOR]

This work experimentally studies the effects of single wall heating on laminar flow in a high-aspect ratio superhydrophobic microchannel. When water that is saturated with air is used as the working liquid, the non-wetted cavities on the superhydrophobic surfaces act as nucleation sites and allow air to effervesce out of the water and onto the surface when heated. Previous works in the literature have only considered the opposite case where the water is undersaturated and absorbs air out the cavities for a microchannel setting. The microchannel considered in this work consists of a rib/cavity structured superhydrophobic surface and a glass surface separated by spacers. The microchannel is 60 mm long by 14 mm wide and two channel heights of nominally 183 μm and 366 μm are explored. The superhydrophobic side is in contact with a heated aluminum block and a camera is used to visualize the flow through the glass side. Thermocouples are embedded in the aluminum to record the temperature profile along the length of the channel. Temperatures are maintained below the boiling temperature of the working liquid. The friction factor-Reynolds product ( fRe ) is obtained via pressure drop and volumetric flow-rate measurements. Five surface types/configurations are investigated: smooth hydrophilic, smooth hydrophobic, superhydrophobic with ribs perpendicular to the flow, superhydrophobic with ribs parallel to the flow, and superhydrophobic with ribs parallel to the flow with several breaker ridges perpendicular to the flow. The surface type/configuration has a significant impact on the mass transport dynamics. For surfaces with closed cell micro-structures, large bubbles eventually form and adversely affect fRe and lead to higher temperatures along the channel. When degassed water is used, no bubble nucleation is observed and the air initially trapped in the superhydrophobic cavities is quickly absorbed by the water. [ABSTRACT FROM AUTHOR]

An abstract of the article "Infrared Visualization of the Cavity Effect Using Origami-Inspired Surfaces" by Mitchell J. Blanc and colleagues is presented.