Condensation of water vapor is an essential process in power generation, water collection, and thermal management. Dropwise condensation, where condensed droplets are removed from the surface before coalescing into a film, has been shown to increase the heat transfer efficiency and water collection ability of many surfaces. Numerous efforts have been made to create surfaces which can promote dropwise condensation, including superhydrophobic surfaces on which water droplets are highly mobile. However, the challenge with using such surfaces in condensing environments is that hydrophobic coatings can degrade and/or water droplets on superhydrophobic surfaces transition from the mobile Cassie to the wetted Wenzel state over time and condensation shifts to a less-effective filmwise mechanism. To meet the need for a heat-transfer surface that can maintain stable dropwise condensation, we designed and fabricated a hybrid superhydrophobic-hydrophilic surface. An array of hydrophilic needles, thermally connected to a heat sink, was forced through a robust superhydrophobic polymer film. Condensation occurs preferentially on the needle surface due to differences in wettability and temperature. As the droplet grows, the liquid drop on the needle remains in the Cassie state and does not wet the underlying superhydrophobic surface. The water collection rate on this surface was studied using different surface tilt angles, needle array pitch values, and needle heights. Water condensation rates on the hybrid surface were shown to be 4 times greater than for a planar copper surface and twice as large for silanized silicon or superhydrophobic surfaces without hydrophilic features. A convection-conduction heat transfer model was developed; predicted water condensation rates were in good agreement with experimental observations. This type of hybrid superhydrophobic-hydrophilic surface with a larger array of needles is low-cost, robust, and scalable and so could be used for heat transfer and water collection applications.

This article presents a fundamental investigation in which velocity amplification is employed in non-resonant structures to enhance the power harvested from ambient vibrations. Velocity amplification is achieved utilising sequential collisions between free-moving masses, and the final velocity is proportional to the number of masses and the mass ratios selected. The governing theory is discussed, particularly how the final velocity scales with the number of masses. This article examines n-mass velocity-amplified vibration energy harvesters and examines their performance relative to single-mass harvesters. Electromagnetic energy conversion is chosen as it is fundamental in allowing the free movement of the masses. Experimental results from two- and three-mass prototypes are presented that demonstrate a wider frequency response and a gain in power of 33 times compared to single-mass configurations under wideband random excitation. The volume of the devices was constrained, which resulted in the two-mass system outperforming the triple-mass system counter to expectations. This was caused by the triple-mass device experiencing an increased number of impact due to the volume constraint, leading to high losses in the system. It is recommended that in order to realise the full benefits of the triple-mass system, additional volume for mass actuation is required.

a b s t r a c tAt present there is significant interest in the development of small scale medical diagnostic equipment. These devices offer faster processing times and require smaller sample volumes than equivalent macro scale systems. Although significant attention has been focused upon their outputs, little attention has been devoted to the detailed fluid mechanics that govern the flow mechanisms within these devices. Conventionally, the samples in these small scale devices are segmented into distinct discrete droplets or slugs which are suspended in an organic carrier phase. Separating these slugs from the channel wall is a very thin film of the organic carrier phase.The magnitude of this film is the focus of the present study and the effects of sample slug length and carrier phase fluidic properties on the film are examined over a range of Capillary numbers. A non-intrusive optical technique was used to capture images of the flow from which the magnitude of the film was determined.The experimental results show that the film is not constant along the length of the slug; however above a threshold value for slug length, a region of constant film thickness exists. When compared with existing correlations in the literature, the experimental data showed reasonable agreement with the Bretherton model when the Capillary number was calculated based on the mean two phase flow velocity. However, significant differences were observed when the Capillary number was redefined to account for the mean velocity at the liquid interface, i.e., the mean slug velocity.Analysis of the experimental data revealed that it fell into two distinct flow regimes; a visco-capillary regime and a visco-inertial regime. A modified Taylor expression is presented to estimate the magnitude of the film for flows in the visco-capillary regime while a new model is put forward, based on Capillary and Weber numbers, for flows in the visco-inertial regime. Overall, this study provides some novel insights into parameters, such as aqueous slug length and carrier phase fluidic properties, that affect the thickness of the film in liquid-liquid slug flow regimes.

This paper describes the results of copper coupons exposed to a class III mixed flowing gas environment ͑MFG͒ following the guidelines given by the Battelle Laboratory and the International Electrotechnical Commission for environmental testing. Corrosion products were studied in detail using scanning electron microscope, energy dispersive X-ray spectroscopy ͑EDS͒, X-ray diffraction ͑XRD͒, focused ion beam ͑FIB͒, secondary ion mass spectroscopy ͑SIMS͒, and transmission electron microscope. The weight gain measured after each exposure was compared with the weight gain calculated from the cathodic reduction of the corrosion layers and cross sectioning using an FIB. The result shows a relatively good correlation between the measured and the calculated experimental values of weight gain. As expected, within the first week, the different corrosion layers thickened until they formed a thick layer that became the determining step for further growth. After several days of exposure the Cu coupons developed a complex multilayered structure consisting of cuprous oxide ͑Cu 2 S͒, cupric oxide ͑CuO͒, copper sulfide ͑Cu 2 S͒, covellite ͑CuS͒, and evidence of antlerite ͑3CuO SO 3 2H 2 O͒. No Cl-containing corrosion products were identified using XRD. However, EDS and SIMS analysis showed that Cl was distributed throughout the corrosion products, indicating that although Cl is inside the corrosion products, it is not part of the crystalline structure. Also, this suggests that Cl plays an important role in accelerating the corrosion of Cu during exposure to the MFG class III test. In the early 1980s, with the discovery of significant printed wiring board ͑PWB͒ and component failure modes ͑mainly due to corrosion͒, a number of firms and laboratories set out to develop accelerated corrosion test methods with a known acceleration factor. The aim of such efforts was to shrink years of service into days of testing, and prove that the field failure modes would be replicated during the tests. The PWB and its components were exposed to different levels of a mixture of gases, temperature, and relative humidity, which would simulate the environment during operating conditions. IBM, AT&T, and Battelle Laboratories participated in this effort.1 The result of this work was the development of a mixed flowing gas ͑MFG͒ test, which is primarily a laboratory test in which the temperature, relative humidity, and concentration of gaseous pollutants are carefully defined, monitored, and controlled.

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To achieve reductions in the power consumption of the data center cooling infrastructure, the current strategy in data center design is to increase the inlet temperature to the rack, while the current strategy for energy-efficient system thermal design is to allow increased temperature rise across the rack. Either strategy, or a combination of both, intuitively provides enhancements in the coefficient of performance (COP) of the data center in terms of computing energy usage relative to cooling energy consumption. However, this strategy is currently more of an empirically based approach from practical experience, rather than a result of a good understanding of how the impact of varying temperatures and flow rates at rack level influences each component in the chain from the chip level to the cooling tower. The aim of this paper is to provide a model to represent the physics of this strategy by developing a modeling tool that represents the heat flow from the rack level to the cooling tower for an air cooled data center with chillers. This model presents the performance of a complete data center cooling system infrastructure. After detailing the model, two parametric studies are presented that illustrate the influence of increasing rack inlet air temperature, and temperature rise across the rack, on different components in the data center cooling architecture. By considering the total data center, and each component's influence on the greater infrastructure, it is possible to identify the components that contribute most to the resulting inefficiencies in the heat flow from chip to cooling tower and thereby identify the components in need of possible redesign. For the data center model considered here it is shown that the strategy of increasing temperature rise across the rack may be a better strategy than increasing inlet temperature to the rack. Part II of this work expands on this paper with further parametric studies to evaluate the robustness of this data center cooling strategy, with conditions for optimal strategy deployment.

Small-scale vibration energy harvesters that respond efficiently at low frequencies are challenging to realize. This paper describes the design and implementation of one such harvester, which achieves a high volumetric Figure of Merit (FoM v ¼ 2.6% at 11.50 Hz) at the scale of a C-type battery and outperforms other state-of-the-art devices in the sub 20 Hz frequency range. The device employs a 2 Degree-of-Freedom velocity-amplified approach and electromagnetic transduction. The harvester comprises two masses oscillating one inside the other, between four sets of magnetic springs. Collisions between the two masses transfer momentum from the heavier to the lighter mass, exploiting velocity amplification. The paper first presents guidelines for designing and optimizing the transduction mechanism, before a nonlinear numerical model for the system dynamics is developed. Experimental characterisation of the harvester design is then presented to validate both the transducer optimization and the dynamics model. The resulting high FoM V demonstrates the effectiveness of the device for low frequency applications, such as human motion. V C 2016 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4939545] Wireless sensor networks are currently widespread in many aspects of everyday life.1 Typically, each sensor is independently powered by batteries, which potentially leads to some major issues: batteries have a limited lifetime, and their disposal is polluting. Moreover, their replacement in a large network can be costly due to their high numbers and practically difficult, because they may be embedded in structures, and so difficult to reach.2 Battery limitations have led to the interest in converting energy which is already present in the environment into electrical energy. Among all the possible sources, kinetic energy from ambient vibrations is one of the most common forms. Conventional Vibrational Energy Harvesters (VEHs) are based on simple linear spring-mass resonator designs, for which the resonant frequency of the device has to be tuned to the dominant frequency of the ambient vibration. To overcome this problem, a 2 Degree-ofFreedom (2DoF) electromagnetic velocity amplified VEH is presented in this paper. These configurations have been shown to naturally enhance the frequency response and power generated in VEHs due to the nonlinear effects introduced by impacts within the device, which enable momentum transfer between masses.3-8 To enable effective operation at the low frequencies typical of human motion applications (typically under 5 Hz), a nonlinear contribution to the system dynamics of the device described in this paper was added through the use of magnetic springs. Such an approach results in an efficient but small-scale VEH device that couples the high power and wide frequency response of the velocity amplified VEHs but enables operation at low frequencies and leads to high volumetric Figure of This paper outlines a design methodology to electrically optimize the harvester. A numerical model for predicting the...

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