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•Two-tier copper structures of nanowires-microgrooves.•Pool boiling CHF at 246.3 W/cm2, 119.3% enhancement over that on a plain surface.•Combined effects of liquid wicking and ...hydrodynamic instability on CHF.•New theoretical correlations between liquid wicking and modulated wavelength on CHF.
Two-tier structures of Copper Nanowires (CuNWs) and microgrooves have been fabricated on copper substrates. Pool boiling heat transfer experiments have been conducted to study the combined effects of wicking by CuNWs and modulated wavelength by microgroove pitches on pool boiling Critical Heat Flux (CHF). First, 70 nm diameter CuNWs with three different heights of 5 µm, 15 µm, and 25 µm have been synthesized on plain copper surfaces via in-house made Anodic Aluminum Oxide (AAO) templates. The wicking effect of CuNWs at different heights on pool boiling CHF has been measured. Second, microgrooves of 262 µm wide and 518 µm deep at three different mesh pitches of 0.5 mm, 1.0 mm, and 4.0 mm have been fabricated to study the modulated wavelength effect on pool boiling CHF. Last, 25 µm height CuNWs have been integrated with various microgroove meshes to create two-tier structures. The highest pool boiling CHFs of each type of structures are achieved on CuNWs at 25 µm height, microgrooves at 0.5 mm mesh pitch, and two-tier structures of CuNWs at 25 µm height and microgrooves at 0.5 mm mesh pitch are 189.3 W/cm2, 236.4 W/cm2, and 246.3 W/cm2, respectively. The CHF enhancements of these structures over that on a plain surface are 68.6%, 110.5%, and 119.3%, respectively. The experimental results have demonstrated that both wicking effect and modulated wavelength effect contribute to pool boiling CHF enhancement on the two-tier structures, and the collective effect depends on the combination between CuNWs height and microgroove mesh pitch. An existing theoretical model of pool boiling CHF has been extended to include liquid wicking effect and contact angle effect on CuNWs surfaces, and a new correlation for pool boiling CHF on the two-tier structures has been developed to include contributions from both liquid wicking by CuNWs and modulated wavelength by microgrooves.
•Constructal design.•Leaf venation architecture.•Combined effects of in-plane liquid supply by Constructal flow architecture, liquid wicking and bubble nucleation site density on pool boiling ...enhancements.•Pool boiling CHF at 343.1 W/cm2, an increase of 206% over that on a plain surface.•New theoretical correlation to predict pool boiling CHF through by a liquid inflow factor.
Novel biomimetic structures are designed and synthesized to enable liquid water flowing from the periphery to the center of a heating surface, and promote local liquid wicking and active nucleation site density, which enhance the critical heat flux (CHF) and heat transfer coefficient (HTC) of pool boiling heat transfer. The biomimetic structures are designed by mimicking leaf vein growth from an initial point of petiole to an architecture that supplies water and nutrition to the whole leaf lamina. The in-plane flow channels are developed to distribute liquid toward bubble nucleation sites on a heating surface. The in-plane flow channels are then fabricated by: 1) machining a solid biomimetic groove structure at 250 µm height on plain copper surfaces, 2) depositing ~70 nm diameter copper nanowires (CuNWs) of height 25 µm on the solid biomimetic groove structure, and 3) sintering ~25 µm diameter copper powder to create porous biomimetic structures at various heights. The experimental pool boiling results on different structures show that the sintered porous biomimetic structure at 1.0 mm height yields the highest CHF of 343.1 W/cm2, an increase of 206% over that on a polished plain copper surface, which is due to the biomimetic channels providing easier access of liquid water flow toward the dry-out spots as indicated by a liquid inflow factor. The biomimetic structure of CuNWs has dramatically increased CHF and heat transfer coefficient (HTC) than that of a plain surface and a solid biomimetic structure. A theoretical analysis of the liquid thin film beneath hovering bubbles reveals that the population density of vapor stems in the liquid thin film increases with a decrease of the vapor stem diameter as heat flux increases. Moreover, the porous biomimetic structures take advantage of active nucleation sites and their wicking effect to delay the hydrodynamic instability of the liquid thin film, thus increasing the pool boiling heat transfer.
An experimental investigation of the combustion behavior of nano-aluminum (n-Al) and nano-aluminum oxide (n-Al
2
O
3
) particles stably suspended in biofuel (ethanol) as a secondary energy carrier ...was conducted. The heat of combustion (HoC) was studied using a modified static bomb calorimeter system. Combustion element composition and surface morphology were evaluated using a SEM/EDS system. N-Al and n-Al
2
O
3
particles of 50- and 36-nm diameters, respectively, were utilized in this investigation. Combustion experiments were performed with volume fractions of 1, 3, 5, 7, and 10% for n-Al, and 0.5, 1, 3, and 5% for n-Al
2
O
3
. The results indicate that the amount of heat released from ethanol combustion increases almost linearly with n-Al concentration. N-Al volume fractions of 1 and 3% did not show enhancement in the average volumetric HoC, but higher volume fractions of 5, 7, and 10% increased the volumetric HoC by 5.82, 8.65, and 15.31%, respectively. N-Al
2
O
3
and heavily passivated n-Al additives did not participate in combustion reactively, and there was no contribution from Al
2
O
3
to the HoC in the tests. A combustion model that utilized Chemical Equilibrium with Applications was conducted as well and was shown to be in good agreement with the experimental results.
Static nanodroplets and dynamic contact line (CL) movements were visualized by an in situ transmission electron microscope (TEM) liquid cell technique at nanometer spatial resolution. Crawling and ...sliding movements of nanoscale CL were observed. The crawling happened at a capillary number (Ca) range of ∼10–9 to ∼10–8, and the sliding happened at a Ca range of ∼10–8 to ∼10–7. Three dimensional (3D) image construction had been employed to study static and dynamic contact angles (CAs) at nanoscale. CA hysteresis at nanoscale was observed in the sliding but not in the crawling. The energies associated with sliding were analyzed to investigate the CA hysteresis. An empirical model of the relationship between nanoscale CAs and Ca was developed. Both the experimental observation and the empirical analysis suggested that the competition among substrate defect, CL elastic, and molecular activation energies dictated different CL movements at nanoscale.
Spherical Cu nanocavity surfaces are synthesized to examine the individual role of contact angles in connecting lateral Rayleigh-Taylor wavelength to vertical Kevin-Helmholtz wavelength on ...hydrodynamic instability for the onset of pool boiling Critical Heat Flux (CHF). Solid and porous Cu pillar surfaces are sintered to investigate the individual role of pillar structure pitch at millimeter scale, named as module wavelength, on hydrodynamic instability at CHF. Last, spherical Cu nanocavities are coated on the porous Cu pillars to create a multiscale Cu structure, which is studied to examine the collective role and relative significance of contact angles and module wavelength on hydrodynamic instability at CHF, and the results indicate that module wavelength plays the dominant role on hydrodynamic instability at CHF when the height of surface structures is equal or above ¼ Kelvin-Helmholtz wavelength. Pool boiling Heat Transfer Coefficient (HTC) enhancements on spherical Cu nanocavity surfaces, solid and porous Cu pillar surfaces, and the integrated multiscale structure have been investigated, too. The experimental results reveal that the nanostructures and porous pillar structures can be combined together to achieve even higher enhancement of HTC than that of individual structures.
The critical heat flux (CHF) and heat transfer coefficient of de-ionized (DI) water pool boiling have been experimentally studied on a plain surface, one uniform thick porous structure, two modulated ...porous structures and two hybrid modulated porous structures. The modulated porous structure design has a porous base of 0.55
mm thick with four 3
mm diameter porous pillars of 3.6
mm high on the top of the base. The microparticle size combinations of porous base and porous pillars are uniform 250
μm, uniform 400
μm, 250
μm for base and 400
μm for pillars, and 400
μm for base and 250
μm for pillars. Both the CHF and heat transfer coefficient are significantly improved by the modulated porous. The boiling curves for different kinds of porous structures and a plain surface are compared and analyzed. Hydrodynamic instability for the two-phase change heat transfer has been delayed by the porous pillars which dramatically enhances the CHF. The highest pool boiling heat flux occurring on the modulated porous structures has a value of 450
W/cm
2, over three times of the CHF on a plain surface. Additionally, the highest heat transfer coefficient also reaches a value of 20
W/cm
2
K, three times of that on a plain copper surface. The study also demonstrates that the horizontal liquid replenishing is equally important as the vertical liquid replenishing for the enhancement of heat transfer coefficient and CHF improvement in nucleate pool boiling.
For mobile users who seek extra power or computing resources to perform computation-intensive tasks, code offloading to remote infrastructures is a promising solution. However, most of the recent ...works mainly target on code offloading from single mobile devices to remote cloud servers, which restricts the potential of offloading only to devices with available Internet access. Moreover, offloading to remote cloud computing platforms is not always guaranteed to be time efficient and energy conserving. In this paper, we propose
Circa
, a framework that demonstrates the feasibility of code offloading among multiple mobile devices in the same vicinity, leveraging the presence of
iBeacons
.
Circa
eliminates the costs incurred by connecting to a remote cloud and running virtual machine instances in the cloud. With iBeacons, neighbouring devices can discover and support one another through collaborative code offloading with short-range communication, obviating the need for centralized servers . We also propose task allocation algorithms to select reliable collaborators among nearby mobile devices and disseminate the computation intensive tasks among them efficiently in a fair fashion. The performance of the task allocation algorithm is evaluated based on three different mobility models. We also implement a prototype of the
Circa
framework on an iOS platform and validate its feasibility and efficiency using iOS devices . According to the experimental results, by involving nearby mobile devices as collaborators,
Circa
is able to reduce the total execution time of an offloaded task substantially, while preserving the satisfactory performance of mobile applications .
In recent decades, cyber security issues in IEC 61850-compliant substation automation systems (SASs) have become growing concerns. Many researchers have developed various strategies to detect ...malicious behaviours of SASs during the system operational stage, such as anomaly-based detection. However, most existing anomaly-based detection methods identify an abnormal behaviour by checking every single network packet without any association. These traditional methods cannot effectively detect "stealthy" attacks which modify legitimate messages slightly while imitating patterns of benign behaviours. In this paper, we present feature selection and extraction methods to generalise and summarise critical features when detecting insider attacks triggering from untrusted control devices within SASs. By applying a sliding window-based sequential classification mechanism, our detection method can detect anomalies across multiple devices without the need to learn datasets collected from all devices. Firstly, to generalise critical features and summarise systems' behaviours so that it is unnecessary to collect all datasets, we selected and extracted six critical network features from generic object-oriented substation events (GOOSE) messages and seven summarised physical features based on the general architecture of the primary plant of distribution substations. After that, to improve detection accuracy and reduce computational costs, we applied sliding window algorithms to divide datasets into different overlapped window-based snippets. Then we applied a sequential classification model based on Bidirectional Long Short-Term Memory networks to train and test those datasets. As a result, our method can detect insider attacks across multiple devices accurately with a false-negative rate of less than 1%.
An experimental study has been conducted to examine the effects of macroscale, microscale, and nanoscale surface modifications in water pool boiling heat transfer and to determine the different heat ...transfer enhancing mechanisms at different scales. Nanostructured surfaces are created by acid etching, while microscale and macroscale structured surfaces are synthesized through a sintering process. Six structures are studied as individual and collectively integrated surfaces from nanoscale through microscale to macroscale: polished plain, flat nanostructured, flat porous, modulated porous, nanostructured flat porous, and nanostructured modulated porous. Boiling performance is measured in terms of critical heat flux (CHF) and heat transfer coefficient (HTC). Both HTC and CHF have been greatly improved on all modified surfaces compared to the polished baseline. Hierarchical multiscale surfaces of integrated nanoscale, microscale, and macroscale structures have been proven to have the most significant improvements on HTC and CHF. The CHF and HTC of the hierarchical multiscale modulated porous surface have achieved the most significant improvements of 350% and 200% over the polished plain surface, respectively. Experimental results are compared to the predictions of a variety of theoretical models with an attempt to reveal the different heat transfer enhancing mechanisms at different scales. It is concluded that models for the structured surfaces at all scales need to be further developed to be able to have good quantitative predictions of CHFs on structured surfaces.
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•The genotoxicity of TiO2 NPs was confirmed at the single-cell level.•CINP, RPA3, and PRKACA served as biomarkers of TiO2 NPs exposure.•5 nm and 14 nm TiO2 NPs caused a response ...similar to antivirus defense.•Unique gene expression patterns were found within the same exposure of TiO2 NPs.
Titanium dioxide nanoparticles (TiO2 NPs) have attracted substantial attention in various applications, including environmental remediation and nanomedicine. A detailed understanding of the toxicity of TiO2 NPs and the underlying mechanisms is fundamental to further development of their environmental and biomedical applications. Herein, we determined the changes in the transcriptional profile of human bone marrow stromal cells (BMSCs) after a set of different sized TiO2 NPs exposure using single-cell RNA-seq (scRNA-seq). By taking advantage of sensitivity of scRNA-seq, we found that TiO2 NPs exposure led to profound changes in gene expression and DNA damage signaling played a pivotal role in cellular responses to TiO2 NPs exposure. Smaller TiO2 NPs (5, 14 nm) led to a response similar to antivirus defense, and bigger TiO2 NPs (54, 135, and 228 nm) induced proliferation and differentiation changes in transcriptional levels. Validation experiments on representative genes showed that some genes might be served as biomarkers of TiO2 NPs exposure. Furthermore, in vitro and in vivo analyses showed remarkable over-production of reactive oxygen species (ROS) but did not affect osteogenic differentiation potential in BMSCs. The results unveiled landscape changes in transcriptomes in BMSCs induced by TiO2 NPs, which provides new insights into toxicity assessments of nanomaterials.
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