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Hypothesis Due to the complex hydrodynamics of droplet impact on ridged superhydrophobic surfaces, quantitative droplet spreading characteristics are unrevealed, limiting the practical applications of ridged superhydrophobic surfaces. During droplet impacting, the size ratio (the ratio of the ridge diameter to the droplet diameter) is an important factor that affects droplet spreading dynamics. Experiments We fabricated ridged superhydrophobic surfaces with size ratios ranging from zero to one, and conduct water droplet impact experiments on these surfaces at varied Weber numbers. Aided by the numerical simulations and theoretical analysis, we illustrate the droplet spreading dynamics and reveal the law on the maximum axial spreading coefficient. Finds The results show that the droplet spreading and retraction dynamics on ridged superhydrophobic surfaces are significantly asymmetric in the axial and spanwise directions. Focusing on the maximum axial spreading coefficient, we find it decreases first and then increases with increasing size ratios, indicating the existence of the critical size ratio. The maximum axial spreading coefficient can be reduced by 25-40% at the critical size ratio compared with that on flat surfaces. To predict the maximum axial spreading coefficient, two theoretical models are proposed respectively for size ratios smaller and larger than the critical size ratio.

Land Surface Temperature (LST) derived from space-borne Thermal-infrared (TIR) sensors is a key parameter of urban climate studies. Current studies are inefficient to capture the spatial and temporal variations of LST for only one snapshot adopted at one time. Focusing on the characterization of the spatial and temporal of LST variations at local scales, the latent patterns, and morphological characteristics are extracted in this study. Technically, sixteen MODerate-resolution Imaging Spectroradiometer (MODIS) eight-day synthesized LST products (MYD11A2) in 2002, 2007, 2012, and 2017 are employed. First, the non-parametric Multi-Task Gaussian Process Model (MTGP) is used to extract the smooth and continuous Latent LST (LLST) patterns using one LST subset and its temporally adjacent images. Second, the Multi-Scale Shape Index (MSSI) is then applied to quantify the morphological characteristics at the optimal scale. Then, the LLST patterns and MSSI maps are clustered into multiple spatial categories. The specific clusters with the highest LLST and MSSI values are considered as local LLST hotspots. The Hotspots Weighted Mean Center (HSWMC) and standard deviation ellipse are adopted to further investigate the spatiotemporal change of hotspots orientation, direction, and trajectories. Results revealed that Impervious Surfaces (IS) composition is the most significant external forcing of local LST anomalies. The configuration factors (e.g., shape index, aggregation index) also have a noticeable local warming effect. This study represents a latent pattern and morphology-based framework for LST hotspots spatial and temporal variations characterization, catering to the zoning and grading strategies in urban planning.

The life of transformer oil-paper insulation is determined by the insulation property of transformer cellulosic insulation. Existing studies have shown that sulfur corrosion poses a serious threat to the properties of transformer oil-immersed cellulosic insulation due to the reaction of corrosive sulfur dissolved in oil with copper winding. This paper investigated the effect of sulfur corrosion on the characteristics of transformer cellulosic insulation, cellulosic insulation winding samples with different degrees of sulfur corrosion were obtained through an accelerated thermal aging experiment. Then, short-/long-term withstand voltage tests for the cellulosic insulation winding were conducted. Experimental results showed that the contamination of sulfur corrosion had a minimal effect on the power–frequency breakdown voltage of the cellulosic insulation winding but considerably decreased its long-term electrical life. Combined with the analysis of the electrical/physical and chemical parameters of oil-immersed cellulosic insulation, the failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion was synthetically discussed and analyzed.

The essential characteristic of the self-propelled jumping droplet is the jumping velocity, which determines its application value in heat transfer enhancement, antifrosting, self-cleaning, and so on. The jumping velocity is directly related to the energy conversion efficiency (i.e., the ratio of jumping kinetic energy surface energy released by coalescence to surface energy released by coalescence) and it is affected by the initial state of droplets but there is no unified theory to describe the relationship between the initial state of droplets and the energy conversion efficiency. In this paper, the projection of the initial chemical potential and the final chemical potential difference of droplets in the direction of jumping is defined as jumping potential by theoretical analysis of the chemical potential evolution. The effects of droplet number, distribution, and radius ratio on energy conversion efficiency can be synthetically characterized by jumping potential. The larger the jumping potential is, the higher the energy conversion efficiency is. Finally, the rationality and universality of the jumping potential are verified by numerical simulations and comparison with previous studies. The jumping potential can explain phenomena that cannot be explained in previous studies and can provide a synthesis critical value of droplet jumping.

This paper investigated the influence of paper-wrapped on the deposition of sulfur corrosion on the insulation winding (paper-wrapped copper conductor) in oil-immersed insulation containing dibenzyl disulfide (DBDS). The distribution of sulfur corrosion deposit on a 500-kV electric reactor was investigated and analyzed, and the properties of oil were analyzed. Aging experiments were also conducted for oil-paper insulation with 500-mg/kg DBDS under IEC 62535 corrosion test condition. The corrosion degree of insulation winding was evaluated, and the properties of oil were also discussed. Results showed that the enwinding pattern of the insulating paper on the copper conductor affected the deposition distribution of copper sulfide to be an alternate appearance of a non-corroded crevice and a strip-shaped corrosion deposit on the copper conductor according to the results of field and laboratory. Furthermore, for the insulation winding, the increased number of paper layers on the copper conductor led to the aggravation of sulfur corrosion, prevented the diffusion of copper ions in the oil, and accelerated the formation of acids, which co-affected the properties of the oil. The enwinding of insulating paper on the copper conductor led to the appearance of a tiny crevice on the insulation winding, which is an important occurrence precondition of crevice corrosion. Finally, the mechanism underlying crevice corrosion for insulation winding was theorized as electrochemical corrosion leading to copper sulfide formation.

There is a growing demand for urban form optimization to mitigate urban heat island (UHI) effect. Nevertheless, how UHI responds to various urban morphological patterns is still limitedly understood, especially for cities in developing countries. Here, based on 1288 urban clusters identified automatically across China, we created a consistent analytical environment to recognize both homogeneity and heterogeneity of the relationship. Specifically, urban form was characterized from aspects of size, shape, and centrality, respectively measured using metrics of urban size, area-weighted mean shape index (AWMSI) and dispersion index (DI), entropy and Moran’s I. Then, relationships between the metrics and surface UHI (SUHI) were modelled using the ordinary least squares (OLS). Results reveal that in nighttime when stronger relationships were observed than in daytime, centrality, a feature of intra-urban development structure largely neglected by previous studies, dominated as the most influential aspect. The relationships also varied across climate zones. A two-step OLS regression further reveals them to be size-dependent. As city expanded, the rising shape irregularity created a cooling impact, while the rising centrality leaded to urban warming. Overall, this study suggests that, in addition to controlling urban expansion, the optimal urban form for SUHI mitigation is moderately dispersed, polycentric, and decentralized.

The failures of high-voltage transformers and reactors can result from the deposition of copper sulfide on the oil-impregnated paper insulation. This study explored the influence of electric fields on copper sulfide deposition and the properties of insulating oils as a result of the reaction of dibenzyl disulfide (DBDS) with copper in oil-immersed transformers. Accelerated aging experiments were conducted for pig-tail windings under two sets of conditions, namely, thermal aging only at 140 °C and AC/DC electrical and thermal aging at 5 kV/140 °C under low oxygen condition. This was combined with oil containing dibenzyl disulfide (DBDS) added to the non-corrosive oil to a concentration of 200 mg/kg. The corrosive sulfur deposits were examined by observing the surface morphology of the windings and measuring the copper and sulfur contents on the windings via inductively coupled plasma-atomic emission spectrometry and a precipitation method. The properties of the insulating oils were measured and analyzed at intervals. The results indicate that electric fields increased the amount of deposited copper sulfide on the windings. The higher the electric field strength, the higher the amount of deposited copper sulfide on the insulating paper surface. Subjecting the oil-paper insulation to electric field and elevated temperature generates dissolved copper in the insulating oil. The electric field can promote the dissolution of copper in oil. The ageing oil under thermal and electrical stresses promote copper dissolution and increase corrosion rate.

Numerous failures of high-voltage transformers and reactors are caused by copper sulfide formation in oil-immersed insulations. This study explored the effect of electrical fields on copper sulfide formation. Accelerated aging experiments were conducted for mineral oil that contains dibenzyl disulfide, which was aged along with insulation windings under different conditions, including single thermal aging and electrical–thermal aging. The corrosive sulfur deposits were quantified using SEM/EDX and ICP-AES. The properties of the insulation oils were also measured and analyzed. Corrosion mechanisms for electrical fields leading to the acceleration of copper sulfide deposition on the oil-immersed insulation were proposed.

Numerous failures of high-voltage transformers occur because of sulfur corrosion in oil-impregnated paper insulation containing dibenzyl disulfide (DBDS). This study determines the effects of DBDS, the copper-insulating paper contact, the insulating paper and the atmosphere on copper sulfide formation. Accelerated thermal aging experiments were conducted for mineral oil that contained DBDS and insulation windings under atmospheric (nitrogen/air) conditions. The corrosive sulfur deposits were quantified using SEM/EDX, ICP-AES and precipitation methods. Oil acidity was measured and analyzed. The study proposes a corrosion mechanism for local enrichment of acids and copper ions in copper-insulating paper contacts leading to the acceleration of sulfur corrosion.

Conspicuous expansion and intensification of impervious surfaces accompanied by rapid urbanization are widely recognized to have exerted evident impacts on the urban thermal environment. Investigating the spatially and temporally varying relationships between Land Surface Temperature (LST) and impervious surfaces (IS) at multiple scales is of great significance for steering IS expansion and intensification. This study proposes an analytical framework to investigate the spatiotemporal variations of LST and its responses to IS in Wuhan, China at both city scale and sub-region scale. The summer LST patterns in 2002&ndash, 2017 are extracted by Multi-Task Gaussian Process (MTGP) model from raw 8-day synthesized MODerate-resolution Imaging Spectroradiometer (MODIS) LST data. At the city scale, the weighted center of LST (LSTWC) and impervious surface fraction (ISFWC), multi-temporal trajectories and coupling indicators are utilized to comprehensively examine the spatial and temporal dynamics of LST and IS within Wuhan. At the sub-region scale, urban heat island ratio index (URI), impervious surfaces contribution index (ISCI) and sprawl rate are introduced for further quantifying the relationships of LST and IS. The results reveal that IS and hot thermal landscapes expanded by 407.43 km2 and 255.82 km2 in Wuhan in 2002&ndash, 2017 at city scale. The trajectories of LSTWCs and ISFWCs are visually coherent and both heading to southeast direction in general. At the sub-region scale, the specific cardinal directions with the highest ISCI variations are examined to be the exact directions of ISFWC trajectories in 2002&ndash, 2017. The results reveal that the spatiotemporal variations of LST and IS are highly correlated at both city and sub-region scales within Wuhan, thus testifying the significance of steering IS expansion and renewal for controlling urban thermal environment deterioration.

Coalescence-induced droplet jumping on superhydrophobic surfaces has good application potential in the fields of heat transfer enhancement, self-cleaning, water collection, and microfluidics. However, the direction of droplets jumping on the plane is uncontrollable and the energy conversion rate is low, which limits the application of droplets jumping. In this work, we report a flexible approach to efficiently regulate the direction and velocity of droplet jumping on superhydrophobic surfaces using superhydrophobic string. By changing the inclination angle of the string, the jumping direction of the coalescent droplet shows a maximum deviation of 68.1° from the surface normal. The maximum energy conversion rate is 32.1%, which is about 5.4 times higher than that of jumping on the plane. We analyzed the mechanism of the efficient regulation and found that the string affected the hydrodynamic characteristics of the jumping process, thus changing the jumping direction and the speed of the coalescent droplet.

This paper investigated the influence of 2, 6-di-tert-butyl-p-cresol (DBPC) on the long-term ageing characteristics of oil–paper insulation and the deposition and migration of copper sulphide induced by the reaction of copper and dibenzyl disulphide (DBDS) in oil-immersed transformers. A thermal ageing experiment at 130°C were designed and conducted by adding different concentrations of DBPC to the naphthenic mineral oil and after the experiments a set of characteristic parameters were tested. The result indicates that the excessive concentration (≥0.6%) of DBPC will slightly increase the ageing of insulating oil and increase the carboxyl group in the mineral oil, affecting the reaction of DBDS and copper, and the insulating paper wrapped on the copper strip was also aged seriously accordingly. Meanwhile, the deposition of copper sulphide on the insulating paper is found to be slightly increased with the increase of the concentration of DBPC. When insulating oil contains DBPC and DBDS, due to migration of DBDS–Cu and DBPC–Cu in oil, the copper ions in the oil would be fluctuating at a certain period of time. Although the addition of a small concentration of DBPC can improve the antioxygenic property of the insulting oil to some extent, the transformers that contain excessive concentration of DBPC may be more harmful than beneficial.

This study investigated the inhibitory effects of adsorption treatment on the degradation of oil–paper insulation and corrosive sulphur in oil. A thermal ageing experiment at 130°C was conducted using five different adsorbents to adsorb some impurities in oil on the 15th day of a 30 day ageing test, the related characteristics of oil–paper insulation before and after the adsorption treatment were measured and analysed. Meanwhile, a thermal ageing experiment at 150°C was also conducted before using five different adsorbents to adsorb dibenzyl disulphide (DBDS) in oil, energy dispersive X-ray was used to evaluate the degree of the corrosion of windings. The result indicates that adsorption treatment is an effective method to inhibit the degradation of oil–paper insulation. A molecular sieve, silica gel and activated alumina can effectively improve the performance of oil–paper insulation. By contrast, the antioxidant (2, 6-di-tert-butyl-p-cresol) can be also adsorbed by adsorbents, which accelerate the ageing degradation of the insulating oil. On the other hand, the adsorbents can adsorb DBDS in oil, but cannot remove all DBDS. The effect achieved from silica gel is especially significant, which not only effectively improves the performance of oil–paper insulation but also adsorbs DBDS in oil.

As, a kind of sulfur antioxidant, dibenzyl disulfide (DBDS) generally exist in the mineral insulating oil, which can improve the oxidation stability of oil. However, DBDS can also lead to corrosion problems of transformer oil. To investigate the effect of the corrosive action and the antioxidant action of DBDS on the oil-paper insulation, oil samples with different DBDS concentration (0–200mg/kg) were made and conducted to perform thermal aging experiments together with insulation windings at 130Ό under the condition of nitrogen. After the aging experiment, the copper strip and insulating paper surface were observed, the properties of insulating oil were also measured and analyzed. Experimental results showed that when the insulating oil contained less than 25mg/kg DBDS, the antioxidant effect of DBDS would be dominant, the oxidation stability of insulating oil was improved, the performance of oil was better than that of excluding DBDS samples. When the insulating oil contained more than 25mg/kg DBDS, the corrosive effect of DBDS will be greater than its antioxidant effect, copper ions and acid will be accelerated, it will lead to more serious of aging oil.

Water icing is a natural phase change phenomenon which happens frequently in nature and industry and has negative effects on a variety of applications. Deicing is essential for iced surfaces, but even for a nanoengineered superhydrophobic surface, deicing may be incomplete with many adherent unmelted ice droplets which have potential for reicing. Here, we focused on the reicing characteristics of droplets on a solid superhydrophobic surface, which has lacked attention in previous studies. Our results show that the nucleation and ice crystal growth characteristics of a reicing droplet are quite different from those of a first-time icing droplet. During reicing, secondary nucleation due to fluid shear always occurs first on the edges of unmelted ice, accompanied by fast-growing ice crystals that can trigger heterogeneous nucleation when in contact with the solid surface. The reicing takes place under very small supercooling (less than 0.5 °C), and the superhydrophobic surface does not play a key role, meaning that any current icephobic surfaces lose their features, which poses great challenges for anti-icing. In addition, because of the small supercooling, no recalescence phenomenon appears during reicing and the droplet remains transparent instead of clouding. Owing to the unmelted ice floating on the top of the droplet, the droplet shape after reicing is also distinguishing from that after normal icing, but the pointy tip formation during reicing and normal icing shows a uniformity. These results shall deepen the understanding of the anti-icing and deicing physics.

The coalescence-induced droplet jumping on superhydrophobic surfaces has attracted considerable attention over the past several years. Most of the studies on droplet jumping mainly focus the droplet jumping on almost flat surfaces or ignore the effect of the microstructure. However, the microstructure often exists on superhydrophobic surfaces, and this effect remains little noticed and poorly understood. In this work, a simulation is carried out to investigate the effect of microstructure on droplet jumping. The microstructure with a similar scale to the jumping droplet on superhydrophobic will affect the jumping direction. The microstructure will improve the jumping velocity and change the jumping direction of the droplet. This work will provide effective guidelines for the design of functional SHSs with controlled and enhanced droplet jumping for a wide range of industrial applications.

Droplets on the superhydrophobic surface can fall off the surface spontaneously, which greatly promote dropwise condensation. This study considers a continuous droplet condensation process including droplet growth and droplet jumping. A droplet growth model considered NCG is developed and droplet jumping is simulated using VOF (Volume Of Fluid) model. Al–based superhydrophobic surfaces are prepared using chemical deposition and etching method. The Al-based superhydrophobic surface has a contact angle of 157°±1° and a rolling angle of 2°±1°. An observation experiment is designed to observe droplet jumping on superhydrophobic surface using a high– speed camera system. The result of droplet growth model shows a good match with experimental data in mid-term of droplet growth. Fordroplet jumping, simulation and experiment results show that droplet jumping of different diameter hasa universality in a non–dimensional form. The jumping process can be divided into 3 stages and droplet vibration is observed.

Many failures of high-voltage transformers and reactors occurred because of copper sulfide formation and deposition, this study aims to explore the influence of the insulating paper and atmosphere on the deposition of copper sulfide on the insulating windings as a result of the reaction of dibenzyl disulfide (DBDS) with copper in oil-immersed insulation. The accelerated thermal aging experiments under atmosphere (Nitrogen and air) conditions at 130°C were conducted by adding 1000mg/L dibenzyl disulfide (DBDS) and the insulation windings which half of copper strip were wrapped insulating paper to the mineral insulation oil. The deposition of copper sulfide were studied by observing the surface morphology of windings and analyzing the surface components of the windings by EDX. The results indicate that copper sulfide is prone to deposition on the copper strip surface which is wrapped by the insulating paper. Few copper sulfide is deposited on the bare copper strip under nitrogen, but copper sulfide is deposited on the copper strip under air. Meanwhile, the sediments of copper sulfide deposition on the copper strip under air are much higher than that under nitrogen, and more sediments of copper sulfide are deposited on the insulating paper under air than on the insulating paper under nitrogen. Moreover, the effect of wrapped paper and atmosphere on the mechanism of copper sulfide deposition will be discussed.