Добірка наукової літератури з теми "Condensation frosting"

A numerical model for coupled heat and moisture transfer with sorption, condensation, and frosting in rotary energy exchangers is presented and validated with experimental data. The model is used to study condensation and frosting in energy wheels. Condensation/frosting increases with humidity and at some humidity level, water/frost will continually accumulate in the wheel. The sensitivity of condensation and frosting to wheel speed and desiccant type are studied. The energy wheel performance is also presented during both sorption and saturation conditions for a desicant coating with a Type I sorption isotherm (e.g., molecular sieve) and a linear sorption isotherm (e.g., silica gel). Simulation results show that the desiccant with a linear sorption curve is favorable for energy recovery because it has better performance characteristics and smaller amounts of condensation/frosting for extreme operating conditions.

Condensation frosting usually causes a negative influence on heat exchangers employed in engineering fields. As the relationships among the first three typical condensation frosting stages in the edge regions of cold plates are still unclear, an experimental study on the localized condensation frosting characteristics in the edge region of a cold plate was conducted. The edge effects on the water droplet condensation (WDC), water droplet frozen (WDF) and frost layer growth characteristics were quantitatively investigated. The results showed that the number of droplets coalescing in the edge-affected regions was around 50% greater than in the unaffected regions. At the end of the WDC stages, the area-average equivalent contact diameter and coverage area ratio of water droplets in the edge-affected regions were 2.69 times and 11.6% greater than those in the unaffected regions under natural convection, and the corresponding values were 2.24 times and 9.9% under forced convection. Compared with the unaffected regions, the WDF stage duration in the edge-affected regions decreased by 63.6% and 95.3% under natural and forced convection, respectively. Additionally, plate-type and feather-type frost crystals were, respectively, observed in natural and forced convection. The results of this study can help in the better understanding of the condensation frosting mechanism on a cold plate, which provides guidelines for optimizing the design of heat exchanger structures and system control strategies facing frosting problems.

Hydrophilic–hydrophobic hybrid wettability structures, inspired by desert beetles, have been widely designed to enhance the dewdrops’ migration under subcooled or/and high-humidity environment. However, it is still a challenge to regulate the graded distribution of the hydrophilic micro-regions for condensation applications. In this paper, we design a simple spray method to prepare the superamphiphilic–superamphiphobic hybrid wettability coatings by controlling the mass ratio (MR) of superamphiphobic SiO2 nano-powder and superamphiphilic gypsum micro-powder. We compare the macroscopical wettability, condensation heat transfer efficiency, frosting delayed time and water harvesting rate to demonstrate the unique advantage of hybrid wettability structures. The results show that the condensation heat transfer efficiency, frosting delayed time and water harvesting rate can be respectively promoted to about 131.50% [Formula: see text], 134.74% [Formula: see text] and 135.62% [Formula: see text], although their macroscopical wettability will gradually reduce with the MR increase. This work will provide substantial insights into the fabrication of efficient superhydrophilic–superhydrophobic hybrid wettability surfaces for condensation heat transfer, anti-frosting and water harvesting applications.

In this study, a novel, thorn-shaped, containing, hydrophilic, and hydrophobic surface is proposed to have a better condensate drainage characteristic and to delay the required time for frosting. By using a hydrophilic and hydrophobic mixed thorn-shaped surface created by screen printing, the design makes use of the differences in the wettability gradient to achieve rapid condensate drainage and to lengthen the time for frosting. The results of a frosting experiment indicated that the droplet adsorption and combination and discharge effect in the thorn sample were substantial. The drainage effect increased the surface renewal rate and inhibited ice layer growth on the thorn sample by 52.4% compared with that on pure copper surface. The heat transfer coefficient of the thorn sample during frosting was approximately 16.2% higher than that of pure copper surface. In addition, the defrosting results indicated that the defrosting time of the thorn sample was almost equal to that of the pure copper sample. However, large droplets were easily stagnated at the structural junction due to contact angle hysteresis after defrosting.

Condensation frosting is a type of icing encountered ubiquitously in our daily lives. Understanding the dynamics of condensation frosting is essential in developing effective technologies to suppress frost accretions that compromise heat transfer and system integrity. Here, we present an experimental study on ice dendrite growth atop a single frozen drop, an important step affecting the subsequent frosting process, and the properties of fully-developed frost layers. We evaluate the effect of natural convection by comparing the growth dynamics of ice dendrites on the surface of a frozen drop with three different orientations with respect to gravity. The results show that both the average deposition rate and its spatial variations are profoundly altered by surface orientations. Such behavior is confirmed by a numerical simulation, showing how gravity-assisted (hindered) vapor diffusion yields the deposition outcomes. These findings benefit the optimization of anti-/de- frosting technologies and the rational design of heat exchangers.

Superhydrophobic films fabricated on copper and aluminum surfaces have potential applications to solve water condensation and frosting problems on chilled ceiling system. The rough surfaces of copper foils obtained by solution immersion method exhibit the existence of fractal structures. The hydrophobicity of copper surfaces is enhanced with fractal structures. The relationship between contact angles (CAs) and the fractal dimensions (FDs) for surface roughness of Cu samples with different etching time is investigated. Moisture condensation and frosting experiments on the two kinds of surfaces are conducted in natural environment under different chilling temperatures. During condensation, micro water condensate droplets drift down the surface like dust floating in the air. Several larger condensate droplets about 1–2 mm appear on the substrates after 3 h condensation. This continuous jumping motion of the condensate will be beneficial in delaying frosting. The results demonstrate that dense nanostructures on copper surfaces are superior to loose lattice-like microstructures on aluminum surfaces for preventing the formation of large droplets condensate and in delaying the icing. The large water droplets of 2–3 mm in diameter that would form on a common metal foil are sharply decreased to dozens of microns and small droplets are formed on a modified surface, which will then drift down like a fog.

The most ubiquitous mode of frost formation on substrates is condensation frosting, where dew drops condense on a supercooled surface and subsequently freeze, and has been known since the time of Aristotle. The physics of frost incipience at a microscopic scale has, nevertheless, eluded researchers because of an unjustified ansatz regarding the primary mechanism of condensation frosting. It was widely assumed that during condensation frosting each supercooled droplet in the condensate population freezes in isolation by heterogeneous nucleation at the solid-liquid interface, quite analogous to the mechanism of icing. This assumption has very recently been invalidated with strong experimental evidence which shows that only a single droplet has to freeze by heterogeneous nucleation (typically by edge effects) in order to initiate condensation frosting in a supercooled condensate population. Once a droplet has frozen, it subsequently grows an ice bridge towards its nearest neighboring liquid droplet, freezing it in the process. Thus ensues a chain reaction of ice bridging where the newly frozen droplets grow ice bridges toward their nearest neighbor liquid droplets forming a percolating network of interconnected frozen droplets. Not always are these ice bridges successful in connecting to their adjacent liquid droplets. Sometimes the liquid droplet can completely evaporate before the ice bridges can connect, thus forming a local dry region in the vicinity of the ice bridge. In this work, we first formulate a thermodynamic framework in order to understand the localized vapor pressure gradients that emerge in mixed-mode phase-change systems and govern condensation and frost phenomena. Following this, we study droplet pair interactions between a frozen droplet and a liquid droplet to understand the physics behind the local ice bridge connections. We discuss the emergent scaling laws in ice bridging dynamics, their relative size dependencies, and growth rates. Thereafter, we show how with spatial control of interdroplet distances in a supercooled condensate and temporal control of the first freezing event, we can tune global frost propagation on a substrate and even cause a global failure of all ice bridges to create a dry zone. Subsequently, we perform a systematic study of dry zones and derive a scaling law for dry zones that collapses all of our experimental data spanning a wide parameter space. We then show that almost always the underlying mechanism behind the formation of dry zones around any hygroscopic droplet is inhibition of growth and not inhibition of nucleation. We end with a discussion and preliminary results of our proposed anti-frosting surface that uses ice itself to prevent frost.
Master of Science

The wettability and phase change phenomena of water are ubiquitous on biological and artificial surfaces. Properties like water repellency, self-cleaning, coalescence induced condensation jumping, anti-frosting, and dew harvesting arise on surfaces with particular structures and chemistry and are of particular interest for energy and water saving. This thesis collects different studies of wettability and phase change on natural and artificial surfaces: growth and self-ejection of condensation droplets on micro and nanostructured surfaces we fabricated, their applications, the Sliding on Frost of condensation droplets observed on the Cotinus Coggygria leaf, the dew harvesting property of the Old Man of the Andes Cactus enhanced by distance coalescence through microgrooves and finally, a theoretical study of viruses viability in sessile droplets. The first chapter introduces the theoretical framework of wettability and phase changes on surfaces. In the second chapter, we present the self-ejection of condensation droplets from hydrophobic nanostructured microstructures. We modelled analytically the droplets jumping and fabricated surfaces to verify the predictions. The fabricated geometry was inspired by the modelling and the available fabrication techniques. We tested the surfaces in condensation conditions. Using a high frame rate camera coupled with a long working distance microscopy objective, we investigated the growth and ejection transient. We then compared the experimental self-ejection velocity for various structures geometry with our analytical models. In Chapter 3, we investigated the applications of the fabricated surfaces reported in Chapter 2. In Chapter 4, we explore the condensation frosting on the leaf of Cotinus Coggygria, native of our woods and with interesting hydrophobic properties. Covered by wax nanotubules, it exhibits coalescence-induced condensation jumpings that may be a useful cleaning tool. Furthermore, the frost is delayed but not only for the jumping. Surprisingly, at temperatures some degrees below zero, we observed what we called ‘droplet Sliding on Frost bridges’, that further delays frosting. We described the feasibility of this sliding in terms of dynamic contact angles of the surface and contact angles of supercooled water on ice. By capturing high temporal and spatial resolution videos we investigated the sliding on frost and droplet recalescence (fast dendrite growth that partially solidify the liquid). The responsible for the failure of sliding for temperatures from about -8 ° C down appears to be the advancing angle of water on ice that increases with the subcooling rather than the recalescence that blocks the drop in place. These results add a piece to the fundamental research on the supercooled water-ice-vapour interfaces. As it often happens, biological surfaces offer a starting point for the study of fundamental mechanisms and the development of artificial surfaces with optimized properties. In the Chapter 5, the multifunctional roles of hairs and spines in Old Man of the Andes Cactus (Oreocereus trolli) are studied. We study the morphology of the appendages, the hairs wettability, mechanical properties of both, and the dew formation on spines. The longitudinal microgrooves on the spines cause a particular phenomenon of distant coalescence (DC), in which smaller droplets flow totally or partially into larger ones through the microgrooves, with consequent accumulation of water in a few large drops. An earlier study has shown artificial micro-grooved surfaces that exhibit DC are more efficient than flat ones at collecting and sliding dew, and thus these cactus spines could act as soil dew conveyors. The agreement between our analytical model and experimental data verifies that the flow is driven by the Laplace pressure difference between the drops. This allowed us to obtain a general criterion for predicting the total or partial emptying of the smaller drops as a function of the dynamic contact angles of a surface. Based on this criterion, an hydrophilic material with small contact angle hysteresis would allow a greater number of complete drops emptying. The COVID-19 pandemic has raised the problem of contagion from airborne and deposited droplets. In the last chapter, we report the state of the art of experiments on the viability of viruses in deposited droplets. Up to date, it has been experimentally highlighted that the relative viability of some viruses (RV) depends on the material chemistry, temperature, and interestingly, on relative humidity (RH) with a U-shaped trend. One of the current hypotheses is that the cumulative dose of salt concentration (CD) affects RV. We model the RV of viruses in sessile droplets by inserting a RV-CD relation in a model of droplet evaporation. By considering a saline water droplet (one salt) as the simplest approximation of real solutions, we analytically simulate the time evolution of salt concentration, vapor pressure, and droplet volume varying contact angles, droplet sizes, and RH in the range 0–100%. The results elucidate some previously not yet well-understood dynamics, demonstrating how three main regimes—directly implicated in nontrivial experimental trends of virus RV—can be recognized as the function of RH. The proposed approach could suggest a chart of a virus fate by predicting its survival time at a given temperature as a function of RH and contact angle. We found a good agreement with experimental data for various enveloped viruses and predicted in particular for the Phi6 virus, a surrogate of coronavirus, the characteristic U-shaped dependence of RV on RH. Given the generality of the model, once experimental data are available that link the vulnerability of a certain virus (such as SARS-CoV-2) to the concentrations of salts or other substances in terms of CD, it is envisioned that this approach could be employed for antivirus strategies and protocols for the prediction/reduction of human health risks associated with SARS-CoV-2 and other viruses.

博士
國立交通大學
機械工程系所
105
Phase change is a commonly seen process in a wide range of systems, including desalination, power generation, water-harvesting, and electronics cooling. Micro/nanostructured surfaces have been recognized to have a huge potential in promoting the efficiency of phases interaction in phase change processes. This thesis aims to improve the heat and mass transfer in condensation by using micro/nanostructured surfaces, and to promote the anti-frosting and de-frosting abilities by using micro/nanostructured surfaces. Condensation is a common phenomenon and is widely exploited in power generation and refrigeration systems. We reported a new concept to enhance condensation by controlling heterogeneous nucleation on superhydrophobic (SHB) surfaces. Condensation on plain silicon surface (plain Si), silicon nanowire coated (SiNW) surface and microgroove with silicon nanowire coated (MG/SiNW) surfaces have been examined. Heterogeneous nucleation on the MG/SiNW surface could be spatially controlled by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale. Moreover, the nucleation site density (NSD) can also be manipulated by tailoring the density of the microgroove on the surface. Our experimental results show that the MG/SiNW surfaces can effectively promote condensation by utilizing the spatial control of nucleation. This suggests that potentially high heat and mass transfer rates can be achieved on the MG/SiNW surfaces. It is worth noting that utilizing micro/nanostructured surface is not necessarily advantageous because the apparent Cassie droplets are usually in fact partial Wenzel in condensation. The Wenzel droplets would result in an increase in droplet departure diameter and thereby deteriorating the condensation performance on the micro/nanostructured surfaces. To attain the efficient shedding of Cassie droplets in condensation on a SHB surface, a Bond number (a dimensionless number for appraising dropwise condensation) and a solid−liquid fraction smaller than 0.1 and 0.3, respectively, were suggested. Ice formation is a catastrophic problem affecting our daily life in a number of ways. At present, de-icing methods are costly, inefficient, and environmentally unfriendly. Ice can be formed on a solid surface either by condensation-freezing process or by frosting process. Although SHB surfaces can potentially be an ice-phobic surface in the condensation-freezing process, frosting is expected at a very cold temperature. Thus, indiscriminate frost formation is found everywhere on the solid surfaces through the frosting process, eliminating the ice-phobic function on the SHB surfaces. Frosting on plain Si surface, SiNW surface, v-shaped microgroove (VMG) surfaces and trapezoid microgroove (TMG) surface have been systematically investigated. It was found that ice embryos could preferentially nucleate at the microgroove on the VMG surfaces and TMG surface. Ice NSD could also be manipulated by tailoring the number of microgrooves on the surfaces. Besides, ice crystals grew and stacked along the direction of grooves on VMG surfaces. The spatial control of frost formation and the confinement of ice growing kinetics on VMG surfaces could effectively improve the anti-frosting and de-frosting performances. The VMG surface possessed the longest ice-covering time (the time required for ice to cover the whole surface area in the frosting experiments) and the shortest dwell time (the time period during which ice covered the whole surface area after switching off the Peltier cooler in the de-frosting experiments) among various kinds of surfaces. This implied that v-shaped microgroove surface exhibited the best anti-frosting and de-frosting performances among the studied surfaces. This thesis has demonstrated the concepts of designing micro/nanostructured surfaces, which can improve condensation performance and anti-frosting/de-frosting abilities. We anticipate that the concept could be adopted in the other phase change process such as boiling or evaporation process.

Minimization of condensate (frost melt water) retention on a surface operating under frosting/defrosting condition is of tremendous importance in a wide range of air conditioning and refrigeration applications. In the present study, the wetting characteristics, condensation and frosting pattern and the drainage of frost melt water from aluminum surfaces with parallel microgrooves have been examined and compared to the flat baseline surfaces. These surfaces are fabricated by topographical modification only, via standard photolithographic process. The microgrooved samples exhibit wetting anisotropy and static contact angles are as high as 149 and 112° when viewed from parallel and perpendicular directions to the grooves, respectively. Frost is grown on the samples inside a thermally controlled chamber at 3 different plate temperatures of −8°C, −13°C and −18°C, air temperature of 20±2°C and for 3 relative humidity conditions (50%, 70% and 90%). The duration of the frosting cycle is 45 minutes and tests are continued up to 5 frosting cycles, each time defrosting for a certain length of time at the end of frosting period. Significantly different size, shape and distribution of condensed and frozen water droplets on the grooved surfaces are observed from that on the flat baselines. The microgrooved samples are found to manifest better water drainage behavior and drained up to 50% more melt water compared to the flat baseline surfaces. While the amount of water retention on the baseline surfaces increases in the subsequent refrost cycles and is highest in the 5th frost cycle, the microgrooved surfaces show consistently improved water drainage in all cycles.

Despite that using surface-roughness-induced superhydrophobic surface as a solution for ice/snow accretion issues has achieved extensive progresses, its icephobicity breaks down in case of condensation frosting, while the high aspect ratio structure brings more concerns on its durability and sustainability. In this work we investigated condensate frosting on substrates fabricated with patterned micropillars having a small aspect ratio, and studied the freezing propagation with different pattern sizes. The results show that a coarse patterned substrate can effectively suppress the freeing propagation while a fine patterned one can drastically promote the freezing propagation. Frost coverage can also be reduced with proper pattern design. A theoretical model was developed to explain the mechanism of surface ice propagation, and agrees well in tendency with experiment measurements. The aim of this study is to provide some new insights on the influence of surface morphology on ice growth.

Minimizing water retention on the air side of aluminum surfaces is important in the design and operation of efficient heat exchangers for heating, ventilation, air-conditioning and refrigeration (HVAC&R) systems. Accumulation of water degrades the performance of heat exchangers by lowering the heat transfer rate and increasing the pressure drop. As a result, power consumption in such systems increases. In this work, a method of fabricating liquid-infused slippery surfaces with honeycomb-like superhydrophobic micro-/nano-structure substrate via an anodization process is developed. The slippery surface exhibits superhydrophobicity with a contact angle of 155° and a sliding angle smaller than 5°. The delay of ice formation is observed during condensation/frosting experiment. Frost-melt retention experiments show that the liquid-infused slippery surface reduces the water retention by 90% compared to an untreated specimen. The longevity of the slippery surface is also explored. The water retention ratio does not show a significant change after 60 frosting/defrosting cycles, and is still only one third that of the baseline. The slippery surface has potential in HVAC&R applications.

Abstract Autonomous vehicles are part of an expanding industry that encompasses various interdisciplinary fields including but not limited to dynamics and control, thermal engineering, sensors, data processing, and artificial intelligence. Autonomous vehicles require the use of various sensors, such as optical cameras, RADAR (radio detection and ranging), or LiDAR (light detection and ranging), to navigate on the road with the aim of self-driving. However, the exposures to environmental conditions related to the combination of surrounding temperature and humidity lead to challenges in sensor performance. For example, the sensor’s temperature will increase as the heat is generated during the vehicle’s usage. On the other hand, the sensor system will undergo thermal shock from the temperature difference the due to sudden changes in temperature, such as moving from an indoor garage at room temperature to −10°C environments. Furthermore, the consistent exposure to the cold weather may occur frosting, which can obstruct the optical sensor’s visibility. Those issues limit the potential of data processing from optical cameras and consequence autonomous driving reliability at extreme environmental conditions. To review the requirements for sensor performance used in autonomous vehicles and to formulate solutions addressing potential concerns to improve autonomous driving safety, we simulate camera operating conditions in the real world. First, we correlate the common placements of optical sensors, mainly focusing on cameras, in autonomous vehicles to naturally occurring environmental conditions in relation to temperature and humidity. With this correlation, we aim to provide an understanding of potential areas on the vehicle that may be more prone to environmental factors of thermal shock or humidity variations. Second, we examine the condensation and frosting mechanism and formation sequence on the vehicle surfaces (e.g., windshield and camera lenses), which is then used to determine the level of water on the lenses before the sensor vision is impeded. Third, we introduce and conceptualize machine learning models that can extract features by employing object detection algorithms that perform image restoration to reconstruct areas with deterioration despite the presence of the droplets or frosts on the camera. With this research, we aim to provide a better understanding of the potential caveats and algorithm solutions that can help the capability for autonomous driving even under extreme environmental conditions.