Boiling is an effective energy‐transfer process with substantial utility in energy applications. Boiling performance is described mainly by the heat‐transfer coefficient (HTC) and critical heat flux ...(CHF). Recent efforts for the simultaneous enhancement of HTC and CHF have been limited by an intrinsic trade‐off between them—HTC enhancement requires high nucleation‐site density, which can increase bubble coalescence resulting in limited CHF enhancement. In this work, this trade‐off is overcome by designing three‐tier hierarchical structures. The bubble coalescence is minimized to enhance the CHF by defining nucleation sites with microcavities interspersed within hemi‐wicking structures. Meanwhile, the reduced nucleation‐site density is compensated for by incorporating nanostructures that promote evaporation for HTC enhancement. The hierarchical structures demonstrate the simultaneous enhancement of HTC and CHF up to 389% and 138%, respectively, compared to a smooth surface. This extreme boiling performance can lead to significant energy savings in a variety of boiling applications.
Extreme pool boiling performance is achieved by manipulating liquid–vapor transport at three length scales by engineering surface structures: 1) at millimeter scale, bubble coalescence is minimized and the complete capillary wicking is exploited with separated nucleation sites, 2) at micrometer scale, microcavities promote vapor nucleation, and 3) at nanometer scale, nanostructures extend the liquid–vapor interface for enhanced evaporation.
Heat and mass transfer in hygroscopic hydrogels Díaz-Marín, Carlos D.; Zhang, Lenan; Fil, Bachir El ...
International journal of heat and mass transfer,
10/2022, Letnik:
195
Journal Article
Recenzirano
Odprti dostop
•Developed model capable of capturing sorption and desorption of hygroscopic hydrogels.•Validated model against experimental data and demonstrated good agreement between theory and experiments.•Model ...shows the relevance of simultaneous vapor, water, and heat transport.•Identified key differences in transport behavior depending on how the hydrogel is being heated during desorption.•Performed parametric analysis of the hydrogel thickness, the effective thermal conductivity, the heat transfer coefficient to the ambient, and the hydrogel shear modulus to guide the practical design of hydrogels for sorption applications.
Sorption and desorption with hygroscopic hydrogels hold significant promise for thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting. However, a comprehensive understanding of the energy and mass transport mechanisms in hygroscopic hydrogels remains missing, impeding accurate modeling and optimization. In this work, we develop a model for the simultaneous vapor, water, and heat transfer in hygroscopic hydrogels during sorption and desorption processes. We show that by considering vapor diffusion in the hydrogel micropores, water diffusion in the polymer mesh, and heat transfer in the porous hydrogel, we can accurately capture experimentally observed thermally-driven desorption rates in these hydrogels. Furthermore, we consider three typical operating configurations of hydrogels and elucidate the differences in the transport mechanisms depending on the configuration. Finally, for each of these configurations, we identify key design parameters, including hydrogel thickness, hydrogel shear modulus, heat transfer coefficient, and thermal conductivity, and we parametrically show that by varying these parameters, a hygroscopic hydrogel can desorb up to 128.5%, 14.9%, 69.7%, and 9.6% more water, respectively, relative to the initial water content. This work provides a generic framework to model sorption and desorption processes in hygroscopic hydrogels which can guide the design and optimization in applications of thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting with hydrogels.
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Bubble nucleation is ubiquitous in gas evolving reactions that are instrumental for a variety of electrochemical systems. Fundamental understanding of the nucleation process, which is critical to ...system optimization, remains limited as prior works generally focused on the thermodynamics and have not considered the coupling between surface geometries and different forms of transport in the electrolytes. Here, we establish a comprehensive transport-based model framework to identify the underlying mechanism for bubble nucleation on gas evolving electrodes. We account for the complex effects on the electrical field, ion migration, ion diffusion, and gas diffusion arising from surface heterogeneities and gas pockets initiated from surface crevices. As a result, we show that neglecting these effects leads to significant underprediction of the energy needed for nucleation. Our model provides a non-monotonic relationship between the surface cavity size and the overpotential required for nucleation, which is physically more consistent than the monotonic relationship suggested by a traditional thermodynamics-based model. We also identify the significance of the gas diffuse layer thickness, a parameter controlled by external flow fields and overall electrode geometries, which has been largely overlooked in previous models. Our model framework offers guidelines for practical electrochemical systems whereby, without changing the surface chemistry, nucleation on electrodes can be tuned by engineering the cavity size and the gas diffuse layer thickness.
Micro-EDM milling is an effective machining process for three-dimensional micro-cavity of high hardness materials. However, tools wear sharply in micro-milling, thus several compensation methods are ...applied. The present study examines the fix-length compensation method, and the initial experiments show that a cone-shaped tool end is formed with this compensation method. Because the cone angle is of great importance in the determination of the fix-length compensation parameters in the machining procedure, a clear explanation of the forming mechanism and precise prediction are of great necessity. First, the tool and the workpiece were geometrically and mathematically modeled as two-dimensional matrices. Second, the machining process was divided into three parts including sparking, horizontal feeding and vertical feeding. Finally, a series of experiments were conducted in order to verify the accuracy of the simulation. The results show that the relative error of the simulation compared to the experimental data is within 4% under most machining conditions. The developed model can thus be used to predict the machined surface of the tool and the workpiece and can also provide a better understanding for the mechanism of the cone shaped tool end.
•Cone angel forming mechanism is included in micro-ED milling with fix-length compensation.•Cone angle and workpiece profile are precisely predicted for most real conditions.•Relationship between cone angle and layer thickness is analyzed.
Sorption-based atmospheric water harvesting (SAWH) offers a sustainable strategy to address the global freshwater shortage. However, obtaining sorbents with excellent performance over a wide relative ...humidity (RH) range and devices with fully autonomous water production remains challenging. Herein, magnesium chloride (MgCl
) is innovatively converted into super hygroscopic magnesium complexes(MC), which can effectively solve the problems of salt deliquescence and agglomeration. The MC are then integrated with photothermal aerogels composed of sodium alginate and carbon nanotubes (SA/CNTs) to form composite aerogels, which showed high water uptake over a wide RH range, reaching 5.43 and 0.27 kg kg
at 95% and 20% RH, respectively. The hierarchical porous structure enables the as-prepared SA/CNTs/MC to exhibit rapid absorption/desorption kinetics with 12 cycles per day at 70% RH, equivalent to a water yield of 10.0 L kg
day
. To further realize continuous and practical freshwater production, a fully solar-driven autonomous atmospheric water generator is designed and constructed with two SA/CNTs/MC-based absorption layers, which can alternately conduct the water absorption/desorption process without any other energy consumption. The design provides a promising approach to achieving autonomous, high-performance, and scalable SAWH.
Haze in optically transparent aerogels severely degrades the visual experience, which has prevented their adoption in windows despite their outstanding thermal insulation property. Previous studies ...have primarily relied on experiments to characterize haze in aerogels, however, a theoretical framework to systematically investigate haze in porous media is lacking. In this work, we present a radiative transfer model that can predict haze in aerogels based on their physical properties. The model is validated using optical characterization of custom-fabricated, highly-transparent monolithic silica aerogels. The fundamental relationships between the aerogel structure and haze highlighted in this study could lead to a better understanding of light-matter interaction in a wide range of transparent porous materials and assist in the development of low-haze silica aerogels for high-performance glazing units to reduce building energy consumption.
The simultaneous imaging of magnetic fields and temperature (MT) is important in a range of applications, including studies of carrier transport, and semiconductor device characterization. Techniques ...exist for separately measuring temperature (e.g., infrared (IR) microscopy, micro-Raman spectroscopy, and thermo-reflectance microscopy) and magnetic fields (e.g., scanning probe magnetic force microscopy and superconducting quantum interference devices). However, these techniques cannot measure magnetic fields and temperature simultaneously. Here, we use the exceptional temperature and magnetic field sensitivity of nitrogen vacancy (NV) spins in conformally-coated nanodiamonds to realize simultaneous wide-field MT imaging at the device level. Our "quantum conformally-attached thermo-magnetic" (Q-CAT) imaging enables (i) wide-field, high-frame-rate imaging (100 - 1000 Hz); (ii) high sensitivity; and (iii) compatibility with standard microscopes. We apply this technique to study the industrially important problem of characterizing multifinger gallium nitride high-electron-mobility transistors (GaN HEMTs). We spatially and temporally resolve the electric current distribution and resulting temperature rise, elucidating functional device behavior at the microscopic level. The general applicability of Q-CAT imaging serves as an important tool for understanding complex MT phenomena in material science, device physics, and related fields.
•Experimentally demonstrated that pool boiling CHF enhancement on hemi-wicking surfaces cannot be explained by roughness or wickability alone.•Experimental results of systematically designed ...micropillar surfaces showed that CHF depends on both roughness and wickability.•Performed a scaling analysis to derive a relationship for CHF with a unified descriptor associated with thin film density and volumetric wicking rate.•Thin film density and volumetric wicking rate enhance CHF by accelerating bubble departure frequency and delaying the dry-out at bubble base.•CHF values from our experiments and literature data showed a positive linear correlation with the unified descriptor.
Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.
Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical ...heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. Here, in this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.
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