Phase Change Heat Transfer

Major increases of the selectivity and/or reactivity in aminosugar and sugar O-alkylation were observed in the presence of tetrabutylammonium fluoride (TBAF) in comparison to TBAI under phase-transfer conditions or in solution. The presence of TBAF allowed the selective and rapid alkylation of the 6hydroxyl function of neamine and efficient preparation of protected intermediates useful in synthesis and potent or potential antimicrobial O-alkylated derivatives of neamine and paromamine. In regard to the observed strong effects of TBAF, the alkylation and acylation of carbohydrates merit to be studied in the presence of TBAF under phase-transfer conditions.

Ionic surfactants are widely used for the phase transfer of nanoparticles from aqueous to organic phases; however, a model that can be used to select ionic surfactants based on the nanoparticle solution properties has yet to be established. Here, we have studied the phase transfer of a variety of nanoparticles and have identified hydrophobicity, steric repulsion, and interfacial tension as key factors in determining whether or not phase transfer will occur. Based on these studies, we have developed a simple model for phase transfer wherein the success of the surfactant depends only on three criteria. The phase transfer agents must (i) efficiently load onto or cross the interface, (ii) solubilize the nanoparticles in the receiving phase, and (iii) sterically stabilize the nanoparticles to prevent aggregation due to van der Waals forces between the inorganic cores. Using these criteria, the effectiveness of ionic surfactants could be predicted based on their molecular geometry and the properties of the nanoparticle solutions. These rules provide a basis for choosing surfactants for phase transfer of spherical nanoparticles up to 16 nm in diameter and advances the development of a general model of nanoparticle phase transfer, which would include all nanoparticle shapes, sizes, and solvents.

A series of binary mixtures of Methyl Palmitate (MP) and Lauric Acid (LA) were prepared and investigated, aiming for potential phase change material (PCM) for thermal energy storage systems. The thermal analysis of the PCM binary mixtures was investigated by means of Differential Scanning Calorimetry (DSC). A theoretical and experimental determination of the eutectic mixture was established. The results indicated that the eutectic binary mixture of 60%MP and 40%LA has desirable properties of phase transition temperatures within the comfort temperature range (T m = 25.6 C, T f = 20.2 C) and high latent heat capacity (DH m = 205.4 J/g, DH f = 205.8 J/g). The paper experimentally studied the other important thermo-physical properties required for modelling and stimulating the PCM in any storage systems such as thermal conductivity, enthalpy curve, phase diagram, specific heat, thermal diffusivity, and density. The thermal stability test indicated that the eutectic mixture had reliable thermal performance upon thermal cycling. Based on all these results, the MP-LA eutectic mixture is a promising material for thermal energy storage.

This study presents the development of form-stable eutectic mixtures, modified with nanoscale structures for enhanced thermal performance. These additives may result in the next generation of phase change materials (PCMs) for thermal energy storage systems. An appropriate gelling or thickening agent (2-hydroxypropyl ether cellulose) is introduced so that the PCM will lose its fluidity, become form-stable, and the liquid leakage problem will be overcome. Nano-graphene platelets (NGPs) are added in order to enhance the thermal properties and overall heat transfer. Differential scanning calorimetry (DSC) was carried out for the thermal analysis of the PCMs. The paper experimentally studied in detail the enhanced thermo-physical properties required for stimulating and modelling the PCM in energy storage applications such as specific heat, thermal diffusivity, thermal conductivity, enthalpy, and density. The principle of the T-history method was applied using a parallel plate heating/cooling guarded plate apparatus to determine the true phase transition temperatures of bulk PCM. The supercooling of the enhanced shape stable mixture was found to be less than 0.1 C. The thermal reliability test indicated that the enhanced form-stable eutectic mixture had reliable thermal performance over a postulated lifetime of 80 years. As a result, the developed form stable PCM eutectic mixture is a promising material for thermal energy storage.

This paper describes the influence of the chain length and the functional group steric accessibility of thiols modifiers on the phase transfer process efficiency of water synthesized gold nanoparticles (AuNPs) to toluene. The following thiols were tested: 1-decanethiol, 1,1-dimethyldecanethiol, 1-dodecanethiol, 1-tetradecanethiol and 1-oktadecanethiol. Nanoparticles (NPs) synthesized in water were precisely characterized before the phase transfer process using Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). The optical properties of AuNPs before and after the phase transfer were studied by the UV-Vis spectroscopy. Additionally, the particle size and size distribution before and after the phase transfer of nanoparticles were investigated using Dynamic Light Scattering (DLS). It turned out that the modification of NPs surface was not effective in the case of 1,1-dimethyldecanethiol, probably because of the difficult steric accessibility of the thiol functiona...

We have fabricated an eco-friendly, affordable, and recyclable heat-releasing chemical bag (hence forth termed as hand warmers), which melts when exposed to solar radiation and can be suitable for warming the hands and feet of mountain dwellers, hikers, and motorbike/vehicle riders during winter. Our hand warmer uses sodium acetate trihydrate (CH 3 COONa•3H 2 O, SAT)-based composites consisting of a suitable photothermal-responsive additive to shorten the melting time of the hand warmer for practical applications. Under simulated solar radiation, activated charcoal powder, graphite powder, graphene flakes, and copper nanoparticles (Cu-NPs) were tested as photothermalresponsive additives, which reduced the melting time of the hand warmer by a factor of 0.68, 0.65, 0.63, and 0.57 respectively, when compared to the sample without any additives. Moreover, the stability of the hand warmer was maintained for two−three months (at which point the samples were deliberately discharged) by incorporating 9 mL of extra water (8% wt) in the SAT composites, which demonstrates the long-term heat storage capacity of the hand warmer.

Understanding heat transfer characteristics of phase change and enhancing thermal energy transport in nanoscale are of great interest in both theoretical and practical applications. In the present study, we investigated the nanoscale vaporization and condensation by using molecular dynamics simulation. A cuboid system is modeled by placing hot and cold walls in the bottom and top ends and filling with working fluid between the two walls. By setting two different high temperatures for the hot wall, we showed the normal and explosive vaporizations and their impacts on thermal transport. For the cold wall, the cuboid nanostructures with fixed height, varied length, width ranging from 4 to 20 layers, and an interval of 4 layers are constructed to study the effects of condensation induced by different nanostructures. For vaporization, the results showed that higher temperature of the hot wall led to faster transport of the working fluid as a cluster moving from the hot wall to the cold wall. However, excessive temperature of the hot wall causes explosive boiling, which seems not good for the transport of heat due to the less phase change of working fluid. For condensation, the results indicate that nanostructure facilitates condensation, which could be affected not only by the increased surface area but also by the distances between surfaces of the nanostructures and the cold end. There is an optimal nanosctructure scheme which maximizes the phase change rate of the entire system.

In this work, we have carried out a systematic study of nucleation of a supercooled nanofluid droplet on a graphite substrate using molecular dynamics simulations. In particular, the effect of nanoparticle (NP) loading (ϕ s up to 12.0 vol%) in the supercooled liquid and the interaction strength between water and NP (N P −W) on the behavior of ice nucleation is investigated. At lower N P −W , the nucleation rate is indifferent, while at higher N P −W , the nucleation rate is found to reduce with the addition of nanoparticles. We found the maximum rate of ice nucleation is at ϕ s = 1.91% and N P −W = 0.40 kcal/mole, which is approximately 45 times more than that seen in the bulk water. We present in detail the effect of nanoparticle and nanoparticle-water interactions on the structure and composition of ice. The results demonstrate that the number of icelike water molecules in the nanofluid droplet decreases with increasing ϕ s and N P −W , which correlates well with the lowering of the rate of ice nucleation at higher vol% of particle and stronger water-nanoparticle interaction. Therefore, the hydrophilicity of the nanoparticles inhibits nucleation. We further investigate the effect of the shape of nanoparticles on ice nucleation. The results suggest that the rate of ice nucleation is independent of particle shape of size ∼1.2 nm. Finally, we try to draw a quantitative comparison with the water activity based ice nucleation theory.

The paper presents experimental investigations on liquid metal (sodium and potassium) heat pipe operating and startup. Thus, is studied the wickless gravity assisted heat pipes, that presents a high non-uniformity of the temperature profile along the entire pipe. Because the temperature increasing of the evaporator lower end, it is compulsory to provide wick in the liquid metal heat pipes. There are recommended composite wicks with grooves and screen, or some screen layers.

Condensation heat transfer in a bundle of horizontal enhanced surface copper tubes (Gewa C+ tubes) has been experimentally investigated, and a comparison with trapezoidal shaped fin tubes with several fin spacing has been made. These tubes have a specific surface three-dimensional geometry (notched fins) and the fluids used are either pure refrigerant (HFC134a) or binary mixtures of refrigerants (HFC23/HFC134a). For the pure fluid and a Gewa C+ single tube, the results were analyzed with a specifically developed model, taking into account both gravity and surface tension effects. For the bundle and for a pure fluid, the inundation of the lowest tubes has a strong effect on the Gewa C+ tube performances contrary to the finned tubes. For the mixture, the heat transfer coefficient decreases dramatically for the Gewa C+ tube.

This paper aims to examine thermal characteristics of a novel composite PCM that is consist of Arachidic acid and graphene nano-platelets. DSC, TGA and thermal conductivity measurements have been conducted to reveal the influence of graphene loading with various fractions, namely 0.5%, 1.0% and 2.0%. Results reveal that while the graphene loading does not cause any major change on the phase change temperature and the latent heat values even for the long term utilization, significant increment on the thermal conductivity is achieved. It is also evaluated that the degree of sub-cooling decreases with increasing the graphene fraction. Moreover, numerical analyses have been also conducted for three typical capsule geometries, slab, cylinder and sphere, to reveal the influence of the increment of thermal conductivity on the total time for solidification.

An enthalpy-based model of thermocline operation general to both single phase and encapsulated phase change filler materials was created in MATLAB. The method of characteristics is applied in space and time, mapping fluid temperature and filler enthalpy to a numerical grid, and in the case of a melting filler, allowing accurate tracking of phase state interfaces to fractional positions of the grid. Careful consideration of various possible heat transfer conditions along with placement of phase state interfaces in the numerical grid allows for extreme versatility and accuracy in model application. Input of specific fluid and filler properties, tank size, time of operation, and initial and boundary conditions returns a full representation to any desired amount of charge/discharge processes or cycles. The paper covers mathematical formulation, certain intricacies of numerical implementation, model verification, and the beginnings of application to prove proper operation and generality.

This article investigates the thermo-physical property enhancement of eutectic (oleic and capric acid) based latent thermal energy storage materials for cold storage applications. Highly porous activated carbon (AC) nanosheets were synthesized, which act as nucleators to enhance thermo-physical properties. FT-IR results confirm that the interaction of AC was only physical and no exo/endothermic reactions. Thermo-physical property enhancement was depicted based on the results obtained from the freezing/solidification, DSC, and thermal diffusivity analysis. The heat transfer rate linearly increases for 0.02 wt% to 0.1 wt% of nano dispersed PCM and the maximum time saving was 54% with respect to pure eutectic. Thermal conductivity of the pure PCM were 0.194 W/mK (liquid), 0.201 W/mK (solid) and has been improved by 55% for 0.1 wt% nano dispersed PCM. Linear decrement in the sub-cooling was found to vary between 30% to 63% up to 0.08 wt% of AC, high nucleation rate eradicated sub-cooling from the PCM at 0.1 wt% of AC in base fluid. Slender crystallinity decrement of 4.6% was observed. In addition to thermo-physical properties, theoretical prediction of the PCM properties has also been discussed. The thermo-physical properties of the AC charged eutectics render them elegant candidates for cold storage applications.

Liquid-vapour phase change is a useful and efficient process to transfer energy in nature, as well as in numerous domestic and industrial applications. Relatively recent advances in altering surface chemistry, and in the formation of micro-and nanoscale features on surfaces, have led to exciting improvements in liquid-vapour phase-change performance and better understanding of the underlying science. In this Review, we present an overview of the surface, thermal and material science to illustrate how new materials and designs can improve boiling and condensation. There are many parallels between boiling and condensation, such as nucleation of a phase and its departure from a surface; however, the particular set of challenges associated with each phenomenon results in different material designs used in different manners. We also discuss alternative techniques, such as introducing heterogeneous surface chemistry or direct real-time manipulation of the phase-change process, which can offer further control of heat-transfer processes. Finally, long-term robustness is essential to ensure reliability and feasibility but remains a key challenge.

A large number of different approaches for the aqueous phase transfer of quantum dots have been proposed. Surface ligand exchange with small hydrophilic thiols, such as L-cysteine, yields the lowest hydrodynamic diameter. However, cysteine is prone to dimer formation, which limits colloidal stability. We demonstrate that precise pH control during aqueous phase transfer dramatically increases the colloidal stability of InP/ZnS quantum dots. Various bifunctional thiols have been applied. The formation of disulfides, strongly diminishing the fluorescence QY has been prevented through addition of appropriate reducing agents. Bright InP/ZnS quantum dots with a hydrodynamic diameter <10 nm and longterm stability have been obtained. Finally we present in vitro studies of the quantum dots functionalized with the cellpenetrating peptide maurocalcine.

The natural convection cooling system of a fully immersed server in a dielectric liquid is analysed numerically, where the servers are in sealed capsules and submerged in a dielectric fluid. A reduced order flow model is developed under a saturated porous media flow assumption using a Darcy flow regime and the Stokes equations solved numerically using successive over relaxation and time marching techniques. The simplified model is shown to agree well with predictions from full Navier-Stokes flow analyses and then used to study the role of spatial parameters on the convective heat transfer, in particular the effect of the locations and separations of two heat sources representing two central processing units (CPUs). The flow and heat transfer characteristics are analysed for a range of modified Rayleigh numbers between 0.5 and 300 and a correlation for Nusselt number is obtained which shows that the thermal behaviour is most strongly influenced by the modified Rayleigh number and that the vertical separation of the CPUs is more influential than the vertical position of the lower CPU.

• A Novel waste-to-power generation system combines with an ammonia dilution Rankine cycle is investigated. • The energy-exergy analysis is utilized to study the effects of critical parameters on the efficiency. • Engineering Equation Solver (EES) software is used for delivering such accurate outlines. • Includes providing a solution for calculating the calorific value of waste.

Nearly a century of research on enhancing critical heat flux (CHF) has focused on altering the boiling surface properties such as its nucleation site density, wettability, wickability and heat transfer area. But, a mechanism to manipulate dynamics of the vapor and liquid interactions above the boiling surface as a means of enhancing CHF has not been proposed. Here, a new approach is implemented to limit the vapor phase lateral expansion over the heat transfer surface and actively control the surface wetted area fraction, known to decline monotonically with increasing heat flux. This new degree of freedom has enabled reaching unprecedented CHF levels and revealed new details about the physics of CHF. The impact of wickability, effective heat transfer area, and liquid pressure on CHF is precisely quantified. Test results show that, when rewetting is facilitated, the CHF increases linearly with the effective surface heat transfer area. A maximum CHF of 1.8 kW/cm2 was achieved on a copp...

A novel vapor chamber was fabricated to assess the feasibility of combining hydrophobic and hydrophilic wettabilities in the evaporator to optimize thermal performance. The proposed vapor chamber included a separate layer of hydrophilic sintered copper powder wick that was pressed in intimate contact with a hydrophobic evaporator substrate with a water contact angle around 140°. The contact between the wick layer and the evaporator was provided by sixteen posts implemented on the condenser, which pushed the wick layer toward the evaporator. The thermal performance was evaluated based on the thermal resistance, source temperature, and temperature uniformity across the condenser. Results were compared with those of a baseline vapor chamber that was fabricated by sintering hydrophilic copper particles on a hydrophilic copper evaporator substrate. The wick size and the copper powders used to fabricate the wick structure were the same in both vapor chambers. Overall, the performance of the proposed vapor chamber was lower than that of the baseline vapor chamber, possibly due to microscale gaps between the wick layer and the evaporator substrate. However, the concept of using a hydrophilic wick to force liquid in contact with a hydrophobic evaporating surface could enable a new family of vapor chambers with low thermal resistance, if more efficient techniques for improving the mechanical contact between the wick layer and the evaporator are introduced through further detailed research. If successful, the fabrication cost of vapor chambers would be reduced as well, by using prepared wick structures, which do not require high-temperature sintering processes on evaporators.

An experimental investigation has been carried out to find the condensing side heat transfer coefficient, ho, during condensation of steam over a plain tube, a circular integral-fin tube(CIFT) and a spine integral-fin tube(SIFT). The CIFT and SIFT have enhanced the ho by a factor of 2.5 and 3.2 respectively. The experimental values of ho for CIFT were compared with those predicted from different models. The Honda and Nozu model underpredicted the values of ho in a range of 10 to 20 percent.

Two new types of vapor chambers were fabricated to investigate the influence of engineering the wettability of evaporators for possibility of simultaneously reducing the thermal resistances and increasing the critical heat flux (CHF). One vapor chamber was fabricated by pressing a hydrophilic screen mesh wick structure into a hydrophobic evaporator through support posts integral to the condenser portion of the vapor chamber. The second vapor chamber included a biphilic evaporator without an evaporator wick, as such, the superhydrophilic zone surrounding the central hydrophobic area directly above the heater provided the capillary forces. Despite their lower thermal performances compared with those of the baseline vapor chamber fabricated by sintering the hydrophilic copper particles onto a hydrophilic copper substrate, both novel vapor chambers exhibited promising performances, as such, the vapor chamber with a hydrophobic evaporator approached CHF at high heat fluxes around 105 W/cm 2 , and the one with a biphilic evaporator did not approach the incipience of CHF until 162 W/cm 2. The mechanical contact between the screen mesh and the hydrophobic evaporator, and the patterning of the biphilic evaporator were identified as the key roles in thermal performances of the tested vapor chambers. Suggestions are given to improve the thermal performances of the proposed vapor chambers in this study. If successful, the fabrication cost of these vapor chambers will be reduced by skipping high-temperature sintering process on the evaporators, the method that is used to fabricate the commonly used sintered-powder vapor chambers.

A novel vapor chamber was fabricated to assess the feasibility of combining hydrophobic and hydrophilic wettabilities in the evaporator to optimize thermal performance. The proposed vapor chamber included a separate layer of hydrophilic sintered copper powder wick that was pressed in intimate contact with a hydrophobic evaporator substrate with a water contact angle around 140°. The contact between the wick layer and the evaporator was provided by sixteen posts implemented on the condenser, which pushed the wick layer toward the evaporator. The thermal performance was evaluated based on the thermal resistance, source temperature, and temperature uniformity across the condenser. Results were compared with those of a baseline vapor chamber that was fabricated by sintering hydrophilic copper particles on a hydrophilic copper evaporator substrate. The wick size and the copper powders used to fabricate the wick structure were the same in both vapor chambers. Overall, the performance of the proposed vapor chamber was lower than that of the baseline vapor chamber, possibly due to microscale gaps between the wick layer and the evaporator substrate. However, the concept of using a hydrophilic wick to force liquid in contact with a hydrophobic evaporating surface could enable a new family of vapor chambers with low thermal resistance, if more efficient techniques for improving the mechanical contact between the wick layer and the evaporator are introduced through further detailed research. If successful, the fabrication cost of vapor chambers would be reduced as well, by using prepared wick structures, which do not require high-temperature sintering processes on evaporators.

In recent years, slippery lubricant infused porous surfaces (SLIPSs) have received important attention due to their excellent performance in applications such as condensation, low friction, self-cleaning and anti-icing, which is owed to the presence of an infused lubricant or oil effectively decreasing the liquid-solid interactions and enhancing droplet mobility when compared to hydrophobic and/or to superhydrophobic surfaces. In this work, we fabricate and make use of hierarchical micro-/nano-structured and nano-structured SLIPSs for condensation phase-change. Optical microscopy and macroscopic experimental observations are coupled to extract the droplet size distribution at different condensation times. Heat transfer resistance Abbreviations

Microencapsulated phase change material (MPCM) and carbon-nanotube based nanofluids have both been investigated as heat transfer coefficient enhancers separately. In this paper, we investigate the potential manifold benefit of using a blend of both as a new heat transfer fluid. The effect of percentage of MPCM that undergoes phase change and the composition of the new blend of heat transfer fluid have been investigated. A computer simulation code reveals that the best composition for the MPCM-nanofluid blend depends on the actual percentage of phase change that takes place in the process. Better heat transfer rates can be achieved when high quantity of MPCM undergoes phase change.

Condensation is ubiquitous in nature and industry. Heterogeneous condensation on surfaces is typified by the continuous cycle of droplet nucleation, growth, and departure. Central to the mechanistic understanding of the thermofluidic processes governing condensation is the rapid and high-fidelity extraction of interpretable physical descriptors from the highly transient droplet population. However, extracting quantifiable measures out of dynamic objects with conventional imaging technologies poses a challenge to researchers. Here, an intelligent vision-based framework is demonstrated that unites classical thermofluidic imaging techniques with deep learning to fundamentally address this challenge. The deep learning framework can autonomously harness physical descriptors and quantify thermal performance at extreme spatio-temporal resolutions of 300 nm and 200 ms, respectively. The data-centric analysis conclusively shows that contrary to classical understanding, the overall condensation performance is governed by a key tradeoff between heat transfer rate per individual droplet and droplet population density. The vision-based approach presents a powerful tool for the study of not only phase-change processes but also any nucleation-based process within and beyond the thermal science community through the harnessing of big data.

Droplet transport on, and shedding from, surfaces is ubiquitous in nature and is a key phenomenon governing applications including biofluidics, self-cleaning, anti-icing, water harvesting, and electronics thermal management. Conventional methods to achieve spontaneous droplet shedding enabled by surface−droplet interactions suffer from low droplet transport velocities and energy conversion efficiencies. Here, by spatially confining the growing droplet and enabling relaxation via rationally designed grooves, we achieve single-droplet jumping of micrometer and millimeter droplets with dimensionless jumping velocities v* approaching 0.95, significantly higher than conventional passive approaches such as coalescence-induced droplet jumping (v* ≈ 0.2−0.3). The mechanisms governing single-droplet jumping are elucidated through the study of groove geometry and local pinning, providing guidelines for optimized surface design. We show that rational design of grooves enables flexible control of droplet-jumping velocity, direction, and size via tailoring of local pinning and Laplace pressure differences. We successfully exploit this previously unobserved mechanism as a means for rapid removal of droplets during steam condensation. Our study demonstrates a passive method for fast, efficient, directional, and surface-pinning-tolerant transport and shedding of droplets having micrometer to millimeter length scales.

Investigation of thermal transport characteristics of thin-film liquid evaporation over nanostructured surface has been conducted using molecular dynamics simulation with particular importance on the effects of the nanostructure configuration for different wall–fluid interaction strengths. The nanostructured surface considered herein comprises wall-through rectangular nanoposts placed over a flat wall. Both the substrate and the nanostructure are of platinum while argon is used as the evaporating liquid. Two different wall–fluid interaction strengths have been considered that essentially emulate both hydrophilic and hydrophobic wetting conditions for three different nanostructure configurations. The argon–platinum molecular system is first equilibrated at 90 K and then followed by a sudden increase in the wall temperature at 130 K that induces evaporation of argon laid over it. Comparative effectiveness of heat and mass transfer for different surface wetting conditions has been stud...

Vapor condensation is a widely used industrial process for transferring heat and separating fluids. Despite progress in developing low surface energy hydrophobic and micro/nanostructured superhydrophobic coatings to enhance water vapor condensation, demonstration of stable dropwise condensation of low-surface-tension fluids has not been achieved. Here, we develop rationally designed nanoengineered lubricant-infused surfaces (LIS) having ultralow contact angle hysteresis (<3º) for stable dropwise condensation of ethanol (≈ 23 mN/m) and hexane (≈ 19 mN/m). Using a combination of optical imaging and rigorous heat transfer measurements in a controlled environmental chamber free from non-condensable gases (< 4 Pa), we characterize the condensation behavior of ethanol and hexane on ultra-scalable nanostructured CuO surfaces impregnated with fluorinated lubricants having varying viscosities (0.496 < µ < 5.216 Pa•s) and chemical structures (branched vs. linear, Krytox and Fomblin). We demonstrate stable dropwise condensation of ethanol and hexane on LISs impregnated with Krytox 1525, attaining about 200% enhancement in condensation heat transfer coefficient for both fluids compared to filmwise condensation on hydrophobic surfaces. In contrast to previous studies, we use 7 hours of steady dropwise condensation experiments to demonstrate the importance of rational lubricant selection to minimize lubricant drainage, and maximize LIS durability. This work not only demonstrates an avenue to achieving stable dropwise condensation of ethanol and hexane, it develops the fundamental design principles for creating durable LISs for enhanced condensation heat transfer of low-surface-tension fluids.

To improve condensation heat transfer technologies, various coating techniques have been used to generate rough hydrophobic surfaces. Condensation on micro-textured hydrophobic surfaces can demonstrate variable heat transfer enhancement compared to smooth hydrophobic surfaces. This is primarily directed towards the improvement in efficiency of condensers for direct dry cooling power plants. Here we investigate the use of a thiol-based self-assembled monolayer deposited on variably-sized microporous copper powder wick monolayers. A custom condensation chamber was fabricated to demonstrate the effect of enhanced dropwise condensation. Although rough hydrophobic surfaces have shown advantageous droplet growth dynamics, precise heat transfer measurements are underdeveloped at high heat flux. At consistent operating conditions, we experimentally demonstrated a 23% improvement in the local condensation heat transfer coefficient for a 4μm hydrophobic microporous copper powder surface compared to a smooth hydrophobic copper surface. This improvement is attributed to the reduction in contact angle hysteresis as evidenced by the decrease in departing droplet size. For larger powder sizes, surface flooding inhibited thermal performance due to larger departing droplet sizes and a decreased conduction pathway. With microporous powder sizes greater than 119µm, a transition of dropwise condensation to intrapowder droplet removal is evident. The use of enhanced two-phase thermal management technologies for high heat flux applications can benefit from the specific design of textured hydrophobic surfaces.

This paper reports the results of a numerical study on the heat transfer during the process of solidification of water inside a spherical capsule. The governing equations of the problem and associated boundary conditions were formulated and solved using a finite difference approach and a moving grid scheme. The model was optimized and the numerical predictions were validated by comparison with experimental results realized by the authors. The model was also used to investigate the effects of the size and material of the shell, initial temperature of the phase change material and the external temperature of the spherical capsule on the solidified mass fraction and the time for the complete solidification.

This paper details a numerical study of solid-liquid phase change heat transfer in a rectangular enclosure with the aim of optimizing the number, dimension, and positioning of fins in the enclosure. The enclosure is exposed on one side to a constant heat flux of 1000 W/m 2 for 3 h and both the total fin mass and the PCM mass are kept constant in all simulated cases. The heat diffusion equation in the PCM uses the equivalent heat capacity method while natural convection driven PCM motion in the enclosure is modeled through the buoyancy force, using a modified viscosity and an additional volume force term in the Navier-Stokes momentum conservation equation. Three efficiency assessment parameters are used: (i) the height-averaged temperature of the front hot plate which should remain as low as possible for the longer possible time, (ii) the energy stored inside the PCM as a function of time, and (iii) the heat transfer rate and heat flux between the front plate and the PCM. For each simulated case, complementary data such as the standard deviation of temperature along the heated plate and the ratio of sensible heat to latent accumulation are also calculated. Results show that increasing the number of fins diminishes both the stabilization temperature and the stabilization time of the front plate during phase change, and accelerates the sensible and latent storage of energy in the PCM. Using thinner but longer fins provides the same impact except that the stabilization time also increases as the fin length is increased. Besides, at constant fin mass, varying fin spacing have marginal impact on the latent heat storage performance and on the hot plate temperature stabilization. The study also shows that the surface area plays the largest role in the increase of the heat transfer rate between the front plate and the PCM, while configurations which can promote stronger natural convection lead to higher heat flux. Finally, it was observed that systems allowing easier heat transfer to the back plate during melting provide a higher ratio of sensible heat to latent accumulation, while the standard deviation of the front plate temperature decreases as the number of fins or the spacing between fins increases.

h i g h l i g h t s An eco-friendly material was prepared to enhance the heat capacity of buildings. Coconut fat was found as a suitable bio-based PCM for TES in building envelopes. Recycled cardboards and natural clay powder were used to prepare the PCM's matrix. The prepared material shows the potential for enhancing thermal inertia of building envelopes.

Professor Jungho Kim, Department of Mechanical Engineering Better understanding of phase change phenomena can be obtained through local measurements of the heat transfer process, which cannot be attained by traditional thermocouple point measurements. Infrared (IR) technology, which has been used by many researchers in the past, cannot be used under certain circumstances due to spectral transparency issues present in some materials. In the current study, the optical properties of fluorescing materials are proposed as a novel tool for heat transfer measurements. Two fluorescing materials were examined within the framework of the current dissertation: Namely Quantum dots and Ruthenium based temperature sensitive paint, which tend to fluoresce upon excitation by blue or Ultraviolet (UV) light. The light intensity emitted by those fluorescing materials tends to drop with temperature, which can be utilized to obtain the surface temperature distribution at a pixel resolution, for a given monochromic camera. Advantages of the fluorescing materials include feasibility, applicability to various surface geometries, and the ability to resolve submicron features. The main objective behind the current research work was to develop and assess the optical measurement technique of fluorescing materials, where phase change heat transfer applications, including ethanol drop evaporation and pool boiling, were used to quantify the advantages and limitations of the current temperature measurement technique. Furthermore, a thermofluid study was conducted in order to examine the mechanism of rapid vapor patch formation near critical heat flux (CHF) conditions. Results from the current research work show a correlation between the fluid velocity gradient near the wall and surface heat flux, where both tend to follow similar trend with surface super heat. Thus, it's believed that the incomplete wetting of previous vapor patches near CHF is associated with restricted capillary motion near the surface, where the wetting liquid fails to reach the dry areas with the increased bubble generation activity, due to the local heating caused by the mushroom bubble ebullition.

Condensation heat transfer is significant in many applications such as such as desalination, energy conversion [1], atmospheric water harvesting [2, 3], electronics cooling, and other high heat flux applications [4]. However, condensate on the surface adds a thermal resistance that limits condensation rates. The rate of condensation heat transfer is inversely proportional to the diameter of the condensate drops[5]. In industrial condensing systems, the resistance is minimized by removing the condensate via gravity or a vapor shear, but the minimum size of droplet removal is typically on the order of the capillary length of the condensate, about 2.7 mm for water. Properly designed superhydrophobic surfaces have been shown to promote the removal of condensate at drop sizes significantly below the capillary length due to the low contact angle hysteresis (indicating high drop mobility) and coalescence-induced jumping of condensate drops. With the removal of condensate drops due to coale...

This paper details a numerical study of solid-liquid phase change heat transfer in a rectangular enclosure with the aim of optimizing the number, dimension, and positioning of fins in the enclosure. The enclosure is exposed on one side to a constant heat flux of 1000 W/m 2 for 3 h and both the total fin mass and the PCM mass are kept constant in all simulated cases. The heat diffusion equation in the PCM uses the equivalent heat capacity method while natural convection driven PCM motion in the enclosure is modeled through the buoyancy force, using a modified viscosity and an additional volume force term in the Navier-Stokes momentum conservation equation. Three efficiency assessment parameters are used: (i) the height-averaged temperature of the front hot plate which should remain as low as possible for the longer possible time, (ii) the energy stored inside the PCM as a function of time, and (iii) the heat transfer rate and heat flux between the front plate and the PCM. For each simulated case, complementary data such as the standard deviation of temperature along the heated plate and the ratio of sensible heat to latent accumulation are also calculated. Results show that increasing the number of fins diminishes both the stabilization temperature and the stabilization time of the front plate during phase change, and accelerates the sensible and latent storage of energy in the PCM. Using thinner but longer fins provides the same impact except that the stabilization time also increases as the fin length is increased. Besides, at constant fin mass, varying fin spacing have marginal impact on the latent heat storage performance and on the hot plate temperature stabilization. The study also shows that the surface area plays the largest role in the increase of the heat transfer rate between the front plate and the PCM, while configurations which can promote stronger natural convection lead to higher heat flux. Finally, it was observed that systems allowing easier heat transfer to the back plate during melting provide a higher ratio of sensible heat to latent accumulation, while the standard deviation of the front plate temperature decreases as the number of fins or the spacing between fins increases.

This paper details a numerical study of solid-liquid phase change heat transfer in a rectangular enclosure with the aim of optimizing the number, dimension, and positioning of fins in the enclosure. The enclosure is exposed on one side to a constant heat flux of 1000 W/m 2 for 3 h and both the total fin mass and the PCM mass are kept constant in all simulated cases. The heat diffusion equation in the PCM uses the equivalent heat capacity method while natural convection driven PCM motion in the enclosure is modeled through the buoyancy force, using a modified viscosity and an additional volume force term in the Navier-Stokes momentum conservation equation. Three efficiency assessment parameters are used: (i) the height-averaged temperature of the front hot plate which should remain as low as possible for the longer possible time, (ii) the energy stored inside the PCM as a function of time, and (iii) the heat transfer rate and heat flux between the front plate and the PCM. For each simulated case, complementary data such as the standard deviation of temperature along the heated plate and the ratio of sensible heat to latent accumulation are also calculated. Results show that increasing the number of fins diminishes both the stabilization temperature and the stabilization time of the front plate during phase change, and accelerates the sensible and latent storage of energy in the PCM. Using thinner but longer fins provides the same impact except that the stabilization time also increases as the fin length is increased. Besides, at constant fin mass, varying fin spacing have marginal impact on the latent heat storage performance and on the hot plate temperature stabilization. The study also shows that the surface area plays the largest role in the increase of the heat transfer rate between the front plate and the PCM, while configurations which can promote stronger natural convection lead to higher heat flux. Finally, it was observed that systems allowing easier heat transfer to the back plate during melting provide a higher ratio of sensible heat to latent accumulation, while the standard deviation of the front plate temperature decreases as the number of fins or the spacing between fins increases.

In polymer protection pieces, polycarbonate is often used for its transparency and its high impact resistance. However, when dew appears it degrades light transmission and thus transparency. To solve this problem, anti-fogging surfaces are developed. To further understand the dew formation in natural environment in order to design high efficiency anti-fogging surfaces on polycarbonate, a recently designed dew tracking setup giving dew pictures and a digital imaging procedure allowing reliable and highly accurate droplet measurements were developed. Thanks to this setup, dew formation conditions such as relative humidity level and air temperature can be chosen and studied. Particular attention is paid to the evolution of the number of droplets according to the relative humidity and how it explains the droplet growth rate. The influence of the relative humidity (RH) on the growth rate of non-coalescing dropwise condensation has already been observed. This paper is based on these existing results as a comparison to validate the experimental setup and the post-processing analysis. We shall also use the number of droplets to further explain the role of the RH on condensation dynamics. Moreover, by applying a light subcooling, the growth kinetics were slowed down in order to observe every characteristic phase in detail and even particular intermediate phases.

An enthalpy-based model of thermocline operation general to both single phase and encapsulated phase change filler materials was created in MATLAB. The method of characteristics is applied in space and time, mapping fluid temperature and filler enthalpy to a numerical grid, and in the case of a melting filler, allowing accurate tracking of phase state interfaces to fractional positions of the grid. Careful consideration of various possible heat transfer conditions along with placement of phase state interfaces in the numerical grid allows for extreme versatility and accuracy in model application. Input of specific fluid and filler properties, tank size, time of operation, and initial and boundary conditions returns a full representation to any desired amount of charge/discharge processes or cycles. The paper covers mathematical formulation, certain intricacies of numerical implementation, model verification, and the beginnings of application to prove proper operation and generality.

Unusual condensation dynamics occur on the hierarchical Black Silicon metasurface Near-zero water removal time and good water yield are simultaneously achieved Great scalability and multifunctionality are accessible to the proposed water panel

Understanding heat transfer characteristics of phase change and enhancing thermal energy transport in nanoscale are of great interest in both theoretical and practical applications. In the present study, we investigated the nanoscale vaporization and condensation by using molecular dynamics simulation. A cuboid system is modeled by placing hot and cold walls in the bottom and top ends and filling with working fluid between the two walls. By setting two different high temperatures for the hot wall, we showed the normal and explosive vaporizations and their impacts on thermal transport. For the cold wall, the cuboid nanostructures with fixed height, varied length, width ranging from 4 to 20 layers, and an interval of 4 layers are constructed to study the effects of condensation induced by different nanostructures. For vaporization, the results showed that higher temperature of the hot wall led to faster transport of the working fluid as a cluster moving from the hot wall to the cold wall. However, excessive temperature of the hot wall causes explosive boiling, which seems not good for the transport of heat due to the less phase change of working fluid. For condensation, the results indicate that nanostructure facilitates condensation, which could be affected not only by the increased surface area but also by the distances between surfaces of the nanostructures and the cold end. There is an optimal nanosctructure scheme which maximizes the phase change rate of the entire system.

Radiation-mixed convection boundary layer analysis in power law fluids along vertical wedge embedded in a saturated porous medium has been performed using an implicit finite difference method. Darcian, steady, 2-D laminar flow has been considered under prescribed surface heat flux condition. The single mathematical model represents the case for vertical wedge, cone and plate by introducing the geometry parameter 'ε'. The governing equations and boundary conditions are transformed into dimensionless form to obtain similar and non-similar solutions. The results for non-similar case have been presented and plotted. Effects of radiation parameter 'Rd', variable heat flux parameter 'λ', wedge angle parameter 'm' and mixed convection parameter 'χ' have been studied for both Newtonian and Non-Newtonian fluids. These results have also been verified with available literature in defined range of parameters and found in good agreement. Results for both Newtonian and Non-Newtonian fluids have been presented for varying parameters like local Nusselt number, temperature and velocity fields.

Polypyrrole dispersions in several organic media were synthesized by using polymeric ionic liquids (PILs) both as steric stabilizers and as phase transfer agents. Firstly, PPy aqueous dispersions were prepared by polymerizing pyrrole in the presence of poly(1-vinyl-3ethylimidazolium bromide) as steric stabilizer. By addition of different salts: lithium triflate, bis(trifluoromethane)sulfonimide lithium salt, bis(pentafluoroethane)sulfonimide lithium salt, dodecylbenzenesulfonic acid sodium salt and p-toluene-4-sulfonic acid monohydrate, the polymeric stabilizer becomes hydrophobic, precipitating in water and trapping the PPy dispersed microparticles inside. Polypyrrole organic dispersions were obtained by dispersion of the corresponding powders in a variety of organic solvents such as methanol, N,N-dimethylformamide (DMF), acetone, tetrahydrofurane (THF), butanone and toluene. The dispersability or non-dispersability of the PPy powders in each of the organic solvents depended on the nature of the added coagulating salt. Characterization of the PPy organic dispersions by photon correlation spectroscopy and atomic force microscopy (AFM) showed individual particles with sizes between 100 and 400 nm. After casting the organic dispersions, the films showed a granular nanostructured surface and electrical conductivity values as high as 10 −1 S/cm.