Phase Change Characteristics Research Articles

Non-monotonously tuning thermal conductivity of graphite-nanosheets/paraffin composite by ultrasonic exfoliation

Organic phase change materials (PCMs) have drown continuous attentions over time due to their large latent heat and constant-temperature solid-liquid phase transition with promising applications in thermal energy storage. Nevertheless, they suffer from the relatively low intrinsic thermal conductivity. Filling the PCMs with graphite-nanosheets (GNs) by ultrasonic exfoliating could alleviate this problem, and GNs with longer ultrasonic exfoliation time is reported to possess larger effective thermal conductivity (ETC) monotonously. In this paper, we discover a non-monotonous variation of ETC for the first time, when enhancing the ETC of paraffin with ultrasonic exfoliated GNs. The mechanism behind this phenomenon is explained by the variation of GNs morphologies over time in the paraffin. Experimental results reveal that longer exfoliation time can increase the aspect ratio and ETC, but over a critical time, the bending stiffness of GNs decrease and the particles tend to be folded with increased interfacial thermal resistance and decreased ETC. The ETC as a function of ultrasonic time shows an obvious peak value at ∼2 min, and the ETC could be increased from 0.3 to 3.0 W/(m·K) at GNs loading of 4 wt% with negligible effect on the phase change characteristics. In addition, the GNs/paraffin composite exhibits the quick thermal response and longer working time. The present non-monotonous discovery reveals the underlying mechanism and provides suggestions on the improvement of ultrasonic exfoliation process.

Liquid phase oxidation quantitative analysis of biodiesel/diesel blends by differential TG and DTA

The oxidative stability of biodiesel/diesel blends is usually evaluated from the released gaseous phase change characteristics, without analyzing what happens in liquid phase. Depending on the storage and/or heating conditions, gums and insoluble organic particulates may be formed in liquid phase by oxidation. To contribute to the evaluation of this oxidized heavier components formation in liquid phase, a new thermal analysis quantitative method was developed by the authors, which was applied to diesel oil/soybean biodiesel blends having from 5 to 20 vol% of the latter. The method consists on obtaining differential thermogravimetric curves (DIFTG), subtracting the TG curve obtained in inert atmosphere from the TG curve of the same sample obtained in air, which, actually, shows how and how much accumulated oxidized mass is being formed in liquid phase during analysis. Corresponding thermal effects are quantified by differential DTA (DIFDTA) curves, subtracting respective DTA curves in N2 from those in air. The results show that, as the biodiesel content is increased, a higher amount of heavy oxidized products remains in liquid phase of the blend, up to the respective much higher decomposition temperatures. This fact is confirmed by estimating the activation energies as a function of the mass loss conversion degree, which shows that the higher is the formation rate of the liquid phase oxidized mass in a blend, the higher is the activation energy needed for its decomposition and the higher is the blend ignition temperature, which is a polynomial function of second order of the biodiesel content.

CFD simulation of a concentrated salt nanofluid flow boiling in a rectangular tube

Improvement of heat transfer and critical heat flux (CHF) via nanofluid in pool boiling is well known. Flow boiling is usually represented by heat transfer correlations. But in the case of a concentrated salt solution nanofluid, there is no suitable representation. A model is generated using commercial CFD code with additional user defined function, (UDF) to address this for the case of determining the heat and mass transfer in a boiler duct for a vapour absorption refrigerator. Computational fluid dynamics (CFD) simulations are performed to assess the effect of varying nanoparticle concentrations, fluid velocity and boiler temperature on the boiling and phase change characteristics of the system. Four phases are treated in this case, which were liquid acetone, vapour acetone, liquid Acetone/ZnBr2 solution and solid nanoparticles cloud. Zinc oxide nanoparticles are represented as a cloud-like phase in the mixture. In addition, this study evaluates the key characteristics of the nanofluid system, and how the different components and phases behave when the single component evaporates. Previous research concentrates on water based fluids or commonly found refrigerants and heat transfer fluids. In this work the process was modelled using ANSYS® Fluent V.15 using the mixture multiphase flow model, however, the volume of fluid (VOF) method is also used to show the behaviour of the vapour phase. A UDF was applied from the literature (LEE, 1980) for boiling of nanofluids to model the mass transfer on boiling. It was found that increase in nanoparticle loading (0, 0.1, 0.3, 0.5 & 1 vol%) leads to an increase in the exiting vapour volume fraction and the heat transfer coefficient. This is primarily due to an increase in the heat transfer in the system due to the increased thermal conductivity in the nanofluid. Incremental increases in the boiler temperature (330, 333, 335 K) creates an increase in both vapour volume fraction and heat transfer coefficient because the process still in the nucleate boiling which, the heat flux increase with increasing the temperature difference. This new approach for the four phase system is capable to demonstrating concentrated salt solution nanofluid boiling.

Ultrafast and efficient photothermal conversion for sunlight-driven thermal-electric system

Thermoelectric battery is effective in converting thermoelectricity based on Seebeck effect. As an all-solid-state energy conversion model, this battery is highly significant in improving the utilization of solar energy using a clean and noiseless process. However, efficient photothermal conversion and temperature difference control are the key challenges in enhancing solar-thermal-electric conversion. Herein, we constructed a novel sunlight-driven thermoelectric system with temperature difference control characteristic. In which the light-driven phase change heat storage material (PCHSM) was used as the hot side, and the phase change cool storage material (PCCSM) as the cold side. Ultrafast photothermal conversion of the PCHSM was achieved through the nonradiative decay of the excited state, which was revealed by means of femtosecond transient absorption technique. Solar radiation was efficiently utilized as shown by the excellent visible light absorption, ultrafast photothermal transformation, and phase change heat storage characteristics of the PCHSM. The PCCSM with high phase change cool storage performance could provide enough cold energy to maintain the cold-side temperature. Temperature gradient was controlled using the cold-side and hot-side materials based on the phase-change temperature control characteristics.

Relationships between dynamic behavior and properties of a single droplet of water-emulsified n-dodecane

Water-fuel emulsion liquids are used in variety of combustion systems to suppress pollutant emission and to control flame characteristics. However, the characteristics of fluid-dynamics, phase change, and combustion were significantly affected by the variation of the emulsion properties. In this study, three representative properties (i.e., surface tension, dynamic viscosity, and light absorbance) were investigated for water-emulsified n-dodecane as a diesel fuel surrogate. First, these properties were preliminarily measured with three independent ordinary methods, and the property variation coupled with the micro-structures within the emulsion was discussed. After that, a new method able to estimate the same three properties was suggested. An emulsion droplet was disturbed using a pulse-laser, and its dynamic behavior was analyzed based on the Taylor’s analogy breakup model. Thus, more reliable properties could be proposed for determining the dynamic behaviors of an emulsion droplet, and the relationship between the properties and dynamic behaviors could be better understood.

Preparation and characterization of indium chalcogenide thin films: A material for phase change memory

The versatile resistive switching behavior of metal chalcogenide thin films is highly desired for the fabrication of phase change memory (PCM) - electronic devices. The In2(Te1-xSex)3 thin films might be one of the prime candidates for the application of PCM devices. In this paper, the effects of Se-Te in the phase change characteristics of In-Se-Te material are investigated by X-ray diffraction, Field emission scanning electron microscopy, the I–V measurement and the optical transmittance. The resistance ratio between the amorphous and crystalline states of In2(Te0.98Se0.02)3, In2(Te0.94Se0.06)3 and In2(Te0.90Se0.1)3 thin films are about two orders of magnitude. The stoichiometric phase change from In2(Te1-xSex)3 to In2Se3 and In2Te3 phases in all the samples have been observed from the temperature dependent I–V characteristics. The electrical switching occurs at 80 °C, Further rise in temperature leads no change in the threshold voltage after switching. For the In2(Te0.98Se0.02)3, In2(Te0.94Se0.06)3 and In2(Te0.90Se0.1)3 films, the forward biased current/threshold voltages are found to be about 8 μA/4.2 V, 6 μA/3.00 V and 4 μA/2.8 V respectively. The In2(Te1-xSex)3 thin film showed the high resistance state at low voltage region. However, when it reaches the threshold voltage, the resistance drastically reduced through the formation of conducting path. From these studies, it can be concluded that the effective way to enhance the comprehensive performance of In-Se-Te system is by varying the concentrations of the Te and Se and it may find potential application in switching volatile PCM devices.

Thermal Performance Test of a Phase-Change-Material Cool Roof System by a Scaled Model

General cool roof is effective for reduction of cooling load, but it has a problem of increasing heating load. Therefore, the purpose of this study is to complement the disadvantages of the cool roof system by utilizing phase change characteristics of phase change material (PCM). The study was carried out to verify the thermal performance of the PCM cool roof system by measuring the temperature on the top and bottom of the PCM cool roof system by making a miniature model (600 × 600 × 600 mm). PCM was inserted and not inserted, and the temperature difference according to the finish color (brown and white) was compared. As a result, the plate surface temperature using PCM was lower than that without PCM, and time-lag of temperature increase occurred. As a result of the comparison of temperature according to the finish color (brown and white), white showed a low temperature distribution up to 16.35°C. Even at room temperature, white maintained a low temperature distribution of 5.40°C than brown. The use of PCM cool roof system in roof finishes could lower the surface temperature and keep the room temperature low.

Diurnal Thermal Behavior of Photovoltaic Panel with Phase Change Materials under Different Weather Conditions

The electric power generation efficiency of photovoltaic (PV) panels depends on the solar irradiation flux and the operating temperature of the solar cell. To increase the power generation efficiency of a PV system, this study evaluated the feasibility of phase change materials (PCMs) to reduce the temperature rise of solar cells operating under the climate in Seoul, Korea. For this purpose, two PCMs with different phase change characteristics were prepared and the phase change temperatures and thermal conductivities were compared. The diurnal thermal behavior of PV panels with PCMs under the Seoul climate was evaluated using a 2-D transient thermal analysis program. This paper discusses the heat flow characteristics though the PV cell with PCMs and the effects of the PCM types and macro-packed PCM (MPPCM) methods on the operating temperatures under different weather conditions. Selection of the PCM type was more important than the MMPCM methods when PCMs were used to enhance the performance of PV panels and the mean operating temperature of PV cell and total heat flux from the surface could be reduced by increasing the heat transfer rate through the honeycomb grid steel container for PCMs. Considering the mean operating temperature reduction of 4 °C by PCM in this study, an efficiency improvement of approximately 2% can be estimated under the weather conditions of Seoul.

Droplet phase change model and its application in wave-type vanes of steam generator

When an entrained droplet travels through the steam-water separator of the steam generator in a nuclear power station, pressure decreases continuously along the droplet trajectories due to the local flow resistance and structure variation. This movement-induced pressure drop will result in droplet evaporation, which eventually affects the steam-water separation performance. To investigate the influence of the droplet motion on its phase change, a phase change model for single moving droplet is developed by combining static droplet phase change model and droplet motion model. The model well reproduces the droplet evaporation process, showing a fast evaporating stage followed by a thermal equilibrium evaporation stage. The discrepancies between the predicted results and the experimental measurements are within ±2%. Furthermore, the model is adopted in the Euler-Lagrange frame to obtain the phase change characteristics of droplets moving in the wave-type vanes. Based on the simulation, the effects of phase change on the droplet movement trajectory, radius, velocity, terminal position and separation efficiency are examined. In addition, the critical pressure difference that could affect the wave-type vanes separation efficiency about 0.01 % is proposed. The theoretical and numerical work can provide guidance to the design and optimization of the steam-water separating apparatuses as well as other applications where both of the droplet movement and phase change occur simultaneously.

Simulation of the Melting Process of Ice Slurry for Energy Storage Using a Two-Fluid Lattice Boltzmann Method

Abstract: Ice slurry can be used as the thermal storage media in latent cool storage systems for both residential and commercial buildings. This paper presents the investigation of the phase change characteristics of the ice slurry using a two-fluid Lattice Boltzmann Method (TFLBM). The melting and migration processes of the ice slurry are simulated by improving the equilibrium distribution function and matching the relevant parameters such as the kinetic viscosity of ice particle cluster and cross-collision coefficient. The sensitivity analysis of the ice slurry viscosity and cross-collision coefficient are achieved through six numerical experiments, and the ice melting in the internal-melt ice-on-coil thermal storage device is then calculated. The results could be potentially used to guide the design of the ice slurry for cooling both residential and commercial buildings.

Phase Change Characteristics of Ultra-Thin Liquid Argon Film over different Flat Substrates at High Wall Superheat for Hydrophilic/Hydrophobic Wetting Condition: A Non-Equilibrium Molecular Dynamics Study

Non-equilibrium molecular dynamics simulations have been conducted to understand the effect of solid-liquid interfacial wettability and surface material on the phase change phenomena of the thin liquid argon film placed over flat substrate at high wall superheat. The molecular system consists of a three phase simulation domain involving solid wall, liquid argon and argon vapor. After the system is thermally equilibrated at 90K and kept in equilibrium for a while, a high wall superheat (250K that is far above the critical temperature of argon) is induced at the liquid boundary so that the liquid undergoes ultrafast heating. Both hydrophilic and hydrophobic surfaces were considered in the present study in order to observe the effect of surface wettability on phase change characteristics for three different solid substrate materials namely, Platinum (Pt), Silver (Ag) and Aluminium (Al). Results obtained in the present study are discussed in terms of transient atomic distribution inside system domain, heat flux characteristics across the solid-liquid interface together with evaporative mass flux from liquid argon. Simulation results show that, depending on the surface wetting condition, the phase change process appears to be very different (explosive/ diffusive) for all three substrate materials under consideration. Among three materials considered herein, Al is found to offer the least favourable condition for phase change process while Pt and Ag show similar heat and mass transfer characteristics for both hydrophilic and hydrophobic wetting conditions. Surface wettability effect is found to be more prominent than the effect of substrate material in thin film liquid phase change phenomena.Journal of Chemical Engineering, Vol. 29, No. 1, 2017: 49-55

Novel hybrid nanofluid with tunable specific heat and thermal conductivity: Characterization and performance assessment for energy related applications

The utilization of heat transfer fluids with improved specific heat and thermal conductivity will be beneficial for enhanced energy collection in solar thermal systems. Accordingly, hybrid nanofluids comprising ZnO nanoparticles and encapsulated paraffin wax in propylene glycol-water mixture were prepared and characterized. The influence of encapsulated paraffin wax concentration (4–16 wt. %) and ZnO nanoparticle concentration (0–2 vol %) revealed that the thermal conductivity and specific heat could be improved to a maximum of 10.4% and 18.7% respectively, in comparison with those of propylene glycol-water mixture, at their appropriate concentrations. The presence of ZnO nanoparticles was found to influence the phase change characteristics of encapsulated paraffin wax, in such a way that the latent heat due to phase change was increased at an optimum concentration of ZnO nanoparticle concentration. While the maximum enhancement in overall heat transfer coefficient of 15.37% was obtained for the hybrid nanofluid sample that had both thermal conductivity and specific heat higher than the base fluid, the maximum enhancement in heat transfer rate (13.54%) was obtained for the hybrid nanofluid that had 18.65% enhancement in specific heat. The results of present investigation reveal the potential of hybrid nanofluids for energy management.

MgCl2·6H2O-Mg(NO3)2·6H2O eutectic/SiO2 composite phase change material with improved thermal reliability and enhanced thermal conductivity

MgCl2·6H2O-Mg(NO3)2·6H2O eutectic has a proper melting point and high latent heat for solar water heating systems with latent heat storage. In this paper, the exact composition of the eutectic and the corresponding phase change temperature are determined by measuring the phase change characteristics of mixtures of MgCl2·6H2O and Mg(NO3)2·6H2O in various ratios. MgCl2·6H2O-Mg(NO3)2·6H2O eutectics, the melting temperature of which is around 58°C, can be obtained with the mass fraction of MgCl2·6H2O ranging from 35% to 50%. The eutectics are composited with porous fumed silica to improve its thermo-physical properties. The eutectic/SiO2 composite retains a similar melting point of the pure eutectic, is of no liquid leakage even after melting while the SiO2 weighs more than 15wt%, and the specific enthalpy reaches 88.13kJkg−1. The eutectic/SiO2 composite has a thermal conductivity 5% higher than pure eutectics and better thermal stability and cycle stability. The thermogravimetric analysis shows its weight loss is 8.03% less than pure eutectic at 300°C. And the eutectics suffers a decrease in phase change enthalpy after 100 thermal cycles by 9.2%, much lower compared with the 37.23% for pure eutectics due to the phase separation during the thermal cycles. Thermo-physical properties and stability of MgCl2·6H2O-Mg(NO3)2·6H2O eutectic have been enhanced by fumed silica.

Controllable Fabrication of Two-Dimensional Patterned VO2 Nanoparticle, Nanodome, and Nanonet Arrays with Tunable Temperature-Dependent Localized Surface Plasmon Resonance.

A universal approach to develop various two-dimensional ordered nanostructures, namely nanoparticle, nanonet and nanodome arrays with controllable periodicity, ranging from 100 nm to 1 μm, has been developed in centimeter-scale by nanosphere lithography technique. Hexagonally patterned vanadium dioxide (VO2) nanoparticle array with average diameter down to sub-100 nm as well as 160 nm of periodicity is fabricated, exhibiting distinct size-, media-, and temperature-dependent localized surface plasmon resonance switching behaviors, which fits well with the predication of simulations. We specifically explore their decent thermochromic performance in an energy saving smart window and develop a proof-of-concept demo which proves the effectiveness of patterned VO2 film to serve as a smart thermal radiation control. This versatile and facile approach to fabricate various ordered nanostructures integrated with attractive phase change characteristics of VO2 may inspire the study of temperature-dependent physical responses and the development of smart devices in extensive areas.

Preparation, thermal properties and thermal reliability of a novel mid-temperature composite phase change material for energy conservation

Condensation heat of air-conditioner in household and public is a kind of indispensable waste heat, which is necessary to recover and reuse it. Herein, phase change material is widely used in exhaust heat recover and storage. In the present work, expanded graphite(EG) was introduced to stearic acid-acetamide(SA-AC) eutectic mixture, aiming at obtaining composite phase change material(CPCM) with high thermal conductivity, large heat storage capacity and favorable thermal repeatability for efficient heat recover. DSC results exhibited its remarkable energy storage capacity with a latent heat of CPCM of 186.8 J g−1 compared to most of the organic eutectic composite. The second law of thermodynamics was used to explain the phase change characteristics of the SA-AC/EG CPCMs corresponding to the pristine SA-AC eutectic mixture. The thermal conductivity of the CPCM was enhanced by 17.59 times comparing to pristine SA-AC. The results of thermal conductivity and infrared thermal images confirmed the CPCM possessed prominent heat storage efficiency. The thermo-physical properties of the SA-AC/EG CPCM after 500 accelerated thermal cycles were slightly decreased which did no distinct influence on heat storage. Due to the low cost and remarkable properties, the SA-AC/EG CPCM was a promising candidate for energy conservation by condensation heat recovery of air-conditioner.

Migration and phase change phenomena and characteristics of molten salt leaked into soil porous system

Molten salt is important heat transfer and storage medium in various high temperature industrial systems, but its leakage and pollution have been seldom investigated. In this paper, migration and phase change phenomena and characteristics of molten salt leaked into soil porous system are numerically investigated using volume of fluid model. During the leakage stage, molten salt expands above the soil, migrates inside the soil and begins to solidify, and finally it solidifies as a solid layer during the post-leakage stage. The vertical velocity of molten salt inside the soil linearly decreases near the surface during the leakage stage, so molten salt gradually migrates into the soil, while the whole flow velocity rapidly approaches to zero after leakage. The temperature and heat flux in the soil near the surface both increase during the leakage stage, and then they decrease during the post-leakage stage. Because of solidification, there exist maximum migration radius and migration depth, so the environment pollution can be limited. As the inlet temperature rises, the maximum migration radius and migration depth both increase for low viscosity of molten salt, while the inlet velocity increment only increases the maximum migration radius. As the soil porosity or particle diameter increases, the maximum migration radius decreases, while the maximum migration depth increases.

GeSbTe계 이중층의 상변화 특성에 미치는 열처리 온도 효과

GeSbTe계 이중층의 상변화 특성에 미치는 열처리 온도 효과

Nucleate boiling inside small evaporating droplets: An experimental and numerical study

Evaporating and boiling of droplets on heated surfaces are widely involved in many industrial applications. In this work, the phase change characteristics inside the small droplets in the nucleate boiling regime are studied experimentally and numerically. Confined by the free surface of the droplet, the bubble dynamics in the small evaporating droplet is found experimentally to be greatly different from those observed in pool boiling. After the droplet is deposited on the heated surface, nucleate bubbles generate immediately on the droplet bottom, and then they grow fast and coalesce into large bubbles. However, unlike pool boiling, once the bubbles grow to a specific size, they stop growing and always attach to the heated surface until the droplet evaporates to a thin liquid film, subsequently the bubbles start shrinking, collapsing and finally disappearing due to the confined effects induced by the thin film free surface. With such bubble dynamics, the heat transfer rates of the fast initial bubble nucleation stage and the last thin film evaporation stage are almost four times as large as the middle stable bubble stage. A numerical model is proposed to understand the confined boiling mechanisms with flow and temperature details. The results show that the confined boiling can be attributed to either the evaporation-condensation competition, or the jetting flows induced by the Marangoni convection inside the droplet. The bubble bottom evaporates while the top condensate when its top enters the droplet subcooled region; thus, the bubble seems to stop growing. The jetting flow due to the Marangoni flows further inhibits the bubble growing by pumping the cooled liquids from the top of the droplet onto the bubble surface. For hydrophobic surfaces, the Marangoni effect dominates the bubble dynamics within the evaporating droplet. For hydrophilic surface, the evaporation-condensation competition dominates the bubble dynamics inside the evaporating droplet.

Preparation and characterization of nano-sized phase change emulsions as thermal energy storage and transport media

Phase change emulsion (PCE) is a kind of two-phase heat transfer fluid with phase change material (PCM) dispersed in carrier fluid. It has received intensive attractions in recent years due to the fact that it can be used as both the thermal energy storage material and transport medium simultaneously in a thermal energy storage system. In the present study, nano-sized PCEs are prepared by the D-phase method with n-hexadecane and n-octadecane as PCMs. The thermo-physical and transport properties are characterized to facilitate the applications. The droplet size distribution of the PCE is measured by a Photon Correlation Spectroscopy, and the results show that the droplet size distributions are similar at different mass fractions. The rheological behavior and viscosity of the PCE are measured by a rheometer, which shows that the PCEs at mass fractions below 30.0wt% are Newtonian fluids, and the viscosities are dependent on both the mass fraction and temperature. The differential scanning calorimetry (DSC) is employed to analyze the phase change characteristics of the PCE, and the results indicate large supercooling degree of water and PCM in the PCE. The melting temperature and latent heat of water in the PCE are much smaller than those of pure water. The thermal conductivities of the PCE with different mass fractions at different temperatures are measured by the transient hot-wire method. Furthermore, the energy transport characteristics of the PCEs are evaluated on the basis of the measured thermo-physical and transport properties. The results suggest that the PCEs show a drastic reduction of pumping power compared with water at the same heat storage capacity.

Variations of Local Motifs around Ge Atoms in Amorphous GeTe Ultrathin Films

Phase-change materials, the highly promising candidate for nonvolatile data recording, present a different phase-change property when film thickness shrinks to very deep submicron scale. The local structure of amorphous GeTe ultrathin films, which contributes to the characteristics of phase change, is examined using X-ray absorption measurements. Ge atoms are found to be low-coordinated when the film thickness decreases. Ge atoms are linked to neighbor atoms by covalent bond, and the weaker Ge–Te bonds are more easily broken, which suggests that Ge atoms are located in the defective Ge2Te3 local arrangement. The mixture of sp3/sp2 hybridization and 3-coordinated Ge found in ab initio molecular dynamics simulations also supports this local motif. The exponential rise of crystallization temperatures of ultrathin films with decreasing film thickness, which is a vital parameter for phase change process, can be well explained by the proposed defective GeTe local arrangement.