Continuous liquid–vapor phase transition in microspace

Molecular dynamics and Monte Carlo methods are adopted to predict thermodynamic parameters of small argon systems. Simulation results of isotherms are similar to the van der Waals loop in the P–V diagram, which implies the continuity between the liquid and gas states. Both simulation and experiment of argon system indicate that the density variation inside the liquid–vapor interface is gradual and the interface may become considerably thick as the saturation temperature is close to the critical point. The temperature in the interface region is not uniform and higher than the saturation temperature, which is another example of the continuous phase transition between liquid and vapor.     

Does nanoparticles dispersed in a phase change material improve melting characteristics?

Nanoparticles dispersed in a phase change material alter the thermo-physical properties of the base material, such as thermal conductivity, viscosity, and specific heat capacity. These properties combined with the configuration of the cavity, and the location of the heat source, influence the melting characteristics of the phase change material. In this paper, an assessment of the influence of the nanoparticles in the base material subjected to a heat generating source located in the center of an insulated square cavity, which is a common configuration in thermal capacitors for temporal heat storage is investigated. The interplay between heat conduction enhanced due to an increase in thermal conduction and buoyancy driven heat convection damped by the increase in viscosity of nanoparticles dispersed in the phase change materials is studied with the calculated streamlines and isotherms. We observed three regimes during the melting process, first at an early time duration dominated by heat conduction, later by buoyancy driven convection till the melting front levels with the center of the cavity, and lastly once again heat conduction in the bottom portion of the cavity. During the first two regimes, addition of nanoparticles have no significant performance gain on the heat storage cavity, quantified by maximum temperature of the heat source and average Nusselt number at the faces of the heat source. In the late regime, nanoparticles provide a slight performance gain and this is attributed to the increase in the specific heat of the melt due to the nanoparticles.     

Stability investigation of the γ-MgH2 phase synthesized by high-energy ball milling

Structural investigations showed that the crystal structure influences the hydrogen storage properties of MgH₂. The crystal structure and dehydriding temperature of MgH₂ activated by high energy ball milling under an argon atmosphere were studied. The X-ray diffraction characterization showed that a small amount of tetragonal α-MgH₂ phase transforms into crystalline γ-MgH₂ of an orthorhombic structure during milling, and γ-MgH₂ peak intensity increased with increasing ball milling time within a certain limit, but disappeared early in the hydrogenation cycle at a temperature of 300 °C. We studied the phase structure and the stability of the MgH₂ during absorption/desorption cycles as well as thermal stability in a context of sustainable use of MgH₂.     

Hydrogen production by gasification of Kenaf under subcritical liquid‒vapor phase conditions

In this study, Kenaf biomass was gasified under subcritical liquid-vapor conditions and effect of initial water amount on gas composition and hydrogen production was evaluated at different temperatures. Gasification was carried out in a batch type reactor, with various initial water volumes (0, 12, 25, 50, 75 mL), at different temperatures (250, 275, 300, 325 °C) and with different catalysts (Ru/C, Pt/C, Na₂CO₃, Fe₂O₃, CaO, CaCO₃ and RuCl₃). The results revealed that the gasification efficiency and hydrogen selectivity are dramatically varied with initial water percentage that was fed to the reactor. The gaseous products obtained from hydrothermal gasification of kenaf were mainly H₂, CO₂, and in lower but varying amounts, CH₄ and C₂H₄. Water soluble RuCl₃ was found to be more effective than water insoluble Ru/C catalyst on biomass gasification. RuCl₃ catalyst has a high catalytic activity for kenaf gasification with greater H₂ selectivity. Maximum total gas (462.5 mL) and H₂ mole fraction in the gaseous product (44.5%) were obtained at 250 °C with RuCl₃ catalyst. The maximum carbon conversion (%) was also obtained with RuCl₃ as 71.3%. Water behaves as both the reaction medium and the second main reactant beside biomass, during the subcritical liquid-vapor phase gasification process.     

Evolution of lamellar α phase during two-phase field heat treatment in TA15 alloy

Two-phase field heat treatment experiments have been performed to study the evolution of the primary lamellar α phase in TA15 alloy during the whole heat treatment process. Meanwhile both the evolutions of the primary and secondary lamellar α during air cooling with different insulation histories have been examined. Results indicate that the transformation mechanism for the primary lamellar α phase during heating is the transformation of the thin lamellae at lower α/β region and the dissolving of the small lamellae originated from the decomposition of the branched lamellae at higher α/β region. During the holding process, the volume fraction of the primary lamellar α decreased, the average thickness increased and the variation trend of the average length is complex. When the cooling mode after two-phase field heat treatment is coupled with the insulation history, the variation trend of the volume fraction shows a non-monotonous way.     

Facile aqueous phase synthesis of 3D-netlike Pd–Rh nanocatalysts for methanol oxidation

The structure-activity relationship between the morphology and composition of Pd-based nanocatalysts is an important fundamental issue in direct methanol fuel cells (DMFC). Three dimensional (3D) netlike Pd–Rh bimetallic catalysts with different atomic ratios (Pd₁Rh₃, PdRh, Pd₃Rh₁) are synthesized through a simple wet chemical way using P123 as a reducing agent and KBr as morphological regulator. The morphology, structure and composition of the catalysts are proved by a series of physicochemical test technology. It is shown that the 3D-netlike structure is composed of short self-assembly nanochains. Electrochemical results display that their application towards methanol oxidation reaction (MOR) in alkaline solution. The MOR activity of the optimized Pd₃Rh₁ nanocatalyst is improved to about 4.0 mA cm⁻², which is much higher than that of the commercial Pd/C catalyst.     

Effect of the water–steam phase transition on the electrical conductivity of porous rocks

The effect of the water–steam phase transition on electrical conductivity was experimentally investigated in volcanic and sandstone samples to support the interpretation of resistivity data to determine changes in steam saturation in geothermal reservoirs. The measurements were performed at simulated in situ conditions with controlled pore fluid chemistry, temperature, and confining and pore pressures. At constant temperature (150 °C) and confining pressure, pore fluid was withdrawn from the sample by steadily increasing the volume of the pore fluid system. At the vapor saturation pressure, the pore water progressively boiled to steam, resulting in a continuous conductivity decrease by a factor of approximately 20. The study showed that: (1) for rocks in which conduction is controlled by the pore fluid, the concurrent changes in both electrical conductivity and pore (vapor) pressure are defined by the pore size distribution; the changes in liquid–steam saturation are approximately proportional to those in conductivity and can thus be quantified; and (2) for rocks in which surface conduction is predominant there is no direct relation between conductivity, pore pressure and drained fluid volume; this implies that the conduction mechanism controls the pattern of electrical conductivity variations as steam saturation changes.     

Kinetics of the hydrogen-induced diffusive phase transformation in Nd–Fe–B industrial alloy

The kinetics of the hydrogen-induced diffusive phase transformation in industrial alloy Nd–Fe–B has been investigated. The isothermal kinetic curves were received for temperatures from 750 to 620°C at the hydrogen pressure 0.1 MPa. An isothermal kinetics diagram was obtained. This diagram is similar to the ones of the transformations in steels during heating. It is shown that the investigated phase transformation is a diffusion-controlled one with the mechanism of nucleation and growth.     

Numerical study on melt fraction during melting of phase change material inside a sphere

This paper presents a numerical study on the constrained melting of phase change material (PCM) inside a sphere to investigate the effect of various factors on the melt fraction. A mathematical model of melting processes of the PCM inside a sphere is developed. And experiments are conducted to verify the numerical method. On the basis of the model, the effects of the sphere radius, the bath temperature, the PCM thermal conduction coefficient and the spherical shell material on the melt fraction of PCM inside a sphere are discussed. The results show that the PCM inside a sphere melts fast as the sphere radius is small, the bath temperature increases, and the PCM thermal conductivity is high. And the metal shell with high thermal conductivity should be adopted preferentially. The present study provides theoretical guidance for the design and operation of the phase change heat storage unit with sphere containers.     

Aqueous phase reforming of ethylene glycol over bimetallic platinum-cobalt on ceria–zirconia mixed oxide

The roles of cobalt promoter on the bimetallic platinum-cobalt supported on the CeO₂-ZrO₂ mixed oxide were investigated for aqueous phase reforming reaction of ethylene glycol (EG) to verify the effects of cobalt to an enhanced activity and stability of PtCo/CeO₂-ZrO₂, where the CeO₂-ZrO₂ mixed oxide was prepared by sol–gel method at a fixed Ce/(Ce + Zr) molar ratio of 0.4. The enhanced catalytic activity for an aqueous-phase reforming (APR) as well as water gas shift (WGS) reaction of EG was observed on the PtCo/CeO₂-ZrO₂ at a Co/Pt molar ratio of 0.5. The higher activity and stability on the PtCo/CeO₂-ZrO₂ at an optimal Co/Pt ratio were mainly attributed to an enhanced WGS activity and less aggregation of active metals due to a high dispersion of platinum nanoparticles with a proper interaction between platinum and oxophilic cobalt nanoparticles. A facile reduction behavior of the supported bimetallic platinum-cobalt nanoparticles through an enhanced hydrogen spillover on the platinum nanoparticles also increased CC cleavage reaction of EG with the suppressed deposition of inactive coke precursors on the partially reduced active nanoparticles. These synergy effects of platinum-cobalt nanoparticles with a close interaction and a less aggregation of active metals were responsible for an enhanced APR and WGS activity on the PtCo/CeO₂-ZrO₂ at an optimal Ce/(Ce + Zr) ratio as well.     

Passive room conditioning using phase change materials—Demonstration of a long-term real size experiment

The thermal properties of lightweight buildings can be efficiently improved by using phase change materials (PCMs). The heat storage capacity of the building can be extended exactly at the desired temperature level, which leads to an enormous increase in residential comfort. This is shown in the present paper using the example of a prefabricated wooden house. The house was divided into two identical rooms. One of them was equipped with almost one ton of phase change material based on salt hydrates with a melting temperature of approx. 21°C. The material was encapsulated in 1-l Polyethylene containers and installed in two back-ventilated layers inside of the walls. The house was monitored for a period of 87 days in terms of temperatures, solar radiation and air velocity inside the PCM wall system. A considerable temperature buffering could be observed in the PCM room compared to the reference room. An overall reduction of the temperature fluctuations of 57% and a reduction of the day/night fluctuations of 62% compared to the reference room could be obtained. In addition, a prediction regarding the energy demand of such buildings is discussed on the basis of a simulation program. Thus, the annual cooling capacity can be reduced by 36.5% compared to the regular timber construction technique by introducing PCM. Furthermore, the good correlation of the simulation results with the experimental ones allows using the simulation as a tool to design a house with additional thermal storages.     

Microstructure and hydrogen storage properties of non-stoichiometric Zr–Ti–V Laves phase alloys

The non-stoichiometric C15 Laves phase alloys namely Zr_0.9Ti_0.1V_x (x = 1.7, 1.8, 1.9, 2.1, 2.2, 2.3) are designed and expected to investigate the role of defect and microstructure on hydrogenation kinetics of AB₂ type Zr-based alloys. The alloys are prepared by non-consumable arc melting in argon atmosphere and annealed at 1273 K for 168 h to ensure the homogeneity. The microstructure and phase constitute of these alloys are examined by SEM, TEM and XRD. The results indicate the homogenizing can reduce the minor phases α-Zr and abundant V solid solution originating from the non-equilibrium solidification of as-cast alloys. Twin defects with {111}<011 > orientation relationship are observed, and the role of defects on hydrogenation kinetics is discussed. Hydrogen absorption PCT characteristics and hydrogenation kinetics of Zr_0.9Ti_0.1V_x at 673–823 K are investigated by the pressure reduction method using a Sievert apparatus. The results show the hypo-stoichiometric alloys preserve faster hydrogenation kinetics than the hyper-stoichiometric ones due to the decrease of dendritic V. The excess content of Zr₃V₃O phase decreases the hydrogenation kinetics and the stability of hydrides. In addition, the different rate controlled mechanisms during hydrogen absorption are analyzed. The effects of non-stoichiometry on the crystal structure and hydrogen storage properties of Zr_0.9Ti_0.1V_x Laves alloys are discussed.     

An enhanced thermal conduction model for the prediction of convection dominated solid–liquid phase change

An enhanced thermal conduction model for predicting convection dominated solid–liquid phase change is presented. The main feature of the model is to predict (1) the overall thermal behavior of the system and (2) the phase front position without recurring to the full solution of the Navier–Stokes equations. The model rests entirely on the conduction equation for both the solid and liquid phases. The effect of convection in the melt is mimicked via an enhanced thermal conductivity that depends on the dimensionless numbers and the geometry of the flow. The model is tested and confronted to full CFD solutions for a freezing duct flow problem and for buoyancy driven melting in an enclosure. In both cases, the predictions of the enhanced thermal conduction model show excellent agreement with that of the CFD model. Not only is the enhanced thermal conduction model simpler to implement but its simulations run at least ten times as fast as those of the CFD model. Consequently, the enhanced thermal conduction model is well suited for controlling real-time solid–liquid phase change processes that occur in industrial applications as well as in latent heat thermal energy storage systems.     

Exact solution of two phase stratified flow through the pipes for non-Newtonian Herschel–Bulkley fluids

Steady state, laminar and fully developed stratified two phase flow including two immiscible fluids through the pipe has been studied analytically. One of the phases is Newtonian and the other one is non-Newtonian which obeys the Herschel–Bulkley fluid model. The dimensionless velocity distribution, Martinelli correction factor and non-Newtonian liquid holdup have been reported. The effect of interface curvature and wide range of viscosity ratio of two phases on flow behavior has been investigated. The results illustrate that the non-Newtonian rheological properties have significant effects on dimensionless velocity and consequently on two phase flow pressure drop specially for larger viscosity ratio.     

Experimental study on the purification of HIx phase in the iodine–sulfur thermochemical hydrogen production process

Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable side reactions between HI and H₂SO₄ in the sulfur–iodine thermochemical cycle. In this paper, a series of experiments on HIx phase purification were conducted by means of a stirred reactor using N₂ as stripping gas. The effects of the iodine concentration, reaction temperature, and the striping gas flow rate on HIx phase purification were investigated systematically in terms of the conversion of H₂SO₄ and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating the reactions during purification. The quantitative analysis of the compositions of the initial and purified HIx phases showed that not only the conversion of H₂SO₄ was enhanced but also the side reactions were effectively impeded by increasing the iodine concentration, temperature and the stripping gas flow rate. Based on the experimental data, the suitable operating conditions for HIx phase purification were proposed.     

Study of the NaBH4–NaBr system and the behaviour of its low temperature phase transition

A mechano-chemical method was used to synthesize solid solution Na(BH₄)_1-xBr_x with 0 ≤ x ≤ 1. Samples with compositions of x ≤ 0.333 were annealed, in order to form a single phase material. Bromide substitution leads to smaller unit cell size and lower temperature and enthalpy of the order-disorder phase transition of NaBH₄. There is a linear relation between the amount of substitution, the temperature, the enthalpy and the kinetics of the phase transition. This linear relation between enthalpy and amount of substitution can be expressed by the function ΔH = ?6.268x + 1.206 where x is the amount of substitution and ΔH is the enthalpy.