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.     

Solidification inside a sphere—an experimental study

This paper presents the investigations of the solidification of an n-hexadecane inside a spherical enclosure. The effect of solidification process was investigated for three different constant surface temperature conditions (13 °C, 8 °C and 3 °C) and for three different initial superheats of n-hexadecane (8 °C, 2 °C and 0 °C). It was observed that the solidification phase front propagates uniformly inwards towards the centre of the sphere. The concentricity of the solid–liquid phase front deteriorates as time progresses due to shrinkages causing formation of voids inside the sphere. A lower constant surface temperature results in a larger solidified mass fraction. The effect of the initial liquid superheats of the PCM on the solidification is insignificant.     

Effect of internal void placement on the heat transfer performance – Encapsulated phase change material for energy storage

The effect of an internal air void on the heat transfer phenomenon within encapsulated phase change material (EPCM) is examined. Heat transfer simulations are conducted on a two dimensional cylindrical capsule using sodium nitrate as the high temperature phase change material (PCM). The effects of thermal expansion of the PCM and the buoyancy driven convection within the fluid media are considered in the present thermal analysis. The melting time of three different initial locations of an internal 20% air void within the EPCM capsule are compared. Latent heat is stored within an EPCM capsule, in addition to sensible heat storage. In general, the solid/liquid interface propagates radially inward during the melting process. The shape of the solid liquid interface as well as the rate at which it moves is affected by the location of the internal air void. The case of an initial void located at the center of the EPCM capsule has the highest heat transfer rate and thus fastest melting time. An EPCM capsule with a void located at the top has the longest melting time. Since the inclusion of a void space is necessary to accommodate the thermal expansion of a PCM upon melting, understanding its effect on the heat transfer within an EPCM capsule is necessary.     

Numerical study of a dense gas–solid jet flow

A new approach using both Eulerian and Lagrangian coordinates and taking account of inter-particle interactions has been developed for the study of gas–solid flows. A numerical algorithm is presented. Comparison with experimental results shows reasonably good agreement.     

Electrochemical synthesis of a hydrogen storage material based on lanthanum and nickel intermetallics in the equimolar KCl–NaCl melt

Co-reduction of lanthanum and nickel ions on inert tungsten and active nickel electrodes were studied using the methods of cyclic chronovoltammetry, chronopotentiometry and open circuit chronopotentiometry in equimolar KCl–NaCl at 973 K. When both lanthanum and nickel ions are present in equimolar KCl–NaCl, the voltammetry dependences demonstrate the nickel ions reduction wave in the region of potentials –(0.00–0.1) V and lanthanum ions reduction wave in the region of potentials –(1.85–1.9) V relative to a silver chloride reference electrode. There are two more reduction waves on the opposite sides of voltammograms in the regions of potentials –(1.5–1.6) V and –(1.75–1.8) V. These waves are associated with the reduction of lanthanum ions and their depolarization on metallic nickel preliminarily deposited on the tungsten electrode, which results in the formation of intermetallic La_xNi_y phases. The (E–t) dependences of open circuit chronopotentiometry demonstrate a delay plateau, which corresponds to the time required for dissolution of separate phases of intermetallic compounds. LaNi₂ and LaNi₅ intermetallic phases were electrochemically synthesized at a definite concentration of lanthanum and nickel chlorides in equimolar KCl–NaCl. The synthesized lanthanum and nickel intermetallic compounds were characterized by the X-ray diffraction analysis, scanning electron microscopy and photon correlation spectroscopy.     

Experimental study on the performance of a heat pump system with refrigerant mixtures’ composition change

An experimental study on the capacity control of a heat pump system has been performed using refrigerant mixtures of R32/134a. A test apparatus was made of a refrigeration part and two different types of composition changing parts; a single separator system and a separator–rectifier combined system. Analysis of the separation process was made for a basic single separator system. In order to pursue a wider range of composition change, a separator–rectifier combined system with packed-type distillation column was designed with Raschig ring as packing material. The composition changing part was connected to the condenser outlet and the evaporator inlet. Heating capacity, cooling capacity, and coefficient of performance (COP) of the system were measured under heating and cooling conditions. When the single separator system was used as a composition changing part, the range of composition change in the refrigeration system was approximately 13%. Around 26% of composition change was obtained using the separator–rectifier combined system. From the composition change with the separator–rectifier combined system, the capacity improved from 2.6 to 3.4 kW in the cooling test and from 1.8 to 2.4 kW in the heating test. As the composition of R32 increases, heating and cooling capacities were improved, whereas the value of COP with the refrigerant mixture is enhanced due to a temperature glide effect. It is concluded that the system capacity can be adjusted to meet load requirements by controlling the composition of the refrigerant mixture.     

Numerical investigation on combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body

Combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body were numerically investigated. Results reveal that the blow-off limit of the micro-combustor with a regular triangular pyramid bluff body is 2.4 times of that in the micro-combustor without bluff body. With the increase of inlet velocity, the recirculation zone expands and preferential transport effect behind the bluff body is intensified. Therefore, the local equivalence ratio in the recirculation zone increases when Φ = 0.8, but the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds 10 m/s. When Φ < 1.0, adding small amount of hydrogen into gas mixture can speed up the significant elementary reaction, leading to an increase of methane conversion. It's found that both the methane conversion rate and the temperature behind the bluff body reaches the highest when blockage ratio increase to 0.22.     

Numerical investigation on location of protrusions and dimples during slot jet impingement on a concave surface using hybrid ANN–GA

Numerical simulations using ANSYS Fluent 17.2 are explored for a detailed discussion on jet impingement over a compound dimpled and protrusioned concave surface. Previous researchers have proved that both dimples and protrusions help in heat transfer augmentation. However, the combined effect of dimple and protrusion over a concave surface is still not studied, which may show a significant improvement in heat transfer. Therefore, the present study is devoted to the alternate location of dimple and protrusion over a concave surface for enhancement in impingement heat transfer. Simulations are performed for several arrangements of dimples and protrusions over a concave surface. It is noticed that a particular arrangement of dimples and protrusions leads to an increase in heat transfer compared with a fully protruded or fully dimpled concave surface. Furthermore, it is also noticed that protrusions/dimples at the stagnation region degrade overall impingement heat transfer. The wall shear stress profile is found to be similar to its corresponding local Nusselt number profile. Lastly, the optimal location of dimples and protrusions is predicted using an artificial neural network and a specially formulated discrete version of genetic algorithm.     

Numerical study of the effect of hydrogen addition on methane–air mixtures combustion

The stoichiometric methane–hydrogen–air freely propagated laminar premixed flames at normal temperature and pressure were calculated by using PREMIX code of CHEMKIN II program with GRI-Mech 3.0 mechanism. The mole fraction profiles and the rate of production of the dominant reactions contributing to the major species and some selected intermediate species in the flames of methane–hydrogen–air were obtained. The rate of production analysis was conducted and the effect of hydrogen addition on the reactions of methane–air mixtures combustion was analyzed by the dominant elementary reactions for specific species. The results showed that the mole fractions of major species CH₄, CO and CO₂ were decreased while their normalized values were increased as hydrogen is added. The rate of production of the dominant reactions contributing to CH₄, CO and CO₂ shows a remarkable increase as hydrogen is added. The role of H₂ in the flame will change from an intermediate species to a reactant when hydrogen fraction in the blends exceeds 20%. The enhancement of combustion with hydrogen addition can be ascribed to the significant increase of H, O and OH in the flame as hydrogen is presented. The decrease of the mole fractions of CH₂O and CH₃CHO with hydrogen addition suggests a potential in the reduction of aldehydes emissions of methane combustion as hydrogen is added. The methane oxidation reaction pathways will move toward the lower carbon reaction pathways when hydrogen is available and this has the potential in reducing the soot formation. Chemical kinetics effect of hydrogen addition has a little influence on NO formation for methane combustion with hydrogen addition.     

Numerical Simulation of the Flow over a Model of the Cavities on a Butterfly Wing

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Numerical and experimental study on granular flow and heat transfer characteristics of directly-irradiated fluidized bed reactor for solar gasification

Solar gasification is one of the promising techniques to convert the carbonaceous materials to clean chemical fuels, which offers the advantages of being transportable as well as storable for extended period of time. In this study, thermal performance of a recently developed 5 kW_th fluidized bed reactor for solar gasification has been investigated and reported. Discrete element method (DEM) has been used for modeling the granular flow, and computational fluid dynamics (CFD) method has been used for modeling the multiphase flow. To validate the developed model, experiments were preformed and compared with modeling results. Discrete ordinate radiation model has been used to solve the radiative transfer equation. The thermal performance of the reactor and particulate flow behavior have been predicted and the effect of particle size, particle size distribution and gas flow rate are analyzed. The results indicate that the performance of the bed increases when fluidizing the annulus region particles as the high porosity increases the diffusion rate of radiation throughout the bed.     

Microencapsulation of phase change material in water dispersible polymeric particles for thermoregulating rubber composites—A holistic approach

Summary To avoid the leakage of phase change materials (PCM) to its surrounding, microencapsulation of PCM in a polymeric shell is highly desirable. These microcapsules ideally should provide a platform to store and release latent heat of the PCM without undergoing any physicochemical transformation of core (PCM) as well as shell (polymer) materials. Several characteristics such as heat transfer efficiency, thermal conductivity, water dispersibility, and durability of the PCM capsules are dependent on the nature of shell materials. In the present study, a random copolymer of poly (methyl methcrylate-co-2-hydroxyethyl methacrylate) poly (MMA-co-HEMA) with an optimum ratio of 75/25 (methyl methacrylate (MMA)/2-hydroxyethyl methacrylate (HEMA)) was used as shell material to encapsulate paraffin wax (PCM), using emulsion solvent evaporation method. The microcapsules of ~5-μm size with a shell thickness of ~0.8 μm with high encapsulation efficiency (~92.34%) and thermal storage capability (99.85%) were fabricated. In addition to ease of water dispersibility, PHEMA (poly(2-hydroxyethyl methacrylate)) containing water absorbable shells also exhibit enhanced thermal conductivity from 0.1 to 0.49 W/(m·K) at 25°C in wet state compared with the dry capsule. The capsules show good durability by displaying no significant change in thermal properties and water dispersibility after running through 500 heating/cooling cycles. To test the feasibility of this novel water dispersible microencapsulated PCM, these were mixed with natural rubber latex at various blend ratios, and their thermal behaviour was evaluated. The obtained rubber composite showed good thermoregulation property with enhanced mechanical strength.     

The influence of the addition of sodium on the transformation of alkali and alkaline-earth metals during oxy-fuel combustion

Alkali and alkaline-earth metals (AAEM) of coal directly affect the coal combustion properties and ash formation during coal oxy-fuel combustion. To further understand the influence of adding sodium on the transformation of AAEM, sodium chloride (NaCl) and sodium acetate (NaAc) were added to Shenmu coal in this study. A drop-tube reactor and ion chromatography were adopted in this study and a serial dissolution method was used to clarify the occurrence modes of the AAEM. The results showed that all types of AAEM can release and the release rates were increased with an increase in temperature during oxy-fuel combustion. Water-soluble (W-type) alkali metals react with SiO₂ and Al₂O₃ in coal and are converted into acid-soluble (H-type) silicate or acid-insoluble (I-type) aluminosilicate under certain experimental conditions. The addition of sodium can promote the release of AAEM via promoting coal combustion; the promotion effect was significant at 600 °C, and the effect of NaCl was more noticeable than that of NaAc. Furthermore, the promoting effect on alkali metals was more noticeable than that on alkali-earth metals. The added sodium can also react with SiO₂ and Al₂O₃ to form H-type sodium silicate or I-type sodium aluminosilicate.     

Numerical Analysis of Thermal Convections in a Three-dimensional Cavity: Influence of Difference in Approximation Models on Numerical Solutions

In order to develop the practical approximation models suitable to flow fields at low Mach number with large temperature difference, the influence of difference in approximation models on numerical solutions was investigated by solving the natural convection in the 3-D enclosures with vertical sidewalls differentially heated and the heated bottom wall using 3 approximation models, that is Boussinesq approximation, low Mach Number approximation and approximation model proposed by Mlaouah. As results of the simulation, the effects of the differences in the three approximation models on the numerical solutions become clear.     

Numerical study of the effects of non-equilibrium plasma on the ignition delay of a methane–air mixture using detailed ion chemical kinetics

Researches on non-equilibrium plasmas in ignition and combustion processes have drawn attention of many scientists, because a non-equilibrium plasma-assisted approach provides a useful method to ignite a combustible mixture and stabilize the combustion process. The ignition delay times of methane–air mixtures have been investigated experimentally and numerically; however, the influence of non-equilibrium plasma on the ignition of argon-free methane–air mixtures has seen relatively little discussion. Here, we investigate the ignition delay time of methane–air mixtures via numerical analysis using detailed chemical kinetics. Discharge process and following ignition process are simulated separately, because of significant differences in their time scales and mechanisms. Data on the concentration of atoms and radicals produced in the discharge processes were used as the initial input data to determine the subsequent ignition process because they play an important role in the subsequent ignition process. We focus on the effects of the strength of the reduced electric field, the discharge duration, and the initial temperature on the ignition delay time for zero-dimensional and axisymmetric one-dimensional models. The simulation results showed that the reduced electric field was important in promoting chemical reactions for both the one-dimensional model and the zero-dimensional model; for a constant reduced electric field, longer discharge durations provided more energy to excite the nitrogen, leading to a larger mole fraction of excited nitrogen species during discharge; the gaps between ignition delay times for E/N = 0 and E/N ? 50 Td were very small at high initial temperatures; however they became very large at low initial temperatures.     

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.     

Experimental analysis of hydrogen storage performance of a LaNi5–H2 reactor with phase change materials

Energy storage, especially thermal energy storage, has an important place in terms of efficient use of energy. Systems in which phase change materials (PCMs) are used are among the thermal energy storage (TES) options, thanks to their advantages such as energy storage at almost constant temperature. The use of PCM as a TES material in the metal hydride (MH) reactor is an influential method to store the heat released by the exothermic reaction occurring in the hydrogen charging process and to recover this heat with the endothermic reaction occurring in the hydrogen discharge process. In the present study, hydrogen charge and discharge processes in a LaNi₅–H₂ reactor were experimentally investigated and compared with and without PCM. Therefore, a hybrid system was designed by integrating PCM around the cylindrical MH reactor filled with LaNi₅ alloy. The hydration process was carried out at both constant pressure and variable pressure. The temperature changes on the reactor surface and inside the PCM were measured over time. In experiments to determine the change in the amount of hydrogen stored in MH reactors over time, it was determined that the hydrogen storage pressure and reactor design significantly affect the hydrogen charge-discharge rate. Considering the use of MH reactors in transportation vehicles such as automobiles and submarines, designing a hybrid MH-PCM storage system is promising for the development of hydrogen storage technologies and transportation technologies.     

Three Dimensional Numerical Investigation on the Effect of Wedge Angle of a Passage with Pin-fin Arrays

Heat transfer in passage with pin-fin arrays for cooling blade trailing edge was studied numerically. Three-dimensional numerical simulations were carried out for steady laminar flow in passages with different wedge angles between pressure surface and suction surface of cooling blade trailing edge to study the effect of different wedge angles (from 0°to 30°) on heat transfer and pressure losses. Research was carried out for both in-line array and staggered array. From this investigation, wedge angle 10°gives the best heat transfer performance.     

Numerical study of nanofluid heat transfer for different tube geometries – A comprehensive review on performance

The heat transfer performance of a system can be improved using a combination of passive methods, namely nanofluids and various types of tube geometries. These methods can help enhance the heat transfer coefficient and consequently reduce the weight of the system. In this paper, the effect of tube geometry and nanofluids towards the heat transfer performance in the numerical system is reviewed. The forced convective heat transfer performance, friction factor and wall shear stress are studied for nanofluid flow in different tube geometries. The thermo-physical properties such as density, specific heat, viscosity and thermal conductivity are reviewed for the determination of nanofluid heat transfer numerically. Various researchers had measured and modelled for the determination of thermal conductivity and viscosity of nanofluids. However, the density and specific heat of nanofluids can be estimated with the mixture relations. The different tube geometries in simulation work are analyzed namely circular tube, circular tube with insert, flat tube and horizontal tube. It was observed that the circular tube with insert provides the highest heat transfer coefficient and wall shear stress. Meanwhile, the flat tube has greater heat transfer coefficient with a higher friction factor compared to the circular tube.