Search

Graphical abstract Highlights • A broad set of nanofluids were tested in a DASC under two radiation intensities. • Nonspherical and hybrid GO/silver metal nanofluids presented the best behaviors. • At high radiation intensity the effect of the nanoparticles was much more relevant. • The thermal conductivity of nanofluids was until 4% higher than pure fluid. Abstract Nanofluids based on spherical gold, silver, and copper nanoparticles, nonspherical silver nanoparticles, layered graphene oxides (GO), and GO/silver hybrid structures, were synthesized to analyze their effect on the thermal behavior of direct absorption solar collectors. The thermal conductivities at room temperature of all the nanofluids were similar, with values 4% higher than pure water, meaning up to three orders of magnitude differences compared to the values expected from the Maxwell model. Photothermal conversion experiments under simulated solar radiation with 1 Sun (=1 kW/m2) of intensity showed that all the spherical metal nanofluids presented up to 5 K higher temperatures and 35% increase of the efficiencies than pure water after 3000 s of irradiation. A much larger effect was seen with nonspherical silver and GO/silver hybrid particles, as these nanofluids presented around 20% higher efficiencies than pure spherical silver nanofluids. The large effect of silver morphology and the addition of GO led to further tests of these nanofluids under natural solar radiation using a parabolic dish mirror with a concentration ratio of 60. Under this high radiation, nanofluids showed up to 50 K higher temperatures and 100% increase in the efficiencies than pure water. Indeed, a boiling process after just 200 s was seen in nanofluids, while in pure water this phase change did not occur. Non-spherical silver nanofluids and GO/silver hybrid structures produced steam faster than spherical silver nanofluids. Our results confirm the relevance of the nanoparticles on the thermal conversion of nanofluids, with nonspherical silver nanofluids and GO/silver hybrid nanofluids presenting the best behaviors under low and high solar radiation. [ABSTRACT FROM AUTHOR]

Highlights • An integrated modified-evaporator of a household refrigerator with PCMs was tested. • The average storage temperatures of the refrigerator compartments were reduced. • The average temperature of the M-packs in the evaporator increased. • The power consumption was reduced by 2% and 6% with the studied eutectic PCMs. • The compressor running time was reduced by 4% and 9% with the studied eutectic PCMs. Abstract Domestic refrigerators are among the most energy-consuming appliances. In many countries, efforts have been undertaken to enhance the thermal efficiency of domestic refrigerators, and such improvements have been focused mainly on compressor operation and electronic control. The use of phase change materials (PCMs) in domestic refrigerators decreases the cycling period of the compressor and reduces internal temperature fluctuations. Two eutectic PCMs are thermally characterised (PLUSICE E-10 and 19.5 wt% NH 4 Cl). Their phase-change temperatures and latent heat values are determined, and their thermal conductivity and density are measured at 20 °C. The main goal of the present work is to study the behaviour of an integrated modified-evaporator of a household refrigerator with two different eutectic PCMs by performing standardised tests. In general, the experimental results help enhance our understanding and improve the operation of household refrigerators in terms of the power consumption and the inner temperature variation. [ABSTRACT FROM AUTHOR]

The main results of an experimental study of the effect of temperature and nanoparticle concentration on thermal conductivity and viscosity of a nanofluid are shown. The nanofluid was prepared with Octadecane, an alkane hydrocarbon with the chemical formula CH 3 (CH 2 ) 16 CH 3 , as a base fluid and 75-nm CuO spherical nanoparticles. Since the base fluid is a phase change material (PCM) to be used in thermal storage applications, the engineered nanofluid is referred to as Nano-PCM. Three Nano-PCMs were prepared by the two-step method (2.5% w/v, 5.0% w/v, and 10.0% w/v). In order to increase the stability of the Nano-PCM, the surface of the CuO nanoparticles were modified with Sodium oleate, and it was verified by IR analysis. The modified CuO nanoparticles were dispersed with an ultrasonic horn. The thermal conductivity was measured with a thermal properties analyzer in the temperature range of 30–40 °C. The viscosity was measured in the temperature range of 30–55 °C. The results for the Nano-PCM showed that thermal conductivity is almost constant in the analyzed temperature range, and the viscosity decreases non-linearly with temperature. With respect to the effect of nanoparticle concentration, both thermal conductivity and viscosity increased with nanoparticle concentration. Thermal conductivity increased up to 9% with respect to the base fluid, and viscosity increased up to 60%, in both cases with increasing concentration. Finally, the viscosity measurements for different deformation rates (30–80 RPM) showed that the addition of nanoparticles modifies the rheological behavior of the base fluid, from a Newtonian to a shear thinning (power-law) non-Newtonian behavior. [ABSTRACT FROM AUTHOR]

Nanofluids are a type of composite material with a demonstrated potential for improving heat transfer processes present in industries such as computers, electronics, and automobile. However, they have a limitation, which is that the suspended nanoparticles tend to agglomerate and in that way decrease their thermophysical properties. The present work studies experimentally the stability of a nanofluid synthesized with TiO 2 nanoparticles (6 nm) dispersed by continuous ultrasonication in water, determining the effect that exposure of the nanofluid to an atmosphere pressurized with N 2 at 1000 kPa has on its stability. A method is proposed for the quantitative measurement of the stability of a nanofluid based on the transient study of its thermal conductivity and the implementation of a model that describes such behavior. The results allow inferring statistically that the pressure treatment improves the stability of the nanofluid due to a presumed decrease of the average diameter of the agglomerations of the suspended nanoparticles. However, this improvement depends on the temperature. [ABSTRACT FROM AUTHOR]

Unsteady natural convection process of a fluid inside an inner walled vessel caused by an external forced turbulent convective flow of air in a rectangular duct has been analyzed numerically for Re = 5000; Re = 22,000. Fluid mechanics and conjugate convective heat transfer, for both inner fluid (water) and outer fluid (air), described in terms of continuity, linear momentum, k – ε turbulence model and energy equations were predicted by using the finite-volume method (FVM). Results for the streamlines, isotherms, local Nusselt numbers along the cavity walls and the average friction factor are presented. A comparison between obtained results for both Reynolds number is made. In general, it was found that the cooling process is improved up to 79% when Re number is increased from 5000 to 22,000; and the fluid mechanics characteristics inside the vessel is not considerable affected by the velocity inlet of the cooling air. [ABSTRACT FROM AUTHOR]

The inadequate use of passive thermal conditioning alternatives, such as phase change materials (PCM), in Chile, despite the climatological advantages in various regions of the country and the need to improve thermal comfort indexes in many houses and buildings, is mainly due to cultural and economic reasons. Passive thermal conditioning measures entail economic benefits and an improvement in the quality of life. However, it is perceived as a luxury and not as an investment by many Chileans. We propose an optimized implementation methodology for PCM using a house belonging to the National Housing Monitoring Network (ReNaM) in Santiago, Chile. The strategic use of PCM is based on thermal comfort and occupancy. The proposed methodology improves the thermal comfort indexes and reduces the thermal loads of the building, resulting in lower energy consumption and CO 2 emission. The proposed methodology also considers economic aspects to promote PCM as a passive alternative for thermal conditioning. [Display omitted] • A methodology to implement PCM using actual data of dwellings in Chile was used. • Walls with higher gains of rooms with higher thermal stress hours were intervened. • Thermal stress hours are reduced by 11%–23% after using PCM. • PCM implementation decreases the cooling load by 45.4%–60.3% in the summer period. • A payback period of 9.64 years is determined after implementing the PCM. [ABSTRACT FROM AUTHOR]

• LHTES system was implemented along with a conventional chiller system of a building. • Strategies of operation were assessed by computational simulation with EnergyPlus. • A hybrid operation strategy was evaluated, and the cooling loads were obtained. • LHTES reduces by 7.8% the cooling energy consumption of the cooling system. Energy consumption of commercial buildings has increasingly gained attention worldwide, because of its significant electricity consumption and peak power demand. Chiller plants are mostly used in commercial buildings to generate cold water for air conditioning. However, because the chiller plants and the refrigeration systems tend to be oversized with respect to their critical design load conditions, they certainly lead to higher energy consumption than a properly sized chiller plant. One possible way to reduce the power consumption and redistribute energy use is through the integration of latent heat thermal energy storage (LHTES) systems with air-cooling system in buildings. In the present work, a LHTES system based on ice is implemented along with a conventional chiller system of an existing commercial building located in Santiago, Chile. Different strategies of operation in a design day are assessed by computational simulation with EnergyPlus. A hybrid operation strategy is evaluated for the hottest summer week, and the results of the cooling loads are obtained. Finally, we founded that the implementation of a LHTES reduces by 7.8% the cooling energy consumption of the cooling system during the analyzed period. [ABSTRACT FROM AUTHOR]

This paper presents the results of a numerical study on the unsteady three-dimensional natural convection of a power-law fluid in a thick walled vessel being driven by natural heat convection in the surrounding air inside a cubical cavity. Continuity, linear momentum and energy equations for the internal Otswald the Waele shear thinning fluid, with a power index n = 0.4, are used to describe fluid mechanics and natural heat transfer along the conjugate continuity, Navier–Stokes and energy equations for the external Newtonian air flow and the transient heat diffusion equation in the walls of the inner fluid container. Unsteady 3D thermal convection of the inner shear-thinning fluid, with n = 0.4, is analyzed in comparison with the fluid mechanics and heat transfer results for the Newtonian fluid, with n = 1, for two values of the Rayleigh number: Ra = 2.09 × 10 4 and 2.09 × 10 6 . Thermo-physical properties of the inner fluid are allowed to change with temperature for the two values of the Rayleigh number and for both the Newtonian and the power law non-Newtonian inner fluids. Numerical simulations were carried out by an -in house- Fortran 90 Finite Volume Method parallelized code. Results of the evolution of the fluid mechanics are described in terms of the streamlines and those related to the unsteady convective heat transfer in terms of the isotherms. [ABSTRACT FROM AUTHOR]

Abstract: Transient phase change of a power law non-Newtonian fluid inside an inner thin walled container caused by external mixed convection in a square cavity has been analyzed numerically. Air was chosen as external cooling fluid and modified non-Newtonian water as the phase change fluid. Fluid mechanics and conjugate convective heat transfer, described in terms of continuity, linear momentum and energy equations, were predicted by using the finite volume method. Solidification was treated in terms of a phase change function varying linearly with temperature. The effect of the external Reynolds number, for Re=200 and 1000 on solidification was studied along the influence of the non-Newtonian power law index (n =0.5, n =1.0). Results for the time evolution of streamlines, isotherms and freezing curves are analyzed. The effect of the Reynolds number on streamlines of the external fluid is remarkable, principally near the region close to the internal water filled container. Differences between cooling and freezing times are found for Newtonian (n =1.0) and non-Newtonian modified (n =0.5) water. [Copyright &y& Elsevier]

The development of long-time stable nanofluids for practical use in heat transfer processes is a tremendous scientific challenge because nanoparticles tend to precipitate and agglomerate when in a solution, affecting both their thermophysical properties and their stability. This work experimentally investigates the role of the electro-repulse force by electric charges around the nanoparticle, as a way of improving the stability of an electrolyte-based nanofluid. Nanofluid samples were prepared in a two-step method, with 1 wt% and 3 wt% concentrations (mass fraction) of titanium oxide (TiO 2) nanoparticles added to a base fluid consisting of an electrolyte solution with a different concentration of potassium chloride (KCl) and deionized water. The pH of the base fluid was maintained constant, adding HEPES as a buffering agent. The stable condition of the nanofluid was established when the temporal variation of the thermal conductivity was negligible. When stability was established, the dynamic viscosity, zeta potential and the enhancement of the thermal conductivity were measured under controlled temperatures. Experimental results showed that the stable behavior of the nanofluid was directly influenced by the electric charge around the nanoparticles and the electro-repulse force between the nanoparticles (represented by the zeta potential), producing a consistent and homogenous stable condition for an extended 30-day period. Due to the greater number of nanoparticles in the 3 wt% solution, the dynamic viscosity of the nanofluid at 3 wt% was higher than at 1 wt%. It was noted that the addition of the nanoparticles did not affect the Newtonian nature of the fluid (except that it was slightly for higher KCl concentrations) and it produced an increase of a 41.75 ± 2.4% for 1 wt% and 59.32 ± 2.1% for 3 wt% of the nanofluid dynamic viscosity, with respect to that of the pure water. Significant enhancement of thermal conductivity enhancement was also obtained, ranging from 0.46 ± 0.11% to 1.47 ± 0.12% for the 1 wt%; and, 2.15 ± 0.11% to 4.7 ± 0.13% for the 3 wt% of nanoparticles added. This noteworthy improvement was attributed to the higher level of homogeneity of the nanofluid, caused by the high electro-repulse force between nanoparticles. Stable electrolyte-based nanofluids, such as KCl, which increase the electro-repulse forces between nanoparticles, can bolster the application of this type of nanofluid in energy conversion and electronic cooling. Enhanced stability properties (particularly in microchannel heat sinks) give these nanofluids the ability to use electric fields for the fluid motion, rather than traditional pumping devices. [ABSTRACT FROM AUTHOR]

In this work, a computational homogenization-based multiscale modeling strategy is adopted to compute the thermal conductivity of wood and to study the thermal performance of cross-laminated timber. In order to determine the thermal conductivity of wood within a multiscale modeling framework, the behavior of its microscopic constituents is investigated. By following a bottom-up approach, the computational homogenization of the material is sequentially carried out at the nanometer level and then, at the micrometer scale, resulting in the effective thermal conductivity of wood. Furthermore, the influence of the cellulose volume fraction and the microfibril angle on the cell-wall material's thermal conductivity is analyzed. These studies lead to the effective thermal conductivity components of 0.308 W/(m K), 0.107 W/(m K), and 0.115 W/(m K), along the longitudinal, radial and tangential axes of wood, respectively. These values are compared to published data and are validated successfully. Several macroscopic applications are investigated. In particular, the influence of two types of panel-to-panel connections on the thermal dissipation is assessed in a cross-laminated timber panel. • A multiscale model is adopted to compute the thermal conductivity of wood. • The homogenization is sequentially carried out at the nano and micrometer scales. • The influence of microscopic features on the thermal conductivity is analyzed. • The influence of the connection is assessed on the thermal dissipation in CLT walls. [ABSTRACT FROM AUTHOR]

TiO 2 nanotubes (TiO 2 -Ntbs) synthesized by a hydrothermal method with a diameter of ca. 5 nm were used as filler to prepare polyethylene (PE) and linear low density polyethylene (LLDPE) composites by melt blending. Nanotubes were used either as synthesized or organically modified with hexadecyltrimethoxysilane (Mod-TiO 2 -Ntbs). In some cases nanoparticles form secondary structures with sizes around 100 nm and agglomerates larger than 2 μm are also seen by transmission electron microscopy (TEM). In terms of mechanical properties, the addition of TiO 2 -Ntbs resulted in a composite, LLDPE/TiO 2 -Ntbs, with improved properties. Compared with the pure polymer, Young's modulus increased by ca. 50%, while yield stress increased by ca. 35%. On the other hand, PE/Mod-TiO 2 -Ntbs and LLDPE/Mod-TiO 2 -Ntbs with 5 wt% filler loading showed higher E′ values in the storage modulus at low temperatures than either neat PE and LLDPE. The photodegradation properties of the PE/Mod-TiO 2 -Ntbs and LLDPE/Mod-TiO 2 -Ntbs composites were studied. The largest increase of chemiluminescence emission (CL) and carbonyl index (CI) was found for LLDPE/Mod-TiO 2 -Ntbs during photoaging. That behavior may be due to large branching of the polymer, and to the presence of the nanotubes incorporated into the LLDPE, which promoted and accelerated its photodegradation due to reactive species generated during irradiation. [ABSTRACT FROM AUTHOR]

Numerical simulations of a geothermal reservoir often assume that the primary heat source is a magmatic system; however, heat from the host rock and the coupled complex transport phenomena between magma chamber and host rock also need to be considered. This research numerically simulated the cooling history of an enclosed magma chamber and the thermal effects on the host rock around it from which geothermal energy could be extracted. Modeling of the magma body included natural convection, the effect of latent heat of phase change when the crystals are being formed between the liquidus temperature and the solidus temperature, and heat conduction when the temperature is below the solidus temperature. This study takes as an example a geothermal reservoir in southern Peru (Western Cordillera) whose heat source is a rhyolitic magma chamber like those that gave rise to the intrusive rocks of the Peru coastal cordillera. This analysis varies the chamber shape and uses three solidification temperature ranges for convection and conduction models above the rock formation temperature (solidus) to study the implications for heating of the host rock. This is the first study of its kind in this area. The center of an average-sized magma chambers takes approximately 500 ka to cool from 800 °C to 300 °C. Simulated cooling times between intrusion and solidus temperatures decreased 3 ka when convection was modeled along with conduction cooling. Cooling times decreased by up to 6 ka when the solidification temperature range was increased. The host rock temperature pattern depends strongly on the stage at which the magma chamber is modeled to begin cooling. The temperatures results near the surface of the host rock obtained in this work match well with measurements at hot springs founded in several places in the Western Cordillera. Application of the methodology proposed in this study can reduce uncertainties in planning geothermal energy extraction wells. The accuracy of the numerical model described here could be improved by including more ground data from exploration wells, e.g., soil stratigraphy and temperatures variation with depth. [ABSTRACT FROM AUTHOR]

Starch nanoparticles (SNp) with a diameter of ca. 70 nm were synthesized and used as fillers to prepare high density polyethylene (PE) composites by in situ polymerization. The effect of these particles on the thermal degradation, isothermal and nonisothermal crystallization, and photodegradation of PE was studied. SNp decreased the thermal degradation temperature of PE as tested by thermogravimetric analysis and increased the relative crystallinity and crystallization rate under isothermal conditions. This nucleating agent effect was confirmed by nonisothermal crystallizations as composites presented higher crystallization temperatures than neat PE. The photodegradation tests under UV radiation during 28 days showed that NPp promoted the polymer degradation by increasing the amount of carbonyl groups and by forming cavities at the nanoparticle/PE interface. Our findings open up new strategies for using SNp as filler in PE matrices to increase not only its photodegradation, but also its thermal properties. [ABSTRACT FROM AUTHOR]