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This numerical research study focuses on impacts of using twisted rib geometry on the hydrothermal performance and entropy generation characteristics of biologically synthesized Ag/water nanofluid flowing in heat sink ‌minichannels. To this end, twelve different twisted rib geometries (Case 1 to 12) were designed where the double-row ribs were twisted at the bottom and top portions with twist angles between − 45° and + 45°. To elucidate the cooling performance comprehensively, four different Reynolds numbers (Re = 500, 1000, 1500, and 2000) and four different nanoparticle concentrations ( φ = 0%, 0.1%, 0.5%, and 1%) were included in the numerical computations. Results revealed that using twisted ribs significantly smoothen the fluid flow and reduces pressure drop remarkably depending on twist angles, nevertheless, it deteriorates the cooling performance. In Case 12, the convective heat transfer coefficient reduces by up to 15.8% at Re = 2000 and φ =1%, compared to the base case. However, pumping power requirement is decreased by 43.9% in this case. Nanoparticle incorporation contributes to enhancement of convective heat transfer coefficient by up to 13.5% at the lowest Re. Frictional entropy generation considerably decreases (up to 31%) when twisted ribs are utilized. However, thermal entropy generation can increase up to 36%, due to ineffective cooling with twisted ribs. This work can be regarded as a first study in the field concerning the effects of top and bottom twist angles on hydrothermal performance and entropy generation in nanofluid-cooled mini-channel heat sinks. [ABSTRACT FROM AUTHOR]

Hybrid nanofluids are emerging as an alternative to conventional heat transfer fluids and nanofluids for improving the thermal efficiency of heat exchanging devices synergistically due to their outstanding thermophysical properties associated because of the dispersion of different types of nanoparticles as compared to mono nanofluids. This will help in optimizing fluid characteristics in different flow regimes for several applications. However, enhancing the thermal energy efficiency of heat exchangers is challenging owing to the deprived stability of hybrid nanofluids at greater volume concentrations. This work concentrated on the synthesizing, thermophysical depiction, and thermal performance estimation of stable water-based Cu-graphene hybrid nanofluids using very low volume concentrations of Cu and graphene hybrid nanostructures. Cu-graphene hybrid nanofluid was successfully synthesized by dispersing the synthesized Cu and graphene nanostructures (keeping the Cu concentration constant at 0.04 vol % and varying the graphene concentration from 0.01 to 0.1 vol %) in water. Hybrid nanofluids display excellent stability against aggregation for up to 7 weeks, as proven by higher zeta potential values. Thermophysical characteristics of the prepared hybrid nanofluids were effectively measured. The thermal conductivity of Cu-graphene hybrid nanofluids shows exceptional enrichment (~ 35%) at minimal concentrations of hybrid nanostructures. Viscosity of the water-based hybrid nanofluids shows remarkable enhancement as compared to water and represents the increasing trend in viscosity of the base fluid with respect to the increase in concentration of hybrid nanostructures. The thermal and rheological properties of hybrid nanofluids are effectively validated with existing theoretical models. In addition, the specific heat and pumping power of Cu-graphene hybrid nanofluids with respect to the volume concentration of hybrid nanostructures are calculated using the existing theoretical equations. A figure of merit (FOM) analysis was conducted for the synthesized hybrid nanofluids to gauge thermal efficiency and evaluate the applicability of these hybrid nanofluids under laminar and turbulent flow conditions. [ABSTRACT FROM AUTHOR]

As one of the phase change materials (PCMs), paraffin is widely used in solar energy storage and utilization; however, there are limitations to its use due to poor photothermal properties of paraffin with low thermal conductivity and weak spectral absorption. In this present work, an experimental investigation on improving the photothermal properties of PCMs by producing hybrid PCM-based nanofluid with paraffin wax incorporating zinc (Zn) and zinc oxide (ZnO) nanoparticles was carried out. The two-step method was considered for the preparation of the Zn–ZnO/paraffin hybrid nanofluid, and the thermal and optical properties of the hybrid nanofluid were experimentally investigated. The impacts of nanoparticle concentration and solar radiation intensity on the photothermal properties of hybrid nanofluid were also studied. The results show that the addition of Zn and ZnO nanoparticles enhances thermal conductivity and specific heat capacity compared with pure paraffin. On the other hand, the transmission of nanofluid reduces with optical length and nanoparticles concentration increasing. Zn–ZnO/paraffin hybrid nanofluid can promote the photothermal properties in lower concentrations and higher irradiance range, and the maximum photothermal conversion efficiency of Zn–ZnO/paraffin hybrid nanofluids is 42.97%. [ABSTRACT FROM AUTHOR]

In the present work, the effect of natural convection caused by immiscible intermediate fluid on melting process in a horizontal shell-and-tube latent heat storage is examined numerically. The intermediate fluid is placed between the section filled with phase change material and the storage outer hot wall (as heat source). Water is considered as the intermediate fluid to transfer heat from the hot wall to PCM (n-octadecane) by the buoyancy impact of heated water. Water and the liquid phase of the n-octadecane are immiscible. Numerical analysis is performed employing the enthalpy-porosity method with the assistance of a commercial CFD code, ANSYS FLUENT 18.2, based on the finite volume method. The present work consists of two parts. In the first part, a simple 2D latent heat storage with various volume proportions of water as intermediate fluid, including; 25, 50, 75, and 100%, is considered. In the second part, the effect of the number and arrangement of heat transfer fluid channels is evaluated and analyzed. In all investigated models, the volume of the storage is constant and the amount of the PCM varies. The solid PCM was subcooled to 1 ℃, and the temperature of the surface of the storage was maintained at 329 K. The pertinent dimensionless parameters, including; Prandtl, Rayleigh, and Stefan numbers, are 59.5, 6.3 × 108, and 0.27, respectively. The results depicted that the maximum saved energy inside the proposed latent heat storage belongs to the case with 50% of storage volume capacity filled by water. Moreover, the case with four heat transfer fluid tubes indicated the highest absorbed energy, and among them, the four tubes with diamond pattern arrangement stored the highest amount of energy. [ABSTRACT FROM AUTHOR]

The solar air heater (SAH) is a flow device of various forms. The presence of rib turbulators is one of the basic characteristics of heat transfer changes. Two-dimensional numerical simulation of heat transfer provided by a SAH with inclined ribs at different angles (60°, 90°, 120°, 150°, 180°, and 210°) is investigated to determine the optimum angle of the inclined ribs leading to the maximum qualification of the SAH. The model is validated by comparing computed results with experimental data obtained under the same conditions. A good agreement using the standard k-epsilon model is assigned. Additionally, different Reynolds numbers (from 3800 to 18,000) are used to evaluate its effects on Nusselt number and friction factor, providing insights into the optimal operating conditions of the SAH. The design of inclined ribs significantly affects the thermal performance of SAHs. Results indicate that the thermal performance is improved when using an angle of the ribs greater than 90°. Specifically, an angle of 180° for the ribs was found to provide the optimum thermal performance. Results also demonstrate that the optimum angle is 180° for a Reynolds number of 18,000 where the Nusselt number and the friction factor are improved by 25% and 93% respectively, compared to its value obtained for an angle of 90°. The findings revealed the effect of increasing the angle of the inclined ribs on the thermal hydraulic performance parameter that the optimal angle of 180° enhances convection heat transfer and improves the efficiency of the system. The variation of the Reynolds numbers can impact the performance of the SAH, providing useful insights for the optimal operation. [ABSTRACT FROM AUTHOR]

In the present research paper, the thermal performance of helical ground air heat exchanger (HGAHE) has been experimentally analyzed under Saharan climate conditions during summer season. A flexible PVC pipe with a diameter of 0.06 m and a length of 30 m has been arranged as a helical-coil layout to design the HGAHE, and it is inserted into a borehole of 5 m depth. The study acknowledges that a drop as high as 11.0 °C in air temperature can be achieved with an air velocity of 10 m s−1 and an inlet air temperature of 38 °C. The study also revealed that, by incrementing the air velocity from 10 to 20 m s−1, the heat exchange rate of the HGAHE system increases by 158% after 3 h of operation time; however, the efficiency of the HGAHE decreases by 21.51%. Moreover, thermal performance derating factor (TPDF) increments with increasing the operation time and air velocity. After 3 h of operation time, the highest TPDF is attained as 0.27 with a velocity of 20 m s−1, whereas the lowest TPDF is observed as 0.14 for a velocity of 10 m s−1. [ABSTRACT FROM AUTHOR]

The solar collector has received great interest in many industrial applications. This paper investigates numerically the influence of the absorber shape on the thermal performance of the solar air heat collector. Turbulent 3D air flow is simulated using the turbulence RNG k-epsilon model. The validation of the used model is achieved by comparing the simulated Nusselt numbers to the experimental ones obtained under the same conditions in the case of the simple shape. A good agreement between numerical results and experimental data is obtained. Three other geometric shapes including oval, rectangular, and hexagonal forms of the absorber are also tested. A maximum improvement in the Nusselt number of 36% is obtained using the hexagonal shape of the absorber and a Reynolds number of 12,000. Predicted distributions of temperature, velocity, and pressure are also improved by using the hexagonal shape of the absorber. [ABSTRACT FROM AUTHOR]

Thermal resistance of ice slows down the charging/discharging process of ice storage systems which results in long operating cycles and thus high energy consumption. To overcome this drawback, various heat transfer enhancement methods have been investigated in the literature. In this paper, a systematic review of the studies dealing with heat transfer enhancement methods is presented. The enhancement methods covered in this review paper include adding fins, embedding metal foam, dispersing nano-additives as well as hybrid methods combining at least two of the mentioned techniques. The literature review shows the significant potential of each enhancement method in reducing the charging/discharging time. Hybrid enhancement methods showed higher impact for reducing the time of operating cycles. Fins combined with metal foam are shown to achieve the highest enhancement with heat transfer rate increase of 200.9% but a trade-off is often required to keep the storage capacity. On the other hand, the review shed light on limitations of the literature, research gaps, and future challenges. The future of heat transfer enhancement methods is headed toward optimization approaches that can play a significant role in improving the performance of ice storage units. [ABSTRACT FROM AUTHOR]

Enhancement in properties of thermal storage materials improves their performance and contributes to reducing the greenhouse gas emissions. The enhancement can be made in a passive way, which is cost-effective and hardly requires management. For decades, phase change materials (PCMs) have been used in many applications for thermal storage, thermal control and thermal insulation purposes. PCM can store a huge amount of energy with low or no temperature swing. However, the major drawback of PCM is the low thermal conductivity. Techniques of different scales (macroscopic, microscopic and nanoscopic) have been adopted in thermal engineering to enhance the thermal conductivity of PCM. This paper presents a comprehensive review of the literature dealing with micro/nano enhancement techniques of PCMs. Enhancement effects as well as limitations of each technique are discussed in detail. Moreover, direction for future research and possible challenges are pointed out along with conclusions drawn from the studies. [ABSTRACT FROM AUTHOR]

Natural convection characteristics in an enclosure having U-shape that contains nanofluid are studied by extensively examining the effect of aspect ratio. The three outer walls of cavity are at isothermal high temperature, and facing inner walls are at a uniform low temperature. Computations are performed to explore the effect of the aspect ratios of cold wall to hot walls of the U-shaped cavity in the horizontal and vertical directions on the velocity and temperature characteristics in the cavity. The impact of various Rayleigh numbers and nanoparticle volume fractions of CuO is also studied. The computational results showed that all the considered parameters have a considerable influence on the heat transfer performance. Furthermore, the augmentation in heat transfer rate introduced by nanoparticles is more profound for larger vertical aspect ratios and low Rayleigh numbers. In addition, a noticeable relationship between the contribution of nanoparticles and the two different aspect ratios studied in this work is observed. Furthermore, the influence of vertical aspect ratio on the heat transfer is more pronounced compared to the horizontal aspect ratio, i.e., increasing the height of the cold wall is more effective than increasing the width of the cold wall for most of the cases. As a result of the comprehensive analysis, a correlation for mean Nusselt number including the influence of aspect ratio of cold rib, Rayleigh number and nanoparticle concentration is proposed. [ABSTRACT FROM AUTHOR]

This paper aims to compare the thermal performance of four different configurations of absorber wall for the solar chimney-shaped channels. A detailed energy and exergy analysis has been performed for various solar radiation intensities. Using Computational Fluid Dynamics technique, the four configurations are examined in order to determine the optimal configuration. The configurations are described by a flat corner, rounded corner, triangle corner and trapeze corner. The Computational Fluid Dynamics results were validated against experimental data from the literature, and a good agreement between the prediction and measurement was achieved. The results indicate that (1) the exergy and energy efficiencies increase with solar radiation and the energy efficiency is always higher than the exergy efficiency, (2) the energy efficiency of triangle corner is 50% higher than that of the trapeze corner configuration, 67% higher than that of the rounded corner and 2% higher than that of the flat corner when the solar radiation intensity is 480 W m−2, (3) the triangle corner increases the air temperature and pressure in the solar chimney which results in an optimum air mass flow rate which enhances the ventilation and (4) the comparison among the four configurations shows that the triangle corner configuration results in higher efficiency when a solar chimney is used as a natural ventilator. [ABSTRACT FROM AUTHOR]

In this work, thermal performance of a conventional brick incorporating phase change material (PCM) is studied. The influence of brick containing PCM on heating and cooling loads is examined considering different fusion temperatures, locations and quantities of PCM. Seasonal and annual thermal performance analysis of brick filled with PCM is evaluated and quantified for climatic conditions of Marmara region, Turkey, by an established and verified numerical model. The obtained results are compared with those of conventional brick and brick filled with phase stabilized material to identify the contribution of latent heat to energy saving. The results showed that filling the gaps of brick near the indoor ambient provides a higher energy conservation. The optimum fusion temperature of PCM varied from season to season in the range of 18–26 °C. An adverse effect of the latent heat activation was observed in summer season, causing higher cooling energy demand by an inappropriate selection of phase transition temperature. Then, an annual analysis was performed to determine the optimum melting temperature which was found to be 18 °C. By incorporating PCM to the brick, the annual thermal load decreased by 17.6%, 13.2% of which was attained due to the utilization of latent heat. The outcomes of this study suggest that the integration of PCM with optimum fusion temperature into the brick can reduce heating and cooling loads considerably in every season of the year and provide thermal comfort for the occupants. [ABSTRACT FROM AUTHOR]