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The present study aims to compare the energy and exergy performances of photovoltaic/thermal-thermal wheel (PT) and photovoltaic/thermal-heat pipe-thermal wheel (PHT) systems. These systems are able to preheat/precool the outside air in the cold/hot season and supply part of the electricity needed by a building. In the PT system, the ambient air exchanges heat directly with the photovoltaic panels, while in the PHT system, the evaporator section of heat pipes located under the panels transfers the heat received from the panels to the air stream through the condenser section of heat pipes. The annual useful energy and exergy produced by the system were taken as objective functions, the two-objective optimization of the systems was performed, and the performances of the optimal systems were assessed. Overall annual analysis of outcomes showed that the highest useful energy output is achieved by the optimal PT system which outperforms by 2.1% than that of the optimal PHT system, while the maximum useful exergy yield is attained by the optimal PHT system, which is 5.1% higher than that of the optimal PT system. The thermodynamic performance improvement of the PHT system is due to the electricity production increment by recovering the waste heat of photovoltaic panels. [ABSTRACT FROM AUTHOR]

In this paper, the two-phase mixture model is used to determine the influence of the number of ribs on the first-law and second-law performance features of the laminar forced convection flow of a biologically prepared silver/water-ethylene glycol nano-fluid (NF) in a rifled tube. The influence of the Reynolds number (Re), nanoparticle concentrations ( φ) on the outcomes are also evaluated. Two parameters of Performance Evaluation Criterion (PEC) and Figure of Merit (FOM) are defined to analyze the merit of using rifled tube and NF over the plain tube and water-ethylene glycol mixture (50:50 by volume) for the same Re and φ. The results showed that the increase in the number of ribs is desirable from the first-law viewpoint and undesirable from the second-law viewpoint. It was found that for all the examined cases, the hydrothermal performance of the rifled tube is always better than the plain tube, and boosting the Re and decreasing the φ leads to an increase in FOM. Moreover, it was observed that the PEC for the six-head and eight-head rifled tubes is always greater than unity and for the two-head rifled tube is always less than unity. [ABSTRACT FROM AUTHOR]

This study numerically investigates the effectiveness of concurrently applying nano-enhanced phase change material (NEPCM) and an ultrasonic field for the thermal management of a pin fin heat sink. The role of the NEPCM is to absorb heat from the heat sink wall, while the ultrasonic field generated by ultrasonic transducers facilitates the accelerated melting of the NEPCM. The study investigated how varying the number of ultrasonic transducers positioned near each side wall of the square cross-section heat sink, along with adjusting the concentration of nanoparticles in the NEPCM, impacts the heat sink's performance. The total power consumption of the transducers is assumed to be constant and an increase in their number is associated with a decrease in the power consumption of each transducer. It was observed that raising the number of transducers and lowering the nanoparticle concentration both contributed to a decrease in the CPU's highest temperature. Additionally, it was found that by using the combination of ultrasonic field and NEPCM, the average temperature of CPU can be reduced by 12.33–15.91 °C to the case without the ultrasonic field. Moreover, raising the number of transducers and lowering the nanoparticle concentration both contributed to a decrease in the CPU's average temperature. • Performance of a NEPCM-based heat sink with ultrasonic transducers is examined. • Effect of transducer arrangement and nanoparticle concentration is investigated. • The best heatsink performance belongs to the case with sixteen 15 W transformers. • Boosting NEPCM concentration entails an increment in the average CPU temperature. [ABSTRACT FROM AUTHOR]

One of the most important and simple ways to improve the performance of heatsinks is to make small changes in their designs. In this work, considering a reference structure for a pin-fin heatsink, different modes are examined through simulation by shifting the location of input/output currents. The operating fluid is water, and by raising its Reynolds number (Re) number in the laminar flow range (500–2000), functional parameters such as convective heat transfer coefficient (h), distribution of temperature in fluid/heatsink, thermal resistance, performance evaluation criterion value as well as the thermal and frictional entropy generation rates are evaluated. The significant point of the results was that, the geometry in which the inlet and outlet streams were perpendicular to the top plate and on either side of the diameter, and the geometry in which the input and output were placed on two opposite sides (rectangular widths) showed the highest efficiencies. In contrast, the placement of the input/output currents in one direction of the side plate had the lowest performance. Although boosting the inlet flow velocity increased heat transfer, it augmented the production of thermal and frictional entropy (especially frictional) as well as raised the pumping power. The highest PEC value obtained was equal to 1.131, which belongs to the geometry in which the input and output were placed on two opposite sides (rectangular widths) and occurred at Re = 1000. [ABSTRACT FROM AUTHOR]

With the aim of an efficient thermal energy storage, this study was carried out on the phase change process in a double-pipe latent heat storage system incorporating sinusoidal wavy fins during both melting and solidification and the outcomes were compared with that for smooth and non-finned units. The phase change material is in the outer side while the water (heat transfer fluid) is passed through the inner side which formed a vertical double tube. The investigation on the performance of different wave-amplitudes and wavelengths of the wavy fins lead to the qualification of the best wave profile. The characteristics of the water flow were also examined. The numerical results reflect that, for the best wavy profile, with the wave-amplitude and wavelength of 2 and 1 mm, respectively, the time for melting and solidification reduces by 43.49% and 17.81%, compared with that of the non-finned unit while they are 7.7% and 4.45% compared to the smooth fin case. Furthermore, sensitivity analysis of the Reynolds number and inlet temperature of water indicates that higher Reynolds number and the temperature difference between the inlet water and melting point result in a time reduction in the charging and discharging. [ABSTRACT FROM AUTHOR]

In this experimental research, the laminar forced convection of water–magnetite nanofluid (NF) in a horizontal twisted tube (TT) is examined under a rotating magnetic field (MF). The findings are compared with those of the plain tube (PT). The influence of nanoadditive concentration (φ ), Reynolds number (Re), twist pitch (P) and MF arrangement on the heat transfer, friction factor and overall thermohydraulic features of NF is assessed. The MF consists of two magnets that rotate around the tube. For each of the magnets, three modes of clockwise rotation, counterclockwise rotation and without rotation are considered. The findings showed that the combined use of TT and rotating MF entails an increase in the overall thermohydraulic features of water–magnetite NF. In addition, it was found that the overall thermohydraulic features of NF augment with boosting φ , while they decline with boosting P and Re. Moreover, it was revealed that the best thermohydraulic features of the water–magnetite NF belonged to the case of φ = 2%, Re = 500 and P = 10 mm in the presence of a rotating magnetic field resulting from the clockwise rotation of the first magnet and the counterclockwise rotation of the second magnet. [ABSTRACT FROM AUTHOR]

In this study, an attempt is made to assess the feasibility of several machine learning techniques for forecasting the energetic performance of a hybrid renewable energy unit consisting of an earth-air heat exchanger (EAX) and a building-integrated photovoltaic thermal (BPV/T) unit. The unit provides preheating/precooling of outdoor air in cold/warm days and generates electricity throughout the year. The employed methods are artificial neural network (ANN), support vector machine network (SVMN), and fuzzy network (FN). These techniques are employed to develop a relationship between the input and output parameters of the unit. The annual total energy output of the unit is taken as the essential output of the unit, while the input parameters were the length, depth, and width of the BPV/T unit, the air mass flowrate, and length and diameter of the EAX unit. The results indicated that all the methods are successful at the prediction of the annual total energy output of the unit; however, the SVMN outperforms other methods in the test phases where the non-trained data sets are examined. Finally, it is demonstrated that the SVMN model can successfully predict the output for any arbitrary combination of the inputs within the training intervals. [ABSTRACT FROM AUTHOR]

Summary: Considering the significance of the appropriate selection of boundary conditions (BCs) in modelling step for practical engineering applications, this study presents a numerical work focusing on the impacts of BCs on the natural convection of molten phase change material (PCM) used for cooling of photovoltaics (PV). A rectangular enclosure loaded with liquid PCM (Pr = 41.22) is considered at an inclination angle of θ = 30° to simulate a PV‐PCM system. Heat flux is applied to the top wall, and impacts of two different BCs at the bottom wall, namely isothermal and convective BCs, on the flow and heat transfer characteristics are compared by taking six different Biot numbers (0.1 ≤ Bi ≤ 100) into account, while the reference case is considered as isothermal BC. Furthermore, the effects of various aspect ratios (AR = 1, 2 and 4) and Rayleigh numbers (Ra = 104, 105 and 106) are also included. The results revealed that Biot number has a significant effect on mean Nusselt number. Compared to the isothermal case, low Biot numbers (Bi < 10) significantly restrict convection motions inside the enclosure and result in remarkably different mean Nusselt numbers, while high Biot numbers give significantly similar results due to low thermal resistance outside the bottom wall. Moreover, the heat transfer enhancement by increasing AR and Ra is considerably high at high Biot numbers, while it is remarkably constricted at low Biot numbers. As a result of comprehensive analyses, it is deduced that utilization of isothermal BCs instead of convective BC for easiness of modelling can be a reasonable approach providing that Biot number is sufficiently large (Bi > 10). Although the inspiration of the present study is PV/PCM systems, the results can be generalized for any kind of fluids used in similar natural convection applications. [ABSTRACT FROM AUTHOR]

In the present work, the entropy production of Al 2 O 3 -water nanofluid in an oblique cavity enclosure is examined. The side edges of the enclosure are at lower temperature and the top and down ones are insulated. Two thick conductive walls are placed on the low temperature edges. Furthermore, the center of the enclosure is equipped with a constant high temperature heat source. The enclosure is exposed to an inclined magnetic field. The governing nonlinear partial differential equations are continuity, Navier–Stokes and energy equations. These equations are solved using an open-source CFD software package (OpenFOAM). The influence of effective parameters includes Ra number, Hartman number, the angles of magnetic field and enclosure, nanoadditives concentration, and aspect ratio on the entropy production and the Bejan (Be) number are investigated. The results show that the minimum entropy production happened at low power magnetic fields. With the increase of the slope of the cavity, the entropy production rises. The addition of nanoadditives leads to an intensification in the entropy production and a reduction in the Be number. [ABSTRACT FROM AUTHOR]

Natural convection and entropy generation of a power-law non-Newtonian fluid in a tilted triangular enclosure subjected to a magnetic field was investigated. A part of the enclosure's right or left wall is at a high temperature while the top wall is cold. The remaining walls are insulated. The results indicate that when the hot wall is at the left wall and the Rayleigh number is increased from 103 to 105, the heat transfer rate of the shear-thinning fluid goes up 1.5 times and its entropy generation rate rises >2 fold. For the Newtonian fluid, these changes mean an increase of 71% in heat transfer and a surge of 80% in entropy generation. With the increase of Rayleigh number, Bejan number diminishes. A higher Hartmann number results in a lower average Nusselt number and entropy generation rate and the rise in the Bejan number in the considered enclosure. Unlabelled Image • Study the natural convection and entropy generation of a non-Newtonian fluid • Study on a tilted triangle with partially heated wall under the magnetic field • An increase of 71 and 80% in heat transfer and entropy generation by increasing Ra • Reduction in the average Nusselt number and entropy generation rate for a higher Ha • A maximum drop of 56 and 65% in heat transfer and entropy generation for a higher n [ABSTRACT FROM AUTHOR]

• MHD natural-convective heat transfer of aluminum/water nano-fluid between two blades of a heat sink. • Irreversibility generation augments and the Bejan number reduces by augmenting the Rayleigh number. • The heat transfer enhanced up to 16% by doubling of the aspect ratio of the pipes. • As the distance between constant-temperature pipes intensified, Nusselt number enhances. • When cavity became vertical, the heat transfer and total irreversibility decreased by 22% and 21%. In the present study, natural-convective heat transfer along with the effects of radiation of aluminum/water nano-fluid between two blades of a heat sink, which is under the impact of a uniform magnetic-field, is studied numerically. The space between two blades of the heat sink is considered as a two-dimensional square enclosure. In the square cavity, there are four pipes with constant temperature T h with a circular cross section. The RSM method is used to optimize the geometric parameters of the pipes. The results show that the heat transfer rate from the pipes and the irreversibility generation augment and the Bejan number reduces by augmenting the Rayleigh number. The heat transfer intensified 7% and 16% by doubling of the aspect ratio of the pipes at the Rayleigh number of 103 and 106, respectively. As the distance between constant-temperature pipes intensified, Nusselt number augments. As the horizontal enclosure rotates 90°, i.e., it becomes a vertical enclosure, the heat transfer decreases by 22% and total irreversibility decreases by 21%. The optimum physical conditions of the pipes are is in the diameter of 0.15 and 0.25 of distance from each other to have maximum heat transfer and the minimum irreversibility generation. [ABSTRACT FROM AUTHOR]

In the present study, the flow field and heat transfer of a water–copper nanofluid with variable properties in a trapezoidal enclosure saturated with porous media are studied. The governing equations are solved by finite volume method and the SIMPLER algorithm. The nanofluid flow is assumed to be laminar, steady and incompressible. Simulations are performed for sidewall (trapezoid legs) angles of 30°, 45° and 60° with respect to horizontal axis, Reynolds numbers from 10 to 1000, Darcy numbers of 10−2, 10−3, 10−4 and volume fractions of 0 to 0.04 of nanoparticles. Numerical results show that the average Nusselt number increases with increasing volume fraction of nanoparticles for all studied Darcy numbers. The convection and motion of the nanofluid decrease by reducing the Darcy number which leads to a reduction in the velocity and local Nusselt number. The average Nusselt number increases by increasing the Darcy number for all aspect ratios. Also, the average Nusselt number increases with increasing Reynolds number for all Darcy numbers, aspect ratios and volume fractions of nanoparticles. [ABSTRACT FROM AUTHOR]

A numerical investigation is carried out to assess the hydrothermal performance of a water-based hybrid nanofluid containing both Fe3O4 (magnetite) nanoparticles and carbon nanotubes (CNTs) in a heated tube in the presence of a constant non-uniform magnetic field. The magnetic field is created by three pairs of permanent magnets. The effects of Reynolds number, magnetite, and CNT volume concentrations as well as magnetic field strength are investigated. The acquired data for the case of without magnetic field confirmed higher values of heat transfer and pressure drop as a result of utilizing nanofluid compared with water. Additionally, it was found that the Nusselt number and pressure drop of the studied nanofluid samples increase significantly under the magnetic field. Moreover, the influence of magnetic field increases with an increase in the nanoparticle concentrations and magnetic field strength and decrease in the Reynolds number. The maximum increments of 109.31% and 25.02% in comparison with the case of without field were obtained in the average Nusselt number and pressure drop for hybrid nanofluid containing 0.9% magnetite and 1.35% CNT at Reynolds number of 500. [ABSTRACT FROM AUTHOR]

The objective of this study is to assess the hydrothermal performance of a non-Newtonian hybrid nanofluid with temperature-dependent thermal conductivity and viscosity compared with a Newtonian hybrid nanofluid with constant thermophysical properties. A counter-current double-pipe mini-channel heat exchanger is studied to analyze the effects of the hybrid nanofluid. The nanofluid is employed as the coolant in the tube side, while the hot water flows in the annulus side. Two different nanoparticles including tetramethylammonium hydroxide-coated Fe3O4 (magnetite) nanoparticles and gum arabic-coated carbon nanotubes are used to prepare the water-based hybrid nanofluid. The results demonstrated that the non-Newtonian hybrid nanofluid always has a higher heat transfer rate, overall heat transfer coefficient, and effectiveness than those of the Newtonian hybrid nanofluid, while the opposite is true for the pressure drop, pumping power, and performance evaluation criterion. Supposing that the Fe3O4-carbon nanotube/water hybrid nanofluid is a Newtonian fluid with constant thermal conductivity and viscosity, there leads to large error in the computation of pressure drop (1.5–9.71%), pumping power (1.5–9.71%), and performance evaluation criterion (18.24–19.60%), whereas the errors in the computation of heat transfer rate, overall heat transfer coefficient, and effectiveness are not considerable (less than 2.91%). [ABSTRACT FROM AUTHOR]

The aim of the present study is an investigation of the impact of nanoparticle shape on the hydrothermal characteristics of boehmite alumina nanofluid flowing through a horizontal double-pipe minichannel heat exchanger. Boehmite alumina (γ-AlOOH) nanoparticles of different shapes (i.e. cylindrical, brick, blade, platelet, and spherical) are dispersed in a mixture of water/ethylene glycol as the nanofluid. The effects of the Reynolds number and nanoparticle concentration on the heat transfer rate, overall heat transfer coefficient, effectiveness, pressure drop, pumping power, and performance index are numerically analyzed for different nanoparticle shapes. The results reveal that the nanofluids containing cylindrical and platelet shaped nanoparticles have the highest and lowest thermal conductivity, respectively. Additionally, it is found that the highest and lowest viscosity belong to the nanofluids with platelet shaped and spherical nanoparticles, respectively. Furthermore, it is depicted that, among the considered nanoparticle shapes, platelet shaped demonstrates better heat transfer characteristics, while performance index of the heat exchanger for nanofluid containing spherical nanoparticles is higher. Finally, it is inferred from the obtained results that the increase of Reynolds number and nanoparticle concentration result in a higher heat transfer rate, overall heat transfer coefficient, pressure drop, and pumping power and a lower performance index. [ABSTRACT FROM AUTHOR]

This paper presents a feasibility investigation of integrating an air-based photovoltaic/thermal (PV/T) system with a thermal wheel (TW) system for residential applications. The innovative system is capable of pre-heating/pre-cooling the ambient fresh air in winter/summer as well as producing electricity. The performance of the system is numerically evaluated and compared with the conventional building integrated PV/T (BIPV/T) and TW systems. Then, a multi-objective optimization approach is utilized to find the optimum values of geometric and operating parameters in order to maximize the annual average effectiveness of the TW and the first-law efficiency of the BIPV/T collector. The performances of the optimized and un-optimized BIPV/T-TW systems are compared for a complete year. The results demonstrated that the BIPV/T-TW system has a better thermal performance compared with the BIPV/T and TW systems, while it has a slightly lower electrical performance compared with the BIPV/T system. Furthermore, it was found that the annual average first-law efficiency and TW effectiveness of the optimized BIPV/T-TW system is 118.3% and 59.7% higher than that of the un-optimized system. [ABSTRACT FROM AUTHOR]

This research attempts to investigate the effects of concentration and radius ratio on convective heat transfer and entropy generation of a non-Newtonian hybrid nanofluid flowing through a concentric annulus. The nanofluid is prepared by suspending tetramethylammonium hydroxide (TMAH) coated Fe 3 O 4 (magnetite) nanoparticles and gum arabic (GA) coated carbon nanotubes (CNTs) in water. Variable thermal conductivity and viscosity are used in simulations. The convective heat transfer coefficient of inner and outer walls, and total entropy generation augment with increasing Fe 3 O 4 and CNT concentrations. Increasing radius ratio from 0.2 to 0.8, at CNT concentration of 1.1% and Fe 3 O 4 concentration of 0.7%, decreases the heat transfer coefficient of inner wall by 85.05%, while increases that of outer wall by 35.49%. Models of convective heat transfer coefficient of both walls and total entropy generation are developed using neural network. Genetic algorithm is used with compromise programming to achieve optimal cases with maximum heat transfer and minimum entropy generation. In this method, the objective functions are mixed and the problem transforms into a single-objective optimization. Finally, applying the nanofluid with high concentrations is recommended for all conditions except the cases in which importance of entropy generation is considered much greater than that of heat transfer. [ABSTRACT FROM AUTHOR]

• Effect of mechanical vibration on the ferrofluid flow in a rifled tube is examined. • Flow of ferrofluid is studied in the presence of two rotating magnetic fields. • Effect of vibration acceleration and magnetic field configuration is assessed. • Using vibration and magnetic field boosts the cooling performance of ferrofluid. • The PEC parameter increases with boosting vibration acceleration. Vibration can enhance the hydrothermal performance by disturbing the thermal boundary layer. Also, the magnetic field increases the ferrofluid mixing, thereby enhancing the heat transfer rate. In this study, an experimental analysis of ferrofluid flow inside a rifled tube under the vibration and rotational magnetic field (RMF) effects was conducted by considering different Reynolds numbers (Re), nanoparticle concentrations (φ), and rifled tube pitches (P). In the first stage, the effect of Re and φ on the hydrothermal performance of the system in the absence of the vibration and RMF was explored. In the second stage, the effect of vibration on the performance evaluation criterion (PEC) of the system was investigated. Finally, the RMF effect was considered. Based on the results, the system with P = 5 mm showed the highest PEC in all experiments. The highest PEC without the vibration and RMF effects was obtained as 1.62 for P = 5 mm and φ = 0% at Re = 2000. The highest PEC under the vibration effect (1.28) was also found for Re = 2000 but at φ of 2%, when the highest vibration acceleration (5 m/s2) was applied. Among the RMFs examined, the RMF with the counter clock-wise along with the counter clock-wise fluid flow inside the rifled tube resulted in the highest PEC of 1.62. RMF improved the PEC of the system from 1.28 to 1.62, corresponding to a 21.32% increase, under the vibration. [ABSTRACT FROM AUTHOR]

• Numerical study of using ultrasonic wave for thermal management of CPUs. • Totally 31 scenarios are examined by altering location and number of transducers. • Impact of Reynolds number and transducer frequency on the outcomes are assessed. • Case with transducers at ceiling and lateral walls (case#27) has the highest PEC. • The maximum PEC belongs to the case#27 and occurs at Re = 1000 for F = 30 kHz. The suitable thermal management of electrical devices leads to their reliable operation and durability. The present study investigates the application of ultrasonic surface vibration in a pin–fin heatsink to improve hydrothermal performance. To this end, a totally 31 scenarios were examined by altering the location of the vibrating transducers at four lateral walls and the top plate of the heatsink. The numerical calculations were performed within Reynolds number (Re) ranges of 500–2000 and frequency magnitude (F) of 15–30 kHz. Our results showed that the heatsink with three transducers at the top plate and two lateral adjacent walls (near and the opposite side of the inlet), namely case#27, exhibits the highest heat transfer coefficient and performance evaluation criteria (PEC) of 1.38 over the base case without transducers (WOT) at Re = 2000 and F = 30 among the studied cases. In addition, case#27 has its highest heat transfer coefficient and pressure drop (Δ P) at Re = 2000, while its maximum PEC is obtained as 1.78 at Re = 1000 for F = 30 kHz over the base case without vibration. Moreover, the maximum heat transfer coefficient and the lowest Δ P for case#27 at Re = 2000 were obtained at = 25 kHz. In such a case, the PEC was obtained as 1.06 over the base case. [ABSTRACT FROM AUTHOR]

This work is devoted to the experimental evaluation of the impact of a rotating magnetic field (RMF) on the thermal-hydraulic characteristics of laminar forced convection of aqueous magnetite ferrofluid through a rifled tube. The RMF consists of two small permanent magnets that revolve around the rifled tube. Considering the clockwise and counterclockwise directions for the rotation of each magnets, four different arrangements were considered for the RMF. The results were determined for different values of Reynolds number (Re), nanoparticle concentration (φ) and rifling pitch (P). Nusselt number (Nu), friction factor and Performance Evaluation Criterion (PEC) were considered as performance parameters. The analysis of the PEC parameter showed that from an overall hydrothermal point of view, the use of rifled tube and RMF is desirable. In the absence of RMF, the highest PEC was equal to 1.813, which occurred in the case of Re =2000, φ =0% and P =5 mm. In addition, among the cases with RMF, the maximum PEC was equal to 1.601, which belonged to the case of Re =2000, φ =2% and P =5 mm. • Effect of magnetic field on the ferrofluid flow in a rifled tube is examined. • Magnetic field consists of two magnets that rotate around the rifled tube. • Using rifled tube and magnetic field boosts the cooling performance of ferrofluid. • The PEC parameter increases with boosting Re and φ and decreasing rifling pitch. [ABSTRACT FROM AUTHOR]

In the present work, natural convective heat transfer of water/Al 2 O 3 nano-fluid in an inclined square enclosure is investigated. The side walls of the cavity are cold and the upper and lower ones are insulated. A wall with a thermal-conductivity of 100 and a thickness of 0.5 is located on the cold walls. Moreover, there is a constant temperature heat source in the center of the enclosure. The enclosure is located under the influence of an inclined magnetic field (MF). The governing equations were solved using the finite volume method (FVM) and solved using the SIMPLE algorithm. The results show that the heat transfer rate intensifies up to 3.11 times with intensifying the Rayleigh number (Ra). The maximum heat transfer occurred at weak magnetic fields. By augmenting the angle of the enclosure, the heat transfer rate on the right and left walls intensifies by 33% and declines by 55%, respectively. The heat transfer rate on the right wall intensifies by 14% by augmenting the angle of the MF. The addition of nano-additives also results in intensification in the heat transfer rate. • Using FVM for natural convection of a nanofluid in an inclined square enclosure. • Effect of magnetic field and enclosure geometry on heat transfer. • Heat transfer rate intensifies up to 3.11 times with intensifying the Rayleigh number. • The maximum heat transfer occurred at weak magnetic fields. [ABSTRACT FROM AUTHOR]

A numerical analysis was performed to evaluate the hydrothermal performance of a pin-fin mini-channel heatsink (MCHS) with six different inlet/outlet flow arrangements from the first and second laws of thermodynamics point of views. The results are presented in terms of several important parameters such as convective heat transfer coefficient (h), CPU mean temperature (T m , CPU), pumping power (W ̇ p), frictional and thermal entropy generation rates (S ̇ fr and S ̇ th) as well as Performance Evaluation Criterion (PEC). The results demonstrated that configurations with two inlets and outlets have the lowest W ̇ p , S ̇ fr , h , and highest T m , CPU among the studied cases. The vorticity intensification is high at the inlet/outlet headers and over the pin-fin bodies for the case with one inlet from the left and two outlets located at opposite sides, namely configuration d. The highest values of S ̇ fr and h , the lowest value of T m , CPU , and a moderate value of W ̇ p are obtained for configuration d as compared to the other studied cases. In consequence, the maximum PECs (1.61–2.20) is obtained for case d at different Re numbers (500–2000). The PEC of configuration d is nearly 19.77% higher than those of configurations with two inlets and two outlets. Also, the PEC of configuration d is nearly 66.95% higher than those obtained for two other configurations with one inlet and two outlets. [ABSTRACT FROM AUTHOR]

In this paper, the 3D numerical calculations of the water flow inside a pin-fin heat sink (PFHS) to determine the influence of tip clearance on the hydrothermal behavior and frictional & thermal irreversibilities of the fluid flow. The numerical simulations were conducted for five different fin heights (ξ = 1.5 mm to 2.5 mm) considering four different Reynolds numbers (Re =500, 1000, 1500, and 2000). The results demonstrated that the highest heat transfer coefficient is associated with ξ = 2.25 mm due to more uniform vorticity distributions, while the minimum central processing unit (CPU) average temperature is obtained for ξ = 2.5 mm due to the higher heat transfer surface area. The minimum thermal and frictional entropy generation rates are associated with PFHS with ξ = 1.5 mm. However, the PFHS with ξ = 2.25 mm has the highest Performance Evaluation Criterion (PEC) of nearly equal to 1.28 as compared to the other ξs. Besides, the fluid flow from the pin-fin base to the top and vice versa leads to create the vortices especially at the entrance of the PFHSs with tip clearance. [ABSTRACT FROM AUTHOR]

In this work, numerical investigation of the natural convection and entropy generation characteristics of biological Ag-water nanofluid in a horizontal annulus having fins attached to the inner cylinder is aimed. The impacts of nanoparticle volume concentration (φ = 0, 0.1, 0.5 and 1%), Rayleigh number (Ra = 103, 104 and 105), and number of fins (n = 0, 2, 4 and 8) on the flow structure, temperature distribution, average Nusselt number (Nu) as well as on the thermal, frictional and total entropy generation rates are examined comprehensively. The computational outcomes indicate that the fin attachment and the increment of its number significantly improve Nu up to 35.50%, while the entropy generation rate sourced by heat transfer significantly increases up to 39.07% accordingly, compared to the finless annulus. The frictional entropy generation is remarkably decreased at low Ra by the increment of the number of fins, nevertheless, it is notably increased at high Ra. Besides, the inclusion of nanoparticles significantly contributes to the augmentation of Nu , being attained up to around 30% with a volume fraction of φ = 1%, whereas it significantly increases both thermal and frictional entropy generation rates, which can respectively reach up to 30.25 and 77.60% at the same volume fraction in the finless annulus. • Natural convection of biological Ag-water nanofluid in a finned annulus is investigated. • Entropy generation with respect to fin number is emphasized. • Fin addition increases Nu up to 35.50% and Ṡ g up to 39.07%. • At low Ra , fin attachment reduces frictional entropy generation significantly. • Nanoparticle addition augments Nu , Ṡ g,h and Ṡ g,f , regardless of fin attachment. [ABSTRACT FROM AUTHOR]

The main objective of the present paper is to investigate the effect of using connecting holes on the hydrothermal performance and irreversibility behavior of laminar forced convection flow of a nanofluid (NF) inside a microchannel heat sink (MCHS). The biologically synthesized silver/water NF is employed as coolant. The influences of the volume concentration of NF (φ), the Reynolds number (Re) and the number of connecting holes on the performance features of MCHS and NF are considered numerically. It was observed that the CPU temperature is reduced and became more uniform due to boosting of Re and φ , and the consequence augmentation of the convective heat transfer. However, the pumping power and frictional irreversibility get raised as Re and φ are increased. The figure of merit is above unity for all Re and φ which indicate that the utilization of connecting holes is a suitable option to improve the MCHS performance from the first-law point of view. The heat transfer irreversibility, however, increased by using the connecting holes. Also, the merit of NF over the pure water is observed for Re of 500 and 2000 and φ of 0.1% and 1%. [ABSTRACT FROM AUTHOR]

In the current study, the two-phase mixture model is employed to assess the influences of nanoparticle shape on the hydrothermal aspects and entropy production of turbulent forced convection of MoS 2 /water nanofluid (NF) flow through a flat plate solar collector. The considered nanoparticle shapes include blade, brick, platelet and cylinder. The studied performance features include the heat transfer, outlet fluid temperature, pressure loss, friction factor, performance evaluation criterion (PEC) and entropy production. In addition, the effect of volume fraction of nanoparticles (ϕ) on the above-mentioned parameters is evaluated at two different NF mass flow rates (ṁ). According to the results, the maximum PEC corresponds to the case of ϕ = 3%, ṁ = 0.5 kg/s and brick-shaped nanoparticles, while the minimum entropy production corresponds to the case of φ = 4%, ṁ = 0.5 kg/s and blade-shaped ones. [ABSTRACT FROM AUTHOR]

The aim of this work is to study the hydrothermal and irreversibility characteristics in forced convection flow of Ag-MgO/water hybrid nanofluid through a sinusoidal hairpin heat-exchanger, numerically. The impact of nano-additive concentration (φ), Reynolds number (Re) and amplitude of the sinusoidal tube are investigated on the heat-exchanger performance form both the first law and second law points of view. The considered performance matrices are heat transfer rate, total heat transfer coefficient, heat-exchanger effectiveness, pressure loss, pumping power as well as the irreversibilities due to flow friction and heat transfer. The findings indicated that boosting the Re and φ causes an enhancement in the heat transfer, while the reverse is true about the remaining performance aspects. In addition, it was found that the irreversibility due to flow friction intensifies by boosting either Re or φ. Moreover, the outcomes revealed that the heat transfer is the main source of irreversibility in the flow of hybrid nanofluid (NF) inside a sinusoidal hairpin heat-exchanger. Furthermore, it was reported boosting the amplitude results in a decrease in the performance index of the heat-exchanger. [ABSTRACT FROM AUTHOR]

This paper aims to evaluate the hydrothermal and irreversibility behaviour of a biological water-Ag nano-fluid in a new heatsink with helical microchannels. Two-phase mixture model is applied to precisely simulate the behavior of nanofluid in the nanoadditive concentration (φ) range of 0–1% and Reynolds number (Re) range of 500–1500. The influences of φ and Re on the convective heat transfer coefficient, CPU surface temperature, pumping power, as well as the irreversibilities due to heat transfer and fluid friction are examined. The findings depict that boosting the Re and φ augments the performance of heatsink by intensifying the convective heat transfer coefficient of the working fluid which favourably declines the CPU surface temperature and the heat transfer irreversibility and importantly results in the temperature uniformity of the CPU surface. However, intensification in Re adversely affects the pumping power, the fluid friction and total irreversibilities in the system. Furthermore, it is revealed that the nano-fluid always has a superior cooling performance as compared with the pure water. Finally, it is found that the best hydrothermal performance of the nano-fluid in the proposed heatsink occurs at Re = 1500 and φ = 1%, while the minimum total irreversibility occurs at Re = 500 and φ = 1%. [ABSTRACT FROM AUTHOR]

The objective of this paper is to study the entropy generation and natural convection of water- Al2O3 nanofluid in an inclined cavity which consists of two connected inclined triangular enclosures under a horizontal magnetic field. The horizontal diameter of the cavity is small in thickness and has the temperature Tc. Half of the bottom wall and also the right wall are at the temperature Th. The rest of the walls are adiabatic. The governing equations of the nanofluid flow are algebrized and solved using the FVFEM method. The effective parameters are 103

• The presence of PCMs reduces temperature fluctuations and heat transfer to the room. • Performing sensitivity analysis for HTR based on variation in properties. • Thermal conductivity of PCM plays most significant role in selecting the appropriate PCM. • For equal thermal conductivity, closer melting point to room temperature is preferred. • When PCM is closer to the exterior, a larger portion of it melts, HTR is greater. Passive latent heat thermal energy storage approach incorporating phase change materials (PCM) is a brilliant technique to tackle high energy consumption issue in buildings. This paper investigated the thermal performance of the conventional walls of buildings in Isfahan, Iran with the inclusion of thirteen different phase change materials. The studied base wall was composed of plaster (2 cm), clay brick (15 cm), and cement (3 cm). The effect of PCM position inside the wall on the heat transfer was assessed in two scenarios, namely: close to the interior and close to exterior. The nonlinear governing equations were solved using the finite volume method. The results show that the performance of PCM-based wall is strongly influenced by the thermal conductivity, phase-change enthalpy and melting temperature of PCM. A PCM can more efficiently reduce the heat transfer to the interior space in case it has a lower thermal conductivity, has a higher latent heat of phase-change, and its phase-change temperature is closer to the room temperature. Moreover, the thermal conductivity has priority over other PCM thermophysical properties. The lower PCM thermal conductivity leads to transfer the lower amount of heat to the interior space. A two-fold increase in the thickness of the PCM leads to less than a twofold reduction in the heat transfer. Among the studied PCMs, the heat transfer reduction by Enerciel 22 was within the range 15.6–47.6%, while this range was 2–7.8% for CaC l 2. 6 H 2 O. [ABSTRACT FROM AUTHOR]

This numerical study aims to focus on the effect of difference between the inlet temperatures of working fluids on the hydrothermal characteristics of a counter-current minichannel hairpin heat exchanger. The water flows in the annulus side and the water based hybrid nano-fluid containing Fe 3 O 4 and carbon nanotubes (CNTs) passes through the tube side of heat exchanger. Temperature-dependent thermal conductivity and viscosity are considered for the non-Newtonian hybrid nano-fluid. The effects of Fe 3 O 4 and CNT volume fractions as well as the Reynolds number on the performance metrics of the heat exchanger are also assessed. The results revealed that the increase of difference between the inlet temperatures of working fluids leads to the augmentation of heat transfer rate, overall heat transfer coefficient (except at Reynolds number of 500), heat exchanger effectiveness and PEC; while the pumping power diminishes with the increase of inlet water temperature. Unlabelled Image • A hybrid nanofluid containing coated CNT/Fe 3 O 4 nanoadditives is prepared. • Variable thermal conductivity and viscosity are employed. • Effect of inlet temperature are assessed on the hydrothermal aspects of a hairpin heat exchanger. • Merit of using nano-fluid in the heat exchanger is higher at higher inlet temperatures. [ABSTRACT FROM AUTHOR]

• Simulation of nanofluid in a heat exchanger equipped with twisted tape using a two-phase mixture model. • Studying effect of nanofluid and twisted tape on the thermal performance of heat exchanger. • Evaluating effect of material type of twisted tape on heat transfer and pressure drop in the heat exchanger. • The use of twisted tape improved the Nusselt number up to 22%. • Alumina nanoparticles increased heat transfer and pressure drop up to 30% and 40%, respectively. The present study dealt with the numerical simulation of nanofluid flow in a double tube heat exchanger equipped with twisted tape. Alumina/water nanofluid and pure water are considered to be working fluids. A two-phase mixture model was employed for nanofluid flow simulation. The effect of nanofluid and twisted tape on the hydrodynamic and thermal performance of the heat exchanger was studied. Next, the focus was placed on the effect of the surface roughness (material type) of twisted tape on heat transfer and pressure drop in the heat exchanger. The results are presented as the temperature distribution, velocity field, Nusselt number, and pressure drop for various Reynolds numbers, nanofluid concentrations, pitch ratios, and tape insert materials. The results revealed that the use of twisted tape improved the Nusselt number up to 22%; also, adding alumina particles to water augmented heat transfer up to 30% and increased pressure drop up to 40%. Tapes with more roughness provided heat transfer enhancement of up to 16%, whereas the friction factor increased up to 21%. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This study experimentally and numerically investigated the effects of heater power and rotation speed of cylinders on the Nusselt number in a tank containing water. Two aluminum cylinders were placed horizontally at a certain height in the water tank with an isoflux heater inside each cylinder. Using the realizable k - ε turbulence model, the problem was numerically evaluated in 3D, steady-state, and incompressible conditions. Isotherms, streamlines, and Nusselt number variation around the cylinders were reported for each test case in the range of 1 < Ri < ∞, 3 × 108 < Ra < 7 × 108 and Re = 0, 1570, 3140, where Ri , Ra, and Re indicate Richardson, Rayleigh, and Reynolds numbers, respectively. The results showed that the rotation speed was the dominant factor. In addition, the Nusselt number increased by about 10% when the speed of rotation (ω) changed from half of the maximum to the maximum value. However, this increase was approximately 5% when the heater power (Q) changed. In the best scenario with Q = 1200 W and ω = 30 rpm, the Nusselt number was 108.1 by enhancing almost 19% heat transfer. Changes in average Nusselt number based on dimensionless parameters like Ra , Ri , and Re were observed, and the experimental data and correlations were compared. [ABSTRACT FROM AUTHOR]

In phase-change applications, once the melting process is completed, the natural convective heat transfer of liquid material may be promoted or degraded by the fins, which is the main motivation of this research. For this reason, the natural convection heat transfer inside a PCM container that can be a representative model for PV/PCM systems is numerically investigated in this study by considering three aspect ratios (AR = 1, 2 and 4), three Rayleigh numbers (Ra = 104, 105 and 106), two types of fins as rectangular and tree-like branching fin, and three different length-to-height ratio of rectangular fin (w/H = 0.3, 0.4 and 0.5). The rates of increment and decrement are presented taking the finless enclosure as the reference case. The computed results revealed that the natural convection is promoted up to 20%, depending on Ra and fin length by the inclusion of fins when the AR = 1, while it is degraded down by 5.5% for AR = 4. Interestingly, at AR = 2, the percentage increase and decrease of mean Nu numbers are slighter compared to other aspect ratios. Besides, it is also noticed in this study that utilizing a tree-like branching fin is not as effective as a rectangular fin with the same mass. Image 1 • Natural convection of molten PCM in a finned PV/PCM system investigated. • The impacts of rectangular and three-like branching fins compared. • Fin attachment can improve or impair heat transfer. • Balance exists between the strength of natural convection and blockage effect of fins. • There is a critical AR and Ra where the fin attachment becomes disadvantageous. [ABSTRACT FROM AUTHOR]