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In this work, the cooling effectiveness and entropy production of aluminum oxide–water nanofluid flow and heat transfer in the circular tube with the wall conduction effects are explored. All values of heat transfer effectiveness ratios are more than unity in this study. For a constant value of nanoparticles concentration, both heat transfer effectiveness ratios increase as the input temperature of fluid is increased. Both heat transfer effectiveness ratios are increased as the concentration of nanoparticles increases. Predictions show that all values of dimensionless local entropy production ratio are less than unity in this study except in the areas near the entrance of the heating part. This means that the usage of nanofluid is beneficial for decreasing the irreversibility of the system. The dimensionless local entropy production ratio decreases when the input temperature of fluid increases. The maximum value of Bi w = 5 × 10-4 was reported for the Biot number in this study, which is far less than 0.1. As a consequence, the tube wall can be regarded as a Lumped system, and it is a reasonable assumption to ignore the radial thermal resistance. [ABSTRACT FROM AUTHOR]

• Thermal resistance circuit model of forced convection in a concentric double tube is developed. • Numerical simulations have been performed to validate the thermal circuits developed. • The resulting local total thermal resistance fully demonstrated the local convective effect. • Applications of the convection thermal circuit derived have been demonstrated. Enhancing convection heat transfer performance of internal flows have been a subject of interest relevant to thermal engineering applications. The present study aims to elucidate, via a novel thermal circuit modeling, the heat flow paths and the heat transport mechanisms of thermally developing forced convection of concurrent fluid flows through the outer annulus and inner tube of an externally iso-flux heated concentric double tube, which was demonstrated as an effective heat transfer enhancement configuration with respect to its parent single tube under identical operation conditions, including the inlet fluid temperature, the total volumetric flow rate, the length of heated section, as well as the wall heat flux imposed. To examine the validity of the model developed, corresponding numerical simulations based on the finite volume method were conducted to evaluate the local thermal resistance relationships. The resulting local total thermal resistance fully demonstrated the local convective effect of the fluid flows in managing the heated wall temperature at the outer-ring wall, from which the local heated wall temperature can be readily evaluated from the given imposed heat flux and the inlet fluid temperature. [ABSTRACT FROM AUTHOR]

Abstract This work investigates experimentally the cooling performance of Al 2 O 3 /water nanofluid with 8 wt% concentration flowing through a minichannel heat sink with a MEPCM layer in its ceiling. The hydrothermal characteristics of the nanofluid-cooled minichannel heat sink are analyzed and the dimensionless wall temperature distribution, Nusselt number, heat transfer effectiveness, coefficient of performance, figure of merit, and wall temperature thermal resistance are assessed. The measured results show that the dispersion of alumina nanoparticles inside pure water declines the wall temperature and consequently enhances the heat transfer effect and Nusselt number augments. The maximum values of average heat transfer effectiveness and FOM at the inlet of the MCHS are achieved 1.4 and 1.27 by flowing nanofluid with 8 wt% nanoparticle concentration and Re = 1549, respectively. The results revealed that embedding MEPCM layer in the ceiling of the MCHS has little influence on the cooling performance of Al 2 O 3 /water nanofluid in the minichannel heat sink. [ABSTRACT FROM AUTHOR]

Abstract The present study demonstrates, via a numerical simulation, the feasibility of achieving enhanced forced convection heat transfer of laminar water flow in an isoflux heated circular tube by inserting a concentric circular tube and controlling the concurrent flow distribution through the resulting concentric double-tube duct. Under identical operation conditions to those for the parent single-tube flow configuration, including the inlet fluid temperature, the total volumetric flow rate, the length of heated section, as well as the wall heat flux imposed, numerical simulations have been undertaken for the thermally developing convective heat transfer characteristics of water flow in the concentric double-tube duct featuring geometrically by three different relative radius ratio r o (= 1.2, 1.5, 1.8), compared with its parent single-tube duct of l h,ST = 0.1 at Re ST = 100, 500, and 1000, respectively. In terms of the local and length-averaged heat transfer effectiveness gauged against that obtained for the parent single-tube duct, numerical results clearly demonstrate that the double-tube duct of smaller r o operating with relative larger flow rate than that in the inner tube can serve as highly effective heat transfer enhancement configuration. [ABSTRACT FROM AUTHOR]

In this work, the augmentation in the hydrothermal performance of parallel and divergent minichannel heat sinks (MCHSs) containing microencapsulated phase change material (MEPCM) suspensions is investigated. Accordingly, the effect of diverging minichannels with the angles of 1.38° and 2.06° on the pressure drop and heat transfer performance in the presence of 2%, 5%, and 10% MEPCM particles at different Reynolds numbers from 238 to 1375, under various different heat fluxes of 3.2 × 10 4 , 4 × 10 4 , and 4.8 × 10 4  W/m 2 , with the inlet temperature of 34 ± 0.2 °C is evaluated. The results also compared with the corresponding data in a parallel MCHS. It was found that the implementation of MEPCM suspension yielded to the increase of both the heat transfer and pressure drop. Diverging the minichannels and incrementing its angle significantly reduced the pressure drop penalty, especially at higher Reynolds number and greater mass fractions. The heat transfer effectiveness was also increased by diverging the minichannels with the angle of 1.38°, especially for higher concentrations and higher heat fluxes. The increment of the divergence angle to 2.06° had a detrimental impact on the thermal performance of the suspension and decreased the heat transfer coefficients to even less that that in the parallel MCHS. The results reveal that diverging the minichannels can effectively intensify the contribution of MEPCM suspensions in heat sinks in a certain range of parameter combinations. [ABSTRACT FROM AUTHOR]

In the present study, experimental efforts have been performed to explore the forced convection heat transfer using water-based suspension of Al 2 O 3 nanoparticles (nanofluid) to replace the pure water as the working fluids in circular tubes. The nanofluid was prepared as a functional forced convection fluid and the thermal properties including the density, thermal conductivity, and dynamic viscosity were investigated experimentally. Besides, forced convection heat transfer in circular tubes was investigated with water-based nanofluid containing various mass fractions of the Al 2 O 3 nanoparticles (2, 5, and 10 wt%) under the following operating conditions: the volume flow rate Q f = 23.6–183.5 cm 3 /min (the Reynolds number Re f ,0 = 188–2095), the heating power applied at the outer wall of the tube q o , eff . ″ = 1908–7362 W/m 2 , and the inlet fluid temperature T in = 24.5–25.5 °C or 49.5–50.5 °C. Measured data showed that the dispersion of increasing mass fraction of Al 2 O 3 nanoparticles can effectively improve the thermal conductivity relative to the pure water. Besides, higher average heat transfer effectiveness ε − h , btd and figure of merit FOM are noted for the cases with higher inlet fluid temperature T in . [ABSTRACT FROM AUTHOR]

This paper considers a thermal phase change actuator filled with a solid–liquid phase change material (PCM), in which its ceiling wall can be thermally activated to move as a result of the volumetric changes associated with thermally induced melting and freezing processes of the PCM contained. Specifically, the present study aims to explore experimentally the dynamic response characteristics of the actuator’s movable wall during the melting process of a paraffin (n-Octadecane) as the PCM filled in a rectangular container. Melting experiments for the phase change actuator heated laterally at various isothermal wall temperatures with the solid PCM filled at different initial subcooling temperatures below its melting point have been undertaken to examine the melting heat transfer characteristics as well as the displacement behavior of its movable wall. The experimental results obtained reveal clearly that the melting progress of the PCM within the actuator has a dominant bearing on the induced moving speed and thus the total displacement of its movable wall. Moreover, the subcooling condition for the solid PCM filled in the actuator during the melting process may give rise to significant time lag for its thermally driven response, hence increasing the total time required for the solid PCM to be fully melted. [ABSTRACT FROM AUTHOR]

In this study, a numerical simulation is performed to explore oscillatory natural convection in a square enclosure with a phase change material (PCM) suspension using the implicit finite difference method. The following parameters are considered: aspect ratio of the physical model = 1; ratio of the buoyancies caused by temperature and concentration gradients = 1; Rayleigh number = 103-106; initial mass fraction (or concentration) of PCM particles = 0–0.1; Stefan number = 0.005–0.1; and subcooling factor = 0–1.0. The results show that high Rayleigh numbers can easily cause oscillations in the flow field and temperature distribution. However, even under low Rayleigh number conditions, when the initial mass fraction or subcooling is increased or the Stefan number is decreased, oscillations occur. • Heat transfer in an enclosure with PCM suspension was numerically investigated. • Oscillatory natural convection phenomena are found. • High Rayleigh numbers can easily cause the oscillations in the flow and temperature. • High initial mass fraction or subcooling can cause, even under low Rayleigh numbers. • Low Stefan numbers cause too, under low Rayleigh numbers. [ABSTRACT FROM AUTHOR]

Abstract: In this work, an experimental study is carried out to examine the cooling performance of a minichannel heat sink using water-based suspensions of alumina nanoparticles (nanofluid), and/or microencapsulated phase change material (MEPCM) particles in terms of the averaged Nusselt number , the wall temperature control effectiveness , and the averaged thermal resistance control effectiveness for the relevant parameters in the following ranges: the Reynolds number, Re =133–1515; the mass fractions of the nanoparticles and/or MEPCM particles dispersed in the water-based suspensions, ωnp or ωmepcm =0–10wt.%; and the heating power. The heat sink was fabricated from copper with 10 rectangular minichannels which were maintained at a uniform base heat flux. The resulting experimental data for the various effectivenesses of using the water-based suspensions formulated in the minichannel sink were found well correlated with the relevant dimensionless parameters in general forms. [Copyright &y& Elsevier]

Abstract: Melting experiments in a vertical square enclosure are undertaken with a solid–liquid phase-change material (n-octadecane) dispersed with nanoparticles (Al2O3) as the nano-phase-change-material (nano-PCM). The enclosure has a cross-section dimension of 25 by 25mm and it was 60mm long. The vertical side walls were differentially heated isothermally while the remaining side walls were thermally insulated. The effect of dispersing nanoparticles in the base phase change material (PCM) on the melting heat transfer characteristics in the enclosure is examined for the relevant parameters in the following ranges: the mass particle fraction of nanoparticles, ωp =0, 5, 10wt.%; the Rayleigh number, Raf =1.71×106–5.67×107; the Stefan number, Stef =0.037–0.108; and the subcooling parameter, Sb =0.042–0.537. The surface-averaged heat transfer results obtained during the melting experiments indicate that natural convection heat transfer into the melted region of the enclosure tends to degrade markedly with increasing mass fraction of nanoparticles dispersed in the nano-PCM, when compared with that of the base PCM. [Copyright &y& Elsevier]

Abstract: This work compares experimentally the effectiveness of substituting the water-based suspensions of Al2O3 nanoparticles (nanofluid) and of micro-encapsulated phase change material particles (PCM suspension) for the pure water for laminar forced convective cooling through an iso-flux heated circular horizontal tube. Convection heat transfer experiments have been undertaken for the tube using the pure water, the PCM suspensions (ω PCM = 2, 5, and 10 wt.%) and the nanofluids (ω np = 2, 6, and 10 wt.%) as the working fluids under the following operating conditions: the volume flow rate Q = 25–240 cm3/min (the Reynolds number Ref  = 195–1801), the heating power applied at the outer wall of the tube qo  = 20, 30, 40 W, and the inlet fluid temperature Tin  = 32.8–33.2 °C. In addition, a scale analysis together with integral energy balance is presented concerning thermally developing heat transfer characteristics of the water-based suspension flow in the tube. The convective cooling performance of the water-based suspensions was gauged in terms of local and surface-averaged temperature control effectiveness along the tube wall compared with that obtained for the pure water. Furthermore, the water-based suspensions were compared based on the figures of merit for their heat transfer performances against the corresponding pressure drop penalty. [Copyright &y& Elsevier]

Abstract: An experimental study has been undertaken concerning natural convection heat transfer of a nanofluid in vertical square enclosures of different sizes, whose dimensions, width × height × length (mm), are 25 × 25 × 60, 40 × 40 × 90, and 80 × 80 × 180, respectively. The nanofluid formulated in the present experiment is water dispersed with various volumetric fractions of the alumina (Al2O3) nanoparticles ranging from 0.1 vol.% to 4 vol.%. The Rayleigh number varies in the range of 6.21 × 105–2.56 × 108. A correlation analysis based on the thermophysical properties of the nanofluid formulated shows that efficacy of applying the nanofluid for natural convection heat transfer enhancement in enclosure is inferred to be generally infeasible. The experimental results for the average heat transfer rate across the three enclosures appear generally consistent with the assessment based on the changes in thermophysical properties of the nanofluid formulated, showing systematic heat transfer degradation for the nanofluids containing nanoparticles of c v ≥ 2 vol.% over the entire range of the Rayleigh number considered. However, for the nanofluid containing much lower particle fraction of 0.1 vol.%, a heat transfer enhancement of around 18% compared with that of water was found to arise in the largest enclosure at sufficiently high Rayleigh number. Such enhancement cannot be explained simply based on the net influence due to relative changes in thermophysical properties of the nanofluid containing such low particle fraction, thus strongly suggesting other factors may come into play. [Copyright &y& Elsevier]

This article considers the problem of conjugate heat transfer in circular pipes with finite heated length to examine the effects of wall conduction on the heat transfer characteristics of solid-liquid phase-change material suspension flow. A mixture continuum approach is adopted in the formulation of the energy equation, with an approximate enthalpy model describing the phase-change process in the phase-change material particles. From numerical simulations via the finite-volume approach, it was found that the conduction heat transfer propagating along the pipe wall results in significant preheating of the suspension flow in the nondirectly heated region upstream of the heated section, where melting of the particles may occur and therefore the contribution of the latent heat transfer to convection heat dissipation over the heated section is markedly attenuated. Contributions of the sensible and latent heat transfer to the total heat transfer rate of the suspension flow over the heated section were delineated quantitatively for various sets of the relevant dimensionless parameters, including the particle volumetric concentration, the modified Stefan number, the Peclet number of suspending fluid, the wall thickness ratio, and the wall-to-fluid thermal conductivity ratio. [ABSTRACT FROM AUTHOR]

In this investigation, the cooling characteristics of mini-channel heat sink with the water-based nano-phase change material emulsions are examined experimentally. The n-eicosane nanoparticles and the pure water are considered as the as the phase change material and base fluid, respectively. The fluid flow and convection heat transfer in the mini channel heat sink are studied at different parameter ranges: the volumetric flow rate varied from Q ˙ = 60 cm3 min−1 to 600 cm3 min−1, three values of the heat flux imposed on the bottom wall of the heat sinks including q h ″ = 3.2 W cm−2, 3.95 W cm−2, and 4.78 W cm−2, and the mass fraction of n-eicosane nanoparticles varied from ω pcm = 0%–10%. The influences of these parameters on the friction factor, wall temperature, heat transfer effectiveness, Nusselt number, figure of merit, coefficient of performance, and thermal resistances are investigated. It is concluded that for the low volumetric flow rate and the heat fluxes of heat flux of 3.20 W cm−2, 3.95 W cm−2, and 4.78 W cm−2, using the n-eicosane nanoparticles with larger values of mass fraction is more proper to decrease the wall temperature. For low value of heat flux, the value of the convective heat transfer effectiveness is larger than unity for most values of Reynolds number. The maximum figure of merit index of 1.098 can be achieved at the Reynolds number of 1381, the heat flux of 4.78 W cm−2, and the mass fraction of 2%. The figure of merit index is decreased with boosting the mass fraction of n–eicosane particles. The figure of merit index is decreased about 44% with boosting the mass fraction of n–eicosane particles from 2% to 10% for the heat flux of 3.20 W cm−2. The usage of n-eicosane nanoparticles with the concentration of 2% results in the lower thermal resistances in comparison to the case of pure water. The difference between the thermal resistances for various mass fractions of n-eicosane nanoparticles decreases as the heat flux increases. [ABSTRACT FROM AUTHOR]

• Cooling performance of nanofluid flow in a horizontal circular tube was investigated. • Boosting the concentration of nanoparticleswould increase the heat transfer effectiveness ratio. • The heat transfer effectiveness ratio increases as the inlet temperature of fluid is increased. In this experimental study, the cooling performance of alumina/water nanofluid flow in a heated copper tube with constant heat flux is explored. The experiments are conducted for the nanoparticle's concentration in the range of 0 to 10%, the Reynolds number in the range of 168 to 2031, two values of inlet temperature of fluid, 25 °C and 50 °C, and the imposed heat flux in the range of 3979 to 7957 W. m − 2. The influences of these parameters on the friction factor, Nusselt number, temperature on internal wall of tube, the bulk temperature of fluid, the profit index, and the heat transfer effectiveness ratio are investigated. The experimental data and compared with the numerical results. The experimental results clearly exhibit a good agreement with the theoretical data existing in the literature and the numerical results. The outcomes indicate that for a constant value of nanoparticles concentration, the heat transfer effectiveness ratio and the profit index increase as the inlet temperature of fluid is increased. In addition, boosting the concentration of nanoparticles causes the increase in heat transfer effectiveness ratio. The maximum heat transfer effectiveness ratio of 1.105 is achieved by using the nanofluid. The maximum profit index of 1.065 can be achieved at the concentration of 10% and the inlet temperature of 50 °C. The heat flux has no considerable effect on the temperature on the internal wall of tube and the bulk temperature of fluid. [ABSTRACT FROM AUTHOR]

• The heat transfer of NEPCM-water suspension in a divergent microchannel was investigated. • The presence of NECPM-particles can improve heat transfer at low Reynolds number flows and low heating powers. • The phase change heat transfer of NEPCM particles promoted the heat transfer by 82% compared to pure water. The convection heat transfer of nano-encapsulated phase change material (NEPCM)-water suspensions in a divergent minichannel heatsink was experimentally investigated. The mini heatsink was made of eight minichannels with a divergent angle of 1.38°. The minichannel was heated at the bottom, and the NEPCM-water was used as the cooling working fluid. The average Nusselt number, the index of performance, and the pressure drop of NEPCM-water compared to the pure water were examined. The experiments show that employing NEPCMs could be advantageous in low Reynolds number flows and low heating powers. The phase change heat transfer of NEPCM particles promoted the heat transfer by 82% compared to pure water. Moreover, using the NEPCM particles was not advantageous at large Reynolds numbers, where the sensible heat was the dominant mechanism of heat transfer. [ABSTRACT FROM AUTHOR]

• Conjugate heat transfer of PCM suspensions in a circular pipe was numerically investigated. • The circular pipe is confined to external cooling convection. • PCM concentration, bi o and ste affected the overall thermal performance. • Pe only had a significant effect on the axial heat transfer of the wall. In this paper, a numerical method was used to investigate the conjugate heat transfer of phase change material (PCM) suspensions in a circular pipe under external cooling convection. The parameters considered in this study and their ranges were the volumetric fraction of PCM particles (c v) = 0–0.2, the Biot number (Bi o) = 0.1–100, the Stefan number (Ste) = 0.01–0.2, and the Peclet number (Pe) = 100–1000. The results showed that the PCM concentration, Bi o , and Ste had significant effects on the fluid temperature, inner/outer wall temperature, and inner/outer wall heat flux, while Pe only had a significant effect on the axial heat transfer of the wall. When a PCM-containing working fluid was used for cooling, the PCM latent heat could directly affect the fluid outlet temperature and the cooling effect. If the latent heat is changed, other thermal system parameters should be adjusted accordingly. [ABSTRACT FROM AUTHOR]

• Cooling effects of a thermosyphon with PCM suspension were numerically and experimentally investigated. • Cooling effects are the subcooling at the cooling section and PCM supercooling. • The cooling effects affected the overall thermal performance. • Controlling the cooling wall temperature can achieve an appropriate subcooling effect. A numerical and experimental study is presented to explore the effects of cooling (the section on subcooling and phase-change material (PCM) supercooling) on the thermal performance of a rectangular thermosyphon containing PCM suspensions. In the numerical simulation, a continuum mixture flow model is used for the buoyancy-driven circulation flow of the PCM suspensions together with an approximate enthalpy model to detail the solid-liquid phase change process of the PCM particles in the loop. Parametric simulations are conducted in the following ranges: heat input at the heating section = 12 W, average temperature of the outer wall of the cooling section = 10–25 °C, and PCM mass concentration = 0–15%. The results show that in the configuration of this study, the downward flow of the high-density fluid in the cooling section became the main driving force of the circulation loop. Therefore, the cooling effects affected the overall thermal performance. To allow the proper melting/freezing of PCM particles, it is critical to control the wall temperature at the cooling section to achieve an appropriate subcooling effect. [ABSTRACT FROM AUTHOR]

• The heat transfer of NEPCM-water suspension in a parallel microchannel investigated. • The presence of NECPM-particles improves heat transfer by up to 70%. • Using NEPCMs could improve the index of performance by up to 45%. • NEPCM particles at high Reynolds numbers reduced the heat transfer rate. In the present experimental study, Nano-Encapsulated Phase Change Material (NEPCM) nanoparticles with particle sizes in the range of 250–350 nm are synthesized. The core of nanoparticles is made of eicosane and can undergo liquid-solid phase change by absorbing/releasing latent heat. The eicosane core of the NEPCM particles is enclosed in a formaldehyde shell, and the particles are suspended in the water as the base fluid. The synthesized NEPCM-water suspension is employed as the working-fluid for heat removal from a microchannel heatsink. The heatsink is made of red-copper, and it consists of eight rectangular microchannels with an aspect ratio of 1.5 and a hydraulic diameter of 1.2 mm. Under the heatsink, a heating plate is embedded, which produces a uniform heat flux. The working-fluid, NEPCM-water, enters the microchannel and absorbs the heat from the microchannel walls in the form of sensible and latent heat. The impact of the nanoparticle's concentration, the heating-power, and the flow rate is investigated on the channel wall temperature, Nusselt number, convection ratio, performance index, and coefficient of performance. The results show that the presence of NECPM-particles improves heat transfer and the index of performance up to 70% and 45%, respectively. The observed enhancement of heat transfer is particularly notable at low Reynolds numbers. However, at the high Reynolds numbers, the presence of NECPM particles may reduce the convection ratio and performance index, which is mainly due to the increase of the viscosity and reduction of the sensible heat of the working-fluid in the presence of NEPCM nanoparticles. [ABSTRACT FROM AUTHOR]

• Convective heat transfer of nanofluid in a minichannel heat sink with micro-encapsulated PCM-cooled ceiling is investigated. • Thermal resistance decreases about 10.88% by using the nanoparticles for the case of bare celling. • The heat flux received by the ceiling in heating section decreases by using the MEPCM. In this work, the convective heat transfer of Al 2 O 3 -water nanofluid in a three-dimensional minichannel heat sink is exaimed numerically. The finite volume method (FOV) is applied to discrete the governing equations, and the LINE SOR and TDMA algorithms are used for solving the equations. The FORTRAN program has been developed for the numerical calculation. Ceiling of the minichannel is covered with the micro-encapsulated phase change material. The N-eicosane with the melting temperature of 34.7 °C and the latent heat of 24,300 J/kg is considered as the phase change material. The purpose of placing this material on the ceiling of the minichannel is to cool the working fluid by absorbing the heat from the fluid during melting. All simulations are performed for three values of solid volume fractions of nanoparticles including 0%, 2%, and 10%, two outer surface temperatures of ceiling including 28 °C and 30 °C, and the Reynolds number in the range of 500–2000. The effects of different parameters including the usage of the phase change material, the solid volume fractions of nanoparticles, the outer surface temperature of ceiling, and the Reynolds number on the thermal field, heat flux, melting rate of micro-encapsulated phase change material, and thermal resistance in the mini-channel heat sink are studied. The results reveal that the thermal resistance decreases about 10.88% by using the nanoparticles with solid volume fraction of 10% at Re bf = 500 and T cw,0 = 28 °C for the case of bare celling. The heat flux received by the ceiling in heating section decreases by using the micro-encapsulated phase change material(MEPCM). In addition, the MEPCM melts faster at lower values of the Reynolds number. [ABSTRACT FROM AUTHOR]

• Water-based nano-PCM emulsion flow and heat transfer in divergent mini-channel heat sink is investigated. • The mean heat transfer effectiveness is higher than unity for smaller values of the Reynolds number. • The FOM is decreased about 23.13% by increasing the concentrations of PCM nanoparticles from 2% to 10% at Re = 305 and q h ″ = 4.79 W / c m 2. • The friction factor increases about 21.36% by increasing the concentrations of PCM nanoparticles from 2% to 10% at Re = 1000. In this work, an experimental investigation is performed to study the water-based nano-PCM emulsion flow and heat transfer in the divergent mini-channel heat sinks (MCHSs). The divergent rectangular MCHS is selected in this study as it provides a higher heat transfer along with smaller value of pressure drop as compared with the parallel one. The water-based microencapsulated n -eicosane PCM suspension is considered as the coolant. Indeed, the particles of microencapsulated n -eicosane PCM are suspended in the water. The diameter of these particles is 130 nm. The divergent angle of 2.06° is used for fabricating the divergent mini-channels. The influences of various parameters such as the Reynolds number, the heat flux, and the concentration of nano-PCM particles on the thermal resistances, the figure of merit (FOM), the friction factor, coefficient of performance (COP), and the heat transfer effectiveness are investigated. The findings of this study indicate that the mean and maximum thermal resistances decrease about 0.5% and 4.92%, respectively by increasing the input heat flux from 40.38 W to 60.08 W at Re = 100 for the case of base fluid. The maximum FOM value is equal to 1.02 that can be achieved at ω PCM = 2%, Re = 305, and q h ″ = 4.79 W / c m 2. Using the nano-PCM emulsion with a large concentration increases considerably the pressure drop penalty and provides a poor FOM index. The mean heat transfer effectiveness is higher than unity for smaller values of the Reynolds number. However, the mean heat transfer effectiveness diminishes for larger values of the Reynolds number. [ABSTRACT FROM AUTHOR]

The minichannel heatsink is one of the small heat exchangers, having high heat capacity and being able to remove substantial heat within a very small volume, in line with the current demand for heat dissipation of electronic components. In the present study, the convection cooling characteristics in a rectangular minichannel considering wall conduction effect was investigated numerically. Numerical simulations are performed under 2% and 10% volume fractions of PCM particles (n-eicosan). The results reveal that the axial wall conduction plays a more pivotal role in the minichannels at the lower Reynolds number. Besides, the employment of phase change nano-emulsion in the working fluid in the rectangular minichannel suppresses the wall temperature and improves the heat transfer performance up to 4.23% and 9.36%, respectively, compared with the application of pure water. In addition, it is demonstrated that a high concentration of PCM particles may have detrimental effect on pressure drop and extended wall heat dissipation efficiency under certain circumstances thus the concentration of PCM particles needs careful selection. [ABSTRACT FROM AUTHOR]

• Thermal performance of MCHSs with parallel and divergent minichannel heat sinks is studied. • The divergent mini-channel increases the wall temperature as compared with the parallel one. • The heat transfer is improved by adding the PCM nanoparticles to the base fluid. In this work, an experimental study is arranged to investigate the cooling efficacies of water-based nano-PCM emulsion flow in the multi-channel heat sinks with parallel and divergent rectangular mini-channels. N-eicosane particles with size of 130 nm are considered as the phase change material (PCM) nanoparticles. Two multi-channel heat sinks with eight parallel and divergent mini-channels are fabricated. The divergent channel has a divergent angle of 2.06°. The effects of different parameters including volumetric flow rate of working fluid (60 cm3/min < Q ̇ < 600 cm3/min), heat flux (3.2 W/cm2 < q h ′ ′ < 4.8 W/cm2), Reynolds number (100 < Re < 1000), and mass fraction of PCM nanoparticles (0% < ω PCM < 10%) on the dimensionless wall temperature, the Nusselt number, the cost of performance (COP), and the pressure drop are investigated. The experimental results show that the nano-PCM emulsion can improve heat transfer in both parallel and divergent mini-channel heat sinks as compared with the pure water. At Re bf = 965 and q h ′ ′ = 3.21 W/cm2, the average Nusselt number in the parallel mini-channel heat sink improves about 15.2% by adding the PCM nanoparticles with mass fraction of 10% to the base fluid. This enhancement is up to 13.8% in the divergent mini-channel heat sink at Re bf = 295 and q h ′ ′ = 3.21 W/cm2. Moreover, the divergent mini-channel heat sinks provide a higher Nusselt number along with lower pressure drop as compared with the parallel ones. Accordingly, the value of COP increases by diverging the mini-channel. [ABSTRACT FROM AUTHOR]

In this study, a numerical simulation of natural convection between two horizontal differentially heated pipelines inside a circular air-filled enclosure is performed using the finite difference method. The relevant parameters of the problem are the inclinations of the two cylinders (positioned vertically in this study, with the cold cylinder above the hot cylinder), the distance between cylinders and the Rayleigh number. The results show that transient irregular fluctuations in the flow field and heat transfer occur when the Rayleigh number increases or the distance between cylinders decreases. Under the current test conditions, increasing the Rayleigh number significantly increases the average heat transfer coefficient between the cold and hot cylinders. [ABSTRACT FROM AUTHOR]

Highlights: [•] We integrated architectural design, a heat flow mechanism, and hot water generation with a wall. [•] We proposed an innovative curtain-wall-integrated solar heater. [•] The heat transfer performance was investigated experimentally. [•] The system-wide energy harvest ratio of the test cell is between 0.45 and 0.78. [ABSTRACT FROM AUTHOR]