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An experimental study is conducted to investigate the effects of inclination angle on the natural convection of nanofluids inside a cubic cavity with the side size of 10cm. One of the surfaces of the cavity is kept in cold temperature and another one (opposite side) in hot temperature while the other four surfaces are insulated. The mixtures of three different types of nanoparticles including Al2O3, TiO2, and CuO within turbine oil (TO) are used as the heat transfer fluid. The heat transfer in the cavity is investigated in three inclination angles with respect to the horizontal position including 0°, 45° and 90° where the weight fractions of nanoparticles are 0.2%, 0.5%, and 0.8%. The Nusselt number results are presented for the three types of nanofluids, and different angles of inclination, Rayleigh number, and weight fraction of nanoparticles. The results reveal that the turbine oil has the highest Nusselt number in any inclination angle of the cavity compared to the nanofluids. Also, it is found that at the inclination angle of 90°, and the weight fraction 0.2%, the application of TiO2 particles results in the maximum Nusselt number while for weight fraction of 0.8%, the maximum Nusselt number is associated with the CuO nanopowders. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an experimental study is performed to assess the heat transfer characteristics and pressure drop of low concentrations of a new class of nanotubes, i.e. COOH-functionalized double-walled carbon nanotubes (DWCNTs) suspended in water under turbulent flow in a double tube heat exchanger. First, the thermal conductivity and viscosity of nanofluids at volume fractions of 0.01%, 0.02%, 0.05%, 0.1%, 0.2%, and 0.4% are measured and corresponding correlations are presented. Next, the heat transfer and pressure drop of the nanofluids through the double tube heat exchanger are evaluated. The results indicate that even the use of low concentration of the nanofluid, i.e. 0.4%, leads to a remarkable increase in heat transfer coefficient (by 32%) in comparison with the distilled water. On the other hand, the pressure drop due to using the volume fraction of 0.4% raised by 20%. Finally, analysis of the heat transfer and pressure drop data via thermal performance factor reveals that in spite of the pressure drop penalty, the COOH-functionalized DWCNTs/water nanofluid with volume fraction of 0.4% is a good option to use in the double tube heat exchanger. [Copyright &y& Elsevier]

Abstract: In this paper, an analytical study of the second law of thermodynamics is performed for the flow and heat transfer of TiO2/water nanofluid in a vertical annulus with isoflux walls and under the influence of magnetohydrodynamic (MHD) field. The governing equations in cylindrical coordinates are written and then simplified with reasonable assumptions. The simplified equations are solved analytically to obtain expressions for the velocity, temperature, and entropy generation distributions. The results are presented for different values of nanofluid volume fractions , Hartmann number M and the flow parameter Gr/Re. To calculate the thermal conductivity and viscosity of TiO2/water nanofluid, relations based on experimental data are used. Finally, using the well-known theoretical models for the thermal conductivity and viscosity, the effects of Hartmann number on the results are investigated. [Copyright &y& Elsevier]

Abstract: In this study, the condensation heat transfer characteristics of R-134a flowing inside multiport minichannels were investigated. The multiport minichannel-tested tubes have 14 channels with a 1.1mm hydraulic diameter, and eight channels with a 1.2mm hydraulic diameter were designed as a counterflow tube-in-tube heat exchanger. The experiment was performed with mass fluxes of refrigerant between 340 and 680kg/m2s, with 15, 20, and 25kW/m2 heat fluxes, and saturation temperatures of 35–45°C. The flow pattern for the experimental data was initially predicted using existing flow pattern maps. It could be noted that the annular flow pattern existed for most of the experimental data. Results showed that the average heat transfer coefficient increased with the increase of vapor quality, mass flux, and heat flux, but decreased as saturation temperature rose. When compared with two correlations obtained from condensation inside the multiport minichannels, the heat transfer coefficient could be predicted within an acceptable range. [Copyright &y& Elsevier]

Abstract: The purpose of this experimental investigation is to investigate the effects of fin pitch (i.e., fp of 2.4, 3.2, and 4.2mm) on the air-side heat transfer performance and frictional characteristics of L-footed spiral fin-and-tube heat exchangers at high Reynolds numbers (Redc ) of 4000–15,000. A determinant of the parallel cross-flow and the counter cross-flow is the flow arrangement of the test heat exchangers. Ambient air and hot water are used as a working fluid on the air- and the tube-side, respectively. The results indicate that the air-side heat transfer coefficient and Colburn factor are independent of fin pitch. However, fin pitch does have an influential effect on the average heat transfer rate, pressure drop, and friction factor. In terms of industrial applications, the correlation of the Colburn factor and friction factor are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: The momentum and forced convection heat transfer for a laminar and steady free stream flow of nanofluids past an isolated square cylinder have been studied numerically. Different nanofluids consisting of Al2O3 and CuO with base fluids of water and a 60:40 (by mass) ethylene glycol and water mixture were selected to evaluate their superiority over conventional fluids. Recent correlations for the thermal conductivity and viscosity of nanofluids, which are functions of particle volumetric concentration as well as temperature, have been employed in this paper. The simulations have been conducted for Pe =25, 50, 100 and 200, with nanoparticle diameters of 30 and 100nm and particle volumetric concentrations ranging from 0% to 4%. The results of heat transfer characteristics of nanofluid flow over a square cylinder showed marked improvement comparing with the base fluids. This improvement is more evident in flows with higher Peclet numbers and higher particle volume concentration, while the particle diameter imposes an adverse effect on the heat transfer characteristics. In addition, it was shown that for any given particle diameter there is an optimum value of particle concentration that results in the highest heat transfer coefficient. [Copyright &y& Elsevier]

Abstract: The air-side performance of crimped spiral fin and tube heat exchangers at high Reynolds number (3000–13,000) is investigated in this study. The test heat exchangers have a new type of multipass parallel and counter cross-flow water flow arrangement which is a combination of parallel cross-flow and counter cross-flow. The test samples are made from copper and aluminium with different number of tube rows (Nrow =2, 3, 4 and 5). The effects of number of tube rows and fin material on the heat transfer and friction characteristics are studied. The results show that no significant effect for either number of tube rows or fin materials on the heat transfer performance is found at high Reynolds number. In addition, the correlation of the air-side performances of this type of the heat exchangers at high Reynolds number is developed for industrial applications. [Copyright &y& Elsevier]

This study proposes a new method, namely the "fully wet and fully dry tiny circular fin method," for analyzing the heat and mass transfer characteristics of plain fin-and-tube heat exchangers under dehumidifying conditions. The present method is developed from the tube-by-tube method proposed in the previous study by the same authors. The analysis of the fin-and-tube heat exchangers is carried out by dividing the heat exchanger into many tiny segments. A tiny segment will be assumed with fully wet or fully dry conditions. This method is capable of handling the plain fin-and-tube heat exchanger under fully wet and partially wet conditions. The heat and mass transfer characteristics are presented in dimensionless terms. The ratio of the heat transfer characteristic to mass transfer characteristic is also studied. Based on the reduced results, it is found that the heat transfer and mass transfer characteristics are insensitive to changes in fin spacing. The influence of the inlet relative humidity on the heat transfer characteristic is rather small. For one and two row configurations, a considerable increase of the mass transfer characteristic is encountered when partially wet conditions take place. The heat transfer characteristic is about the same in fully wet and partially wet conditions provided that the number of tube rows is equal to or greater than four. Correlations are proposed to describe the heat and mass characteristics for the present plain fin configuration. [ABSTRACT FROM AUTHOR]

Abstract: This article presents a numerical investigation on heat transfer performance and pressure drop of nanofluids flows through a straight circular pipe in a laminar flow regime and constant heat flux boundary condition. Al2O3, CuO, carbon nanotube (CNT) and titanate nanotube (TNT) nanoparticles dispersed in water and ethylene glycol/water with particle concentrations ranging between 0 and 6vol.% were used as working fluids for simulating the heat transfer and flow behaviours of nanofluids. The proposed model has been validated with the available experimental data and correlations. The effects of particle concentrations, particle diameter, particles Brownian motions, Reynolds number, type of the nanoparticles and base fluid on the heat transfer coefficient and pressure drop of nanofluids were determined and discussed in details. The results indicated that the particle volume concentration, Brownian motion and aspect ratio of nanoparticles similar to flow Reynolds number increase the heat transfer coefficient, while the nanoparticle diameter has an opposite effect on the heat transfer coefficient. Finally, the present study provides some considerations for the appropriate choice of the nanofluids for practical applications. [Copyright &y& Elsevier]

Abstract: This study investigates the effect of fin pitches and fin materials on the air-side performance of crimped fin-and-tube heat exchangers in the range of high Reynolds numbers (4000–13000). The test samples are made from copper and aluminium with different fin pitches (f p =3.2,4.2 and 6.2mm). It is found that the proposed simple average effectiveness equation from the pure counter and parallel circuitry arrangement can well represent the effectiveness-NTU relationship for the current z-shape arrangement. The experimental results reveal that the fin pitch casts insignificant effect on the heat transfer characteristics (Colburn j factor). However, a detectable rise of the friction factor is seen when the fin pitch is increased to f p =6.2mm. On the other hand, the effect of fin material on the airside performance is negligible. [Copyright &y& Elsevier]

• The crimped spiral fin for small diameter tube is experimentally investigated. • Effects of fin pitch and fin outside diameter on air-side performance are studied. • The fin pitch and fin outside diameter are important key of friction characteristic. • New correlations of the Colburn factor and friction factor are proposed. The performance of crimped spiral fin-and-tube heat exchangers (CSFTHXs) with a small tube diameter at a Reynolds number of 1500-6400 based on the tube's outer diameter is experimentally investigated. An important factor in this study is the small-diameter tubing, with an outer diameter of 9.53 mm. Copper tubes and aluminum fins are assembled as CSFTHXs. Test sections consist of a fin thickness of 0.5 mm, two tube rows, and various fin pitches and outer diameters of 3.18-6.35 mm and 23.0-28.5 mm, respectively. Hot water and fresh air are used as the working fluids. The effects of fin pitch and fin outer diameter on the heat transfer characteristics and friction factor (f) are also studied. The results show that the fin's pitch and outer diameter have little effect on the Nusselt number (Nu) and Colburn factor (j). On the other hand, they have a significant effect on the f. Furthermore, correlations between the Nu, j , and f for a CSFTHX with a small tube diameter are developed and proposed for designing heat exchangers in the thermal industry. [ABSTRACT FROM AUTHOR]

The enhancement of heat transfer between parallel surfaces, including parallel plates, parallel disks, and two concentric pipes, is vital because of their wide applications ranging from lubrication systems to water purification processes. Various techniques can be utilized to enhance heat transfer in such systems. Adding nanoparticles to the conventional working fluids is an effective solution that could remarkably enhance the heat transfer rate. No published review article focuses on the recent advances in nanofluid flow between parallel surfaces; therefore, the present paper aims to review the latest experimental and numerical studies on the flow and heat transfer of nanofluids (mixtures of nanoparticles and conventional working fluids) in such configurations. For the performance analysis of thermal systems composed of parallel surfaces and operating with nanofluids, it is necessary to know the physical phenomena and parameters that influence the flow and heat transfer characteristics in these systems. Significant results obtained from this review indicate that, in most cases, the heat transfer rate between parallel surfaces is enhanced with an increase in the Rayleigh number, the Reynolds number, the magnetic number, and Brownian motion. On the other hand, an increase in thermophoresis parameter, as well as flow parameters, including the Eckert number, buoyancy ratio, Hartmann number, and Lewis number, leads to heat transfer rate reduction. [Display omitted] • Nanofluid flow and heat transfer between parallel surfaces are studied. • Parallel plates, parallel disks, and concentric pipes are considered. • Physical phenomena and influential parameters are evaluated. [ABSTRACT FROM AUTHOR]

Due to their superior thermophysical properties, there is a growing body of work on nanofluids in the field of thermal systems. However, there is no specific review of the role of the nanoparticle shape, which has been found crucial to their performance adjustment. A comprehensive literature review of the effect of nanoparticle shape on the hydrothermal performance of thermal systems utilizing nanofluids was compiled. The review covered the forced, mixed, and natural convection regimes and included heat exchangers, boundary layer flows, channel flows, peristaltic flows, impinging jets, cavity flows, and flows of hybrid nanofluids. It indicated that the control of nanoparticle shape is a promising technique for the optimization of heat exchange and the required pumping power. However, no uniform conclusion was reached for the role of nanoparticle shape on the hydrothermal performance of thermal systems. In most of the previous studies in the natural and forced convection regimes, the platelet–like nanoparticle acquired the highest heat transfer rate. However, most of the works in the mixed convection regime reported the best heat transfer performance for the blade–like nanoparticle. More research studies are required in future to determine the role of nanoparticle shape for thermal management of energy systems. • Nanoparticle shape is crucial to the performance control of various nanofluids. • The effects of nanoparticle shape on thermal system performance were reviewed. • Nanoparticle shape control is a promising technique for heat transfer optimization. • More research is required for the role of nanoparticle shape in thermal systems. [ABSTRACT FROM AUTHOR]

Heat transfer and pressure drop of turbulent fluid flow were studied numerically and experimentally in a quad-channel with twisted tapes placed in a circular tube under uniform heat flux boundary condition. All the tests were conducted with water as the working fluid where the turbulent flow range of Reynolds numbers were maintained at 4250 to 11000. Quad-channel twisted tapes (QCTT) were manufactured by 3D printer with the material PLA, the lengths of 50 mm, 100 mm, 200 mm with the twist ratio of 4.48. The tapes were identified with their tape/pipe length ratios (TPR) of 0.025, 0.05 and 0.1. Validation of the experimental data was performed by the comparison of the experimental results with the data from the well-known empirical correlations. Alteration of the friction factors and Nusselt numbers with the Reynolds numbers are presented in this paper. The results showed that the Nusselt number and the friction factor increased with the increasing of the length of QCTTs. On the other hand, the performance evaluation criteria (PEC) should be declining with the increase of the lengths of the tapes. The evaluated temperature distributions inside the tested tubes with the inserts were plotted in order to investigate the observed data. Based on the obtained experimental data, the empirical correlations for the Nusselt number within the deviation of ±2% and the friction factor within the deviation of ±5% were developed for the practical usage. [ABSTRACT FROM AUTHOR]

• Heat transfer enhancement inside the plate heat exchanger using a rough surface is studied. • A commercial plate heat exchanger with three different surface roughness is tested. • The surface roughness has a strong effect on the two-phase flow mechanism. • The optimum conditions of the plate heat exchangers with rough surfaces are evaluated. New experimental data were presented on the effect of heat exchanger plate surface roughness, mass flux, average quality, heat flux, and temperature on the evaporative heat transfer coefficient (HTC) and frictional pressure gradient (FPG) of R-134 flowing in a plate heat exchanger (PHE). Three different plate surface roughness of 0.594 μm, 1.816 μm, and 2.754 μm, were used. A commercial herringbone PHE was tested under the counterflow arrangement. The experiments were done at the heat fluxes ranging between 5 and 15 kW/m2, the mass fluxes were between 67 and 96 kg/m2s, and the saturation temperatures ranging from 10 to 20 °C. The results showed that the HTC and FRG increased with increasing mass flux, heat flux, average quality and surface roughness. Conversely, they were slightly decreased with an increase in saturation temperature. The optimum thermal performance of a PHE appeared at high plate surface roughness and average quality by about 0.2. The correlations for predicting the Nusselt number and friction factor were proposed for practical applications. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer coefficients and critical heat fluxes of the R134a fluid in two-phase flow was studied in a 27-channel, 421 μm hydraulic diameter and 40 mm long microchannel experimental set-up. Data were collected at constant mass flux of 1000 kg m−2 s−1, saturation inlet temperature range of 20–28 °C and at inlet vapor quality range of 0.01–0.20. The heat flux was initiated at 250 kW m−2 and gradually increased until reaching the critical heat flux (CHF). The effect of inlet vapor quality, saturation inlet temperature and heat flux on heat transfer coefficient and critical heat flux were investigated in detail and presented graphically. It can be observed that the CHF decreases with the increasing of saturation inlet temperature and inlet vapor quality. However, the heat transfer coefficient at the CHF point increased with the increasing of saturation temperature and the heat transfer coefficient at the CHF point was decreased with the increasing of vapor quality. The test results from the correlations, suggested in well-known articles, were compared with the experimental data, where the Kosaz et al. proposed correlation was showing well fit with the experimental data having absolute error of 9.86%. In this investigation, the new correlations for the heat transfer coefficient for the heat transfer coefficient and the critical heat flux have been proposed by taking into account the effect of the vapor quality. [ABSTRACT FROM AUTHOR]

• The effects of nanomaterial addition to the refrigerants/lubricants. • Nanorefrigerants and nanolubricants in the refrigeration and HVAC systems. • On the basis of heat transfer enhancement and thermophysical properties behavior. • Considering various influential applicable parameters of nanomaterials. Over the last century, nanomaterial-based cooling/heating working fluids have been considered one of the most potent alternatives to the conventional heat transfer medium. Likewise, as one prevalent series of the applications and by utilization of much more potent eco-friendly fluids, the constant everyday attempt in the area of refrigeration, HVAC, and other associated systems has involved the usage of nanorefrigerants/nanolubricants. Indeed, the addition of high thermal conductive nanoparticles to the low thermal conductive traditional base fluids is one of the most attention-grabbing areas of research. In this current scientific review article, by considering boiling, condensation, or the related phenomena, the heat transfer rate and the thermophysical properties have been targeted to investigate the substantial effect of nanoparticle addition to common refrigerants or lubricants. At the end, the relevant research gaps, namely the heat transfer rate at an atomic level, the inconsistent behavior of nanofluids, the phase change enthalpy level, and other possible fields, were detected as a helpful benchmark for future studies. [ABSTRACT FROM AUTHOR]

• Heat transfer and pressure drop of R-134a in dimpled tube are investigated. • The effects of mass flux, heat flux, and saturation temperature are examined. • The dimpled tube enhances the heat transfer coefficient up to 1.7 times. • The efficiency index ranges from 0.18 to 0.42. The heat transfer and pressure drop characteristics of R-134a during evaporation in smooth and dimpled tubes are experimentally examined. All test sections used in this study are double-tube heat exchangers, where the hot water in an annulus transfers its heat to R-134a in the inner tube. The experiments are conducted in a mass flux range of 300–500 kg/m2 s, heat flux range of 20–30 kW/m2, and evaporating temperature range of 7–13 °C. Smooth and dimpled tubes with a length of 1500 mm and an inner diameter of 8.1 mm are tested. The results indicate that the tube with protrusions enhance the heat transfer up to 70% along with a pressure drop increase up to 587% over the smooth tube. The effects of relevant parameters on heat transfer and pressure drop are explored. In this experiment, the heat transfer enhancement factor falls in the range of 1.26–1.7, while the pressure drop penalty factor fluctuates between 3.78 and 6.87. In addition, the efficiency index is applied to estimate the performance of dimpled tube. It is found that all efficiency index values are in the range of 0.18–0.42. [ABSTRACT FROM AUTHOR]

The role of surface modification technology in augmenting condensation heat transfer from surfaces had been recently of significant interest due its application in various industrial systems. In this work, steam condensation test was conducted to estimate the heat transfer characteristics of bare vertical copper tube, helically grooved vertical copper tube with and without coating. The pitch helical grooves were varied by 3.17 mm, 4.35 mm, 6.35 mm each with depth 0.4 mm and the bare, grooved copper tube coated with chlorotriethylsilane of thickness 150 μm. Experiments were conducted by varying the degree of sub-cooling from 3.8 °C ≤ ∆ T ≤ 28.5 °C. Results showed that helically grooved copper tube outperformed bare copper tube for the range of sub-cooling. Among the grooved copper tubes, tube with pitch of 6.35 mm showed an enhancement in condensation heat transfer coefficient up to 37.1% compared to the bare copper tube at a degree of sub-cooling ∆ T = 28 °C. Furthermore, coating the grooved copper tube of pitch 6.35 mm chlorotriethylsilane reported a heat transfer improvement of 67.7% at a degree of sub-cooling ∆ T = 28 °C compared to bare copper. However at low degree of sub-cooling (∆ T ≤ 17 °C) even though similar trend was observed it cannot be quantified due to the large experimental uncertainties. [ABSTRACT FROM AUTHOR]

• Latest developments on sonication characteristics on nanofluid properties reviewed. • In general, thermal conductivity increases with sonication time. • Viscosity can be reduced with the increase of sonication. • Finding optimal sonication time and power is still challenging. The most crucial step towards conducting experimental studies on thermophysical properties and heat transfer of nanofluids is, undoubtedly, the preparation step. It is known that good dispersion of nanoparticles into the base fluids leads to having long-time stable nanofluids, which result in having higher thermal conductivity enhancement and lower viscosity increase. Ultrasonic treatment is one of the most effective techniques to break down the large clusters of nanoparticles into the smaller clusters or even individual nanoparticles. The present review aims to summarize the recently published literature on the effects of various ultrasonication parameters on stability and thermal properties of various nanofluids. The most common methods to characterize the dispersion quality and stability of the nanofluids have been presented and discussed. It is found that increasing the ultrasonication time and power results in having more dispersed and stable nanofluids. Moreover, increasing the ultrasonication time and power leads to having higher thermal conductivity and heat transfer enhancement, lower viscosity increase, and lower pressure drop. However, there are some exceptional cases in which increasing the ultrasonication time and power deteriorated the stability and thermophysical properties of some nanofluids. It is also found that employing the ultrasonic horn/probe devices are much more effective than ultrasonic bath devices; lower ultrasonication time and power leads to better results. [ABSTRACT FROM AUTHOR]