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Abstract: Nanofluids are a suspension of particles with ultrafine size in a conventional base fluid that increases the heat transfer performance of the original base fluid. They show higher thermal performance than base fluids especially in terms of the thermal conductivity and heat transfer coefficient. During the last decade, many studies have been carried out on the heat transfer and flow characteristics of nanofluids, both experimentally and theoretically. The purpose of this article is to propose a dispersion model for predicting the heat transfer coefficient of nanofluids under laminar flow conditions. TiO2 nanoparticles dispersed in water with various volume fractions and flowing in a horizontal straight tube under constant wall heat flux were used. In addition, the predicted values were compared with the experimental data from He et al. . In the present study, the results show that the proposed model can be used to predict the heat transfer behaviour of nanofluids with reasonable accuracy. Moreover, the results also indicate that the predicted values of the heat transfer coefficient obtained from the present model differ from those obtained by using the Li and Xuan equation by about 3.5% at a particle volume fraction of 2.0%. [Copyright &y& Elsevier]

In the present article, the effect of heat source temperature, heat sink temperature, short-tube orifice diameter and short-tube orifice length on the performance characteristics of HFC-140A and HFC-134a refrigeration system using a short-tube orifice as expansion device, i.e., mass flow rate, cooling capacity, compressor pressure ratio, power consumption, and second law efficiency are experimentally studied. The short-tube orifices diameters ranging from 0.849 to 1.085 mm with length ranging from 10 to 20 mm are used in this examination. The test run are done at heat source temperature ranging between 16.5 and 18.5°C, and heat sink temperature ranging between 30 and 35°C. The results show that the tendency of second law efficiency is increased as the short-tube orifice diameter and heat source temperature are enhanced, but it is decreased by increasing the short-tube orifice length and heat sink temperature. Under the similar conditions, the mass flow rate, cooling capacity, and compressor power consumption obtained from HFC-410A are higher than those obtained from HFC-134a. [ABSTRACT FROM AUTHOR]

This article describes experimental investigations of the heat transfer coefficient and pressure drop of R-134a flowing inside internally grooved tubes. The test tubes are one smooth tube and four grooved tubes. All test tubes are made from type 304 stainless steel, have an inner diameter of 7.1 mm, are 2,000 mm long and are installed horizontally. The test section is uniformly heated by a DC power supply to create evaporation conditions. The groove depth of all grooved tubes is fixed at 0.2 mm. The experimental conditions are conducted at saturation temperatures of 20, 25 and 30°C, heat fluxes of 5, 10 and 15 kW/m, and mass fluxes of 300, 500 and 700 kg/m s. The effects of groove pitch, mass flux, heat flux, and saturation temperature on heat transfer coefficient and frictional pressure drop are discussed. The results illustrate that the grooved tubes have a significant effect on the heat transfer coefficient and frictional pressure drop augmentations. [ABSTRACT FROM AUTHOR]

Abstract: This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46mm. Similarly, the corrugation depths are 1, 1.25, and 1.5mm, respectively. The test conditions are performed at saturation temperatures of 40–50°C, heat fluxes of 5–10kW/m2, mass fluxes of 200–700kg/m2 s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes. [Copyright &y& Elsevier]

Abstract: The effects of corrugation pitch on the evaporation of R-134a flowing inside horizontal corrugated tubes are investigated. The test section is a tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water flowing in the annulus. Smooth and corrugated tubes having similar inside diameters are used as the inner tube. Three corrugated tubes having a corrugation depth fixed at 1.5mm and corrugation pitches of 5.08, 6.35 and 8.46mm are examined. The data obtained from smooth tube are plotted and compared with the flow pattern map established by and . The effects of average vapour quality, equivalent Reynolds number and corrugation pitch are discussed. The maximum ratios of heat transfer enhancement factor to pressure drop penalty factor (Nuc/Nus)/((ΔP/L)c/(ΔP/L)s) is approximately 1.2. [ABSTRACT FROM AUTHOR]

Abstract: A study of nucleate pool boiling heat transfer of TiO2–water nanofluids is experimentally conducted. Nanofluids with various concentrations of 0.00005, 0.0001, 0.0005, 0.005, and 0.01vol.% are employed. Horizontal circular plates made from copper and aluminium with different roughness values of 0.2 and 4μm are used as heating surfaces. The experiments are performed to explore the effects of nanofluids concentration as well as heating surface material and roughness on nucleate pool boiling characteristics and the heat transfer coefficient under ambient pressure. The results show that based on the copper heated surface which is tested with a concentration of 0.0001vol.%, higher nucleate pool boiling heat transfer coefficient is obtained when compared with the base fluid. A 15% increase is obtained for the surface roughness of 0.2μm and a 4% increase is obtained for roughness of 4μm. For concentrations higher than 0.0001vol.%, however, the higher the concentration, the lower the heat transfer coefficient. In the case of aluminium heated surface, the corresponding heat transfer coefficients are larger than for the copper surface by around 30% with a roughness of 0.2μm and around 27% with a roughness of 4μm. Moreover, the results also indicate that the heat transfer coefficient obtained based on a roughness of 4μm is higher than that for a roughness of 0.2μm by around 12% for aluminium and by around 13% for copper. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the jet liquid impingement heat transfer characteristics in the mini-rectangular fin heat sink for the central processing unit of a personal computer are experimentally investigated. The experiments are tested with three different channel width heat sinks under real operating conditions: no load and full load conditions. The jet liquid impingement cooling with mini-rectangular fin heat sink system is introduced as the active and passive heat transfer enhancement techniques. Effects of relevant parameters on the central processing unit temperature are considered. It is found that the central processing unit temperatures obtained from the jet liquid impingement cooling system are lower than those from the conventional liquid cooling system; however, the energy consumption also increases. The results of this study are of technological importance for the efficient design of cooling systems of personal computers or electronic devices to enhance cooling performance. [ABSTRACT FROM AUTHOR]

Abstract: New experimental data of HFC-410A flow mechanisms, including pressure and temperature distributions, flow pattern, and mass flow rate inside a short-tube orifice are studied. Different short-tube orifice lengths of 10mm, 15mm, and 20mm, with short-tube orifice diameters ranging from 0.849 to 1.096mm were manufactured for this experiment. The test runs are done at upstream pressures ranging from 2100 to 2600kPa, and with a degree of subcooling between 4 and 12°C. The results show that the refrigerant flow mechanisms in the entrance region and inside a short-tube orifice agree with incompressible flow theory, but the refrigerant flow mechanisms in the exit region are similar to choked flow in a theoretical compressible flow. The temperature distribution inside a short-tube orifice is directly related to the pressure distribution. The mass flow rate is directly proportional to the upstream pressure and degree of subcooling, whereas it is strongly affected by the short-tube diameter. The downstream pressure has insignificant effects on the mass flow rate and pressure distribution inside a short-tube orifice. However, the downstream pressure has a slight effect on the temperature distribution inside a short-tube orifice. [Copyright &y& Elsevier]

Abstract: This article reports a comparison of the differences between using measured and computed thermophysical properties to describe the heat transfer performance of TiO2–water nanofluids. In this study, TiO2 nanoparticles with average diameters of 21nm and a particle volume fraction of 0.2–1vol.% are used. The thermal conductivity and viscosity of nanofluids were measured by using transient hot-wire apparatus and a Bohlin rotational rheometer, respectively. The well-known correlations for calculating the thermal conductivity and viscosity of nanofluids were used for describing the Nusselt number of nanofluids and compared with the results from the measured data. The results show that use of the models of thermophysical properties for calculating the Nusselt number of nanofluids gave similar results to use of the measured data. Where there is a lack of measured data on thermophysical properties, the most appropriate models for computing the thermal conductivity and viscosity of the nanofluids are the models of Yu and Choi and Wang et al., respectively. [Copyright &y& Elsevier]

Abstract: The heat transfer coefficient and pressure drop of R-134a inside a horizontal smooth tube and corrugated tubes are experimentally investigated. The test section is a 2.0m long counter-flow concentric double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. A smooth tube and corrugated tubes having inner diameters of 8.7mm are used as an inner tube. The corrugation pitches are 5.08, 6.35, and 8.46mm, respectively. The corrugation depth of all corrugated tubes is fixed at 1.5mm. The outer tube is made from smooth copper tube having an inner diameter of 21.2mm. The test runs are performed at the saturation temperatures of 40, 45, and 50°C, heat fluxes of 5 and 10kW/m2, and mass fluxes ranging from 200 to 700kg/m2 s. The results obtained from the corrugated tubes are compared with that of the smooth tube. It is found that the corrugation pitches have a significant effect on the heat transfer coefficient and pressure drop augmentations. [Copyright &y& Elsevier]

Abstract: This study presents the airside performance of the fin-and-tube heat exchangers having plain fin geometry with a larger diameter tube (Dc =15.88mm) under dehumidifying condition. A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch are made and tested. It is found that the effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row. However, there is a unique characteristic of fin pitch at a shallow tube row, the heat transfer performance is first increased at a wider pitch but a further increase of fin pitch lead to a falloff of heat transfer performance due to interactions amid flow development and bypass flow. The influence of tube row on the airside performance is rather small for both heat transfer and frictional characteristics at a fin pitch of 2.1mm and when the Reynolds number is less than 4000. A slight deviation of this effect is encountered when fin pitch is increased to 2.54mm or 3.1mm due to condensate adhered phenomena. [Copyright &y& Elsevier]

This article concerns the pressure drop caused by using the electrohydrodynamic (EHD) technique during evaporation of pure R-134a inside smooth and micro-fin tubes. The test section is a counter-flow concentric tube-in-tube heat exchanger where R-134a flows inside the inner tube and hot water flows in the annulus. A smooth tube and micro-fin tube having an inner diameter of 8.12 mm and 8.92 mm, respectively, are used as an inner tube. The length of the inner tube is 2.50 m. The outer tube is a smooth copper tube having an inner diameter of 21.2 mm. The electrode, which is a cylindrical stainless steel wire having diameter of 1.47 mm, is placed in the center of the inner tube. The electrical field is established by connecting a DC high voltage power supply of 2.5 kV to the electrode while the inner tube is grounded. Experiments are conducted at saturation temperatures of 10-20°C, mass fluxes of 200-600 kg/m2s, and heat fluxes of 10-20 kW/m2. The experimental results indicate that the application of EHD introduces a small pressure drop penalty. New correlations for the pressure drop are proposed for practical applications. [ABSTRACT FROM AUTHOR]

Abstract: Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids. This article presents the heat transfer coefficient and friction factor of the TiO2-water nanofluids flowing in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions, experimentally. TiO2 nanoparticles with diameters of 21nm dispersed in water with volume concentrations of 0.2–2vol.% are used as the test fluid. The results show that the heat transfer coefficient of nanofluid is higher than that of the base liquid and increased with increasing the Reynolds number and particle concentrations. The heat transfer coefficient of nanofluids was approximately 26% greater than that of pure vol.%, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 2.0vol.% was approximately 14% lower than that of base fluids for given conditions. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increases with increasing the volume concentrations. Finally, the new correlations were proposed for predicting the Nusselt number and friction factor of the nanofluids, especially. [Copyright &y& Elsevier]

Abstract: Nanofluid is an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of conventional fluids. Many attempts have been made to investigate its thermal conductivity and viscosity, which are important thermophysical properties. No definitive agreements have emerged, however, about these properties. This article reports the thermal conductivity and dynamic viscosity of nanofluids experimentally. TiO2 nanoparticles dispersed in water with volume concentration of 0.2–2vol.% are used in the present study. A transient hot-wire apparatus is used for measuring the thermal conductivity of nanofluids whereas the Bohlin rotational rheometer (Malvern Instrument) is used to measure the viscosity of nanofluids. The data are collected for temperatures ranging from 15°C to 35°C. The results show that the measured viscosity and thermal conductivity of nanofluids increased as the particle concentrations increased and are higher than the values of the base liquids. Furthermore, thermal conductivity of nanofluids increased with increasing nanofluid temperatures and, conversely, the viscosity of nanofluids decreased with increasing temperature of nanofluids. Moreover, the measured thermal conductivity and viscosity of nanofluids are quite different from the predicted values from the existing correlations and the data reported by other researchers. Finally, new thermophysical correlations are proposed for predicting the thermal conductivity and viscosity of nanofluids. [Copyright &y& Elsevier]

Abstract: This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2vol.% TiO2 nanoparticles. The heat transfer coefficient and friction factor of the TiO2–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO2 nanoparticles of about 21nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop. [Copyright &y& Elsevier]

Abstract: Nucleate pool boiling heat transfer of a refrigerant-based-nanofluid was investigated at different nanoparticle concentrations and pressures. TiO2 nanoparticles were mixed with the refrigerant HCFC 141b at 0.01, 0.03 and 0.05vol%. The experiment was performed using a cylindrical copper tube as a boiling surface. Pool boiling experiments of nanofluid were conducted and compared with that of the base refrigerant. The results indicate that the nucleate pool boiling heat transfer deteriorated with increasing particle concentrations, especially at high heat fluxes. At 0.05vol%, the boiling heat transfer curves were suppressed. At high pressures of 400 and 500kPa, the boiling heat transfer coefficient at a specific excess temperature was almost the same. [Copyright &y& Elsevier]

Abstract: The evaporation heat transfer coefficient and pressure drop of R-410A flowing through a horizontal aluminium rectangular multiport mini-channel having 3.48mm hydraulic diameter are experimentally investigated. The test runs are performed at mass flux ranging between 200 and 400kg/m2 s. The heat fluxes are between 5 and 14.25kW/m2 and the saturation temperatures range between 10 and 30°C. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of the imposed wall heat flux, mass flux, vapour quality, and saturation temperature on the evaporation heat transfer and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the existing correlation. New correlations for the evaporation heat transfer coefficient and pressure drop of R-410A flowing through a multiport mini-channel are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail. [Copyright &y& Elsevier]

Abstract: This paper presents a mathematical model for determining optimal configuration of plate finned tube condensers widely used in air conditioning system. The condenser investigated in this work is a cross-flow air-refrigerant type with aluminium plain fin geometry. Refrigerant HFC 134a flows inside while air flows outside the copper tubes. Minimum entropy change of air and refrigerant is used as the criterion for analysis. This entropy change resulting from losses due to heat transfer and pressure drop can be represented in a form of entropy generation number, . The proposed mathematical model is divided into three regions: superheated, two-phase and sub-cooled regions. Effects of heat transfer and pressure drop of air side and refrigerant side on entropy generation rate are simultaneously analyzed through the implicit method. Results from the mathematical model show that entropy generation number decreases with increasing fin pitch and decreasing number of rows and tube diameter. In addition, this work also presents an optimal configuration of condenser. The optimal tube diameter and the number of fin per inch are 6.35–12.7 mm ( inch) and 11–12, respectively. [Copyright &y& Elsevier]

Abstract: A nanofluid is a suspension of ultrafine particles in a conventional base fluid which tremendously enhances the heat transfer characteristics of the original fluid. Furthermore, nanofluids are expected to be ideally suited in practical applications as their use incurs little or no penalty in pressure drop because the nanoparticles are ultrafine, therefore, appearing to behave more like a single-phase fluid than a solid–liquid mixture. About a decade ago, several published articles focused on measuring and determining the effective thermal conductivity of nanofluids, some also evaluated the effective viscosity. There are only a few published articles on deriving the forced convective heat transfer of nanofluids. The purpose of this article is to summarize the published subjects respect to the forced convective heat transfer of the nanofluids both of experimental and numerical investigation. [Copyright &y& Elsevier]

Abstract: This paper is a part in a series that reports on the experimental study of the performance of the two-phase ejector expansion refrigeration cycle. In the present study, three two-phase ejectors are used as an expansion device in the refrigeration cycle. The effects of throat diameter of the motive nozzle, on the coefficient of performance, primary mass flow rate of the refrigerant, secondary mass flow rate of the refrigerant, recirculation ratio, average evaporator pressure, compressor pressure ratio, discharge temperature and cooling capacity, which have never before appeared in open literature, are presented. The effects of the heat sink and heat source temperatures on the system performance are also discussed. [Copyright &y& Elsevier]

Abstract: Researches in heat transfer have been carried out over the previous several decades, leading to the development of the currently used heat transfer enhancement techniques. The use of additives is a technique applied to enhance the heat transfer performance of base fluids. Recently, as an innovative material, nanometer-sized particles have been used in suspension in conventional heat transfer fluids. The fluids with these solid-particle suspended in them are called ‘nanofluids’. The suspended metallic or nonmetallic nanoparticles change the transport properties and heat transfer characteristics of the base fluid. The aim of this review is to summarize recent developments in research on the heat transfer characteristics of nanofluids for the purpose of suggesting some possible reasons why the suspended nanoparticles can enhance the heat transfer of conventional fluids and to provide a guide line or perspective for future research. [Copyright &y& Elsevier]

Abstract: The two-phase heat transfer coefficient and pressure drop of pure HFC-134a condensing inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786m long helically coiled double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth copper tubing of 9.52mm outer diameter and 8.3mm inner diameter. The outer tube is made from smooth copper tubing of 23.2mm outer diameter and 21.2mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305mm. The pitch of coil is 35mm. The test runs are done at average saturation condensing temperatures ranging between 40 and 50°C. The mass fluxes are between 400 and 800kgm−2 s−1 and the heat fluxes are between 5 and 10kWm−2. The pressure drop across the test section is directly measured by a differential pressure transducer. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy rejected from the test section. The effects of heat flux, mass flux and, condensation temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 33–53% and 29–46%, respectively. New correlations for the condensation heat transfer coefficient and pressure drop are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: The performance of heat exchangers can be improved to perform a certain heat-transfer duty by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration, etc. The passive techniques require fluid additives or special surface geometries. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are the most widely used tubes in several heat transfer applications. This article provides a literature review on heat transfer and flow characteristics of single-phase and two-phase flow in curved tubes. Three main categories of curved tubes; helically coiled tubes, spirally coiled tubes, and other coiled tubes, are described. A review of published relevant correlations of single-phase heat transfer coefficients and single-phase, two-phase friction factors are presented. [Copyright &y& Elsevier]

Abstract: Effects of electrohydrodynamic (EHD) on the two-phase heat transfer enhancement and pressure drop of pure R-134a condensing inside a horizontal micro-fin tube are experimentally investigated. The test section is a 2.5m long counter flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from micro-fin horizontal copper tubing of 9.52mm outer diameter. The electrode is made from cylindrical stainless steel of 1.47mm diameter. Positive high voltage is supplied to the electrode wire, with the micro-fin tube grounded. In the presence of the electrode, a maximum heat transfer enhancement of 1.15 is obtained at a heat flux of 10kW/m2, mass flux of 200kg/m2 s and saturation temperature of 40°C, while the application of an EHD voltage of 2.5kV only slightly increases the pressure drop. New correlations of the experimental data based on the data gathered during this work for predicting the condensation heat transfer coefficients are proposed for practical application. [Copyright &y& Elsevier]

Abstract: The two-phase heat transfer coefficient and pressure drop of HFC-134a during evaporation inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786-m long helically coiled tube with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from copper tubing of 9.52mm outer diameter and 7.2mm inner diameter. The heat exchanger is fabricated by bending a straight copper tube into a spiral coil. The diameter of coil is 305mm. The test run are done at average saturated evaporating temperatures ranging between 10 and 20°C. The mass fluxes are between 400 and 800kgm−2 s−1 and the heat fluxes are between 5 and 10kWm−2. The inlet quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of heat flux, mass flux and, evaporation temperature on the heat transfer coefficients and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the straight tube reported in the literature. New correlations for the convection heat transfer coefficient and pressure drop are proposed for practical applications. [Copyright &y& Elsevier]

Abstract: The present study proposes a new reduction method to calculate the heat and mass transfer characteristics of the wavy fin-and-tube heat exchangers under dehumidifying conditions. For fully wet conditions, the sensible heat transfer and mass transfer characteristics are relatively insensitive to the inlet relative humidity. The heat and mass transfer performances show appreciable influence of fin spacing at 1-row configuration. Both the heat and mass transfer performances increase when the fin spacing is reduced. However, the difference becomes less noticeable when Re Dc >3000. For 1-row configuration, larger wave height shows much larger difference with the fin spacing. However, the effect of inlet conditions and geometrical parameters on the heat and mass performance becomes less significant with the rise of number of tube rows. Test results show that the heat and mass transfer analogy is roughly applicable (the ratios of h c,o/h d,o C p,a are in the range 0.6–1.1, and is insensitive to change of fin spacing). The correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 94.19% of the j h factors within 15% and 83.72% of the j m factors within 15%. Correspondingly, 93.02% of the ratios of h c,o/h d,o C p,a are predicted by the proposed correlation within 15%. [Copyright &y& Elsevier]

In the present study, the experimental and theoretical results of the heat transfer characteristics and performance of a spirally coiled, finned-tube heat exchanger under sensible cooling conditions are investigated. The test section is a spiral-coil heat exchanger, which consists of a steel shell and a spirally coiled finned-tube unit. The spiral-coil unit consists of six layers of concentric, spirally coiled, finned tubes. Each tube is fabricated by bending a 9.6 mm-diameter straight copper tube into a spiral-coil of four turns. The innermost and outermost diameters of each spiral-coil are 145.0 and 350.4 mm, respectively. Aluminium annular fins with a thickness of 0.6 mm and outer diameter of 28.4 mm are placed helically around the tube. The edge of each fin at its inner diameter is corrugated. Air and water are used as working fluids in the shell side and tube side, respectively. A mathematical model based on mass and energy conservation is developed to simulate the heat transfer characteristics and performance of the heat exchanger. The results obtained from the present model show reasonable agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Abstract: This paper is a continuation of the authors’ previous work. The paper presents new experimental data of the system performance of the two-phase ejector refrigeration cycle (TPERC). The TPERC uses a two-phase ejector as an expansion device while the conventional refrigeration cycle (CRC) uses an expansion valve. The TPERC enables the evaporator to be flooded with refrigerant, resulting in a higher refrigerant-side heat transfer coefficient. The experimental study shows that the TPERC gives a higher cooling capacity and a higher coefficient of performance. Moreover, the pressure ratio and the discharge temperature of the compressor of the TPERC are lower than those of the CRC. [Copyright &y& Elsevier]

Abstract: An experimental study is conducted to investigate the effects of a fin pitch and number of tube rows on the air side performance of fin and tube heat exchangers having herringbone wavy fin configuration at various fin thicknesses. A total of 10 samples of fin and tube heat exchanger with a tube outside diameter of 9.53mm, transverse tube pitch of 25.4mm and longitudinal tube pitch of 19.05mm, having various fin pitches, number of tube rows and fin thicknesses, are tested in a well insulated open wind tunnel. The heat exchangers are made from aluminium plate finned, copper tube. Ambient air is used as a working fluid in the air side while hot water is used for the tube side. The results are presented as the variation of the Colburn factor and the friction factor with the Reynolds number based on the fin collar outside diameter (). The experimental results reveal that the fin pitch has an insignificant effect on the heat transfer characteristic. The friction factor increases with increasing fin pitch when >2500, approximately. The Colburn factor and the friction factor decrease with increasing number of tube rows when <4000, approximately. These results remain the same when the fin thickness is changed. [Copyright &y& Elsevier]

Abstract: The heat transfer characteristics and the performance of a spiral coil heat exchanger under cooling and dehumidifying conditions are investigated. The heat exchanger consists of a steel shell and a spirally coiled tube unit. The spiral-coil unit consists of six layers of concentric spirally coiled tubes. Each tube is fabricated by bending a 9.27mm diameter straight copper tube into a spiral-coil of five turns. Air and water are used as working fluids. The chilled water entering the outermost turn flows along the spirally coiled tube, and flows out at the innermost turn. The hot air enters the heat exchanger at the center of the shell and flows radially across spiral tubes to the periphery. A mathematical model based on mass and energy conservation is developed and solved by using the Newton–Raphson iterative method to determine the heat transfer characteristics. The results obtained from the model are in reasonable agreement with the present experimental data. The effects of various inlet conditions of working fluids flowing through the spiral coil heat exchanger are discussed. [Copyright &y& Elsevier]

Abstract: In the present study, the average tube-side and air-side heat transfer coefficients in a spirally coiled finned tube heat exchanger under dry- and wet-surface conditions are experimentally investigated. The test section is a spiral-coil heat exchanger, which consists of six layers of concentric spirally coiled tube. Each tube is fabricated by bending a 9.6-mm outside diameter straight copper tube into a spiral coil of four turns. Aluminium fins with thickness 0.6 mm and outside diameter 28.4 mm are placed helically around the tube. The chilled water and the hot air are used as working fluids. The test runs are done at the air and water mass flow rates ranging between 0.02 and 0.2 kg/s and between 0.04 and 0.25 kg/s, respectively. The inlet-air and -water temperatures are between 35 and 60 °C and between 10 and 35 °C, respectively. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. New correlations based on the data gathered during this work for predicting the tube-side and air-side heat transfer coefficients for the spirally coiled finned tube heat exchanger are proposed. [Copyright &y& Elsevier]

Abstract: This paper presents the results of simulations using a two-phase separated flow model to study the heat transfer and flow characteristics of refrigerants during evaporation in a horizontal tube. A one-dimensional annular flow model of the evaporation of refrigerants under constant heat flux is developed. The basic physical equations governing flow are established from the conservation of mass, energy and momentum. The model is validated by comparing it with the experimental data reported in literature. The present model can be used to predict the variation of the temperature, heat transfer coefficient and pressure drop of various pure refrigerants flowing along a horizontal tube. It is found that the refrigerant temperature decreases along the tube corresponding to the decreasing of its saturation pressure. The liquid heat transfer coefficient increases with the axial length due to the reducing thickness of the liquid film. The evaporation rate of liquid refrigerant tends to decrease with increasing axial length, due to the decreasing latent heat transfer through the liquid–vapor interface. The developed model can be considered as an effective tool for evaporator design and can be used to choose appropriate refrigerants under designed conditions. [Copyright &y& Elsevier]

In the present study, new experimental data on the performance of a never before seen two-phase ejector refrigeration cycle (TPERC) is presented. In this cycle, a two-phase ejector is used as an expansion device. The TPERC enables the evaporator to operate as in a liquid-recirculation system. The results are compared with those of the conventional refrigeration cycle (CRC). The effects of external parameters, i.e., heat sink and heat source temperatures on the system performance are discussed. The results show that the coefficient of performance of the TPERC is higher than that of the CRC over the whole range of experimental conditions. This is due to a higher refrigerant-side heat transfer coefficient in the evaporator, resulting from the higher refrigerant mass flow rate passing through the evaporator. However, the increase becomes relatively smaller as the heat sink temperature increases. [Copyright &y& Elsevier]

An introduction is presented in which the editor discusses various reports within the issue on topics including the thermal design of plate fin heat exchangers, studies on dropwise condensation, and the modification of Wilson Plots.

• A novel heat sink for cooling a Photovoltaic panel has been studied. • Combination of CFD, differential evaluation and sensitivity analysis has been employed. • A new concept of engineered porous media was explored. • Thermal radiation has been considered. • An optimum porosity has been obtained for two type of porous heat sink. Temperature is one of the most important factors of solar photovoltaic panel (PV) efficiency. İncreasing the temperature causes serious damage to the solar panel. In this research, the effect of the new model of the porous fin as rows of connected spheres on surface temperature was investigated. For achieving this target, the rigid fins and this model of porous fin were compared. Thermal radiation was also examined to increase accuracy in the calculations. After validation and grid study, two models of heat sink with distinct vertical layers and two horizontal layers of connected spheres have been investigated to cool down the PV panel. After that, the new concept of porous heat sink was optimized based on sensitivity analysis (SA) and Differential Evolution (DE) optimization algorithm. A combination of numerical modeling and D.E optimization can improve the final efficiency and increase heat transfer. In this study, by considering the porosity range between (0–70%), we try to find the optimal value of porosity. The results show that by using the special form of porosity on the back layer of the P.V panel, the heat transfer can be increased up to 15%. It's also indicated that the optimization process can improve the Nu and reduce calculation cost. This optimization algorithm is 34 times much faster than another optimization process in CFD method. [ABSTRACT FROM AUTHOR]

• Geometry of double pass SAH with helical flow path is optimized. • CFD, sensitivity analysis and deferential evaluation optimization are employed. • This work is a continuation of the previous research of the authors. • Thermo-hydraulic performance parameter is increased 16.5% due to optimization. Solar air heaters (SAH) have been widely studied due to their extensive applications in solar energy extraction. This research is in line with the previous study of the authors of this article in which the energy efficiency and heat transfer performance of a double pass solar air heater with helical flow path (HFP) were experimentally and numerically explored in a triangular cross-section channel. Here, the effect of different geometric parameters of this SAH on heat transfer performance is numerically investigated. The optimization procedure was also carried out by the differential evolution (DE). The results of this study were developed by referring to the grid independency and validation of the previous work of the authors. Finally, the effects of triangular section angles and dimensions and the location of the entrance section of each channel were investigated and optimized. The results indicated that optimized geometry resulted in a considerably higher thermo-hydraulic performance as compared with reference geometry for all Reynolds numbers which showed a minimum increment of t 16.5% upon optimization. The inactive vortices produced at the edge of entrance flow passage were reduced in each channel due to optimization which also declined the pressure drop. [ABSTRACT FROM AUTHOR]

Flow pattern, heat transfer, and pressure drop data for different flow orientations was presented in this study. The data was obtained based on flow boiling experiments with R-134a flow through a 1 mm diameter channel which was aligned in different orientations, i.e. horizontal flow, vertical upward flow, and vertical downward flow. A constant surface heat flux condition was performed under a saturation pressure of 8 bar, a heat flux range of 1–60 kW/m 2 , and a mass flux range of 250–820 kg/m 2 s. The experimental results showed the importance of the change in the flow direction. The shape of the gas slug during horizontal flow did not look the same as in the vertical orientations. Heat transfer coefficient and pressure drop became increased when the refrigerant flowed in the vertical downward direction. The experimental data was also compared with the existing prediction methods. [ABSTRACT FROM AUTHOR]

This study examined the pool boiling heat transfer of distilled water on a copper heating surface with passive rotating blades installed above the heating surface. The rotating blades were made from copper material, with a diameter of 30 mm, a core of 5 mm, a length of 50 mm, and a blade angle of 90°. The number of blades in this experiment varied between 2, 3, and 4. The study examined the effects of a varying number of blades and the distance between the heating surface and rotating blades (L SB ) on the pool boiling heat transfer coefficient. The experimental results show that, when compared under the same conditions, the rotor with four blades yielded a higher heat transfer coefficient than those with two and three blades. This is because the added blades increased the area that received strike force from the bubbles. As a result, the rotating blades created more disturbance of the working fluid over the heating surface. Furthermore, when compared with the same number of blades, the L SB of 5 mm yielded a higher heat transfer coefficient than the L SB of 15 or 25 mm. This is because the increased L SB provided less chance for the bubbles to strike the rotating blades. Hence, the rotating blades did not create a disturbance of the working fluid over the heating surface. [ABSTRACT FROM AUTHOR]

The entropy generation analysis in a viscous dissipative flow of a Newtonian fluid through a hyper-porous microchannel formed between two heated parallel plates is conferred. Employing an analytical method, which is consistent with the perturbation analysis, the transport equations governing the thermo-hydrodynamics are studied. The effects of nonlinear Forchheimer drag and conjugate heat transfer on the thermal transport characteristics of heat are considered, while the thermal boundary conditions of third kind have been employed at the outer boundaries of channel for the conjugate heat transfer analysis. The explicit alterations are made in the thermal transport of heat in the system as attributable to the effect of dissipative heat generated due to the non-linear effect Forchheimer drag, Darcy frictional effect and the viscous shearing stress in the flow field. To account these effects, the explicit variation of Forchhiemer constant, Darcy number, porosity, thickness and conductivity of upper wall and Biot number of upper wall are carried out to shows the changes in the available energy of the system. Also, it is shown that the effect of non-linear drag mainly stemming from the presence of complex porous structure in the flow field and its interaction with the conjugate transport of heat alters the heat transfer rate in the system non-trivially, which, in turn, gives rise to the entropy generation in the system. The individual contribution of two different effects viz., the heat transfer and viscous dissipation on the system entropy generation rate for different cases are studied. It is believed that the implications of the present analysis may have direct bearing on the design of micro devices/systems typically used in electronic cooling, micro-heat pipes. [ABSTRACT FROM AUTHOR]

Heat exchanger is a basic component used in thermal processes such as industrial fluid heating, household appliances, waste heat recovery units (WHRU), etc. Presently, the spiral fin-and-tube heat exchanger is used as a favored type of heat exchanger for the waste heat recovery unit system, i.e., the economizer heat exchanger. The study investigates the effect of fin pitches on the air-side performance of serrated welded spiral fin-and-tube heat exchangers having Z-shape flow arrangement and the number of tube rows of 2 in range of high Reynolds numbers (4000–15,000). The test samples are made from steel (iron) with different fin pitches of 3.6, 4.2, and 6.2 mm. The experimental results reveal that the fin pitch has a significant effect on the air-side performance. It is confirmed that the fin pitch of 3.6–6.2 mm has the same trend in heat transfer coefficient at the tested condition. For f -friction factor, a detectable rise of the friction factor is found when the fin pitches increase to 6.2 mm. Moreover, the air-side performance correlations of serrated welded spiral finned tube heat exchangers are proposed for practical industrial applications. The proposed air-side performance correlation describes 100% for j -Colburn factor and f -friction factor within 10%. [ABSTRACT FROM AUTHOR]

• Vertical downward condensation plate heat exchanger. • Electrochemical etching generates microscale roughness. • Enhance thermal-hydraulic performance of plate heat exchanger. • Analyze effect of heat flux, mass flux, vapor quality and surface roughness on heat transfer. • Multi-objective generic algorithm optimized designed roughness. • Pareto front solutions for maximizing performance. This research employed electrochemical etching to generate microscale roughness on a plate heat exchanger (PHE) thereby enhancing condensation heat transfer in a vertical downward flow. The effects of the mass flux, vapor quality, heat flux, saturation temperature and microscale surface roughness on the condensation heat transfer coefficient (HTC) and frictional pressure drop (FPD) of R-513a flowing through a PHE was experimentally investigated. Experimental measurements were made under various heat fluxes of 5–15 kW/m2, mass fluxes of 60–80 kg/m2s, saturation temperatures of 40–50 °C, and mean vapor qualities of 0.1–0.7. HTC increased by up to 28, 26, 74, and 32.5 % as the heat flux, mass flux, average quality, and microscale surface roughness were respectively increased, but decreased by 19 % with the saturation temperature. FPD increased as the mass flux and average vapor quality increased, whereas it was reduced as the refrigerant saturation temperature increased. In contrast, the heat flux had little to no influence on FPD. Correlations for predicting the Nusselt number and friction factor were postulated by considering influential dimensionless numbers during heat transfer. Maximizing the Nusselt number and thermal performance factor was ultimately resolved using a multi-objective genetic algorithm to find a Pareto front solution. Consequently, the optimal surface roughness was found to be 9.94 μm. [ABSTRACT FROM AUTHOR]

In the present study, an attempt has been made to summarize and analyze the results of an examination of the air-side performance of spiral (or helical) fin-and-tube heat exchangers. Currently, the spiral fin-and-tube heat exchanger is a favored type of heat exchanger for the waste heat recovery unit (WHRU), a kind of economizer system. The present paper is broadly divided into an experimental section and numerical and simulation sections. A significant fraction of the papers herein reviewed pertains to the effect of fin configurations, tube arrangements, operating conditions, and other factors on the air-side performance of the spiral fin-and-tube heat exchangers. Approximately 40 published articles related to spiral fin-and-tube heat exchangers are briefly described. Moreover, the air-side performance correlations of spiral fin and circular fin-and-tube heat exchangers are compiled into this work for practical industrial applications. [ABSTRACT FROM AUTHOR]

Highlights: [•] Slug flow gives better air–water heat transfer results, compared with the gas core flow. [•] For a given flow pattern, the Nusselt number is dependent on superficial Reynolds numbers. [•] Two-phase air–water flow results in heat transfer enhancement up to 80% in comparison to the liquid flow. [ABSTRACT FROM AUTHOR]

• 20.2% reduction in evaporator temperature noted with the use of internal condenser. • Rate of condensation enhanced by 4.15% with the use of internal condenser. • Lowest thermal resistance is noted with the use of internal condenser at 90° angle. In conventional heat pipe designs, cooling in condenser region is typically achieved externally through forced or natural convection. This approach limits surface area and often leads to condensation inefficiency. This inefficiency prolongs vapour residence time, hinders liquid reflux and causing problems like increasing operating temperatures, and pressures. To tackle this problem, an innovative flat heat pipe (FHP) design with internally cooled condenser is proposed. The design includes a tube-in-tube assembly where the coolant is circulated inside the condenser region to enhance the condensation rate. Test are conducted at different heat loads (75 W, 195 W, 315 W), coolant flow rates (ṁ=10–25LPH), and inclination angles (IA=0°-90°). Results demonstrated a superior performance of the proposed FHP design compared to conventional heat pipes. At IA-0° and ṁ-25LPH, it is observed a reduction of 20.2% and 48.1% in the evaporator-wall temperature and evaporator pressure. In addition, at IA-90°, the lowest thermal resistance of 0.0237 °C/W is noted at 315 W. An enhancement of 4.15% in the rate of condensation is noted, when the inclination angle changes from 0° to 90°. Notably, at 315 W heat load with inclination angle of 90°, a minimum evaporator pressure of 0.25 bar and a maximum vapour velocity of 1.7 m/s is observed. [ABSTRACT FROM AUTHOR]

• Condensation flow of R-134a and R-513A in a horizontal smooth tube is studied. • Condensation flow of R-513A in horizontal smooth and micro-fin tubes is studied. • R-513A can replace R-134a with a lower heat transfer coefficient and pressure drop. • The micro-fin tube can enhance the heat transfer coefficient by 69 %. • The enhancement in pressure drop for a micro-fin tube is up to 120 %. An experimental investigation to determine the average heat transfer coefficient (h av) and frictional pressure drop (ΔP F) characteristic of R-513A and R-134a in a smooth tube (ST) and micro-fin tube (MFT) is carried out. A 2500 mm long counter-flow tube-in-tube heat exchanger made of copper is used for conducting experiments. The ST and MFT with inner diameters (D i) of 6.74 and 6.82 mm, respectively, are experimentally tested for various operating conditions such as saturation temperatures (T sat = 40 to 50 °C), heat fluxes (q" =9.8 to 19.6 kW/m2), mass fluxes (G = 575 to 770 kg/m2s) and average vapor qualities (x av = 0.2 to 0.9). The influence of q", G, and T sat on h av and ΔP F are discussed. First, a comparison between R-513A and R-134a in an ST showed maximum reductions of 16 and 20 % in h av and ΔP F, respectively for R-513A, when compared to R-134a. Second, the MFT, due to its higher heat transfer area, resulted in 69 % and 120 % enhancements in h av and ΔP F , respectively, compared to that of an ST. Hence, the findings of this research provide information that is currently useful to the thermal industries, affirming that R-513A can be a suitable long-term alternative refrigerant. [ABSTRACT FROM AUTHOR]

• Multi-channel heat pipe is experimentally studied with acetone as working fluid. • Optimum liquid reservoir L res = 5 mm and fluid volume of 2.5 ml dissipated 90 W. • 35.4 % enhancement in heat dissipation rate noted for axial face (α a) condition. • 24.5 % reduction in temperature fluctuation noted for lateral side inclination (α l). • Antigravity heat transport capability noted up to 15 W at α a = −15o. The heat transfer characteristics of a miniatured flat heat pipe (MFHP) with multi-channels, featuring a port diameter of 1.18 mm, is investigated experimentally. Various operating parameters are considered, including the working fluid volume (V f = 1.5, 2.5, and 3.5 ml), length of the liquid reservoir (L res = No reservoir, 5, and 10 mm), orientation such as axial face (α a) or lateral side (α l), inclination angles (α = −15 to 90o), and cooling water flow rates (ṁ i = 10, 15, and 20 LPH). Based on the experiments, the optimal values for the working fluid volume, reservoir length, and flow rate are determined as V f = 2.5 ml, L res = 5 mm, and ṁ i = 20 LPH, respectively. Further analysis reveals that, the heat dissipation rate for the axial face is significantly higher than that of the lateral side, with an average percentage increase of 35.4 %. However, the lateral side outperforms the axial face in terms of stabilizing the evaporator wall temperature, reducing fluctuations by an average of 24.5 %. Moreover, the presence of multi-channels allows the MFHP in axial face orientation to dissipate a maximum heat load of 15 W against gravity at an inclination angle of α a = −15o. Finally, the variations in MFHP operation based on the orientation and its underlying physical mechanisms that contribute to enhancing heat transfer are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This study presents the improvement in heat transfer performance of a heat pipe using silver nanoparticles dispersed in DI (De-Ionized) water. The nanoparticles suspended in conventional fluids have superior heat transfer capability due to improved thermal conductivity. The heat pipes are tested for heat inputs ranging from 20W to 100W in five steps, which is suitable for removing heat from power transistors in electronics and processors in computers. The effect of various operational limits and test parameters such as heat inputs, volume fraction, vapour temperature on the thermal resistance, evaporation and condensation heat transfer coefficients, are experimentally investigated. The tested silver nanoparticles volume concentration ranged from 0.003% to 0.009% with average nanoparticle diameter of 58.35nm. The experimental results are evaluated in terms of performance metrics by direct measurement of vapour temperatures in the centre core of heat pipe. A substantial reduction in thermal resistance of 76.2% is observed for 0.009vol.% concentration of silver nanoparticles. Further an enhancement in the evaporation heat transfer coefficient of 52.7% is observed for the same concentration. The use of nanoparticles enhances the operating range of heat pipe by 21% compared with that of DI water. [Copyright &y& Elsevier]