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Highlights • Heat transfer and pressure drop of R-134a in dimpled tubes are investigated. • The effects of dimpled depth on the heat transfer and pressure drop are examined. • The heat transfer enhancement factor is up to 1.81 times of smooth tube. • The correlations to predict the Nusselt number and friction factor are proposed. Abstract An experimental investigation is conducted to determine the effect of dimpled depth on the condensation heat transfer coefficient and pressure drop of R-134a flowing inside dimpled tubes. The test condenser is a double tube heat exchanger where the refrigerant flows inside and water flows in the annulus. The inner tube is a 1500 mm long and 8.1 mm inside diameter. The experiments are carried out for one smooth tube and three dimpled tubes having dimpled depth of 0.5, 0.75, and 1.0 mm. For each test tube, several test runs are performed over mass flux range of 300–500 kg/m2s, heat flux range of 10–20 kW/m2, and condensing temperature range of 40–50 °C. The experimental results reveal that the dimpled tube presents the significant heat transfer enhancement and pressure drop penalty. The tube with the highest dimpled depth yields the highest heat transfer enhancement and pressure drop penalty up to 83% and 892% higher than those of the smooth tube, respectively. Additionally, the overall performance of dimpled tube is evaluated in term of efficiency index. The new correlations including the effect of dimpled depth, dimpled pitch, and helical pitch on the Nusselt number and friction factor of R-134a in dimpled tube are developed. [ABSTRACT FROM AUTHOR]

The heat transfer enhancement and pressure loss penalty of condensing refrigerant R-134a in dimpled tubes with different helical and dimpled pitches are experimentally investigated. The test section is a 1500-mm-long tube-in-tube heat exchanger placed horizontally. The inside and outside diameters of the inner tubes are 8.1 and 9.52 mm, respectively. One smooth tube and five dimpled tubes with different helical and dimpled pitches are tested. The experimental runs are done at saturated condensing temperatures ranging from 40 to 50 °C. The mass fluxes and heat fluxes vary from 300 to 500 kg/m 2  s and from 10 to 20 kW/m 2 , respectively. The results show that the dimpled tubes yield higher heat transfer enhancement and pressure drop penalty compared to the smooth tube under the same operating condition. By decreasing helical and dimpled pitches, both the heat transfer coefficient and frictional pressure drop increase significantly. The maximum heat transfer coefficient and frictional pressure drop are achieved through the tube with the lowest helical and dimpled pitches (p = 5.08 mm, z = 3.24 mm) up to 88% and 630% higher than those of the smooth tube, respectively. The new correlations for predicting the Nusselt number and friction factor of R-134a condensing in dimpled tubes with different helical and dimpled pitches are proposed. [ABSTRACT FROM AUTHOR]

This article presents two mathematical models for estimating the performances of finned tube heat exchangers with wavy configuration under partly and fully wet surface conditions. The actual dry bulb temperature method is the first one and the other is the method of equivalent dry bulb temperature. For the first one, the actual dry bulb temperature method that was firstly presented by our research team in 2015 is more uncomplicated than the well-known enthalpy potential method and is also easier than the equivalent dry bulb temperature method. Moreover, it has never been applicable to the finned tube heat exchanger. For accuracy, the finned tube heat exchangers are distinguished into teeny segments and every segments are separated by conditions of surface into three groups: partially wet segments, fully wet segments, fully dry segments. This procedure is called the finite circular fin method that was firstly presented by our research in 2007. From the results, the heat and mass transfer characteristics estimated by the method of equivalent dry bulb temperature are greater than those estimated by the method of actual dry bulb temperature. This is the influence of the non-constant terms in both methods. Moreover, the correlations for the conditions of party and fully wet surface are revealed for predicting the heat and mass transfer coefficients of the finned tube heat exchanger with wavy configuration. The users can apply our presented models with the proposed correlation for predicting the heat and mass transfer performances of finned tube heat exchangers with wavy configuration under the conditions of fully and partly wet surface. [ABSTRACT FROM AUTHOR]

This study investigates the heat transfer and pressure drop of R-134a during condensation inside a dimpled tube. The test section is a horizontal counter-flow double-tube heat exchanger with refrigerant flowing in the inner tube and cold water flowing in the annulus. The inner tubes consist of one smooth tube and one dimpled tube, which are made from copper. The length and inner diameter of the test tube are 1500 mm and 8.1 mm, respectively. The test runs are performed at saturation temperatures of 40, 45, and 50 °C, heat fluxes of 10, 15, and 20 kW/m 2 , and mass fluxes of 300, 400, and 500 kg/m 2 s. The effects of heat flux, mass flux, and saturation temperature on the heat transfer coefficient and frictional pressure drop are examined. Comparisons between smooth and dimpled tubes on the heat transfer and frictional pressure drop are also discussed. It is observed that the heat transfer coefficient and frictional pressure drop obtained from a dimpled tube are higher than that of the smooth tube. In addition, as the equivalent Reynolds number increases, the dimpled tube enhances the Nusselt number around 1.3–1.4 times in comparison to the smooth tube. Whereas, when the equivalent Reynolds number decreases, the two-phase friction factor rises around 2.8–4.1 times as compared to the smooth tube. [ABSTRACT FROM AUTHOR]

A heat sink is a kind of heat exchanger used as a cooling system for electronic components due to simplicity, low cost, and a reliable manufacturing process. The heat sink can be divided by plate and pin fin shapes. In the past decade, several published articles have focused on studying the effects of fin shapes and arrays of heat sinks on flow characteristics and thermal performance with forced convection. However, only a few publications have focused on the flow directions (parallel and impingement flows) and flow behaviors (bypass and non-bypass flows) through the heat sink, which are necessary for fin and overall cooling system design. The purpose of this article is to summarize the publications with respect to experimental research on the fluid flow directions and behaviors through heat sinks as well as offering guidelines for future research. [ABSTRACT FROM AUTHOR]

Abstract: This paper is a continuation of the authors’ previous work on the condensation of R134a flowing inside a multiport minichannel (Sakamatapan et al., 2013) [22]. The pressure drop’s characteristics during condensation were investigated in an experiment for R134a flowing inside a multiport minichannel. The pressure drop characteristics were observed under mass flux range of 345–685kg/m2, heat flux of 15–25kW/m2, and saturation temperature of 35–45°C. Two types of multiport minichannels having 14 channels, one with a 1.1mm hydraulic diameter and another with 8 channels with a 1.2mm hydraulic diameter, were designed as a counter flow tube in a tube heat exchanger. The results showed that the total pressure drop is dominated by the frictional pressure drop. The frictional pressure gradient increased with the augmentation of mass flux and vapor quality, but the increase of the saturation temperature and channel size play an important role in the decrease of frictional pressure gradient. On the other hand, the heat flux had an insignificant effect on the frictional pressure gradient. Moreover, the miniscale correlations were conducted to predict the frictional pressure gradient, and it was found that only the multiport minichannel correlations gave a reasonable result. A new two-phase friction factor correlation was proposed using the equivalent Reynolds number concept to predict the frictional pressure gradient during condensation. [Copyright &y& Elsevier]

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]

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: 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: 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]

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: 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 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 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]

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]

This study numerically examines the elliptical winglets (EW) on the overall performance of fin-and-tube heat exchanger. The major geometrical parameters characterizing the EW has been investigated thoroughly, including the winglet location, attack angle and the trailing angle of the elliptical winglets with two different winglet lengths which are 1.0 mm and 1.5 mm. The numerical results indicate that the elliptical winglets with the 30° attack angle, 0 mm horizontal distance, 6 mm vertical distance and 120° trailing angle of both winglet lengths shows the highest performance. The heat transfer coefficients and pressure drops obtained by both winglet sizes are larger than that of the plain fin by about 13% and 35%, respectively. In term of the performance efficiency index, the 1.5 mm winglet length indicates about 1.8–2.9% improvement relative to plain fin geometry and the efficiency index of 1.0 mm winglet length is comparable with that of the plain fin. [ABSTRACT FROM AUTHOR]

The fin and tube heat exchanger with a semi-dimple pair is numerically studied in terms of the air side’s thermal performance. The investigated parameters consist of the diameter, attack angle and the placed location of the semi-dimple. The results show that the 30° attack angle, 5.5 mm vertical distance and 7.5 mm horizontal distance can perform the highest j/f ratio or “goodness parameter” of every semi-dimple’s diameter. Additionally, the thermal performance of the fin with a semi-dimple pair is also compared with that of the fin with existing semi-dimples and the plain fin within the Reynolds number ranging from 813 to 4019. Obviously, the existing semi-dimple exhibits the highest heat transfer coefficient and pressure drop. However, the semi-dimples’ pair demonstrates a goodness factor of 33–37% greater than the existing semi-dimples and 15–20% greater than the plain fin, approximately. [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]

Entropy generation analysis of hybrid nanofluid in a two pass multiport minichannel heat exchanger coupled with a thermoelectric cooler is experimentally investigated. Alumina (Al 2 O 3 , 50 nm), graphene (5 nm) and the hybrid of these two in equal portions with 0.1% volume concentrations is separately dispersed in to the base fluid and tested. The hydraulic diameter and aspect ratio of the channel are 1.184 mm and 0.689 respectively. The heat flux is varied from 6250 W/m 2 to 25,000 W/m 2 and the flow regime is considered to be laminar with the Reynolds number varying from 200 to 1000. The results showed an enhancement of 17.32% in cooling capacity and coefficient of performance (COP) with the use of pure graphene–water nanofluid when compared with that of the other tested combinations of nanofluids. Total entropy generation decreased from 0.0361 W/K to 0.0184 W/K with increase in Reynolds number from 200 to 1000 for the maximum applied heat flux of 25,000 W/m 2 . Similarly an enhancement of 88.62% in the convective heat transfer coefficient and a reduction of 4.7 °C in the device temperature are achieved when pure graphene–water nanofluid is used as the coolant. Among the tested nanofluids, graphene–water nanofluid shows better performance in terms of heat transfer, thermodynamic and exergic analysis. [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]

• 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]

Abstract: This study conducted experiments on the optimized fin pitch for crimped spiral fin-and-tube heat exchangers. The experiments covered a size range of 2.4–6.5mm, which is the manufacturing limitation for this kind of fin. The water-flow arrangement used in this experiment combined the parallel cross-flow and the counter cross-flow in a two-row configuration. Ambient air was used as the working fluid on the air-side, and hot water was used on the tube-side. The effects of fin pitches on the heat transfer coefficient and pressure drop characteristics were studied. The results clearly showed that the convective heat transfer coefficient (ho ) for a fin pitch of 2.4mm is relatively low compared with that of other fin pitches with the same air frontal velocity. Using larger fin pitches (i.e., 4.2, 6.2, and 6.5mm) resulted in negligible differences in the pressure drop. Moreover, this work introduces the parameter of three performances indexes, which can be expressed as the ratio of the desired output to the required input, for optimization purposes. Due to the difference in optimum fin pitch obtained by these performance indexes, an intersection analysis was conducted. The results indicated that the optimum fin pitch is 4.2mm for this work, which could be valuable for the effective design for industrial thermal-system applications. [Copyright &y& Elsevier]

Abstract: Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600mm long with a diameter of 1.75mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000kg/m2 s, a heat flux range of 1–83kW/m2 and saturation pressures of 8, 10, and 13bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented. [Copyright &y& Elsevier]

Abstract: Correlations are proposed for the wet surface ratio of a fin-and-tube heat exchanger with plain and wavy fin geometry under dehumidifying conditions. The ‘Finite Circular Fin Method’ (FCFM) is used for data reduction. It is found that the percentage of wet surface area increases with increasing fin spacing or number of tube rows and decreasing Reynolds number. Despite the addition of tube rows or reduced fin spacing the effective surface area is increased, and its influence on a partially wet surface is apparently the opposite. This is because adding tube rows will provide a more effective temperature drop in air flow than adding fins, Moreover, the heat and mass transfer characteristics of jh and jm increase with an increase in the area of wet surface. Correlations for prediction of the percentage of wet surface area are proposed. These correlations can describe 83.81% of the area of wet surface to within ±10%. [Copyright &y& Elsevier]

Abstract: This paper presents a review of data reduction method for heat and mass transfer characteristics of fin-and-tube heat exchangers with dehumidification. There are many reduction methods for fin-and-tube heat exchangers under dehumidifying conditions. The data reduction methods being reviewed includes the original Threlkeld method, direct method, the equivalent dry-bulb method, tube by tube method, the fully wet and fully dry tiny circular fin method, and the finite circular fin method. Among these methods, the original Threlkeld method, direct method, the equivalent dry-bulb method are lumped method while others can divide the fin-and-tube heat exchangers into small segments for more accuracy in handling the surfaces to be fully dry, fully wet, or partially wet. In addition, the mass transfer characteristics can be obtained from the modified process line equation incorporated with the preceding methods. It should be noted that the conventional assumption of constant ratio (hc,o /hd,oCp,a ≈constant) is actually incorrect. This present paper can be used as the first guideline for the researcher for reducing the experimental data for fin-and-tube heat exchangers under dehumidifying conditions. [Copyright &y& Elsevier]

Abstract: The present study proposes the finite circular fin method for analyzing the heat and mass transfer characteristics of wavy fin-and-tube heat exchangers under fully and partially wet surface conditions. The analysis is carried out by dividing the wavy fin-and-tube heat exchanger into many tiny segments. The tiny segments can be analyzed based on surface conditions, i.e. fully wet, fully dry or partially wet surface condition. From the experimental results, it is found that the heat and mass transfer characteristics are insensitive to the inlet relative humidity but the effect of relative humidity on mass transfer characteristic become more pronounced when the partially wet surface condition takes place. The heat transfer characteristic is independent of the fin spacing. Effect of fin spacing on mass transfer characteristic is small when fin spacing is larger than 2.5mm. However, at smaller fin spacing, the mass transfer characteristic slightly decreases when the relative humidity increases. The ratios of h c,o/h d,o C p,a are in the range of 0.6–1.2. Correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 95.63% of the heat transfer characteristic within 15% and 95.14% of the mass transfer characteristic within 20%. Correspondingly, 94.68% of the ratios of h c,o/h d,o C p,a are predicted by the proposed correlation within 20%. [Copyright &y& Elsevier]

Abstract: This study proposes a new method, namely the “finite circular fin method” (FCFM), to analyze the performance of fin-and-tube heat exchangers having plain fin configuration under dehumidifying conditions. The analysis is done by dividing the heat exchanger into many tiny segments (number of tube rows×number of tube passes per row×number of fins). The tiny segments are distinguished into three types: the fully dry, partially wet or fully wet surface conditions. The proposed method is capable of handling fully and partially wet surfaces. From the test results, it is found that the sensible heat transfer performance and the mass transfer performance are insensitive to changes of fin pitch. The influence of inlet relative humidity on the sensible heat transfer performance is small, and is almost negligible when the number of tube rows is above four. For one and two row configurations, considerable increase of mass transfer performance is encountered when partially wet condition takes place. The sensible heat transfer coefficient is about the same for those in fully wet and partially wet conditions provided that the number of tube row is equal or greater than four. Correlations applicable for both fully wet and partially wet conditions are proposed to describe the heat and mass performance for the present plain fin configuration. [Copyright &y& Elsevier]

Abstract: This study investigated the effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions. A total of 10 samples were tested with associated fin thickness (δ f) of 0.115mm and 0.25mm, respectively. For a heat exchanger with two rows (N =2) and fin pitch F p of 1.41mm, the effect of fin thickness on the heat transfer coefficient is more pronounced. The heat transfer coefficients for δ f =0.25mm is about 5–50% higher than those for δ f =0.115mm whereas the pressure drop for δ f =0.25mm is about 5–20% higher. The unexpected difference in heat transfer coefficient subject to fin thickness is attributable to better interactions between the directed main flow and the swirled flow caused by the condensate droplet for δ f =0.25mm. The maximum difference in heat transfer coefficients for N =2 and F p =2.54mm subject to the influence of fin thickness is reduced to about 20%, and there is no difference in heat transfer coefficient when the frontal velocity is above 3m/s. For N ⩾4 and F p =2.54mm, the influence of fin thickness on the heat transfer coefficients diminishes considerably. This is because of the presence of tube row, and the unsteady/vortex shedding feature at the down stream of wavy channel. Based on the present test results, a correlation is proposed to describe the air-side performance for wavy fin configurations, the mean deviations of the proposed heat transfer and friction correlations are 7.9% and 7.7%, respectively. [Copyright &y& Elsevier]

• Alternating cross-section flattened tubes with various aspect ratios are studied. • This tubes with various aspect ratios are the first study in two-phase flow. • The necessary parameters that affect thermal performance are discussed. • Alternating cross-section flattened tubes outperform the round tube up to 2.1 times. • Nu and f TP correlations for ACF tube are proposed. In this experiment, we investigated the condensation of R32 flowing inside alternating cross-section flattened (ACF) tubes with various aspect ratios. ACF tubes were made from a copper circular tube with a 4.31-mm inner diameter. Three ACF tubes with aspect ratios of 5.82 (ACF1), 4.11 (ACF2), and 3.23 (ACF3) were used as test tubes. The experimental conditions covered a mass flux of 280–580 kg/m2s, a heat flux of 10–20 kW/m2, a saturation temperature of 40 °C, and a vapor quality of 0.1–0.8. The results revealed that most experimental data fell into the annular flow regime when using existing flow patterns. In comparison, the ACF1 tube obtained the highest heat transfer coefficient (HTC) and pressure drop (ΔP). The HTC of ACF tubes outperformed those of the circular tube by about 1.7–6.9 (for ACF1), 1.1–2.8 (for ACF2), and 1.2–2.1 (for ACF3) times. In addition, ACF tubes provided a greater frictional pressure drop than circular tubes, around 1.0–2.3 (for ACF1), 1.2–1.9 (for ACF2), and 1.0–1.4 (for ACF3) times. However, the ACF1 achieved the best overall performance by approximately 2.1 times, superior to the circular tube. Since the existing correlations failed to predict the Nusselt number and two-phase friction factor, we propose new correlations for the ACF tube with mean deviations of 14.9% and 7.3%, respectively. [ABSTRACT FROM AUTHOR]

• Plate-fin heat sinks with lateral perforation is numerically studied. • The study on thermal performance of perforated plate-fin heat sinks is proposed. • Diameters and numbers of circular perforations on a plate fin are varied. • Flow behavior of air inside laterally perforated plate-fin heat sink is presented. • Optimum design parameters of perforated plate-fin heat sink are reported. This study presents the thermal performance of laterally perforated plate-fin heat sinks (LAP-PFHSs) with different numbers (N p) and diameters (D p) of circular perforations. Based on the same plate fin dimension, the available diameter of circular perforations is limited by their number. The largest diameters of perforation for N p = 14, 27 and 75 are 10 mm, 7 mm and 4 mm, respectively. The computational results of solid fin heat sinks (SFHSs) and LAP-PFHSs are validated with the measured data and experimental data from the available literature. The comparison results of LAP-PFHSs give the mean absolute error of about 3.6% and 5.3% for the pressure drop and thermal resistance, respectively. According to the numerical results, the LAP-PFHS with D p = 3 mm and N p = 75 exhibits the highest heat transfer rate, about 11.6% higher than that of the SFHS. Finally, the thermal performance factor, which is a consideration of the Nusselt number and friction factor, is proposed to find the suitable design parameters. Under the same conditions, the optimized LAP-PFHS demonstrates 10.6% greater thermal performance and 28% lower volume of heat sink material than SFHS. [ABSTRACT FROM AUTHOR]

• Sinusoidal wavy plate fins with phase shift of 0°, 90° and 180° are investigated. • Thermal performance of SW-PFHS and CCSW-PFHS are compared. • The phase shift has significant effect on the thermal performance. • The proper working conditions of sinusoidal wavy fins heat sinks are reported. This article presents new experimental data on the influence of phase shift, air velocity, heat sink base surface temperature on heat transfer coefficient, and pressure drop of airflow through sinusoidal wavy plate fin heat sinks (SW-PFHS) and crosscut sinusoidal wavy plate fin heat sinks (CCSW-PFHS). Sinusoidal wavy plate fins with a wave length of 18.68 mm, amplitude of 2.0 mm, and phase shift of 0°, 90°, and 180° are used. For CCSW-PFHS, the sinusoidal wavy plate fin is cut transversely at crests and troughs with a 2 mm length. The test runs are performed at an air velocity ranging between 1 and 5 m/s and a heat sink base surface temperature of 70 °C, 90 °C, and 110 °C. The results show that the higher phase shift and air velocity lead to the enhancement of the heat transfer coefficient and pressure drop. Conversely, the heat sink base surface temperature has a slight effect on the heat transfer coefficient and pressure drop. The heat transfer coefficient of SW-PFHS is enhanced when increasing the phase shift compared with a phase shift of 0°. Under the same phase shift, the Nusselt number of CCSW-PFHS is higher than that of SW-PFHS by about 5.9–19.1%. The CCSW-PFHS with a phase shift of 180° yields the highest TPF. [ABSTRACT FROM AUTHOR]

Highlights • The Venturi nozzle design with entry and exit angles, which leads the self-sucked air flow. • Microbubbles generation regarding to entry and exit angles. • High speed visualization study to track the self-sucked air flow stream broken into microbubbles. Abstract We studied the effect of varying the entry and exit angles of Venturi nozzles on the formation of microbubbles in Venturi nozzle-type microbubble generators. We 3D-printed nozzles with five entry angles (15, 22, 30, 38 and 45°) and five exit angles (15, 22, 30, 38 and 45°). For the visualization experiment, we inserted the nozzles into a cover case made of aluminum and transparent acrylic. We measured the pressure drop and the air flow rate with respect to the entry and exit angles, determined the diameters of the bubbles using a digital camera, and analyzed bubble breakage by observing the behavior of the bubbles using a high-speed camera. We confirmed that the exit angle (not the entry angle) is dependent on the pressure drop and found that the air flow rate did not vary linearly with the fluid flow rate, as expected according to Bernoulli's theorem. Instead, it tended to remain constant or decrease as the fluid flow rate increased due to the abnormal flow. The sizes of the bubbles decreased as the exit angle increased, except in cases where the outlet angle was greater than 30° at high flow rates (260–300 LPM). We observed a change in bubble size with respect to exit angle. According to our visualization, the bubbles were broken by the flow separation at the beginning of the divergence at the exit. [ABSTRACT FROM AUTHOR]

Highlights • Experimental study of heat transfer of ferrofluid under influence of magnetic field. • Infrared thermography used for measurement of the heat transfer coefficient. • Heat transfer augmentation takes place under influence of magnetic field. • Augmentation process depends on the interplay between the involved times scale. Abstract Convective flow of single-phase ferrofluids under the influence of constant and alternating magnetic field has attracted attention as an effective strategy for enhanced heat transfer in mini/micro thermal systems. In the present study, an attempt has been made to gain deep insight of the heat transfer characteristics of single-phase ferrofluid flow in a heated stainless steel tube under the influence of constant and time-varying magnetic field. The governing parameters are mainly the magnetic flux density (B) and perturbation frequency (f) of the applied magnetic field. Three magnetic flux density value of B = 0 G, 700 G and 1080 G have been used for constant magnetic field. Constant value of B = 1080 G was used for alternating magnetic field while, frequencies of applied magnetic field has been varied from 0.1 Hz to 5 Hz. Flow Reynolds number was kept constant to Re = 66. Some 2-D numerical simulations have also been performed to qualitatively support the experimental data. The study is focused to delineate the mechanism of augmentation of heat transfer through the interaction of available force fields, i.e., interplay of magnetic force and inertia of the flow, and also the effect of various time scales on the flow and thermal behavior. Major inferences of the study are (a) on the application of external magnetic (constant and alternating), heat transfer augments (b) existence of a threshold frequency of external magnetic field for maximum augmentation as outcome of advective time-scale and magnetic perturbation time-scale. InfraRed Thermography (IRT) has been used to measure the wall temperature, while, some bright field visualizations have also been done to qualitatively support the explanations of experimental data. [ABSTRACT FROM AUTHOR]

Highlights • Using a 3D analytical model for evaluating thermal performance of air-cooled heat sink. • Simulation can predict spatial variation of plate fins in forced convection temperatures. • Studying the Nusselt number, various heat sink heights and different number of fins. • Investigating the structural stability method in this high-temperature device. • Chosen range of air flow velocity is totally suitable for this high-temperature device. Abstract In the present study, the thermal performance of an air-cooled, flat-plate heat sink in forced convection was performed using a three-dimensional analytical model. To validate the present model, this solution is compared with numerical and experimental results that agree. This research studies the influence of heat sink thermal performance and of environmental parameters on the heat sink's stability against thermal stress, which is essential for manufacturers' goals. The effective parameters on heat transfer rate from the surface of heat sink were investigated. Reynolds and Nusselt numbers, heat transfer coefficient, fins height, and fins number were studied under various air flow velocities. In addition, the thermal stress (structural stability method) was investigated. Enhancement of the airflow velocity caused the Nusselt number value to incline, which increases the heat transfer coefficient. Increasing the Reynolds number from 3.80*103 to 2.28*104 leads to less thermal stress by 47.36%, less deformation by 50%, less surface temperature by 28.57%, and, consequently, less geometry failure. Results depicted that pure convection has less heat transfer by 1.66% compared with convection-radiation heat transfer, and enhancing the fins number and fins height reduces the surface temperature by 25% and 20%, respectively. As a result, the chosen range of the Reynolds number is totally suitable for this high-temperature device that does not allow thermal stress to deform the fins of the heat sink. [ABSTRACT FROM AUTHOR]

Highlights • A brief introduction of nanofluid and its applications were discussed. • The role of effective parameters on the nanofluid viscosity was reported. • Viscosity increased with increasing nano-additives amounts and reducing temperature. • Nano-additives size had different effects on nanofluids viscosity have been reported. • Hybrid nanofluids can well cover nanofluid weaknesses and improve its strengths. Abstract Nanoparticles are an evolution in improving heat transfer by fluids that have the good potential for heat transfer. Thus, in the last two decades, interest in researching them has increased dramatically. It has been proved that the thermophysical properties of nanofluids are different from those of common fluids. One of these properties is viscosity, which has a significant contribution to the calculation of the fluid heat transfer. In the present study, a brief introduction of nanofluid and its applications have been discussed in the first. Then, the classical equations suggested for predicting the viscosity of nanofluids and their accuracy have been reviewed. The role of effective parameters on the nanofluid viscosity has also been reported, indicating an increase in the viscosity of the common fluids by increasing the nano-additives volume fraction and reducing it with increasing temperature. For the effect of increasing nano-additives size, different results (decreasing or increasing) on fluid viscosity have been reported. The effect of changing the type of nano-additives has also been described for a variety of metal/metal oxides nanoparticles, and nanomaterial extracted from nature. In addition, nanofluids made with carbon nanotubes are described according to the high heat transfer coefficient. Finally, the introduction of hybrid nanofluid and their comparison with mono nanofluids has been explained by the conclusion that hybrid nanofluids can well cover nanofluid weaknesses and improve its strengths. [ABSTRACT FROM AUTHOR]

We investigate the influence of viscous dissipation on the convective heat transfer and entropy generation characteristics for the Poiseuille flow of a viscoelastic fluid in an asymmetrically heated slit microchannel. We use the simplified Phan –Thein –Tanner constitutive equations to describe the rheology of the fluid. Considering hydrodynamic slip at fluid–solid interface, we analytically solve the transport equations to obtain the velocity and temperature fields inside the flow domain, which are further used to evaluate the Nusselt number and the thermodynamic irreversibility generation rate. The results show that the present thermo-fluidic transport process is strongly influenced by the governing dimensionless parameters viz. , the Brinkman number, heat flux ratio, viscoelastic group and slip coefficient. Finally, this study reveals that the Brinkman number and viscoelastic parameter have reverse effects on the Nusselt number and entropy generation rate as compared to the slip coefficient. [ABSTRACT FROM AUTHOR]

Condensation heat transfer on surfaces can be enhanced by altering the surface topography. Surface modification technology can promote dropwise condensation which can exhibit higher heat transfer rate than filmwise condensation. This paper presents experimental data on the condensation of steam on vertical bare copper tubes and lead coated copper tubes for degree of sub-cooling in the range 0.5 °C ≤ ΔT ≤ 20 °C. The chemical texture of the tube surface was altered by coating with lead of thickness 10 µm and the physical texture of the surface was transformed by providing four grooves each having equal depths 0.10 mm, 0.15 mm and 0.30 mm. The condensation heat transfer characteristics of the tube surface is explained based on contact angle hysteresis and sliding velocity of the droplet. The results of the study reveal that for the tubes tested the average condensation heat transfer coefficient decreases with increase in degree of sub-cooling. It is also found that for copper tubes, providing grooves aids condensation heat transfer for the range of sub-cooling. However, for lead coated copper tube with/without grooves the heat transfer performance at ΔT < 2 °C shows marked difference in contrast to ΔT > 2 °C. [ABSTRACT FROM AUTHOR]

The main objective of the present study is to assess the heat transfer efficiency of Al 2 O 3 -MWCNT/thermal oil hybrid nanofluid over different temperatures (25–50 °C) and solid concentrations (0.125%–1.5%). To this end, first of all, the stability of the nano-oil has been studied through the Zeta potential analysis. Then, the dynamic viscosity and thermal conductivity of the nanofluid have been experimentally investigated. It was found that the nanofluid showed Newtonian behavior over the studied range of temperatures and solid concentrations. The dynamic viscosity showed increasing trend as the solid concentration increased. It is found that the minimum increase in dynamic viscosity is at the temperature of 50 °C in all the studied solid concentrations except 0.5% and 1%. As for the thermal conductivity, it showed increasing trend as the temperature and solid concentration increased. The maximum enhancement was at the temperature of 50 °C and solid concentration 1.5% by approximately 45%. Based on the experimental data, two new highly precise correlations to predict the dynamic viscosity and thermal conductivity of the studied nanofluid have been proposed. Moreover, the heat transfer efficiency of the nanofluid has been evaluated based on different figures of merit. It is revealed that using this nanofluid instead of the base fluid can be beneficial in all the studied solid concentrations and temperatures for both the internal laminar and turbulent flow regimes except the solid concentrations of 1 and 1.5% in internal turbulent flow regimes. The effect of adding nanoparticles on pumping power and convective heat transfer coefficient has also been theoretically investigated. [ABSTRACT FROM AUTHOR]

In this work, heat transfer and pressure drop characteristics of nanofluids flowing through a horizontal circular tube have been investigated experimentally. The test tube was made of stainless steel type 304 with an inner diameter of 7.1 mm. The working fluid was SiO 2 /water nanofluid where the average diameter of nanoparticles was 7 nm. Nanofluids at three different volume concentrations of 0.5, 1, and 2% have been prepared and tested. The experiments have been performed for Reynolds numbers ranging from 3800 to 12000, inlet temperatures of 25, 30, and 35 °C where a constant heat flux was imposed on the tube. The effects of particle volume concentrations, inlet temperature and mass flow rate on convective heat transfer and pressure drop characteristics have been evaluated. The results revealed that with increasing Reynolds number, volume concentration, and inlet temperature the heat transfer coefficient and Nusselt number increased. Moreover, pressure drop increased with increasing volume concentration; conversely, decreased with increasing inlet temperature. The efficiency index reached its maximum quantity (i.e. 1.6) at Reynolds numbers higher than 9000, the volume concentration of 2%, and inlet temperature of 35 °C. On the other hand, the minimum values of efficiency index were obtained for Reynolds numbers less than 7000, the volume fraction of 0.5%, and inlet temperature of 25 °C. Finally, new correlations for predicting the Nusselt number and friction factor of SiO 2 /water turbulent flow have been proposed. [ABSTRACT FROM AUTHOR]

• The alternating cross-section flattened having a small-sized tube is examined. • Two-phase flow inside this tube is the first experimental study. • The effective parameters for thermal performance are discussed. • Alternating cross-section flattened tube shows an outstanding performance. • New correlations for predicting N u and f T P are proposed. This work presents the experimental study of two-phase flow inside an alternating cross-section flattened (ACF) tube. The ACF tube was made from a copper circular tube with an inner diameter of 4.31 mm. We conducted the test under the following experimental conditions: a mass flux of 280, 430, and 580 kg/m2s; a saturation temperature of 40, 45, and 50 °C; a heat flux of 10, 15, and 20 kW/m2; and an average vapor quality of 0.1–0.8. Compared to the existing flow pattern maps, most fell into the annular and semi-annular flow regimes. However, the results were found in the transition and intermittent flow regions at low vapor quality. The results from the circular and ACF tubes were examined and discussed. We found that the ACF tube revealed a prominent condensation heat transfer coefficient and a reasonable pressure loss penalty. The heat transfer coefficient and frictional pressure drop of the ACF tube were higher than that of the circular tube by approximately 25.6–111.1% and 4.3–39.9%, respectively. New correlations for the Nusselt number and two-phase friction factor were proposed. [ABSTRACT FROM AUTHOR]

Researchers in heat transfer field always attempt to find new solutions to optimize the performance of energy devices through heat transfer enhancement. Among various methods which are implemented to reinforce the thermal performance of energy systems, one is the dispersion of solid nanoparticles in common working fluids such as water. The suspension is called nanofluid. On the other hand, utilizing porous media in heat exchangers is another technique to augment of thermal efficiency. Porous media by providing high surface area contact will ameliorate heat transfer rate in ducts. In the present work, a comprehensive review is conducted on the simultaneous application of nanofluids and porous media for heat transfer enhancement purposes in thermal systems with different structures, flow regimes, and boundary conditions. [ABSTRACT FROM AUTHOR]

A miniature loop heat pipe is theoretically modelled and the predicted results are used to compare the thermal performance and entropy generation with the experimental results. The working fluids used are water and two concentrations of graphene–water nanofluid (0.003 Vol.% and 0.006 Vol.%). The analysis shows that the model is in good agreement with experiments with a maximum variation of 6% in evaporator wall temperature. The entropy generation in mLHP due to heat transfer and pressure drop are also estimated from the model and experimental results. The results show that the use of nanofluid diminishes the entropy generation and increases the second law efficiency. The average reduction in total entropy generation with the use of nanofluid is 23.9% and 34.6% for 0.003 Vol.% and 0.006 Vol.% respectively. Similarly the second law efficiency shows an average increase of 19.4% and 37.5% for the same concentrations. The heat transfer is found to be the dominant factor causing the entropy generation in mLHP and the effect of friction on entropy generation is found to be less at very low volume fractions of nanofluid. [ABSTRACT FROM AUTHOR]