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In the present paper, the thermal conductivity of hybrid nanofluids is experimentally investigated. The studied nanofluid was produced using a two-step method by dispersing Cu and TiO 2 nanoparticles with average diameter of 70 and 40 nm in a binary mixture of water/EG (60:40). The properties of this nanofluid were measured in various solid concentrations (0.1, 0.2, 0.4, 0.8, 1, 1.5, and 2%) and temperatures ranging from 30 to 60 °C. Next, two new correlations for predicting the thermal conductivity of studied hybrid nanofluids, in terms of solid concentration and temperature, are proposed that use an artificial neural network (ANN) and are based on experimental data. The results indicate that these two new models have great ability to predict thermal conductivity and show excellent agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation on the thermal cooling of vapor chamber for cooling computer processing unit of the personal computer is performed. Two different configurations of the vapor chambers with de-ionized water as working fluid are tested under the real operating conditions of PCs. Parametric studies including different aspect ratios, fill ratios, and operating conditions of PC on the CPU temperature are considered. It was found that the vapor chamber cooling technique has significant effect on the thermal cooling of CPU. Average CPU temperatures obtained from the vapor chamber cooling system are 4.1%, 6.89% lower than those from the conventional cooling system for no load and 90% operating loads, respectively. In additional, this cooling system requires 6.89%, 10.53% lower energy consumption for no load and 90% operating loads, respectively. The results of this study are of technological importance for the efficient design of cooling systems of the personal computers or electronic devices to enhance cooling performance. [Copyright &y& Elsevier]

Abstract: New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2mm and an acrylic outer tube with an internal hydraulic diameter of 25.4mm. The experiments were performed at heat fluxes between 10 and 35kW/m2, and a refrigerant mass flux between 45 and 155kg/(m2 s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical investigation of the flow characteristics of helical capillary tubes compared with straight capillary tubes. The homogenous two-phase flow model developed is based on the conservation of mass, energy, and momentum of the fluids in the capillary tube. This model is validated by comparing it with the experimental data of both straight and helical capillary tubes. Comparisons of the predicted results between the straight and helical capillary tubes are presented, together with the experimental results for straight capillary tubes obtained by previous researchers. The results show that the refrigerant flowing through the straight capillary tube provides a slightly lower pressure drop than that in the helical capillary tube, which resulted in a total tube length that was longer by about 20%. In addition, for the same tube length, the mass flow rate in the helical capillary tube with a coil diameter of 40mm is 9% less than that in the straight tube. Finally, the results obtained from the present model show reasonable agreement with the experimental data of helical capillary tubes and can also be applied to predict the flow characteristics of straight capillary tubes by changing to straight tube friction factors, for which Churchill''s equation was used in the present study. [Copyright &y& Elsevier]

Abstract: Adiabatic two-phase air–water flow is experimentally studied in this work. Two channels, made of fused silica, with different diameters of 0.53 and 0.15mm are used as test sections. The void fraction data for both tubes are obtained by image analysis. For the larger channel, the void fraction is found to be a linear relationship with the volumetric quality. In the case of the smaller one, however, the non-linear void fraction is obtained. The measured frictional pressure drop data are compared with the predictions regarding the homogeneous flow assumption. Several well-known two-phase viscosity models are subsequently evaluated for applicability to micro-channels. [Copyright &y& Elsevier]

Abstract: This paper presents the effects of various geometries of helical capillary tubes on the flow characteristics of alternative refrigerants flowing through adiabatic helical capillary tubes. The theoretical model is based on the conservation of mass, energy and momentum of fluids in the capillary tube. The two-phase flow model developed was based on a homogenous flow assumption. The model was validated by comparing it with the experimental data of published in literature for R-22, particularly various pairs of refrigerants. It was found conventional refrigerants had lower capillary lengths than alternative refrigerants. For all pairs, the numerical results showed that the traditional refrigerants consistently gave lower pressure drops for both single-phase and two-phase flows, which resulted in longer tube lengths. The results show that coil diameter variation (less than 300mm) for helical capillary tube geometries affected the length of helical capillary tubes. However, pitch variation (more than 300mm) had no significant effect on the length of helical capillary tubes. This adiabatic helical capillary tube model can be used to integrate system models working with alternative refrigerants for design and optimisation. [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: 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 study analyzes several well-known two-phase viscosity models in order to address the appropriate correlations among them for application to micro-channel. Pressure drop data is obtained from adiabatic two-phase air–water flow experiments. A fused silica channel, 320 mm long, with an inside diameter of 0.53 mm is used as the test section. The measured data is compared with the homogeneous flow predictions calculated using the existing viscosity models and detailed comparisons 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: The main concern of the present paper is to simulate the evaporation of defrosted water in domestic refrigerator water trays. The tray is assumed to be located above in vicinity to the compressor head, and part of the heat used to evaporate the water is heat from the compressor. The mathematical model is based on that of Bansal and Xie. In modeling, the basic physical equations are conservations of mass and energy. The model includes the various relevant parameters, e.g., the ambient air temperature, relative humidity of ambient air, initial temperature of defrosted water, area of water tray, initial mass of water in the tray, and characteristic length and area of the compressor shell. The experiments are done in two steps: finding of the correlations between the temperature of the compressor shell and the elapsed time and verifying the simulation results. The results show that the model simulates the experimental data fairly well. The model may be used as a tool to determine a suitable configuration for the evaporation of defrosted water collected in domestic refrigerator water trays. [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]

New experimental data on plate heat exchanger performance including the heat transfer coefficient, pressure drop, and thermal performance factor under different chevron angles, surface roughness, and working conditions are presented. Plate surface roughness ranging between 0.95 μm and 2.75 μm with chevron angles of 30° and 60° are used. The experiments are performed at Reynolds numbers ranging between 1200 and 3500, a hot water temperature of 40 °C, and a cold water temperature of 25 °C. The experimental results show that the heat transfer coefficient and pressure drop increase when the chevron angle is decreased and the surface roughness and Reynolds number are increased. Under the testing conditions, the average thermal performance factors are 1.09 and 1.02 for 30° and 60° chevron angles, respectively. The optimum thermal performance of a plate heat exchanger is obtained at a 30° chevron angle, the highest surface roughness, and the lowest Reynolds number. A correlation of Nusselt number and friction factor for different surface roughness and chevron angles are also proposed for practical applications. [ABSTRACT FROM AUTHOR]

This paper presents an investigation on the thermal conductivity of nanofluids using experimental data, neural networks, and correlation for modeling thermal conductivity. The thermal conductivity of Mg(OH) 2 nanoparticles with mean diameter of 10 nm dispersed in ethylene glycol was determined by using a KD2-pro thermal analyzer. Based on the experimental data at different solid volume fractions and temperatures, an experimental correlation is proposed in terms of volume fraction and temperature. Then, the model of relative thermal conductivity as a function of volume fraction and temperature was developed via neural network based on the measured data. A network with two hidden layers and 5 neurons in each layer has the lowest error and highest fitting coefficient. By comparing the performance of the neural network model and the correlation derived from empirical data, it was revealed that the neural network can more accurately predict the Mg(OH) 2 –EG nanofluids' thermal conductivity. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the single phase pressure drop characteristics of smooth and microfin tubes are investigated experimentally. The horizontal test section is a counter flow double tube heat exchanger with water flowing in the inner tube and cooling water flowing in the annulus. By means of experimental setup, required temperature and pressure measurements are recorded and friction factor coefficient and pressure drop of smooth and microfin tubes are determined. Experiments are conducted for mass flow rates in the range between 0.023kg/s and 0.100kg/s and effect of Reynolds number on pressure drop is investigated. By using experimental results, Blasius type friction factor equations are developed for both smooth and microfin tubes. Experimental results for both smooth and microfin tubes are compared with correlations given in the literature. [Copyright &y& Elsevier]

Abstract: Correlations for the evaporation heat transfer coefficient and two-phase friction factor of R-134a flowing through horizontal corrugated tubes are proposed. In the present study, the test section is a horizontal counter-flow concentric tube-in-tube heat exchanger with R-134a flowing in the inner tube and hot water flowing in the annulus. Smooth tube and corrugated tubes with inner diameters of 8.7mm and lengths of 2000mm are used as the inner tube. The corrugation pitches are 5.08, 6.35, and 8.46mm and the corrugation depths are 1, 1.25, and 1.5mm, respectively. The outer tube is made from smooth copper tube with an inner diameter of 21.2mm. The correlations presented are formed by using approximately 200 data points for five different corrugated tube geometries and are then proposed in terms of Nusselt number, equivalent Reynolds number, Prandtl number, corrugation pitch and depth, and inside diameter. [Copyright &y& Elsevier]

Abstract: Fluid flow and heat transfer in the mini-rectangular fin heat sink for CPU of PC using de-ionized water as working fluid are numerically investigated. Based on the real PC operating conditions, the three-dimensional governing equations for fluid flow and heat transfer characteristics are solved using finite volume scheme. The standard k–ε turbulent model is employed to describe the flow structure and behavior. The predicted results obtained from the model are verified by the measured data. There is a reasonable agreement between the predicted results and experiments. The results of this study are expected to lead to guidelines that will allow the design of the cooling system with improved cooling performance of the electronic equipments increasing reliable operation of these devices. [Copyright &y& Elsevier]

Turbulent heat transfer characteristics of Graphite-SiO 2 /Water hybrid nanofluid flow in a horizontal smooth tube with and without quad-channel twisted tape (QCTT) inserts is investigated experimentally. The hybrid nanofluid is obtained using two different nanoparticles: Siliciumdioxid (60%) and Graphite (40%) with pure water as base fluid. Experiments are conducted for two different volume concentrations, 0.5% and 1%, respectively. The length of quad-channel twist tape inserts are between 0 and 42 cm with constant twist ratios of 5. The Reynolds number is varied from 3400 to 11,000. According to the results, Nusselt number of the case with hybrid nanofluid increased with increasing mass flow rate and volume concentration. Also, heat transfer coefficient increased with increasing length of twisted tape insert. Moreover, friction factor increases with increasing volume concentration for increasing Reynolds number whereas and friction factor increases with increasing the length of tape inserts. Pressure drop increases with increasing mass flow rate and increasing volume concentration. Finally, the regression equations are found to be well-matched with the experimental data within the deviation band of ±5% for Nusselt number and ± 10% for friction factor, respectively. [ABSTRACT FROM AUTHOR]

Abstract This experimental study includes measurement of thermal conductivity of distilled water-based CNT-SiO 2 hybrid nanofluids. Nanofluids were prepared by using two-step method, 3 different concentrations and 4 different mass range of CNT-SiO 2. SiO 2 has 2200 kg m−3 density, 1.4 W m−1 K−1 thermal conductivity and 7 nm average particle size. CNT has 2620 kg m−3 density, 25 W m−1 K−1 thermal conductivity and 6–10 nm average particle size. Samples were placed in ultrasonic homogenizer maximum power capacity for 3 h. Throughout sonication process temperature of nanofluids have been kept under control in order not to chance volumetric fraction of nanofluids. All measurements of thermal conductivity were done by using thermal conductivity meter. Thermal conductivity meter was calibrated by di-water. Measurements of thermal conductivity was done range from 25 °C to 60 °C for every 5 °C. Validation of measurements had been performed by using di-water and shown in a thermal conductivity-temperature figure. Minimum and maximum thermal conductivity enhancements were revealed in detail. Alteration of the thermal conductivity with temperature according to various volumetric fractions were in literature rated and it is found that the thermal conductivity increases with temperature and vol. fraction clearly. Enhancement on the thermal conductivity to di-water were also depicted for various temperatures and vol. fraction in figures. Almost well-known correlations in the literature were given with their predictable rates. Moreover, comparisons with other studies were provided in this present study. A practical correlation was proposed for other researchers. [ABSTRACT FROM AUTHOR]

Abstract The present paper is an overview of the latest developments regarding the application of nanofluids in boiling critical heat flux by means of both pool- and convective-flow boiling. Boiling heat transfer is a significant field in thermal engineering systems, and it is especially used for boiling in power plants as well as the cooling of nuclear reactors and high-tech electronic systems. This concise review contains efforts to show how nanofluids could play an essential role in achieving high heat flux with small temperature differences during the boiling process—which, in turn, improves the critical heat flux (boiling crisis) for such an operation and makes the heat-exchange system's performance safer and more durable. It is also expected that this work could be a helpful new reference guide that will allow investigators to update their knowledge on the topic of boiling critical heat flux using nanofluids. In addition, this work contains concise recommendations for future study directions. [ABSTRACT FROM AUTHOR]

Abstract The hybrid nanofluids were obtained by dispersing Silicon dioxide and Graphite nanoparticles in distilled water. Viscosity is regarded as internal resistant of a fluid, and it affects the pumping power and the convective heat transfer coefficient. This hybridization requires to be experimentally determined since its properties are unknown. For viable real life applications viscosity and density parameters are significant, owing to their influence on energy and cost saving of heat exchangers by affecting pressure drops. The viscosity measurements of hybrid nanofluids were performed using capillary tube viscometer, within a temperature range of 15 °C to 60 °C at several volume concentrations up to 2%. Furthermore, the volumetric fraction ratios within nano composites have also been changed, since there is more than one nano particle. The highest increment of viscosity was observed to be 36.12% in the nanofluid of 2 vol% at 15 °C. The conclusion was supported with SEM images and a regression correlation was proposed and afterward the results were compared with relevant literature. It is able to estimate the viscosity of hybrid nanofluid with 2.4% accuracy. [ABSTRACT FROM AUTHOR]

Water-based graphene quantum dots (GQD) suspension has great potential for different heat transfer applications as a novel coolant due to their unique colloidal stability, high thermal conductivity and low penalty for rheological properties once loading GQD. To this end, graphene quantum dots were firstly prepared through a new and cost-effective exfoliation procedure. Based on the morphological characterization, the average thickness and diameter of the synthesized amine treated-GQD (AGQD) were determined as mostly less than 1 nm and in the range of 5–20 nm, respectively. Case studies show that water-based AGQD nanofluid at very low weight fractions shows a considerably higher thermal conductivity than that of base fluid. In a detailed rheological investigation of the water-based AGQD nanofluid, no noteworthy increase was observed in comparison with the base fluid, which is considered as a major benefit for this novel generation of coolants. The water-based AGQD nanofluids were also found to be especially more effective in the thermosyphon in terms of overall thermal properties such as net heat transfer, and thermal efficiency, and rheological property such as effective viscosity, as well as, total pressure drop in comparison to the distilled water. Since the water-based AGQD nanofluids show no sedimentation, high thermal conductivity and fairly no effect on rheological properties, it would provide an economical approach for enhancing the performance of industrial heat pipes and thermosyphons. [ABSTRACT FROM AUTHOR]

The present study reports on an experimental evaluation of flow boiling of R134a inside a multi-microchannel heat sink. The copper test section consisted of 27 parallel rectangular channels with 0.470 mm depth, 0.382 mm width, 0.416 mm fin thickness and 40 mm length. The experiments were performed at saturation temperatures of 18, 23 and 28 °C, mass fluxes of 800, 1000 and 1200 kg m − 2 s − 1 and constant vapor quality at the heat sink inlet at 0.05. The wall heat flux was increased from 50 kW m − 2 until reaching critical heat flux, and maximum value of 460 kW m − 2 was reached. With the collected data from the experiments, the effects of mass flux, heat flux, saturation temperature and steam quality on the heat transfer coefficient are emphasized and the reasons are discussed. At low vapor quality, heat flux plays a major role in increasing the heat transfer coefficient, while the effect of mass flux is negligible. With rising heat flux, convective boiling begins to control the heat transfer mechanism, and the heat transfer coefficient increases with rising vapor quality and mass flux, until dry-out point. In all conditions, high heat transfer coefficients are obtained for high saturation temperatures. In open literature, the correlation generated by Mortada et al. for heat transfer coefficient is the one that makes the most accurate prediction for current study. Based on the experimental results, a correlation is proposed to calculate the heat transfer coefficients for R134a flow boiling inside the multi-microchannel at high mass and heat flux predicting the measured ones better than those in the literature. [ABSTRACT FROM AUTHOR]

In order to avoid the costs of experimental evaluations, soft computing methods like artificial neural network (ANN) and genetic algorithm have remarkably grabbed the attentions of investigators for predicting the hydrothermal characteristics of different types of nanofluids. In this paper, the implementation of ANN and genetic algorithm for modeling and multi-criteria optimizing the hydrothermal behavior of SiO 2 /water nanofluid has been investigated. Using the data obtained from the experimental analysis, an ANN model is developed to estimate the pressure drop and Nusselt number as a function of volume concentration, Reynolds number, and inlet temperature. Different network structures were assessed and it has been achieved that a network with 2 hidden layers and 6 neurons in every layer provides the most accurate prediction. It is revealed that the developed network is satisfactorily accurate to determine the Nusselt number and pressure drop of SiO 2 /water nanofluid compared to the empirical correlations. To optimize the hydrothermal behavior of the nanofluid (i.e. to find the optimal cases with highest Nusselt number and the relatively least pressure drop), the genetic algorithm coupled with compromise programming approach has been implemented considering decision maker's attitude. [ABSTRACT FROM AUTHOR]

This experimental study aims to investigate the heat transfer characteristics of single-phase turbulent flow of R-134a refrigerant in a rectangular multi-micro channel heat sink having 27 channels where each channel has a hydraulic diameter of 421 μm. Experimental results were obtained for inlet temperatures ranging from 24 to 33 °C, mass fluxes from 1485 to 2784 kg m − 2 s − 1 and wall heat fluxes from 3 to 24 kW m − 2 . The results indicate that the heat transfer coefficients are found to be higher at lower inlet temperatures than those at higher ones. In addition, when equal amount of heat supplied to the heat sink, the heat transfer coefficients increase with increasing the mass flux of refrigerant. They were also compared with 12 well-known correlations and it was seen that 4 of 12 were in good agreement with each other with the average deviation < 10%. The findings demonstrate that well-known correlations in fundamental sources can be used to predict the heat transfer coefficient of R-134a during its single phase flow in a multiport microchannel heat sink under turbulent regime. [ABSTRACT FROM AUTHOR]

In the present paper, in order to understand the accuracy of 38 different correlations derived by various researchers in this field, the correlations were executed for condensation frictional pressure drop. To accomplish this goal, experimental data provided from authors' previous publications encompassing 412 points for two smooth tubes, and 334 points for five corrugated tubes, have been utilized so as to compare the determined results. The experimental setup is composed of a 2.5 m double tube for horizontal configuration and smooth and corrugated tubes at the inner diameters of 0.0081 m, while the applied mass flux range spans between 709 and 1974 kg m − 2 s − 1 . The average quality of vapor and saturation pressure ranges lie within 0.09 to 0.97, and 10 to 13 bar, respectively. Determining the frictional pressure drop in two-phase flows does not involve corrugated tube geometry in the calculation of friction factor, to make this available, a slight alteration that requires the replacement of a correlation with another one in the literature was suggested with regard to friction factor approach. As a result of this, it was noticed that performances of some correlations were optimized to predict the frictional pressure drop in corrugated tubes. Additionally, the most effective correlations have been selected for the horizontal double pipe heat exchanger having smooth and corrugated tubes. Finally, alteration of the condensation pressure drop with Reynolds number are presented using both experimental data and best predictive correlations. [ABSTRACT FROM AUTHOR]

Frost formation and growth is quite important physical phenomena in evaporators since frost can directly affect both capacity and fan performance. In this study, an in-house-code was programmed to determine frost ratio on evaporator surfaces via an image processing technique. Air inlet temperatures along with pressure drop across the evaporators were monitored and both temperature and pressure breaking points were identified for evaporators with flat, hydrophilic flat and corrugated fins. Evaporator capacities and pressure drops across the evaporators were experimentally measured and compared with theoretical findings. It was revealed that the evaporator with hydrophilic fin was the most suitable selection when entire measured and calculated parameters were considered altogether. [ABSTRACT FROM AUTHOR]

The present paper deals with the artificial neural network modeling (ANN) of heat transfer coefficient and Nusselt number in TiO 2 /water nanofluid flow in a microchannel heat sink. The microchannel comprises of 40 channels; each channel has a length of 4 cm, a width of 500 μm, and a height of 800 μm. In the ANN modeling of heat transfer coefficient and Nusselt number 23 and 72 datasets have been used, respectively. The experimental Nusselt number has been calculated based on three different thermal conductivity models, four volume fractions of 0, 0.5, 1, and 2%, two values of Reynolds number i.e. 400 and 1200 and three different heating rates including 50.6, 60.7, and 69.1 W. Therefore, the inputs that are introduced to the neural network are volume fraction of nanoparticles, Reynolds number, heating rate, and model number while the output of network is the Nusselt number. It is elucidated that an appropriately trained network can act as a good alternative for costly and time-consuming experiments on the nanofluid flow in microchannels. The average relative errors in the prediction of Nusselt number and heat transfer coefficients were 0.3% and 0.2%, respectively. [ABSTRACT FROM AUTHOR]

Owing to the generalization problem, there aren't sufficient empirical correlations for two-phase flows. So as to investigate the thermal features of the two-phase flow in smooth and enhanced tubes, a suitable procedure of the models and correlations related with the heat transfer coefficients, friction factors and two-phase multipliers are needed because a significant variation in thermal properties happens during phase-change. Comparison of frictional pressure drop of R134a during flow boiling phenomena occurred in a smooth and 5 enhanced tubes with well-known empirical correlations were performed in this study. The apparatus has 0.85 m long double tube for vertical configuration as a test section that includes smooth and corrugated copper tubing having inner diameters of 0.0087 m, and the range of mass fluxes are between 200 and 400 kg m − 2 s − 1 . The average vapor qualities vary from 0.14 to 0.86, and saturation pressure interval is between 4.5 and 5.7 bar. The mean boiling heat transfer coefficient of R134a is determined via energy balance in the test section. The estimation performance of 36 empirical correlations in literature proposed for convective boiling flows in smooth and corrugated tubes are evaluated by means of authors' database (350 data points for vertical tubes). Boiling trend lines have been plotted for the change of vapor quality, liquid phase Reynolds numbers with gas phase ones. In addition, the most successful correlations are confirmed their predictabilities for the vertical adjusted evaporator having smooth and corrugated tubes using the database of authors' earlier publications in open sources. [ABSTRACT FROM AUTHOR]

The study of pulp suspension flow is one of the most significant scientific interests since the considerable changes in the flow behavior of water were noticed after adding a small amount of fiber to water. The rheological studies on Kenaf pulp (a fiber crop) suspensions revealed that concentration has a great influence on variations of viscosity and yielded stress values. Heat transfer and pressure loss data were obtained from Kenaf suspension flow at different concentrations over a wide velocity range in a straight pipe. Kenaf suspension at the concentration of 0.6 wt.% showed maximum drag reduction. A considerable increase in the heat transfer coefficient of Kenaf suspension (0.2 wt.%) was also observed. Such an increment in the heat transfer coefficient ratio of Kenaf suspension flow is interesting since the enhancement of heat transfer has mostly been reported for suspensions containing nanoparticles and not for natural fibers in microscale. Moreover, the effects of fiber length and flexibility on momentum and heat transfer data were studied. [ABSTRACT FROM AUTHOR]

Nitrogen-doped graphene (NDG) nanofluids are prepared using a two-step method in an aqueous solution of 0.025 wt.% Triton X-100 as a surfactant with various nanosheets at several concentrations (0.01, 0.02, 0.04, 0.06 wt.%). This paper reports results of experiments on thermal conductivity, specific heat capacity, and viscosity of the NDG nanofluids, as well as their convective heat transfer behavior flowing in a double-pipe heat exchanger. To assess the thermal properties, we used various water-based nanofluids as coolants to analyze the total heat transfer coefficient, convective heat transfer coefficient, the percentage of wall temperature reduction, pressure drop, and pumping power in a counter-flow double-pipe heat exchanger. A novel MATLAB code carried out the calculations for Reynolds numbers between 5000 and 15,000 (turbulent flow) and nanosheet weight percentages between 0.00% and 0.06%. An increase in Reynolds number or the percentage of nanomaterial could perhaps enhance the heat transfer of the working fluid. As an example, using 0.06 wt.% nanomaterial in the base fluid led to 15.86% enhancement of the convective heat transfer coefficient in comparison with water. Nonetheless, the penalty in terms of the rise in the pumping power was rather small. For a particular material, increasing Reynolds number or nanomaterial weight percentage would augment pumping power. Power consumption, heat removal, and heat transfer rate were greater for nanofluids than for water in all investigated cases, for a particular pumping power. The average increase in heat transfer coefficient was nearly 16.2%. As a result, choosing NDG/water as the working fluid can improve the performance of double-pipe heat exchangers. [ABSTRACT FROM AUTHOR]

In this paper, at first, a new correlation was proposed to predict the relative viscosity of MWCNTs-SiO 2 /AE40 nano-lubricant using experimental data. Then, considering minimum prediction error, an optimal artificial neural network was designed to predict the relative viscosity of the nano-lubricant. Forty-eight experimental data were used to feed the model. The data set was derived to training, validation and test sets which contained 70%, 15% and 15% of data points, respectively. The correlation outputs showed that there is a deviation margin of 4%. The results obtained from optimal artificial neural network presented a deviation margin of 1.5%. It can be found from comparisons that the optimal artificial neural network model is more accurate compared to empirical correlation. [ABSTRACT FROM AUTHOR]

This paper includes an examination of the dynamic viscosity of single-wall carbon nanotubes (SWCNTs) in ethylene glycol (EG) at temperatures ranging from 30 °C to 60 °C for various suspensions at solid volume fractions of 0.0125%, 0.025%, 0.05%, 0.075%, and 0.1%. Experimental findings revealed that SWCNTs/EG nanofluid behaves as a Newtonian fluid at solid volume fractions of 0.1% and at all considered temperatures. The measurements also indicated that dynamic viscosity increases with increasing solid volume fraction and decreases with increasing temperature. Moreover, relative viscosity results showed that that the viscosity of the nanofluid increases to 3.18 times that of the base fluid at a temperature of 30 °C and a solid volume fraction of 0.1%. Finally, using experimental data to estimate the dynamic viscosity of SWCNTs/EG nanofluid, a new correlation with acceptable accuracy was suggested. [ABSTRACT FROM AUTHOR]

During the past decade, nanotechnology with its rapid development has grabbed the attention of scientists, scholars, and engineers. Nanofluids are one of the surprising outcomes of this technology that could increase the efficiency of thermal systems remarkably. Nanofluids containing solid nanoparticles have a higher viscosity than common working fluids; hence, measuring the viscosity is necessary for designing thermal systems and estimating the required pumping power. In the current review study, an attempt has been made to cover the latest experimental studies performed on the viscosity of nanofluids. An experimental investigation is very vital for the analysis since the theoretical models usually underestimate the nanofluid viscosity. Through experiments, the real effects of volume fraction, temperature, particle size, and shape on the viscosity of nanofluids will be determined. [ABSTRACT FROM AUTHOR]

The pressure-drop characteristics during flow boiling in a single rectangular micro-channel with hydraulic diameter of 0.68 mm are presented. In the present study, pressure drop was studied at heat flux range of 7.63–49.46 kW/m 2 , mass flux range of 600–1400 kg/m 2 s, and saturation temperature of 23, 27 and 31 °C. Experimental results indicated that the total pressure was dominated by frictional pressure drop. The increase of mass flux also increased the frictional pressure gradient, whereas the increase of saturation temperature reduced the frictional pressure gradient. In addition, heat flux also had an insignificant effect on the frictional the pressure gradient. A new correlation was also proposed for effective design of micro-channel heat exchanger. [ABSTRACT FROM AUTHOR]

The present paper deals with the design, development and the measurement of thermal conductivity of graphene–water nanofluid using a transient hot wire technique at temperatures below and above ambient conditions ranging from 10 °C to 50 °C. The equipment is designed to measure the thermal conductivity using a single platinum wire of diameter 50 μm and 100 mm length. The platinum micro-wire acts both as a temperature sensor and heating element. Low volume concentrations (0.05, 0.1 and 0.15%) of graphene, having the size less than 100 nm, dispersed in 100 ml of water with SDBS (sodium dodecyl benzene sulfonate) as surfactant, for prolonged stability, is used in the present study. The results showed an enhancement in the thermal conductivity of 37.2% for 0.15% volume concentration of graphene at 50 °C when compared with that of the water at the same temperature. An interesting observation from this study is that the average thermal conductivity enhancement percentage with the increase in volume concentration (say from 0.05% to 0.15%) is found to be 3.3% higher when compared with that of the average enhancement with the increase in temperature from 10 °C to 50 °C. [ABSTRACT FROM AUTHOR]

In the present work, forced convective heat transfer of water/functionalized multi-walled carbon nanotube (FMWCNT) nanofluid in a two-dimensional microchannel is investigated. To solve the governing Navier–Stokes equations and discritization of the solution domain, the numerical method of finite volume and SIMPLE algorithm have been employed. Walls of the microchannel are under a periodic heat flux, and slip boundary conditions along the walls have been considered. Effect of different values of shear forces, solid nanoparticles concentration, slip coefficient, and periodic heat flux on the flow and temperature fields as well as heat transfer rate has been evaluated. In this study, changes of the variables considered to be from 1 to 100 for Reynolds number, 0–25% for weight percentage of solid nanoparticles, and 0.001–0.1 for velocity slip coefficient. Results of the current work showed good agreement with the numerical and experimental studies of other researchers. Data are presented in the form of velocity and temperature profiles, streamlines, and temperature contours as well as amounts of slip velocity and Nusselt number. Results show that local Nusselt number along the length of microchannel changes in a periodic manner and increases with the increase in Reynold number. It is also noted that rise in slip coefficient and weight percentage of nanoparticles leads to increase in Nusselt number, which is greater in higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO 2 /ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO 2 nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles. [ABSTRACT FROM AUTHOR]

In the present study, the thermal conductivity of CuO/EG–water nanofluid in different solid concentrations and temperatures has been experimentally investigated. Using a two-step method, the nanofluid has been produced in different solid concentrations ranging from 0.1% to 2% and temperatures up to 50 °C. The thermal conductivity of the nanofluid has been experimentally measured using the KD2 Pro instrument. Based on the experimental data, new correlations for predicting the thermal conductivity of CuO/EG–water at different temperatures have been proposed. The results show that with the increase of the solid concentration, the thermal conductivity of the nanofluid increases. Furthermore, the thermal conductivity of the nanofluid increases while the temperature increases. This increase is by far more noticeable in higher solid concentrations compared with lower solid volume fraction. This means that it is the presence of nanoparticles in the base fluid that causes the increase of the effect of temperature on the thermal conductivity. [ABSTRACT FROM AUTHOR]

In the present study, the thermal conductivity of the ZnO-EG nanofluid has been investigated experimentally. For this purpose, zinc oxide nanoparticles with nominal diameters of 18 nm have been dispersed in ethylene glychol at different volume fractions (0.000625, 0.00125, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, and 0.05) and temperatures (24–50 °C). The two-step method is used to disperse nanoparticles in the base fluid. Based on the experimental data, an experimental model has been proposed as a function of solid concentration and temperature. Then, the feedforward multilayer perceptron neural network has been employed for modeling thermal conductivity of ZnO-EG nanofluid. Out of 40 measured data obtained from experiments, 28 data were selected for network training, while the remaining 12 data were used for network testing and validating. The results indicate that both model and ANN outputs are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

In the continuous casting process, natural convection occurs in mold containing a liquid metal. Natural convection in the melt causes the impurities to move and this phenomenon can lead to poor product. Therefore, by reducing natural convection, the quality of the product is improved. In this paper, 3D numerical simulation and multi-objective optimization of natural convection in a cylindrical annulus mold filled with molten potassium under a magnetic field is carried out. The inner and outer cylinders are maintained at uniform temperatures and other walls are thermally insulated. Two objective functions including the natural convection heat transfer rate (average Nusselt number) and magnetic field strength have been considered simultaneously. The multi-objective particle swarm optimization algorithm (MOPSO) has been employed. Four decision variables are the Hartmann number, inclination angle, and magnetic field angles. For the optimization process, the calculations of three-dimensional Navier–Stokes, energy, and electrical potential equations are combined with MOPSO. Using the numerically evaluated objective functions, the optimum frontier is estimated by a second order polynomial based on objective functions. [ABSTRACT FROM AUTHOR]

In this paper, the efficiency of Fe/ethylene glycol nanofluids as coolants is investigated in both laminar and turbulent flow regimes. The effects of particle size, temperature and concentration of nanofluids on the efficiency are examined. To determine the efficiency of nanofluids, the thermal conductivity and viscosity of the nanofluids with concentrations up to 3% in the temperature range of 26–55 °C are measured. Three different average particle sizes including 40 nm, 70 nm, and 100 nm are utilized. The results show that, in laminar flow, the nanofluids with low concentrations and containing particles with big size are not useful. For the turbulent flow, the efficiency analysis reveals that the efficiency of nanofluids increases with an increase in the temperature and solid volume fraction. The nanoparticles with the average size of 70 nm are found to be the best option in the turbulent flow and at the concentration of 3%, while in lower concentrations, the nanoparticles with the average size of 40 nm provide the maximum efficiency. [ABSTRACT FROM AUTHOR]

This paper is a continuation of the authors' previous work on the thermophysical properties, heat transfer, and pressure drop of nanofluids [Experimental Thermal and Fluid Science 52 (2014) 68–78]. In this paper, an experimental study is carried out to study the turbulent flow of COOH-functionalized multi-walled carbon nanotubes/water nanofluid flowing through a double tube heat exchanger. For this purpose, first, the thermophysical properties of the nanofluid, including the thermal conductivity and dynamic viscosity, have been measured at various temperatures and concentrations. Using the measured data, new correlations as a function of temperature and concentration are presented to predict the thermophysical properties. In the next step, the effects of low volume fractions of the nanofluid (from 0.05% to 1%) on the heat transfer rate are studied at the Reynolds numbers between 5000 and 27,000. The experimental results reveal that with increasing the nanofluid concentration, the heat transfer coefficient and thermal performance factor increase. On average, a 78% increase in heat transfer coefficient, a 36.5% increase in the average Nusselt number, and a 27.3% penalty in the pressure drop are recorded for the highest concentration of MWCNTs in water. [ABSTRACT FROM AUTHOR]

An experimental investigation has been carried out to compare the enhancement in the thermal performance of sintered and mesh wick heat pipes by varying the working fluid, inclination angle and heat input. Similar geometrical specifications of 12, 330 and 1 mm respectively are selected for the outer diameter, length and wick thickness and kept constant for both sintered and mesh wick heat pipes. The study focuses on changes in surface temperature distribution, thermal resistance and effective thermal conductivity of heat pipes. The results showed that the maximum reduction in surface temperature is obtained for sintered wick heat pipe at 45° tilt angle and 60° for mesh wick heat pipe with CuO/DI water nanofluid concentration at 1.0 wt.% for both the cases. The reduction in thermal resistance of sintered wick heat pipe is 13.92% higher compared with mesh wick heat pipe for the same heat input, mass concentration and inclination angle. Presence of CuO nanoparticles in DI water and increasing heat input tremendously increases the thermal conductivity of heat pipes. An important observation from this study is the sole effect of sintered wick in heat pipe not only reduces the thermal resistance but also increases the heat transport capacity up to 20 W compared with that of mesh wick. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation, a numerical model having two-dimensional equations was obtained by a CFD program and authors' experimental data were evaluated for the verification procedure of the numerical outputs. The experimental case study includes the single-phase flow of pure water in plain and micro-fin pipes whereas the numerical one has the simulated results of TiO2 particles suspended in single phase water flow in equivalent pipes at a constant heat flux. Hydrodynamics and thermal behaviors of the water–TiO2 flow were calculated by constant heat flux and temperature-dependent settings. Physical specifications of nanofluids were calculated by means of the results of authors' previous ANN analyses. This study illustrates local and average values of temperature, pressure, and velocity distributions in the tested pipes; furthermore, comparisons of pressure drop characteristics are given in terms of nanoparticle concentrations and tube types. [Copyright &y& Elsevier]

Abstract: In this paper, the performance of a microchannel heat sink using TiO2/water nanofluid is experimentally investigated. The dimensions of the microchannel are 500μm width, 800μm height, and 40mm length, where the number of flowing channels is 40.The effects of uncertainties in thermophysical properties on the Nusselt number and friction factor are investigated by using three different sets of thermophysical models, which are based on experimental and theoretical relations. It is concluded that the use of the model which is based on experimental data is very important to estimate the friction factor, while the use of different models to calculate of thermal conductivity has no considerable effect on the prediction of Nusselt number. [Copyright &y& Elsevier]

Abstract: An analytical solution is presented on the entropy generation due to mixed convection between two isothermal cylinders where a transverse magnetic field is applied to the system. The governing equations in cylindrical coordinates are simplified and solved to obtain the distribution of entropy generation and the effects of MHD flow on it. The results for the entropy generation number (NS ), the Bejan number (Be) and average entropy generation number (NS,ave ) are presented for different values of the Hartmann numbers, radius ratios and a flow parameter, Gr/Re. The results show that the entropy generation decreases with increases in the magnetic field. In addition, it is found that with decreases in the radius ratio, the effects of MHD flow on the entropy generation are reduced. [Copyright &y& Elsevier]