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

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

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]

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]

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]

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]

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]

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]

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]

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

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

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