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The paper reports an experimental investigation into the thermal performance and pressure drop characteristics of nanofluid-cooled heat sinks and then compares the results with the data for water-cooled heat sinks. Heat sinks with miniature circular-fin (MCFHS) and square-fin (MSFHS) structures are used and made from aluminum material with dimensions of 28 × 33 mm. Similarly, SiO 2 nanoparticles dispersed in deionized water with particle concentrations of 0.2%, 0.4%, and 0.6% volume are used as working fluids. The effects of pin fin configuration, particle concentration, and flow rate on the heat transfer performance and flow behaviors are presented. Reynolds numbers based on the hydraulic diameter of each flow channel ranging between 700 and 3700, fluid temperature of 15 °C, and heat flux ranging from 2 and 5 W/cm 2 are tested. Hydraulic diameters based on each flow channel are equally designed at 1.2 mm for both heat sinks. The experimental results indicate that the heat transfer coefficient increased with increasing Reynolds numbers and particle concentrations. The MCFHS gave greater heat transfer performance than that of the MSFHS by about 6–9%. For pressure drop data, the measured data showed that the pin fin configuration and particle concentration had small effects on the pressure drop and pumping power. [ABSTRACT FROM AUTHOR]

Experimental data regarding heat transfer coefficient and pressure drop of water flow inside a plate heat exchanger with a rough surface is investigated and compared with that obtained from a smooth surface. Three commercial stainless steel corrugated plates with symmetrical chevron angle of 25° are used. The water flow inside the plate heat exchanger is arranged for a single pass and counter flow. The test runs are performed at Reynolds numbers ranging between 1300 and 3200 and plate surface roughness ranging between 0.936 μm and 3.312 μm. The experimental results show that increase in surface roughness yields an increase in heat transfer coefficient between 4.46% and 17.95% and an increase in pressure drop between 3.90% and 19.24% with respect to a smooth surface. The correlations for predicting the Nusselt number and friction factor of water flow inside a plate heat exchanger with a rough surface are proposed. [ABSTRACT FROM AUTHOR]

In this paper, flow patterns and heat transfer characteristics of R134a refrigerant during flow boiling in a single rectangular micro-channel with 0.68 mm hydraulic diameter are investigated. In this experiment, heat transfer coefficients were studied at a heat flux range of 7.63–49.46 kW/m 2 , mass flux range of 600–1400 kg/m 2 s, and saturation temperature range of 23–31 °C. Results show that there are six different flow patterns occurring during boiling of R134a refrigerants: bubbly flow, bubbly-slug flow, slug flow, throat-annular flow, churn flow, and annular flow. It is also found that flow pattern has a significant relation to heat transfer coefficients. In low heat flux range, variation of mass flux has no significant effect on the heat transfer coefficient. It corresponds to the nucleate boiling heat transfer mechanism in which its flow patterns are bubbly flow, bubbly-slug flow, slug flow, throat-annular flow, and churn flow. The heat transfer coefficient at higher saturation temperatures is greater than that of lower saturation temperatures. In high heat flux ranges, the heat transfer coefficient increases with increasing mass flux. It corresponds to the convective boiling heat transfer mechanism in which its flow pattern is annular flow. Increase of saturation temperature causes the heat transfer coefficient to decrease. At very high saturation temperature, a partial dry-out occurs that results in a lower heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Cooling systems are needed for electronic devices in order to operate efficiently. Whereas the size of the equipment has decreased to the micro scale, research on the heat transfer characteristics in a microchannel heat sink is needed. In this work, we suggest that the segmented air–water flow can enhance the heat transfer rate in a microchannel heat sink as compared to using single-phase water cooling. The experiment was conducted with two-phase air–water flow in a single rectangular microchannel with a hydraulic diameter of 267 μm. The test section was made from copper. For a clear understanding, two-phase flow pattern, pressure drop, and heat transfer characteristics in the low Reynolds number of air–water flow were identified. The results show that segmented, throat-annular, throat-annular/liquid, and annular flow were observed within the test section. A flow pattern map was created and compared with the previous maps. The pressure drop can be predicted by the homogeneous flow model and the Friedel correlation separated flow model. The Nusselt number of segmented flow increases up to 1.2 times over the single-phase flow. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effect of the free rotation of a swirl device is investigated. [•] Five swirl generators are installed equally distant along the test section. [•] Data of fixed turbine and freely rotating turbine are compared. [•] The performance for rotating turbine is 6.3% higher than that for fixed turbine. [•] The tube with RTSG inserts gave the lowest pressure drop. [Copyright &y& Elsevier]

Abstract: Heat transfer enhancement and flow behaviors in horizontal circular tubes installed with five rotating turbine-type swirl generators (RTSG) are presented experimentally. This type of RTSG is an innovative design that has not been seen in the previous research. Deionized water is used as a working fluid and flows through the test section under turbulent flow conditions. A smooth, stainless steel tube 9.2mm in ID, 0.4mm in thickness, and 2.3m in length is used as the test section. RTSG made from aluminum, with a twisted angle of 60° and a length of 2cm, are located at 0.46m equally distant along the test section. When fluid flows through the RTSG, continuously free rotations are observed, and swirl flow after RTSG is created. A DC power supply is used to supply heat load to the test section. Compared with a common, plain tube (without rotating swirl devices), the experimental results indicate that the local heat transfer coefficient of the tube with RTSG inserts is higher than that of the common, plain tube, and they increase when the working fluid flows through the RTSG. Similarly, the wall temperatures of the tube with RTSG inserts are lower than those of the common tube. However, the pressure drop of the tube fitted with RTSG is significantly greater than that of the common, plain tube. [Copyright &y& Elsevier]

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

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

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

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

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

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

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

Abstract: Flow boiling experiments with R-134a flowing in horizontal and vertical mini-channels are performed to obtain flow visualization and heat transfer data. A 1.75mm diameter channel with a length of 600mm is used as test section. Two sets of the horizontal flow and vertical upward flow data are presented based on a mass flux range of 200–1000kg/m2 s, a heat flux range of 1–80kW/m2 and a saturation pressure range of 7–13bar. Flow visualization and heat transfer results show heat transfer coefficient depending on flow pattern. The results also indicate that flow regime transition and heat transfer coefficient tend to depend on flow direction under the certain experimental conditions. In addition, the comparisons between the experimental data and the predictions recommended for either macro-scale flow boiling or micro-scale flow boiling are presented in this work. [Copyright &y& Elsevier]

Abstract: This research focuses on heat transfer to R-134a during flow boiling in a 1.75mm internal diameter tube. Flow visualisation and heat transfer experiments are conducted to obtain heat transfer coefficients for different flow patterns. The measured data in each flow regime are compared with predictions from a three-zone flow boiling model. The calculations are in fair agreement with the experimental results which correspond in particular to slug flow, throat-annular flow and churn flow regimes under conditions of low heat flux. [Copyright &y& Elsevier]

Abstract: The flow boiling heat transfer characteristics of R134a in the multiport minichannel heat exchangers are presented. The heat exchanger was designed as the 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. Two inner tubes were made from extruded multiport aluminium with the internal hydraulic diameter of 1.1mm for 14 numbers of channels and 1.2mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979mm2 and 6171m2, while the inner surface areas are 13,545mm2 and 8856mm2 for 14 and eight numbers of channels, respectively. The outer tube of heat exchanger was made from circular acrylic tube with an internal hydraulic diameter of 25.4mm. The experiments were performed at the heat fluxes between 15 and 65kW/m2, mass flux of refrigerant between 300 and 800kg/m2 s and saturation pressure ranging from 4 to 6bar. For instance the boiling curve, average heat transfer coefficients are discussed. The comparison results of two test sections with different the number of channels are investigated. The results are also compared with nine existing correlations. The new correlation for predicting the heat transfer coefficient was also proposed. [Copyright &y& Elsevier]