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An investigation was experimentally performed to explore the influence of geometrical parameters on the evaporative heat transfer coefficient and pressure drop of R-134a in dimpled tubes. The test sections were the counter-flow tube-in-tube heat exchanger, where the refrigerant was evaporated in the inner tube and hot water flows in the annulus. Seven dimpled tubes with various helical pitches (p), dimpled pitches (z), and dimpled depths (e) were used as the test tubes. The test runs were carried out over mass fluxes of 300, 400, and 500 kg/m2s; heat fluxes of 20, 25, and 30 kW/m2; and evaporating temperatures of 7, 10, and 13 °C. The results show the heat transfer enhancement factor (HF) and pressure drop penalty factor (PF) increase with a rise in e and a decrease in p and z. The HF and PF range between 1.03–1.7 and 1.76–9.00, respectively. The correlations for predicting the Nusselt number and the friction factor of R-134a during evaporation in dimpled tubes are proposed. [ABSTRACT FROM AUTHOR]

A small plate heat exchanger filled with copper foam which can be applied for small scale electronic cooling is presented. The heat transfer coefficient and pressure drop of water flow inside a plate heat exchanger filled with copper foam (PHECF) are experimentally studied. The effect of copper foam pore density and water velocity on the optimum thermal performance is also presented. The experiment is performed with a Reynolds number ranging from 1200 to 2000 and copper foam pore density ranging from 30 to 50 pores per inch (PPI). The results show that the heat transfer coefficient and pressure drop increased when the water velocity and pore density increased. The heat transfer coefficient is enhanced by 20.23%, 29.37%, and 40.28% for PHECF with a pore density of 30 PPI, 40 PPI, and 50 PPI as compared to a plate heat exchanger. The total pressure drop of water flow inside PHECF is dominated by inertial drag pressure drop. Thermal performance of PHECF with 50 PPI is highest with the average thermal performance factor of 1.21. The Nusselt number and friction factor correlation of water flow inside plate heat exchanger filled with copper foam are also proposed for practical applications. [ABSTRACT FROM AUTHOR]

The heat transfer and pressure drop characteristics of SiO2/water nanofluids flowing through a horizontal circular stainless steel tube equipped with free rotating swirl generators (FRSGs) at the entrance of a test tube are reported in this article. The experimental studies were performed with a Reynolds number ranging from 3500 to 13,000; volume concentrations of 0.5, 1, and 2 vol%; and inlet temperatures of 25, 30, and 35 °C, where a constant heat flux was imposed on a test tube. FRSGs made from aluminum, with three different twisted angles (30°, 60°, and 90°) and a length of 2.3 cm, were located at the entrance of the test tube. The results indicated that FRSGs had significant effects on heat transfer and pressure drop characteristics for the flow of both water and nanofluids in the tube. The Nusselt number increased with increasing volume concentration, inlet temperature, and Reynolds number. However, the friction factor of nanofluids flowing through FRSGs was significantly greater than that of water when the Reynolds number was lower than 6000. This is because FRSGs do not rotate under the given conditions. A maximum efficiency index of 1.57–1.7 was obtained when using SiO2/water nanofluids with FRSGs at a volume concentration of 2 vol%, inlet temperature of 35 °C, and Reynolds number greater than 7000. On the other hand, the efficiency index value was lower than unity when the Reynolds number was lower than 7000 and with volume concentrations of 0.5 and 1 vol%. No significant differences were found among twisted angles of FRSGs on heat transfer, pressure drop, and efficiency index. [ABSTRACT FROM AUTHOR]

In this study, convective heat transfer performance of a nanofluids containing graphite is studied in an industrial microchannel. In the experiments, initially, to prepare nanofluids at the volume fraction values of 0.5, 1, 1.5, 2%, distilled water has been employed as the base liquid. To provide sedimentation and stabilization of nanofluids in distilled water, Cetyltrimethylammonium bromide (CTAB) is utilized as surfactant. Thermophysical properties of nanofluids such as thermal conductivity, dynamic viscosity, and specific heat are determined experimentally. Furthermore, by building an experimental setup, in the temperature range of 20–30 °C and with temperature intervals of 2 °C, performance experiments are carried out in a microchannel of which hydraulic diameter is 1.6 × 10−3 m. Additionally, experiments have been conducted using nanofluids at different volumetric rates from 1 to 7 l min−1, heat fluxes from 100 to 1100 W, and volume fractions from 0.5 to 2%. Measuring heat flux, temperature, and flow rate, outcomes such as convective heat transfer coefficient, Reynolds number, and Nusselt number are calculated. The validation process of the experimental results has been performed by plotting the figures of Nusselt numbers vs Reynolds ones, and heat transfer coefficient vs supplied heat considering distilled water and nanofluids having various volumetric proportions. Regarding with the performance of nanofluids against distilled water under similar operating conditions, some proportional positive increase are acquired. Using outcomes attained from experiments, new correlations for Nusselt number have been derived with the R2 values around 0.96, and afterward by means of those correlations experimental data have been compared with those in the literature. A large number of measured and calculated data are given in the paper for other researchers to validate their theoretical models. [ABSTRACT FROM AUTHOR]

In this study, a comparison of the convective heat transfer, pressure drop, and performance index characteristics of heat sinks with a miniature circular pin-fin inline arrangement (MCFHS) and a zigzag flow channel with single cross-cut structures (CCZ-HS) is presented. SiO2-water nanofluids with different particle concentrations are used as the coolant. The effects of the heat sink type, particle concentration and fluid flow rate on the thermal and hydraulic performances are evaluated. The testing conditions are performed at the wall heat fluxes of 10 to 60 kW/m2 and at a mass flow rate ranging from 0.18 to 0.6 kg/s. The dimension of heat sinks is equally designed at 28 × 33 mm. The heat transfer area of MCFHS and of CCZ-HS is 1430 and 1238 mm2, respectively. Similarly, the hydraulic diameter of the flow channel of MCFHS and of CCZ-HS is 1.2 and 1.0 mm, respectively. The measured data indicate that the cooling performances of CCZ-HS are about 24-55% greater than that of MCFHS. The effects of the channel diameter and single cross-cut of the flow channel are more dominant than the effects of the fin structure and heat transfer area. [ABSTRACT FROM AUTHOR]

Investigations on heated ceiling method can be considered as a new research area in comparison to the common wall heating-cooling and cooled ceiling methods. In this work, heat transfer characteristics of a heated radiant ceiling system was investigated experimentally. There were different configurations for a single room design in order to determine the convective and radiative heat transfer rates. Almost all details on the arrangement of the test chamber, hydraulic circuit and radiant panels, the measurement equipment and experimental method including uncertainty analysis were revealed in detail indicating specific international standards. Total heat transfer amount from the panels were calculated as the sum of radiation to the unheated surfaces, convection to the air, and conduction heat loss from the backside of the panels. Integral expression of the view factors was calculated by means of the numerical evaluations using Matlab code. By means of this experimental chamber, the radiative, convective and total heat-transfer coefficient values along with the heat flux values provided from the ceiling to the unheated surrounding surfaces have been calculated. Moreover, the details of 28 different experimental case study measurements from the experimental chamber including the convective, radiative and total heat flux, and heat output results are given in a Table for other researchers to validate their theoretical models and empirical correlations. [ABSTRACT FROM AUTHOR]

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

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

The affiliation of the sixth author [ABSTRACT FROM AUTHOR]