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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]

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]