Experimental investigation of Al2O3/water nanofluids on the effectiveness of solar flat-plate collectors with and without twisted tape inserts

The thermal effectiveness of solar water heaters can be enhanced if passive heat transfer enhancement techniques are used. Among the most effective passive heat transfer enhancement techniques are the increase of the working fluid thermal conductivity and of its flow turbulence. In this paper, Al2O3 nanofluids and twisted tape inserts are the passive techniques used to enhance the heat transfer and, consequently the thermal effectiveness of the solar water heater. In the solar water heating system considered in this study, the collector is essentially mimicked by a tube with or without a twisted tape, with water or nanofluids flowing through it. Results of the heat transfer experiments indicate that for a Reynolds number of 13000 the heat transfer enhancement for 0.3% volume concentration of nanofluid is 21% for the plain tube and it is further enhanced to 49.75% when a twisted tape of HID = 5 is inserted in the tube. The maximum friction penalty of 1.25-times was observed for 0.3% nanofluid with H/D = 5 when compared to water in a plain collector. The thermal effectiveness of the plain collector is enhanced to 58%, when the 0.3% nanofluid is used and it is further enhanced to 76% with a twisted tape of H/D = 5 at a mass flow rate of 0.083 kg/s. Solar water heaters, in which the collectors have twisted tape inserts and use nanofluids, have thermal performance increases that largely outweigh pressure drop losses. Under the same operating conditions, the nanofluids/twisted tape inserts collector outperforms that with water and no twisted tapes. (C) 2017 Elsevier Ltd. All rights reserved.