Experimental study on convective heat transfer and entropy generation of carbon black nanofluid turbulent flow in a helical coiled heat exchanger

Abstract

The convective heat transfer coefficient (CHTC) of a fluid is one of the most effective factors on the performance of a fluid in heat transfer equipment. Due to the higher conductivity of metals than that of liquids, solid metal particles can be mixed in a fluid to improve the CHTC of it. According to recent advances in nanotechnology, using nanofluids is one of the popular methods to increase heat transfer in heat exchangers. In this paper, the main influence factors on increasing the CHTC of the nanofluid, including flow rate and nanofluid concentration, were investigated. For this study, all tests were carried out in the Reynolds range of 10,000 to 31,500 which are definitely in turbulent regime. Furthermore, in order to reduce the number of tests, Design–Expert software using central composite method has been employed. The results indicate that increasing the Reynolds leads to an increase in the CHTC and a decrease in the friction factor. In addition, it was shown that at a constant Reynolds, carbon nanofluid with concentration of 0.21 mass% has about 40.7% higher CHTC than pure water. The variation of friction factor was investigated too and was found that this parameter is increased by increasing the nanofluid concentration and decreasing the Reynolds number. The entropy generation due to heat transfer and friction factor was assessed and finally showed that in 0.21 mass% where the maximum Nusselt is available, the entropy generation is the lowest. By calculating the Bejan number, the portions of heat and friction factors in generating entropy were determined.