Exergy efficiency and entropy analysis of MWCNT/Water nanofluid in a thermosyphon flat plate collector

The thermal efficiency, Nusselt number, exergy efficiency, thermal entropy generation, and frictional entropy generation of natural circulation of water-based multi-walled carbon nanotubes nanofluids flow in a flat plate collector was analyzed experimentally in this paper. The experiments were conducted between the time duration from 09:00 to 16:30 hr, with particle volume loading ranging from 0 % to 0.3 % under outdoor conditions. Results show that the heat transfer coefficient, Nusselt number, and exergy efficiency increase gradually from 09:00 hr reaching a maximum value at 13:00 hr, and then decreases gradually until the end of the test at 16:30 hr, Hence, in the analysis of the results, the experimental time duration is divided into two durations: time duration-1 (09:00 to 13:00 hr) and time duration-2 (13:00 to 16:30 hr). Dynamic regression equations were developed for Nusselt number and friction factor for time durations 1 and 2. At the peak value of 13:00 hr, the thermal efficiency of the flat plate collector increased from 27.78 % for water to 56.69 % for with 0.3 vol% nanofluid; the exergy efficiency increased from 0.5 % to 2.67 %. Compared to water, the results show that the Nusselt number of 0.3 vol% of nanoparticle in water is higher by 18.01 %; and the friction factor is higher by 13.05 %; the thermal entropy generation is lower by 2.378 %. Using genetic algorithm, the thermal and exergy efficiencies of 52.4 % and 2.59 % are found at an optimal mass flow rate of MWCNT/water in flat plate collector of 0.0243 kg/s and an inlet temperature of 48.2 degrees C.