Numerical study of turbulent nanofluid heat transfer in a tubular heat exchanger with twin twisted-tape inserts

In the present study, the turbulent flow of water/Al2O3 nanofluid in a tubular heat exchanger with two twisted-tape inserts has been numerically investigated in the three-dimensional coordinate. This numerical simulation has been done by using FVM, and all of the equations have been discretized by second-order upwind method. For coupling velocity-pressure equations, SIMPLEC algorithm has been used. The investigated parameters of the present study are Reynolds numbers at the range of 10,000-30,000, the effect of twist ratio of twisted-tape inserts from 2.5 to 4, co-swirl flow and counter-swirl flow of two twisted-tapes inside the tube and volume fractions of nanofluid from 1 to 4%. The results of this research revealed that, by decreasing the twist ratio, the counter-swirl flow twisted-tape and the enhancement of volume fraction of Al2O3 nanoparticles in the base fluid, the amount of average Nusselt number increases. In twist ratios of 3.25 and 2.5, the maximum amounts of performance evaluation criterion (PEC) are 1.6 and 1.55, respectively. Also, the amount of PEC improvement in counter-swirl flow state is considerably more than co-swirl flow state. Therefore, the maximum enhancements of PEC improvement in three twist ratios of 4, 3.25 and 2.5 are, respectively, 40.8, 47 and 51% and the maximum enhancements in three mentioned ratios are 26.5, 28.3 and 30.6%, respectively.