Heat transfer, friction factor and effectiveness of Fe^sub 3^O^sub 4^ nanofluid flow in an inner tube of double pipe U-bend heat exchanger with and without longitudinal strip inserts

Heat transfer, friction factor, effectiveness and number of transfer units (NTU) were determined experimentally for a Fe3O4 nanofluid flowing through the inner tube with longitudinal strip inserts of a double pipe U-bend heat exchanger. Different concentrations of the Fe3O4 nanofluid, which is the hot fluid, were used in the present study and cold water circulates in the annulus region of the double pipe heat exchanger. The heat transfer and friction factor experiments were conducted for the Reynolds number range from 15,000 to 30,000 with the Fe3O4 nanofluid volume concentrations of 0.005%, 0.01%, 0.03% and 0.06%. The effect on heat transfer and friction factor of longitudinal strip inserts in the inner tube is studied for three different strip aspect ratios (AR) with the values of 1, 2 and 4, respectively. The results indicate the Nusselt number on the nanofluid side increases with increasing Reynolds number and particle concentration, and with decreasing aspect ratio of the longitudinal strip inserts. The Nusselt number enhancement, compared to the water data, for the 0.06% volume concentration of the nanofluid is 14.7% and it further increases to 41.29% for the same 0.06% concentration with the longitudinal strip insert with AR equal to 1 for the Reynolds number of 28,954. Compared to water data, the friction factor for the 0.06% volume concentration of the nanofluid increases by 1.092-times and it further increases to 1.267-times for the same concentration with the longitudinal strip insert with AR equal to 1 for the Reynolds number of 28,954. The overall performance of the double pipe heat exchanger with longitudinal strip inserts in the nanofluid side is expressed in terms of effectiveness and number of transfer units (NTU). New correlations for the Nusselt number and friction factor are reported and they are based on the obtained experimental data.