The Effect of Threaded Rod Inserts and Aluminum Oxide–Water Based Nanofluids on Performance of a U-Bend Double Pipe Heat Exchanger

Double pipe heat exchangers, owing to their simplicity in construction and ease of maintenance, are being widely used in refineries, food processing units and pharma industries. However, they have a limitation that they can only operate at low heat loads. In order to make them suitable
 for higher loads, it is essentially required to enhance the rate of heat transfer. In this work, an attempt is made to this end using a combination of threaded rods and water-based aluminum oxide nanofluids. Experimental investigations were carried on a counter-flow double pipe return bend
 heat exchanger (DPHE) using water-aluminum oxide (Al2O3) nanofluids with threaded rod inserts. The volume concentration of Al2O3 nanofluid added in water was 0.03% and 0.05%. The entire analysis was carried out under turbulent flow regime by varying
 the mass flow rates of cold water from 3 to 15 LPM in steps of 2 LPM, with the hot water flow rate being fixed at 6 LPM. The Reynolds number range considered was from 3000 to 30000. From the obtained results, it was found that the maximum enhancement in the Nusselt number and friction factor
 for 0.05% concentration of the nanofluid and threaded rod inserts respectively is 89.95% and 32.46% more than the plain tube. The maximum thermal performance factor was found to be 1.75 for the combination of 0.05% nanofluid and inserts.