Performance analysis of an extended-surface double-pipe heat exchanger for heat recovery from hot exhaust gas using water and nanofluids

This study explores enhanced configurations of a double-pipe heat exchanger (DPHE) aimed at recovering waste heat from hot industrial exhaust gases at 600 K. In the proposed designs, conventional longitudinal fins were extended using a thin tubular copper layer attached to the outer surface of the annulus to boost heat transfer. Water and CuO/water nanofluid were employed as the cooling media flowing inside the inner pipe, while hot air flowed counter-currently through the annulus. Numerical simulations were conducted using the quadratic element method. Results show that CuO/water nanofluid increases the Nusselt number by approximately 3 % compared to water. Transitioning from four to six fins in conventional designs led to a 15 % rise in Nu at Re = 7090 and 8 % at Re = 21300, albeit with a 33 % increase in pressure drop. The novel configurations with four and six extended fins achieved Nu enhancements of 26 % and 32 %, respectively, over the traditional four-finned design—without a significant rise in pressure drop due to the minimal cavity modification. The performance evaluation criterion (PEC) improved by 1.27-fold and 1.30-fold for the new four- and six-fin designs, indicating a substantial gain in thermal efficiency relative to pressure loss. These results demonstrate the superior performance of the proposed DPHE configurations for waste heat recovery applications.