Numerical simulation of double-pipe helical coil heat exchangers: Influence of geometry and nanofluid on heat transfer enhancement

• Ten double-pipe helical coil exchangers are studied. • HTC increased by 12%, and pumping power by 70% as the coil curvature increases. • HTC increased by 15% and pumping power by 50% as the coil pitch ratio increases. • The performance evaluation criterion indicates that enhancement of HCx is greater at higher Re. • Contribution of nanofluids to heat transfer enhancement is less than that of HCx. The discrepancies in the effects of the main parameters found in studies of helical coil heat exchangers are largely the result of not establishing a common basis for comparison. In the present study, fifteen helical coil exchanger configurations with different coil pitch sizes and curvature ratios were examined. All configurations had a fixed length to serve as a common basis for comparison. Water and nanofluids are being investigated as coolants under laminar and turbulent conditions. Thus, the impact of helical geometry and nanofluids on heat exchanger performance is examined under various flow conditions. The findings indicate that the impact of secondary flows leads to a 34% augmentation in the Nusselt number when water flows through the helically coiled heat exchangers in comparison to double-pipe heat exchangers comprising straight tubes. This increase is achieved with a large pitch size and curvature ratio. Using MWCNT/W and CuO/W nanofluids can further enhance Nu to 59% and 87%, respectively. The improved Nusselt number is attributed to the enhancement of the secondary flow intensity and induced fluid mixing due to Brownian motion resulting from the nanoparticles suspended in water. The helical HCx has a friction factor that is 97% higher than that of the DPHx with straight tubes. With MWCNT/W and CuO/W nanofluids, the friction factor increased by 112% and 113%, respectively. The highest PEC of 2.4 was achieved with CuO/W nanofluid under laminar flow with a larger pitch size and curvature ratio. Helical coils have a more significant impact on the process than nanofluids do when the flow is turbulent, and a lesser impact when the flow is laminar.