Enhancing the heat transfer in CuO-MWCNT oil hybrid nanofluid flow in a pipe

•Three-dimensional steady-state laminar flow within a horizontal pipe with CuO, multi-walled carbon nanotubes (MWCNT) nanoparticles, and engine oil as a base fluid are performed.•The effect of different nanoparticle volume concentrations of CuO and MWCNT with convective heat transfer was investigated.•Increasing the nanoparticle volume fraction increases both heat transfer and Nusselt number.•For the CuO-oil nanofluids’, a thermal boundary layer thickness formula is given depending on the volume fraction.•A scale analysis of the average heat transfer coefficient is performed. In this study, three-dimensional steady-state laminar flow simulations were conducted in a horizontal pipe using CuO and multi-walled carbon nanotubes (MWCNT) nanoparticles with engine oil as the base fluid. Various nanoparticle volume fractions were examined under a constant heat flux boundary condition applied to the pipe wall. The main goal was to assess and compare the effects of different nanoparticle volume concentrations, including CuO and MWCNT in ratios of 1:1 and 1:2, on convective heat transfer. A second-order discretisation method was employed for solving the equations, and the SIMPLE algorithm was used for pressure–velocity coupling in the CFD code. The study focused on the impact of nanoparticle volume fraction on the convective heat transfer coefficient and the Nusselt number at a Reynolds number of 750. The findings indicate that increasing the nanoparticle volume fraction enhances both the convective heat transfer coefficient and the Nusselt number, with MWCNT having a more pronounced effect compared to CuO. Specifically, adding 2% CuO increases the heat transfer coefficient by 65%, while a mixture of 1% CuO and 1% MWCNT boosts it by 75%. The thermal boundary layer thickness also grows with higher nanoparticle concentrations, with 1% CuO and 3% CuO increasing the thickness by 1.5% and 3.6%, respectively. A formula for the thermal boundary layer thickness in CuO-oil nanofluids is provided based on volume fraction, and a scale analysis of the average heat transfer coefficient confirms that the simulation results are consistent with this analysis.