Numerical Optimization of MWCNT/Water Nanofluids in Turbulent Forced Convection: Trade-offs Between Thermal Enhancement and Hydraulic Penalty

Document Type : Full Length Research Article

Author

The Second Faculty of Mechanical and Electrical Engineering, Damascus University, Swaida, Syrian Arab Republic

Abstract

This study revisits the turbulent forced‑convection performance of multi‑walled carbon nanotube (MWCNT)/water nanofluids. Building on a Eulerian–Eulerian two‑fluid framework, we couple a second‑order orientation tensor with population balance equations (PBEs) to simultaneously resolve shear‑induced alignment, anisotropic thermal conductivity and aggregation kinetics. A rigorous grid convergence study and quantitative comparisons with published experiments demonstrate root‑mean‑square errors below 5 % in Nusselt numbers and friction factors. Micro-level validation of the constitutive models is provided by comparing predicted orientation factors with rheo-optical measurements and simulated cluster size distributions with dynamic light scattering (DLS) data. The new coupled model predicts an optimal operating window (0.8–1.2 vol% % MWCNTs, Reynolds number 20 000–40 000) where the performance evaluation criterion (PEC) exceeds 1.3. Within this window, axial thermal conductivity increases by ~38 % and Nusselt numbers by ~24 % at 1 vol% %, while hydraulic penalties remain manageable. Concentrations above 1.2 vol% % trigger rapid aggregation that reduces thermal conductivity and increases viscosity, producing PEC values below unity. An economic analysis based on a 500-kW cooling system and realistic nanofluid preparation costs indicates payback periods of less than two years when operated in the high-PEC region. The paper closes with a balanced discussion of modelling limitations and future research directions.

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Articles in Press, Accepted Manuscript
Available Online from 25 January 2026
  • Receive Date: 18 November 2025
  • Revise Date: 24 December 2025
  • Accept Date: 25 January 2026