Convective heat transfer with flow characteristics of laminar to transition region for MWCNT-Water based nanofluid

A B S T R A C T Multi-walled carbon nanotube (MWCNT) nanofluids have demonstrated superior thermal performance compared with conventional heat-transfer fluids however, their application in shell-and-tube heat exchangers (STHEs) remains limited, particularly under laminar-to-transitional flow conditions. To the best of the authors’ knowledge, this study is the first to combine industrial-scale probe ultrasonication with controlled centrifugation based on relative centrifugal force (RCF) during nanofluid preparation for STHE applications, and to experimentally investigate MWCNT-water nanofluids operating within the laminar-to-transitional regime. Nanofluids were prepared using a two-step method of probe ultrasonication followed by RCF-controlled centrifugation, with sodium dodecyl benzene sulfonate (SDBS) employed to enhance dispersion stability. The highest concentration (0.06 wt.%) exhibited excellent stability, with a measured zeta potential of -52.12 mV, indicating strong electrostatic repulsion and effective particle dispersion. Experiments were conducted at tube-side flow rates of 0.5-3 L/min and a constant shell-side flow, with nanoparticle concentrations ranging from 0.01 to 0.06 wt.%. Thermal performance was evaluated in terms of heat-transfer rate, convective and overall heat-transfer coefficients, and heat-exchanger effectiveness. Results show consistent enhancement with increasing concentration and flow rate at 3 L/min, the 0.06 wt.% nanofluid achieved a 29.25% increase in overall heat-transfer coefficient, a 34.95% increase in convective coefficient, and an improvement in effectiveness from 29.9% to 36.76% compared with water. Finally, a modified Hausen correlation is proposed to predict the Nusselt number of MWCNT-water nanofluids under laminar-to-transitional conditions.