Assisting/opposing/forced convection flow on entropy-optimized MHD nanofluids with variable viscosity: Interfacial layer and shape effects

This paper aims at investigating the variable viscosity, shape, and interfacial layer effects on entropy-optimized assisting/opposing/forced convection flow of single-walled carbon nanotube (SWCNT)/multi-walled carbon nanotube (MWCNT) nanofluids past a thin needle. The nanoparticles such as SWCNT/MWCNT are used to enhance the heat transfer rate (HTR). Revised Hamilton–Crosser model is implemented in imparting significant augmentation in the thermal conductivity of SWCNT/MWCNT nanoparticles. The energy equation is modeled by including thermal radiation, viscous dissipation, and Newtonian heating mechanisms. Transformed governing equations have been worked out with the help of the bvp4c method along with the shooting technique. The numerical results of velocity, temperature, surface viscous drag (SVD), HTR, entropy generation (EG) rate, and Bejan number are discussed. The flow velocity attains maximum value for a rise in interfacial layer parameter and size of the thin needle, while exhibits declining trend due to hike in shape factor. Surface viscous drag, heat transfer rate, and entropy generation rate enhance in the order opposing, forced convection, and assisting the flow of magnetic fluids while Bejan number shows reverse effect. Interestingly, at lower magnetic parameter $(M=2.0)$, HT enhancement for MWCNT–water nanofluid is 60% higher than that of SWCNT–water nanofluid.