Unsteady stagnation-point flow of CNTs suspended nanofluid on a shrinking/expanding sheet with partial slip: multiple solutions and stability analysis

Abstract

The purpose of the study of single/multi-wall carbon nanotubes (SWCNTs/MWCNTs) mixed water-based nanofluid having unsteady stagnation-point flow on shrinking/expanding sheet with velocity and thermal slip effects is to decode the heat transfer mechanism to know the high cooling rate criteria. Governing boundary layer coupled partial differential equation (PDEs) are converted into ordinary ones. The transformed equations are numerically solved by shooting method with RK-4 scheme. The impacts of different parameters are described graphically and a comparison between current and previous results is made in tabular form. Existence of multiple solutions along with unique solution appears for specific cases of shrinking and expanding velocities. The investigation also reveals that SWCNT-nanoparticles have more dominating heat and momentum transfer rates than MWCNT-nanoparticles. Velocity slip delays the boundary layer separation, but maximum surface drag-force does not alter. Thermal slip and unsteadiness reduce heat transfer rate, whereas velocity slip enhances it. For high shrinking velocity compared to free stream velocity, the surface cooling rate drops down with Prandtl number and thermal boundary layer thickness significantly reduces for all types of nanofluids and for both solution branches. For both nanofluids, the temperature near the sheet decreases with thermal slip. For non-uniqueness of the similarity solution, a linear stability analysis is conducted and it verifies that upper branch of the obtained solutions is stable, while lower branch is unstable for high shrinking velocity and the unique solution is stable for expanding and low shrinking velocities.