CFD simulation and optimization of natural convection in a vertical annulus with nanofluids

Numerical studies have been performed to examine the effect of base fluid, size, and constructive material of nanoparticles on natural convection heat transfer of nanofluids inside a vertical annulus using a finite volume method. The simulations are performed at a constant heat flux of 10,000 W/m² and a volume fraction of nanoparticles of 0.03. Three controllable input parameters, i.e., size of nanoparticle (13 nm, 36 nm, and 59 nm), nanoparticles material (Copper, Aluminium oxide, and titanium dioxide), and base fluids (Water, Ethylene glycol, and Ethanol), were considered for the study. Their effect on four output responses, viz. heat transfer coefficient (HTC), Nusselt number (Nu), rate of mass flow (MFR), and Reynolds number (Re), was investigated using Taguchi's L₂₇ orthogonal array. As a novelty, the study is further extended through Multi-response optimization. Two multi-criteria decision making (MCDM) methods namely Criteria Importance through Inter criteria Correlation (CRITIC) and weighted Aggregated Sum Product Assessment (WASPAS) are implemented to determine optimal setting of the input parameters that yield optimal multiple responses. The optimum combination of the input parameters, which maximizes the output responses simultaneously, is found as A₂B₁C₁ (i.e., Particle size = 36 nm, constructive material of nanoparticles as copper, and base fluid as water). It is also concluded that the base fluids have the maximum influence on the heat transfer rate, followed by nanoparticle size and constructive materials.