Natural Convection of Water/Titanium Oxide Nanofluid Inside a Closed Enclosure at Different Angles of Attack

Abstract

In most industrial applications, several situations are associated with closed enclosures, such as avionics, automotive, cooling/heating systems in buildings, electronic equipment, food, and phase change materials. In this paper, the natural convection (NC) of a Newtonian fluid inside a Non-Square Closed Enclosure (NSCE) is numerically simulated. The working fluid is a water/Titanium oxide nanofluid (NF) with volume fractions in the range of φ = 0 to 4 % and experiences a laminar flow with Rayleigh numbers (Ra) from 103 to 105. To benefit from better flow mixing, NSCE undergoes five different angles of attack -90°, -45°, 0°, 45°, and 90° degrees (cases 1 to 5, respectively). This research was solved using a computer code in two-dimensional space in steady state using the finite volume method. The solid-fluid suspension is considered homogeneous, single-phased, and Newtonian. The Boussinesq approximation is used for the density term. A SIMPLE algorithm is used for decoupling pressure and velocity fields. The results suggest that increasing the Ra number strengthens the fluid velocity components in the Closed Enclosure (CE). In all cases, the maximum Nusselt number (Nu) occurs at the interface between the fluid and the hot surface. In cases (1) and (5), due to the elongation of the fluid path, the circulation effects become more important, creating an anomaly in the friction factor for the Ra = 105. A symmetric pattern in the Nu number diagrams in cases (2) and (4) is evident which is due to the invariance of this parameter in these two cases. Entropy generation is influenced by fluid circulation and rotation. In all cases and conditions, the use of solid nanoparticles reduces the temperature gradient, which significantly affects the removal of hot spots with high entropy and consequently reduces the average entropy generation. Increasing the angle of attack of the closed enclosure compared to the smooth case (case 3) at Rayleigh numbers 103 and 104 can increase the friction coefficient by a factor of 1.62. Also, at Rayleigh number 104, changes in the angle of attack of the closed enclosure will experience a decrease in the Nusselt number and average heat flux by <8 % compared to the smooth case. At Rayleigh number 103, the 10 % increase in the average Nusselt number and heat flux is only due to the increase in the volume fraction of the solid nanoparticle and is somewhat independent of the angle of attack of the closed enclosure. © 2025 The Author(s)