Natural convection in enclosures occurs in a wide range of industrial applications and engineering systems: solar collectors, thermal insulation of buildings, and cooling systems for nuclear reactors and electronic devices [
1,
2,
3,
4,
5,
6,
7]. Because natural convection is less efficient than forced convection, it should be further investigated to be improved. Through many kinds of industrial thermal processes, it is necessary to add, remove, or exchange heat. Therefore, enhancing the rate of heating and cooling inside an industrial operation will help save energy, decrease the processing time, and increase the performance life of machinery. One strategy for enhancing heat transfer that has received tremendous attention from studies over the past decade is the use of nanofluids [
8,
9,
10,
11]. The term “nanofluid” refers to a suspension of conductive nanoparticles in a base fluid such as water. A nanofluid has considerably better thermal conductivity than a base fluid. According to the most recent studies in the field, nanofluids may also increase heat transfer in cavities and channels. Despite the number of studies undertaken, the mechanism by which a nanofluid could enhance natural convection in a cavity is still not completely understood. Certain conclusions of the research are contradictory for several reasons including a lack of valid experimental data, fundamental theoretical investigations, and precise numerical simulations. To simplify simulations, several researchers have assumed a homogenous mixture for nanofluid flow, which is a two-phase flow with a significant relative drift or slip velocity between particles and the base fluid [
12,
13]. In addition, it is possible that the appropriate thermophysical property correlations are not employed in certain cases.
A comprehensive review of studies on free convection in a cavity was carried out by Pandey et al. [
14]. The shape effect of the internal cavity, such as a square, circular, and elliptical cylinder, on free convection heat transfer was summarized. Free convection heat transfer inside two water-filled square enclosures was investigated experimentally by Ali et al. [
15]. Two different aspect ratios, κ (length/height) = 7.143 and 12.0, were used. The Nusselt numbers was correlated with the modified Rayleigh numbers for both enclosures in the range
. They observed that the Nusselt number increased with an increase in the modified Rayleigh number for each of the two enclosures with a higher Nu at a small aspect ratio (κ = 7.143). Almuzaiqer et al. [
16] investigated the effect of tilt angle on free convection inside an enclosure filled with water. The Nusselt number reached a maximum at 60° at a fixed modified Rayleigh number for all four tilt angles considered: 0°, 30°, 60°, and 90°. The Nusselt number was found to be higher at any tilt angle other than at a zero tilt angle with an enhancement range of 7.92–62.38%, depending on the modified Rayleigh numbers and the tilt angle. The same trend was observed through other numerical studies [
17,
18,
19] that showed that the Nusselt number reached its maximum at a certain tilt angle and then decreased again. Ma et al. [
20] used numerical simulations and parameter sensitivity analyses to investigate the performance of fluid flow and heat transfer in rectangular microchannels including the key physical properties of the fluids and the different parameters of the microchannels. They found that at low Reynolds number conditions, the number of channels and the Reynolds number have a significant impact on heat transfer. However, when the Reynolds number increases, the number of channels is the key factor influencing the heat transfer and flow in microchannel heat sinks. Zhao et al. [
21] presented a comprehensive overview of graphene-based studies of energy conversion, energy storage, and heat transfer. A nanofluid of graphene nanoparticles can also be effectively used in heat exchangers and other heat transfer devices. In their review, they reported that when hybrid graphene nanoplatelets and silver in a water base fluid were used in the rectangular duct, the maximum Nusselt number enhancement was 32.7% and the friction factor increased by 1.08 times at 0.1% concentration (by mass) and a Reynolds number of 17,500. Hu et al. [
22] investigated experimentally and numerically the natural convection heat transfer in a vertical square enclosure filled with an alumina nanofluid. Their study showed an enhancement of 2% in the Nusselt number at a low nanoparticle concentration of a 1% mass fraction. However, at a 2% concentration, they found no enhancement and a degradation occurred at a 3% concentration. Ali et al. [
23,
24] investigated natural convection heat transfer in vertical circular cavities using Al
2O
3–water nanofluid at different volume concentrations for heating either from the top or the bottom of the cavity. While heating from the top, alumina–water nanofluid had a lower Nusselt number than the base fluid. On the other hand, when heating from the bottom, the heat transfer coefficients increased with an increase in the volume concentration up to a maximum point; then, they decreased as the volume concentration increased further. The heat transfer coefficient increased by a maximum of 40% for the shallow enclosure at κ (height/diameter) = 0.0635 and only by 8% for κ = 0.127. Solomon et al. [
25] studied the effect of cavity aspect ratio on free convection in alumina–water nanofluid-filled rectangular cavities. The aspect ratio of the cavity has an impact on both the heat transfer coefficient and the Nusselt number. A total of seven volume concentrations (0.0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and 0.6%) were used at a set of temperatures (∆T = 20, 30, 40, and 50 degrees Celsius) between cold and hot surfaces. At low concentrations, nanofluids demonstrated a slight increase in Nu over that of the base fluids, up to 5%, whereas at high volume concentrations, a decrease in Nu was observed. Choudhar and Subudhi [
26] investigated turbulent free convection in an Al
2O
3–water-filled cavity with different aspect ratios of 0.3–2 and 5 for Rayleigh numbers in the range of 10
7 < Ra < 10
12 for very low volume concentrations of 0.01% and 0.1%. It was observed that Nu was enhanced by 29.5% for lower particle concentrations, 0.01 vol.%, where deterioration was caused by increasing the viscosity and decreasing the Brownian motion. The effect of inclination angles on free convection in an enclosure filled with Cu–water was numerically analyzed by Abu-Nada and Oztop [
27]. An enhancement was observed in the Nusselt number of approximately 33% at a 90° tilt angle with a Rayleigh number of 1000 and for a 0.1% nanofluid concentration. Heris et al. [
28] studied the free convection in a cube with a side length of 100 mm. The effect of the tilt angle on free convection was observed. Their study used 0°, 45°, and 90° tilt angles and various types of nanofluids of Al
2O
3, TiO
2, and CuO with turbine oil as a base fluid. However, the influence of the inclination angle on the aspect ratio was not examined. They concluded that no enhancement was observed when using different nanoparticles in turbine oil as a base fluid. In other words, the Nusselt numbers of turbine oil as a base fluid were higher than other nanofluids using turbine oil as a base fluid. The natural convection of double-walled carbon nanotubes–water nanofluid in a cuboid cavity was experimentally and numerically studied in [
29] at a set of different temperatures. It was observed that the heat transfer coefficients and Nusselt numbers reached a maximum at a 0.05% concentration and then decreased as the volume concentration increased. The natural convection heat transfer of SiO
2–water nanofluid in a rectangular cavity was studied experimentally by Torki and Etesami [
30] at various concentrations and inclination angles. It was found that using SiO
2–water nanofluid at low concentrations (0.1%) did not significantly improve natural convection heat transfer coefficients; however, the coefficient of natural convection was reduced at volume concentrations of more than 0.5%. Heat transfer rates also decreased with inclination angle, and Nusselt numbers have a maximum value at a 0° tilt angle. The free convection heat transfer in enclosures with CuO–water nanofluid that was heated from the right side and cooled from the top was numerically analyzed by Bouhalleb and Abbassi [
31], where five small aspect ratios were investigated (i.e., 0.08, 0.1, 0.125, 0.25, and 0.5). The effect of Rayleigh number, aspect ratio, and inclination angle on flow patterns and energy transport was investigated. They found an improvement in heat transfer when using CuO–water nanofluid. The Nusselt number reached its maximum at volume concentrations of 2% and 2.5% for aspect ratios of 0.5 and 0.25 and 0.125, 0.1, and 0.08, respectively. It was also observed that Nu reach its maximum at 30°, then decreasing as the angle increased.
As seen in the literature survey presented above, experiments on the natural convection heat transfer of nanofluids in enclosures that investigate the effect of tilt angle and aspect ratio are limited. Most of the studies in the literature involve only 2D numerical analyses; however, the present study employed 3D analyses using wide enclosures, and the thermophysical properties were determined experimentally and compared to those in the literature. The current experimental investigation aimed to determine the influence of the inclination angle and the aspect ratio on free convection heat transfer using an aluminum oxide–water nanofluid in square cuboid cavities at two different aspect ratios. This extensive study will be valuable for future theoretical, numerical, and practical studies in the field of natural convection inside cavities.