Stability and Natural Convection of TiO₂‑Water Nanofluid in Rotated Enclosures: Experimental Findings

Abstract

This study examines the stability and natural convection heat‑transfer performance of TiO₂‑water nanofluid in square enclosures rotated at –45°, 0°, 45°, and 90°. Stability was optimized by adding 6 wt% dispersant at pH 8, which yielded the lowest transmittance and best dispersion. Experiments varied nanoparticle loading (0.1–0.5 wt %) and heating power (1–20 W). The 0° orientation produced the highest Nusselt numbers, followed by 45° and 90°, with –45° yielding the lowest. Heat transfer increased with particle loading and power, though the relative enhancement declined at higher power due to viscosity effects.

Background

Nanofluids offer superior thermal conductivity, making them attractive for heat‑transfer applications, especially natural convection. Numerous numerical studies have explored Al₂O₃‑water, CuO‑water, and TiO₂‑water systems, yet experimental data on rotated enclosures remain sparse. This work fills that gap by providing empirical evidence on how rotation influences convection in TiO₂‑water nanofluids.

Method

Preparation and Stability Assessment

TiO₂ nanoparticles (≈10 nm, flat morphology) were dispersed in deionized water using a two‑step process: mechanical stirring (30 min) followed by 40 min sonication. Dispersant dosing (6 wt %) and pH adjustment to 8 produced the most stable suspension, confirmed by minimal sedimentation after 72 h and a transmittance minimum of τ ≈ 0.2. The particle size distribution was verified by SEM, TEM, and XRD, with a Scherrer‑derived crystallite size of 6–9 nm.

Experimental Apparatus

Three rectangular enclosures (W × H: 10×20 cm, 5×20 cm, 20×20 cm) were constructed from copper, featuring a heated left wall (silicone sheet) and a cooled right wall (water bath). Six thermocouples measured wall temperatures, and heat loss was quantified by a flow meter. The setup allowed rotation of the enclosure relative to gravity to assess four angles: –45°, 0°, 45°, 90°.

Data Processing

Key equations:

Heat input: Q = I

Net power: Qₙₑₜ = Q – Qₗₒₛₛ

Average hot side temperature: Tₕ* = (T₁+…+T₆)/6

Effective hot side temperature: Tₕ = Tₕ* – Qₙₑₜ·δ/(A·λ_w)

Convection coefficient: h = Qₙₑₜ/[A(Tₕ–T₍C₎)]

Nusselt number: Nu = h·W/λ_f

Uncertainty analysis yielded 5.65 % for h and 6.34 % for Nu, confirming experimental reliability.

Results and Discussion

Validation

Measured Nusselt numbers for pure water matched literature within <9 % error across aspect ratios A = 1:2, 1:4, 1:1, confirming system accuracy.

Aspect Ratio A = 1:2

Nu increased with rotation angle up to 0°, then declined at 45° and 90°. The –45° case showed the lowest Nu due to restricted buoyant flow. Particle loading from 0.1 to 0.5 wt % raised Nu by up to 29 % at 1 W but the relative gain decreased at 20 W because of rising viscosity. Increasing heating power from 1 to 20 W amplified Nu by up to 581 % (0.5 wt %, 0°).

Aspect Ratio A = 1:4

Similar trends as A = 1:2, but absolute Nu values were lower. Enhancement ratios for 0.5 wt % at 0° ranged from 6.5 % (1 W) to 20.7 % (5 W). Heat‑transfer gains from 1 to 20 W reached 242–702 %.

Aspect Ratio Comparison

Nu rose with increasing aspect ratio: A = 1:1 > A = 1:2 > A = 1:4. The 0.5 wt % nanofluid achieved up to 701 % higher Nu than water in the best configuration (A = 1:1, 0°, 20 W). Enhancement from 0.1 to 0.3 wt % was more pronounced than from 0.3 to 0.5 wt %, reflecting the balance between conductivity and viscosity.

Conclusions

Stability is maximized with 6 wt % dispersant at pH 8.
Rotation angle 0° delivers the highest Nu; –45° the lowest.
Higher aspect ratios markedly improve heat transfer (up to 224 % enhancement).
Nu increases with particle loading but the relative benefit diminishes at high power.
Overall Nu can be boosted by up to 701 % relative to pure water.

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Nanomaterials