Heat Transfer and Flow Enhancement of TiO2‑Water Nanofluids in Spirally Fluted vs. Smooth Tubes

Abstract

This study experimentally investigates the flow and heat‑transfer performance of TiO₂‑water nanofluids with varying nanoparticle mass fractions in both spirally fluted and smooth stainless‑steel tubes over a wide Reynolds‑number range. Stability is assessed using a novel transmittance method with an ultraviolet spectrophotometer, revealing optimal dispersion at 6 wt% dispersant and pH 8. Results show that spirally fluted tubes deliver a superior heat‑transfer boost (up to 257.9 % over smooth tubes) while incurring only modest frictional losses. Turbulent flow in the fluted tube yields the greatest thermal‑enhancement to friction ratio. A comprehensive performance index, ξ, peaks at Re ≈ 2300 for the fluted tube, whereas it continuously rises with Re for the smooth tube. These findings provide clear guidance for selecting tube geometry and operating conditions in nanofluid‑based heat exchangers.

Background

Nanofluids—suspensions of nanoparticles in a base fluid—offer remarkable heat‑transfer capabilities, enabling advances in solar thermal systems, desalination, and electronic cooling. While many studies have examined smooth or corrugated tubes, comprehensive comparisons between spirally fluted and smooth geometries remain scarce. This work bridges that gap by combining experimental measurements with a new stability assessment technique and a holistic performance metric.

Methods

Nanofluid Preparation and Stability Assessment

TiO₂ nanoparticles (20–100 nm) were dispersed in de‑ionized water via a two‑step method: 30‑min mechanical stirring followed by 40‑min sonication. A 6 wt% dispersant (PAA) and pH 8 were used for all samples. Stability was evaluated by both the conventional precipitation test and a quantitative transmittance method; the latter demonstrated that samples with 0.1, 0.3, and 0.5 wt% TiO₂ maintain excellent stability over 72 h.

Experimental Setup

The test rig comprised a 1.2 m long stainless‑steel tube (equivalent diameter 0.01 m) with a 1 m central measurement section. A DC power supply heated the tube; 10 T‑type thermocouples measured wall temperature, while K‑type probes captured inlet and outlet fluid temperatures. Pressure drop was recorded with a differential pressure transducer. Flow rates were controlled by a precision pump.

Key Equations

• Heat input: Q₀ = U I
• Fluid heat load: Q_f = c_p ṁ (T_out – T_in)
• Effective specific heat and density calculated via mixture rules (equations 3 & 10).
• Reynolds number: Re = ρ u d_e / μ_f
• Convective coefficient: h_f = Q_f / (π d_e l (T_iw – T_f))
• Nusselt number: Nu = h_f d_e / λ_f
• Friction factor: f = 2 d_e / (ρ u²) (Δp/Δl)
• Comprehensive performance index: ξ = (Nu/Nu_bf) / (f/f_bf)^{1/3}

Results and Discussion

System Validation

Comparisons with literature for water confirm that Nusselt numbers and friction factors match published data within ±3.5 % across laminar and turbulent regimes, validating the experimental rig.

Smooth Tube Performance

Increasing TiO₂ loading raises the Nusselt number by 11.2 % (0.5 wt%) to 16.1 % (0.5 wt%) in turbulent flow. Friction factors rise modestly (≤ 8 % in laminar, ≤ 3 % in turbulent). Thermal conductivity increases 0.17–1.6 % while viscosity rises 2.5–13.6 %, with the former dominating heat transfer in laminar flow and the latter becoming more significant at higher Reynolds numbers.

Spirally Fluted Tube Performance

Fluted geometry amplifies the benefit of TiO₂: Nusselt numbers improve up to 42.8 % (0.5 wt%) in turbulent flow, surpassing the smooth tube by 240–260 %. Friction factors increase more noticeably (≈ 20 % laminar, ≈ 10 % turbulent) due to the screw structure, yet the net performance index ξ peaks at Re ≈ 2300, where heat‑transfer gains outweigh friction penalties.

Comprehensive Evaluation

The ξ metric reveals that the spirally fluted tube delivers its highest performance near the laminar‑to‑turbulent transition, while the smooth tube benefits from progressively higher Reynolds numbers. This insight informs optimal tube selection and operating conditions for nanofluid heat exchangers.

Conclusions

1. Stable TiO₂‑water nanofluids are achieved with 6 wt% dispersant and pH 8; transmittance measurements confirm dispersion quality.
2. In spirally fluted tubes, turbulent flow yields the greatest heat‑transfer enhancement with comparatively lower friction increases.
3. Fluted tubes provide a heat‑transfer boost exceeding 250 % over smooth tubes for 0.5 wt% TiO₂ loading.
4. Optimal comprehensive performance in fluted tubes occurs at Re ≈ 2300; for smooth tubes, performance continuously improves with higher Reynolds numbers.