Effects of screw pitches and rotation angles on flow and heat transfer characteristics of nanofluids in spiral tubes

Abstract

Flow and heat transfer characteristics of nanofluids in spiral tubes are experimentally studied. The effects of screw pitches (s = 10 cm, s = 12.5 cm, s = 15 cm), rotation angles (β = 0°, β = 45°, β = 90°) and nanoparticle mass fractions (ω = 0.0 wt%, ω = 0.1 wt%, ω = 0.3 wt%, ω = 0.5 wt%) on the flow and heat transfer performances are analyzed. Results show that Nusselt number increases with the decreasing screw pitch and the increasing nanoparticle mass fraction. It is also found that spiral tube with rotation angle β = 45° shows the largest heat transfer enhancement ratio, followed by rotation angle β = 0°, and the spiral tube with rotation angle β = 90° shows the smallest heat transfer enhancement ratio. In addition, a comprehensive evaluation index is applied to analyze the thermo-hydraulic performances of spiral tubes. One conclusion is obtained that there is a critical Reynolds number (Rec = 10,000) for the highest evaluation index, and the evaluation index increases with the decreasing screw pitch and increasing nanoparticle mass fraction. The evaluation index of nanofluids in the spiral tube can reach 1.24–1.25 at best.