Experimental and statistical analysis of heat transfer and friction factor for rectangular-toothed v-cut twisted tapes using TiO₂/H₂O nanofluid

This study investigates the performance of a double pipe heat exchanger utilizing TiO₂/H₂O nanofluid at different concentrations (ϕ = 0.05%, 0.15%, and 0.5%) and v-cut twisted tape inserts with varying width-to-depth ratios (w/h) and rectangular tooth depth to depth of v-cut ratios (z/h). Experimental tests are conducted with hot water at 70 °C and cold water in the range of 30–32 °C, covering Reynolds numbers from 6000 to 13,000 for double pipe flow. Twisted tape with a width-to-depth ratio of w/h = 0.67 increases the Nusselt number by 68.75% compared to a plain tube. Furthermore, combining this twisted tape configuration with a rectangular tooth depth to depth of v-cut ratio of z/h = 0.14 leads to an even greater enhancement of 84%. Introducing TiO₂/water nanofluid at a concentration of 0.5% alongside the aforementioned twisted tape arrangement results in a significant increase of 123.64% in the Nusselt number. These findings highlight the positive impact of both twisted tape inserts and nanofluid on heat transfer performance. The friction factor increases up to 7.1 times in the absence of nanofluid, while including nanofluid (0.5% concentration) raises it significantly to 15.41 times relative to a plain tube. Importantly, the established correlations show good agreement with experimental results, with deviations within ± 10% for both the Nusselt number and the friction factor. These findings validate the reliability of the proposed correlations. Overall, this study highlights the effectiveness of toothed v-cut twisted tape inserts and TiO₂/water nanofluid in enhancing heat transfer performance in double pipe heat exchangers.