Evaluation and prediction of flow and heat transfer performance in two-start spirally coiled tube

Abstract

The coiled tube has been widely used in heat exchangers due to its advantages of compact structure, large heat transfer area, and high heat transfer efficiency. In this paper, two specifically-shaped high-efficiency coiled tubes are proposed. The effects of structural parameters on the heat transfer performance and pressure drop of two-start coiled tubes (TCTs) and two-start spirally coiled tubes (TSCTs) are investigated through numerical simulations. Furthermore, the enhanced heat transfer mechanism is analyzed using field synergy theory. Furthermore, the enhanced heat transfer mechanism is analyzed based on the field synergy theory. The practical values of TCT and TSCT are evaluated using the performance evaluation criterion (PEC). The results indicate that the coiled structure can induce the secondary flow of the fluid, enhance the synergy between the velocity and temperature of the secondary flow, thereby enhancing the heat transfer. The spiral structure of the TSCT will cause the fluid to generate vortices, which further strengthens the heat transfer. As apex angle (α₁) increases, both the Nusselt number (Nu) and friction factor (f) of the TCT and TSCT gradually decrease. As spiral angle (α₂) increases, both the Nu and f gradually of the TSCT gradually increase. The Nu and f of the TSCT (α₁ = 30°, α₂ = 10800°) reach their maximum values, which are 1.44–1.65 times and 4.40–4.53 times those of the circular coiled tube, respectively. The corresponding PEC (PEC = 1.68–1.78) is also the largest, which is close to that of TCT (PEC = 1.66–1.70). However, its spiral structure increases its manufacturing cost. Finally, correlations for the TCT and TSCT are proposed through linear fitting. The errors of the correlations for TCT are within 10 %, and the errors for TSCT are within 15 %, providing guidance for their practical engineering applications.