AUTHOR=Ma Xiaoxu , Liu Taicheng , Dai Nan , Tian Maocheng , Li Shijie 

TITLE=Experimental Investigation on the Perturbation Length for Air–Water Flow Upstream and Downstream of U-Bends

JOURNAL=Frontiers in Energy Research

VOLUME=Volume 10 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2022.906809

DOI=10.3389/fenrg.2022.906809

ISSN=2296-598X

ABSTRACT=In this work, the perturbation length of vertical U-bend effects on air-water two-phase flow in their adjacent straight tubes was investigated experimentally at ambient pressure and temperature. The experimental database covered test sections with three inner diameters of U-tubes (9, 12, 16 mm), two curvature ratios (8.33 and 12.5) and two orientations (downward and upward). The air and water superficial velocities were varied from 0.18 to 25.11 m/s and 0.11 to 1.98 m/s, respectively. The dissipation of U-bend effects in the regions upstream and downstream of U-bends is characterized by the variation regularities of segmental pressure gradients along the contiguous straight tubes. It was found that the significant value of dimensionless perturbation length (Lp/D) is less than 60 for the experimental conditions in this study. The influences of inlet volume void fraction, two-phase mixed Froude number, mixed Weber number and mixed Dean number on dimensionless perturbation length (Lp/D) are similar. With the increase of these parameters, Lp/D presents a unimodal distribution which first increases and then decreases. The maximum value of Lp/D takes place at a volume void fraction of around 90%. New dimensionless correlations to predict the influence length of U-bends on the straight tubes upstream and downstream have been given. The prediction errors of 80% data are within ±30% compared with the present experimental data.