Influence of U-bend heterogeneous effects on bubble dynamics and local flow boiling heat transfer in hairpin tubes

The present study describes a series of flow boiling experiments with R141b in coiled tubes conducted to investigate the local heat transfer enhancement mechanism of U-bends compared with bubbles growing in horizontal straight tubes. The study focuses on two-phase flows with boiling in U-bends of a vertical upward coiled tube of inner diameter 6 mm at mass fluxes of 120.94, 181.41 and 241.88 kg m−2 s−1 and heat fluxes between 6191 and 13 929 W m−2. The experimental results show that the bubble growth rates in U-bends are 2-4 times faster than those in the downstream horizontal straight tube. The local heat transfer mechanism was analyzed using a transient CFD model with faster bubble growth rates predicted in a U-bend than in a horizontal straight tube. The heterogeneous effects of reheating and thermal non-equilibrium at the tube walls caused by the U-bend structure were found to play critical roles in the two-phase flow and heat transfer, as the main reason for local heat transfer enhancement in bubbly or intermittent flows.