• influence of u-bend heterogeneous effects on bubble dynamics and local flow boiling heat transfer in hairpin tubes

    نویسندگان :
    جزئیات بیشتر مقاله
    • تاریخ ارائه: 1390/01/01
    • تاریخ انتشار در تی پی بین: 1390/01/01
    • تعداد بازدید: 435
    • تعداد پرسش و پاسخ ها: 0
    • شماره تماس دبیرخانه رویداد: -
     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.

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