An experimental research on the dryout point of flow boiling in narrow annuli is conducted under low mass flux with 1.5mm and 1.0mm gap, respectively. Distilled water is used as work fluid, the range of pressure is from 2.0 to 4.0MPa and that of mass flux is 26.0∼69.0 kg/(m2 · s). The relation of CHF and critical qualities with mass flux and pressure are revealed. It is found that the critical qualities decrease with the mass flux and increase with the inlet qualities in externally heated annuli. Under the same conditions critical qualities in outer tube are always larger than that in inner tube. Кутателадзе’s correlations is cited and modified to predict the location of dryout and proved to be not a proper one. Considering in detail the effects of the geometry of annuli and heat flux on dryout, an empirical correction is finally developed to predict dryout point in narrow annuli under low mass flux condition which has a good agreement with experimental data.

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