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• Thermosyphon in a vertical narrow annulus with external cooling is investigated. • The cooling power has a linear relation with the inlet water temperature. • The relationship between the LMTD and Gr*1/3 shows an excellent linearity. • A correlation is proposed to predict the natural convective heat transfer in annuli. In PWRs, the Control Rod Drive Mechanisms (CRDMs) are heated by coils and coolant, and cooled by external convection. The thermosyphon in the CRDMs has significant influences on the coil temperatures. In this work, the thermosyphon characteristics in a vertical narrow annulus closed at top with external forced convection cooling is experimentally investigated. The effects of the inlet water temperature and the velocity of external air cooling on the thermosyphon and cooling power are analyzed. The results indicate that the cooling power has a linear relation with the inlet water temperature. The external cooling condition has effects on the natural convective heat transfer and transition point in the annulus. The relationship between the Logarithmic Mean Temperature Difference and the 1/3 power of Gr*, a correction of Gr considering external forced convection heat transfer, shows an excellent linearity. A new correlation is proposed to predict the natural convective heat transfer coefficient in annuli. [ABSTRACT FROM AUTHOR]