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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]

• LDV was verified to be used to measure the flow induced vibration of rod bundle. • Flow induced vibration of a 4x4 rod bundle with space grids was tested. • Both turbulence and vortex excited vibration of the rod and space grid were obtained. • Strong vortex induced resonance was found when the flow velocity was 7.35 m/s. In order to develop a feasible method for measuring FIV of both spacer grid and rod, two experiments are conducted in this study. First, the simultaneous verification tests between the LDV and piezoelectric accelerometer have been performed. For the case of vibration occurring in a channel with water, the measured signal by the LDV need to be modified while not affecting the measured vibration value. The results show that the LDV is a good candidate for measuring vibration under flow conditions. Then, the LDV is employed to measure the FIV of a specified 4 × 4 rod bundle with spacer grids. Both turbulence and vortex excite the rod bundle, and a strong vortex induced resonance is found when the flow velocity is 7.35 m/s. The vibration intensity is amplifying with increasing the flow velocity. Moreover, 'lock-in' phenomenon and two types of vortex shedding with St numbers of 0.15 and 0.26 are found. [ABSTRACT FROM AUTHOR]