Abstract: In order to explore heat transfer characteristics out of passive residual heat removal heat exchanger (PRHR HX) tubes, which affects operation safety of nuclear reactors, a single C-shaped tube setup was built and experimental study was conducted with visualization technology. The results show that with the increase of nucleation number and bubble size, the water temperatures at horizontal and vertical observation points increase from 95.1 and 84.7 ℃ to saturation temperature, respectively. Correspondingly, the heat transfer coefficients increase in the range of 7 930.9-14 545.9 W·m^-2·℃^-1 and 2 876.9-8 742.2 W·m^-2·℃^-1, and the subcooling degrees decrease from 4.9 and 15.3 ℃ to nearly 0 ℃. The wall temperature and superheat degree vary within 2.0 ℃. The differences between experimental heat fluxes and predicted values with empirical correlations based on the q - \Delta \theta _\textsat^n and superposition model were obvious. Empirical correlations were fitted for horizontal and vertical sections based on the weakening coefficient model under subcooled boiling conditions, and the average prediction errors were 1.9% and 6.4%, respectively. Horizontal section under saturation boiling conditions was also fitted and the error was within ±5%.