In previous research, several integrated effect experiments such as APEX [
2], ROSA [
3], etc. which adopt PRHR system were performed to verify the safety of advanced reactors in severe accidents. The PRHR HX model equipment works as an important part in those test facilities. Its heat transfer capacity and action were mainly tested and studied to confirm the passive safety characteristics of the designed advanced reactors. In recent research, an increasing number of PRHR HX separate-effects experiments were performed to investigate local thermal-hydraulic behaviors of the C-shape tubes bundle. Men et al. [
4] conducted a series of experiments to investigate the natural convection heat transfer characteristics of a single C-tube, and compared traditional heat transfer correlations for the tube outside and inside heat transfer with the experimental data to find the best-fit correlation. Lu et al. [
5] and Zhang et al. [
6] built an overall scaled-down IRWST and PRHR HX model, which consisted of 12 electrical heating rods bundle located at one side of the IRWST, to investigate the natural convection and pool-boiling heat transfer mechanisms of C-shape rods bundle in different stages during its operation. Moreover, they compared traditional correlations for different bundle sections and heat transfer mechanisms with the experimental data to validate their applicability for system analysis of the PRHRS. Furthermore, they proposed the revised correlations based on the data to match the specific operating conditions of PRHR HX. Tao et al. [
7] investigated the transient single-phase thermal hydraulics of PRHR HX by a miniature model consisting of 6 × 7 C-shape tube bundle, by performing both experiments and numerical simulations. On the one hand, they compared traditional correlations published in literatures with the experimental values to find the best-fit one for the specific heat transfer condition. On the other hand, they validated numerical simulation results by experiments and obtained more details of flow patterns and local heat transfer characteristics. Liu et al. [
8] explored the heat transfer of PRHR HX by a test facility of C-shape tubes bundle immerged in a water tank. They found three heat transfer regions during the heating process on the outer tube surface: natural convection region, transition region, and saturation boiling region. Besides, they analyzed the correlations for different regions in the said paper and their previous work [
4]. Kang [
9] investigated the effects of different major parameters including surface roughness, tube diameter, wall superheat, and dimensionless tube length, which were introduced into pool boiling heat transfer behaviors of a vertically mounted tube. Additionally, he obtained an empirical correlation for pool boiling heat transfer to quantify the tube length effect by using the experimental data.