Experimental and numerical investigation on heat transfer of supercritical water flowing upward in 2 × 2 rod bundles.

• We conducted a heat transfer experiment of SC water flowing upward in 2 × 2 rod bundles. • We used 3 different methods to calculate the HTC of sub-channels of rod bundles. • Spacer effects on heat transfer was observed and CFD simulation of spacer effect was also investigated. • HTCs in 2 × 2 rod bundles were compared with the annuli and circular tube of the same equivalent hydraulic diameter. This paper presents the investigation of heat transfer of supercritical water flowing upward in 2 × 2 rod bundles, both experimentally and numerically. The experiment conditions are as follows: pressures from 22.9 MPa to 26.1 MPa, mass fluxes ranging from 450 kg/m2/s to 1500 kg/m2/s and heat fluxes ranging from 0.4 MW/m2 to 1.5 MW/m2. Four heated-rods of 8 mm outer diameter formed 2 set of experiment sections with pitch to diameter ratio (P/D ratio) 1.18 and 1.3. Heat transfer coefficients between the heated rod and fluid were conducted by 3 different methods and the effect of spacer grids was clearly observed. The experiment results show that heat transfer to supercritical water in rod bundles is strongly affected by flow channels and spacer grids. Moreover, a large database of heat transfer in rod bundles was carried out and HTC were compared with tube and annuli with the equivalent hydraulic diameter. The phenomenon of heat transfer difference in rod bundle circumferential direction and enhancement heat transfer in downstream of spacer grids were observed and investigated. In addition, Computational Fluid Dynamics (CFD) method was applied to give a deep insight of velocity profile, spacer effect and heat transfer enhancement in this research. [ABSTRACT FROM AUTHOR]