Research activities are ongoing for High performance light water reactor (HPLWR) with square double rows fuel assembly to develop nuclear power plants with the purpose to achieve a high thermal efficiency and to improve their economical competitiveness. However, there is still a big deficiency in understanding and prediction of heat transfer in supercritical fluids. This paper evaluates three-dimensional turbulent flow and convective heat transfer in a single-phase and steady-state sub-channel of HPLWR by using general computational fluid dynamics code, Ansys 14 Fluent. The major concern using supercritical water as work fluid is the heat transfer characteristics due to large variations of thermal properties of supercritical water near pseudo-critical line. In order to ensure the safety of operation in High performance light water reactor (HPLWR), heat transfer deterioration (HTD) must be avoided. Numerical results prove that the RNG k-e model with the enhanced near-wall treatment obtained the most satisfactory prediction and lead to satisfactory simulation results. The HPLWR Square fuel assembly has many square-shaped water rods, Out of four types of sub-channels; three sub-channels SC-1, SC-2 and SC-3 are investigated (adjacent to the side of the moderator flow channels (SC-1) (moderator tube and assembly gap), central sub-channels formed by four fuel rods (SC-2), adjacent to the corner of the moderator tube (SC-3). Since coolant flow distribution in the fuel assembly strongly depends on the gap width between the fuel rod and water rod, fuel rod pitch to diameter ratio 1.1–1.4 with 8mm diameter are considered for simulation. Sub-channel analysis clarifies that coolant flow distribution becomes uniform when the gap width is set to 1.0 mm. was less than 620°C. Effects of various parameters, such as boundary conditions and pitch-to-diameter ratios, on the mixing phenomenon in sub-channels and heat transfer are investigated. The effect of pitch-to-diameter ratio (P/D) on the distributions of surface temperature and heat transfer coefficient (HTC) in a sub-channel, it was found that HTC increases with P/D 1.1 first and then decreases with increasing P/D ratio. Apart from the basic geometry sub-channel, a square sub-channel with a wire-wrapped rod inside has been chosen to investigate the “wire effect”.
