ICONE23-1108 CFD PREDICTION OF MIXED-CONVECTION HEAT TRANSFER IN SUPERCRITICAL WATER IN A BARE TUBE

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The demand for clean, non-fossil-based electricity is growing. Therefore, the world needs to develop new nuclear reactors with inherent safety and higher thermal efficiencies in order to increase electricity generation per kilogram of fuel and decrease detrimental effects on the environment. To address these issues a number of countries worldwide are developing next generation or Generation-IV nuclear-reactor concepts (six in total) and, as a result, Nuclear Power Plants (NPPs) will have significantly higher operating parameters, especially, temperatures (550-1000°C). Also, Generation-IV nuclear technology will include SuperCritical-Pressure (SCP) reactor coolants (helium and water) and/or SCP working fluids in power cycles (carbon dioxide, helium and water). Due to this reliable and accurate prediction methods for heat transfer in SC Fluids (SCFs) should be developed and verified. These methods include: 1) empirical correlations; 2) CFD software; and 3) Thermalhydraulics codes. The present paper deals with Computational Fluid Dynamics (CFD) PHOENICS software studies, which intended to predict heat transfer in SuperCritical Water (SCW) flowing upward in a 4-m bare vertical tube (D=10 mm) as an initial approach. The current study is related to lower range of mass fluxes (about 200 kg/m2s) in which heat transfer can be influenced by natural convection. In general, Heat Transfer Coefficients (HTCs) in bare tube can be considered as a conservative approach in predicting minimum possible HTCs in more complex geometries such as bundles. Compared to empirical 1-D correlations, CFD studies allow to look inside flow and to have a better picture of various phenomena related to heat transfer in SCFs. Experimental data on SCW were compared with predictions from CFD calculations in this study. The obtained results show that within some operating conditions CFD PHOENICS can predict experimental HTC values reasonably well. In other conditions, especially, within a Deteriorated-Heat-Transfer (DHT) regime more studies are required. Also, selected velocity and Turbulent Kinetic Energy (TKE) profiles across the radial and axial directions of a tube have been provided.

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