High‐heat flux flow and heat transfer transitions in a supercritical loop: Numerical verification and correlation

Supercritical heat transfer loops are widely used in large-scale thermal equipment, which development is restricted by the thermal-hydraulic characteristics, especially the safety and thermal efficiency under high-heat fluxes. This study is focused on numerical simulation for a 4-m test section of the loop and further analyzes the flow transitions and heat transfer mechanisms from subcritical to supercritical. The heat transfer characteristics of supercritical water in the vertical loop were simulated for conditions: P = 24 MPa, T-in = 320 degrees C, 340 degrees C, mass flux (G) = 1000 kg/m(2)s, 1500 kg/m(2)s, and heat fluxes varied from 100 to 1200 kW/m(2). The heat transfer transitions are analyzed from the perspective of local parameters that affect the status of both core flow and boundary layers. The empirical correlation and the error backpropagation (BP) neural network prediction model are proposed and compared with experimental and numerical results. The results show that transitions of both boundary layer and core flow from subcritical to supercritical contribute to the heat transfer behaviors: normal heat transfer, heat transfer deterioration, and heat transfer enhancement. The zero-velocity gradient ( partial differential u/ partial differential r = 0) on the M-shaped velocity curve causes the turbulent shear stress near the wall to be close to zero, which leads to the suppression of the turbulence. At the same time, the thickness of the thermal boundary layer reaches the maximum, resulting in a large thermal resistance, which affects the heat transfer from the wall to the core area.