Influences of tube wall on the heat transfer and flow instability of various supercritical pressure fluids in a vertical tube

Highlights: • Supercritical water heat transfer and flow instabilities in a vertical tube were studied. • Fluid flow rate oscillations are suppressed by a finite-thickness wall. • Larger wall ρ and c_p or smaller fluid ρ and c_p keep the flow stable longer. • Wall acts as a heat accumulator that adjusts heat transferred to water according to fluid conditions. -- Abstract: The subcritical steam generators in the current high temperature gas-cooled reactor pebble-bed module, HTR-PM, in China can be replaced by supercritical steam generators to better coordinate the reactors and the supercritical steam turbine unit to improve the thermal efficiency with no fluid phase change at supercritical pressures. This study then numerically analyzes the heat transfer and flow instabilities of supercritical pressure fluid flow in a vertical tube with zero or finite-thickness walls. The wall thickness has negligible effect on the heat transfer at supercritical pressure water at steady state. However, for transient calculations, the flow rate oscillates intensely beyond a critical heat flux with the zero-thickness wall, while the flows in tubes with finite-thickness walls are very different. The wall properties including density, specific heat and thermal conductivity are varied to analyze why the oscillations are suppressed by the finite-thickness wall. The flow and heat transfer characteristics and the heat stored in the wall compared with the total power are presented during the heating process at various moments. The flow instabilities are also analyzed for water at various pressures and CO₂ to analyze the effects of the fluid properties.