An investigation on heat transfer deterioration in a tube using supercritical CO2

Supercritical CO2 (sCO2) can be employed as a working fluid in sCO2 power cycle. Supercritical fluids enhance thermal efficiency and enables compact system designs due to their favorable thermophysical properties. However, the intersection of sCO2 thermophysical properties with the two-phase-like regime leads to a pseudo-boiling phenomenon (PBP). During PBP, a thicker vapor-like layer forms, reducing convective heat transfer efficiency and leading to the heat transfer deterioration (HTD). HTD, closely associated with PBP, significantly affects overall system performance. This study investigates the magnitude of HTD by analyzing the characteristics of the vaporlike layer in a heated tube. A wide range of mass and heat fluxes and pressures (mass fluxes (200kg/(m2s)≤ G ≤800kg/(m2s)), heat fluxes (25kW/m2≤ qw ≤100kW/m2), and pressures (7.5MPa≤ P ≤30MPa)) were simulated in vertical and horizontal tubes with 4 mm inner diameter. Computational fluid dynamic simulations were performed using the Eulerian method and the SST k – ω turbulence model. Additionally, behavior of sCO2 was validated against experimental data in vertical and horizontal tubes with 4 mm inner diameter. The results show that HTD can be ignored beyond 10MPa when the Richardson number is less than 0.01. In other words, this study provides a roadmap for identifying critical HTD thresholds in tube heat exchangers and thermal systems.