Heat transfer degradation of buoyancy involved convective RP-3 hydrocarbon fuel in vertical tubes with various diameters under supercritical pressure

The flow and heat transfer characteristics of hydrocarbon fuel flowing upward in vertical circular tubes with various diameters under supercritical pressure were numerically investigated. The buoyance effects in the vertical tubes were analyzed and discussed. A simulation tool for predicting the supercritical turbulent internal flow and heat transfer was developed and validated with experimental data. A four-species surrogate model of the fuel RP-3 was used to calculate the temperature-dependent fluid properties. The turbulence was modelled with the Launder-Sharma (LS) low-Reynolds number turbulence model. The tube diameter ranged from 2 mm to 15 mm in order to analyze the size effect. The simulation results showed that the tube diameter significantly affected the heat transfer deterioration, especially for low mass flux cases. Based on the predicted velocity profiles, the mechanism of the size effect on the heat transfer deterioration was discussed and analyzed. In the tube with a small diameter, the accelerated thin layer took up a significant share of the cross section of the tube. However, the buoyancy effect dominated in the tube with a large diameter due to the large gradient from the thin layer to the core of the main flow. It was found that the threshold for the size effect was around 10 mm, below which the size effect was significant.