Numerical investigation of subcooled flow boiling heat transfer in a horizontal tube with RPI model

Numerical studies of subcooled flow boiling heat transfer and vapor phase characteristics in a horizontal tube were conducted using a non-equilibrium Eulerian–Eulerian Rensselaer Polytechnic Institute (RPI) model. The water in a horizontal tube (hydraulic diameter d_h × length L = 16.05 mm × 2 m) subjected to higher heat loads (heat flux/mass flux) was simulated, wherein the wall superheating temperature was close to the thermal fatigue of the material. The thermophysical properties, formulated as temperature-dependent polynomial equations, were used to accurately capture the boiling and vapor superheating. Flow boiling characteristics were investigated at various combinations of wall heat fluxes (181–578 kW/m²) and mass flow rates (0.1–0.19 kg/s). Parameters such as two-phase flow loss (ΔP), wall temperature (T_w), liquid volume fraction (α_l), void fraction (VF), mixture velocity (U_mix), liquid and vapor superficial velocities (U_sl,U_sg,), and heat transfer coefficient (HTC) are illustrated. Results indicated a presumed transition in the flow regime from intermittent to annular when subjected to higher heat loads. Decreasing the mass flow rate and increasing the heat flux led to a higher wall superheating, as anticipated. The reduction in the mass flow rate by the same percentage as the increase in heat flux possibly resulted in greater wall superheating. The heat transfer coefficient decreased throughout the length for all simulated heat flux cases, indicating heat transfer deterioration owing to buoyancy-induced asymmetric superheating and vapor-phase accumulation. The results emphasize the importance of meticulously evaluating the heat and mass fluxes to prevent thermal fatigue in heat exchangers and nuclear reactor components.