Mechanism of critical heat flux during flow boiling of subcooled water in a circular tube at high liquid Reynolds number

Abstract

The subcooled boiling heat transfer and the steady state critical heat flux (CHF) in a vertical circular tube for the liquid Reynolds numbers (Red=2.77×104–3.08×105) and the flow velocities (u=3.95–30.80m/s) are systematically measured by the experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The SUS304 test tube of inner diameter (d=6mm) and heated length (L=59.5mm) is used in this work. The outer surface temperatures of the SUS304 test tube with heating are observed by an infrared thermal imaging camera and a video camera. The subcooled boiling heat transfers for SUS304 test tube are compared with the values calculated from correlations due to other researchers for the subcooled boiling heat transfer. The influence of flow velocity on the subcooled boiling heat transfer and the CHF is investigated in detail based on the experimental data. Nucleate boiling surface superheats at the CHF are close to the lower limit of the heterogeneous spontaneous nucleation temperature and the homogeneous spontaneous nucleation temperature. A suggestion as to what the dominant mechanism is for the subcooled flow boiling CHF on the SUS304 circular tube is made at high liquid Reynolds number. On the other hand, the RANS equations (Reynolds Averaged Navier–Stokes Simulation) with k–ε turbulent model in a circular tube of a 3mm in diameter and a 526mm long are numerically solved for heating of water on heated section of a 3mm in diameter and a 67mm long with various thicknesses of conductive sub-layer by using PHOENICS code under the same conditions as the experimental ones previously obtained and with temperature dependent thermo-physical fluid properties. The Platinum (Pt) test tube of inner diameter (d=3mm) and heated length (L=66.5mm) was used in this experiment. The thicknesses of conductive sub-layer from non-boiling regime to CHF are measured. The thicknesses of conductive sub-layer at the CHF point are predicted for various flow velocities. The experimental values of the CHF are also compared with the corresponding theoretical values of the liquid sub-layer dry-out models suggested by other researchers, respectively. A suggestion as to what the dominant mechanism is for the subcooled flow boiling CHF on the Pt circular tube is made at high liquid Reynolds number.