Influence of Heating Rate on Subcooled Flow Boiling Critical Heat Flux in a Short Vertical Tube

The subcooled flow boiling critical heat flux (CHF) for the flow velocities (u=4.0 to 13.3m/s), the inlet subcoolings (ΔTsub, in=130 to 161K), the inlet pressure (Pin=812 to 1315kPa), the dissolved oxygen concentration (O2=5.88 and 7.34ppm) and the increasing heat input (Q0exp(t/τ), τ=38.1ms to 8.3s) are systematically measured by the experimental water loop installed the pressurizer. The SUS304 tube of test tube inner diameter (d=6mm), heated length (L=60mm), L/d=10 and wall thickness (δ=0.5mm) with the rough finished inner surface (Surface roughness, Ra=3.18µm) is used in this work. The CHF data for high heating rate were compared with the quasi steady state ones previously obtained and the values calculated by the steady state CHF correlations against outlet and inlet subcoolings. Transient CHF correlation against inlet subcooling has been given based on the experimental data for wide exponentially increasing heat input (Q0exp(t/τ), τ=38.1ms to 8.3s). The influence of heating rate on CHF was investigated into details and the dominant mechanism of subcooled flow boiling critical heat flux for high heating rate was discussed.