Transient Critical Heat Fluxes of Subcooled Water Flow Boiling in a Short Vertical Tube Caused by Exponentially Increasing Heat Inputs

The transient critical heat fluxes (CHFs) of the subcooled water flow boiling for the flow velocities $(u=4.0–13.3m∕s)$⁠, the inlet subcoolings $(ΔTsub,in=68.08–161.12K)$⁠, the inlet pressures $(Pin=718.31–1314.62kPa)$⁠, the dissolved oxygen concentrations (⁠$O2=2.94ppm$ to the saturated one), and the exponentially increasing heat inputs (⁠$Q0exp(t∕τ)$⁠, $τ=16.82msto15.52s$⁠) are systematically measured with an experimental water loop comprised of a pressurizer. The SUS304 tubes of the inner diameters (⁠$d=3mm$⁠, $6mm$⁠, $9mm$⁠, and $12mm$⁠), heated lengths $(L=33.15–132.9mm)$⁠, $L∕d=5.48–11.08$⁠, and wall thickness (⁠$δ=0.3mm$ and $0.5mm$⁠) with the rough finished inner surface (surface roughness, $Ra=3.18μm$⁠) are used in this work. The transient CHF data $(qcr,sub=6.91–60MW∕m2)$ are compared with the values calculated by the steady state CHF correlations against inlet and outlet subcoolings. The transient CHF correlations against inlet and outlet subcoolings are derived based on the experimental data. The dominant mechanisms of the subcooled flow boiling CHF for a high heating rate are discussed.