Influence of heat input waveform on transient critical heat flux of subcooled water flow boiling in a short vertical tube Original Research Article

The transient critical heat fluxes (CHFs) of the subcooled water flow boiling for ramp-wise heat input [Q = αt, α = 6.21 × 108 to 1.63 × 1012 W/m3 s, (q ≅ 1.08 × 107 to 6.00 × 107 W/m2)] and stepwise one [Q = Qs, Qs = 0 W/m3 at t = 0 s and Qs = 2.95 × 1010 to 7.67 × 1010 W/m3 at t > 0 s, (q = 0 W/m2 at t = 0 s and q ≅ 1.61 × 107 to 3.87 × 107 W/m2 at t > 0 s)] with the flow velocities (u = 4.0–13.3 m/s), the inlet subcoolings (ΔTsub,in = 86.8–153.3 K) and the inlet pressures (Pin = 742.2–1293.4 kPa) are systematically measured by an experimental water loop comprised of a pressurizer. The SUS304 tubes of inner diameters (d = 3, 6 and 9 mm), heated lengths (L = 33.15, 59.5 and 49.3 mm), L/d (=11.05, 9.92 and 5.48), and wall thickness (δ = 0.5, 0.5 and 0.3 mm) respectively with the rough finished inner surface (surface roughness, Ra = 3.18 μm) are used in this work. The experimental errors in the subcooling measure and the pressure one are ±1 K and ±1 kPa, while in the heat flux it is ±2%. The transient CHF data for the ramp-wise heat input and the stepwise one are compared with those for the exponentially increasing heat input (Q = Q0 exp(t/τ), τ = 16.82 ms to 15.52 s) previously obtained and the dominant variables on transient CHF for heat input waveform difference are confirmed. The transient CHF data are compared with the values calculated by the steady state CHF correlations against inlet and outlet subcoolings, and the applicability of steady state CHF correlations is confirmed extending its possible validity for the reduced time, ωp, down to 800 ms. The transient CHF data are compared with the values calculated by the transient CHF correlations against inlet and outlet subcoolings, and the influence of heat input waveform on transient CHF is clarified based on the experimental data for the ramp-wise heat input, the stepwise one and the exponentially increasing one. The dominant mechanisms of the subcooled flow boiling critical heat flux for the ramp-wise heat input, the stepwise one and the exponentially increasing one are discussed.