Transient critical heat fluxes of subcooled water flow boiling in a SUS304-circular tube caused by a rapid decrease in velocity from non-boiling regime

• Measured flow transient CHF caused by a rapid decrease in velocity from non-boiling regime.
• Compared with steady-state CHF data for vertical and horizontal SUS304-circular tubes.
• Derived correlations of CHF and flow velocity at flow transient CHF for circular tube.
• The new correlations can describe flow transient CHFs within ±20% difference.
• Discussed mechanism of flow boiling CHF in circular pipe.

The flow transient critical heat fluxes (FT-CHFs, qcr,sub) in a SUS304-circular tube caused by a rapid decrease in velocity from non-boiling regime are systematically measured for various initial flow velocities, initial heat fluxes, inlet liquid temperatures, outlet pressures and decelerations caused by a rapid decrease in velocity by the experimental water loop comprised of a multistage canned-type circulation pump controlled by an inverter. The SUS304-circular tubes of inner diameter (d = 6 mm), heated length (L = 59.5 mm), L/d (=9.92) and wall thickness (δ = 0.5 mm) with average surface roughness (Ra = 3.89 μm) are used in this work. The flow transient CHFs for SUS304-circular tube are compared with authors’ steady-state CHF data for the empty VERTICAL and HORIZONTAL SUS304-circular tubes and the values calculated by authors’ steady-state CHF correlations against outlet and inlet subcoolings for the empty circular tube. The influences of initial flow velocity (u0), initial heat flux (q0) and deceleration caused by a rapid decrease in velocity (α) on the flow transient CHF are investigated into details and the widely and precisely predictable correlations of CHF and flow velocity at the flow transient CHF for the circular tube is given based on the experimental data. The correlations can describe the flow velocity and the CHFs at the flow transient CHFs for SUS304-circular tube obtained in this work within ± 20% difference.