Transient critical heat flux of upward water flow boiling in vertical small tube at slow velocity

The transient critical heat flux (CHF) due to exponentially increasing heat input was experimentally measured for upward water flowing in vertical small tubes. The heat generation rate was increased exponentially with a function of Q(o)exp (t/tau), where, Q(o) is an initial heat generation rate, tau represents time and r is e-folding time. The test tubes were made of platinum with inner diameters of 0.7 mm and 1.0 mm, and heated lengths of 12.0 mm and 40.9 mm. The flow velocities and the inlet liquid temperatures ranged from 2.0 m/s to 8.0 m/s, 90 K to 140 K, and the inlet pressure was 800 kPa. It was obtained that the transient CHF was affected by inner diameter of test tube, liquid subcooling, flow velocity, and e-folding time. The transient CHF values increased with the increases in flow velocity and inlet subcooling. The transient CHFs increased with a decrease in e-folding time at tau <1 s, and they approached steady-state values at tau >1 s. It was understood that the heat transfer is in steady-state at tau >1 s, and it is in transient state at tau <1 s. Moreover, the values of transient CHFs at diameter of 0.7 mm were higher than those at the diameter of 1.0 mm. The correlation by Hata and Noda (2008) can express authors' transient CHFs at e-folding time larger than about 1 s (steady-state region), however, it shows larger values than authors' data at e-folding time smaller than 1 s (transient region).