Heat transfer on a flat plate at one end of a series-connected rectangular duct in pressurized He II

Steady-state and transient heat transfer on a flat plate at one end of a two-series-connected duct with a sudden change of the cross-section were performed in subcooled He II at bath temperature of 1.8 K in order to simulate the effect of the heat flow expansion in the cooling channel. The results of the steady-state critical heat flux (CHF) can be described within 15% by the correlation of CHF derived here. For the stepwise beat flux larger than the CHF, quasisteady-state exists for a certain duration called lifetime on the extension of Kapitza conductance curve. With increase in the stepwise heat flux, the lifetime becomes shorter and approaches that for one-dimensional duct. The experimental results were analyzed by the two-dimensional computer code named SUPER-2D developed by the authors based on the two fluid model and the theory of the mutual friction. The solutions of CHF and lifetime agreed well with the experimental data. The heat flow mechanism in the duct was clarified by the analysis.