Transient heat transfer on a flat plate at one end of a duct with an orifice in pressurized HeII

Transient heat transfer caused by a large stepwise heat input to a flat plate with 10 mm in width and 40 mm in length at one end of a rectangular duct containing subcooled He II was measured at atmospheric pressure. Four types of rectangular ducts with the same inner dimensions of 10 x 40 x 100 mm(3) were used. Three of them are with orifices of 9%, 25% and 50% open mouth at the center of their lengths, and the other is without orifice. Each test plate was pasted on an inner side of an end-plate made of fiber-reinforced-plastic (FRP) and the other end of the duct is open to a pool of pressurized He II. The steady-state critical heat fluxes (CHFs) on the plates are highest for the duct without orifice and are lower for the ducts with smaller orifices. The transient heat transfer for a stepwise heat input is such that the quasi-steady state exists with a certain lifetime on the extrapolation of steady-state Kapitza conductance curve and then jumps to film boiling regime. The lifetime for the duct without orifice agrees well with that predicted by the authors' theoretical correlation already given. The lifetime for a certain quasi-steady heat flux in a relatively lower range is shorter for the ducts with smaller orifices and approaches the value by the correlation with the increase in the step height.