Channel Gap Size Research Articles

Onset of Nucleate Boiling in Research Reactors with Thin Rectangular Channels Under Low-Velocity Upward-Flow Conditions

Currently used correlations to predict the onset of nucleate boiling heat flux q/sup ''//sub ONB/ in research reactor channels determine q/sup ''//sub ONB/ in terms of local channel pressure and wall superheat. Recent experiments show that these correlations may over- or underestimate q/sup ''//sub ONB/ by as much as a factor of 5 in thin rectangular channels for low-velocity upward flows. Such flow conditions are encountered in the natural convection cooling of research reactors with plate-type fuels. A set of experiments are performed to quantify the effect of channel flow velocity and gap size (in the ranges of 2 to 14 cm/s and 2 to 4 mm, respectively) on q/sup ''//sub ONB/ for upward flow in rectangular channels. An adjustable gap between two internally heated aluminum blocks forms the flow channel. A new correlation is proposed that predicts the experimental q/sup ''//sub ONB/ results within 13% for flows with Re < 700 (Re based on channel gap) and that is valid in the 1.40- to 1.46-atm pressure range.