Abstract
While the boundary layer behavior dictates the heat transfer of a single-phase flow, the bubble layer dominates the boiling heat transfer of a low void fraction two-phase flow. The bubble layer is a highly viscous layer of crowded bubbles flowing parallel to the heating surface. In a subcooled or low quality boiling flow, the bubble layer locates between the main liquid stream and the heated wall. It shields the wall from the incoming cooling liquid so that the wall can be overheated and reach the heat flux density causing Departure from Nucleate Boiling (DNB). Increase in the DNB heat flux of the rod bundle in a PWR core can be achieved by breaking the bubble layer.
One way to break the bubble layer is to introduce the coolant flow normal to the fuel rods. Normal flow may induce vibration of the rod banks. However, flow-induced vibration is absent in a crossed-rod matrix when two adjacent banks of rods are in contact at a 90 degree angle. However, this may cause concern to the reactor designer, because the rod-contact point produces a hot spot which might cause a premature DNB. This paper reports the DNB heat fluxes measured in freon flow normal to a crossed-rod matrix having all rods heated. The result is that the DNB heat flux at the rod-contact point is much higher than that in flow parallel to the rod bundles.