Large-Eddy Simulation of convective wall-boiling flow along an idealized PWR rod bundle

Abstract

A LES analysis of turbulent convective boiling flow along the rods of a PWR sub-channel was performed using the code TransAT. The campaign is an extension of a former attempt in which the flow was predicted by means of DNS (or super resolved LES). The extension refers to accounting for wall-boiling heat transfer using the triple flux decomposition model, employed within the LES approach constructed on the basis of the filtered mixture equations. The results in the wall boiling configuration differ from the single-flow case in many ways, but more particularly: the flow seems to be portioned in three zones, (i) pre-boiling, (ii) transitional boiling, and (iii) post-boiling, where turbulence activity increases with distance from the onset of nucleate boiling location, affecting heavily the dynamic and thermal boundary layers. The boundary layers thin quite substantially in the post-boiling section, and the flow accelerates due to higher volumetric flow rate induced by phase change, notably in the gap region. The vorticity distribution between the single flow and boiling flow cases suggests that boiling causes a higher rate of vorticity generation. These results corroborate with the other finding, that is: the average frictional velocity in the boiling case increases by a factor of 1.5. Scrutinizing the power density spectra reveals that the boiling flow embodies about twice more energy than the single phase flow, with marked large-scale energetic eddies at the upper end of the rods, i.e. z/L>0.8. A clear transitional behavior is observed in the boiling flow PSD, in that the amount of energy embodied in the flow increases with vapor production.