Subchannel-averaged void fraction covariance and relative velocity covariance for steam-water boiling flows in NUPEC type I and type II bundles

In subchannel analysis codes, constitutive correlations for subchannel-averaged void fraction covariance and relative velocity covariance are essential to account for the effects of non-uniform void fraction distribution on the subchannel-averaged relative velocity between gas and liquid phases. In this study, NUPEC (Nuclear Power Engineering Center) rod bundle data measured using X-ray CT were processed with a noise-removal method to obtain subchannel-averaged void fraction covariance and relative velocity covariance data for boiling two-phase flows. The experiments covered pressure ranges from 0.981 MPa to 8.63 MPa, with subchannel-averaged void fractions ranging from 0.1 to 0.8. The void fraction covariance in different subchannels decreased with increasing subchannel-averaged void fraction. Meanwhile, the relative velocity covariance initially increased and then decreased as the subchannel-averaged void fraction increased. Across the experimental pressure range, pressure effects on subchannel-averaged void fraction covariance and relative velocity covariance were minimal. Correlations for subchannel-averaged void fraction covariance and relative velocity covariance were developed based on the experimental data. For interior, edge, and corner subchannels, the mean absolute relative deviations between the proposed correlations and the experimental data were within 2.0 %, demonstrating the high accuracy of the correlations. These correlations were also validated for subchannels in a rod bundle containing a large water rod, yielding mean absolute relative deviations within 1.5 % for the interior, edge, and corner subchannels.