Void fraction covariance and relative velocity covariance for steam-water boiling flows in NUPEC Type I and Type II rod bundles

Accurately modeling gas-liquid two-phase flows in rod bundles is critical for process engineering, equipment optimization, and safety assessments involving rod bundle channels. Void fraction covariance and relative velocity covariance are key parameters that indicate the effect of non-uniform void fraction distribution on the value of area-averaged relative velocity. Reliable correlations for these covariances are essential for accurately modeling two-phase flows in system analysis codes. Developing constitutive relations for void fraction and relative velocity covariance requires accurate experimental data. Under light water-cooled reactor conditions, X-ray computed tomography (CT) is suitable for measuring gas-liquid two-phase flows. However, measurement noise is often present in the void fraction distribution data collected by X-ray CT. To address this, a post-processing algorithm was developed to remove noise from the X-ray CT data for rod bundle channels under BWR prototypical conditions. Based on the optimized data, constitutive correlations for void fraction covariance and relative velocity covariance in rod bundles were established for both bulk and subcooled boiling flows. The mean absolute relative deviations of these correlations for void fraction covariance and relative velocity covariance were 1.03 % and 0.956 %, respectively. Furthermore, the developed correlation was validated for a rod bundle containing a large water rod, with mean absolute relative deviations of 0.851 % for void fraction covariance and 1.31 % for relative velocity covariance.