Modeling of void fraction covariance and relative velocity covariance for upward boiling flows in subchannels of a vertical rod bundle.

• Interfacial drag forces for interior, edge, and corner subchannels were formulated in a rod bundle. • Subchannel-average void fraction covariance was empirically modeled for saturated boiling flow. • Subchannel-average void fraction covariance was analytically modeled for subcooled boiling flow. • The covariance models were validated by steam-water data up to 8.6 MPa. • The relative velocity covariances were calculated from the void fraction covariances. Accurate simulation of boiling two-phase flows in a rod bundle is indispensable for the robust, economical, and safe design of various heat transfer systems using the rod bundle configuration. Subchannel analysis codes are used for this purpose. Interfacial drag force modeling significantly affects the prediction accuracy of the void fraction. The void fraction and relative velocity covariances constitute the interfacial drag force. However, the covariances are currently not considered in existing subchannel codes due to a lack of reliable constitutive equations to calculate the void fraction and relative velocity covariances. This study aims to model the subchannel-average void fraction and relative velocity covariances for subcooled and saturated boiling flows in three types of subchannels in a rod bundle. The considered subchannels are interior, edge, and corner subchannels. The subchannel-average void fraction and relative velocity covariances for saturated boiling flow are modeled by the data obtained from local void fraction data collected for saturated boiling flow in an 8 × 8 rod bundle under pressures from 1.0 to 8.6 MPa. The subchannel-average void fraction and relative velocity covariances for subcooled boiling flow are modeled based on the bubble-layer thickness model. The modeled subchannel-average void fraction and relative velocity covariances are well validated with the experimental data. The modeled subchannel-average void fraction and relative velocity covariances are expected to be implemented in subchannel analysis codes to improve the void fraction prediction accuracy in each subchannel type. [ABSTRACT FROM AUTHOR]