Spacer Grid Separate Effects

The nuclear reactor coolant system is composed of various flow geometries and restrictions. Many geometric effects of two-phase flow encountered in a typical reactor system have not yet been fully examined, including the effect of rod bundle spacer grids.

The purpose of this research is to investigate the geometric effects of two- phase flow through a prototypic spacer grid to establish both the local and global database necessary for model development and code validation. The end goal is to develop predictive models that account for the effects of spacer grids and to serve as constitutive relations in the interfacial area transport equation. This model has been developed for various flow conditions and has been validated in an extensive database; however, further study on geometric effects is indispensable to establish a comprehensive and robust model. In view of this, the present test facility is designed to study the effects of rod bundle spacer grids on interfacial area transport in two-phase flow.

The facility employed is a scaled test facility based on a conventional pressurized water reactor. 19 dimensionless geometric parameters and five flow parameters were employed in the design of the facility to achieve similarity between the test facility and the prototypic system. The facility consists of a 1x3 array of 3.18 cm diameter simulant fuel rods with a pitch of 4.45 cm that are arranged in a rectangular channel with a 13.65 cm x 4.45 cm cross-section that spans 3.3 m from injection to outlet. The facility was designed as a narrow rectangular channel with clear acrylic sides and rods to facilitate flow visualization. Seven axial measurement locations are available along the length of the test section, one upstream of the spacer grid and six downstream of the spacer grid, to obtain measurements of local two-phase flow parameters throughout the channel cross-section

A High speed camera is employed to perform flow visualizations to visually observe the effect of the spacer grid on flow regime transition. Four flow regimes are observed by researchers: Bubbly, Cap-Turbulent, Slug and Churn-Turbulent. Two major effects of the spacer grid on the flow regime transition are observed, bubble coalescence, and bubble breakup. The observation of larger bubbles after the spacer grid indicated bubble coalescence for various conditions. While during other conditions the increased quantity of smaller bubbles indicated bubble breakup occurring.