Experimental Studies on the Effects of a Spacer Grid in a 1x3 Rod Bundle Single and Two-phase Flow

Open Access
Wheeler, Joshua Edward
Graduate Program:
Mechanical Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
Committee Members:
  • Seungjin Kim, Thesis Advisor
  • Horacio Perez Blanco, Thesis Advisor
  • Karen Ann Thole, Thesis Advisor
  • spacer grid
  • rod bundle
  • two-phase flow
  • air-water
  • conductivity probe
  • laser Doppler anemometry
  • interfacial area concentration
Understanding the effects of spacer grids on the coolant flow through a nuclear reactor core is required for best-estimate design and analysis of the nuclear power plant. The current work investigates the effects of a spacer grid on the hydrodynamics of adiabatic air-water single-phase and two-phase flow through a scaled 1x3 rod bundle. Thru the use of a high-speed digital movie camera two-phase flow conditions are selected to study the impact of the spacer grid on four separate regimes, bubbly, turbulent-cap, slug and churn-turbulent and their transition boundaries. A state of the art four sensor conductivity probe capable of measuring local time averaged two-phase flow parameters is employed to investigate locally the influence of the spacer grid on two-phase flow at four axial locations, one upstream and three downstream of the spacer grid. 1-Dimensional laser Doppler anemometer (LDA) is employed to measure local axial turbulence distributions in single-phase flow at three locations, one upstream, and two downstream. The local conductivity probe data indicates a redistribution effect caused by the spacer grid, inducing mixing and lateral motion of the bubbles. Furthermore the local data indicates near the cap-turbulent to slug transition boundary an increased spacer grid induced coalescence effect through the decreasing group-I (spherical/distorted bubbles) void fraction. The slug and churn-turbulent transition boundary demonstrates a spacer grid induced breakup of bubbles observed in Sauter mean diameter leading to an increase in the interfacial area concentration. The axial turbulence intensity data indicates the influence that individual structures of the spacer grid have on the flow. The region with spacer grid dimples directly upstream of the measurements has increased turbulence intensity compared to the region without. The study establishes a high fidelity database containing 2,496 local two-phase measurements, and 534 local single-phase measurements around the center rod of the test facility, under the influence of a spacer grid, which can be used to benchmark computational fluid dynamics (CFD) two-phase models.