Effects of Flow Obstacles on Single-Phase Heat Transfer and CHF in a Narrow Rectangular Channel

Abstract

Mixing vane geometries enhance the fuel-to-coolant heat transfer within nuclear reactors, which allows higher power ratings for reactors and improves their economics. At the same time, their presence may affect the critical heat flux (CHF), the upper limit to heat extraction rate of a reactor. Numerical simulations often do not accurately reflect the changes in CHF when mixing vanes are included in nuclear fuel assemblies, suggesting that the CHF models are not resolving the relevant boiling phenomena present with mixing vane geometries. Thus, there is a need for experimental data to inform future simulations for more accurate results. This thesis aims to address this need by modifying an existing experimental apparatus to reproduce and measure the momentum and thermal transport of flows when perturbed by flow obstacles. The capabilities of the apparatus were demonstrated through experiments comparing heat transfer characteristics of single-phase flow in an empty rectangular channel and in a channel with a flow obstacle. Comparisons with simulations of the experiment were done to highlight the ability of simulation tools to elucidate some of the hydrodynamic behavior of fluids near obstacles responsible for the enhancement of heat transfer. Initial two-phase experiments were also done to characterize the changes in CHF in the presence of a flow obstacle, which when compared to an empty flow channel show a degradation of CHF at low mass fluxes and an enhancement at high mass fluxes. However, future research is needed to understand the reason for these changes.