Measurements of the Effects of a Wire-Wrap Spacer on the Thermalhydraulics of Heated Annular Upward Flow of Supercritical R134A in Steady and Transient Conditions

Abstract

Generation IV Supercritical Water-Cooled Reactor fuel rod bundles will make use of wire-wrap spacers to improve heat transfer through turbulence generation and the promotion of inter-channel mixing. However, our current understanding of the effects of wire-wrap spacers on convection heat transfer particularly in the supercritical thermodynamic regime is limited. Accordingly, experiments were conducted on a test section consisting of a 2-m long electrically-heated rod of circular cross-section centered in a circular-tube pressure boundary using R134a as the working fluid. The measurements focused on determining the effects of localized spacers and wire-wrap spacers on the generation of near-wall turbulence for improved convective heat transfer. Measurements were performed on a closed-loop experimental facility in which the test section was mounted in the vertical upflow configuration. Tests were conducted at two values of flow Reynolds numbers, for two variations of discrete spacer geometry and two wire wrap pitch-to-heater rod diameter ratios. The results of these steady-flow steady-state measurements provide insight into the heat transfer and pressure loss characteristics of a simulated fuel rod cooled by a supercritical fluid in an isolated annular upflow configuration as affected by a range of spacer configurations. Additionally, tests were conducted for a range of transient operating conditions, including transients in fluid pressure, mass flux and heat flux. Through these measurements, insight is gained on how such transients affect the deteriorated heat transfer phenomenon observed in heated supercritical fluids.