Experimental Study of Single Phase Turbulent Pipe Flow with Variable Obstructions

Abstract

A considerable amount of effort is expended to better characterize and avoid the critical heat flux (CHF) phenomena in nuclear reactor systems. Most fuel assemblies used in power reactors possess small structures to maintain them in place as well as maintain the physical integrity of the assembly and element to element clearances. These structures act as local flow obstructions that modify the turbulent flow characteristics and in general improve the heat transfer. In CANDU reactors, a series of bearing pads are used to maintain clearances between the fuel and the pressure tube while spacer pads maintain element to element clearances. This thesis experimentally investigates the impact of bearing pads on the downstream flow and turbulence characteristics under room temperature and pressure conditions. The test section is fabricated from a 30 cm long and 2.54 X 2.54 cm squared transparent cast acrylic piece. Machined through the center is a cylindrical flow channel with inner diameter of approximately 11 cm. CANDU fuel bundle bearing pad simulators have been designed and constructed at McMaster University to allow for the study of the local flow characteristics downstream of these obstructions. Measurements are taken with unobstructed flow and a variety of obstruction heights to examine the turbulence features generated. Each bearing pad is removable and is secured in place using high strength magnets. The test section connects to an existing room temperature fluid loop capable of delivering accurate flows up to a Reynolds number of 100000. In order to validate the test section and measurement methods, a series of experiments were performed without obstructions. These measured velocity and turbulence data were compared to literature values to ensure similarity. After this validation study, experiments were carried out using room temperature water as fluid at three different turbulent Reynolds numbers of 30700, 61400 and 99000 with and without obstructions, using Laser Doppler Velocimetry. Results in the streamwise direction were as expected, with velocity perturbations that in general decreased with increasing Reynolds number. Some evidence of local recirculation in the wake of the obstruction was also measured. In the azimuthal direction, velocity and turbulence measurements were largely isotropic downstream of the obstruction for low Reynolds numbers and small obstructions. At higher Reynolds number and/or larger obstructions, the components in the azimuthal direction were systematic and exhibited coherent like structures (i.e., rotating flow or swirl). Of particular importance was the decay of turbulence with downstream length which was significant in cases where no coherent structures were present. With coherent structures, the turbulence levels remained high for much further distances. The source of the structures at high Reynolds number is not clear as all physical geometries were verified to ensure no systematic biases were present.