Heung June Chung

Fluid-Elastic Instability of Rotated Square Array U-Tubes in Air-Water Flow

Fluid-elastic instability characteristics of a U-tube bundle were experimentally investigated in air-water two-phase flow. A total of 39 U-tubes were arranged in a rotated square array with a pitch-to-diameter ratio of 1.633. Vibration responses of four U-tubes were measured with three-axis accelerometers. Two sets of experiments were performed to investigate the onset of fluid-elastic instability, and the damping and hydrodynamic mass of the U-tube. The experiments were performed for a void fraction of 70-95%. Fluid-elastic instability was clearly observed in an out-of-plane mode vibration. The effect of a primary side flow on the vibration of U-tube was investigated separately. The damping ratio of the present U-tube was higher than the damping ratio of the cantilever tubes in the literature. The hydrodynamic mass of the U-tube was generally in accordance with the hydrodynamic mass of the cantilever tubes in the literature. The instability constant (K) of the Connors equation was assessed with a simplified effective gap velocity and the fluid-elastic instability constant was 8.5.

Multiple-Hole Effect on the Performance of a Sparger During Direct Contact Condensation of Steam

An experimental study has been carried out to investigate an I-type sparger performance in view of pressure oscillation and thermal mixing in a pool. Its pitch-to-hole diameter, P/D, varies from 2 to 5. The test conditions are restricted to the condensation oscillation regime. In the present study, two different hole patterns, staggered and parallel types, are employed under various test conditions. The amplitude of the pressure pulse shows a peak for pool temperatures of 45∼85°C, which depends on P/D and the steam mass flux. The effect of hole pattern on the pressure load is smaller than that ofP/D. The dominant frequency increases with the subcooling temperature of pool water and P/D. A correlation for the dominant frequency is proposed in terms of the pitch-to-hole diameter ratio and other dimensionless thermal hydraulic parameters.

Measurements of turbulent flow in axially finned rod bundles

Abstract The hydraulic characteristics in subchannels of axially finned rod bundles installed in the Korea Multipurpose Research Reactor (KMRR) were measured using one-component laser-Doppler velocimetry (LDV). Pressure drops for each component, time mean axial velocity, and axial turbulent intensity were measured. Then the friction factors in rod bundles were estimated from the measured pressure drops. The turbulent crossflow mixing rate between neighboring subchannels was evaluated from the measured data. The results show that (1) the friction factors for axially finned rod bundles are less than those given by Moodys correlation for smooth tubes, and (2) as the flow develops, the turbulent crossflow mixing rate between neighboring subchannels decreases and, in the developed region, the values level out.

Flow-Induced Vibration Responses of U-Tube Bundle in Air-Water Flow

In the present study, a series of experiments have been performed to investigate a fluid-elastic instability of a nuclear steam generator U-tube bundle in an air-water two-phase flow condition. A total of 39 U-tubes are arranged in a rotated square array with a pitch-to-diameter ratio of 1.633. The diameter and other geometrical parameters of U-bend region are the same to those of an actual steam generator, but the vertical length of U-tubes are reduced to 2-span in contrast to 9-span of an actual steam generator. The following parameters were experimentally measured to evaluate a fluid-elastic instability of U-tube bundles in a two-phase flow: a general tube vibration response, a critical gap velocity, a damping ratio and a hydrodynamic mass. Based on the experimental measurements, the instability factor, K, of Connors’ relation was preliminary assessed with some assumptions on the velocity and density profiles of the two-phase flow.Copyright

Fluid-Elastic Instability in Tube Bundles and Effect of Flow Regime Transition

Fluid-elastic instability characteristics in an air-water two-phase cross-flow have been experimentally investigated using two different arrangements of cantilevered straight tube bundles. Rotated triangular array tube bundle is for the supplementary test of the existing work, and normal square array tube bundle is for the investigation of fluid-elastic instability in higher p/d condition. The present paper provides the experimental results of the tube vibration response, hydrodynamic mass, damping ratio, and fluid-elastic instability. As the two-phase gap velocity increased, the fluidic-elastic instability occurred in the lift direction and a strongly coupled tube motion was found. The damping ratio was very dependent on the void fraction, as in the previous works. For a low void fraction flow, the fluid-elastic instability could be predicted by using Connors’ equation. However, the fluid-elastic instability in a high void fraction flow was quite different. The transition between the two fluid-elastic instability regions almost coincided with the flow regime transition criteria from a continuous bubbly flow to an intermittent flow.Copyright