Conference Agenda

A critical heat flux (CHF) occurring on a heat transfer surface under forced flow conditions has different mechanisms depending on the flow channel geometry and flow conditions. In the thermal design of reactor cores, the CHF is an important phenomenon, and it is essential to understand the CHF characteristics under actual flow conditions to improve the CHF prediction method. In this study, steady-state CHF experiments were conducted in forced convection boiling flow at low velocities under high-temperature and high-pressure conditions using a 2 × 2 heated rod bundle with a heated length of approximately 1.2 m. An optical fiber sensor inserted in a 0.5 mm diameter metal tube was mounted on the rod surface and captured the axial distribution of the rod surface temperature at a frequency of 100 Hz and a spatial resolution of 2.6 mm. The experimental results showed intermittent increases and decreases in the rod surface temperature at the top of the heated rod bundle section with stepwise increases in the rod bundle thermal power. This corresponds to repeated localized dry patch formation and rewetting. As the inlet subcooling decreased, the onset of the rod surface temperature increase shifted upstream and dry patches formed over a larger area in the flow direction. A slight increase in the thermal power of the rod bundle near the CHF expanded the area of dry patches or increased the frequency of their occurrence, leading to a transition to a continuous increase in rod surface temperature.

Subchannel analysis is the most competitive approach in thermal hydraulics analysis of rod bundles. It considers transport of mass, momentum and energy axially along the subchannel and laterally across the gaps between the subchannel. Turbulent mixing is an influential parameter for lateral exchange across the gaps which is caused due to velocity fluctuations in the axial direction. Several factors such as rod bundle geometry, coolant flowing properties, gap distance between the subchannels and eddy diffusivity play an essential role in the turbulent mixing parameter. A wide number of experimentally fitted empirical correlations are present to predict turbulent mixing parameter for different subchannel geometry with a significant average mean error among themselves. In 2018, Shen et.al. performed Computational Fluid Dynamics (CFD) for a square bare rod bundle and developed a correlation for square-square center subchannel interaction for the turbulent mixing parameter. In this paper, a similar CFD analysis is performed for a hexagonal bare rod bundle between two triangular center-center subchannel and a center-side subchannel using Reynolds Stress Model (RSM) for a range of Pitch to Diameter ratio varying from 1.1-1.5 for the Reynolds number in the range 8000 to 100000. A new correlation is being developed for turbulent mixing parameter using this numerically generated data. The developed correlation is then compared with the existing empirical correlations.

The void fraction is a key parameter for affecting the coolability and neutron-moderating performance of water-cooled nuclear reactor. More refined experimental data are required to develop multi-fluid dynamics models for determining the void fraction distribution. A Wire Mesh Sensor (WMS) with rod electrodes was developed to measure the cross-sectional distribution of void fraction in a 7 × 7 heated rod bundle with a diameter of 9.5 mm and pitched at 12.6 mm, and applied to a boiling two-phase flow experiment under atmospheric pressure conditions assuming at accident in pressurized water reactor (PWR). The sensor consists of 8-wire by 8-wire and 7-rod by 7-rod electrodes. Wire electrodes with a diameter of 0.2 mm are arranged in a horizontal and vertical crosswise between the rod bundles. For each measurement, the local void fraction in the subchannel center at 64 points were obtained from the wire by wire electrodes and 196 void fraction points near the rod surface were obtained from the wire to rod electrodes. The temporal resolution of the void fraction measurements was 2500 frames. The axial and radial power distribution of the heated rod bundle is uniform.

This study presents the development of a detailed experimental database for two-phase flow interfacial parameters in a 5×5 rod bundle channel featuring a spacer grid. The investigation aims to elucidate the spatial and transport characteristics of gas-liquid interfacial structures and the influence of spacer grids on two-phase flow dynamics.

A comprehensive experimental system was designed, incorporating flow visualization, four-sensor conductivity probe, and two-phase PIV (Particle Image Velocimetry) measurement technologies. Key interfacial parameters, including void fraction, bubble size, interfacial area concentration, and velocities of gas and liquid phases, were systematically measured and analyzed under various flow conditions.

Results reveal distinct distributions of void fractions transitioning from "core-peak" to "gap-peak" patterns as liquid velocity increases, driven by enhanced turbulent mixing. Spacer grids significantly disrupt flow characteristics, causing bubble breakup and coalescence, with effects extending approximately 20 hydraulic diameters downstream. Existing drift and fluctuation velocity models underpredict the impact of spacer grids, highlighting the need for model optimization.

This work provides critical insights into the complex behavior of two-phase flow in rod bundle channels, offering validated datasets to enhance computational models for reactor thermal-hydraulics and guiding the design of spacer grid structures for improved flow stability.

The boundary layer forms the main thermal resistance for heat transfer between the coolant and the fuel rod. Therefore, the structures of the velocity boundary layer greatly affect the thermal hydraulic performance of the fuel rods. The present study performed experimental investigation on effects of pulsating flow on evolution of the velocity boundary layer in a 5×5 rod bundle channel. The Time Resolved Particle Image Velocimetry (TR-PIV) technique is used to directly measure the velocity distributions near the rod surface under different flow conditions. The velocity boundary layer is reconstructed from the measured velocity. The dimensionless velocity distribution over the surface of the fuel rod is obtained by fitting the experimental data to the Spalding formula. The structure of the boundary layer and flow characteristics are analyzed and compared quantitatively. The experimental results indicate that the perturbation introduced by the pulsating flow can disrupt the development of the boundary layer and significantly reduce the thickness of the inner layer of the boundary layer. The larger the amplitude and the smaller the period, the greater the perturbation introduced by the pulsating flow.