Mary V. Holloway

The Effect of Support Grid Features on Local, Single-Phase Heat Transfer Measurements in Rod Bundles

Locally averaged heat transfer measurements in a rod bundle downstream of support grids with and without flow-enhancing features are investigated for Reynolds numbers of 28,000 and 42,000 Support grids with disk blockage flow-enhancing features and support grids with split-vane pair flow enhancing features an examined. Grid pressure loss coefficients and feature loss coefficients are determined based on pressure drop measurements for each support grid design. Results indicate the greatest heat transfer enhancement downstream of the support grid designs with disk blockages. In addition, the local heat transfer measurements downstream of the split-vane pair grid designs indicate a region of decreased heat transfer below that of the hydrodynamically fully developed value. This decreased region of heat transfer is more pronounced for the lower Reynolds number case. A correlation for the local Nusselt numbers downstream of the standard support grid designs is developed based on the blockage of the support grid. In addition, a correlation for the local Nusselt numbers downstream of support grids with flow-enhancing features is developed based on the blockage ratio of the grid straps and the normalized feature loss coefficients of the support grid designs. The correlations demonstrate the tradeoff between initial heat transfer enhancement downstream of the support grid and the pressure drop created by the support grid.

Development of Swirling Flow in a Rod Bundle Subchannel

Experimental measurements of the axial development of swirling flow in a rod bundle subchannel are presented. Swirling flow was introduced in the subchannel from a split vane pair located on the downstream edge of the support grid. Particle image velocimetry using an optical borescope yielded full-field lateral velocity data. Lateral flow fields and axial vorticity fields at axial locations ranging from 4.2 to 25.5 hydraulic diameters downstream of the support grid were examined for a Reynolds number of 2.8×10 4

Mapping of the Lateral Flow Field in Typical Subchannels of a Support Grid With Vanes

Lateral flow fields in four subchannels of a model rod bundle fuel assembly are experimentally measured using particle image velocimetry. Vanes (split-vane pairs) are located on the downstream edge of the support grids in the rod bundle fuel assembly and generate swirling flow. Measurements are acquired at a nominal Reynolds number of 28,000 and for seven streamwise locations ranging from 1.4 to 17.0 hydraulic diameters downstream of the grid. The streamwise development of the lateral flow field is divided into two regions based on the lateral flow structure. In Region I, multiple vortices are present in the flow field and vortex interactions occur. Either a single circular vortex or a hairpin shaped flow structure is formed in Region II. Lateral kinetic energy, maximum lateral velocity, centroid of vorticity, radial profiles of azimuthal velocity, and angular momentum are employed as measures of the streamwise development of the lateral flow field. The particle image velocimetry measurements of the present study are compared with laser Doppler velocimetry measurements taken for the identical support grids and flow condition.

The Effect of Support Grid Design on Azimuthal Variation in Heat Transfer Coefficient for Rod Bundles

Support grids are an integral part of nuclear reactor fuel bundle design. Features, such as split-vane pairs. are located on the downstream edge of support grids to enhance head transfer and delay departure from nucleate boiling in the fuel bundle. The complex flow fields created by these features cause spatially varying hert transfer conditions on the surfaces of the rods. Azimuthal variations in heat transfer for three specific support grid designs, a standard gird, split-vane pair grid, and disc grid, are measured in the present study using a heated, thin film sensor. Normalized values of the azimuthal variations in Nusselt number are presented for the support grid designs at axial locations ranging from 2.2 to 36.7 D h . Two Reynolds numbers, Re = 28,000 and Re = 42,000 are tested

Investigation of Swirling Flow in Rod Bundle Subchannels Using Computational Fluid Dynamics

The fluid dynamics for turbulent flow through rod bundles representative of those used in pressurized water reactors is examined using computational fluid dynamics (CFD). The rod bundles of the pressurized water reactor examined in this study consist of a square array of parallel rods that are held on a constant pitch by support grids spaced axially along the rod bundle. Split-vane pair support grids are often used to create swirling flow in the rod bundle in an effort to improve the heat transfer characteristics for the rod bundle during both normal operating conditions and in accident condition scenarios. Computational fluid dynamics simulations for a two subchannel portion of the rod bundle were used to model the flow downstream of a split-vane pair support grid. A high quality computational mesh was used to investigate the choice of turbulence model appropriate for the complex swirling flow in the rod bundle subchannels. Results document a central swirling flow structure in each of the subchannels downstream of the split-vane pairs. Strong lateral flows along the surface of the rods, as well as impingement regions of lateral flow on the rods are documented. In addition, regions of lateral flow separation and low axial velocity are documented next to the rods. Results of the CFD are compared to experimental particle image velocimetry (PIV) measurements documenting the lateral flow structures downstream of the split-vane pairs. Good agreement is found between the computational simulation and experimental measurements for locations close to the support grid.Copyright

Continuous Wavelet Transforms of Instantaneous Wall Pressure in Slug and Churn Upward Gas-Liquid Flow

Continuous wavelet transforms are employed to determine the time-localized frequency content (scalogram) of instantaneous wall pressure signals in upward gas-liquid flow. The flow conditions correspond to well-defined slug flow, well-defined churn flow, and flows near the transition from slug-to-churn flow. Scalograms demonstrate that the frequency content of the pressure signals is time-dependent, and visual observations of the flow conditions suggest that the time-dependent frequencies are related to identifiable physical behaviors of the flow. In well-defined slug flow, the scalograms are characterized by the presence of a dominant frequency throughout the duration of the signal and by frequency shifting events. Scalograms representing well-defined churn flow contain intermittent frequencies, and the energy density in churn flow is spread over a wider range of frequencies than in slug flow. The present results provide evidence that flows near transition alternately display characteristics of both well-defined slug and well-defined churn flows. @DOI: 10.1115/1.1490376#