Bao-Wen Yang

The effect of spacer grid critical component on pressure drop under both single and two phase flow conditions

Abstract As pressure drop is one of the most critical thermal hydraulic parameters for spacer grids the accurate estimation of it is the key to the design and development of spacer grids. Most of the available correlations for pressure drop do not contain any real geometrical parameters that characterize the grid effect. The main functions for spacer grid are structural support and flow mixing. Once the boundary sublayer near the rod bundle is disturbed, the liquid forms swirls or flow separation that affect pressure drop. However, under two phase flow conditions, due to the existence of steam bubble, the complexity for spacer grid are multiplied and pressure drop calculation becomes much more challenging. The influence of the dimple location, distance of mixing vane to the nearest strip, and the effect of inter-subchannel mixing among neighboring subchannels on pressure drop and downstream flow fields are analyzed in this paper. Based on this study, more detailed space grid geometry parameters are recommended for adding into the correlation when predicting pressure drop.

CFD analysis on mixing effects of spacer grids with different dimples and sizes for advanced fuel assemblies

Abstract The thermal hydraulic characteristics of a mixing vane grid are largely dependent on the structure of key components, such as strip, spring, dimple, weld nugget, as well as the mixing vane configuration. In this paper, several types of spacer grids with different dimple shapes are modeled under subcooled boiling conditions. Prior to the application of CFD on the dimple shape analysis, the mixing effects of spacer grids were studied. After the dimple shape analysis, the side channel effect is discussed by comparing the simulation results of a 3 × 3 and a 5 × 5 spacer grid. The two phase flow CFD models in this study are validated through simple geometry showing that the calculated void fraction is in good agreement with the experimental data. The dimple comparison result shows that varying dimple structures can result in different temperatures, lateral velocities and void fraction distributions downstream of the spacer grids. Comparison of two sizes of spacer grids demonstrate that the side channel generates different flow distribution pattern in the center channel.

Uniform versus Nonuniform Axial Power Distribution in Rod Bundle CHF Experiments

Rod bundle experiments with axially uniform and nonuniform heat fluxes are examined to explore the potential limitations of using uniform rod bundle CHF data for CHF correlation development of light water reactors with nonuniform axial power distribution (APD). The case of upstream burnout is presented as an example of unique phenomena associated with nonuniform rod bundle CHF experiments. It is a result from combined effect of axial nonuniform power shape and different interchannel mixing mechanisms. In addition, several key parameters are investigated with respect to their potential impacts on the thermal-hydraulic behaviors between rod bundles with uniform and nonuniform APDs. This type of misrepresentation cannot be amended or compensated through the use of correction factors due to the lack of critical information in the uniform rod bundle CHF testing as well as the fundamental difference in the underlining driving mechanisms. Other potential issues involved with the use of uniform rod bundle CHF data for nonuniform APD system applications also present strong evidence concerning the limitations and inadequacy of using uniform rod bundle CHF data for the correlation, prediction, and design limit calculation for safety analysis.

The Impact of Mixing Vane Arrangement on Mixing Coefficient β for Subchannel Analysis

Abstract Mixing vanes attached to a space grid play an important role in heat transfer enhancement, thus increasing critical heat flux. Subchannel analysis and computational fluid dynamics (CFD) are usually applied to simulate the coolant flow behavior in a fuel assembly. In subchannel analysis, the mixing effect, mainly turbulent mixing, produced by mixing vane grids (MVGs) is represented by a coefficient β without considering flow direction and mixing vane arrangement. However, in CFD computation, the mixing effect can be simulated more closely. The objective of this paper is to evaluate the mixing coefficient β used in subchannel analysis by a CFD code. Then, the effects of the three MVGs are compared qualitatively and quantitatively. Through the analysis, an effective mixing coefficient adopted in the subchannal codes should be related to the vane arrangement. Improvements for β are needed to better reflect the true mixing function from the spacer grid relevant to its mixing vane arrangement. Besides the lateral velocity distribution, secondary flow intensity, temperature distribution, and thermal nonuniformity are different for different vane arrangement patterns.

Challenges in reactor core thermal-hydraulics: subchannel analysis, CFD modeling and rod bundle CHF

Avital area impacting the safety and economics of most nuclear power plants is the reactor core thermal-hydraulics for all types of reactors. Several issues of concern include: understanding of fundamental physical phenomena, various thermal-hydraulic limits (such as critical heat flux (CHF), flow instability (FI) and natural circulation cooling (NC)), strong multi-physics coupling, severe accidents, flow structure interactions (FSI), as well as the influence of mixing vane spacer grids in fuel bundles. Understanding these complex physical phenomena of multiscale, multidimensional and multiphase flows is limited. The challenges in subchannel thermal-hydraulics continue to draw major focus from academic and industrial researches. Various investigations in advanced simulation, modeling and measurement techniques are carried out to enhance the understanding of the thermal-hydraulic phenomena under complex conditions. Advancements in subchannel codes are being pursued to overcome massive computational limitations for reactor simulation and to account for the complicated mixing vane grid effects on local thermal-hydraulic calculations. Recently, the activities in computational fluid dynamics (CFD) with the advancement in the computational algorithms, fast computers, turbulence modeling, boiling heat transfer, two phase flow mixing and flow structure interactions are attracting a lot of attention in nuclear applications. In this issue 15 articles were selected from over 80 papers submitted to the 2 International Seminar on Subchannel Analysis, CFD Modeling and Verification, CHF Experiment and Benchmarking (ISACC-2015). With continued focus on CFD mixing vane grid modeling and rod bundle CHF evaluation, renewed interests are also geared towards subchannel flow instability, GPU-MPS modeling of fuel coolant interaction (FCI), as well as mixing vane grid impacts on post-accident fuel bundle cooling. These papers cover the broad field of subchannel thermal-hydraulics: from subchannel code development, experimental study and subchannel measurements, CFD modeling , flow structure coupling, post-accident cooling, correlation developments, subchannel flow instability and to other subchannel applications. Riley et al. analyzed steam cooling behavior in the PWR subchannels using RBHT data to examine the effect of spacer grids on heat transfer after accident events. Yang et al. examined separate effects of key components in typical spacer grids using experimentally verified CFD modeling techniques. Park et al. presented experimental investigations examining the impact on heat transfer characteristics and peak cladding temperature through studying the dynamics of a single droplet impacting a hot surface above the Leidenfrost point temperature. Mao et al. presented an approach to improving the subchannel code local condition predictability by adapting mixing vane grid effects through different source terms with mixing parameters derived. Moon et al. investigated the coolability of partially blocked cores during a loss-of-coolant accident in 2 · 2 and 5 · 5 bundles. Han et al. analyzed the influence of key components in mixing vane grids, such as the dimple location, distance of mixing vane to the nearest strip and the effect of inter-subchannel mixing among subchannels on pressure drop and downstream flow fields. Ding et al. designed a new algorithm to decompose the penta-diagonal matrix and improve computational efficiency based on Stone s incomplete LU (ILU) decomposition method. Chen et al. carried out a CFD study to analyze the effects of spacer grid with mixing vanes under single and adiabatic two-phase flow. Han et al. adapted a new CFD modeling approach to enhance the predictability of CHF location in an axially non-uniformly heated bundle. Zhang et al. presented the CFD modeling results for steam generators under various single and two-phase flow conditions to examine the effects of tube support plates (TSP) on thermal hydraulics. Liu et al. presented analysis of experimental results from high heat flux pool boiling and liquid film boiling experiments (BETA) and used to support modeling of micro-layer hydrodynamics under high heat flux. Mao et al. delivered a comprehensive review of the grid-enhanced heat transfer correlations. A predictive model was also presented to account for mixing mechanisms, such as swirl flow and crossflow, associated with the mixing vane. Yang et al. evaluated the performance of various turbulence models for the application of CFD modeling in rod bundles fuel assemblies. Gou et al. applied graphical processing unit (GPU) implementation of the moving particle semi-implicit (MPS) method to simulate the phenomena of fuel coolant interaction (FCI). Wang et al. applied RELAP code to examine the stability in single, 2 · 2, and 3 · 3 channel systems for both closed and opened subchannels. The editors of this special issue would like to acknowledge their most sincere gratitude to all authors that submitted papers for their contribution and efforts. EDITORIAL

Study on effects of mixing vane grids on coolant temperature distribution by subchannel analysis

Abstract Mixing vane grids (MVG) have great influence on coolant temperature field in the rod bundle. The MVG could enhance convective heat transfer between the fuel rod wall and the coolant, and promote inter-subchannel mixing at the same time. For the influence of the MVG on convective heat transfer enhancement, many experiments have been done and several correlations have been developed based on the experimental data. However, inter-subchannel mixing promotion caused by the MVG is not well estimated in subchannel analysis because the information of mixing vanes is totally missing in most subchannel codes. This paper analyzes the influence of mixing vanes on coolant temperature distribution using the improved MVG model in subchannel analysis. The coolant temperature distributions with the MVG are analyzed, and the results show that mixing vanes lead to a more uniform temperature distribution. The performances of split vane grids under different power conditions are evaluated. The results are compared with those of spacer grids without mixing vanes and some conclusions are obtained.

Effects of axial power shapes on CHF locations in a single tube and in rod bundle assemblies

Abstract Currently, the prediction of rod bundle CHF is dependent on CHF correlations derived from CHF data. A simple correction factor, such as F-factor, is often used to account for the axial power shape differences based on a simple accumulated energy concept, which has totally no consideration on the impact of true local condition on CHF mechanism. Subsequently, as expected, large uncertainty is often associated with the CHF value and CHF location predictions. For the purpose of obtaining different power shapes effects on CHF, CFD calculated parameter values were used to predict the possible CHF occurrence location. The possible CHF location prediction method proposed in this paper is calculated void fraction, heat transfer coefficient (HTC), liquid temperature distribution and detailed local parameters. And the uniform and non-uniform CHF were analyzed. The prediction of possible CHF locations in a 5 × 5 rod bundle may provide useful information for the design of a full-length CHF test, enhance the accuracy of CHF and CHF location prediction, and reduce the costs of the experimentation.