Showing papers by "Yassin A. Hassan published in 2018"

Time-resolved PIV measurements in a low-aspect ratio facility of randomly packed spheres and flow analysis using modal decomposition

Abstract: This work experimentally investigated the flow characteristics in a facility with randomly packed spheres at a low-aspect ratio of 4.4. Velocity fields in the near-wall region and in the pores between spheres were obtained by employing the matched-index-of-refraction (MIR) and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of 340, 520, and 720. From the obtained TR-PIV velocity vector fields, flow characteristics including first- and second-order statistics, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress profiles, were computed. The effects of the wall enclosure and Reynolds numbers on the flow patterns were investigated by comparing the computed flow statistics and evaluating two-point cross correlations of the velocities measured adjacent to and far from the wall. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress component showed an increase in flow mixing and turbulent intensity in the gaps between spheres in the packed bed. The re-circulation-region sizes, however, were found to be independent from an increase in Reynolds numbers. Finally, flow modal decompositions, such as the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were applied to the vorticity fields extracted from sub-regions located near and far from the wall to reveal the most dominant POD and DMD flow structures.

Prospective For Nuclear Thermal Hydraulic Created By Ongoing And New Networks

Abstract: This paper introduces the FONESYS, SILENCE and CONUSAF projects run by some of the leading organizations working in the nuclear sector.The FONESYS members are developers of some of the major System Thermal-Hydraulic (SYS-TH) codes adopted worldwide, whereas the SILENCE members own and operate important thermal-hydraulic experimental facilities. The two networks work in a cooperative manner and have at least one meeting per year where top-level experts in the areas of thermal-hydraulic code development and experimentation are gathered.The FONESYS members address various topics such as hyperbolicity and numerics in SYS-TH codes, 3-field modeling, transport of interfacial area, 3D modeling, scaling of thermal-hydraulic phenomena, two-phase critical flow (TPCF), critical heat flux (CHF), and others. As part of the working modalities, some numerical benchmarks were proposed and successfully conducted by the network, addressing some of the most relevant topics selected by the FONESYS members.On the other hand, SILENCE addresses topics such as identification of current measurement needs and main gaps for further SYS-TH and CFD codes development and validation, definition of similar tests and counterpart tests in Integral Tests Facilities (including containment thermal-hydraulics) to be possibly conducted on Members' test facilities, scaling issue, and other subjects. Furthermore, SILENCE organized a Specialists Workshop on Advanced Instrumentation and Measurement Techniques for Nuclear Reactor Thermal-Hydraulics (SWINTH) which was held in Italy on June 2016. A second edition of the Workshop, namely SWINTH-2019, will be held in Italy in 2019 under the umbrella of the OECD/NEA/CSNI/WGAMA.Recently a new initiative is being taken by launching an international consortium of nuclear thermal-hydraulics code users, the CONUSAF. The main idea is to enhance the interactions between the users of computational tools in nuclear TH, noticeably including SYS-TH and CFD codes, the code developers and the experimentalists. The proposed initiative is expected to have a positive impact on the entire ecosystem by pursuing the assessment of the current code limitations and capabilities, analyzing and addressing issues raised by the users and promoting common RandD efforts on topics of high relevance.

Analysis of Pressure Field on Wire-Wrapped Pin Bundle Surface for Concerns of FSI

Abstract: Wire wrapped fuel pins, typical of sodium fast reactor designs, introduce a strong secondary rotational flow and enhanced mixing compared to bare pins found in many other nuclear reactor designs. The transverse flow is created by deflection off of the helical wires, exerting a force onto the wires. The resulting pressure field can be more intricate than found for bare fuel pins. There is an increased concern for fluid structure interactions such as twisting, bending, and vibration due to the fluid deflection and increased transverse flow. The present study uses LES to simulate the fluid flow in a wire-wrapped fuel pin bundle. The simulation is performed using Nek5000, a spectral element LES/DNS code. The time-dependent, fluctuating pressure field at the surface of select fuel pins are specifically captured for analysis. Statistical analysis is performed on the pressure data to find areas of persistent force or bending moment, which over large periods of time may result in deformation of the pins. Further, the pressure data is analyzed in search of areas with large fluctuations in pressure, which may result in flow induced vibrations. Frequency spectra of the force fluctuations are analyzed.

Turbulent Transverse Plane PIV Measurements on a Wire-Wrapped 61-Pin Hexagonal Fuel Bundle

Abstract: This study recaps the experimental effort to characterize a transverse plane flow field in a tightly-packed, 61-pin, wire-wrapped, hexagonal fuel bundle prototypical for a sodium fast reactor. The motive was to produce high spatiotemporal experimental data for computational fluid dynamics turbulence model validation. The matched-index-of-refraction and laser-based optical measurement techniques were utilized on an isothermal experimental flow facility. Measurements were performed on a transverse plane perpendicular to the axial flow. Fluid flow in the three types of subchannels (corner, exterior, and interior) were quantified. All measurements have been performed at a bundle-averaged Reynolds number of approximately 10,400. Results include flow statistics such as ensemble-averaged velocity, root-mean-square fluctuating velocity, and Reynolds stress. Of interest was the flow behavior around the restriction caused by the wire spacer and hexagonal duct wall, where recirculation regions formed. Regions of maximum and minimum momentum transfer coincided with regions of maximum and minimum fluctuations. These regions highlight locations of maximum and minimum cooling of the fuel pins. The experimental data will be used to benchmark computational fluid dynamics simulations of the sodium fast reactor fuel bundle using Reynolds-averaged Navier Stokes and large-eddy simulation turbulence modeling methods.

Simulations of Twin Turbulent Planar-Like Jets Injected into a Large Volume using RANS

Abstract: The complex behavior of thermal fluids in nuclear reactors require the usage of computational fluid dynamics (CFD) codes for design and analysis. In order to use CFD codes, they require regular benchmark problems to ensure the predictions are reasonable representations of reality. The twin jet water facility (TJWF) designed and built at the University of Tennessee, Knoxville was created for this purpose. The facility features twin planar-like turbulent free shear jets injecting fluid into a transparent tank to study a variety of flow behavior. The experimental work using this facility by Texas A&M University was used for the benchmarking activities. This work was conducted using a steady Reynolds-averaged Navier–Stokes formulation to simulate the flow behavior. It was determined that the standard k–ε and elliptic blending Reynolds stress model (EBRSM) turbulence models can be used to simulate the twin jet behavior with reasonable success for design and analysis activities.

Time-Resolved Velocity Measurements in a Matched Refractive Index Facility of Randomly Packed Spheres

Abstract: Complex geometries and randomly connected void spaces within packed beds have hindered efforts to characterize the underlying transport phenomena occurring within. In this communication, we present our experimental studies on a facility of randomly packed spheres that can be a representative of sections within a reactor core in a nuclear power plant. The results of high-fidelity velocity measurements can be seen using Time-Resolved Particle Image Velocimetry (TR-PIV) at the pore scales and near the wall boundary in the Matched Index of Refraction (MIR) facility. The MIR approach allows for a non-invasive analysis of the flow within packed spheres at the microscopic scales with high temporal and spatial resolution. Flow characteristics obtained from the TR-PIV measurements at various Reynolds numbers are presented. The results include the first- and second-order flow statistics, such as mean velocity, root-mean-square fluctuating velocity and Reynolds stresses. Effects of the wall boundary and Reynolds numbers on flow patterns are currently being investigated. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress components show the increase of flow mixing and turbulent intensities within the gaps between spheres in the packed bed. Sizes of recirculation regions, however, seem to be independent of the increase of Reynolds numbers.Copyright © 2018 by ASME

Probability Analysis of Velocity Distribution in a Facility of Randomly Packed Spheres Featuring Matching-Refractive-Index and Time-Resolved PIV Techniques

Abstract: INTRODUCTION Complex geometries developed from a random packing produce intricate flow structures in packed bed systems. These are found in a diverse universe of multiple disciplines such as gas purifiers with petroleum engineering, chemical reactors with chemical engineering, lung tissue and other biological constructs with biological and biomedical engineering, and, rather intuitively, nuclear pebble bed reactors (PBR) with nuclear engineering [1]. With regards to pebble bed reactors, understanding and mapping the flow is of great importance as the influence of the flow structures on transport phenomena such as heat removal is a critical factor. In the loss-of-forced-cooling (LOFC) accidental scenario, the coolant flow could be driven by the natural circulation phenomenon within the reactor core. One would expect the heat removal rate remains acceptable to the reactor design level. Therefore, it is important to gain the fundamental knowledge about heat and mass transfer within the pebble bed reactor core during the LOFC scenarios. For this purpose, multiple points or fullfield measurements of flow characteristics and heat transfers at a high level of spatial and temporal resolutions are needed to fully map the complex flow patterns and to provide data at high spatial density to permit accurate volume averaging in the pebble bed. Texas A&M University is conducting isothermal measurements of pressure drops, and velocity fields in a pebble bed experimental facility to support the research on advanced nuclear reactors sponsored by Department of Energy (DOE). The main purpose of these tests is to perform high spatial and temporal resolution measurements and use the obtained results for code validation and model development [2-3]. In this paper, we analyze the probability distributions of velocity experimentally measured in a facility of randomly packed spheres featuring matching-refractive-index (MRI) and timeresolved particle image velocimetry (TR-PIV) techniques. Further details of the TR-PIV measurements in the MRI facility of packed beds and flow analysis using proper orthogonal decompositions (POD) and dynamic mode decomposition (DMD) can be reviewed in [4].

PIV/PTV Measurements of Lateral Velocity Components in a 5×5 PWR Bundle with Mixing Vanes

Abstract: The core of a PWR consists of multiple fuel assemblies housed inside a reactor pressure vessel. Each fuel assembly is constructed from fuel rods and supports to create a rod bundle. In PWRs, water is used as a coolant and is circulated inside the reactor core along the axial direction of the fuel rods. As the coolant is pressurized and flows in the axial direction, the fuel rods are cooled by forced convection, and the thermal energy of the rods is transferred to the coolant (McClusky [1]). Spacer grids are used in the pressurized water reactor (PWR) to fix, clamp the fuel rods, and enhance the turbulence mixing within the spaces of the rods and overall heat transfer performance. With the presence of the spacer grids, many important factors related to the thermal-hydraulic performance of the fuel rod bundles need to be characterized, such as pressure drop, flow vibration, and flow field development in the bundle. Proper understanding of the thermal-hydraulic characteristics of the turbulent mixing and heat transfer created by the space grids is important. However, spacer grids having complex design structure with dimples, mixing vanes and springs has been a challenge for computational fluid dynamics (CFD) turbulent models. High fidelity experimental data with spatialtemporal resolutions is essential to validate and assess the performances of CFD models. The interest in lateral velocity measurements began with Smith III [2] and Langford [3]. These studies provided experimental and numerical comparisons of the lateral velocity components, flow structure, axial development, and the overall influence of the mixing vanes. Particle image velocimetry (PIV) was the measurement technique used in these studies. PIV allows 2D instantaneous and average velocity fields in a plane. Due to the complex geometry of the bundle in these studies, the PIV measurements were only possible through the use of a borescope. This limits the measurements to single-subchannels and loses the intersubchannels interactions. Dominguez-Ontiveros [4] and Conner et al. [5] also implemented PIV measurements in a bundle. They used the refracted index matching (RIM) technique in order to resolve the optical access problem due to the bundle complex geometry. RIM allows optical access for either illumination or visualization through a relatively long axial distance from the mixing vanes. The main focus of these studies was to measure the velocity fields on planes parallel to the axial direction. Recently, Nguyen and Hassan [6] performed time-resolved stereoscopic PIV (TR-SPIV) measurements of turbulent flows in a 5x5 fuel rod bundle with a spacer grid and mixing vanes. The TR-SPIV measurements provide 3-component velocity fields along the vertical-horizontal planes in the inter-channels between the fuel rods. Later, Busco and Hassan [7] studied turbulent flows in the fuel rod bundle using Partially-Average NavierStokes (PANS) simulations. These authors compared the performances of PANS and Large-Eddy Simulations (LES) with emphasis on the mean flow characteristics and spatiotemporal turbulent flow structures. This report presents the results of experimental efforts to obtain time-resolved and full field (2-D) measurements of the lateral velocity components within a 5×5 rod bundle with Westinghouse designed PWR mixing vanes. The efforts of this study are focused on the development of an experimental technique capable of providing time-resolved full-field (2D) velocity measurements of the lateral velocity components within a 5×5 rod bundle by means of PIV/PTV and RIM techniques.