Elvis E. Dominguez-Ontiveros

Measurements of Flow Modification by Particle Deposition Inside a Packed Bed Using Time-Resolved PIV

In the Advanced Gas Cooled Pebble Bed Reactors for nuclear power generation, the fuel is spherical coated particles. The energy transfer phenomenon requires detailed understanding of the flow and temperature fields around the spherical fuel pebbles. Detailed information of the complex flow structure within the bed is needed. Generally, for computing the flow through a packed bed reactor or column, the porous media approach is usually used with lumped parameters for hydrodynamic calculations and heat transfer. While this approach can be reasonable for calculating integral flow quantities, it may not provide all the detailed information of the heat transfer and complex flow structure within the bed. The present experimental study presents the full velocity field using particle image velocity technique (PTV) in a conjunction with matched refractive index fluid with the pebbles to achieve optical access. Velocity field measurements are presented delineating the complex flow structure.Copyright

Development of a wall shear stress integral measurement and analysis system for two-phase flow boundary layers

An integral system was developed for the measurement and analysis of wall shear stress data from a two-phase boundary layer flow. Three different and independent measurement techniques are incorporated in a synchronized acquisition system. The three measurement techniques are the particle tracking velocimetry, a differential pressure transducer, and an optical wall shear stress sensor. Each of these techniques provides complementary information that helps in the description and understanding of the phenomena involved in a two-phase boundary layer. The practical implementation of the integral measurement and analysis system is demonstrated with the measurement of the wall shear stress of a microbubble laden boundary layer flow in a channel. The agreement in the results of the wall shear stress from the three synchronized measurements techniques was 93% for the single-phase case and 92.8% for the two-phase flow measurements. Moreover, independent measurements with each technique show no appreciable change i...

Experimental Study of Temperature Sensitive Dyes for Planar Laser Induced Fluorescence Thermometer

As a non-intrusive, whole-field temperature measurement technique, LIF (Laser Induced Fluorescence) has been used successfully to measure temperature fields. The performances of dyes are essential of the technique, especially the temperature sensitivity of the dyes. This work presents an analysis to provide a correct choice of temperature sensitive dyes combination (FL27 and RhB). The influences of temperature, excitation wavelength and pH on emission intensity and temperature sensitivity were analyzed. The results show that the temperature dependent tendency of FL27 changed from negative to positive as the excitation wavelength increased. The temperature sensitivity (4.0% per °C) of combination under 532nm laser is better than that of the wide used combination of RhB and Rh110 (2.0% per °C). The emission intensities of dyes are sensitive to pH value; however, the temperature dependence is unaffected.Copyright

Non-Intrusive Experimental Investigation of Flow Behavior Inside a 5X5 Rod Bundle With Spacer Grids Using PIV and MIR

The validity of the simulation results from Computational Fluid Dynamics (CFD) are still under scrutiny. Some existing CFD closure models for complex flow produce results that are generally recognized as being inaccurate. Development of improved models for complex flow simulation require an improved understanding of the detailed flow structure evolution along with dynamic interaction of the flow multi-scales. Thus, the goal of this work is to contribute to a better understanding of presupposed and existent events that could affect the safety of nuclear power plants by using state-of-the-art measurement techniques that may elucidate the fundamental physics of fluid flow in rod bundles with spacer grids. In particular, this work aims to develop an experimental data base with high spatial and temporal resolution of flow measurements inside a 5×5 rod bundles with spacer grids. The full-field detailed data base is intended to validate CFD codes at various temporal-spatial scales. Measurements were carried out using Dynamic Particle Image Velocimetry (DPIV) technique inside an optically transparent rod bundle utilizing the Matching Index of Refraction (MIR) approach. This work presents results showing full field velocity vectors and turbulence statistics for the bundle under single phase flow conditions.Copyright

Detection of vapor nanobubbles by small angle neutron scattering (SANS)

Experiments using boiling water on untreated (roughness 100–300 nm) metal surfaces using small-angle neutron scattering (SANS) show the appearance of structures that are 50–70 nm in size when boiling is present. The scattering signal disappears when the boiling ceases, and no change in the signal is detected at any surface temperature condition below saturation. This confirms that the signal is caused by vapor nanobubbles. Two boiling regimes are evaluated herein that differ by the degree of subcooling (3–10 °C). A polydisperse spherical model with a log-normal distribution fits the SANS data well. The size distribution indicates that a large number of nanobubbles exist on the surface during boiling, and some of them grow into large bubbles.

Experimental Turbulence Analysis of Fuel Bundle With Spacer Grids

Rod bundles are widely used in industry today with applications ranging from nuclear reactors, heat exchangers, and steam generators. Accurately modeling the inherently unsteady and turbulent flow within these rods is essential in order to design for optimal efficiency while controlling vibration, noise and heat transfer. The problem complicates further when spacer-grids are used within the rods to maintain separation and structural rigidity. Computational modeling can be a useful alternative to the costly process of manufacturing and testing prototypes, but its accuracy needs to be checked with detailed experimental data. This paper describes an experimental database obtained using two-dimensional Time Resolved Particle Image Velocimetry (TR-PIV) measurements within a 5 × 5 rod bundle with spacer-grids. One of the unique characteristic of this set-up is the use of the Matched Index of Refraction technique employed in this investigation which consists of immersing plastic rods with a similar index of refraction as the one for water to achieve optical transparency across the spacer grid. This unique feature allows flow visualization and measurement within the bundle without rod obstruction. This approach also allows the use of high temporal and spatial non-intrusive dynamic measurement techniques namely TR-PIV to investigate the flow evolution below and immediately above the spacer. The data base presented includes explanation of the various cases tested such as boundary conditions, rig dimensions, measurement zones, and the test equipment in order to provide a good base for Computational Fluid Dynamics (CFD) simulations. Turbulence analysis of the obtained data is provided in order to gain insight of the physical phenomena and to compare the possible results obtained from computational simulations.Copyright

PIV Measurements of Jet Flow Mixing in the Vicinity of Rod Bundles using Matched-Index of Refraction

In this study, the velocity field of impinging jets within a rod bundle was developed. Velocity measurements were accomplished using Particle Image Velocimetry (PIV). Additionally, Matched-Index of Refraction (MIR) techniques were implemented to allow the visualization of flow characteristics within interior areas of the rod bundle which would typically be obstructed. Such measurements are of importance and essential to the development of new models to predict the systems’ hydraulic behavior.Copyright

PTV Experiments of Subcooled Boiling Flow Through a Rectangular Channel

Experiments were carried out to investigate turbulent sub-cooled boiling flow of Novec-2000 [1] refrigerant through a vertical square channel with one heated wall. Channel dimensions were selected to be similar to those encountered on a Boiling Water Reactor (BWR) channel flow, with an hydraulic diameter of Dh = 8.2 mm. Flow visualization techniques such as Particle Tracking Velocimetry (PTV) and Shadowgraphy were used to measure time-average axial and normal velocities, axial and normal turbulence intensities, and Reynolds Stresses. Results are reported for hydraulic Reynolds numbers at channel inlet of 4638 , 14513 and 24188 for up to thirteen wall heat fluxes (q″ ) ranging from 0.0 to 64.0 kW/m2 . This work is an attempt to enrich the database already collected on turbulent subcooled boiling flow, with the hope that it will be useful in turbulence modeling efforts.Copyright

Dynamic Non‐Obstructed Particle Image Velocimetry Measurements inside a Packed Bed Reactor

The aim of this work is to present the progress of experimental approaches used to investigate the flow pattern towards mass and heat transfer phenomena within a packed bed. Most of the studies on packed beds consider the porous medium approach to obtain lumped parameters from which bulk characteristics of the flow are determined. This conventional approach neglects the complex flow details within individual pores and constrictions, thus obscuring the physical basis of the model. The experimental approach presented uses Particle Image Velocimetry (PIV) and Matching Refractive Index (MRI) techniques to obtain velocity measurements within a packed bed at the microscopic (pore) level. To completely understand and describe the physical process occurring within the system, local information is required. The main results of velocity measurements for single and two‐phase flow conditions are presented under various flow conditions with Reynolds numbers ranging between 50 and 500, based on particle diameter. The r...

Time and Spatial Pressure and Velocity Correlation in a Microbubble Laden Boundary Layer

Measurements of the velocity fields and wall pressure have been performed in a microbubble laden boundary layer in order to have a better understanding of the degree of correlation between these two parameters. Cross-correlation coefficients have been obtained from synchronized measurements of pressure and velocity at different distances from the wall in a channel flow. The results show a high correlation between pressure and both the streamwise and normal components of the velocity vector for the two-phase flow case. In contrast, the correlation coefficient between pressure and velocity is high only for the streamwise component of the velocity vector for single phase flow (no microbubbles in the flow). A practical application of these measurements is obtaining data and information to better describe the mechanism responsible for the microbubble drag reduction phenomenon, which has great potential for energy savings on different transport means.Copyright