Javier Ortiz-Villafuerte

Investigation of Microbubble Boundary Layer Using Particle Tracking Velocimetry

Particle tracking velocimetry has been used to measure the velocity fields of both continuous phase and dispersed microbubble phase, in a turbulent boundary layer, of a channel flow. Hydrogen and oxygen microbubbles were generated by electrolysis. The average size of the microbubbles was 15μm in radius. Drag reductions up to 40% were obtained, when the accumulation of microbubbles took place in a critical zone within the buffer layer. It is confirmed that a combination of concentration and distribution of microbubbles in the boundary layer can achieve high drag reduction values. Microbubble distribution across the boundary layer and their influence on the profile of the components of the liquid mean velocity vector are presented. The spanwise component of the mean vorticity field was inferred from the measured velocity fields. A decrease in the magnitude of the vorticity is found, leading to an increase of the viscous sublayer thickness. This behavior is similar to the observation of drag reduction by polymer and surfactant injection into liquid flows. The results obtained indicate that drag reduction by microbubble injection is not a simple consequence of density effects, but is an active and dynamic interaction between the turbulence structure in the buffer zone and the distribution of the microbubbles.

Three-dimensional measurements of single bubble dynamics in a small diameter pipe using stereoscopic particle image velocimetry

Abstract The particle tracking velocimetry combined with the shadow image measurement techniques were used to study the velocity field generated by the passage of a single air bubble through a small volume, as it was rising in quiescent water, in a small diameter pipe. This technique provides the velocity field in conjunction with the bubble size and shape. The tracer particles were first tracked through four consecutive frames, using a Hough transform and an architecture resonance theory 2 (ART 2) neural network. Then, the three-dimensional reconstruction of the flow field in the whole measurement volume was achieved through a stereoscopic matching technique. Detailed information about the three-dimensional reconstruction of the bubble shape, calibration, particle tracking, and stereoscopic vector matching algorithms is presented. Error analysis to evaluate the accuracy of the measurements is also included. The effect of the wall on the flow field was studied by dividing the test volume into two regions; namely, center region and wall region. This allowed grouping similar bubble rise trajectories to perform conditional averages of the transient behavior of the velocity field. The vorticity generated from the velocity field is described and discussed for the bubble trajectory along the pipe core.

Investigation of three-dimensional two-phase flow structure in a bubbly pipe flow

A measurement study was performed to obtain a full-field quantitative description of a three-dimensional, two-phase bubbly flow. Particle image velocimetry (PIV), a whole-field, non-invasive velocity measurement technique, was utilized. PIV is capable of producing an instantaneous velocity map of steady-state and transient flows of a fluid seeded with microscopically small neutral density particles. The objective of this investigation was to study the turbulence structure in a co-current bubbly flow. The obtained information will help to determine parameters needed for two-phase flow modelling. The study investigated the influence of bubbles on the surrounding flow field (bubble/flow interaction). A stereoscopic reconstruction technique was used to obtain three-dimensional velocity vector data from the recorded planar images. Radial distributions of volume-averaged turbulence intensities and Reynolds stresses were calculated. The volume-averaged turbulent kinetic energy within the measurement zone due to a rising bubble is presented.

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...

Investigation of Microbubble Boundary Layer Using Particle Image Velocimetry

Particle tracking velocimetry has been used to measure the velocity fields of both continuous and dispersed phases, in a microbubble turbulent boundary layer, in a channel flow. Hydrogen and oxygen microbubbles were generated in the flow by electrolysis. The average size of the microbubble radius was 15 μm. Significant drag reductions (above 40%) were observed, when the accumulation of microbubbles took place in a critical zone within the buffer layer. The z-component of the mean vorticity field was derived from the measured velocity fields. There is a decrease in the magnitude of the vorticity, leading to a smother transition from the viscous to the buffer layer. This indicates the increase of the viscous sublayer thickness, as also observed in investigations of drag reduction by polymer and surfactant injection in liquid flows.© 2003 ASME

Rocking motion, trajectory and shape of bubbles rising in small diameter pipes

Abstract The trajectory, rocking motion, and shape of a single air bubble rising in water were investigated utilizing the particle tracking velocimetry (PTV) measurement technique. The three components of the velocity and acceleration vectors of the bubble were obtained by employing a stereoscopic technique. This technique allowed for a better description of rising bubble behavior. Four charged coupled device (CCD) cameras at different view angles acquired the view volume images. Two experimental configurations were employed in this study. In the first setup the bubbles were tracked for a duration of 133.3 ms. The viewing area was 24.6 mm high and 18.6 mm wide, so the bubbles trajectories could be determined. In this configuration, the bubbles rose in a stagnant and a laminar water flow. The average equivalent spherical diameter of the bubbles was 3.6 mm. In the second experimental setup configuration, the bubbles were tracked for 66.7 ms, but the viewing area was reduced to 11.0 mm high and 13.0 mm wide. In addition, a combined shadow particle image velocimetry (SPIV) technique was employed. This technique allowed for an accurate description of the bubble shape. The results indicate that the trajectory of the bubbles was zigzag or helical, and the shape was oblate spheroidal. The period of the cyclic motion and the amplitude decreased as a result of the wall effect. The bubbles rising close to the pipe wall have more rotational behavior than the bubbles flowing through the pipe center. This is a consequence of bubble/wall interaction.

Three-dimensional Bubbly Flow Measurement Using PIV

The experimental flow visualization tool, Particle Image Velocimetry (PIV), is being extended to determine the velocity fields in three-dimensional, two-phase fluid flows. In the past few years, the technique has attracted quite a lot of interest. PIV enables fluid velocities across a region of a flow to be measured at a single instant in time in the whole volume (global) of interest. This instantaneous velocity profile of a given flow field is determined by digitally recording particle (microspheres or bubbles) images within the flow over multiple successive video frames and then conducting flow pattern identification and analysis of the data. This paper presents instantaneous velocity measurements in various three-dimensional, bubbly two-phase flow situations. This information is useful for developing or improving existing computer constitutive models that simulate this type of flow field. It is also useful for understanding the detailed structure of two-phase flows.

Prony’s Method for BWR Stability Analysis

Power uprates are becoming a quite viable option both for improved performance and safety of nuclear power plants. Part of the success in power uprates are new designs of fuel assemblies. Nevertheless, power uprates in BWRs has made mandatory new stability analyses since operational and startup maneuvers are not necessarily independent of the performance of such new fuel assembly designs. For example, it has been inferred that certain operational areas, within the before-to-uprate power-flow map, could be reached for the new expected operational conditions, especially during startup, because certain fuel assembly designs have shown faster response to certain neutronic perturbations and their lower coolant flow area. As a consequence, stability monitors are being taken into account as a great help for operation in such special maneuvering conditions. Currently, power oscillations during startup are of not of much concern since boiling boundary length, decay ratio, and other similar parameters are continuously followed by power stability monitors. However, there still exist certain transients, as a recirculation pump trip suddenly occurring at rated operation, in which the instability range can be difficult to determine in real time. One alternative, for a fast analysis of fast events, is Prony’s Method, which is mostly employed in power and energy transport systems problems. This analysis method provides information about stability through a short-time series, in quasi-steady conditions, which may be expected in certain BWR transients. The method computes the complex dampening coefficient, which is the most instable pole in an autoregressive analysis of the time series. However, not many applications in BWR operation have reported and supported to establish the scope of using such analysis for actual events. This work presents the response method to the impulse of an autoregressive model and its relation to the damping coefficient of Prony’s method. It is found a second order behavior with respect to the decay ratio. To support such result, three different noise signals are analyzed where BWR transient events in which low coolant flow and high power conditions are assumed.Copyright

Application of the Relative Power Contribution Methodology to the Analysis of a Control System Failure

The Relative Power Contribution methodology has been applied to delineate the initiating event leading to a BWR transient. Diverse reactor signals were analyzed to calculate the coefficients required on the relative power contribution method. Those coefficients were computed from an autoregressive multivariable model. Among the signals used in the analysis of the transient event are total flow through the core, pressure drop across the core, feedwater flow, and reactor power. Analyses of the same type of transient event showed a resonance of the main event frequency on the range within which it has been considered and observed frequencies related to some failures of certain control systems of a nuclear power plant. Those analyses employed the short-time Fourier transform or the power spectral density, for time-frequency and frequency-only domains, respectively. In this work, the same value of the frequency of the resonance mentioned above was obtained through the relative power contribution analysis, but, furthermore it was found that the feedwater flow behavior had an important impact on the transient event, and also that the transient event was not initiated by a reactivity-related instability.© 2009 ASME

ICONE15-10105 INERT MATRIX FUEL ASSEMBLY AS AN OPTION FOR THE LAGUNA VERDE NPP FUEL RELOADS

The availability of large amounts of reactor and weapons grade plutonium in the world shows the necessity of anticipating situations for the use and disposition of it. Because Light Water Reactors (LWRs) prevail on the stage of electric energy generation by nuclear power, it is important to take into account the potential of these reactors to reduce the plutonium inventory. Several studies performed in Pressurized Water Reactors (PWRs) show that reactor and weapons grade plutonium can effectively be burned in these reactors, in assemblies with fertile-free fuel, and maintaining reactivity control and other safety issues at least comparable to those related to the standard fuel normally used. The Instituto Nacional de Investigaciones Nucleares, currently carries out research on diverse alternatives to use Inert Matrix Fuel (IMF) as an option to fuel reloads for the two BWR/5 Units at the Laguna Verde Nuclear Power Plant. This work presents first the neutronic analysis of a fuel assembly conceptual design, which contains a combination of plutonium oxide (in an inert matrix) fuel rods, uranium oxide fuel rods, and uranium oxide with gadolinia fuel rods. Then, simulations for three different fuel assembly reload options were performed for Unit 1. Results of reactor operation from the different reload options are presented. The results obtained with reload fuel using inert matrix fuel assemblies observe a decrease in the length of operation cycle in the plant. However, the mass of uranium used is minor to require for make all fuel assemblies.