Derek H. Lister

SPRITE MRI of bubbly flow in a horizontal pipe.

Bubble flow is characterised by numerous phase interfaces and turbulence, leading to fast magnetic resonance signal decay and artefacts in spin-warp imaging. In this paper, the SPRITE MRI pulse sequence, with its potential for very short encoding times, is demonstrated as an ideal technique for studying such dynamic systems. It has been used to acquire liquid velocity and relative intensity maps of two-phase gas-liquid dispersed bubble flow in a horizontal pipe at a liquid Reynolds number of 14,500. The fluids were air and water and a turbulence grid was used to generate a dispersed bubble flow pattern. The SPRITE technique shows promise for future research in gas-liquid flow.

Evaluation Methods for Corrosion Damage of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics (III) : Evaluation of Wall Thinning Rate with the Coupled Model of Static Electrochemical Analysis and Dynamic Double Oxide Layer Analysis

Flow accelerated corrosion (FAC) is divided into two processes: a corrosion (chemical) process and a flow dynamics (physical) process. The former is the essential process to cause FAC and the latter is the accelerating process to enhance FAC occurrence. The chemical process in the surface boundary layer is analyzed to evaluate FAC rate. Contributions of flow dynamics on wall thinning rate due to FAC are expressed as a function of mass transfer coefficient but not that of flow velocity. FAC evaluation procedures were divided into 5 steps as follows. (1) Flow pattern and temperature in each elemental volume along the flow path were obtained with 1D computational flow dynamics (CFD) codes, (2) corrosive conditions, e.g., oxygen concentration and electrochemical corrosion potential (ECP) along the flow path were calculated with a hydrazine oxygen reaction code, (3) precise flow patterns and mass transfer coefficients at the structure surface were calculated with 3D CFD codes, (4) danger zones were evaluated by coupling major FAC parameters, and then, (5) wall thinning rates were calculated with the coupled model of static electrochemical analysis and dynamic double oxide layer analysis at the identified danger zone. Anodic and cathodic current densities and ECPs were calculated with the static electrochemistry model and ferrous ion release rate determined by the anodic current density was used as input for the dynamic double oxide layer model. Thickness of the oxide film and its characteristics determined by the dynamic double oxide layer model were used for the electrochemistry model to determine the resistances of cathodic current from the bulk to the surface and anodic current from the surface to the bulk. Two models were coupled to determine local corrosion rate and ECP for various corrosive conditions. The calculated results of the coupled models had good agreement with the measured ones.

Flow-Assisted Corrosion of Carbon Steel under Neutral Water Conditions

Abstract Flow-assisted corrosion (FAC) often has caused serious damage to carbon steel piping in nuclear power plants. As a first stage of experiments to determine the effects of water chemistry parameters on FAC, corrosion rates of carbon steel were measured in 140°C pure water with an online corrosion rate monitor based on electrical resistance measurement as [O2] and flow velocity were changed. The data were compiled as a function of electrochemical corrosion potential (ECP) and flow velocity. It was concluded that the FAC rate was below the detectable limit in highly oxygenated conditions, where [O2] was >50 ppb and ECP was above −0.2 VSHE; and the effects of preoxidation treatment disappeared rapidly under deaerated conditions.

Verification and Validation of Evaluation Procedures for Local Wall Thinning Due to Flow-Accelerated Corrosion and Liquid Droplet Impingement

A six-step evaluation procedures have been proposed to evaluate the local wall thinning due to flow-accelerated corrosion (FAC) and that due to liquid droplet impingement (LDI). Corrosive conditions were calculated with a N2H4-O2 reaction analysis code. Precise flow turbulence at major parts of the system was analyzed with the three-dimensional computational flow dynamics code to obtain mass transfer coefficients at structure surfaces. Then, wall thinning rates were calculated with the coupled model of electrochemical analysis and oxide layer growth analysis by applying the corrosive conditions and the mass transfer coefficients. To apply computer simulation codes for wall thinning due to FAC and LDI to evaluate residual life and the effectiveness of countermeasures, accuracy and applicability of the codes were confirmed based on verification and validation processes. From comparison of the calculated wall thinning rates due to FAC with hundreds of measured results for secondary piping of an actual pressurized water reactor plant, it was confirmed that the calculated wall thinning rates agreed with the measured ones within a factor of 2 and the accuracy of the evaluation model for residual pipe wall thickness after 1 year of operation had an error of <20%. Finally, just the FAC simulation code was applied to evaluate the effects of oxygen injection into the feedwater line. From comparison of the calculated wall thinning rates due to LDI with measured results for vent lines of an actual boiling water reactor plant, it was confirmed that the calculated local wall thinning rates agreed with the measured ones within about a factor of 2, though there were still some outside that region.

Modeling Two-Phase Flow in Pipe Bends

In order to have a better understanding of the interaction between the two-phase steam-water coolant in the outlet feeder pipes of the primary heat transport system of some CANDU reactors and the piping material, themalhydraulic modelling is being performed with a commercial computational fluid dynamics (CFD) code-FLUENT 6.1. The modeling has attempted to describe the results of flow visualization experiments performed in a transparent feeder pipe with air-water mixtures at temperatures below 55°C. The CFD code solves two sets of transport equations-one for each phase. Both phases are first treated separately as homogeneous. Coupling is achieved through pressure and interphase exchange coefficients. A symmetric drag model is employed to describe the interaction between the phases. The geometry and flow regime of interest are a 73 deg bend in a 5.9 cm diameter pipe containing water with a Reynolds number of ∼IE5-IE6. The modeling predicted single-phase pressure drop and flow accurately. For two-phase flow with an air voidage of 5-50%, the pressure drop measurements were less well predicted. Furthermore, the observation that an air-water mixture tended to flow toward the outside of the bend while a single-phase liquid layer developed at the inside of the bend was not predicted. The CFD modeling requires further development for this type of geometry with two-phase flow of high voidage.

Modeling of Particulate Fouling on Heat Exchanger Surfaces: Influence of Bubbles on Iron Oxide Deposition

Studies of iron oxide deposition on Alloy-800 heat exchanger tubes have been part of a continuing research program at the University of New Brunswick (UNB); the present work formulates mechanisms for the effect of bubbles on deposition in water under boiling conditions. To supplement results from earlier deposition experiments in a fouling loop at UNB, measurements of bubble frequency and departure diameter as a function of heat flux were performed. High-speed movies of bubbling air/water systems indicated that a pumping action moved particles from adjacent areas at the surface to bubble nucleation sites. To explain the observations, the model considers deposition and concomitant removal. Deposition includes microlayer evaporation and filtration through the porous deposit. The deposit is sparse in the first stage, when the dominant process is microlayer evaporation including particle trapping and pumping, creating spots of deposit. Filtration becomes more important as the deposit thickens to a stage when microlayer evaporation becomes negligible. Chimney effects then control. Turbulence due to detaching and collapsing bubbles affects removal. In subcooled boiling, collapsing bubbles generate enough turbulence to maintain much of the deposit labile, while in bulk boiling bubble detachment from the nucleation site is dominant and a smaller portion of the deposit is labile and subject to removal. Model predictions are presented and shown to agree quite well with experimental data.

Surface Dissolution and the Development of Scallops

Abstract Flow-assisted corrosion (FAC) is a significant problem with carbon steel components exposed to rapidly moving water or water/steam mixtures. Such components often develop distinctive patterns of surface damage called scalloping, so to gain further insight into FAC it is of interest to understand the formation and significance of scallops. Experiments were carried out on the dissolution of pipes made of plaster of Paris (CaSO4.½H2O) to study the evolution of scalloping patterns as well as to explore the link between scalloping and hydrodynamics and scalloping and dissolution rate. The conductivity and pH of water flowing through the test sections were recorded and posttest examination was carried out. Scallops were observed along the plaster surface at the end of the tests. Their characteristics are strongly related to the flow rate; scallop size decreases with increasing flow rate whereas surface density of scallops increases with increasing flow rate. Imperfections such as voids on embedded particles seem necessary for scallops to develop at all.

Kinetics of corrosion product release from carbon steel corroding in high-temperature, lithiated water

Abstract An experiment to measure the release of Fe-59 radiotracer from an irradiated carbon steel tube in a loop containing flowing lithiated water at 270°C to 280°C was conducted. Measurements we...

In situ measurement of corrosion of type 316L stainless steel in 553 K pure water via the electrical resistance of a thin wire

A system for the in situ monitoring of corrosion depth via electrical resistance measurements was applied to study the corrosion rate of type 316L stainless steel at 553 K in pure water. Corrosion depth was measured using a 50 μm diameter wire probe mounted axially in the tube. Measurements were in good agreement with literature data for both the hydrogen water chemistry (HWC) condition and the normal water chemistry (NWC) condition. Oxide film analyses by scanning electron microscopy and laser Raman spectroscopy on the wire probe and the tube showed no effects from shape of the test specimens or the application of electric current. Corrosion kinetics was evaluated by fitting equations to the measurements. Data for the HWC condition could be fitted by a two-step logarithmic–parabolic law. A single-step logarithmic law fitted data for the NWC condition. Changes in corrosion rate by the water chemistry changes were readily detected with the technique. Corrosion depth change could be observed for the water chemistry change from the NWC condition to the HWC condition with electrochemical corrosion potential (ECP) of −0.56 V vs. standard hydrogen electrode, which is lower than the ECP that the phase of iron oxide changes from α-Fe2O3 to Fe3O4.

Determination Procedures of High Risk Zones for Local Wall Thinning Due to Flow-Accelerated Corrosion

A modified six-step evaluation procedure has been proposed to evaluate local wall thinning due to flow-accelerated corrosion (FAC). In step 1, the one-dimensional (1-D) distribution of flow turbulence and the temperature along pipes in cooling systems were analyzed with a 1-D system simulation code to obtain approximate mass transfer coefficients at structure surfaces, prior to using a three-dimensional (3-D) computational fluid dynamics (CFD) code for precise flow turbulence analysis of the major parts. In step 2, corrosive conditions were calculated with a N2H4-O2 reaction analysis code. In step 3, high FAC risk zones were determined for further evaluation for wall thinning rates, based on five parameters: temperature, pH, oxygen concentration, mass transfer coefficient, and chromium content. Then, in step 4, the 3-D CFD code was used to calculate precise mass transfer coefficients at the high FAC risk zones. In step 5, the wall thinning rates were calculated using a coupled model of electrochemical analysis and oxide layer growth analysis by applying the corrosive conditions and the mass transfer coefficients. Finally, in step 6, the residual lifetime of the pipes and the applicability of countermeasures against FAC were evaluated. This paper introduces procedures for determining major FAC parameters and evaluation procedures for high FAC risk zones by synthesizing the parameters in step 3. The procedures for determination of high FAC risk zones in a pressurized water reactor secondary cooling system are also demonstrated.