Kimitoshi Yoneda

Flow structure and bubble characteristics of steam–water two-phase flow in a large-diameter pipe

The flow structure and bubble characteristics of steam–water two-phase upward flow were observed in a vertical pipe 155 mm in inner diameter. Experiments were conducted under volumetric flux conditions of JG<0.25 m s−1 and JL<0.6 m s−1, and three different inlet boundary conditions to investigate the developing state of the flow. The radial distributions of flow structure, such as void fraction, bubble chord length and gas velocity, were obtained by horizontally traversing optical dual void probes through the pipe. The spectra of bubble chord length and gas velocity were also obtained to study the characteristics of bubbles in detail. Overall, an empirical database of the multi-dimensional flow structure of two-phase flow in a large-diameter pipe was obtained. The void profiles converged to a so-called core-shaped distribution and the flow reached a quasi-developed state within a relatively short height-to-diameter aspect ratio of about H/D=4 compared to a small-diameter pipe flow. The PDF histogram profiles of bubble chord length and gas velocity could be approximated fairly well by a model function using a gamma distribution and log–normal distribution, respectively. Finally, the correlation of Sauter mean bubble diameter was derived as a function of local void fraction, pressure, surface tension and density. With this correlation, cross sectional averaged bubble diameter was predicted with high accuracy compared to the existing constitutive equation mainly being used in best-estimate codes.

New Interfacial Drag Force Model Including Effect of Bubble Wake, (II) Model Validation Using Experimental Data of Steam-Water Bubbly Flow in Large-Diameter Pipes

In the previous study, we proposed a new interfacial drag force model based on experimental data of steam-water bubbly flow in a large-diameter pipe. This is because our experimental results had suggested that effect of bubble wake should be included in the interfacial drag force model, although it had not been taken into account in the existing models. A preliminary method for including the effect of bubble wake was hence developed and used in the new model. A new bubble size prediction method was also adopted in the model. In the present study, after improving the measuring equipment and signal processing procedure, another series of experiments was carried out. Using the new experimental data, the methods of predicting bubble size and effect of bubble wake were slightly modified. To test the validity of the new model, predicted results were compared with available experimental data sets of steam-water bubbly two-phase flow in large-diameter pipes. One-dimensional and two-dimensional two-fluid models were used for the calculation. Comparisons showed that the new model is in good agreement with the experimental data, whereas the model which does not take into account the effect of bubble wake overestimated the void fractions.

New interfacial drag force model including effect of bubble wake, (I). Model development for steam-water bubbly flow in large-diameter pipes

Several experimental results show that bubbles can easily be captured in the wake formed by leading bubbles when multiple bubbles are rising in a liquid. It is suggested from this experimental result that the effect of bubble wake should be included in the constitutive relationships representing the interfacial drag force. In the present study, steam-water bubbly flow experiments were performed to develop a new interfacial drag force model including the effect of bubble wake. Since the validity of the existing constitutive equations have been tested mainly for two-phase flow in small-diameter pipes, our study focused on two-phase flow in a large-diameter pipe. Using a one-dimensional two-fluid model, the applicability of the new interfacial drag force model to our experimental conditions was investigated. As a result, it was shown that the present model markedly improves the accuracy of the predicted results. It was therefore demonstrated that the present bubble wake model is effective at least for the conditions which were used for model development. Its applicability to different conditions will be discussed in a subsequent study.

Bubble characteristics of steam–water two-phase flow in a large-diameter pipe

Bubble characteristics of steam–water two-phase upward flow were observed in a vertical pipe 155 mm in inner diameter. Experiments were conducted under volumetric flux conditions of JG<0.25 m/s and JL<0.6 m/s, and three different inlet boundary conditions to investigate the developing state of the flow. The radial distributions of flow structure were obtained by horizontally traversing optical dual void probes through the pipe. The spectra of bubble chord length and gas velocity were also obtained to study the characteristics of bubbles in detail. The flow reached a quasi-developed state within relatively short height to diameter aspect ratio of about H/D=4 compared to a small-diameter pipe flow. The PDF profiles of bubble chord length and gas velocity were able to approximate fairly well by model function using Gamma distribution and log-normal distribution, respectively.

Evaluation of Hydraulic Factors Affecting Flow Accelerated Corrosion and Its Verification With Power Plant Data

Flow Accelerated Corrosion (FAC) is well known as a complex phenomana of hyraulics and electro-chemicals. Among the two, this study focused on the hydraulic factors affecting FAC. FAC experiments with small rectangular flow duct were conducted in PWR condensate condition. Flow field for the experiment was calculated with numerical simulation using LES (Large Eddy Simulation) turbulence model. From both experimental and numerical results, new model of mass transfer coefficient, as the essential parameters of hydraulics, was proposed considering local turbulent velocity, so as to evaluate the effect of eccentric flow on FAC. To verify the applicability of the model, FAC plant data of actual PWR (Pressurized Water Reactor) condensate line (146 degC) and BWR (Boiling Water Reactor) condensate line (35 degC) were referred. Mass transfer coefficients for each pipe lines were calculated from flow numerical analysis. The new proposed model showed good correlation with the data of FAC thinnng rate, and its applicability was confirmed. In addtion, comparing the two plant cases, electro-chemical effect could be estimated as a similar level, which suggests the possibility of low-temperature FAC in BWR condensate lines.Copyright

An evaluation model to predict steam concentration in a BWR reactor building

When there is no power for cooling the spent fuel pool and conditioning the air in a boiling water reactor (BWR) reactor building, water vapor is generated from the pool and it affects the atmosphere in the building. To consider the impact of the steam in preparing emergency operation procedures, the building atmosphere under various conditions is to be evaluated with reasonably low computational cost. A lumped parameter model to predict the transient behavior of the building atmosphere was developed, in which the evaporation from the spent fuel pool and the condensation to the wall were taken into consideration. A transient behavior of temperature and vapor concentration in a BWR operating floor was predicted with the model. The results and the prediction speed were compared to those of a three-dimensional computational fluid dynamic calculation, and it was confirmed that the model could obtain almost the same results about 280,000 times faster. Parameter studies are conducted with the model, and dominant parameters to the evaporation and the condensation were clarified.

Development of Pipe Wall Thinning Evaluation Method and Prediction Tool

Flow accelerated corrosion (FAC) and liquid droplet impingement erosion (LDI) are the main pipe wall thinning phenomena in piping system of power plants in Japan. Authors have promoted the development of prediction method to evaluate local thinning trend by FAC/LDI. To apply the method to pipe wall thinning management in power plants, it is required to be transformed into practical tools for easy usage. In Japan, discussion is being made to considerate the introduction of prediction tools into wall thinning management based on wall thickness measurement at present.Authors have simplified their FAC/LDI models to predict wall thinning trend one-dimensionally along piping layout, and applied to actual thinning data of power plants. With PWR’s FAC data and BWR’s LDI data, maximum thinning rate for each pipe elements were roughly predictable with considerable accuracy. Especially for high thinning rate data, which is important in plant management, the model was able to evaluate within the factor of 2. By installing this model, prediction software “FALSET” was developed, equipped with practical functions for the management. With the further verification and improvement of each function, there are prospects for this software to be utilized as a management tool in power plants.Copyright

Development of Evaluation System for Liquid Droplet Impingement Erosion (LDI)

An evaluation system for liquid droplet impingement erosion (LDI) has been developed to predict the LDI location and the wall thinning rate. The results from previous studies and knowledge are organized and the LDI evaluation system that involves the “1. Flow Evaluation Step” and “2. LDI Evaluation Step” is suggested. The flow evaluation step includes the mass flow rate evaluation, flow distribution evaluation and droplet behavior evaluation. The LDI evaluation is conducted with LDI sensitivity function. The LDI sensitivity was test-evaluated in the steam piping model. The results show that the 1st and last elbows have higher LDI sensitivity because of the velocity condition. We also found that the droplet collision frequency is important when evaluating the LDI sensitivity.Copyright

A Study on Fluid Excitation Forces Acting on a Rotated Square Tube Bundle of T∕D=3.1 in Cross-Flow

The local fluid excitation force acting on a rotated square tube bundle having transverse pitch-to-diameter ratio of T/D=3.1, in a single-phase cross-flow was measured, and the normalized power spectral density (NPSD) and correlation length in the axial direction of a tube were examined. The fluid excitation force acting on the interior tube was from three to ten times larger than that acting on the upstream tube. The fluid force was almost fully developed after the third row. NPSD of the fluid excitation force could be almost plotted on a single universal curve. Regarding the lift direction, there was a peak in NPSD at fD/u∼0.3 caused by vortex shedding. Regarding the drag direction, there could be another peak in NPSD around twice the vortex shedding frequency. In the region of fD/u>0.5, where the effect of the vortex shedding was assumed to be small in the lift direction, the correlation length of the lift direction was ∼1.1D. NPSD was a little larger than previous results for tube bundles of relatively small pitch to diameter ratios summarized by Axisa, Antunes, and Villard (1990, J. Fluid Struct., 4, pp. 321-341).

Numerical Evaluation of Wall Thinning Profile in Separation and Union Pipe due to Flow Accelerated Corrosion

Flow Accelerated Corrosion (FAC) is a pipe wall thinning phenomenon to be monitored and managed in power plants with high priority. In Japan, its management has been conducted with conservative evaluation of thinning rate and residual lifetime of the piping based on wall thickness measurements. However, noticeable case of the wall thinning occurred at separation and union pipe. In such pipe system, it is a problem to manage a section beneath reinforcing plate of T-tube pipe and a crotch of T-joint pipe; wall thickness measurement with high accuracy is difficult to conduct in the region by using ordinary ultrasonic testing devices. In this study, numerical analysis for separation and union parts of T-tube and T-joint pipes was conducted, and wall thinning profile by FAC was evaluated by calculating mass transfer coefficient and geometry factor. Based on these results, applicable wall thinning management for T-tube and T-joint pipes was considered. In the case of union flow from main and branch pipe, the wall thinning profile of T-tube showed the tendency of increase at main pipe like semielliptical region. On the other hand, noticeable profile appeared at crotch in T-joint although it was found that geometry factor of T-joint in this flow pattern was half the value of T-tube. An alternative evaluation method to previous one might be needed for such semielliptical region in T-tube and crotch in T-joint.Copyright