Tomio Okawa

New entrainment rate correlation in annular two-phase flow applicable to wide range of flow condition

Abstract Assuming the rate of droplet entrainment is characterized by the ratio of the interfacial shear force to the surface tension force acting on the phase interface, new correlation representing the rate of droplet entrainment in annular-dispersed two-phase flow was developed. Although the correlation is based on the simple assumption, the quasi-equilibrium droplet flow rates measured in many experiments were predicted reasonably well (root mean square error of entrainment fraction was roughly halved comparing with several existing correlations). Its applicability to the non-equilibrium situation was also demonstrated by the numerical calculations using a one-dimensional three-fluid model.

Prediction of Critical Heat Flux in Annular Flow Using a Film Flow Model

A new set of correlations for the film flow analysis was developed to predict the critical heat flux in annular flow regime accurately. All the correlations adopted here were based on experimental data or considerations of the processes in annular flow; the resulting model required no parameters that should be adjusted from the measured data of critical heat flux. The 4,375 data of critical heat flux in forced flow of water in vertical uniformly heated round tubes were used to test the basic performance of the model. The comparisons between the calculated and measured critical heat fluxes showed that the predicted results by the present model agree with the experimental data fairly well if the flow pattern at the onset of critical heat flux condition is considered annular flow. The predictive capability was not deteriorated even in the cases of small diameter tube, short length tube as well as low vapor quality. Good agreements were also achieved in the preliminary tests against the critical heat flux data for Freon 12 and Freon 21. It is expected from these results that the present model satisfactorily expresses the important phenomena to predict the critical heat flux in annular flow regime.

Temperature effect on single bubble rise characteristics in stagnant distilled water

Abstract Rise characteristics of spherical and ellipsoidal bubbles in normal- and high-temperature distilled water were visually observed. In the high-temperature experiments, the measured results of the rise velocity of a single bubble and the existence of rise path oscillation roughly agreed with the correlations for the bubbles in contaminated liquid. Also, applicability of an available correlation for the frequency of rise path oscillation was confirmed and a new correlation was developed to evaluate the amplitude of oscillation. It is expected that these results are to contribute to the further improvement of the prediction methods of multidimensional void distribution.

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.

Photographic Study on Bubble Motion in Subcooled Pool Boiling

Using a static contact angle of a vertical heated wall as a main experimental parameter, a photographic study was carried out to elucidate the mechanisms to determine the vapor bubble dynamics during subcooled pool boiling. The test fluid was distilled water and the experiments were performed under the atmospheric pressure; liquid subcooling was set to around 5 K. To enable clear observation of bubble behavior with a high speed camera, the experiments were conducted in an isolated bubble regime near the onset of nucleate boiling. Distinctly different bubble behaviors were observed on hydrophobic and hydrophilic surfaces: the bubbles were adhered to the surface for a long period of time when the contact angle was large while lifted-off the surface within a short period of time after the nucleation when the contact angle was small. Since buoyancy does not remove the bubble from the vertical surface, the mechanisms of bubble lift-off were investigated. It was indicated that the change in bubble shape induced by the surface tension force, unsteady growth force, and local liquid flow induced by heterogeneous condensation around the bubble are considered to promote the bubble lift-off while the surface tension force acting on the three-phase common line prevented the lift-off. Effects of the surface wettability on the lift-off bubble diameter, the elapsed time from the nucleation at the lift-off, and the condensation rate after the lift-off were also investigated.

Effects of a Flow Obstacle on the Deposition Rate of Droplets in Annular Two-Phase Flow

Double film extraction technique was used to elucidate the effects of a flow obstacle on the deposition rate of droplets in annular two-phase flow. In the present experiments, the test section was a round tube of 5 mm in inside diameter, air and water were used as test fluids and the flow direction was vertical upward; a short tube that was 2 mm in inside diameter, 3 mm in outside diameter and 20 mm in length was used as a flow obstacle. It was shown that the deposition rate of droplets was markedly increased if the present flow obstacle was concentrically placed in the flow channel. In all the experimental conditions tested, the deposition rates when the flow obstacle was placed were approximately 1.5 times larger than the deposition rates when no obstacle was placed. An empirical correlation was developed to elucidate the influence of flow parameters on the enhancement of deposition rate by the present flow obstacle of simple geometry.

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.

Numerical implementation of interfacial drag force for one-dimensional, two-way bubble tracking method

A two-way bubble tracking method, in which each bubble is individually tracked on the Lagrangian coordinates, is one of the most promising two-phase flow models to partly take the place of a fully-averaged two-fluid model. In this method, conservation equations on different coordinate systems must be solved simultaneously, so that construction of the phase coupling models becomes particularly important. In the present study, taking into account the downward liquid flow formed around a bubble rising in a vertical pipe and using the bubble shape function proposed by Trapp et al. at the same time, we developed simple phase coupling models for one-dimensional, two-way bubble tracking method. The calculated results of a single bubble rising in a circular pipe filled with stagnant liquid showed that the proposed model not only successfully mitigates the numerical fluctuation of the bubble rising velocity but also naturally predicts the effect of containing wall on the terminal bubble velocity; the present model would therefore be reasonable from both numerical and physical points of view. It was also revealed that the mean void fraction in bubbly two-phase flow is reasonably to be predicted if the effect of other bubbles on the downward liquid flow is appropriately evaluated.

Numerical Simulation of Two-Dimensional Bubbles Initially Flattened Along a Flat Plate

ABSTRACT Several researchers have reported the detachment of vapor bubbles from a vertical heated wall in upward flow boiling of water. In these experiments, the bubble shape was frequently flattened along the wall at inception but became more rounded before the onset of detachment. In this study, the motion of two-dimensional bubbles is investigated numerically to elucidate the mechanism of bubble detachment. The numerical results show that the change in bubble shape due to surface tension force induces the liquid flow from the periphery to the base of bubble and consequently the liquid inertia lifts the bubble from the wall.