Shigebumi Aoki

Flow Behavior and Pressure Drop of Two-Phase Flow through C-Shaped Bend in Vertical Plane, (II): Downward Flow

Experimental results are presented on the flow behavior, average void fraction and pressure drop in air-water two-phase flow mixture flowing upward through a C-shaped bend curved in vertical plane. The curved test section of transparent acrylic resin tubing was varied in four versions of (a) 90 mm, (b) 132.5 mm and (c) 180 mm radii of curvature with 16 mm inside diameter tube, and (d) 135 radius with 24 mm diameter tube. The combined action of gravity and centrifugal force acting on the two-phase flow is expressed in terms of a modified Froude number representing the balance of radial forces between those acting on the liquid and the gaseous phases of the flow passing through the reclined vertical U- bend. The average void fraction in the curved test section was determined, and empirically correlated to the pressure drop, by means of a series of nondimensional numbers.

Instabilities in Parallel Channel of Forced-Convection Boiling Upflow System, (II)

The instabilities observed in a parallel-channel system carrying boiling fluid in forced upward flow have been studied theoretically and experimentally, using water as test fluid. The limits of the stable flow in such a parallel-channel system (the stable boundary) are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results obtained are compared with calculations based on the mathematical model reported in a companion paper, and good agreement is seen between analysis and experiment in respect of the stable boundary. The experimental results are further examined through phase analysis using the same model.

Studies on two-phase cross flow. Part I: Flow characteristics around a cylinder

Abstract A two-phase flow around a body has scarcely been studied until now, though the flow is used in many industrial components. The cross flows around a spacer in a fuel assembly of light water reactors (LWR) and tube supports in a steam generator are closely related to the long-term reliability and the safety. The present study has been planned to clarify the two-phase flow and heat transfer characteristics around a body including the unknown complicated flow behavior. In the first report, the flow characteristics near and behind a cylinder which was located in a vertical upward air-water bubbly flow were investigated. From the observation of the flow patterns and the measurements of the distributions of void fraction, liquid velocity and static pressure, it is revealed that the vortex flow and the change of the static pressure and liquid velocity distribution around the cylinder resulted in the large distortion of the void fraction distribution around the cylinder. The most noticeable phenomena in the wake were that the peaks of the local void fraction appeared in the vicinity of the cylinder surface near the separation point and in the wake behind the cylinder.

Instabilities in Parallel Channel of Forced-Convection Boiling Upflow System, (II): Experimental Results

The instabilities observed in a parallel-channel system carrying boiling fluid in forced upward flow have been studied theoretically and experimentally, using water as test fluid. The limits of the stable flow in such a parallel-channel system (the stable boundary) are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results obtained are compared with calculations based on the mathematical model reported in a companion paper, and good agreement is seen between analysis and experiment in respect of the stable boundary. The experimental results are further examined through phase analysis using the same model.

Experimental study on the boiling phenomena within a narrow gap

Abstract The experiments were carried out with annular narrow gaps having the gap widths 0.2,0.3,0.4,0.5, 1.0 and 1.5 mm for the following two cases: (a) for the “open bottom” case, the heat transfer coefficient was improved as the gap width decreases, but it was not affected by gap lengths in the range 40

Studies on two-phase cross flow. Part II: Transition Reynolds number and drag coefficient

L . (b) for the “closed bottom” case, the heat transfer coefficient was not affected by gap width or length. The transition heat flux could be correlated by the equivalent gap length defined in terms of the cross-sectional area of the open end.

Instabilities in Parallel Channel of Forced-Convection Boiling Upflow System, (I): Mathematical Model

Abstract A reduction of the transition Reynolds number from laminar separation to turbulent one of cross flow around a body is anticipated in a gas—liquid bubbly flow, since there exists intensive turbulence in the main flow. A decrease in the drag coefficient of the body can also be expected. This report was aimed to classify flow patterns of the two-phase wake flow behind a cylinder and to investigate quantitatively the change of the drag coefficient corresponding to the transition of the flow patterns. From measurements of the static pressure distribution on the cylinder surface and the drag coefficient of the cylinder, it was found that the flow pattern was sure to change finally into a new one similar to the transcritical type in the single-phase flow with an increase of the mean void fraction in the main flow. It was concluded that a large reduction of the upper transition Reynolds number occurred in a two-phase flow, because the transition could be realized by a little increase of the void fraction even below the lower transition Reynolds number in the case of a large cylinder diameter.

Studies on two-phase cross flow. Part III: Characteristics of unsteady flow behavior

The instabilities occurring in forced-convection upflow passing through a parallelchannel system have been studied theoretically and experimentally, using water as test fluid. A non-linear mathematical model is proposed, that explains the results quite satisfactorily. Close agreement is obtained between calculations based on this model and measurements of the stable boundary in a system of two channels. The model is further applied to three and four parallel channels, the results of which indicate that the flow instability behavior in such multiple channels are quite similar to that observed in the two-channel system, in such aspects as the stable boundary, as well as the period and the development of flow oscillations in the unstable region.

Thermo-hydraulic behavior of inverted annular flow

Abstract Basic characteristics in a two-phase wake flow, in particular, fluctuations of bubble number density, were investigated experimentally using mainly flat plates. New measuring procedures of the periodicity of the bubble number density and of the spatial distribution of the phase-averaged void fraction were developed. The experimental results indicated that two kinds of flow patterns appeared alternately in the wake. Moreover, it was confirmed that one of them corresponded to the Karman vortex type in a single-phase flow and the other was similar to the twin eddies type which was observed under low Reynolds number ( Re 2 ) in the flow. The alternation process of the two flow patterns depended on the shape of obstacles in the cross flow. In case of a cylinder, numbers of the Karman vortices formed continuously were very few.

Thermo-hydraulic instabilities in parallel boiling channel systems: Part 1. A non-linear and a linear analytical model

Abstract The thermo-hydraulic behavior of inverted annular flow was investigated experimentally under various heat flux, inlet velocity, inlet subcooling and heating length conditions using freon 113. Empirical correlations are proposed concerning the net vaporization rate from the interface, heat flux from the interface to the liquid phase, interfacial shear stress and heat transfer coefficient from the wall to fluid. It is found that the roughness of the interface between the vapor film and liquid jet increases with the thickness of the vapor film and that an increase in the vapor film thickness causes the average Nusselt number and interfacial friction factor to rise linearly. On the other hand, the wall shear stress in inverted annular flow is lower than that in liquid single-phase flow due to the existence of a vapor film at the wall.