Air-water Bubbly Flow Research Articles

A Model for Sound Velocity in a Two-Phase Air-Water Bubbly Flow

In this paper, wave propagation in a homogeneous, low void fraction, two-phase air-water bubbly flow is analyzed through the compressibility of a single bubble to derive a P({rho}) relation; the dispersion relation is then derived by a homogeneous model. The phase velocity and attenuation calculated from the model are compared with existing data and are in good agreement. The momentum transfer effect is considered through the virtual mass term and is significant at a higher void fraction. The interfacial heat transfer between phases is significant at low frequency, while bubble scattering effects are important at high frequency (near resonance). Bubble behavior at both low and high frequency is derived based on the isothermal and the adiabatic cases, respectively. The phase velocity occurs at the limiting condition in both cases. Furthermore, resonance is present in the model, and the resonant frequency is determined.

An experimental study on local interfacial parameters in a horizontal bubbly two-phase flow

The internal phase distribution of cocurrent, air-water bubbly flow in a 50.3 mm dia transparent pipeline has been experimentally investigated by using a double-sensor resistivity probe. Liquid and gas volumetric superficial velocities ranged from 3.74 to 5.71 and 0.25 to 1.37 m/s, respectively, and average void fractions ranged from 4.30 to 22.5%. The local values of void fractions, interfacial area concentration, mean bubble diameter and bubble interface velocity, chord-length and frequency distributions were measured. The experimental results indicate that the void fraction, interfacial area concentration and bubble frequency have local maxima near the upper pipe wall, and the profiles tend to flatten with increasing void fraction. The observed peak void fraction can reach 0.65, the peak interfacial area can go to 900–1000 m 2/m 3, and the bubble frequency can reach a value of 2200/s. These ranges of values have never been reported for vertical bubbly flows. It is found that either decreasing the liquid flow rate or increasing the gas flow would increase the local void fraction, the interfacial area concentration and the bubble frequency. The axial bubble interface velocity and the Sauter mean diameter profiles show a relatively uniform distribution except near the upper pipe wall, where a sharp reduction in the velocity and mean diameter occurs. The local bubble velocity and the mean diameter generally increase with the gas flow rate.

Effects of Wall Roughness and Entry Length on Void Profile in Vertical Bubbly Flow

An experimental study of upward air-water bubbly two-phase flow in an entry region was performed with various rough wall test tubes. The objective of the work is to clarify the effects of wall roughness and entry length on void profile. The fluid flows in the vertical circular test tube of 25 mm I.D. under nearly atmospheric pressure, at room temperature. The void profile changes from a pattern similar in appearance to the saddle shape which has local void peaks near the wall, into the power law shape whose curve is approximated by a power law formula, with increasing wall roughness and/or entry length. That is, wall roughness and entry length have a similar effect upon void profile. There are two patterns in the power law shape, a pattern with sharp center peak and a pattern with obtuse center peak. As wall roughness and/or entry length increase, the void profile changes from the former pattern to the latter pattern. At enough long entry length (L/D ≃ 150), every void profile has almost the same power law shape independent of wall roughness. Some void profiles are asymmetric to the axis.

Effects of wall roughness and entry length on void profile in vertical bubbly flow.

An experimental study of upward air-water bubbly two-phase flow in an entry region was performed with various rough wall test tubes. The objective of the work is to clarify the effects of wall roughness and entry length on void profile. The fluid flows in the vertical circular test tube of 25 mm I.D. under nearly atmospheric pressure, at room temperature. The void profile changes from a pattern similar in appearance to the saddle shape which has local void peaks near the wall, into the power law shape whose curve is approximated by a power law formula, with increasing wall roughness and/or entry length. That is, wall roughness and entry length have a similar effect upon void profile. There are two patterns in the power law shape, a pattern with sharp center peak and a pattern with obtuse center peak. As wall roughness and/or entry length increase, the void profile changes from the former pattern to the latter pattern. At enough long entry length (L/D ≃ 150), every void profile has almost the same power law shape independent of wall roughness. Some void profiles are asymmetric to the axis.

Air-water two-phase flow through a pipe junction. Experiment on the local void fraction distribution adjacent to the branching part.

The distribution of gas and liquid phases was studied experimentally in air-water two-phase flow flowing through a pipe junction with the branching angle of 90°and the area ratio of unity. The main of the junction was set up vertically and upward air-water bubbly and slug flows were arranged in the main upstream section. As the result, the local void fraction distribution in the main upstream section is almost axisymmetry. More air in the main upstream section flows into the lateral than that in the main downstream section. Therefore, nonsymmetry of the local void fraction distribution in the lateral is more remarkable than that in the main downstream section.

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

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.

Momentum and heat transfer in two-phase bubble flow—II. A comparison between experimental data and theoretical calculations

Momentum and heat transfer process in a two-phase air-water bubble flow is investigated experimentally to confirm the applicability of the theoretical model proposed in Part 1 of this study. Comparisons are made between the measurements and the predictions for both velocity and temperature profiles, and then satisfactory agreement is obtained. Also, the results for bubble flow simulation are presented to clarify the interrelationship of the frictional pressure gradient and the heat transfer coefficient with the void fraction profile.

Forced Convective Heat Transfer in Vertical Air-Water Bubble Flow

垂直円管内の空気・水系気ほう流の相分布,液体温度分布,熱伝達係数に関する実験値が得られた.断面内相分布の差異は熱伝達に影響し,同一流動条件では壁面近傍のボイド率の高い流れほど熱伝達係数が高くなる.ボイド率分布には気水流量,気ほう群の寸法のほか,加熱による壁面近傍の水の温度変化も関係することがわかった.実験情報をもとに管の半径方向への熱輸送に与える気ほうのかく乱作用についても考察した.

Forced Convective Heat Transfer in Vertical Air-water Bubble Flow

The dependence of heat transfer on the cross-sectional distribution of void fractions in a diabatic two-phase bubble flow has been explored experimentally. A vertical tube of 16.9 mm ID was employed in the investigation. Two different types of the mixer were installed to realize the flows with different types of the mixer were installed to realize the flows with different distributions of void fractions even at the same flow rates of both phases. It was found that these two types of flows present different heat transfer coefficients due to the difference in the void fraction distributions. Increasing the void fraction near the tube wall causes an increase in heat transfer coefficient. The effect of bubble agitation on the thermal diffusivity in the radial direction is discussed based on the distributions of both void fractions and liquid temperatures.

Turbulence structure of air-water bubbly flow—III. transport properties

An experimental study of heat and bubble transport in turbulent air-water bubbly flow was carried out by means of tracer techniques. Helium tracer gas concentration data and temperature distributions were used to extract bubble and heat diffusivity information. The results indicated that the turbulent velocity components of the liquid phase play a predominant role in the turbulent transport process. A systematic increase of diffusivity of heat, ϵ H , with quality and water velocity was observed. An empirical correlation for the diffusivity ratio ϵ H, TP / ϵ H, SP is presented. The Péclet number, u′ c ovbar| dφ, for bubble dispersion can be approximated by 2.0, independent of the flow variables. The bubble-to-heat diffusivity ratio, φ/ ϵ H , approaches unity with increasing quality and water velocity. Momentum transport is also discussed, based on a mixing length theory.

Turbulence structure of air-water bubbly flow—I. measuring techniques

This paper, the first in a series describing our work on the turbulence structure of air-water bubbly flow, describes the principles of measurement and specially developed electronic instrumentation for determining various important local parameters, and the rates of turbulent transport of heat and bubbles in air-water two-phase bubbly flow. These instruments indicate the phase distribution, the bubble velocity and its spectrum, the water velocity and the turbulent intensity, and the turbulent dispersion coefficient of bubbles. Brief discussions are also presented on the accuracy of these techniques.

Turbulence structure of air-water bubbly flow—II. local properties

Microstructure was studied experimentally in air-water two-phase bubbly flow flowing upward in a vertical pipe of 60 mm diameter under atmospheric pressure. The results indicate that over a large portion of fully-developed bubbly flow the phases, the velocities of bubbles and water, and the ratio between the velocities of the phases have fairly flat radial profiles. In the wall region a maximum void fraction was observed. Spectra of the velocities of bubbles and water showed a Poisson distribution and a normal distribution function, respectively. The experimental evidence indicated a trend for the turbulent intensity to decrease first with increasing gas flow rate for constant water velocity and to increase again with a further increase in the gas flow rate. This phenomenon was more significant for a higher water velocity.