The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 2. Effect on void fraction and liquid film thickness

An experimental study has been carried out to clarify the characteristics of the void fraction and the liquid film thickness of the air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter and pitch. The flow pattern in the experiment on the average void fraction and the local void fraction distribution in cross section was a bubble flow, and the liquid film thickness was in the region of semiannular and annular flows. It is clarified from these experiments that the average void fraction in tubes with wire coils is lower than that in a smooth tube and decreases with the wire diameter owing to the centrifugal force of the swirl flow which concentrates bubbles at the center of the tube, that the local liquid film thickness becomes more uniform with a decrease in the pitch of the wire coil, and that the liquid film becomes thicker after the passage through the wire coil with an increase in the wire diameter. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 652–664, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10067     

Effects of liquid viscosity on inception of disturbance waves and droplets in gas–liquid annular two‐phase flow

The structure of gas–liquid two‐phase flow is investigated in order to establish a reliable criterion for the development of disturbance waves and droplets considering the effects of liquid viscosity. The structure of the gas–liquid interface and the flow rate of droplets entrained in gas are measured simultaneously at five kinematic viscosities (1.0, 3.2, 9.9, 30, 70 mm²/s). The time‐series traces of liquid film thickness measured by five holdup probes reveal that the inception of disturbance waves occurs at a liquid Reynolds number of 200 or a non‐dimensional liquid film thickness of 6.5. It is also shown that droplets are generated before the inception of disturbance waves with increasing liquid kinematic viscosity at a liquid velocity of 0.02 to 0.03 m/s. As previously published criteria for the inception of droplets are found to be unsatisfactory, a new critical condition for droplet generation balancing the interfacial shear stress $τi$ with the wave height h and surface tension σ is proposed: $τih/σ=0.025$. This relation describes the action of shear force and surface tension on wave crests, and is notably independent of liquid viscosity. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 529–541, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20176     

A simplified model of gas–liquid two‐phase flow pattern transition

An experiment of upward gas–liquid two‐phase flow was conducted in an air–water isothermal system under atmospheric pressure. The differential pressure was measured at the fully developed section by using a variable reluctance type transducer to classify the flow patterns and their transitions. The flow behavior was observed with a high‐speed video camera. The probability density function (PDF) of the differential pressure signal was employed to identify the flow pattern. A simplified one‐dimensional flow model was proposed to clarify dominant factors affecting the formation and transitions of flow patterns. The model dealt with the gas‐component advection based on the spatiotemporal void fraction behaviors by considering the gas compressibility, the wake, and the liquid phase redistribution mechanism. The simulation results of the model indicated four kinds of the void wave patterns (ripple‐like, rectangular, distorted rectangular, and uniform wave patterns) depending on gas and liquid volumetric fluxes. These void wave patterns corresponded well to the experimentally observed flow patterns. The transitions among void wave patterns agree well with the Mishima–Ishii flow pattern map. The friction loss estimated by the present model coincides fairly well with Chisholm's empirical formula. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 445–461, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20029     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 1. Experimental results of flow pattern,void fraction,and pressure drop

Experiments were carried out to investigate the flow pattern, average void fraction, and pressure drop of an upward air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter, pitch, and number of coils in cross section. Five kinds of flow patterns—bubble, slug, churn, semiannular, and annular flow—were defined based on the observation of flow behavior in the experiments. At higher water flowrates, the bubble‐to‐slug transition occurred at lower air flowrates in tubes with wire coils than in smooth tubes. The average void fraction was found by using the drift flux model. Further, the experimental results of the friction pressure drop were compared with the Lockhart‐Martinelli correlation. As a result, a correlation with the constant C in Chisholm's equation was obtained as a function of the wire coil pitch‐to‐diameter ratio. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 639–651, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10066     

Development of an ultrafast X‐ray computed tomography scanner system: Application for measurement of instantaneous void distribution of gas–liquid two‐phase flow

An ultrafast X‐ray computed tomography (CT) scanner has been developed. This scanner overcomes problems that occur in a transient or unsettled state, which make the conventional CT scanner inappropriate. To reduce the scanning time, this X‐ray CT system uses electronic switching of electron beams for X‐ray generation instead of the mechanical motion adopted by conventional CT scanners. The mechanical motion is a major obstacle to improving scanning speed. A prototype system with a scanning time of 3.6 ms was initially developed and confirmed to measure the dynamic events of two‐phase flow. However, an increased scanning speed is generally required for practical use in the thermal hydraulics research field. Therefore, an advanced type which can operate under the scanning time of 0.5 ms and can measure two‐phase flow with a velocity up to 4 to 5 m/s was developed. © 2000 Scripta Technica, Heat Trans Asian Res, 29(3): 155–165, 2000     

A study on un‐fully developed slug flow in a vertical tube

Gas‐liquid co‐current vertical slug flow was studied in a vertical Plexiglas tube. Taylor bubbles and liquid slug lengths and their rising velocities were measured by means of a pair of conductivity probes under un‐fully developed flow conditions. The influences of the superficial velocity of gas and liquid on slug flow parameters were examined. Using statistical analysis on the length of Taylor bubbles, the probability distribution of the length of the Taylor bubbles was obtained, which obeyed a normal distribution under a significance level of α = 0.05. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 235–242, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20063     

Study of the behavior of ephemeral waves in gas–liquid two‐phase flow: I. Determination of sub‐wave‐veins

Time‐spatial measurements of liquid holdup distributions along the axis of a tube were carried out over the length of 1325 mm in upward gas–liquid two‐phase flow. In order to clarify the characteristics of the behavior of ephemeral waves, a method of determining sub‐wave‐veins, that is, the traces of ephemeral waves on the time‐spatial behavior charts of the interface, was developed. This method was applied to the flow conditions in huge wave flow and annular flow regimes, and the sub‐wave‐veins in these flow regimes were successfully determined. Time‐spatial behavior charts of the interface with determined sub‐wave‐veins were systematically presented and the characteristics of sub‐wave‐veins were discussed. Close inspection of the behavior of sub‐wave‐veins reveals that there are two types of ephemeral waves: one has a shorter life span and the other has a longer life span during which absorption and discharge of small ephemeral waves occurs. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 114–125, 2001     

Characteristics of interfacial profiles in upward and downward gas-liquid two-phase plug flow

Gas-liquid interfacial profiles in plug flow for both upward and downward flows were obtained using semi-supermultiple point-electrode probes, comprising 67 sensing tips arranged on a tube diameter. Typical interfacial profiles are demonstrated for both flows. Close inspection of the profiles reveals that four zones exist in a pair of gas and liquid slugs for upward plug flow and a high slip velocity region in downward plug flow. The lengths of the swelling liquid front zone and the wake zone were determined. The length of the wake zone strongly depends on the relative velocity between the liquid film around the gas slug and the liquid phase in the liquid slug. Characteristic distributions of bubbles within liquid slugs were found, i.e., three types of radial distributions of void fraction, namely saddle-shaped, trapezoidal and bullet-shaped distributions, exist for upward flow. The two types for downward flow exclude the saddle-shaped distribution. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25 (8): 568–579, 1996     

Numerical study of two‐phase flow of liquid helium in a vertical converging‐diverging nozzle

The fundamental characteristics of the two‐dimensional gas‐liquid two‐phase flow of liquid helium through a vertical converging‐diverging duct near the lambda point are numerically investigated to realize the further development and high performance of new multiphase superfluid cooling systems. First, the governing equations of the two‐phase flow of liquid helium based on the unsteady thermal nonequilibrium multifluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two‐dimensional structure of the gas‐liquid two‐phase flow of liquid helium though vertical converging‐diverging nozzle is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid‐ to vapor‐phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 432–448, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20071     

Study on behavior of ephemeral wave in gas‐liquid two‐phase flow: Part 2. Characteristics in flow parameter of ephemeral waves

In order to clarify the behavior of ephemeral waves, flow parameters of ephemeral waves, such as the number and residence zone lengths per unit axial length, mean values and standard deviations of wave velocity, width, and maximum holdup, were determined using a wave‐vein analysis in upward huge wave flow and annular flow. The mean values of wave velocities, widths, and maximum holdups of ephemeral waves are compared with those of liquid lumps having main wave‐veins, and the differences in the parameters between those liquid lumps are discussed. Furthermore, the parameters of both active ephemeral waves and inactive ephemeral waves are determined, and the characteristics in the flow parameters of two types of ephemeral waves are presented. © 1999 Scripta Technica, Heat Trans Asian Res, 29(1): 1–14, 2000     

The experimental study on the flow characteristics for a swirling gas–liquid spray atomizer

A new spray atomizer, that is, swirling gas–liquid spray atomizer, is presented in this paper based on the analysis of various air-assist atomizers and their atomization mechanism. The air-to-liquid mass flow rate ratio (ALR) of the atomizer during the hot-state tests is 4–6% (by compressed air for emulsified heavy oil). And it is of excellent atomization. The discharge coefficient and the spray cone angle of the atomizer are studied systematically. The influence factors considered are as follows: the structure parameters, ALR and the viscosity of the liquid. Through the experiments, the formula of discharge coefficient and the spray cone angle of the atomizer are given in the paper. And it can give good direction on the atomizer design. The atomizer can be employed in the industrial furnaces and other equipments where the liquid must be atomized.     

Numerical simulation of steam–water two‐phase flow under dynamic load

A separated‐phase physical model for steam–water two‐phase flow on a rotating platform was developed. The mesh generation for a horizontal pipe was conducted, and the finite volume method was used to discretize the equations. Equations were solved with the SIMPLE algorithm after setting the initial and boundary conditions. Predicted results were compared with experimental data, and they agreed well with each other. The results showed that the fluid outlet pressure and pressure drop in the test section increased with increasing dynamic load. However, the effective heat transferred to the fluid decreased with the increase of dynamic load. The developed model can be used to simulate the gas–liquid two‐phase flow under different gravity or rotary conditions.     

Modelling of stratified gas–liquid two-phase flow in horizontal circular pipes

This paper reports numerical and experimental investigation of stratified gas–liquid two-phase flow in horizontal circular pipes. The Reynolds averaged Navier–Stokes equations (RANS) with the k–ω turbulence model for a fully developed stratified gas–liquid two-phase flow are solved by using the finite element method. A smooth interface surface is assumed without considering the effects of the interfacial waves. The continuity of the shear stress across the interface is enforced with the continuity of the velocity being automatically satisfied by the variational formulation. For each given interface position and longitudinal pressure gradient, an inner iteration loop runs to solve the non-linear equations. The Newton–Raphson scheme is used to solve the transcendental equations by an outer iteration to determine the interface position and pressure gradient for a given pair of volumetric flow rates. Favorable comparison of the numerical results with available experimental results indicates that the k–ω model can be applied for the numerical simulation of stratified gas–liquid two-phase flow.     

Influence of liquid injection on two‐phase flow convective boiling of refrigerant mixtures

In this paper, an analytical study on the influence of liquid injection on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of the liquid injection the thermophysical properties of refrigerant mixtures as well as the heat transfer characteristics such as average heat transfer coefficient of R‐507, R‐404A, R‐410A, and R‐407C in two‐phase flow boiling inside enhanced surface tubing. It was also evident that the proposed correlations and the experimental data that the liquid injection has significant impact on the heat transfer coefficient. In addition, the proposed correlations were applicable to the entire heat and mass flux, investigated in the present study under the liquid injection conditions. The deviation between the experimental and predicted under liquid injection were less than ±20, for the majority of data. Copyright © 2004 John Wiley & Sons, Ltd.     

Characteristics of particle mixtures in a liquid‐fluidized bed

Behavior of particle mixtures was investigated in a liquid‐fluidized bed experimentally. In the experiment, two kinds of particles of different diameters, and of the same or different materials, were charged in a fluidization column and fluidized by water. Based on the observation of fluidization, a flow pattern map was proposed using the Archimedes number ratio and the terminal velocity ratio, which classifies the two patterns, that is, separation and homogeneous mix. Measured mean void fractions of particle mixtures agreed well with the values calculated from those for each particle independently. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 175–184, 2001     

Influence of gas/liquid injection on two‐phase flow convective boiling of refrigerant mixtures

An analytical study on the influence of gas/liquid injection on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of the gas/liquid injection on the heat transfer characteristics such as average heat transfer coefficient of R‐507, R‐404A, R‐410A and R‐407C in two‐phase flow boiling inside enhanced surface tubing. The data also revealed that gas/liquid injection is beneficial at certain gas/liquid injection ratios to the heat transfer coefficient depending upon the Reynolds number and the boiling point. It was also evident that the proposed correlations and the experimental data that the gas/liquid injection has significant impact on the heat transfer coefficient. In addition, the proposed correlations were applicable to the entire heat and mass flux, investigated in the present study under gas/liquid injection conditions. The deviation between the experimental and predicted under gas/liquid injection were less than ±20, for the majority of data. Copyright © 2004 John Wiley & Sons, Ltd.     

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

A theoretical study was performed to investigate the evaporative heat transfer of high‐velocity two‐phase flow of air–water in a small vertical tube under both heating conditions of constant wall temperature and constant heat flux. A simplified two‐phase flow boundary layer model was used to evaluate the evaporative heat transfer characteristics of the annular two‐phase flow. The analytical results show that the gravitational force, the gas–liquid surface tension force, and the inertial force are much smaller than the frictional force and hence can be neglected for a small tube. The evaporative heat transfer characteristics of the small tube with constant wall temperature are quite close to those of the small tube with constant heat flux. The mechanism of the heat transfer enhancement is the forced convective evaporation on the surface of the thin liquid film. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 430–444, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10110     

Eulerian simulations of bubble behaviour in a two‐dimensional gas–solid bubbling fluidized bed

In the present study, the CFD model is based on a two‐fluid model extended with the kinetic theory of granular flow. The simulation results of bubble diameter and bubble rise velocity are compared to the Darton equation and the Davidson model in a free bubbling fluidized bed. The predicted values are in reasonable agreement with the values from the Darton bubble size equation and the Davidson model for isolated bubbles. It is shown that the break‐up and direct wall interaction effects influence the dynamic bubble behavior in the free bubbling fluidized beds. Copyright © 2002 John Wiley & Sons, Ltd.     

Predictions of turbulent flow and heat transfer in gas–droplets flow downstream of a sudden pipe expansion

Droplets-laden turbulent flow downstream of a sudden pipe expansion has been investigated by using Euler/Euler two-fluid model for the gaseous and dispersed phases. Significant increase of heat transfer in separated flow at the adding of evaporating droplets has been demonstrated (more than 2 times compare with one-phase flow at the value of mass concentration of droplets M_L1  0.05). Addition of dispersed phase to the turbulent gas flow results in insignificant increase of the reattachment length. Low-inertia droplets (d₁  50 μm) are well entrained into the circulation flow and present over the whole pipe section. Large particles (d₁ ≈ 100 μm) go through the shear layer not getting into the detached area. Comparison with experimental data on separated gas–droplets flows behind the plane backward-facing step has been carried out.