Annular Two-phase Flow Research Articles

B209 垂直上昇環状二相流における液滴飛散特性に及ぼす壁面濡れ性の影響(OS8 熱流動),動力エネルギーシステム部門20周年,次の20年への新展開)

To evaluate the effect of wall surface wettability on the characteristics of upward gas-liquid annular two-phase flow in a vertical pipe, an experimental study was performed using two test pipes: an acrylic pipe and a FEP pipe. Measured contact angles on the acrylic and FEP pipe surfaces were 60° and 80°, respectively. Basic flow characteristics such as liquid film thickness and liquid entrainment ratio were respectively measured by a laser focus displacement meter and a suction method. At relatively high gas flow rate and low liquid flow rate conditions, a reduction of the surface wettability by the FEP pipe enhanced the interfacial waves on liquid film, and caused an increase in the liquid entrainment ratio and a decrease in the liquid film thickness.

A singular integro-differential equation model for dryout in LMFBR boiler tubes

A 2D steady model for the annular two-phase flow of water and steam in the steam-generating boiler pipes of a liquid metal fast breeder reactor is proposed. The model is based on thin-layer lubrication theory and thin aerofoil theory. The exchange of mass between the vapour core and the liquid film due to evaporation of the liquid film is accounted for using some simple thermodynamics models, and the resultant change of phase is modelled by proposing a suitable Stefan problem. Appropriate boundary conditions for the flow are discussed. The resulting non-linear singular integro-differential equation for the shape of the liquid film free surface is solved both asymptotically and numerically (using some regularization techniques). Predictions for the length to the dryout point from the entry of the annular regime are made. The influence of both the traction τ provided by the fast-flowing vapour core on the liquid layer and the mass transfer parameter η on the dryout length is investigated.

Prediction of the entrained liquid fraction in vertical annular gas–liquid two-phase flow

The study considers the prediction of the entrained liquid fraction in adiabatic gas–liquid annular two-phase flow in vertical pipes. Nine empirical correlations have been tested against an experimental data bank drawn together in this study containing 1504 points for 8 different gas–liquid combinations and 19 different tube diameters from 5.00 mm to 57.1 mm. The correlation of Sawant, Ishii and Mishima and the one of Oliemans, Pots and Trompé were found to best reproduce the available data. A new correlating approach, derived from both physical intuition and dimensional analysis and capable of providing further physical insight into the liquid film atomization process, was proposed and worked better than any of the existing methods. This new correlation is based on the core flow Weber number that is also a controlling dimensionless group in determining the wall shear stress and associated frictional pressure gradient of annular flows.

Liquid film behavior in annular two-phase flow under flow oscillation conditions

Abstract Experiments were carried out to investigate the effects of sinusoidal forced oscillation of the inlet flow rate on the time variations of local liquid film thickness and the frequencies of large wave’s passing in steam–water annular two-phase flows. The liquid film thickness oscillated with the same period as the inlet flow rate. The mean film thickness in the thin film regions decreased and approached to an asymptotic value with an increase in the oscillation period of the inlet flow rate. This result was consistent with the experimental results of the occurrence of liquid film dryout under flow oscillation conditions reported in the literature. It was hence considered that the axial liquid transport from the thick to thin film regions mitigates the reduction of the critical heat flux caused by the flow oscillation. It was also found that the wave frequency in the thin film region increased with a decrease in the oscillation period. This observation suggested that the disturbance waves contribute to the enhancements of the liquid transport and consequently the critical heat flux associated with the liquid film dryout under flow oscillation conditions.

Numerical analysis of the onset of droplet entrainment in annular two-phase flow by hybrid method

A particle-grid hybrid method for simulating the onset of droplet entrainment is described. The air and liquid phases are expressed by grid and particle separately. The effects of surface tension and wall adhesion are calculated by the continuum surface force model. The fluid film variation and the droplet disengaging process are analyzed. The onset of entrainment criterion is obtained which has a good agreement with the correlation of Ishii and Grolmes (AIChE J., 21, 308–318). The influences of surface tension and gas phase density on the criterion are also studied. Several simulation flow fields are given for illustrating the efficiency of the method.

Analytical and Numerical Simulations of the Two-Phase Flow Heat Transfer in the Vent and Scavenge Pipes of the CLEAN Engine Demonstrator

Advanced aircraft engine development dictates high standards of reliability for the lubrication systems, not only in terms of the proper lubrication of the bearings and the gears, but also in terms of the removal of the large amounts of the generated heat. Heat is introduced both internally through the rotating hardware and externally through radiation, conduction, and convection. In case where the bearing chamber is in close proximity to the engine’s hot section, the external heat flux may be significant. This is, for example, the case when oil pipes pass through the turbine struts and vanes on their way to the bearing chamber. There, the thermal impact is extremely high, not only because of the hot turbine gases flowing around the vanes, but also because of the hot cooling air, which is ingested into the vanes. The impact of this excessive heat on the oil may lead to severe engine safety and reliability problems, which can range from oil coking with blockage of the oil tubes to oil fires with loss of part integrity, damage, or even failure of the engine. It is therefore of great importance that the oil system designer is capable of predicting the system’s functionality. As part of the European Research program efficient and environmentally friendly aero-engine, the project component validator for environmentally friendly aero-engine (Wilfert, et al., 2005, “CLEAN–Validation of a GTF High Speed Turbine and Integration of Heat Exchanger Technology in an Environmental Friendly Engine Concept,” International Symposium on Air Breathing Engines, Paper No. ISABE-2005-1156;Gerlach et al., 2005, “CLEAN–Bench Adaptation and Test for a Complex Demo Engine Concept at ILA Stuttgart,” International Symposium on Air Breathing Engines, Paper No. ISABE-2005-1134) was initiated with the goal to develop future engine technologies. Within the scope of this program, MTU Aero Engines has designed the lubrication system and has initiated an investigation of the heat transfer in the scavenge and vent tubes passing through the high thermally loaded turbine center frame (TCF). The objective was to evaluate analytical and numerical models for the heat transfer into the air and oil mixtures and benchmark them. Three analytical models were investigated. A model that was based on the assumption that the flow of air and oil is a homogeneous mixture, which was applied on the scavenge flow. The other two models assumed annular two-phase flows and were applied on the vent flows. Additionally, the two-phase flow in the scavenge and vent pipes was simulated numerically using the ANSYS CFX package. The evaluation of the models was accomplished with test data from the heavily instrumented test engine with special emphasis on the TCF. Both the analytical and the numerical models have demonstrated strengths and weaknesses. The homogeneous flow model correlation and the most recent correlation by Busam for vent flows have demonstrated very good agreement between test and computed results. On the other hand the numerical analysis produced remarkable results, however, at the expense of significant modeling and computing efforts. This particular work is unique compared with published investigations since it was conducted in a real engine environment and not in a simulating rig. Nevertheless, research in two-phase flow heat transfer will continue in order to mitigate any deficiencies and to further improve the correlations and the CFD tools.

Unified macro-to-microscale method to predict two-phase frictional pressure drops of annular flows

The study considers the prediction of pressure gradients in adiabatic gas–liquid annular two-phase flow in the macro-to-microscale range. Twenty-four empirical correlations have been tested against an experimental data bank drawn together in this study containing 3908 points for eight different gas–liquid combinations and 22 different tube diameters, covering microscale and macroscale channels from 0.517 to 31.7 mm in diameter. The correlations of Lombardi, Friedel and Baroczy-Chisholm were found to be the best existing methods when considering macroscale data only, while the microscale database was best predicted by the correlations of Lombardi, Müller-Steinhagen and Heck and the homogeneous model with the two-phase viscosity defined according to Cicchitti. A new correlating approach based on the vapor core Weber number, capable of providing physical insight into the flow, was proposed and worked better than any of the existing methods for the macroscale database. This new macroscale method was then extended to cover microscale conditions, resulting in one unified method for predicting annular flows from the macroscale to the microscale covering both laminar and turbulent liquid films. The macroscale method optimized for microchannels worked better than any of the other methods considered.

Experimental Study of DRA for Vertical Two-Phase Annular Flow

Experimental investigation of drag reduction in vertical two-phase annular flow is presented. The work is a feasibility test for applying drag reducing additives (DRAs) in high production-rate gas-condensate wells where friction in the production tubing limits the production rate. The DRAs are intended to reduce the overall pressure gradient and thereby increase the production rate. Since such wells typically operate in the annular-entrained flow regime, the gas and liquid velocities were chosen such that the experiments were in a vertical two-phase annular flow. The drag reducers had two main effects on the flow. As expected, they reduced the frictional component of the pressure gradient by up to 74%. However, they also resulted in a significant increase in the liquid holdup by up to 27%. This phenomenon is identified as “DRA-induced flooding.” Since the flow was vertical, the increase in the liquid holdup increased the hydrostatic component of the pressure gradient by up to 25%, offsetting some of reduction in the frictional component of the pressure gradient. The DRA-induced flooding was most pronounced at the lowest gas velocities. However, the results show that in the annular flow the net effect will generally be a reduction in the overall pressure gradient by up to 82%. The findings here help to establish an envelope of operations for the application of multiphase drag reduction in vertical flows and indicate the conditions where a significant net reduction of the pressure gradient may be expected.

Prediction of amount of entrained droplets in vertical annular two-phase flow

Prediction of amount of entrained droplets or entrainment fraction in annular two-phase flow is essential for the estimation of dryout condition and analysis of post dryout heat transfer in light water nuclear reactors and steam boilers. In this study, air–water and organic fluid (Freon-113) annular flow entrainment experiments have been carried out in 9.4 and 10.2 mm diameter test sections, respectively. Both the experiments covered three distinct pressure conditions and wide range of liquid and gas flow conditions. The organic fluid experiments simulated high pressure steam–water annular flow conditions. In each experiment, measurements of entrainment fraction, droplet entrainment rate and droplet deposition rate have been performed by using the liquid film extraction method. A simple, explicit and non-dimensional correlation developed by Sawant [Sawant, P.H., Ishii, M., Mori, M., 2008. Droplet entrainment correlation in vertical upward co-current annular two-phase flow. Nucl. Eng. Des. 238 (6), 1342–1352] for the prediction of entrainment fraction is further improved in this study in order to account for the existence of critical gas and liquid flow rates below which no entrainment is possible. Additionally, a new correlation is proposed for the estimation of minimum liquid film flow rate at the maximum entrainment fraction condition. The improved correlation successfully predicted the newly collected air–water and Freon-113 entrainment fraction data. Furthermore, the correlations satisfactorily compared with the air–water, helium–water and air–genklene experimental data measured by Willetts [Willetts, I.P., 1987. Non-aqueous annular two-phase flow. D.Phil. Thesis, University of Oxford]. However, comparison of the correlations with the steam–water data available in literature showed significant discrepancies. It is proposed that these discrepancies might have been caused due to the inadequacy of the liquid film extraction method used to measure the entrainment fraction or due to the change in mechanism of entrainment under high liquid flow conditions.

Liquid film dryout in a boiling channel under flow oscillation conditions

A simple theory was developed to elucidate the influence of sinusoidal oscillation of the inlet flow rate on the occurrence of liquid film dryout in an annular two-phase flow regime in a boiling channel. The theory assumes that the critical heat flux (CHF) under an oscillatory condition can be calculated from values in steady states provided that the effect of axial mixing of the liquid film is appropriately considered. The trends of CHFs calculated using a one-dimensional three-fluid model and those experimentally measured under atmospheric pressure were in reasonable agreement with the proposed theory. However, the CHF values measured under oscillatory conditions were usually higher in the experiment than in the numerical simulation, which indicated that axial liquid transport induced by disturbance waves might enhance axial mixing of the liquid film.

Prediction of evaporation heat transfer coefficient based on gas–liquid two-phase annular flow regime in horizontal microfin tubes

A physical model of gas–liquid two-phase annular flow regime is presented for predicting the enhanced evaporation heat transfer characteristics in horizontal microfin tubes. The model is based on the equivalence of a periodical distortion of the disturbance wave in the substrate layer. Corresponding to the stratified flow model proposed previously by authors, the dimensionless quantity Fr 0 = G/[ gd eρ v ( ρ l − ρ v )] 0.5 may be used as a measure for determining the applicability of the present theoretical model, which was used to restrict the transition boundary between the stratified-wavy flow and the annular/intermittent flows. Comparison of the prediction of the circumferential average heat transfer coefficient with available experimental data for four tubes and three refrigerants reveals that a good agreement is obtained or the trend is better than that of the previously developed stratified flow model for Fr 0 > 4.0 as long as the partial dry out of tube does not occur. Obviously, the developed annular model is applicable and reliable for evaporation in horizontal microfin tubes under the case of high heat flux and high mass flux.

Liquid film dryout model for predicting critical heat flux in annular two-phase flow

Gas-liquid two-phase flow and heat transfer can be encountered in numerous fields, such as chemical engineering, refrigeration, nuclear power reactor, metallurgical industry, spaceflight. Its critical heat flux (CHF) is one of the most important factors for the system security of engineering applications. Since annular flow is the most common flow pattern in gas-liquid two-phase flow, predicting CHF of annular two-phase flow is more significant. Many studies have shown that the liquid film dryout model is successful for that prediction, and determining the following parameters will exert predominant effects on the accuracy of this model: onset of annular flow, inception criterion for droplets entrainment, entrainment fraction, droplets deposition and entrainment rates. The main theoretical results achieved on the above five parameters are reviewed; also, limitations in the existing studies and problems for further research are discussed.

Algebraic turbulence modeling in adiabatic gas–liquid annular two-phase flow

The study considers algebraic turbulence modeling in adiabatic gas–liquid annular two-phase flow. After reviewing the existing literature, two new algebraic turbulence models are proposed for both the liquid film and the droplet laden gas core of annular two-phase flow. Both turbulence models are calibrated with experimental data taken from the open literature and their performance critically assessed. Although the proposed turbulence models reproduce the key parameters of annular flow well (average liquid film thickness and pressure gradient) and the predicted velocity profiles for the core flow compare favorably with available core flow velocity measurements, a more accurate experimental database is required to further improve the models accuracy and range of applicability.

Pressure Drop in Two-Phase Swirling Flow in a Steam Separator

Pressure drop and liquid film thickness in air-water swirling flows in a one-fifth scale model of the steam separator are measured for a wide range of gas and liquid volume fluxes. Numerical simulations based on one-dimensional single-fluid and two-fluid models are also carried out to examine the feasibility of predicting the pressure drop and film thickness in swirling flows. The pressure drop in a single-phase swirling flow is about five times as large as that in a non-swirling flow due to the increase in the frictional pressure drop. The pressure gradient and liquid film thickness in a two-phase swirling annular flow at the inlet of the pick-off-ring of the separator are well evaluated by using a standard one-dimensional two-fluid model, provided that the interfacial and wall frictions in an ordinary two-phase annular flow are multiplied by appropriate constant values.

Estimation of pressure drop in venturi scrubbers based on annular two-phase flow model, artificial neural networks and genetic algorithm

Abstract Pressure drop is the most important factor affecting the dust collection efficiency in venturi scrubbers. The model described by Viswanathan et al. [S. Viswanathan, A.W. Gnyp, C.C. St. Pierre, Annular flow pressure drop model for Pease–Anthony type venturi scrubbers, AIChE J. 31 (1985) 1947–1958] predicts pressure drop according to an annular two-phase flow model, but there is a parameter lacking in order to implement this model. In this work, some correlations are suggested for use in Viswanathan's model. The results are compared with experimental data extracted from two different venturi scrubbers with different conditions. In these ranges of conditions, good agreement between the results of this modified model and experimental data shows the ability of the model to predict pressure drop. In the next step, artificial neural network was used to predict pressure drop in venturi scrubbers and acceptable results were obtained. For increasing the efficiency of neural networks, genetic algorithm was used to optimize parameters of the neural network such as the number of neurons in the hidden layer, the momentum rate and the learning rate. Finally, the model of neural network optimized by genetic algorithm was selected as the best model due to its agreement with experimental data and greater flexibility compared to mathematical models.

Prediction of wall shear for horizontal annular air–water flow

In this study, non-intrusive pressure drop, liquid base film thickness distribution, and wave behavior measurements have been obtained for 206 horizontal annular two-phase (air–water) flow conditions in 8.8, 15.1, and 26.3 mm ID tubes. Wall shear was correlated to within 8% by a friction factor involving flow quality and gas Reynolds number. This correlation was found to perform better than those available in the literature, including film roughness correlations, two-phase multiplier methods, and pure data fits. Among published relations, the Müller-Steinhagen and Heck correlation was found to be the most accurate, while the Lockhart–Martinelli correlation can be modified to provide reasonable results. The gas friction velocity is found to be similar to the disturbance wave velocity, which suggests that waves are important sources of shear.

Properties of disturbance waves in vertical annular two-phase flow

Disturbance waves play an important role in interfacial transfer of mass, momentum and energy in annular two-phase flow. In spite of their importance, majority of the experimental data available in literature on disturbance wave properties such as velocity, frequency, wavelength and amplitude are limited to near atmospheric conditions (Azzopardi, B.J., 1997. Drops in annular two-phase flow. International Journal of Multiphase Flow, 23, 1–53). In view of this, air–water annular flow experiments have been conducted at three pressure conditions (1.2, 4.0 and 5.8 bar) in a tubular test section having an inside diameter 9.4 mm. At each pressure condition liquid and gas phase flow rates are varied over a large range so that the effects of density ratio, liquid flow rate and gas flow rate on disturbance wave properties can be studied systematically. A liquid film thickness is measured by two flush mounted ring shaped conductance probes located 38.1 mm apart. Disturbance wave velocity, frequency, amplitude and wavelength are estimated from the liquid film thickness measurements by following the statistical analysis methods. Parametric trends in variations of disturbance wave properties are analyzed using the non-dimensional numbers; liquid phase Reynolds number ( Re f), gas phase Reynolds number ( Re g), Weber number ( We) and Strouhal number ( Sr). Finally, the existing correlations available for the prediction of disturbance wave velocity and frequency are analyzed and a new, improved correlation is proposed for the prediction of disturbance wave frequency. The new correlation satisfactorily predicted the current data and the data available in literature.

Investigation of Waves Interaction in Annular Gas–Liquid Flow Using High-Speed Fluorescent Visualization Technique

High-speed fluorescent visualization complex has been developed for quantitative investigation of the process of interaction of waves on liquid film in annular two-phase flow. Evolution of ripples on disturbance waves surface and disturbance waves coalescence are investigated. It is shown that all the ripples in presence of disturbance waves appear at the base of the back front of disturbance waves and then either decelerate, travel on substrate and are overtaken by the following disturbance wave or accelerate, grow and then disappear at the front of disturbance wave. The disappearance happens due to entrainment of liquid into the core of gas stream. Several scenarios of coalescence of disturbance waves were identified. For disturbance waves with close velocities different types of remote interaction were observed.

Nonstationary heat transfer crisis in annular dispersed flows

A model of motion and heat transfer of annular dispersed two-phase flows is developed. The results of an investigation of the heat transfer crisis in a vertical steam-generating channel in nonstationary regimes with a decreasing heat transfer rate of the heater are presented

Droplet entrainment correlation in vertical upward co-current annular two-phase flow

Upward annular two-phase flow in a vertical tube is characterized by the presence of liquid film on the tube wall and entrained droplet laden gas phase flowing through the tube core. Entrainment fraction in annular flow is defined as a fraction of the total liquid flow flowing in the form of droplets through the central gas core. Its prediction is important for the estimation of pressure drop and dryout in annular flow. In the following study, measurements of entrainment fraction have been obtained in vertical upward co-current air–water annular flow covering wide ranges of pressure and flow conditions. Comparison of the experimental data with the existing entrainment fraction prediction correlations revealed their inadequacies in simulating the trends observed under high flow and high pressure conditions. Furthermore, several correlations available in the literature are implicit and require iterative calculations. Analysis of the experimental data showed that the non-dimensional numbers, Weber number ( We = ρ g〈 j g〉 2 D/ σ(Δ ρ/ ρ g) 1/4) and liquid phase Reynolds number ( Re f = ρ f〈 j f〉 D/ μ f), successfully collapse the data. In view of this, simple, explicit correlation was developed based on these non-dimensional numbers for the prediction of entrainment fraction. The new correlation successfully predicted the trends under the high flow and high pressure conditions observed in the current experimental data and the data available in open literature. However, in order to use the proposed correlation it is necessary to predict the maximum possible entrainment fraction (or limiting entrainment fraction). In the current analysis, an experimental data based correlation was used for this purpose. However, a better model or correlation is necessary for the maximum possible entrainment fraction. A theoretical discussion on the mechanism and modeling of the maximum possible entrainment fraction condition is presented.