Air-water Mixture Flow Research Articles

Characterization of Two-Phase Flow Patterns in Small Round Tubes

To investigate flow characteristics by high speed video systems, experimental study was conducted to gas-liquid two-phase flow in horizontal round small tube with diameter of 5.5 and 2.6 mm, the typical flow pattern images were obtained, but stratified flow of the conventional size horizontal channel had not been discovered. Gas and liquid superficial velocities range from 0.1 to 100 ms-1, and 0.01to 10.0 ms-1 respectively. Flow patterns for co-current flow of air-water mixtures in horizontal round tubes are determined by high-speed video analysis to develop flow regime maps and the transitions between these flow regimes. Comparisons with the relevant literatures show that diameter and surface tension effects play an important role in determining the flow patterns and transitions between them.

Application of spectral analysis in radiometric measurements of twophase liquid-gas flow

This article presents use of the classical spectral analysis to identify a type of flow in investigation of gas transportation by liquid with a measurement of gamma radiation absorption. During numerous experiments it was found that a magnitude of the cross-spectral density distribution of recording signals reveals type of air-water mixture flow in a horizontal pipeline. As an example, some results of laboratory measurements equipped in 241 Am radiation source and scintillation probes with of NaI(Tl) crystals are presented. Moreover attached figures facilitate interpretation of observed results and in details illustrating the proposed method.

Effect of rolling motion on transient flow resistance of two-phase flow in a narrow rectangular duct

As a typical ocean environment, rolling motion can induce additional forces on a flow system. Experimental study of the characteristics of air–water mixture flow resistance in narrow rectangular channel(40×1.41mm2) in rolling motions was carried out with the rolling periods and amplitudes of 8s, 12s, 16s, and 10°, 15°, 30°, respectively. Several homogeneous and separated flow models were evaluated against the experimental data to clarify their applicability in predicting the two-phase flow resistance in vertical rectangular channels. The comparison showed that McAdams correlation for calculating the viscosity in homogeneous model is suitable for the present situation to calculate the averaged two-phase flow resistance. Transient flow resistance under rolling condition changed periodically, as a result, attempt was conducted to illustrate the effect of rolling motion. Transient frictional pressure drop strongly depends on the mass quality, Reynolds number, rolling period and amplitude. The fluctuation amplitude of the frictional pressure drop increased with increasing mass quality and rolling amplitude. It also increased with reduction of the rolling period. A new correlation for predicting the transient frictional factor was given by including the influence of rolling parameters and was validated against the experimental data.

Numerical investigation of single phase and two phase flow through thin orifices in horizontal pipes

Pressure drop due to single phase flow of water and two phase flow of air-water mixture through thin orifices in horizontal pipes have been numerically investigated. Two-phase computational fluid dynamics (CFD) calculations, using Eulerian–Eulerian model have been employed to calculate the pressure drop through orifices. The operating condition covers the gas and liquid superficial velocity ranges Vsg=0.3–4 m/s and Vsl=0.6–2 m/s, respectively. The local pressure drops have been obtained by means of extrapolation from the computed upstream and downstream linearized pressure profiles to the orifice section. Simulations for the single-phase flow of water have been carried out for local liquid Reynolds number (Re based on orifice diameter) ranging from 3×104 to 2×105 to obtain the discharge coefficient and the two-phase local multiplier, which when multiplied with the pressure drop of water (for same mass flow of water and two phase mixture) will reproduce the pressure drop for two phase flow through the orifice. The effect of orifice geometry on two-phase pressure losses has been considered by selecting eight different orifice plates with two area ratios (σ=0.73 and σ=0.54) and four different thicknesses (s/d = 0.025-0.59). The results obtained from numerical simulations are validated against experimental data from the literature and are found to be in good agreement. Keywords: Orifice, Pressure drop, Two phase flow, Computational fluid dynamics, Area ratio, Discharge coefficient

Exploratory study on the roller effect of direct hydraulic jumps

Can a direct hydraulic jump also be generated if the surface roller is substituted with a corresponding inset? This and related questions are considered in the note, to understand the role of the roller. Based on recent findings relating to the classical hydraulic jump, its modification using bottom and surface insets is reconsidered using laboratory experimentation. The results of these tests are reported below using mainly photography and basic hydraulic computations. It is further demonstrated that the roller may not be separated from the hydraulic jump in a free surface arrangement. This would greatly simplify observations, because the standard two-phase flow including an air–water mixture flow would become a one-phase flow consisting of only water flow. The implications of this result are discussed. This work has exploratory character and should stimulate a further discussion on the main flow features of direct hydraulic jumps.

Two-phase gas–liquid flow characteristics inside a plate heat exchanger

In the present study, the air–water two-phase flow characteristics including flow pattern and pressure drop inside a plate heat exchanger are experimentally investigated. A plate heat exchanger with single pass under the condition of counter flow is operated for the experiment. Three stainless steel commercial plates with a corrugated sinusoidal shape of unsymmetrical chevron angles of 55° and 10° are utilized for the pressure drop measurement. A transparent plate having the same configuration as the stainless steel plates is cast and used as a cover plate in order to observe the flow pattern inside the plate heat exchanger. The air–water mixture flow which is used as a cold stream is tested in vertical downward and upward flow. The results from the present experiment show that the annular-liquid bridge flow pattern appeared in both upward and downward flows. However, the bubbly flow pattern and the slug flow pattern are only found in upward flow and downward flow, respectively. The variation of the water and air velocity has a significant effect on the two-phase pressure drop. Based on the present data, a two-phase multiplier correlation is proposed for practical application.

Quantitative measurement of size and three-dimensional position of fast-moving bubbles in air-water mixture flows using digital holography

We present a digital in-line holographic imaging system for measuring the size and three-dimensional position of fast-moving bubbles in air-water mixture flows. The captured holograms are numerically processed by performing a two-dimensional projection followed by local depth estimation to quickly and efficiently obtain the size and position information of multiple bubbles simultaneously. Statistical analysis on measured bubble size distributions shows that they follow lognormal or gamma distributions.

Usage of artificial intelligence methods in free flowing gated closed conduits for estimation of oxygen transfer efficiency

Oxygen transfer is the process which oxygen is transferred from the gaseous to the liquid phase. The oxygen transfer efficiency depends almost entirely on the amount of surface contact between the air and water. This surface contact can be increased by conduit flow that involves air-water mixture flow. In reality, the physical structure of the air-water interface is complex and still awaits clarification. In the past few years, many artificial intelligence methods have been successfully applied to the solution of complex problems. In this study, models based on Adaptive Network based Fuzzy Inference Systems and Least Squares Support Vector Machines methods were developed to predict oxygen transfer efficiency in free flowing gated closed conduits. Experimental results were compared with the results of these artificial intelligence methods. The best performance was obtained with the Least Squares Support Vector Machine model. Average correlation coefficient (R^2) and average root mean square error (RMSE) in the Least Squares Support Vector Machine model were achieved equal to 0.9927 and 0.0073, respectively. Extremely good agreement between the predicted and measured values proves the validity of the Least Squares Support Vector Machine model.

Reply to Discussion by M. Pfister on “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators”Appears in the Canadian Journal of Civil Engineering, 36(5): 826-838.

In our study, we compared the results of computational fluid dynamics (CFD) using FLUENT software with a small-scale physical model including considerable scale effects and some prototype data only in terms of the air entrainment rates. The verifications of the CFD analyses were based on Richardson extrapolation and other methods in the study. It is noted that the study should be considered in the above framework. In FLUENT, three different Euler–Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. Spillway aerator flows is a free-surface flow and a multiphase flow at the same time. For free-surface flow and large bubble flow, the best multiphase model is VOF, but the VOF model is not appropriate for air–water mixture and small bubble flow. Since there are small bubbles and air–water mixture in spillway flows downstream of an aerator, we suppose that the ASM model is more suitable for modeling of aeration. FLUENT also recommends using the Algebraic Slip Mixture (ASM) model for bubble flows as aeration. It is stated in the tutorial of FLUENT 5.3 (FLUENT 2000) that ASM model can be used at all values of volume fraction. However, it is true that it restricts to relatively small bubbles and low volume fraction when primary phase density is much smaller than the secondary phase density for gas–liquid flows. Among available multiphase models, we think that the ASM model is the most suitable for both free surface and air–water mixture flows since flow aerated by an aerator includes small bubbles. Low gas (air) fraction was only an assumption in our study. Turbulence model

Reply to discussion by Chanson and Lubin on “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators” Appears in the Canadian Journal of Civil Engineering, 36(5): 826-838.

In our study, we compared the results of computational fluid dynamics (CFD) using FLUENT software with a small-scale physical model including considerable scale effects and some prototype data only in terms of the air entrainment rates. The verifications of the CFD analyses were based on Richardson extrapolation and other methods in the study. It is noted that the study should be considered in the above framework. In FLUENT, three different Euler–Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. Spillway aerator flows is a free-surface flow and a multiphase flow at the same time. For free-surface flow and large bubble flow, the best multiphase model is VOF, but the VOF model is not appropriate for air–water mixture and small bubble flow. Since there are small bubbles and air–water mixture in spillway flows downstream of an aerator, we suppose that the ASM model is more suitable for modeling of aeration. FLUENT also recommends using the Algebraic Slip Mixture (ASM) model for bubble flows as aeration. It is stated in the tutorial of FLUENT 5.3 (FLUENT 2000) that ASM model can be used at all values of volume fraction. However, it is true that it restricts to relatively small bubbles and low volume fraction when primary phase density is much smaller than the secondary phase density for gas–liquid flows. Among available multiphase models, we think that the ASM model is the most suitable for both free surface and air–water mixture flows since flow aerated by an aerator includes small bubbles. Low gas (air) fraction was only an assumption in our study. Turbulence model

Impact of flow patterns in a heterogeneous air–water mixture flow in a microchannel on in situ physical parameters

Space transportation systems require high-performance thermal protection and fluid management techniques for systems ranging from cryogenic fluid management devices to primary structures and propulsion systems exposed to extremely high temperatures, as well as for other space systems such as cooling or environment control for advanced space suits and integrated circuits. Although considerable developmental effort is being expended to bring potentially applicable technologies to a readiness level for practical use, new and innovative methods are still needed. One such method is the concept of Advanced Micro Cooling Modules (AMCMs), which are essentially compact two-phase heat exchangers constructed of microchannels and designed to remove large amounts of heat rapidly from critical systems by incorporating phase transition. The development of AMCMs requires fundamental technological advancement in many areas, including: (1) development of measurement methods/systems for flow-pattern measurement/identification for two-phase mixtures in microchannels; (2) development of a phenomenological model for two-phase flow which includes the quantitative measure of flow patterns; and (3) database development for multiphase heat transfer/fluid dynamics flows in microchannels. This paper focuses on the results of experimental research in the phenomena of two-phase flow in microchannels. The work encompasses both an experimental and an analytical approach to incorporating flow patterns for air–water mixtures flowing in a microchannel, which are necessary tools for the optimal design of AMCMs. Specifically, the following topics are addressed: (1) design and construction of a sensitive test system for two-phase flow in microchannels, one which measures ac and dc components of in situ physical mixture parameters including spatial concentration using concomitant methods; (2) data acquisition and analysis in the amplitude, time, and frequency domains; and (3) analysis of results including evaluation of in situ physical parameters of the mixture and their validity for application in flow pattern determination.

Classification of two-phase flow regimes via image analysis and a neuro-wavelet approach

A non-intrusive method of two-phase flow identification is investigated in this paper. It is based on image processing of data obtained partly from dynamic neutron radiography recordings of real two-phase flow in a heated metal channel, and partly by visible light from a two-component mixture of water and air. Classification of the flow regime types is performed by an artificial neural network (ANN) algorithm. The input data to the ANN are some statistical moments of the wavelet pre-processed pixel intensity data of the images. The pre-processing used in this paper consists of a one-step multiresolution analysis of the 2-D image data. The investigations of the neutron radiography images, where all four flow regimes are represented, show that bubbly and annular flows can be identified with a high confidence, but slug and churn-turbulent flows are more often mixed up in between themselves. The reason for the faulty identifications, at least partially, lies in the insufficient quality of these images. In the measurements with air-water two-component mixture, only bubbly and slug flow regimes were available, and these were identified with nearly 100% success ratio. The maximum success ratio attainable was approximately the same whether the raw data was used without wavelet preprocessing or with a wavelet preprocessing of the input data. However, the use of wavelet preprocessing decreased the training time (number of epochs) with about a factor 100.

The Behaviour of Gas-Liquid and Vapour-Liquid Upward Bubbly Flows Passing Through a Vertical Venturi

A comparison between the flow of air-water and steam-water mixtures through a venturi has established key differences between the two systems. The variations of local void fraction of two-phase flows through a converging-diverging venturi have been studied using gamma ray attenuation. The independent variables are the liquid and gas flow rates, the temperature and the axial position in the flow direction. The difference between the two systems is found to be most evident at, and downstream of the throat. In steam-water, void fraction increases substantially at, and beyond the throat as a result of the low local pressure at the throat. On the other hand, with air-water the void fraction at the throat decreases at low temperature because of local changes in the slip ratio of the two phases. The effect of temperature on flow through a venturi is found to be more significant than it is in a straight pipe. This phenomenon becomes more important as the system approaches its boiling point.

Hydrodynamics Characteristics of High Speed Multi-Phase Flow in AWJ Nozzles. 1st Report, Analytical Model and Comparison with Experimental Data.

This paper describes the hydrodynamic characteristics of the abrasive water jet (AWJ) flowing within smallbored nozzles. For analyzing high-speed water jet containing fine solid particles, we introduce the slip velocity model to the one-dimensional momentum equation and estimate the axial distribution of pressure and phase-velocities within the nozzle. The analytical result shows that (1) the pressure within nozzle is always in the vacuum side in case of high-speed water jet and (2) the pressure along the nozzle axis is greatly affected with the change in slip ratio. Experimental data using high-speed two speed two-phase flow of air-water mixture coincide with these predicted tendency. The data also show that there is steep pressure rise at the nozzle exit. From the data analysis, it is concluded that gas phase velocity exceeds its sonic velocity in some region within the nozzle and the phenomena like as normal shock wave may occur in the AWJ nozzle.

Experimental comparison of flow pattern detection techniques for air–water mixture flow

Abstract The complicated and complex nature of two-phase mixture flow phenomenon itself requires detection, monitoring and description of flow pattern influence on the dynamic parameters of mixture flow. The experimental research documentation and control of real mixture flow requires detection, monitoring and measurement of many parameters associated with concentration, velocity and pressure with their distributions in time and space. In addition to the complexity of two-phase mixture flow, there is difficulty in comparing results of experiments from different publications due to the limitation in documentation in the used measurement techniques as well as experimental methodologies. Four often-used measurement methods for flow pattern detection were identified, developed and implemented in the constructed vertical experimental system. This computer-aided experimental system (CAES) was used to generate all typical flow patterns in vertical flow and measure all chosen dynamic parameters (concentration, pressure, and interfacial phenomenon) simultaneously. In this CAES the flow pattern detection system is composed of the application of two different electromagnetic methods (capacitive and resistive), pressure fluctuation method and an optical method. In the reported experimental research on vertical adiabatic air–water flow, results were obtained using all four chosen measurement methods simultaneously. The results allowed a direct comparison of the efficiency of each method in terms of individual ability and sensitivity in the detection of flow patterns. The different sensitivity and abilities of individual methods to detect flow patterns were documented and discussed.

The Effect of Surface Coating Layer on Air-Water Mist Cooling of Hot Metals

This paper deals with transient boiling heat transfer to mist flow of air-water mixture from hot metals coated with a thin layer of low thermal conductivity material. The test specimens used for the present experiment are silver and stainless steel disks whose heat transfer surface is coated with a refractory paint over the thickness range, 3–38μm. The heated disk of 750°C is plunged vertically into the mist flow and is cooled down to the temperature of air-water mixture under atmospheric pressure. The mass velocity of water ranges between 0.04 and 0.78kg/m2.s. The air velocity is varied from 3 to 7m/s. The coating produces a great enhancement in heat transfer, especially in transition and film boiling regions. This enhancement becomes higher with increasing coating thickness since minimum film boiling temperature becomes higher and an earlier transition from film to transition boiling occurs. Heat transfer is more enhanced as mass velocity of water increases although it has a weak dependence on air velocity.

The Effect of Surface Coating Layer on Air-Water Mist Cooling of Hot Metals.

This paper deals with transient boiling heat transfer to mist flow of air-water mixture from hot metals coated with a thin layer of low thermal conductivity material. The test specimens used for the present experiment are silver and stainless steel disks whose heat transfer surface is coated with a refractory paint over the thickness range, 3–38μm. The heated disk of 750°C is plunged vertically into the mist flow and is cooled down to the temperature of air-water mixture under atmospheric pressure. The mass velocity of water ranges between 0.04 and 0.78kg/m2.s. The air velocity is varied from 3 to 7m/s. The coating produces a great enhancement in heat transfer, especially in transition and film boiling regions. This enhancement becomes higher with increasing coating thickness since minimum film boiling temperature becomes higher and an earlier transition from film to transition boiling occurs. Heat transfer is more enhanced as mass velocity of water increases although it has a weak dependence on air velocity.

Air Influence on Similarity of Hydraulic Transients and Vibrations

The effects of wave speed variation due to air content on the validity of similarity laws for model studies of hydraulic transients and hydraulic vibrations were investigated. Studies show that hydraulic similarities between geometrically similar model and prototype are reduced substantially for cases involving two- or three-phase flow. For flow with intensive cavitation, analysis shows that there is no hydraulic similarity between model and prototype. For pump discharge pipeline and turbine draft tube where two phase flow of air-water mixture occurs, analysis shows that the natural frequency and the response characteristics of the fluid system are a strong function of the local wave speed variation within the fluid system. This local wave speed variation is a function of the local transient air content and transient pressure.

Properties of Gas and Liquid Two Phase Swirling Flow in a Vertical Long Tube. 1st Report, Flow Pattern and Properties of Pressure Loss.

Performance improvement of the steam separator used in BWR (boiling water reactor) is one of the important items that must be realized in advanced nuclear reactors. Performance evaluation of the separator, however, has been concentrated on operational properties such as carry over, carry under and water level effect of the reactor. In order to obtain detailed information on the structural flow properties inside the separator, a basic experiment was conducted using a long vertical pipe through which swirling air-water two-phase mixture flows. In this paper, some results on flow pattern, void fraction distribution and friction loss are presented.

The effects of contact angle on two-phase flow in capillary tubes

A two-phase gas—liquid flow was studied in small diameter (1.6 mm i.d.) horizontal tubes. Adiabatic flow of air-water mixtures was tested over a range of flow rates, 1000:1 for air and 600:1 for water. Flow maps were established for four systems with different contact angles. Each system contained the same fluids (air and water) and had the same tube diameter, however, the tube material was different for each system. The four tube materials were pyrex, polyethylene, polyurethane and FEP, a fluoropolymer resin. Three of the contact angles, measured on a flat sample of each tube material, were partially wetting (34° 61° and 74°) and one was partially non-wetting (106°). The flow maps for the systems were very similar except for a region of rivulet flow. Rivulet flow is a new flow regime that replaces wavy flow when the contact angle becomes large. On the other hand, the transition boundaries for the non-wetting system, except for the plug—slug transition, were significantly changed.