Volumetric Void Fraction Research Articles

Numerical simulation of unsteady cloud cavitation in an orifice nozzle by a compressible gas-vapor/liquid mixture cavitation model

Abstract Focused on the behaviour of unsteady cloud cavitation in a submerged orifice nozzle, this work presents a numerical investigation by using a compressible gas-vapor/liquid mixture cavitation model. The mean flow of the two-phase mixture is calculated by URANS for compressible fluid and the intensity of cavitation is evaluated by the gas volumetric void fraction considering the effect of fluid compressibility varying with phase change. Water jets issuing from a submerged orifice nozzle are investigated under different cavitation conditions. Computation results are compared with experiment data and the validity of computation is confirmed. The periodical shedding of cavitation clouds is captured reliably. Numerical simulations reveal that a leading cloud principally split by the shear flow and a subsequent cloud detached by the re-entrant jet are successively shed downstream, and they coalesce over the range of approximately x/d ≈ 2∼3. This process is repeated and the flow rate coefficient pulsates periodically under the effect of cavitation clouds.

Experimental investigations of gas–liquid two-phase flow in a horizontal mini pipe: Flow regime, void friction, and frictional pressure drops

Experimental studies of air–water two-phase flows in a mini tube with an inner diameter of 3.12 mm have been conducted in terms of the flow pattern, void fraction, and pressure gradients. The fluid velocities range from 0.065 to 21.78 m/s and from 0.109 to 1.835 m/s, respectively. A right-angle prism is applied to capture images simultaneously from two perpendicular directions. Three-dimensional gas–liquid interfaces are reconstructed with the obtained images from two directions. The gas void fraction is investigated using this method, which is experimentally validated with the quick closing valves method. Three flow patterns are obtained in the present work, which is bubbly, slug, and annular flows. The Probability Density Functions of the cross-sectional void fraction show that the intermittent flow has two peaks and the annular flow has only one peak. Volumetric void fraction is affected by the flow pattern and the flow rate. However, at very high gas velocities, volumetric void fraction is independent of the liquid rate. A new frictional pressure drop correlation is proposed based on the experimental data.

Tailorable regulation of mass transfer channel in environmentally friendly calcium alginate membrane for dye removal

Environmentally friendly polymer membranes for dye contamination removal were prepared from edible calcium alginate. The stable gel structure was formed by molecular chains of sodium alginate cross-linked by chelate-formation with calcium ions. To improve the water permeability of the membranes and make them suitable for membrane separation processes, their mass transfer channels were precisely regulated to form fine voids via the addition of oligoethylene glycol (OEG) or polyethylene glycol (PEG) to a casting polymer solution and subsequent removal of the OEG or PEG from the membranes by a washing process after cross-linking. Fourier transform infrared spectroscopy confirmed that the chemical properties of the membranes with voids formed by OEG or PEG were the same as the properties of the original calcium alginate membrane without void forming. Despite their same chemical properties, the performance of the membranes clearly differed. The effect of the different void structures on the mass transfer channels was investigated using pure water permeation, Methyl Orange dye rejection, and volumetric void fraction measurements, as well as morphological observations by field-emission scanning electron microscopy. The membrane with voids formed by OEG exhibited to an increase in water permeability compared with the original membrane, whereas the molecular separation performance was maintained within the nanofiltration region. Meanwhile, the membrane with voids formed by PEG was suitable for use in high-flux ultrafiltration processes for pretreating wastewater. The biocompatible membranes with tailorable separation ability presented herein are candidates for use in purification processes for recovering dye-polluted water resources.

An Experimental Study on the Void Fraction for Gas-Liquid Two-Phase Flows in a Horizontal Pipe

The flow patterns and the void fraction related to a gas-liquid two-phase flow in a small channel are experimentally studied. The test channel is a transparent quartz glass circular channel with an inner diameter of 6.68 mm. The working fluids are air and water and their superficial velocities range from 0.014 to 8.127 m/s and from 0.0238 to 0.556 m/s, respectively. The void fraction is determined using the flow pattern images captured by a high-speed camera, while quick closing valves are used for verification. Four flow patterns are analyzed in experiments: slug flow, bubbly flow, annular flow and stratified flow. For intermittent flows (bubbly flow and slug flow), the cross-sectional void fraction is in a borderline condition while its probability distribution function (PDF) image displays a bimodal structure. For continuous flows (annular flow and stratified flow) the cross-sectional void fraction behaves as a fluctuating continuous curve while the (PDF) image displays a single peak structure. The volumetric void fraction data are also compared with available predictive formulas, and the results show that the agreement is very good. An effort is also provided to improve the so-called Gregory and Scott model using the available data.

Studies of gas-liquid two-phase flows in horizontal mini tubes using 3D reconstruction and numerical methods

In order to study the characteristics of 3D gas-liquid interfaces, flow pattern transitions, and void fraction in mini tubes, virtual multi-vision experiments and numerical simulations are conducted in a 5.16 mm horizontal tube with air as the dispersed phase and water as the continuous phase. Air and water flow velocities range from 0.024 to 12.330 m/s and from 0.039 to 0.942 m/s, respectively. A simple setup is adopted to capture images synchronously from two orthogonal views with a prism. 3D gas-liquid interfaces of bubbly, slug, and annular flows in horizontal mini tubes are reconstructed in Matlab based on the captured images. Flow pattern transitions and void fractions with 3D interfaces are comprehensively investigated both experimentally and numerically. The numerical results fit well with experimental data in terms of 3D interfaces, cross-sectional and volumetric void fractions. The results show that three flow patterns (bubbly, slug, and annular) are observed experimentally and numerically. The increases in both bubble size and interfacial fluctuation are the main reasons for flow pattern transitions. Two peaks are observed in the Probability Density Functions figures for intermittent flows, while only one peak is obtained for annular flow. The volumetric void fraction fits well with the homogenous model when the gas quality is lower than 0.8, while the slip ratio is around 1. The velocity difference between the two phases increases dramatically at a higher gas quality, leading to a lower void fraction than the predicted value by the homogenous model. The results agree well with empirical correlations with a mean average deviation about 10%, which validates the experimental and numerical methods in the present work.

Signal characteristics of IR transceiver pair during two phase flow of varying liquid film thickness in a mini-channel

Taylor bubble/slug flow is a type of multiphase flow that is predominant in many natural and industrial applications. A thorough understanding of two phase flows and void fraction measurement is essential to understand thermal hydraulic behavior during accident conditions in nuclear reactors. The performance of mini-reactors/microreactors and microthrusters requires a complete knowledge on shape and size of the void fraction distribution, which governs the flow characteristics in mini-channels/microchannels. The present study focuses on analyzing the signal characteristics of two IR transceivers simultaneously during two-phase bubble/slug regime having varying liquid film thickness. The transceivers are positioned at 90° and 0° to the axis of a tube of 1.5 mm diameter and 2.5 mm thickness. The experimental signal output is compared with the results obtained from a numerical model developed using COMSOL Multiphysics package. The present study exposes the critical parameters that need to be considered while developing a 3D reconstruction methodology based on IR transceivers for volumetric void fraction measurement.

Experimental study on transient heat/mass transfer characteristics during static flash of aqueous NaCl solution

The instant height/temperature of waterfilm were selected as a pair of characteristic parameters to study the transient heat/mass transfer properties during static film flash of aqueous NaCl solution though experiments with waterfilm concentration ranging between 0 and 0.15 (mass fraction), superheat between 5.1 and 35.0 K, and initial height of waterfilm between 0.10 and 0.20 m. The heat transfer mainly happened at bubble interface within waterfilm. The mass transfer included two steps of vaporization at bubble interface and vapor overflow from top free surface. The transient intensity of heat transfer was measured by volumetric evaporation coefficient. It shared the same experience with volumetric void fraction of quick increase to a peak value and gradual falling back. But its change always lagged behind that of void fraction. The transient intensity of overflow was measured by overflow speed of flash vapor. It was found to increase with rising superheat or initial height of waterfilm, but dropped significantly with increasing waterfilm concentration. On basis of these results, a calculation model for static film flash was set up. The evolution of waterfilm temperature/height could be computed simultaneously from given evolution of flash chamber pressure and other necessary initial conditions.

Volumetric Reconstruction of Taylor Slug Gas Flow Using IR Transceiver in Minichannels

Taylor slug flow commonly occurs in crude oil pipelines, mini-/microreactors, space propulsion thrusters, heat exchangers, and nuclear reactors. The flow is an intermittent gas regime which occurs in flowing liquid inside pipelines and capillary tubes. Measurement of liquid-film thickness and cross-sectional and volumetric void fraction is necessary for an effective operation and safety of many industrial processes involving heat and mass transfer operations. In industrial applications such as miniheat exchangers, lab-on-chip devices, and pulsating heat pipes, gas–liquid Taylor flow enhances heat and mass transfer. The dominance of surface tension force is responsible for the occurrence of Taylor flow. Online measurement of void fraction is essential to determine heat and mass transfer characteristics of systems involving the two-phase flow. This article focuses on developing an instrumentation technique for the measurement of volumetric voids in minichannels. The effect of thickness of liquid films and nose/tail shape on infrared (IR) transceiver irradiation is studied. The experiments are done in two borosilicate glass tubes (3.40 and 1.50 mm inner diameter and 0.6 and 2.5 mm thickness, respectively) to measure liquid-film thickness and volumetric void fraction using a pair of IR transceiver. A numerical model is developed using COMSOL Multiphysics for the same experimental conditions. The effect of nose/tail curvature shape and liquid-film thickness on volumetric void fraction is predicted using a novel simple algorithm. A 3-D reconstruction technique is developed by measuring the undulations at the sides and top/bottom of the Taylor slug using a pair of IR transceiver output current (mA).

Experimental study on the void fraction during two-phase flow of R1234yf in smooth horizontal tubes

Void fraction is an important parameter to design and simulate thermal systems involving two-phase flows. In this research, an experimental investigation of the void fraction in adiabatic two-phase flow of R1234yf in horizontal and smooth tubes with an internal diameter of 4.8 mm was carried out. For the experimental tests, the vapor quality ranges from 0.1 to 1 while two saturation temperatures (15 and 25 °C) and two mass flow rates (180 and 280 kg.m − 2s−1) are investigated. The quick-closing valve(s) method is used to measure the volumetric void fraction. Tests are also undertaken with R134a used as a reference in this study. The results highlight that the void fraction of R1234yf is 5% lower than that for R134a. In addition, the experimental data of R1234yf were compared against seven correlations from the literature: the Baroczy and the Hughmark correlations were shown to provide the best prediction, with a mean absolute error of 2% and 3.2%, respectively.

Application of time–frequency entropy from wake oscillation to gas–liquid flow pattern identification

Gas–liquid two-phase flow abounds in industrial processes and facilities. Identification of its flow pattern plays an essential role in the field of multiphase flow measurement. A bluff body was introduced in this study to recognize gas–liquid flow patterns by inducing fluid oscillation that enlarged differences between each flow pattern. Experiments with air–water mixtures were carried out in horizontal pipelines at ambient temperature and atmospheric pressure. Differential pressure signals from the bluff-body wake were obtained in bubble, bubble/plug transitional, plug, slug, and annular flows. Utilizing the adaptive ensemble empirical mode decomposition method and the Hilbert transform, the time–frequency entropy S of the differential pressure signals was obtained. By combining S and other flow parameters, such as the volumetric void fraction β, the dryness x, the ratio of density φ and the modified fluid coefficient ψ, a new flow pattern map was constructed which adopted S(1–x)φ and (1–β)ψ as the vertical and horizontal coordinates, respectively. The overall rate of classification of the map was verified to be 92.9% by the experimental data. It provides an effective and simple solution to the gas–liquid flow pattern identification problems.

Experimental investigation of heat transfer from a horizontal flat surface to aqueous foam flow

Usage of aqueous foam flow as a coolant allows relatively large heat transfer intensity with very small coolant mass flow rate in foam apparatus and heat exchangers. Our previous experimental investigation of the heat transfer between a vertical flat surface and longitudinal upward aqueous foam flow showed dependence of the surface cooling intensity on the foam flow velocity and the volumetric void fraction. In the case of an inclined flat surface, liquid drained from the foam and formed a film on the inclined heated surface. The direction of movement of liquid particles in this film depended on experimental conditions.An experimental investigation of the heat transfer between a horizontal flat surface and longitudinal foam flow was performed and the results are presented and discussed in this paper. It was observed that the particles of drained liquid film moved in the same direction as the foam flow. The cross-sectional velocity distribution of the foam bubbles influenced on the amount of draining liquid across the channel. Higher inflow of draining liquid of lower temperature gave rise to better conditions for the cooling of the side parts of the heated surface.

Experimental study on evolutions of temperature and height of waterfilm during static flash

The temperature and height of waterfilm were a pair of coupled characteristic parameters indicating the instant heat/mass transfer during static film flash of pure water. Their evolutions and interactions were studied through experiments with initial height of waterfilm ranging between 0.15 and 0.3 m, initial temperature between 80.2 and 93.0 °C, initial pressure of vacuum chamber between 0.013 and 0.061 MPa. Results suggested both height and decrease rate of temperature of waterfilm experienced a rapid increase to peak value and a gradual decline after that. Considering most of heat transfer happened within waterfilm, volumetric evaporation coefficient was introduced as the total heat flux transferred through all bubble interfaces within unit volume of waterfilm under unit superheat. This coefficient increased significantly with rising volumetric void fraction of waterfilm. Except vapor generation, mass transfer during static flash also included the overflowing of vapor. Its overflowing speed increased with initial height of waterfilm. According to these results, a model was set up to calculate the evolutions of temperature and height of waterfilm from given evolution of saturation temperature corresponding to the pressure of flash chamber.

Simulation of turbulent non-isothermal polydisperse bubbly flow behind a sudden tube expansion

The results of numerical simulation of the structure of non-isothermal polydisperse bubbly turbulent flow and heat transfer behind a sudden tube expansion are presented. The study was carried out at a change in the initial diameter of the air bubbles within d m1 = 1–5 mm and their volumetric void fraction β = 0–10 %. Small bubbles are available in almost the entire cross section of the tube, while the large bubbles pass mainly through the flow core. An increase in the size of dispersed phase causes the growth of turbulence in the liquid phase due to flow turbulization, when there is a separated flow of liquid past the large bubbles. Adding the air bubbles causes a significant reduction in the length of the separation zone and heat transfer enhancement, and these effects increase with increasing bubble size and their gas volumetric flow rate ratio.

Bubble Tolerant Liquid Management

Initially CMP slurries were hand mixed in small quantities and drawn from buckets at the back of the tool using peristaltic pumps. Batch to batch variations adversely affected yields as did pulsatile dispense. Eventually, automated, batch verified, basement blending and loop distribution systems came into use. These systems reduced batch to batch variation but, still relied upon peristaltic pumps for dispense volume control. Particle generation from tubing and high chemical cost drove a need for a change at sub-micron (90 Nanometer) geometries. Various flow control schemes were evaluated including paddle wheel, turbine and differential pressure based systems ultimately settling on ultrasonic based flow controllers as the preferred technology. However, as feature sizes continue to shrink, the chemistries required to achieve smaller feature sizes becomes more and more exotic. Mixtures including peroxides and hydroxides naturally out-gas which produce bubbles in the dispensed flow streams, and in some cases results in a two-phase flow regime. At comparatively low volumetric void fractions bubble concentrations (less than 10%) can adversely affect measurement accuracy of DP, Paddle Wheel, Turbine or Ultrasonic flow measurement technologies to the extent that many of these flow measurement technologies cease to operate satisfactorily. Malema has developed a mass based, flow controller for slurry and clear chemical dispense applications. Based on Coriolis mass flow rate measurement, the Malema Coriolis Mass Flow Controller (CMFC) is essentially bubble tolerant. Flow meter / flow controller manufacturers used an assortment of software techniques to filter the effects of these entrained bubbles with limited results. Since Coriolis measures mass directly, bubbles have very little effect on measured or dispensed volumes. A brief history of the Coriolis Effect, how it is applied to direct mass flow rate measurements will be presented and discussed including a description of Malema’s family of CMFCs and associated performance data.

CFD analysis of the two-phase bubbly flow characteristics in helically coiled rectangular and circular tube heat exchangers

Due to their ease of manufacture, high heat transfer efficiency and compact design, helically coiled heat exchangers are increasingly being adopted in a number of industries. The higher heat transfer efficiency over straight pipes is due to the secondary flow that develops as a result of the centrifugal force. In spite of the widespread use of helically coiled heat exchangers, and the presence of bubbly two-phase flow in a number of systems, very few studies have investigated the resultant flow characteristics. This paper will therefore present the results of CFD simulations for the two-phase bubbly flow in helically coiled heat exchangers as a function of the volumetric void fraction and the tube cross-section design. The CFD results are compared to the scarce flow visualisation experimental results available in the open literature.

Numerical simulation of two-phase gas–liquid flow through gradual expansions/contractions

In the present work, turbulent two-phase flows of air and water are numerically simulated through a smooth area expansion/restriction with a constant opening angle. To model the multiphase flow, a two-fluid approach is adopted in which two sets of governing equations are solved simultaneously for continuous and dispersed phases and the coupling between these phases is forced by proper source terms. An extended version of two-equation k–ε turbulence model is used and the numerical result was validated in comparison to previously published experimental data. The effect of volumetric void fraction, Reynolds number and the opening angle is examined on the pressure distribution across the flow domain.

Vertical channel’s heat transfer to the upward foam flow

Paper presents the semi empirical model of the heat transfer process between the internal surface of the vertical channel (tube) and the laminar longitudinal upward macro foam flow. Model is based on the assumption that the amount of the liquid which drains from the macro foam per second in the n vertical zones of the channel is proportional to the nth part of the liquid which comes to the appropriate zone with the foam.The relationship between the foam flow parameters, such like foam flow rate (velocity), volumetric void fraction and temperature was analyzed. Investigation showed that the heat transfer rate monotonically increases with the increase of the foam flow velocity, with decrease of the volumetric void fraction and with increase of the temperature difference between foam flow and channel wall.Comparison of the results of the modeling of the vertical channel with the experimental data, obtained for the vertical flat surface, showed its satisfactory agreement.

Experimental investigation of heat transfer from vertical flat surface to upward aqueous foam flow

Paper presents the results of the experimental investigation of the local and average heat transfer process between the vertical flat surface and upward macro foam flow. Macro foam was generated by bubbling gas (air) into the liquid made from the 0.5% concentration of the washing powder (Tide Absolute) in water. Macro foam flow parameters: velocity 0.10÷0.25m/s; volumetric void fraction 0.996÷0.998; bubbles size 0.005÷0.015m. Surface temperature varied from 25 to 40°C; macro foam flow temperature 12.5÷18.0°C.Investigation showed that the heat transfer rate depends on the foam flow parameters: velocity and volumetric void fraction. It was stated that the heat transfer rate increased with the increase of the foam flow velocity and with the decrease of the volumetric void fraction. The local heat transfer rate is lower at the sides and is higher at the middle part of the surface. The maximum of the local heat transfer rate (up to 1137W/(m2K)) is identified at the inlet region. Average heat transfer rate for the whole surface varied from 152 to 331W/(m2K).The results of the experimental investigation were generalized using the criterion equations.

Experimental investigation of flow accelerated corrosion under two-phase flow conditions

The main objective of this paper is to experimentally study the effect of two-phase flow on flow-accelerated corrosion (FAC) downstream an orifice. FAC is a major safety and reliability issue affecting carbon-steel piping in nuclear and fossil power plants. This is because of its pipe wall wearing and thinning effects that could lead to sudden and sometimes catastrophic failures, as well as a huge economic loss. In the present study, FAC wear of carbon-steel piping was simulated experimentally by circulating air–water mixtures through hydrocal (CaSO4·1/2H2O) test sections at liquid superficial Reynolds number, Re=20,000, and different air mass flow rates. Experiments were performed for a test section with different orifice to pipe diameter ratios (do/D=0.25, 0.5 and 0.74). The observed flow patterns were compared with the available flow pattern maps. Surface wear patterns downstream the orifices were also analyzed. The maximum FAC wear was found to occur at approximately 2–5 pipe diameters downstream of the orifice. The obtained results were found to be consistent with those from a single-phase flow study reported earlier. Moreover, FAC was found to depend on the relative values of the mixture mass quality and the volumetric void fraction. Lower values of FAC wear rate were obtained for higher values of mass quality. A modified correlation is developed in order to predict FAC wear rate downstream of the pipe-restricting orifice with an average RMS accuracy of ±10%. However, the location of maximum wear rate is well predicted. The current study is considered as an integrated effort to develop guidelines to FAC engineers in power plants in order to prepare more reliable plant inspection scope.

Experimental investigation of heat transfer from inclined flat surface to aqueous foam

Paper presents the results of the experimental investigation of heat transfer process between the inclined flat surface and the longitudinal upward macrofoam (foam) flow. Investigation was performed on the special laboratory stand using macrofoam, which was generated inside the channel on the perforated plate. Foaming solution was made from the 0.5% concentration of the washing powder (TIDE Absolute) solution in water and gas (air). Macrofoam parameters: velocity 0.10÷0.30m/s; volumetric void fraction 0.996÷0.998; bubble dimensions 0.005÷0.015m. Inclination angle of the flat surface 45°. Experimental surface was heated using electric current; surface temperature varied from 16.8 to 36.2°C; foam flow temperature varied from 14.6 to 19.5°C.Investigation showed that the heat transfer rate depends on the foam flow characteristics (velocity and volumetric void fraction) and on the drained liquid film parameters (film thickness, film flow velocity and direction). It was stated that the heat transfer rate increases with the increase of the foam flow velocity up to the critical value. When the foam flow velocity exceeds the critical value, the thickness of the drained liquid film begins to grow and heat transfer rate decreases. Further augmentation of the foam flow velocity forms a concurrent drained liquid flow and heat transfer intensity increases again.The experimental results were compared with the results obtained for the vertical flat surface and for the different types of the tube bundles as well.