Air-water Flow Research Articles

Visualization of local two-phase flow structure in a packed bed of spheres

A two-phase flow structure in porous media is essential to clarify the debris cooling characteristics during a severe accident in nuclear power plants. In previous experimental studies, a packed bed of spheres has been used instead of the porous debris bed. Many models to predict the flow and heat transfer characteristics in the packed bed have been proposed based on the experimental data in the sphere-packed bed. However, those models have not considered the local flow structure, which should be studied to enhance the prediction accuracy. In this study, the local flow structure of the air-water two-phase flow in the unit model of the sphere-packed bed was measured by neutron computed tomography. The distribution of the gas phase was reconstructed by acquired images. From these results, the breakup and coalescence of the bubbles in the packed bed were evaluated.

Development of a one-dimensional system code for the analysis of downward air-water two-phase flow in large vertical pipes

In nuclear thermal-hydraulic system codes, most correlations used for vertical pipes, under downward two-phase flow, have been developed considering small pipes or pool systems. This suggests that there could be uncertainties in applying the correlations to accident scenarios involving large vertical pipes owing to the difference in the characteristics of two-phase flows, or flow conditions, between large and small pipes. In this study, we modified the Multi-dimensional Analysis of Reactor Safety KINS Standard (MARS-KS) code using correlations, such as the drift-flux model and two-phase multiplier, developed in a plant-scale air-inflow experiment conducted for a pipe of diameter 600 mm under downward two-phase flow. The results were then analyzed and compared with those based on previous correlations developed for small pipes and pool conditions. The modified code indicated a good estimation performance in two plant-scale experiments with large pipes. For the siphon-breaking experiment, the maximum errors in water flow for modified and original codes were 2.2% and 30.3%, respectively. For the air-inflow accident experiment, the original code could not predict the trend of frictional pressure gradient in two-phase flow as ⟨jg⟩/⟨j⟩ increased, while the modified MARS-KS code showed a good estimation performance of the gradient with maximum error of 3.5%.

Improving air-water two-phase flow pumping in centrifugal pumps using novel grooved front shrouds

The present investigations aim to improve the gas-liquid two-phase transport in centrifugal pumps using a novel front shroud design with macroscopic grooves. This increases the secondary flow strength and the two-phase mixing, boosting the performance. A total of 23 different circumferential and radial groove profiles were numerically investigated to identify the best configuration leading to the highest mixing efficiency. Three different loads were considered, i.e., part load, optimal point, and overload. Additionally, the performance of the novel designs was compared with that of the established alternatives, involving two different clearance gaps and/or an inducer. While circumferential grooves already lead to a slightly improved specific delivery work, radial grooves improve noticeably the pump efficiency and deliver a very high two-phase mixing compared to other traditional pump configurations. Under conditions of optimal loading, the utilization of circumferential grooves led to a phase mixing enhancement of approximately 19.5%, whereas the implementation of radial grooves exhibited a notable improvement in mixing efficiency by 64.8% compared to the standard gap. The radial design R14 is superior to all other designs regarding efficiency and phase mixing. It is therefore recommended for transporting gas-liquid two-phase flows.

CFD Simulation of isothermal upward two-phase flow in a vertical annulus using interfacial area transport equation

This work presents a numerical simulation of a vertical, upward, isothermal two-phase flow of air bubbles and water in an annular channel applying a Computational Fluid Dynamics (CFD) code. For this, the Two-Fluid model is applied considering interfacial force correlations, namely: drag, lift, wall lubrication, turbulent dispersion, and virtual mass. The turbulence k-ε model effects and the influence of One-group Interfacial Area Transport Equation (IATE) are taken into account, in this case, the influence of two source term correlations for the bubble breakup and coalescence IATE is analysed. The work assesses whether the code properly represents the physical phenomenon by comparing the simulation results with experimental data obtained from the literature. Six flow conditions are evaluated based on two superficial liquid velocities and three void fractions in the bubbly flow regimen. The annular channel adopted has an outer pipe with an internal diameter of 38.1 mm and an inner cylinder of 19.1 mm. To represent this geometry, a three-dimensional mesh was generated with 160,000 elements, after a mesh sensitivity study. The void fraction distribution, taken radially to the flow section, is the main parameter analysed as well as interfacial area concentration, interfacial gas velocity, and bubble sizes distribution. The CFD model implemented in this work demonstrates satisfactory agreement with the reference experimental data but indicates the need for further improvement in the phase interaction models.

Experimental investigation and one-way coupled fluid-structure interaction analysis of gas-liquid two-phase flow conveyed by a subsea rigid M-shaped jumper

The subsea rigid M-shaped jumpers provide reliable and flexible connections in subsea production systems by using deflection to reduce the impact of internal flow and providing installation tolerances. During their service, the gas-liquid mixed multiphase flow conveyed by the jumpers will cause flow-induced vibration (FIV) and reduce their fatigue life. In this paper, the air-water mixed multiphase flow conveyed by a rigid M-shaped jumper at different mixture velocities (1 m/s to 4 m/s) and volume fraction of water (0.2–0.8) is investigated to analyze the factors affecting the flow patterns and identify the most dangerous regions. Flow patterns in each segment and historical pressure changes are obtained by simulation and experiment. The dynamic response of the jumper excited by FIV is obtained by one-way coupled fluid-structure interaction (FSI). The evolution of the air-water two-phase flow pattern from inlet to outlet is visualized and analyzed. In particular, the regularity of Taylor bubble generation in the first ascending segment is discussed. The pressure tends to decrease from inlet to outlet, and a consistent change in pressure difference is observed between simulation and experiment. The elbows are the most notable segments, with the maximum deformation and stress concentration.

Numerical validation of novel scaling laws for air–water flows including compressibility and heat transfer

Air–water flows are among the most important flow types in hydraulic engineering. Their experimental modelling at reduced size using Froude scaling laws introduces scale effects. This study introduces novel scaling laws for compressible air–water flows in which the air is considered compressible. This is achieved by applying the one-parameter Lie group of point-scaling transformations to the governing equations of these flows. The scaling relationships between variables are derived for the fluid properties and the flow variables including temperature. The novel scaling laws are validated by computational fluid dynamics modelling of a Taylor bubble at different scales. The resulting velocity, density, temperature, pressure and volume of the bubble are shown to be self-similar at different scales, i.e. all these variables behave the same in dimensionless form. This study shows that the self-similar conditions of the derived novel scaling laws for compressible air–water flows have the potential to improve laboratory modelling.

Discussion of “Air–Water Flows and Head Losses on Stepped Spillways with Inclined Steps”

Discussion of “Air–Water Flows and Head Losses on Stepped Spillways with Inclined Steps”

Closure to “Air–Water Flows and Head Losses on Stepped Spillways with Inclined Steps”

Closure to “Air–Water Flows and Head Losses on Stepped Spillways with Inclined Steps”

Flow and heat transfer characteristics of submerged impinging air-water jets

Flow and heat transfer characteristics of submerged impinging water jets were experimentally investigated. Air was mixed with water using a venturi tube installed before a pipe nozzle. The effects of the volumetric air fraction (β = 0.0–0.7) and jet impingement distance (L/D = 2–8) were studied at a water jet Reynold number, Rew, of 24,000. The air-water flow in the pipe nozzle and impinging air-water jet was recorded using a high-speed camera. An infrared thermal imaging camera measured local heat transfer on the impingement surface. Results show that heat transfer depends strongly on the volumetric air fraction. Impingement distance also affects the behavior of air bubbles in the water jet. Air bubbles can increase the heat transfer on the impingement region by increasing turbulence intensity in the jet with small air bubbles or an intermittent effect of large air bubbles impinging on the surface. Impinging jets where L/D = 4 with β = 0.3 and L/D = 2 with β = 0.2 give the highest heat transfer enhancement, about ∼47% compared to the case of β = 0.0.

Experimental insights into energy savings and future directions of drag reducing polymers in multiphase flow pipelines

Frictional pressure drop has been grasping the attention of many industrial applications associated with multi-phase and academia. Alongside the United Nations, the 2030 Agenda for Sustainable Development calls for the exigency of giving attention to economic growth, a considerable reduction in power consumption is necessary to co-up with this vision and to adhere to energy-efficient practices. Thereinto, drag-reducing polymers (DRPs), which do not require additional infrastructure, are a much better option for increasing energy efficiency in a series of critical industrial applications. Therefore, this study evaluates the effects of two DRPs—polar water-soluble polyacrylamide (DRP-WS) and nonpolar oil-soluble polyisobutylene (DRP-OS)—on energy efficiency for single-phase water and oil flows, two-phase air–water and air-oil flows, and three-phase air–oil–water flow. The experiments were conducted using two different pipelines; horizontal polyvinyl chloride with an inner diameter of 22.5 mm and horizontal stainless steel with a 10.16 mm internal diameter. The energy-efficiency metrics are performed by investigating the head loss, percentage saving in energy consumption (both per unit pipe length), and throughput improvement percentage (%TI). The larger pipe diameter was used in experiments for both DRPs, and it was discovered that despite the type of flow or variations in liquid and air flow rates, there was a reduction in head loss, an increase in energy savings, and an increase in the throughput improvement percentage. In particular, DRP-WS is found to be more promising as an energy saver and the consequent savings in the infrastructure cost. Hence, equivalent experiments of DRP-WS in two-phase air–water flow using a smaller pipe diameter show that the head loss drastically increases. However, the percentage saving in power consumption and throughput improvement percentage is significantly compared with that found in the larger pipe. Thus, this study found that DRPs can improve energy efficiency in various industrial applications, with DRP-WS being particularly promising as an energy saver. However, the effectiveness of these polymers may vary depending on the flow type and pipe diameter.

Experimental study on sub-regimes of air-water slug flow in a rectangular micro-channel

In mini/micro-channel two-phase flow, slug flow occurs easily because of the significant influence of surface tension and the low gravitational effect on the flow. Although an irregularity of slug flow has been reported, previous studies on mini/micro-channel slug flow have mainly concerned the slug flow with the alternate of film and liquid slug zones regularly. Therefore, the present study investigates sub-regimes of micro-channel slug flow with regular or irregular alternating film and liquid slug zones to understand their principal characteristics. Experiments are performed to visualize and simultaneously measure the transient local pressure of the air-water mixture flow in a horizontal rectangular channel with a 2 mm height and 1 mm width. Superficial Reynolds numbers for liquid and gas phases range from 56 to 1116 and 5.75 to 245, respectively. With the aid of visualization data, slug flow is categorized into two types of stable slug flow and four types of unstable slug flow. Present slug flow data is compared with previous correlations for predicting the regime of bubble formation mechanism, flow regime boundaries, and frictional pressure gradient. Measured temporal pressure records are analyzed using a fast Fourier transform and digital Butterworth filter, while a dominant frequency is identified from the normalized average power spectrum. Then, the local pressure fluctuation at the dominant frequency is examined for different types of slug flow using the visualization results. A morphological repetition is confirmed to be associated with the pressure fluctuation: the repeated film and liquid slug zones in both types of stable slug flow, the alternative between liquid-only flow and a group of gas bubbles and liquid slugs in type 3 unstable slug flow, and the periodic variation of bubble length in type 4 unstable slug flow. However, no correlation between morphological characteristics and pressure fluctuation is found in type 1 and 2 unstable slug flows.

2D Modelling and energy analysis of entrapped air-pocket propagation and spring-like geysering in the drainage pipeline system

Transient (highly unsteady) air–water two-phase flows and spring-like geysers have been one of the critical concerns in drainage pipeline systems, which may cause or exacerbate drainage flooding problems and associated damage consequences. In this paper, the flow dynamics and energy evolution mechanism of the induced spring-like geysers are innovatively investigated through a two-phase full-2D numerical model developed in this study. After full validation by laboratory experimental tests conducted in this study, the proposed 2D model is systematically applied to simulate transient air–water flows in drainage pipelines. The results have shown acceptable accuracy of this full-2D model to capture the complex flow interactions between the air and water phases, and indicated that the velocity and pressure distribution patterns are highly relevant to the air–water interface deformation and energy exchange. The in-depth energy analysis demonstrates that the intermittent eruption of geysers could be attributed to the conservation and release of different energy forms during the transient air–water two-phase flow process. Besides, the numerical applications for the systems with different boundaries and initial conditions indicate that the different ventilation conditions and initially entrapped air volume may significantly affect the velocity distribution of the air phase, thereby playing an essential role to provide effective measures to mitigate unexpected geyser events and pressure oscillations in the system. The results and findings of this paper could provide insights to improve the theory and practice of transient air–water two-phase flows in drainage pipeline systems.

Development of a System for Materials Science Studies on the T-15MD Tokamak

One of the main objectives pursued in carrying out materials science studies on the T-15MD tokamak is to insert samples, including large-scale ones, both for their long-term life cycle tests in the near-wall plasma areas and for studying the effect of impulse loads in the divertor section. A conceptual design of a system for carrying out materials science studies on the T-15MD tokamak has been developed at the Department of General Physics and Nuclear Fusion of the National Research University Moscow Power Engineering Institute. The system will open the possibility to carry out unique experiments on studying the state and composition of the divertor and first wall surfaces without the need to disturb the tokamak chamber vacuum and dismantle the components. It will be possible to perform such studies in the time intervals between tokamak pulses, which will make it possible to reveal changes on the divertor surface after each pulse and estimate correlations between the plasma parameters and changes on the first wall surface. In the attachment assembly, means are provided for cooling the mockups with both water flow and dispersed water-air flow. In addition, various diagnostic devices for monitoring the first wall and divertor surfaces can be installed on the attachment assembly.

Effect of inlet gas and liquid velocity profiles on one-group interfacial area transport equation in a vertical large rectangular channel

Bubbly flows appear in many heat and mass transfer equipment. The interfacial area concentration is a critical parameter in determining the performance of the heat and mass transfer systems. The one-dimensional one-group interfacial area transport equation (IATE) was developed based on area-averaged bubbly flow data collected under uniform inlet flow boundary conditions. The applicability of the IATE to the flow systems with non-uniform inlet flow boundary conditions was not comprehensively examined. This study investigated the effect of inlet liquid and gas velocity profiles on measured interfacial area concentration changes along the flow direction. The data used in this study were collected for vertical upward air-water two-phase flow in a rectangular channel with a gap length of 10 mm and width of 200 mm. The test conditions were the superficial gas velocity ranging from 0.0870 to 0.288 m/s and the superficial liquid velocity ranging from 0.515 to 2.53 m/s. The interfacial area concentrations with uniform and non-uniform inlet flow boundary conditions measured in a large vertical rectangular channel were compared under the same area-averaged flow rate conditions. Then, the predictive capability of the IATE to the bubbly flow test data with uniform and non-uniform inlet flow boundary conditions was assessed. Some non-uniform inlet flow boundary conditions, including channel-center peaking of liquid flow (CPF), channel-center peaking of gas flow (CPG), single-side-wall peaking of liquid flow (SPF), and single-side-wall peaking of gas flow (SPG), significantly affected the interfacial area concentration changes along the axial location at relatively high superficial gas or liquid velocity conditions. The IATE's predictive capability deteriorated for several non-uniform inlet flow boundary conditions, including double-side-wall peaking of gas flow (DPG) and single-side-wall peaking of liquid flow (SPF), specifically at relatively high superficial liquid velocity conditions. The maximum percentage error of the interfacial area concentration prediction by the IATE for these non-uniform inlet flow boundary conditions reached 75%.

Investigation of the geometry effect on air-demand ratio in conduits with a sluice gate

Gated conduits involve high-velocity air–water flow. When studies on gated conduits are examined, it is determined that the air-demand ratio changes according to the hydraulic and geometric parameters. However, no study has investigated the effect of the cross-sectional geometry of high-head conduits with a sluice gate on the air-demand ratio. In this study, the effect of conduit cross-sectional geometry on the air-demand ratio is examined. Results indicate that a conduit's cross-sectional geometry has an important effect on the air-demand ratio, especially at 10% and 15% gate opening rates. It is seen that the effect of the conduit geometry on the air-demand ratio decreases at 20%, and for greater gate opening rates. Moreover, a formula for estimated the air-demand ratio is presented, relating the air-demand ratio to the gate opening rate, Froude number, hydraulic radius and conduit length.

Advances in Spillway Hydraulics: From Theory to Practice

Over the past decades, significant advances have been achieved in hydraulic structures for dams, namely in water release structures such as spillway weirs, chutes, and energy dissipators. This editorial presents a brief overview of the eleven papers in this Special Issue, Advances in Spillway Hydraulics: From Theory to Practice, and frames them in current research trends. This Special Issue explores the following topics: spillway inlet structures, spillway transport structures, and spillway outlet structures. For the first topic of spillway inlet structures, this collection includes one paper on the hydrodynamics and free-flow characteristics of piano key weirs with different plan shapes and another that presents a theoretical model for the flow at an ogee crest axis for a wide range of head ratios. Most of the contributions address the second topic of spillway transport structures as follows: a physical modeling of a beveled-face stepped chute; the description and recent developments of the generalized, energy-based, water surface profile calculation tool SpillwayPro; an application of the SPH method on non-aerated flow over smooth and stepped converging spillways; a physical model study of the effect of stepped chute slope reduction on the bottom-pressure development; an assessment of a spillway offset aerator with a comparison of the two-phase volume of fluid and complete two-phase Euler models included in the OpenFOAM® toolbox; an evaluation of the performance and design of a stepped spillway aerator based on a physical model study. For the third topic of spillway outlet structures, physical model studies are presented on air–water flow in rectangular free-falling jets, the performance of a plain stilling basin downstream of 30° and 50° inclined smooth and stepped chutes, and scour protection for piano key weirs with apron and cutoff wall. Finally, we include a brief discussion about some research challenges and practice-oriented questions.

Fuzzy neural network for mapping CFD data of air-water flow in chemical reactors: Computational simulation

ANFIS (Adaptive Neuro-Fuzzy Inference Systems) artificial intelligence is used to enhance the computational fluid dynamic (CFD) modeling of an air-water bubble column reactor for the first time. Air temperature and velocity are 500 K and 0.006 m/s, respective in the process. The water temperature is considered at 295 K. This turbulent kinetic energy prediction is performed to measure the mixing flow within the reactor. The ANFIS projection is examined to see what impact the number of membership functions and input has. The findings showed that the most intelligent output is calculated by inputting two numbers and using a membership function with a value of 10. With respect to that, under this condition the ANFIS is capable of estimating the turbulent kinetic energy with more nodes on the domain while skipping mesh refinement in the CFD domain. The hybrid model was indicated to be robust in predicting the performance of liquid-phase reactor.

Design Methodologies for Sizing of Drainage Stacks and Vent Lines in High-Rise Buildings

Diameters for drainage stacks and vent lines within high-rise building drainage systems are determined by consulting building standard agencies’ design codes. While these are critical design decisions, codes are based upon dated research (1940s to 1970s), which has numerous inherent limitations, and the methodologies employed within the codes are unclear. Thus, a new methodology is presented which is based upon an analogy with other forms of multiphase flow transport systems. This methodology assumes, as a pre-condition, that flows of air and the flow of water within the stack are reasonably steady over time. Component diameters must then be chosen which ensure an acceptably large air supply or air–water flow ratio, and an acceptably small pressure excursion within the stack. Two ways to implement this methodology are presented: an ‘explicit approach’, in which component diameters are directly calculated using empirical correlations, and an ‘implicit approach’, in which component diameters are determined by iteration, using a hydraulic model. The methodology pre-conditions of the approach are then discussed. The physical geometry of the stack and branches tends to promote steady water flow but to render air flow very susceptible to temporary interruptions. A need to maintain the air pathway within high-rise drainage systems using components to supplement the air feed drawn in through the roof vent as required is highlighted.

Unified two-component interfacial area concentration model for narrow channel two-phase flow

The conventional interfacial area concentration (IAC) models do not distinguish narrow and wide side interfaces when applied in narrow channels, which hinders accurate modeling of interfacial transfer mechanisms at different interfaces. In order to overcome such deficiency, void fraction data obtained in narrow-channel air-water flow with the flexible printed circuit (FPC) wire-mesh sensor (WMS) is analyzed to establish a two-component IAC model which consists of two essential parameters, i.e. the projected interfacial area concentration (PIAC) and projected interfacial length concentration (PILC). Based on the refined and sharpened void fraction distribution, the postprocessing algorithm is developed to obtain the PIAC and PILC for different flow patterns. Analysis of experimental data indicates that the PIAC is approximately proportional to void fraction, while variation of the PILC with void fraction is complicated. The ratio of PILC to PIAC, which characterizes topology of interfaces, shows distinct value in bubbly flow compared with other flow patterns. Such difference is attributed to the existence of large bubbles, typically with spanwise length over 70% of the channel width, in flow patterns other than bubbly flow. Hence, bubbles are separated into two groups with the critical ratio of bubble width to channel width in the model derivation. The semi-empirical PIAC and PILC models are derived for each bubble group. Based on volume fraction of two bubble groups, the unified PIAC and PILC models are obtained to realize the accurate prediction in all the flow patterns. The unified two-component IAC model and other IAC models are evaluated with the present IAC experimental data and the experimental data of Shen et al. (2012). The evaluation shows the superiority of the unified two-component IAC model and that the proposed unified two-component model can be applied in the narrow-channel air-water two-phase flow patterns with α¯≤0.9.

Numerical simulation of hydrodynamics in wet cooling tower packings

Wet cooling towers are used in many industrial processes but hydrodynamic behaviour of air-water counter flows in towers packing remains unknown. Hydrodynamics is directly linked with main operating issues (e.g. packing fouling reducing cooling efficiency). The objective of this work is to use Computational Fluid Dynamics (CFD) simulations to characterize local hydrodynamic parameters such as water film thickness, velocity or wall shear stress and system scale parameters such as wetting rate or interfacial area. After a theoretical study of fluid mechanics, Volume of Fluid (VOF) simulations, with dedicated models for air/water interface capture and solid/liquid contact angle, successfully reproduced water falling films on packing surface. Simulations allowed the calculation of main water flow local and system parameters. The results have pointed out trends of increasing liquid film thicknesses and wetting rates with increasing water flowrate in agreement with experimental observations. A correlation of the interfacial area has been proposed as a function of the hydrodynamic adimensional numbers to characterize transfer phenomena in the considered packing.