Gas Entrainment Research Articles

Energy efficiency and performance of bubble generating systems

The energy efficiency of bubble generating systems of gas-liquid contacting reactors is reviewed and analyzed quantitatively. The key metrics that emerge from a new theoretical analysis are: i) gas fraction of the output dispersion (εgen), ii) interfacial area density of the output dispersion (agen), iii) the energy necessary to create interfacial area (ξ), and iv) cost and maintenance. These metrics are then used to create a literature review of the performance of existing technologies for bubble generation. thus producing a conceptual framework for process intensification and future designs for bubble generating systems. A novel micro-ejector device is then studied within this framework, and is found to have desirable performance, hypothetically because it limits fluid shear to be located on at the gas injection site, on micron size scales. A physical model of the gas entrainment and bubble creation mechanisms in the micro-ejector is described and validated with empirical data.

Exploring the Relationship between Natural and Artificial Modes of Supercavitation

A supercavity can be attained via two distinct routes: vaporous and ventilated supercavitation. A vaporous supercavity is one that is obtained by the coalescence of individual vapor bubbles formed by cavitation. On the other hand, a ventilated supercavity results from the coalescence of bubbles of non-condensable gas formed through gas injection at the rear part of the cavitator. A typical strategy of operation of a supercavitating underwater vehicle entails an interplay between these two modes viz. accelerating it to a high speed using ventilation, when a natural supercavity can be sustained. This fact necessitates a systematic study of the synergistic relationship between the process of supercavity formation under these two modes. Thus, in the current work, we have systematically carried out water tunnel experiments to study the effect of vaporous and ventilated modes of supercavitation on each other during supercavity formation. The results show a systematic dependence between the relevant parameters which include the natural cavitation number and the gas entrainment coefficient.

Intrusive measurements of air-water flow properties in highly turbulent supported plunging jets and effects of inflow jet conditions

A plunging jet is an efficient device to entrain gas into liquid flow. In many practical occasions, the gas entrainment needs to be carefully controlled, and the interaction between the shear flow turbulence and entrained bubbles has to be better understood. This paper presents a physical study of vertical supported two-dimensional plunging jets using a relatively large-size facility. The air-water flow and turbulence properties were measured with an intrusive phase-detection probe and a total pressure sensor simultaneously. The inflow pre-aeration and turbulence level of the falling jet were carefully characterised, and the effects of jet impact velocity and jet length on air entrainment in plunging pool were investigated. The experimental results were systematically compared to relevant studies. A discussion was developed on the quantification of turbulence intensity in highly-aerated flow based on total pressure measurement. The flow turbulence properties were derived respectively from the interfacial phase-detection signals and total pressure signals. The results highlighted difference in terms of the turbulence intensities between interfacial motions and water-phase turbulence. The present work showed that the jet impact velocity, jet length, inflow disturbance and pre-entrainment of air had considerable effects on air entrainment capacity and subsurface air-water flow properties in plunging jets hence should be carefully characterised in relevant studies.

NO Emission of a Non-premixed Oxygen-Enhanced Flame in Burned Gas Entrainment above the Self-Ignition Temperature (BEST) Conditions

ABSTRACTThe emission characteristics of non-premixed oxy-fuel and oxygen-enhanced flames in burned gas entrainment above the self-ignition temperature (BEST) conditions were experimentally investigated in a lab-scale slot-type combustor. The inside geometry of the furnace was width = 250 mm, depth = 250 mm, and height = 350 mm. A slot-type burner was located on the center of the furnace bottom. The widths of the fuel jet and oxidizer nozzle exit were 0.24 mm and 0.52 mm. The separation distance between the nozzle exits was 8 mm and the depth of the nozzle exit was 20 mm. Light emission data were obtained with a digital single lens reflex camera, spectrometer, and intensified charge-coupled device camera to visualize the flame appearance in a furnace. The distribution of the furnace temperature in the BEST conditions was measured with R-type thermocouples. The pollutant emission trend was analyzed when the inlet velocity for fuel jet and oxidizer is 10 m/s to 30 m/s and 10 m/s to 55 m/s, the volumetric mole fraction of oxygen in an oxidizer is 40% to 100%, and the global equivalence ratio is 0.55 to 1.03. The objectives of the present work were to study the flame spectra in normal temperature and pressure (NTP) and the BEST conditions and to determine the global production rate of nitrogen oxides in non-premixed oxy-fuel and oxygen-enriched flames. The visible flame of a non-premixed oxy-methane flame disappeared and temperature deviation in a furnace decreased by 300°C in the BEST conditions. The global production rate of nitrogen oxides ([NOx]/τG) level decreased while increasing the volumetric mole fraction of oxygen in an oxidizer. The slope of [NOx]/τG was similar to that of a non-premixed methane-air flame in NTP conditions. The level of [NOx]/τG was lower in oxygen-enriched flames than in CH4-air flames.

Experimental investigation of ventilated partial cavitating flows with special emphasis on flow pattern regime and unsteady shedding behavior around an axisymmetric body at different angles of attack

The objective of this paper is mainly to investigate the ventilated partial cavitating flow structure at different angles of attack with experimental methods. A high-speed camera technique is used to record cavity evolution patterns. The numerical simulation is performed by CFX Solver with a free surface model and filter-based turbulence model. Three flow pattern maps are constructed to depict the flow pattern and structures at different angles of attack α, i.e. the range of Froude number Fr and gas entrainment coefficient CQ. The maps show that the angle of attack α has a significant effect on ventilated cavitating flow structures. With the increasing value of α, the asymmetry of ventilated cavity becomes obvious and the angle between the axis of the body and the closure line θ is decreased. The value of the included angle θ is found to vary with Fr number and CQ. The cavity break-off may be mainly determined by the momentum and the direction of re-entrant jet flow.

A characteristic scale for cold gas

We find that clouds of optically-thin, pressure-confined gas are prone to fragmentation as they cool below $\sim10^6$ K. This fragmentation follows the lengthscale $\sim{c}_{\text{s}}\,t_{\text{cool}}$, ultimately reaching very small scales ($\sim{0.1} \text{pc}/n$) as they reach the temperature $\sim10^4$ K at which hydrogen recombines. While this lengthscale depends on the ambient pressure confining the clouds, we find that the column density through an individual fragment $N_{\text{cloudlet}}\sim10^{17} \text{cm}^{-3}$ is essentially independent of environment; this column density represents a characteristic scale for atomic gas at $10^4$ K. We therefore suggest that "clouds" of cold, atomic gas may in fact have the structure of a mist or a fog, composed of tiny fragments dispersed throughout the ambient medium. We show that this scale emerges in hydrodynamic simulations, and that the corresponding increase in the surface area may imply rapid entrainment of cold gas. We also apply it to a number of observational puzzles, including the large covering fraction of diffuse gas in galaxy halos, the broad line widths seen in quasar and AGN spectra, and the entrainment of cold gas in galactic winds. While our simulations make a number of assumptions and thus have associated uncertainties, we show that this characteristic scale is consistent with a number of observations, across a wide range of astrophysical environments. We discuss future steps for testing, improving, and extending our model.

Air entrainment driven by a converging rotational field in a viscous liquid

A thorough physical understanding of gas entrainment within a liquid pool is established using a pair of fully submerged cylinders with an opposite but symmetric rotational field. An asymmetric version of such entrainment of gaseous filaments inside a liquid has been demonstrated in our earlier work [P. Kumar, A. K. Das and S. K. Mitra, Phys. Fluids 29, 022101 (2017)]. Here, our efforts focus on revealing the governing factors and understanding the stages of alteration from a stratified to an undulating interface for a wide range of symmetric cylinder rotations. Interfacial configurations such as an upper rounded crest, a bubble-ejecting jet, and a non-collapsible jet with an air pocket in the stagnant zone are obtained as the cylinder rotation increases. Near the critical capillary number, air pinch-off into a filament and subsequent stable collapse of this filament into bubbles were observed. As the rotation-driven capillary number increased, we noted the formation of an entrained jet of gaseous-phase forming air pockets in the region, which resulted in a diverging rotational field. Our analysis of the fluid stream explains the interfacial configurations and corresponding entrainment patterns based on fundamental physics. The power-law fit (Y = KXm) of the cusp profiles revealed close agreement with numerically obtained interfaces. We propose correlation coefficients as a function of the capillary number. We also assess the dependence of the entrainment pattern on cylinder submergence and spacing.

Premixed flames subjected to extreme levels of turbulence part I: Flame structure and a new measured regime diagram

This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity (DT=u′Lx) to the molecular diffusivity within the preheat layer (D*=SLδF,L). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

Filament formation in wind–cloud interactions– II. Clouds with turbulent density, velocity, and magnetic fields

We present a set of numerical experiments designed to systematically investigate how turbulence and magnetic fields influence the morphology, energetics, and dynamics of filaments produced in wind-cloud interactions. We cover 3D magnetohydrodynamic systems of supersonic winds impacting clouds with turbulent density, velocity, and magnetic fields. We find that log-normal density distributions aid shock propagation through clouds, increasing their velocity dispersion and producing filaments with expanded cross sections and highly-magnetised knots and sub-filaments. In self-consistently turbulent scenarios the ratio of filament to initial cloud magnetic energy densities is ~1. The effect of Gaussian velocity fields is bound to the turbulence Mach number: Supersonic velocities trigger a rapid cloud expansion; subsonic velocities only have a minor impact. The role of turbulent magnetic fields depends on their tension and is similar to the effect of radiative losses: the stronger the magnetic field or the softer the gas equation of state, the greater the magnetic shielding at wind-filament interfaces and the suppression of Kelvin-Helmholtz instabilities. Overall, we show that including turbulence and magnetic fields is crucial to understanding cold gas entrainment in multi-phase winds. While cloud porosity and supersonic turbulence enhance the acceleration of clouds, magnetic shielding protects them from ablation and causes Rayleigh-Taylor-driven sub-filamentation. Wind-swept clouds in turbulent models reach distances ~15-20 times their core radius and acquire bulk speeds ~0.3-0.4 of the wind speed in one cloud-crushing time, which are three times larger than in non-turbulent models. In all simulations the ratio of turbulent magnetic to kinetic energy densities asymptotes at ~0.1-0.4, and convergence of all relevant dynamical properties requires at least 64 cells per cloud radius.

Hydrodynamics around a shroud tube assembly of a fast breeder reactor

Control plug assembly of a liquid sodium cooled fast breeder reactor is partially submerged inside the liquid sodium coolant pool, free surface of which is blanketed with cover gas, mainly Argon to provide a non-reactive environment. It receives high velocity coolant flow from the fuel subassembly which travels through annular gap inside Control Safety Rod Driving Mechanism (CSRDM) assembly leading to swelling of free surface and gas entrainment inside the coolant pool. Entrained gas bubbles travel through different submerged components of the reactor and induces numerous complications including reactor power fluctuation, cavitation in inlet pumps and heat transfer inhibition in heat exchangers. Study of hydrodynamics around the shroud tube is crucial to examine cover gas entrainment inside the pool. The present study reports an experimental study on a 1:9 scale down model of CSRDM shroud tube assembly based on Froude number similarity. Eccentricity of the control rod inside the shroud tube is also an important parameter as temperature gradient along the CSRDM assembly leads to longitudinal thermal stress causing displacement of control rod from its center position in the shroud tube. Flow behavior around shroud tube, hump formation at the axial exit of shroud tube, air entrainment inside coolant pool have been reported for various parameters viz. inlet flow rate (qi), free surface height (hf) and eccentricity of central rod (e). Dye visualization, Schlieren visualization, Bulk visualization have been carried out and digital image processing techniques have been adopted for quantification of hump parameters and entrained air fraction inside the coolant pool.The axial flow from the exit of the shroud tube impinges on the free surface like a wall jet. At low Froude number, the impinging jet flow travels radially outward on the interface like a wall jet boundary layer. The shear of the free surface flow drags the jet like flow from the pores upward leading to mixing between the vortices inside the shear layer and the free vortices from the pores of the shroud tube. At high Froude number, the impinging jet like flow penetrates through the interface leading to hump formation. The downward motion of the hump reduces the radial free surface flow and blankets the radial flow from the pores of the shroud tube. The eccentricity of the central rod leads to asymmetric axial flow from the shroud tube and thus the resulting hump becomes asymmetric due to impingement of asymmetric jet. Therefore, the impingement angle of the downward hump motion becomes different at both sides of the shroud tube with asymmetric flow field and bubble formation process inside the liquid pool. However, the synchronization between the hump unsteadiness and the surface wave motion leads to mixing of air bubbles inside the liquid pool. Average entrained air fraction increases with increase in inlet flow rate and decreases with increase in free surface height.

Two-phase discharge from a stratified region through two horizontal branches with centerlines falling on an inclined plane

The main objective of this study was to obtain new experimental data for conditions not previously tested for discharging two-phase flow through two 6.35 mm diameter branches with centerlines falling in an inclined plane. The present results are relevant to many industrial applications including headers and manifolds, multichannel heat exchangers and small breaks in horizontal pipes. In the experimental investigation, the critical heights for the onsets of liquid and gas entrainment (OLE and OGE, respectively) were obtained, analyzed and correlated for two different branch spacings and two different angles between the branches. For each combination of branch spacing and angle between the branches, a wide range of Froude numbers was used. Two-phase mass flow rate and quality results were also obtained and analyzed for a range of interface heights for 16 different combinations of branch spacing, inclination angle, test section pressure and pressure drop across each branch. New empirical correlations were developed to predict the dimensionless mass flow rate and quality. The new correlations show good agreement with the present data and with previous correlations.

Internal and near nozzle measurements of Engine Combustion Network “Spray G” gasoline direct injectors

Gasoline direct injection (GDI) sprays are complex multiphase flows. When compared to multi-hole diesel sprays, the plumes are closely spaced, and the sprays are more likely to interact. The effects of multi-jet interaction on entrainment and spray targeting can be influenced by small variations in the mass fluxes from the holes, which in turn depend on transients in the needle movement and small-scale details of the internal geometry. In this paper, we present a comprehensive overview of a multi-institutional effort to experimentally characterize the internal geometry and near-nozzle flow of the Engine Combustion Network (ECN) Spray G gasoline injector. In order to develop a complete picture of the near-nozzle flow, a standardized setup was shared between facilities. A wide range of techniques were employed, including both X-ray and visible-light diagnostics. The novel aspects of this work include both new experimental measurements, and a comparison of the results across different techniques and facilities. The breadth and depth of the data reveal phenomena which were not apparent from analysis of the individual data sets. We show that plume-to-plume variations in the mass fluxes from the holes can cause large-scale asymmetries in the entrainment field and spray structure. Both internal flow transients and small-scale geometric features can have an effect on the external flow. The sharp turning angle of the flow into the holes also causes an inward vectoring of the plumes relative to the hole drill angle, which increases with time due to entrainment of gas into a low-pressure region between the plumes. These factors increase the likelihood of spray collapse with longer injection durations.

Numerical and experimental investigation on surface air entrainment mechanisms of a novel long‐short blades agitator

We investigate numerically and experimentally the mechanisms of surface air entrainment in the vessels equipped with the long‐short blades agitator. VOF method coupled with LES model is used to visualize the surface air entrainment process. In the case of partial submergence of the long blades (LBs), the interaction of the LBs with the liquid free surface creates a depression behind the LBs. Backfilling of the liquid into the depression leads to gas separation and entrapping into the liquid. The critical tip velocity of the LBs, utip,c, for the onset of gas entrainment is measured in vessels with diameters, T = 200∼600 mm. It is found that when H/T ≥ 1.0, utip,c is determined by the LBs, independent of the liquid level. utip,c is also affected by the size of the vessel through the diameter of the sweeping circle of the LBs, but for substantially large vessels, it approaches a constant value. © 2017 American Institute of Chemical Engineers AIChE J, 63: 316–325, 2018

22aBU-10 Theoretical derivation of the gas entrainment rate coefficient from the ideal atomizing liquid jet model

22aBU-10 Theoretical derivation of the gas entrainment rate coefficient from the ideal atomizing liquid jet model

Experimental study and mechanistic modeling of pressure surging in electrical submersible pump

Gas entrainment is frequently encountered in crude oil production with electrical submersible pumps (ESP). Previous studies revealed that the increase of gas entrainment rate in ESPs results in mild degradation of boosting pressure followed by a drastic drop. This critical condition, termed as pressure surging, significantly affects ESP's operational stability and run-life. In this paper, the pressure surging phenomenon in ESPs is studied through experimental measurements and mechanistic modeling. A 7.62-cm two-phase flow loop with a 14-stage radial-type ESP is used for testing pump performance under single- and two-phase flow conditions. The stage-by-stage boosting pressure with different gas entrainment rates is measured. Effects of intake pressure, gas volumetric fraction (GVF) and rotational speeds on the ESP two-phase pressure increment are investigated. Experimental results show that the boosting pressure of ESP under gassy flow conditions varies significantly with inlet GVFs and fluid properties. For low GVFs (<6%), the ESP pressure increment deteriorates gradually with the increase of gas flow rate. However, severe degradation of pump boosting pressure is observed if the inlet GVF exceeds a certain value (>7%), which triggers ESP's unstable operations. A mechanistic model based on the critical bubble diameter in rotating multiphase flow field is developed to predict the surging initiation in ESPs. Compared with experimental results, the model predictions demonstrate good agreement.

Experimental study on two-phase flow natural circulation in a core catcher cooling channel for EU-APR1400 using air-water system

Ex-vessel core catcher cooling system driven by natural circulation is designed using a full scaled air-water system. A transparent half symmetric section of a core catcher coolant channel of a pressurized water reactor was designed with instrumentations for local void fraction measurement and flow visualization. Two designs of air-water top separator water tanks are studied including one with modified ‘super-step’ design which prevents gas entrainment into down-comer. In the experiment air flow rates are set corresponding to steam generation rate for given corium decay power. Measurements of natural circulation flow rate, spatial local void fraction distribution and re-wetting time near the top wall are carried out for various air flow rates which simulate boiling-induced vapor generation. Since heat transfer and critical heat flux are strongly dependent on the water mass flow rate and development of two-phase flow on the heated wall, knowledge of two-phase flow characteristics in the coolant channel is essential. Results on flow visualization showing two phase flow structure specifically near the high void accumulation regions, local void profiles, rewetting time, and natural circulation flow rate are presented for various air flow rates that simulate corium power levels. The data are useful in assessing the cooling capability of and safety of the core catcher system.

Experimental investigations on a common centrifugal pump operating under gas entrainment conditions

This paper presents an experimental study on the effects of additional gas entrainment in centrifugal pumps designed for conveying liquid phases only. The pump performance has been evaluated for several gas entrainment conditions, and for various operational settings of the pump, such as its alignment and the rotational speed of the impeller. As a main performance indicator the impact of entrained gas on the hydraulic power of the pump has been analyzed using experimental data. Additionally, high-resolution gamma-ray computed tomography (HireCT) operated in time-averaged rotation-synchronized scanning mode has been applied to quantify local phase fraction distributions inside the rapidly rotating pump impeller. Based on these quantitative tomographic measurements, gas holdup profiles along selected streamlines have been calculated and gas accumulation areas inside the impeller chambers have been visualized. Thus, various internally accumulated gas holdup patterns have been identified and, eventually, associated with characteristic pump performance behaviors. Moreover, the tomographic measuring method allowed an enhanced gas holdup analysis in specified pump compartments. As a result, the related specific gas and liquid phase holdup profiles have been evaluated.

Effects of micro-hole nozzle and ultra-high injection pressure on air entrainment, liquid penetration, flame lift-off and soot formation of diesel spray flame

Increasing the injection pressure and downsizing the nozzle orifice diameter have been major measures for diesel engines to facilitate fuel–ambient gas mixture formation and combustion processes. The objective of this investigation is to carry out a quantitative analysis on the effects of micro-hole nozzle and ultra-high injection pressure on the mixing and combustion characteristics of diesel spray flame. Hence, laser-induced fluorescence and particle image velocimetry technique was employed to quantitatively access the gas entrainment of diesel spray emerging from nozzle with orifice diameter down to 80 µm under injection pressure up to 300 MPa, together with OH* chemiluminescence imaging and two-color pyrometry techniques to resolve the combustion and soot formation processes. Additionally, numerical simulation on the multi-phase flow inside injector nozzle was conducted to obtain information on internal flow dynamics. Experimental results show that over 80% of the ambient gas entrained into a spray pl...

Numerical Study on Electrical-Submersible-Pump Two-Phase Performance and Bubble-Size Modeling

Summary The electrical submersible pump (ESP) is widely used in the petroleum industry to maintain high crude-oil production rate. Its performance deteriorates with the increase of gas entrainment. Previous studies showed that bubble size is an important factor affecting ESP boosting pressure under multiphase flow. In this paper, a novel indirect approach is proposed to obtain the representative bubble sizes inside a rotating ESP impeller by matching numerical-simulation results with experimental performance curves. A 3D computational-fluid-dynamics (CFD) model is implemented on a three-stage ESP geometry to simulate the pump-pressure increment under various flow conditions. By use of structured hexahedral grids and the frozen-rotor technique, the mesh independence and numerical accuracy are verified. ESP boosting pressure in single-phase simulations is found to match well with experimental data. For two-phase simulations, the Eulerian-Eulerian model is used. At low inlet gas volumetric fractions (GVFs), the numerically simulated pump-pressure increment using constant bubble sizes agrees well with experimental measurements. At higher GVFs, the simulation results deviate from experimental pump-performance curves considerably. By increasing bubble sizes, the simulated ESP performance can be tuned to match experimental results. Through this process, a bubble-size change trend with varying GVFs is obtained. A mechanistic model that is based on the maximum stable bubble in a rotating turbulent flow field is developed to correlate the CFD simulated bubble sizes. Further, numerical-simulation results incorporating the proposed bubble-size prediction model agree well with experimental data.

Management of Low Tidal Volume and ETCO2 during Thoracoscopy in the Treatment of Tracheo-Esophageal Fistula: A Case Report

Esophageal atresia with tracheo-oesophageal fistula (TEF) occurs in 1 in 2500-3000 live births. Management of airway and ventilation in TEF repair is challenging. Nowadays, advances in minimally invasive surgical techniques and neonatal intensive care for neonates have allowed endoscopically repair of TEF, which lends another level of complexity to airway management. Usually, a sudden increase in ETCO2 should alert the anesthesiologist to possible surgical trauma and entrainment of thoracoscopic gas. However, low ETCO2 emerging with low tidal volume can also alerts the anesthesiologist to possible surgical trauma. We had an uncommon situation, where a sudden low ETCO2 emerged with low tidal volume when doing the complete anastomosis. After communicating with surgeons, they stopped and tried to find the possible causes. Finally, it appeared to be a surgical injury on the right lung. Continuous communication between the surgeon and the anesthesiologist is imperative when performing these operations.