Gas Entrainment Research Articles

Robust technique using magnetohydrodynamics for safety improvement in sodium-cooled fast reactor

Among Generation IV reactors, the sodium-cooled fast reactor (SFR) is attracting attention as a system having great potential for commercial use. Gas entrainment is a thermal-hydraulic issue related to the safety problem of the reactor core in the SFR. Typically, a dipped plate or baffles are installed under the free surface to suppress gas entrainment. However, these approaches can cause gas entrainment in other locations and require many trial-and-error and verifications. In this study, a new strategy using magnetohydrodynamics to suppress gas entrainment in the SFR is proposed. In a counter-flow model, a judgment criterion of gas entrainment occurrence was developed for both water and liquid metal. Moreover, the gas entrainment can be completely suppressed by applying a magnetic field.

Drop Bouncing Dynamics on Ultrathin Films

For drops to contact various surfaces, the removal of the interstitial fluid is the prerequisite to contact. While the conventional understanding is for drops to irreversibly spread on a film made of the same substance, we describe the dynamics of drops initiating contact yet carrying enough momentum to completely lift off of the substrate which we label as contact bouncing. We report new experimental results of the dynamics between drops impacting thin films described by the ratio of the liquid film hL to the drop with diameter D0 for the range of 0.004 < hL/D0 < 0.08. Using high-speed interferometry, we visualize the interfacial gas layer spatiotemporal signatures across the various film thicknesses and Weber numbers. We find that while increasing the deformability of the thin films enhances the gas entrainment phenomenon at early times, it also increases the rate of the gas purging rate, increasing the chance of contact just prior to the gas film retraction and drop lift off sequence. Drops which contact the liquid film during the retraction stage are able to bounce with <5% volume loss.

Analysis of mechanism of ducted fuel injection under non-vaporizing condition

Diesel engines play an important role in road transportation. However, they are facing strict regulations in reducing soot and harmful substances emissions. Ducted fuel injection (DFI) is a new method that effectively attenuates soot on a large scale. How this technology reduces emissions by boosting spray atomization remains unknown. This study focuses on the spray characteristics of DFI, which are difficult to obtain in experimental conditions by using numerical methods. Results showed that DFI increases the spray velocity by the less total entrainment of the ambient gas and the restriction of radial development of the spray. After the jet penetrates the duct, the higher speed leads to higher turbulent kinetic energy, thus enhancing the mixing process of ambient gas and fuel. The analysis of the formation of the mushroom-shaped head was also conducted. This phenomenon is attributed to the influence of pressure difference inside and outside of the duct.

EFFECTS OF GAS VOLUME FRACTIONS ON ELECTRICAL SUBMERSIBLE PUMP PERFORMANCE UNDER TWO-PHASE FLOW

An Electrical Submersible Pump (ESP) is the most commonly used artificial lifting method in the oil and gas industry through conversion of kinetic energy to pump pressure head; however, issues like gas entrainment and shifting production rates tend to cause ESP pressure degradation. Flow behaviours inside the ESP, such as gas pockets or pressure surging tend to diminish the pump pressure head significantly. Observation of gas-liquid flow in the ESP is challenging due to the compact, sophisticated geometry and highly turbulent flow strucutres. This paper observes two-phase flow in the ESP through CFD simulation, and illustrates its pressure degradation through Air Volume Fraction contours showing formations of gas pockets and recirculation bubbles. This research utilized Mixture Flow as its main two-phase model with 1%, 6% and 10% Gas Volume Fraction (GVF) ratios inside the pump at a constant flow rate of 250 L/min. The results show a clear pump head degradation from 3.062 m to 2.251 m overall. The centrifugal pump under two-phase flow is not able to generate the same amount of pressure head as it normally does due to the bubble point pressure and this decrease in pump pressure head is a potentially unstable behaviour, which is acknowledged as pressure surging.

A Gas Separation Model for a Downhole Gravitational Separator

Summary Pumping systems reduce their efficiency and lifetime when working with free gas at the pump inlet. Downhole gas separators are usually installed upstream of the pump in oil production wells to avoid free gas, but they cannot handle high void fraction. Studies on the so-called inverted-shroud gravitational separator indicate that it is possible to achieve gas separation efficiencies (GSEs) higher than 97%, even in the presence of very high void fraction. Gas separation inside this kind of separator works in two stages. The most critical stage is related to gas entrainment caused by a plunging free-surface flow in an annular channel formed by the production tubing and the separator itself, where strong kinetic energy dissipation takes place. Studies regarding this phenomenon in inverted-shroud separators are scanty. In this study, we present a semianalytical model supported by an empirical turbulent energy-dissipation correlation. The proposed correlation is based on dimensional analysis. New experimental data of the statistical distributions of bubble diameters during the gas entrainment process are presented. We measured bubbles diameters using a high-speed video camera and an edge detection algorithm. Data of both bubble diameters and GSE were used to adjust and validate the proposed model. The model establishes a transition boundary that separates a region of total gas separation (TGS) from a region of partial gas separation (PGS). In other words, it provides the operating envelope of the separator. The results can be used as a starting point for the design of inverted-shroud separators for field applications.

Rankine-vortex model based assessment of CFD methods for simulating the effect of gas entrainment observed in the hot-pool of sodium coold fast breeder reactors

The flow behavior in the hot pool of a compact Sodium Cooled Fast Breeder Reactor (SFR) is investigated in the MICAS facility of the CEA. Computational Fluid Dynamics (CFD) is used to assist the interpretation of the experiments by multi-scale calculations. Special attention is given to the risk of gas entrainment into the reactor core due to the formation of drain-type vortexes at the intermediate heat exchanger intakes. After theoretical considerations based on the Rankine-vortex model, the single effect gas entrainment experiment of Monji is analyzed by different CFD methods. The well-known incapacity of linear RANS models to simulate swirling flow is emphasized and limitations regarding the representation of gas cores by single-phase models are discussed. Then, the flow in the MICAS facility is analyzed by CFD using RANS and LES. The quantitative comparison between experiment and calculation shows that RANS can simulate correctly the global flow patterns in the hot pool. Small-scale drain-type vortexes have not been observed in the RANS calculations. LES can predict the onset of the formation of drain-type vortexes. However, single-phase models cannot simulate stable drain-vortexes.

Behavior of the Free Surface of Two-Phase Fluid Flow Near the Taphole in a Tank

The present study numerically investigated the deformation of the free-surface of two-phase fluid flow in a tank which is considered as a simplified blast furnace hearth. Actually, the fluids existing in a blast furnace hearth are gas, slag and hot metal from top to bottom. However, the present study considered only gas and cold molten iron in the tank. The porosity is considered as a substitute for void volume formed by the packed bed of the particles such as cokes. The single-phase flow and two-phase fluids flow without the porosity are analyzed for comparison. The porosity contributed the free surface to forming a viscous finger near the taphole. The axi-symmetry nature of the interface of two-phase fluids flow in the cylindrical tank is broken by viscous finger as the interface instability by the gas entrainment into taphole, which has been identified by the visualization of the free surface formation. The acceleration of the free surface falling velocity and the outflow near the taphole are associated by the viscous finger by the gas entrainment. The dimensionless gas break-through time is linear with respect to the porosity magnitude.

Investigation of Reverse Flow Slinger Combustor With Jet A-1 and Methanol

Abstract This paper describes an experimental investigation of the reverse flow slinger (RFS) combustor that has been developed in order to attain high flame stability and low emissions in gas turbine (GT) engines. The combustor employs centrifugal fuel injection through a rotary atomizer and performs flame stabilization at the stagnation zone generated by reverse flow configuration. The design facilitates entrainment of hot product gases and internal preheating of inlet air, which enhances flame stability and permits stable lean operation for low NOx. Moreover, a rotary atomizer eliminates the need for high injection pressures, resulting in a compact and lightweight design. Atmospheric pressure combustion was performed with liquid fuels, Jet A-1 and methanol, at ultralean fuel–air ratios (FARs) with thermal intensity varying from 30 to 52 MW/m3 atm. Combustor performance was evaluated by analyzing the lean blowout, emissions, and combustion efficiency. A very low lean blowout corresponding to global equivalence ratio of 0.1 was observed, which showed the combustor's high flame stability. Sustained and stable combustion at low heat release was attained, and NOx emissions as low as 0.4 g/kg and 0.1 g/kg were achieved with Jet A-1 and methanol, respectively. Combustion efficiency of around 55% and 90% was obtained in operation with Jet A-1 and methanol. The overall combustor performance was significantly better with methanol in terms of emissions and efficiency.

Comparison of supercavity geometry and gas leakage behavior between the double-cavity flow pattern and the wake-closure flow pattern

The objective of this paper is to investigate the effect of two typical supercavity flow patterns (double-cavity and wake-closure) on ventilated supercavity geometry and supercavitating flow structure and further reveal the different mechanisms of gas leakage for these two supercavity flow patterns. An inhomogeneous multiphase model with shear stress transport turbulence was employed to validate the investigation by experimental results. The supercavity geometry was investigated quantitatively between the two supercavity flow patterns by the contrast, and corresponding gas leakage mechanisms were determined. The results show that compared with those of the wake-closure flow pattern, the maximum diameter and full-length of the supercavity with the double-cavity flow pattern decreased by 9.83% and 35.8%, respectively, under the same gas entrainment coefficient. The reason for the apparent difference in the supercavity dimension between the two supercavity flow patterns is that the amount of gas leaking from the closure region is different. For the wake-closure flow pattern, about 6.0% of the ventilated gas leaks from the closure region. However, for the double-cavity flow pattern, more gas leaks from the closure region, which is about four times that of the wake-closure flow pattern. As a result, the mass flow rate of the reverse gas decreases, leading to a decrease in the size of the region where gas flows reversely.

Norwegian Sea net community production estimated from O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and prototype CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; optode measurements on a Seaglider

Abstract. We report on a pilot study using a CO2 optode deployed on a Seaglider in the Norwegian Sea from March to October 2014. The optode measurements required drift and lag correction and in situ calibration using discrete water samples collected in the vicinity. We found that the optode signal correlated better with the concentration of CO2, c(CO2), than with its partial pressure, p(CO2). Using the calibrated c(CO2) and a regional parameterisation of total alkalinity (AT) as a function of temperature and salinity, we calculated total dissolved inorganic carbon content, c(DIC), which had a standard deviation of 11 µmol kg−1 compared with in situ measurements. The glider was also equipped with an oxygen (O2) optode. The O2 optode was drift corrected and calibrated using a c(O2) climatology for deep samples. The calibrated data enabled the calculation of DIC- and O2-based net community production, N(DIC) and N(O2). To derive N, DIC and O2 inventory changes over time were combined with estimates of air–sea gas exchange, diapycnal mixing and entrainment of deeper waters. Glider-based observations captured two periods of increased Chl a inventory in late spring (May) and a second one in summer (June). For the May period, we found N(DIC) = (21±5) mmol m−2 d−1, N(O2) = (94±16) mmol m−2 d−1 and an (uncalibrated) Chl a peak concentration of craw(Chl a) = 3 mg m−3. During the June period, craw(Chl a) increased to a summer maximum of 4 mg m−3, associated with N(DIC) = (85±5) mmol m−2 d−1 and N(O2) = (126±25) mmol m−2 d−1. The high-resolution dataset allowed for quantification of the changes in N before, during and after the periods of increased Chl a inventory. After the May period, the remineralisation of the material produced during the period of increased Chl a inventory decreased N(DIC) to (-3±5) mmol m−2 d−1 and N(O2) to (0±2) mmol m−2 d−1. The survey area was a source of O2 and a sink of CO2 for most of the summer. The deployment captured two different surface waters influenced by the Norwegian Atlantic Current (NwAC) and the Norwegian Coastal Current (NCC). The NCC was characterised by lower c(O2) and c(DIC) than the NwAC, as well as lower N(O2) and craw(Chl a) but higher N(DIC). Our results show the potential of glider data to simultaneously capture time- and depth-resolved variability in DIC and O2 concentrations.

Experimental investigation on coal-breakage performances by abrasive nitrogen-gas jet with a conical nozzle

With the increasing energy and environmental protection demands, coalbed methane (CBM) have attracted worldwide attentions. A cost-effective way of using nitrogen-gas as the fluid media of abrasive jet for radial jet drilling (RJD) in CBM exploitation was proposed in this paper. Laboratory experiments were carried out to investigate the influence of parameters including jetting pressure, nozzle diameter, dimensionless standoff distance, bedding direction, abrasive diameter, duration time and coal strength on coal-breakage efficiency. Besides, sensitivity analysis was conducted to evaluate the effect of jetting parameters on erosion performance. Moreover, the surface morphologies of jet-drilled holes were measured by a 3D profilometer. Finally, the coal-breakage mechanisms of abrasive nitrogen-gas jet were illustrated. The results indicated that the key factors for improving coal-breakage efficiency by abrasive nitrogen-gas jet were dimensionless standoff distance and nozzle diameter. Four major factors that contribute to the coal-breakage mechanisms can be summarized as: fractures generation, edge chipping, repeated erosion and gas entrainment effect. Moreover, jetting direction plays an important role in generating fractures. Fractures tend to be generated along the parallel bedding direction on the coal since tensile failures are easier to be occurred. Furthermore, a regular and smooth hole generated by the abrasive nitrogen-gas jet made it suitable for RJD drilling in CBM wells. The experimental results are expected to provide a viable alternative for efficient, economical and safe exploitations in CBM reservoirs.

CFD modeling of liquid entrainment through vertical T-junction of fourth stage automatic depressurization system (ADS-4)

In this study, a three-dimensional (3D) transient computational fluid dynamics (CFD) model is presented to investigated liquid entrainment in gas–liquid flow through vertical T-junction of fourth stage automatic depressurization system (ADS-4) in AP1000. A specialized CFD model for the liquid entrainment phenomenon study, which is based on coupled Eulerian-Eulerian VOF (volume of fluid) formulation with suitable interfacial drag and standard κ-ε turbulence model for each phase was proposed. The entrainment rate was calculated and results were validated with ADETEL experimental data, which was built at XJTU-NuTheL. The good agreement between CFD calculations and experimental data demonstrated that the proposed CFD model could reasonably predict entrainment within the studied range. The effect of vertical to horizontal branch diameter ratio and gas mass flow rates on liquid entrainment were also studied. In addition, the liquid volume fraction distributions and velocity field were investigated to develop an understanding of the entrainment process. The relatively high demand for computational resources due to very small timestep size and small grid size to accommodate high flow velocities was found to be a challenge.

Scaling effects on the free surface backward facing step flow

A set of large eddy simulations for the free surface backward facing step (FSBFS) are carried out to study wave formation behind the step. The volume-of-fluid ghost fluid method is employed to capture the free surface. Previous studies have indicated that the wave physics depend on the step draught-based Froude number (Fr). For small Fr, the rear face of the step (transom) becomes wet, while for large Fr, the wave separates smoothly from the transom. Close to a critical Fr separating wet and dry transoms, both conditions may occur. Here, we study wet, critical, and dry conditions based on the Fr classification with three different inflow boundary layer profiles (ReL=1,2,3×106). For Fr = 1.75 (wet conditions), we observe a weak dependence on the ReL. A proper orthogonal decomposition of the velocity field at Fr = 1.75 shows a coherent vortex street forming beneath the free surface. At Fr = 2.66 (critical conditions), we observe that an increase in the ReL results in a decrease in the wavelength and pronounced gas entrainment due to wave breaking. For Fr = 3.17 (dry conditions), we also observe shorter wavelength at increased ReL. Further, in the dry conditions, a breaking wave is noticed to occur at higher ReL, while breaking waves are not observed for the smallest studied ReL. Based on the results, we conclude that the wave shape for FSBFS cannot be characterized by the Froude number alone.

Hydraulic characteristics of liquid–gas ejector pump with a coherent liquid jet

The gas suction rate of a conventional liquid–gas ejector pump was measured on a modular pilot plant unit utilizing convergent nozzles and a water–air system. Hydraulic characteristics were measured for various water flow rates, suction and discharge pressures, and ejector geometries, covering a wide range of configurations and operating conditions. The device configurations, providing the most stable and reproducible results, were identified. The air flow rates measured with these configurations were correlated by a simple, three-parameter correlation with a relative standard deviation (RSD) of 11%. The performance and behavior of the ejector in less stable configurations are discussed and the recommendations for the design of optimally operating units are provided. Finally, the correlation is tested against available literature data. The ejectors with comparable geometry agree well with the proposed correlation (within 20% RSD). The reasonable agreement with the discussed differences is found for other literature data. This comparison results in the validation of the proposed correlation to provide a safe prediction of minimal gas entrainment in units of various geometries, orientation and operating conditions.

Numerical analysis of liquid fall type gas entrainment in pool-type fast reactor

Argon covering the free surface of the pool-type sodium-cooled fast reactor may be mixed into the coolant in form of entrainment bubbles due to the fluctuation of liquid surface, which introduces positive reactivity to the core, reduces the heat exchange capacity of the core and intermediate heat exchangers (IHXs), and possibly leads to the cavitation of the primary sodium pump (PSP), threatening the safe operation of the reactor. In this study, the computational fluid dynamics (CFD) simulation of liquid sodium fall entrained argon was carried out based on the Volume of Fluid (VOF) interface tracking method to describe the entrainment process in detail, and the influences of geometry size, initial liquid level height and inlet velocity on entrainment were analyzed successively. It was found that the re-submergence velocity, having a linear relation with the re-submergence angle, is the key factor of the liquid fall type gas entrainment. It can be used to judge whether the liquid fall type entrainment occurs conservatively. In addition, the parameters affecting the re-submergence velocity are specifically analyzed from the supplementary cases, according to the results of which, the empirical relation of the re-submergence velocity is obtained. Therefore, the situation of liquid fall type gas entrainment under various working conditions can be predicted numerically.

In-depth analysis of mixing characteristics of stratified gases during spray operation in the TOSQAN Test 113 using OpenFOAM

During the severe accident of a nuclear power plant, the hydrogen is generated by the reactor core oxidation which may be stratified or distributed in the containment building. There are risks such as hydrogen explosion at a certain concentration of hydrogen, and therefore a detailed analysis is needed to assess the hydrogen risk. Meanwhile, the spray system operates as part of a severe accident strategy, and the dynamic effects of spray, in particular, are expected to have a significant impact on the gas entrainment and global mixing. Some parameters such as a buoyancy term in a turbulence model and particle diameter affecting the global behavior, have been reported in the previous researches. However, the detailed mechanisms of the dynamic aspect of spray were not described. Thus, the present study aims to perform an in-depth analysis of the effect of the buoyancy term in a turbulence model and the droplet size on the mixing characteristics (mixing tendency and mixing time) in TOSQAN 113 test.

Numerical assessment of performance characteristics and two-phase flow dynamics of a centrifugal rotor operating under gas entrainment condition

This work presents a numerical study of performance characteristics and flow dynamics of a centrifugal rotor operating with gas-liquid flow. An Euler-Euler polydispersed model is adopted, which was validated in a previous publication and is used in this work to assess quantities not addressed in the earlier study. An analysis of the rotor performance under two-phase is performed, for which suitable expressions are presented. In particular, an analysis of the rotor head components is carried out including the dynamic head evaluation through the rotor, which is often neglected in most studies. The numerical model is explored further to analyze flow patterns and relevant gas-liquid flow quantities. This includes analysis of the three-dimensional distribution of the gas phase inside the rotor, evaluation of gas-liquid interphase forces, and the effect of the intake gas flow rate on the bubble diameter, gas-liquid relative velocities, and the overall turbulence levels through the rotor. Such analyses, which are usually hard to obtain through experimental methods, are also rarely found in related numerical studies. Thus, this work could bring useful information to help in the understanding of the gas-liquid flow behavior in centrifugal rotors, while also contributing to the progress of numerical solution of this problem.

Radiative mixing layers: insights from turbulent combustion

ABSTRACTRadiative mixing layers arise wherever multiphase gas, shear, and radiative cooling are present. Simulations show that in steady state, thermal advection from the hot phase balances radiative cooling. However, many features are puzzling. For instance, hot gas entrainment appears to be numerically converged despite the scale-free, fractal structure of such fronts being unresolved. Additionally, the hot gas heat flux has a characteristic velocity vin ≈ cs, cold(tcool/tsc, cold)−1/4 whose strength and scaling are not intuitive. We revisit these issues in 1D and 3D hydrodynamic simulations. We find that over-cooling only happens if numerical diffusion dominates thermal transport; convergence is still possible even when the Field length is unresolved. A deeper physical understanding of radiative fronts can be obtained by exploiting parallels between mixing layers and turbulent combustion, which has well-developed theory and abundant experimental data. A key parameter is the Damköhler number Da = τturb/tcool, the ratio of the outer eddy turnover time to the cooling time. Once Da &amp;gt; 1, the front fragments into a multiphase medium. Just as for scalar mixing, the eddy turnover time sets the mixing rate, independent of small scale diffusion. For this reason, thermal conduction often has limited impact. We show that vin and the effective emissivity can be understood in detail by adapting combustion theory scalings. Mean density and temperature profiles can also be reproduced remarkably well by mixing length theory. These results have implications for the structure and survival of cold gas in many settings, and resolution requirements for large scale galaxy simulations.

Gas entrainment from gaseous supercavities: Insight based on numerical simulation

The understanding of the internal gaseous flow of artificially ventilated supercavities is developed using a locally homogenous, multiphase computational fluid dynamics model that is benchmarked using experimental data. The solutions indicate that gas leakage from a ventilated supercavity originates from the gaseous shear layers forming at the gas-water interface. Not only do these observations corroborate previous theory developed for cavities with toroidal closure, they also display evidence that shear-layer mechanisms remain important for cavities in the twin-vortex regime and when interacting with bodies. It is also found that the treatment of turbulence in these shear layers affects the outcome of computational fluid dynamics approaches. Lastly, a semi-empirical model considering these shear layers is proposed. Results from the model indicate an improved prediction capability of the relationship between cavity size and ventilation rate for steady, twin-vortex supercavities.

Analysis of gas entrainment phenomenon from free liquid surface for a sodium-cooled fast reactor design (Velocity profile and Strouhal number in a flow field)

Analysis of gas entrainment phenomenon from free liquid surface for a sodium-cooled fast reactor design (Velocity profile and Strouhal number in a flow field)