Flow Entrainment Research Articles

Droplet entrainment analysis of three-phase low liquid loading flow

Most of the commonly used multiphase flow models neglect the amount of droplets entrained in stratified wavy flow. However, the experimental data presented in this study show high entrainment values, exceeding 50% in some cases. This shows that neglecting entrainment phenomenon can introduce a major source of discrepancy into multiphase flow modeling predictions. This study improves the entrainment fraction predictions in three-phase stratified flow.The droplet entrainment in three-phase stratified flow in horizontal pipelines is experimentally investigated using a 0.152-m ID facility. The experiments are conducted under low liquid loading conditions, which is very commonly observed in wet gas pipelines. The oil-aqueous-gas flow experiments are initially performed without mono-ethylene glycol (MEG), and then repeated with 50 wt% of MEG in the aqueous phase to analyze the effects of MEG presence on entrainment. MEG is a commonly used inhibitor in oil industry, applied to avoid hydrate formation in offshore systems. Its impacts on multiphase flow droplet entrainment are investigated.The experimental range of this study covers superficial gas velocity (vSg) values from 17 to 23m/s, superficial liquid velocity (vSL) values of 0.01 and 0.02m/s, and inlet liquid stream aqueous phase fraction (APFin) values between 0 to 100%. Similar test matrix is completed for both water and water and MEG solution as the aqueous phase. An isokinetic probe system is used to measure the entrained droplet flux at different vertical positions in the gas phase. Liquid entrainment fraction is then estimated by means of volumetric averaging. The trends of the data with respect to input parameters are investigated.The two and three-phase entrainment fraction data are used for a correlation evaluation study. Performances of ten correlations for two-phase entrainment fraction are compared to the experimental data, and best performing correlations are identified. The correlation of Pan and Hanratty (2002) is modified in an effort to improve the entrainment fraction estimations. The predictions of the modified correlation are compared with the acquired data and datasets from the literature. In addition, a simple empirical correlation is proposed to predict liquid phase entrainment fraction for three-phase flow systems.

Influence of the Coriolis force on the structure and evolution of wind turbine wakes

In this study, large-eddy simulation combined with a turbine model is used to investigate the effect of vertical wind veer associated with the Coriolis force on the structure and evolution of wind turbine wakes. In order to isolate the Coriolis effect on the wake, two cases are considered. In the first case, atmospheric boundary-layer flow is driven by a geostrophic wind, including the effect of Earth's rotation and the Coriolis force. In the second case, the boundary-layer flow is unidirectional and is forced by an imposed pressure gradient. Both cases have the same mean horizontal velocity and turbulence intensity at the hub height. The simulation results show that the Coriolis force significantly affects the aerodynamics of the wake including the mean velocity deficit, turbulence statistics, and wake-meandering characteristics downwind of the turbine. In particular, when the flow is forced by a geostrophic wind, vertical wind veer causes a skewed spatial structure in the wake. Moreover, the presence of lateral wind shear, in addition to the vertical one, enhances the shear production of turbulent kinetic energy and the turbulent momentum flux. This leads to a larger flow entrainment and, thus, a faster wake recovery compared to the case forced by unidirectional pressure gradient. Consistent with this result, wake meandering is also stronger in both lateral and vertical directions in the case of geostrophic forcing compared to the case with pressure-gradient forcing.

Effect of finite width cavity in axisymmetric supersonic flow field

In this research, the effect of finite width cavities in supersonic flow field is experimentally investigated. The test facility consists of a supersonic nozzle, which provides flow Mach number of 1.9. A circular cross-sectional test section is fastened at the exit of the nozzle. Cavities are incorporated in the test section at a distance of 20 mm from the inlet. Cavities of constant length and width, and varying depth are used for the study. Entrainment of flow in the main stream is observed immediate downstream of the cavity aft edge due to three dimensional effect. As the depth of the cavity is increased, residence time of the fluid as well as the mixing characteristics are enhanced and stagnation pressure loss also increased. Twin cavities are also arranged symmetrically, which further leads to an improvement in mixing with marginal rise in stagnation pressure loss.

Scalar dissipation rate and scales in swirling turbulent premixed flames

Simultaneous Rayleigh scattering and OH LIF imaging measurements of temperature and OH were used to investigate the properties of turbulent premixed flames, including the nature of the 2D thermal structures and scalar dissipation rate in the Cambridge/Sandia swirling bluff body stabilized flames, with and without the effect of swirl. Swirl creates enhanced turbulence as well as outer flow entrainment, and disrupts the pre-flame zone significantly, whilst the high temperature reaction zone as marked by OH remains relatively intact. In particular, the temperature at the location of maximum OH gradient shows very low variance across the flame region.The 2D image analysis of OH and temperature shows that the corresponding 2D gradients are aligned up to a distance of half the laminar flame thickness away from the flame front, deviating significantly in the case of swirling flames beyond that region. As in previous investigations in diffusion flames, the mean width of the observed thermal structures increases from 300 to 600 µm near the flame, with a main mode around the laminar flame thermal width in the unswirled case. The correlation between 2D thermal dissipation and variance of the reaction progress variable extracted from the images shows a direct proportionality, with a slope which agrees well with theory in the region of high turbulence away from the base. At the base of the flame where turbulence is low, the local scalar dissipation becomes a function of the local temperature via the thermal diffusivity.

Numerical and experimental investigation of surface vortex formation in coolant reservoirs of reactor safety systems

Abstract The reliable operation of the emergency coolant pumps and passive gravitational injection systems are an important safety issue during accident scenarios with coolant loss in pressurized water reactors. Because of the pressure drop and flow disturbances surface vortices develops at the pump intakes if the water level decreasing below a critical value. The induced swirling flow and gas entrainment lead to flow limitation and to pump failures and damages. The prediction of the critical submergence to avoid surface vortex building is difficult because it depends on many geometrical and fluid dynamical parameters. An alternative and new method has been developed for the investigation of surface vortices. The method based on the combination of CFD results with the analytical vortex model of Burgers and Rott. For further investigation the small scale experiments from the Institute of Nuclear Techniques of the Budapest University of Technology and Economics are used which were inspired from flow limitation problems during the draining of the bubble condenser trays at a VVER type nuclear power plants.

Friction measurements with yoghurt in a simulated tongue-palate contact

The perception of many food attributes is related to mechanical stimulation and friction experienced in the tongue-palate contact during mastication. Friction in the tongue-palate is determined by the changing film properties (composition, component distribution, thickness) in the conjunction. We suggest this evolution is essentially determined by tongue-palate film loss rather than shear flow entrainment which predominates in conventional bearing lubrication. The paper reports friction measurements in a simulated tongue-palate contact for a range of high and low fat dairy foods. A reciprocating, sliding contact with restricted stroke length (<contact width) was used; under these conditions there is negligible shear-entrainment of fluid from outside the contact area. The tongue-palate contact was simulated by a PDMS ball and glass surface. The effect of hydrophobic and hydrophilic surfaces on friction was investigated for different fat contents (0, 4.2, 9.5%wt fat). Friction was measured over 60s of rubbing. Significant differences were observed in the friction change with time for different fat contents (μ 9.5<μ 4.2<μ 0wt%) and for different surface energy conditions (μ hydrophilic<μ hydrophobic). Post-test visualisation of the rubbed films showed that low friction coefficient was associated with the formation of a thin oil film on deposited particulate solids.

Multi-particle dispersion during entrainment in turbulent free-shear flows

Multi-particle dispersion is studied using direct numerical simulations of temporally evolving mixing layers and planar jets for tetrahedra consisting of four fluid particles which are seeded in the turbulent regions or in the non-turbulent regions near the turbulent/non-turbulent interface (TNTI). The modified Richardson law for decaying turbulence is observed for particle pairs. The size dependence of the mean and relative motions of the entrained tetrahedra indicates that the characteristic length scale of the entrained lumps of fluid is approximately 10 times the Kolmogorov microscale. When the tetrahedra move within the TNTI layer they are flattened and elongated by vortex stretching at a deformation rate that is characterized by the Kolmogorov time scale. The shape evolutions of the tetrahedra show that in free-shear flows, thin-slab structures of advected scalars are generated within the TNTI layers.

A bidirectional valveless piezoelectric micropump with three chambers applying synthetic jet

A new type of valveless piezoelectric micropump is presented. Synthetic jet and Coanda effect are utilized to achieve larger and bidirectional flow rate. The numerical simulation applying the velocity and pressure boundary conditions as well as the SST turbulence model were utilized to research the performance and internal flow state of the micropump. The simulation method was tested by the previous experimental data and the results matched well. The results suggest that the flow rate of the micropump is related to the Reynolds number and frequency. The entrainment flow rate of synthetic jet accounts for over 80% of the total outflow rate. The outflow rate is much larger than the volume change of the micropump chambers. There is an optimal frequency to obtain the maximum flow rate regarding the volume change of the chambers as a constant. The fluctuation of the flow rate decreases with the increase of frequency. When the frequency is higher than 25 Hz, the outflow can be continuous. Working at the Reynolds number of 1000 and optimal frequency of 50 Hz, the flow rate is 6.8 ml/min.

Leading-edge vortex burst on a low-aspect-ratio rotating flat plate

Leading-edge vortex burst on a rotating flat plate wing was investigated experimentally using stereoscopic particle image velocimetry. Vortex burst was found to occur near the point of maximum lift production and was characterized by a concentration of opposite-sign vorticity entrainment and axial flow stagnation within the vortex core.

A mechanistic model of bubble entrainment in turbulent free surface flows

A framework for the development of models of bubble entrainment in free surface turbulent flows is presented. The framework uses mechanistic processes to model the stages involved in the entrainment of bubbles due to interaction of turbulence with a free surface. Entrainment is modeled as a chain of events for a bubble that is formed at the free surface, then pulled into the fluid against buoyancy, and interacts with vortices that break it up into smaller bubbles, or with other bubbles by coalescence. The main entrainment mechanism is modeled as a vortex/free surface interaction process that can entrain bubbles if the vortices located a given distance from the surface are strong enough. This approach overcomes limitations of approaches where the entrainment is determined only by turbulence parameters, which in the case of objects interacting with a free surface entrain bubbles on the boundary layers irrespective of the distance to the free surface. Depending on the computational fluid dynamics approach used to solve the flow, these processes may need different levels of modeling; more resolved approaches like large-eddy simulation with a volume of fluid method of the free surface will require less modeling complexity than a less resolved RANS method, since some of the involved processes of entrainment are directly accounted for. In this paper a standard RANS approach is used with the free surface modeled using a single-phase level set method, and models are presented for each of the relevant processes to produce a complete mechanistic model of turbulent bubble entrainment. The model was calibrated and tested for two relevant problems: a 2D+T breaking wave in model scale, and the full scale bubbly flows around the US Navy Research Vessel Athena. For the second case a grid study is carried out to analyze grid convergence performance of the model. Comparisons with experimental data show that the model predicts well location and magnitude of entrainment.

Entrainment in a compressible turbulent shear layer

Direct numerical simulations (DNS) of temporally evolving shear layers have been performed to study the entrainment of irrotational flow into the turbulent region across the turbulent/non-turbulent interface (TNTI). Four cases with convective Mach number from 0.2 to 1.8 are used. Entrainment is studied via two mechanisms; nibbling, considered as vorticity diffusion across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. The mass flow rate due to nibbling is calculated as the product of the entrained mass flux with the surface area of the TNTI. It is found that increasing the convective Mach number results in a decrease of the average entrained mass flux and the surface area of the TNTI. For the incompressible shear layer the local entrained mass flux is shown to be highly correlated with the viscous terms. However, as the convective Mach number increases, the mass fluxes due to the baroclinic and the dilatation terms play a more important role in the local entrainment process. It is observed that the entrained mass flux is dependent on the local dilatation and geometrical shape of the TNTI. For a compressible shear layer, most of the entrainment of the irrotational flow into the turbulent region due to nibbling is associated with the compressed regions on the TNTI. As the convective Mach number increases, the percentage of the compressed regions on the TNTI decreases, resulting in a reduction of the average entrained mass flux. It is also shown that the local shape of the interface, looking from the turbulent region, is dominated by concave shaped surfaces with radii of curvature of the order of the Taylor length scale. The average entrained mass flux is found to be larger on highly curved concave shaped surfaces regardless of the level of dilatation. The mass fluxes due to vortex stretching, baroclinic torque and the shear stress/density gradient terms are weak functions of the local curvatures on the TNTI, whereas the mass fluxes due to dilatation and viscous diffusion plus the viscous dissipation terms have a stronger dependency on the local curvatures. As the convective Mach number increases, the probability of finding highly curved concave shaped surfaces on the TNTI decreases, whereas the probability of finding flatter concave and convex shaped surfaces increases. This results in a decrease of the average entrained mass flux across the TNTI. Similar to the previous works on jets, the results show that the contribution of the engulfment to the total entrainment is small for both incompressible and compressible mixing layers. It is also shown that increasing the convective Mach number decreases the velocities associated with the entrainment, i.e. induced velocity, boundary entrainment velocity and local entrainment velocity.

Two-dimensional flow effects on soot formation in laminar premixed flames

Experimental measurements on axisymmetric laminar premixed flames have been used extensively for chemical and soot model validation. Numerical simulations of these flames always rely on the assumption of one-dimensionality. However, the presumed one-dimensionality has not been justified in general, and may not be valid under all circumstances. In the current work, two-dimensional flow effects are investigated in four representative ethylene/air laminar premixed flames, which have been selected as validation targets for the International Sooting Flame workshop. These flames cover all typical experimental arrangements, namely stabilizing plate, steel plate with centered hole, and enclosed chamber. To assess the assumption of one-dimensionality, detailed numerical simulations with finite-rate chemistry are performed with the exact experimental set-ups. It is shown that flow entrainment and acceleration are significant for all four flames. Further, it is found that the flame centerlines cannot be approximated as one-dimensional, since the mass flow rates vary substantially along the centerlines. As a consequence, non-negligible differences are found between the soot profiles predicted in two-dimensional simulations and in simulations where one-dimensionality is assumed. Using data extracted from the two-dimensional simulations, a modified one-dimensional model is derived on the flame centerline to include two-dimensional effects. Results from the modified one-dimensional model are compared against detailed, two-dimensional simulation results and experimental measurements.

The integral characteristics of the deceleration and entrainment of water droplets by the counter flow of high-temperature combustion products

This paper presents the results of experimental investigations on the deceleration and entrainment of water droplets during their motion in the counter flow of high-temperature combustion products (up to 1900K). High-speed video cameras (105 frames per second), specialized software applications (with continuous tracking functions), as well as panoramic optical methods (Particle Image Velocimetry, Stereoscopic Particle Image Velocimetry, Particle Tracking Velocimetry, Shadow Photography) registered the processes under study. We used several typical oils, gasoline, kerosene, acetone, and industrial alcohol to generate combustion products with a controlled high temperature. The initial sizes (radius) of droplets and their velocities were varied from 0.05mm to 0.35mm and from 0.5m/s to 5m/s. The velocities of counter motion of combustion products were varied from 0.1m/s to 2.5m/s. In this paper we have also determined the characteristic trajectory length of the droplets of different sizes until their complete stop (and subsequent entrainment) in the counter flow of high-temperature gases. As a result of studies, we summarized the research results through the establishment of Weber and Reynolds numbers ranges for droplets and gases, when the full stop and entrainment of droplets may occur. This paper also covers a comparison of the characteristics of water droplet deceleration under the conditions of non-stationary (when the temperature of combustion products varies from 1900K to 400K in channel) and nearly stationary (when the temperature is 1100±30K) heat transfer. Thus, it has been found out that the values of these parameters correlate well (deviation did not exceed 7%) under such conditions (stationary and nearly stationary) during short heating (less than 0.5s).

A method for estimating the bed-sediment entrainment in debris flow

A method for estimating the bed-sediment entrainment in debris flow

Studies of Two-Phase Flow at a Chute Aerator with Experiments and CFD Modelling

The chute aerator of a spillway is a structure in such a sense that air is, in the intense emulsification, entrained into the high-velocity water flow. Correctly predicting the air entrainment and two-phase flow pattern at the aerator would contribute to reliable spillway operation. Based on experimental data, 2D numerical simulations are preformed to predict streamwise air concentrations in the aerated flow, in which a two-fluid model is used. Depending on the air bubble size, relatively good agreement is seen with the experiments in the air cavity zone. The simulations give rise to higher air concentration downstream of the cavity, which is presumably due to underestimation of the interfacial forces in the two-fluid model.

The Emptying-box problem with a baffle of different porosity percentages

This research studies the Emptying-box problem with a porous baffle just behind the entrance opening. A theoretical model is extended to include the effect of a porous baffle, and the theoretical model taking account of porosity establishes the connections between two typical unidirectional displacement flow types. The salt-bath technique is employed to conduct the experiments using an acrylic reduced-scale model. Dye attenuation technique is used to analyze the light intensity data derived from the recorded images of experiments. According to the baffle porosity percentage (ϕ), the experiments are categorized into two series, EM(79%) and EM(60%). Each series respectively includes three different opening area ratios, which are 1, 0.5 and 0.33. Experimental results show that emptying processes for the cases with a baffle of the larger porosity percentage consist of emptying the dense layer and emptying the mixed layer, but there is only one process of emptying the dense layer for the cases with a baffle of the smaller porosity percentage. Experimental results are in reasonable agreement with the extended theoretical model developed in this research. Two extreme cases, those with and without an impenetrable baffle, are included in this paper to represent two typical flow types of the horizontal inflow denoted as EM(H), as that with ϕ=0%, and the vertical inflow denoted as EM(V), as that with ϕ=100%, i.e. the unidirectional classical displacement flow and the displacement flow with interfacial mixing. As the porosity percentage increases, the emptying time for the dense layer decreases, but the emptying time for the whole box tends to increase. The emptying time for the dense layer decreases, when the total effective opening area or the reduced gravity increases for the cases having the same porosity percentage. The initial interfacial height of the mixed layer increases, as the baffle porosity percentage increases for the cases having the fixed total effective opening area, or the total effective opening area increases for the cases with the same baffle porosity percentage. The initial buoyancy of the mixed layer is dependent on the penetrative entrainment flow rate from the dense layer to the mixed layer and the emptying time for the dense layer. As the total effective opening area or the porosity percentage increases, experimental results show that the initial buoyancy of the mixed layer tends to increase, and the penetrative entrainment flow rate increases as well, but the emptying time for the dense layer decreases.

Modeling of a concentrated-solar, falling-particle receiver for ceria reduction

This paper presents a steady-state model coupling three-dimensional, spectral radiation heat-transfer to a quasi-one-dimensional particle flow model for assessing performance of a directly-irradiated, falling-particle solar receiver with a closed cavity for thermochemical reduction of ceria. A semi-empirical gas-phase flow entrainment model captures the heat and mass exchange between the surrounding gas and the reducing ceria particles. With bulk and surface thermochemistry for the oxide-ion transport and surface reduction kinetics, model results indicate that for particle diameters under 500μm, surface chemistry controls the rate of ceria reduction in the receiver-reactor. For the range of particle inlet temperatures, flow rates, and diameters studied, the degree of ceria reduction correlates well with particle outlet temperature and increases to as high as 6% at maximum outlet temperatures of 1900K. However, with the relatively low absorptivity of ceria, higher outlet temperatures lower the fraction of solar energy absorbed by the particles from approximately 30% at outlet temperatures of 1500K to just above 10% at outlets of 1900K. Most of the heat recovered in the ceria is due to sensible heating, and re-radiation losses can account for as much as 60–75% of the solar energy input due to the required high reduction temperatures and radiation properties of the ceria particles. The predicted low efficiencies show that ceria particle reduction must utilize significant heat recovery and alternative receiver optics other than simple direct radiation to improve the feasibility of solar-driven ceria redox cycles based on falling particle receivers.

Estimating rates of debris flow entrainment from ground vibrations

AbstractDebris flows generate seismic waves as they travel downslope and can become more dangerous as they entrain sediment along their path. We present field observations that show a systematic relation between the magnitude of seismic waves and the amount of erodible sediment beneath the flow. Specifically, we observe that a debris flow traveling along a channel filled initially with sediment 0.34 m thick generates about 2 orders of magnitude less spectral power than a similar‐sized flow over the same channel without sediment fill. We adapt a model from fluvial seismology to explain this observation and then invert it to estimate the level of bed sediment (and rate of entrainment) beneath a passing series of surges. Our estimates compare favorably with previous direct measurements of entrainment rates at the site, suggesting the approach may be a new indirect way to obtain rare field constraints needed to test models of debris flow entrainment.

Computational fluid dynamic simulations on liquid film behaviors at flooding in an inclined pipe

The complex liquid film behaviors at flooding in an inclined pipe were investigated with computational fluid dynamic (CFD) approaches. The liquid film behaviors included the dynamic wave characteristics before flooding and the transition of flow pattern when flooding happened. The influences of the surface tension and liquid viscosity were specially analyzed. Comparisons of the calculated velocity at the onset of flooding with the available experimental results showed a good agreement. The calculations verify that the fluctuation frequency and the liquid film thickness are almost unaffected by the superficial gas velocity until the flooding is triggered due to the Kelvin–Helmholtz instability. When flooding triggered at the superficial liquid velocity larger than 0.15m·s−1, the interfacial wave developed to slug flow, while it developed to entrainment flow when it was smaller than 0.08m·s−1. The interfacial waves were more easily torn into tiny droplets with smaller surface tension, eventually evolving into the mist flow. When the liquid viscosity increases, the liquid film has a thicker holdup with more intensive fluctuations, and more likely developed to the slug flow.

Comparative study on effect of air–water and steam–water mediums on liquid entrainment through ADS-4 in AP1000

For the previous experiments of entrainment at T-junction, the air–cold water two-phase flow is always selected as working medium, and the theoretical models of liquid entrainment are established mainly based on the experimental data of air–water entrainment through a small break. The saturated steam–water entrainment experiments are gravely insufficient, especially the steam–water experiments of entrainment through a large size branch are even fewer. Hence, a scale-down experiment of steam–water entrainment at T-junction has been conducted referring to liquid entrainment through ADS-4 (Fourth Stage Automatic Depressurization System) in AP1000 nuclear plant. The steam–water experimental results are comprehensively compared with the results of the previous similar air–water experiment: The visualization research shows that the same entrainment phenomena can be observed including intermittent flow entrainment, stratified flow entrainment and annular flow entrainment; The transition conditions between the two entrainment phenomena have a little difference but can be negligible; The comparison of experimental data of entrainment onset and stable entrainment indicates that the experimental data of steam–water and air–water have the same change tendency, and the air–water correlation can agree well with the steam–water data. Hence, we can make a conclusion that effect of different working mediums on liquid entrainment is negligible. In addition, based on the analysis of model formation, we suppose that effect of the system pressure on entrainment can be neglected under 0.22MPa.