Flow Entrainment Research Articles

Experimental investigation of entrainment effect on the countercurrent flow in the Hot Leg and Pressurizer Surge Line assembly of third-generation passive nuclear reactors

Abstract The effect of entrainment from the Hot Leg (HL) to the Pressurizer Surge Line (PZR-SL) on the characteristics of Countercurrent Flow (CCF), is experimentally studied using air-water in a transparent test section, 1/4 scaled-down model of the HL and PZR-SL assembly of AP1000. The typical CCF behaviors are recorded with video cameras for understanding of mechanisms of Countercurrent Flow Limitation (CCFL) affected by entrainment. Two entrainment flow regimes, stratified-flow entrainment and slug-flow entrainment, are found to occur under certain HL pipe water levels and air velocity conditions. The entrainment effect on the characteristics of CCF in the test section is analyzed in detail based on the two-phase air and water Kutateladze numbers, the transient hydraulic parameters and the flow behaviors. The stratified-flow entrainment does not always influence CCFL extent, unless the entrainment is strong enough and reverse flow of water occurs in the PZR-SL pipe. However, the slug-flow entrainment markedly influences CCFL extent. Two empirical correlations are developed respectively for high HL pipe water level situation with slugging entrainment and for medium and low HL pipe water level situation without slugging entrainment. The present data are compared and verified with the Small-Break Loss-of-Coolant Accidents (SB-LOCAs) tests data on AP600 Scaled Integral Test Facility (APEX). The present experimental research provide insight on the CCF flow phenomena in the HL and PZR-SL assembly for the third-generation passive nuclear reactors during the Fourth Stage Automatic Depressurization Stage (ADS-4) of SB-LOCAs and support theoretical model development and the validation of computational codes.

Hydrodynamic Drivers of Juvenile-Salmon Out-Migration in the Sacramento River: Secondary Circulation

The entrances to the two lowest-survival migration routes for juvenile Chinook salmon in the tidal Sacramento River are located in the outside of a river bend where secondary circulation occurs. Three-dimensional simulations are conducted, in the Eulerian and Lagrangian frame, to understand tidal and secondary circulation effects on salmon migration route selection within this river reach. Fish were assumed to behave as neutrally-buoyant particles. Findings show that simulated particle entrainment rates into these routes tend to be larger than those expected from flow entrainment calculations alone, particularly during ebb tides, due to several factors. First, the fraction of the flow diverted to these routes tends to be higher in the shallowest layers, as a result of the secondary circulation that develops in the main river. Second, and supporting previous work done at the study site, the secondary circulation acting upstream also causes the surface-biased salmon distribution to be skewed toward the outside of the bend as they approach the entrance to the migration routes. As a result of these effects, the risk of particles being entrained is maximal near the surface, remaining higher than 50% during the course of a tidal cycle.

Stability of boundary layer flow based on energy gradient theory

The flow of the laminar boundary layer on a flat plate is studied with the simulation of Navier–Stokes equations. The mechanisms of flow instability at external edge of the boundary layer and near the wall are analyzed using the energy gradient theory. The simulation results show that there is an overshoot on the velocity profile at the external edge of the boundary layer. At this overshoot, the energy gradient function is very large which results in instability according to the energy gradient theory. It is found that the transverse gradient of the total mechanical energy is responsible for the instability at the external edge of the boundary layer, which induces the entrainment of external flow into the boundary layer. Within the boundary layer, there is a maximum of the energy gradient function near the wall, which leads to intensive flow instability near the wall and contributes to the generation of turbulence.

The effect of heat release on the entrainment in a turbulent mixing layer

Direct numerical simulations of a temporally evolving compressible reacting mixing layer have been performed to study the entrainment of the irrotational flow into the turbulent region across the turbulent/non-turbulent interface (TNTI). In order to study the effects of heat release and interaction of the flame with the TNTI on turbulence several cases with different heat release levels,$Q$, and stoichiometric mixture fractions are chosen for the simulations with the highest opted value for$Q$corresponding to hydrogen combustion in air. The combustion is mimicked by a one-step irreversible global reaction, and infinitely fast chemistry approximation is used to compute the species mass fractions. Entrainment is studied via two mechanisms: nibbling, considered as the vorticity transport across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. As the level of heat release increases, the total entrained mass flow rate into the mixing layer decreases. In a reacting mixing layer by increasing the heat release rate, the mass flow rate due to nibbling is shown to decrease mostly due to a reduction of the local entrainment velocity, while the surface area of the TNTI does not change significantly. It is also observed that nibbling is a viscous dominated mechanism in non-reacting flows, whereas it is mostly carried out by inviscid terms in reacting flows with high level of heat release. The contribution of the engulfment to entrainment is small for the non-reacting mixing layers, while mass flow rate due to engulfment can constitute close to 40 % of the total entrainment in reacting cases. This increase is primarily related to a decrease of entrained mass flow rate due to nibbling, while the entrained mass flow rate due to engulfment does not change significantly in reacting cases. It is shown that the total entrained mass flow rate in reacting and non-reacting compressible mixing layers can be estimated from an expression containing the convective Mach number and the density change due to heat release.

The thermal and hydrodynamic behaviour of confined, normally impinging laminar slot jets

Jet impingement cooling is an effective means of inducing high convective heat transfer coefficients for applications such as the cooling of gas turbine surfaces, drying of textiles, and the tempering of metal and glass. Slot jets can be used for applications which require cooling over a line or strip, and these can be realized at the micro-scale if size is constrained. In this paper the application of these jets is of specific interest for high-density photonic integrated circuits (PICs), which can generate device-level heat fluxes as high as 1kW/cm2. In this context, an understanding of the thermal and hydrodynamic behaviour of low Reynolds number, submerged, confined and normally impinging slot jets is currently unavailable in the literature and would be beneficial for the design of micro-scale cooling systems. This investigation utilized the isoflux heated foil technique and Particle-Image Velocimetry (PIV), to measure the single-phase convective heat transfer and velocity fields associated with confined liquid slot jets. The aspect ratios of the jets were varied from 1 to 8, with a constant hydraulic diameter, in order to determine the influence of aspect ratio on the measured parameters. The study was carried out over a jet exit Reynolds number range of 100–500, and a fixed confinement ratio of H/Dh=1. The results showed that, enhancements of up to 68% in area-averaged heat transfer could be achieved through increasing slot jet aspect ratio, with a corresponding 12% reduction in head loss coefficient. The appearance of off-center peaks in the velocity and Nusselt number distributions were also observed. These peaks were concluded to be as a result of the stagnation zone fluid dynamics and local flow entrainment.

Flame extension lengths beneath a confined ceiling induced by fire in a channel with longitudinal air flow

This paper investigates the length of flame extension for an impinging flame underneath the confined ceiling in a channel with longitudinal air flow. Previous works in this field have been primarily concerned with un-confined ceilings and no forced air flow conditions. Under longitudinal air flow conditions, the flame extension beneath the channel ceiling is non-symmetrical, that is, different in the upstream and downstream directions from the fire source. In this study, experiments were carried out with two channeled ceilings with widths of 1.5 m and 0.5 m. Square porous gas burners of different sizes were employed as the fire source, using propane as fuel, with various heat release rates and source-ceiling heights. The flame extension lengths beneath the ceiling, both upstream and downstream from the fire source, were measured. Their difference as well as their total length was quantified for different magnitudes of forced longitudinal air flow along the channel. Results show that the flame extension lengths beneath the ceiling increases with heat release rate, but decreases with source-ceiling height, channel width, burner size or longitudinal air flow speed. With a longitudinal air flow, the flame extension is longer downstream than upstream. Non-dimensional correlations are proposed for the flame extension lengths (upstream, downstream and their total length), based on the unburnt fuel distribution upstream and downstream, as well as considering air entrainment of the ceiling flow, which further consumes the unburnt fuel along the ceiling. These correlations are shown to fit the data well.

Flow fields and particle trajectories beneath a tidal bore: A numerical study

Tidal bores may appear in some estuaries when the tides quickly reach a high level. This phenomenon is rare but has a strong impact during its short duration: i.e. the river bed is significantly eroded and sediments are then transported. In this paper, the trajectories of suspended particles induced by this flow are numerically studied. Four undular bores with Froude numbers between 1.1 and 1.2 are studied. Despite similar Froude numbers, various initial flow conditions were selected to produce or not an inversion of the flow direction during the bore passage. The particle trajectories associated with each distinct flow configuration are presented and analyzed. These trajectories, estimated by solving the Maxey-Riley equation, appear to be very different even though the Froude numbers of flows are similar. These observations are important because the Froude number is often used to characterize a tidal bore as it describes well the free surface, however, it cannot describe the sediment transport. Finally, Chen's model of wave-current interactions is adapted to fit the cases studied and is applied to the four bores simulated. The results highlight that this latter model can reproduce the observed trajectories and dissociate their different components. From this model, it is shown that the inertial and Basset history effects can be neglected compared to the gravity and flow entrainment effects due to the viscous drag when one wants to determine the long-term trajectories of suspended particles.

Development of correlations for liquid entrainment through a large-scale inclined branch pipe connected to the main horizontal pipe

Many theoretical and experimental studies have been conducted over the years on liquid entrainment through a branch pipe connected to the main horizontal pipe (i.e., entrainment at the T-junction). Several mathematical models have been developed to predict the onset of liquid entrainment and branch quality during the liquid entrainment process with reasonable accuracy. Most of these models are developed based on small-scale branch, where the ratio of the branch diameter to the horizontal pipe diameter is less than or equal to 0.2 (d/D ≤ 0.2). Therefore, these models are not suitable to predict the onset of liquid entrainment through a large-scale branch. In addition, these models are developed for a specific branch angle (typically 0 or 90°) and therefore, are incapable of making good predictions of the onset of liquid entrainment through a large-scale branch inclined at other angles. In order to address these issues, we conducted experiments to investigate the onset of liquid entrainment through a large-scale branch pipe (d/D > 0.6) inclined at three angles (32.2, 47.9, and 62.3°) and we assessed the mass flow quality of the branch. By analyzing the mathematical models for liquid entrainment currently available and by theoretical deduction, we developed new correlations to predict the onset of liquid entrainment and mass flow quality. We found that the values predicted by these correlations fit well with the experimental data with a maximum error of ±35%. Both of these liquid entrainment correlations provide good predictions of liquid entrainment through a large-scale branch inclined at an angle of 32.2, 47.9, and 62.3°. We also obtained some meaningful conclusions based on the experimental data. We found that at the onset of liquid entrainment, under similar experimental conditions, the critical Froude number Frg decreases with an increase in the branch angle. In addition, the larger the branch angle, the lower the effect of the branch scale on the onset of liquid entrainment. There are two primary factors that influence the liquid entrainment process: (1) gas chamber height—the liquid entrainment process is promoted as the gas chamber height decreases, and (2) vertical component of the inertial force—the liquid entrainment process is weakened with a decrease in the vertical component of the inertial force. This indicates that the gas chamber height dominates the liquid entrainment process at relatively large branch angles whereas the vertical component of the inertial force dominates the liquid entrainment process at small branch angles.

Experimental investigation of air entrainment by vertical plunging liquid jet

This paper presents experimental results and predictive model on the air entrainment flow rate due to the vertical plunging liquid jet. Three types of fluids with different physical properties were utilized as a working fluid for the experiment. Air entrainment flow rate was measured using soap meniscus method and various geometrical parameters including nozzle diameter (13–16 mm), fall height (105–210 mm), pool depth (30–180 mm), and angle of the bottom pool surface (0–75°) were changed to investigate the effects on air entrainment rate. Plunging tube was installed around the falling jet and impinging point. For the current experimental conditions, it was found that the air entrainment rate depends on the product of Weber number and Laplace length scale, and new predictive model on air entrainment ratio, defined as the entrained air flow rate divided by impinging jet flow rate, is proposed. The proposed model is capable of predicting air entrainment ratio at a mean absolute relative deviation of 15.9% at the given experimental conditions.

Run, Wander, and Drift: Multi-modal Transport in the Eukaryotic Cytoplasm

Eukaryotic cells utilize a variety of mechanisms to transport particles of different sizes throughout the cytoplasm. Commonly employed transport modes include diffusion driven by stochastic fluctuations in the medium, processive motor-driven transport along cytoskeletal tracks, and advection in a flowing cytoplasmic fluid. We use analytical theory and simulations, grounded in the physics of stochastic processes and fluid dynamics and coupled with analysis of in vivo data on organelle motion, to explore the efficiency and relative contributions of these different modes of transport. Using several example cellular systems, we identify both the physical limits and synergistic nature of multi-modal transport, where individual organelles switch between active and passive regimes. In particular, we highlight the potential importance of hydrodynamic entrainment and large-scale cytoplasmic flow associated with cell deformation to the dispersion of vesicles within fungal hyphae and motile leukocytes, respectively. Peroxisome transport in hyphae is shown to lie in a regime where both passive diffusion and directed “hitch-hiking” runs contribute substantially to the organelles' ability to efficiently find intracellular targets. We also show that spatial localization of neuronal mitochdria by glucose-dependent regulation of transport is sensitive to intracellular glucose levels in a manner dependent on the balance of mitochondrial transport modes. These results highlight the importance of combining together a variety of passive and active transport mechanisms to efficiently distribute, deliver, and organize organelles within eukaryotic cells.

Numerical simulation of 2D granular flow entrainment using DEM

To understand the entrainment process in granular flow, numerical experiments have been conducted using a Discrete Element Method model. A flow channel of 8 m long with $$15^\circ $$ slope is setup with monitoring points located in an erodible bed. Particles, ranging from 3 to 4 mm in diameters, are used in the simulations. In the simulations, translational, rotational and average velocities, total volume, shear stresses are calculated in the measurement circles. The sizes of the measurement circles have been varied to see their effects on the results. It is found the minimum size of the measurement circles should include 20–30 particles. An new analytical model has been developed to calculate entrainment in granular flow. Results of the numerical experiment are compared with analytical model. Shear stresses at the interface between flowing particles in motion and the immobile particles in the channel bed, change of depth of erosion and entrainment rate are used to verify the analytical model. It is found that the calculated shear stresses in the PFC model agree well with the shear stresses calculated using Mohr–Coulomb frictional relationship in the analytical model. The calculated depth of erosion using the new analytical model is also compared with that from dynamic and static entrainment model. The results indicates that the analytical model is able to capture the mechanism of erosion and it can be used in granular flow analysis.

Spatio ante temporal structure of near field of fire plume and entrainment flow under condition of limited ventilation

Spatio ante temporal structure of near field of fire plume and entrainment flow under condition of limited ventilation

Kelvin–Helmholz instability in thermoviscous free shear flow

This paper is focused on the entrainment and mixing in the nonisothermal shear flow, wherein the velocity profile discontinuity arises due to viscosity–temperature relation, i.e. thermoviscosity. Kelvin–Helmholz instability generated at the interface paves the way to large-scale entrainment which is analysed in terms of perturbation growth rate and momentum thickness. The flow is simulated in the plane domain with periodic boundary conditions using second-order accuracy explicit CABARET numerical scheme in weakly compressible formulation. Initial instability evolution is fully governed by a universal scaling parameter kt, composed of Reynolds number Re and kinematic viscosity ratio Rν. There are seven flow configurations determined by nonlinear mode interaction, vorticity convection and diffusion and cascade vortex merger. The problem at issue appears to be analogues to the classical boundary layer theory that totally fortifies its remarkable features.

Experimental validation of pressure drop models during flow boiling of R134a – effect of flow acceleration and entrainment

A crucial step to assure proficient work of power and process apparatus is their proper design. A wide array of those devices operates within boiling or condensation of the working fluid to benefit from high heat transfer rates. Two-phase flows are associated with high heat transfer coefficients because of the latent heat of evaporation and high turbulence level between the liquid and the solid surface. Predicting heat transfer coefficient and pressure drop is a challenging task, and has been pursued by researchers for decades. In the case of diabatic flows, the total pressure drop is due to the change in kinetic and potential energy. The article presents detailed boiling pressure drops data for R134a at a saturation temperature of 19.4°C. Study cases have been set for a mass flux varying from 300 to 500 kg/m2s. Presented data along with the data reduction procedure was used to obtain the momentum pressure drop values during flow boiling. The study focuses on experimental values of momentum pressure drop component and its prediction based on various void fraction models and entrainment effects.

Flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations

The flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations were investigated experimentally. The transversely-oscillating plane jet was generated by a specially designed fluidic oscillator. Isothermal flow patterns were observed using the laser-assisted smoke flow visualization method. Meanwhile, the flame behaviour was studied using instantaneous and long-exposure photography techniques. Temperature distributions and combustion-product concentrations were measured using a fine-wire type R thermocouple and a gas analyzer, respectively. The results showed that the combusting transversely-oscillating plane jets had distributed turbulent blue flames with plaited-like edges, while the corresponding combusting non-oscillating plane jet had laminar blue-edged flames in the near field. At a high Reynolds number, the transversely-oscillating jet flames were significantly shorter and wider with shorter reaction-dominated zones than those of the non-oscillating plane jet flames. In addition, the transversely-oscillating combusting jets presented larger carbon dioxide and smaller unburned hydrocarbon concentrations, as well as portrayed characteristics of partially premixed flames. The non-oscillating combusting jets presented characteristics of diffusion flames, and the transversely-oscillating jet flame had a combustion performance superior to its non-oscillating plane jet flame counterpart. The high combustion performance of the transversely-oscillating jets was due to the enhanced entrainment, mixing, and lateral spreading of the jet flow, which were induced by the vortical flow structure generated by lateral periodic jet oscillations, as well as the high turbulence created by the breakup of the vortices.

A laboratory study of channel sidewall expansion in upland concentrated flows

Gully erosion contributes large amounts of sediment within watersheds around the world. Gully widening constitutes about 80% of total soil loss, especially in the presence of a plow pan which manifests a less or non-erodible soil layer. Current knowledge on sidewall toe scour (scour arcs) and tension crack processes in gully widening is limited. Thus, simulated channel sidewall expansion tests, where the channel bed was fixed to represent a non-erodible layer, were designed to investigate how inflow rate, slope gradient and initial channel width affect channel widening processes. Soil boxes (2.0 m-long, 0.3 m-wide and 0.5 m-deep) with two slope gradients (15° and 20°), four inflow rates (1.0, 2.0, 3.0 and 4.0 L min−1) and two initial channel widths (4 and 8 cm) were subjected to clear-water overland flow. Photogrammetry was used to detect tension crack and width variations of channels. The results show that sediment delivery and channel width increase with the increase of inflow rate, bed slope and the decrease of initial channel width. Exponential equations were used to predict the channel width time series. Time lag occurred between sediment peak and soil block failure. Toe scour, crack development, sidewall failure and block detachment and transport, in sequence, were the four main processes of channel widening. Basal scour arc length, tension crack length and width decreased with initial channel width and increased with time, flow discharge and bed slope. Basal scour arcs were divided into three patterns according to different shapes in comparison to the failure arcs. Sediment delivery equations based on the disaggregation of concentrated flow entrainment and mass failure were also fitted. This study provides new insight on improving gully erosion measurements and prediction technology.

Liquid droplet entrainment in two-phase oil-gas low-liquid-loading flow in horizontal pipes at high pressure

An experimental study is conducted in a 152.4-mm inner diameter (ID) high-pressure facility to investigate the liquid droplet entrainment in two-phase oil-gas flow in horizontal pipe under low-liquid loading condition. Total of 247 tests are carried out at five different operating pressures, namely, 1.48, 1.82, 2.17, 2.51, and 2.86 MPa. The test fluids are nitrogen and mineral oil (Isopar-L). Superficial oil velocity (vSL) ranges from 0.01 to 0.05 m/s, while superficial gas velocity (vSg) ranges from 1.6 to 16.7 m/s depending on the operating pressures. An isokinetic sampling system is used to measure the entrained droplet flux at three different vertical positions in the gas phase. A high-speed video camera is used to visually observe and estimate the onsets of entrainment. This paper provides a unique entrainment data set for two-phase flow in a large diameter pipe at high operating pressure.The experimental data are analyzed to evaluate the effects of high operating pressure and superficial velocities. It is found that the entrainment fraction increases with the operating pressure. In addition, predictions of ten correlations for two-phase entrainment fraction are compared with the experimental data to evaluate the performances. The results show that these correlations over-predict the entrainment fraction data from this study.

Characterization of water-air dispersed two phase flow

The analysis of two phase flow has a great relevance in many industrial sectors such as nuclear and process industry. The study and the measurement of the related phenomena are particularly difficult due to the variety of the parameters that affect the flow (void fraction, flow regime, orientation, etc.). Measurement instrumentation for two phase flow is nowadays very limited even if it would be highly useful in the industrial field. Before approaching the research and development toward the realization of innovative instrumentation for two-phase flow measurement, it is fundamental a precise description of the particular flow regime of interest. Dispersed water droplets in air/gas is a possible flow regime at high gas void fractions; therefore it is important to characterize this particular flow pattern in a detailed way. To reproduce this condition in a laboratory environment it is possible to use nozzles with very small outlet diameters and high pressure water supply. In this paper the characterization of dispersed flow is performed as function of the nozzle characteristic and water inlet pressure. The tests are performed using an experimental setup realized at the Energy Department at Politecnico di Torino. The water jet is observed in a PMMA (PolyMethylMethacrylate) pipe 1.8 meters long with an inner diameter of 78 mm. High pressure water is obtained using a plunger pump and pump inlet water pressure is adjustable in the range 1-4 bar. Water pressure upstream the nozzle and air entrainment flow rate are measured and used as primary parameters of the study. A sensitivity analysis on these two parameters is performed with the purpose to find the conditions that are optimal to reproduce a dispersed flow.

Flow thresholds for leaf retention in hydrodynamic wakes downstream of obstacles

AbstractLeaves are the major component of terrestrial litter input into aquatic systems. Leaves are distributed by the flow, accumulate in low flow areas, and form patches. In natural streams, stable leaf patches form around complex structures, such as large woody debris. Until now, little is known about flow conditions under which leaf patches persist. This study aims to quantify flow conditions for stable leaf patches and entrainment of leaf patches. We hypothesize that entraining flow processes, such as turbulence, Reynolds stress, or lift forcing (vertical flow velocity), best explain local leaf retention. This study was performed in an unscaled flume experiment, which conditions coincide with conditions found in low‐energetic lowland streams. We positioned a wooden obstacle perpendicular to the flow on the bed of the flume. A leaf patch was positioned downstream from the wooden obstacle. The experiment was performed under 5 flow conditions. We monitored leaf patch cover and near‐bed flow conditions in the area downstream of the wooden obstacle. We showed that near‐bed flow velocities explain leaf retention better than more complex flow velocity derivatives such as turbulence, Reynolds stress, and vertical flow velocity. The entrainment near‐bed flow velocity for leaves ranges from 0.037 to 0.050 m/s. Flow velocities frequently exceed those values, even in low‐energetic lowland streams. Therefore, complex structures, such as woody debris, create flow conditions to support stable leaf patches. Thus, adding instead of removing obstacles may be a key strategy in restoring biodiversity in deteriorated streams.

Comparison between hydraulic and structural based anti-vortex methods at intakes

Surface vortices are the main reason of flow instability and entraining air and debris into the intakes. Using structural based anti-vortex facilities is a common method for dissipating vortices over the intakes. In the present experimental study, performance of a new hydraulic based anti-vortex method is compared with available structural based anti-vortex methods over the horizontal intakes. In the new approach dynamically high-risk surface vortices are eliminated by imposing a hydraulic jet at the formed vortex zone over the intake. The injected jet acts as a point source of external momentum in order to change the hydrodynamic condition of the formed vortex flow. Results show that the investigated hydraulic based anti-vortex device is more flexible and more hydraulically efficient than the structural based anti-vortex methods, especially when the intake Froude number is higher than 1.3.