Flow Entrainment Research Articles

Drop entrainment in two-phase non concurrent film flow

When the motion vectors of the media are collinear, well-known experimentally obtained empirical criteria are used to assess the conditions for the drop entrainment by the gas flow from the surface of a moving liquid film. However, blowing a liquid film at an angle with gas causes a tangential component in the film movement, which is an additional factor affecting its thickness and the velocity profile. The paper presents the correctness of the criterion of the conditions for the drop entrainment and the effect of the noncollinearity of the vectors of the media counterflow on the dynamics of a liquid film estimated from the results of numerical simulation.

축류형 및 엘보우형 게이트펌프장 모델의 공기흡입 유동특성

The gate pump has recently received considerable attention as a type of effective drainage equipment. Air can be easily inhaled by the suction pipe, which could cause damage to the pumping system. Therefore, to improve the hydraulic performance of the gate pump, the flow behavior of the pump, including the pump-sump, must be investigated. This study focused on the investigation of the air entrainment and water flow behavior in the horizontal and elbow gate pump-sump models using a computational fluid dynamics analysis. A three-dimensional fluid domain of the horizontal and elbow gate pump models was constructed. Subsequently, the commercial code of ANSYS CFX was adopted to conduct numerical simulations. The unsteady simulations of the two-phase flow (air and water) were performed using the shear stress transport turbulence model. A total of six cases of numerical simulations were implemented. The details of the air-water two-phase flow characteristics in the pump-sump, including the pump models, were then investigated. The improvement of the air entrainment reduction using the elbow gate pump model was discussed.

Condensation, evaporation and boiling of falling films in wickless heat pipes (two-phase closed thermosyphons): A critical review of correlations

Abstract With the objective of estimating the thermal resistance of a two-phase closed thermosyphon (TPCT), the work presented is a state of the art of condensation and falling film evaporation/boiling correlations. A deep analysis of up-to-date reported equations dealing with falling film heat transfer is carried out and recommendations are provided to select the most suitable and reliable correlations. Mechanisms such as filmwise condensation and Nusselt theory, falling film development in the thermal entry length (TEL), film thickness, appearance of waves and turbulence, two-phase flow interaction and entrainment with the shear stress at the liquid-vapour interface, falling film evaporation and boiling, in addition to film breakdown, are comprehensively explained and discussed. For all sections, the advised correlations are reported in a clear and simple way using tables. The objective of this paper is to cover sufficient knowledge on the falling film dynamic in thermosyphons to ease the prediction of heat transfer coefficients. This paper can be taken as a starting point for thermosyphon users in the study of falling film heat transfer in wickless heat pipes.

Flow field evolution and entrainment in a free surface plunging jet

We investigate ambient fluid entrainment and near-field flow characteristics of a free surface plunging jet for five Reynolds numbers ranging from 3000 to 10000 using time-resolved stereo particle image velocimetry (SPIV). We present time-averaged velocities, RMS velocity fluctuations, mean entrainment and unsteady flow features and compare them with previous studies on free jets. We find that plunging jets have a smaller potential core length, and earlier decay of the mean centerline velocity. The peak RMS velocity fluctuations occur at a location significantly upstream compared to the free jets reported in the literature. Near-field ambient fluid entrainment of plunging jets is measured for the first time and is found to be considerably higher than free jets in the low Reynolds number range. For the plunging jet case at Re = 3000, faster jet decay, higher levels of turbulent intensity in the near-field, and augmented mass entrainment result from strong primary vortices that give the turbulent/non-turbulent interface (TNTI) its convoluted shape which facilitates both bulk entrapment of ambient fluid and small scale nibbling because of larger surface area. These primary vortices occur right below the free surface and disintegrate into secondary structures at axial locations that are upstream compared to those of free jets. At higher Reynolds numbers, primary vortices are smaller in size, weak in swirling strength, and disintegrate prematurely, resulting in suppressed mixing and reduced entrainment efficiency.

Impact of gasoline direct injection fuel films on exhaust soot production in a model experiment

The fuel films that can be generated during the injection process in gasoline direct injection engines are the most important factor in carbon particle mass and number. They also have an influence on combustion chamber and injector tip deposits. A model experiment was set up to study a liquid film, its evaporation, and combustion with soot generation on a metal plate in realistic engine conditions. The experiment was conducted in a dedicated constant volume vessel. A liquid fuel injection system (with injection pressures up to 100 bar) directs the spray onto a plate with a controlled temperature in the range of 80 °C–200 °C. The resulting liquid film and vaporization process were studied when subjected to interaction with a laminar spherical flame. A blend of four components was used as a gasoline surrogate. The liquid film spreading, thickness, and evaporation rates were initially measured in ambient conditions. Mie scattering and schlieren measurements in the chamber conditions returned a qualitative correlation of the vaporized area with the surface temperature. Fluorescence of the light and heavy fuel components was used to quantify the influence of the vaporization process on soot production. Simultaneous measurements of natural luminosity and KL factor were analyzed to understand the process of soot production. The results showed a critical wall temperature of 120 °C at which the maximum quantity of soot is generated, which can be due to the quantity and composition of fuel film in interaction with the entrainment flows generated during combustion.

Effects of Back Pressure on Flow Regime and Suction Performance of Gas–Liquid Swirl Ejector

Abstract The back pressure of a gas–liquid swirl ejector is a critical parameter that affects the flow regime and entrainment performance of the system. In the case of a weak swirl injection, the motive jet carries relatively higher axial momentum thus remains in a single phase and the liquids appear glassy. With increasing in the back pressure of the bubble flow, the suction rate drops rapidly. For a strong swirl injection, the liquid jet is disintegrated due to higher angular momentum and the spray is atomized. Augmentation in the back pressure also causes the reduction in the suction rate but it tends to grow more gradual than the weak swirl injection. As a result, the suction rate of the strong swirl is greater than that of the weak swirl in the majority of the back-pressure range. However, owing to the high transition pressure, only the weak swirl entrains the air in the bubble flow regime at low back pressure. The relationship between the suction and the swirl intensity is not fixed and is influenced by the back pressure.

Numerical Investigation of Jet-Wake Interaction for a Dual-Bell Nozzle

The turbulent wake of a planar generic space launcher equipped with a dual-bell nozzle is numerically investigated to examine the interaction of the dual-bell nozzle jet and the wake flow. The simulation is performed at transonic freestream condition, i.e., freestream Mach number $Ma_{\infty }= 0.8$ and freestream Reynolds number based on the launcher thickness ReD = 4.3 ⋅ 105, with the dual-bell nozzle operating at sea-level mode. A zonal RANS/LES approach is used and the time-resolved flow field data is analyzed by classical spectral analysis and modal decomposition techniques, i.e., proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The overall flow topology of the recirculation region downstream of the base and the pressure loads on the outer nozzle fairing are only slightly affected by the modified nozzle shape. However, the changed nozzle flow topology characterized by the flow separation at the nozzle contour inflection leads to a backflow region and an entrainment of the outer flow into the nozzle extension which results in increased pressure loads on the inner nozzle wall. Using spectral, POD, and DMD analyses, the outer wake flow is investigated, revealing a growing and contracting of the separation bubble and an undulating motion of the shear layer similar to the “cross-pumping” and “cross-flapping” motion detected in previous investigations of a configuration with a classical nozzle and a jetless backward facing step setup. The spectral and modal analysis of the nozzle flow shows that the increased pressure loads detected at the inner wall of the nozzle extension are caused by an interaction of the separated shear layer inside the nozzle extension with the shock pattern that leads to a streamwise oscillation of the shock and a pumping or wave-like motion of the shear layer.

Experimental investigation of droplet entrainment and deposition in horizontal stratified wavy flow

An experimental study of the local droplet parameters for an atmospheric horizontal stratified wavy flow was conducted in a horizontal circular pipe with an inner diameter of 40 mm and a length of 5.5 m. The local distributions of the droplet fraction, droplet velocity, droplet diameter, and droplet mass flux in the cross section of the pipe were measured using single optical fiber probes at four locations with length-to-diameter ratios of 17.5, 55.0, 80.0, and 105.0 from the inlet of the test section. The average droplet mass flow rate along the axial direction of the flow channel was also obtained from the droplet parameters. The combination of the droplet entrainment and deposition rate models of Schimpf et al. was evaluated for droplet mass flow rates obtained from five different experiments including the present experiment in the horizontal stratified flow. However, its prediction accuracy was not good for the present and REGARD data. Finally, new droplet entrainment and deposition rate models for the horizontal stratified flow were proposed based on those five experiments. The applicable ranges of the proposed models are 65,500–571,400 and 170–11,000 for the gas (Reg) and liquid (Rel) Reynolds numbers, respectively.

On the mechanism of symmetric vortex shedding

In this paper, we study the vortex-shedding mechanism of a square cylinder subjected to a mean flow with a superimposed pulsatile flow, under symmetric vortex shedding conditions. The near-body vortical events are interpreted in accordance with the two-dimensional vorticity transport equation, and the interactions between different vortical regions are quantified by circulation balance for a control volume in the near wake. Strong wake counter flow that splits into two branches, with one branch considerably disrupting the boundary layer while other branch cuts off supply of vorticity to the shear layer, is found to be an essential feature of symmetric vortex shedding. To further clarify the roles of correlated terms contributing to the vorticity generation and transport in the wake, the Reynolds number was varied while keeping other excitation parameters constant. This results in the intensity of symmetric vortex shedding increasing with Reynolds number. The increase in wake counter-flow strength with increased Reynolds number cannot be explained either by a relative velocity between the mean and pulsatile flow components or relative acceleration between bluff body and fluid. Based on the results it is inferred that the brief rolling up of the shear layer to form a wake vortex during a part of the pulsation cycle, together with the associated unsteady tangential pressure gradient around the body, contribute to the stronger wake counter flow at higher Reynolds numbers. At lower pulsation amplitudes, the tendency of the fluid in the wake to form strong counter flow can interact significantly with the transverse entrainment flow to alter the natural vortex-shedding mechanism, to give different vortex-shedding modes.

The effects of the compressibility of the subsonic flow on heat transfer characteristics in a microscale impinging slot jet

We experimentally investigated the effects of both the compressibility and nozzle width on the local heat transfer distribution of microscale unconfmed slot jets impinging on a uniformly heated flat plate. We made heat transfer measurements under the following experimental conditions; Reynolds numbers of Re = 4000~10000, Mach numbers of Ma = 0.13~0.68, nozzle-to-plate distances of H/B = 3~25, lateral distances of x/B = 0~25, and nozzle widths of B = 300~700 μm having a nozzle aspect ratio of y/B = 30. A thermal infrared imaging technique was used to measure the impingement plate temperature. The experimental results show that for all tested Re and H/B values at a nozzle width of B = 300 μm, the Nusselt number maximum occurred nearly at the stagnation point and then monotonically decreased along the downstream. However, at B = 500 and 700 μm, the maximum Nusselt number point shifted toward x/B ≈ 1.5~2.0. And the Nusselt number increased, as x/B increased, from the stagnation point to the shifted maximum point and monotonically decreased afterward. This shifted maximum point may be attributable to vortex rings promoting sudden flow acceleration and entrainment of surrounding air moving along the jet axis. For the same Reynolds number, the Nusselt number in the stagnation region increased as the nozzle width increased due to a momentum increase of the jet flow caused by the formation of vortices. And, the Nusselt numbers for the smallest nozzle width of B = 300 μm (or highest Mach number at a given Reynolds number) at all H/B and Reynolds numbers tested significantly deviated from those for B = 500 and 700 μm in the downstream region corresponding to x/B > 5, suggesting that the compressibility, when it is high, can affect the heat transfer in the downstream region.

Wake behind a three-dimensional dry transom stern. Part 2. Analysis and modelling of incompressible highly variable density turbulence

We analyse the turbulence characteristics and consider the closure modelling of the air entraining flow in the wake of three-dimensional, rectangular dry transom sterns obtained using high-resolution implicit large eddy simulations (iLES) (Hendricksonet al.,J. Fluid Mech., vol. 875, 2019, pp. 854–883). Our focus is the incompressible highly variable density turbulence (IHVDT) in the near surface mixed-phase region${\mathcal{R}}$behind the stern. We characterize the turbulence statistics in${\mathcal{R}}$and determine it to be highly anisotropic due to quasi-steady wave breaking. Using unconditioned Reynolds decomposition for our analysis, we show that the turbulent mass flux (TMF) is important in IHVDT for the production of turbulent kinetic energy and is as relevant to the mean momentum equations as the Reynolds stresses. We develop a simple, regional explicit algebraic closure model for the TMF based on a functional relationship between the fluxes and tensor flow quantities.A prioritests of the model show mean density gradients and buoyancy effects are the main driving parameters for predicting the turbulent mass flux and the model is capable of capturing the highly localized nature of the TMF in${\mathcal{R}}$.

Measurements and analysis of adhesive forces for micron particles on common indoor surfaces

Dust particle resuspension from indoor surfaces is an important source of particulate matter in indoor environments. The adhesive force represents the resistance of the particle to resuspension in an air stream and is a key factor in the resuspension process. The present study used an atomic force microscope (AFM) to measure the adhesive forces between three particle samples and four common indoor surfaces including acrylic, marble, epoxy flooring and wood flooring. The effects of different indoor surfaces were investigated using 12 particle–surface combinations. The results show that the average adhesive forces range from 8.2 nN to 448.1 nN for different combinations with the surface roughness being the main factor. The average adhesive force increases with the contact radius and is larger on the wood flooring surface than on the other three surfaces. Then, the resuspension process was simulated using the moment balance method with the measured adhesive forces as the model inputs. The model predictions show that the wood flooring surface has the largest resistance to the air flow entrainment, followed by the epoxy flooring surface, then the marble surface and the least is the acrylic surface.

The effect of check dams on the dynamic and bed entrainment processes of debris flows

Bed entrainment plays a significant role in the formational process of a debris flow. Thus the influence of bed entrainment may be an important factor which cannot be neglected when assessing the prevention effect of check dams. However, since few studies have investigated the interaction between check dams and debris flows with considering bed entrainment, the interactive effect of check dams on the dynamic and bed entrainment processes of debris flows remains unclear. Therefore, in this paper, an improved depth-averaged model is proposed to overcome this weakness. In the improved model, the impeding effect of a check dam is simplified as a rigid constraint, and a new computational scheme is adopted to improve the simulation efficiency. Using this model, the dynamic and bed entrainment processes of the catastrophic 2010 Hongchun gully debris flow are analyzed, and the effects of check dams on this debris flow are studied. The results show that the present model can properly depict the dynamic and bed entrainment processes of the Hongchun gully debris flow. Without bed entrainment, the flow quantity tends to decrease gradually from the upstream to the downstream, while the flow quantity will show an opposite tendency if bed entrainment is considered. The check dams can largely reduce the bed entrainment scale and flow quantity of this debris flow. Additionally, the prevention effect of check dams tends to be better when they are constructed at the upper part of the gully by constraining bed entrainment.

Pressure evaluation of spray-induced flow in the framework of a statistical approach based on URANS and ensemble averaging

Using a statistical approach based on the unsteady Reynolds-averaged Navier–Stokes equations (URANS) and ensemble averaging, pressure evaluation of spray-induced flow is conducted by means of stereo-particle image velocimetry. The method allows the determination of the full velocity gradient tensor in order to characterize the governing equations. The spray-induced flow of a two-hole gasoline direct injection (GDI) research sample is investigated at 100 bar injection pressure, 1 bar ambient gas pressure, 25 °C fuel and ambient gas temperature. Low- and high-pressure regions are observed representing entrainment, displacement and recirculation flow. In reference to the ambient pressure, the mean pressure field indicates small pressure differences in the order of 0.1 mbar. For the assessment of pressure evaluation, a comparative measurement with a pressure sensor is carried out, which shows good agreement of the temporal pressure course in the intermediate spray region. The propagation of instantaneous velocity uncertainties to the pressure field indicates low pressure uncertainties for an ensemble average of 50 measurement samples. As a result of scale analysis, the pressure gradient is mainly described by the local acceleration, whereas the contribution of convective acceleration and Reynolds stresses focuses in the spray proximity. The analysis shows negligible effects of viscosity and gravity. The spray-induced flow is regarded as incompressible in case of low mass and heat transfer. The observations justify a simplification of the governing equations minimizing measurement and computational expense.

The role of eddies on pathways, transports, and entrainment in dense water flows along a slope

The flow of dense water along slopes has been investigated in several numerical investigations based on the Dynamics of Overflow Mixing and Entrainment (DOME) setup. In the present study, we try to obtain further insight into the pathways, transports, dynamics, and entrainment of such flows by performing numerical model studies with horizontal grid sizes of 10 km and 2.5 km. It is found that the rates of descent of the plumes along the slope are robust to the horizontal resolution. With a high vertical resolution and a bottom boundary condition that facilitates the representation of Ekman drainage, the plumes will follow a deeper path than when using quadratic bottom drag with a constant drag coefficient. In the results from the studies with 2.5 km horizontal grid, ambient lighter water inside anticyclonic eddies is sucked downward. Due to Ekman drainage, this water flows outwards near the bottom and underneath denser plume water. The water column around the core of the anticyclonic eddies becomes unstable, and ambient water is entrained into the plume. Due to the increased mixing and entrainment in the eddy-permitting regime, there is a substantial increase in the along slope plume transports when we reduce the grid size from 10 km (the laminar case) to 2.5 km (the eddy-permitting case).

Insights into the catastrophic Xinmo rock avalanche in Maoxian county, China: Combined effects of historical earthquakes and landslide amplification

A large and catastrophic rock avalanche occurred at the Xinmo village in the Maoxian county of the Sichuan province of China on June 24, 2017. The avalanche destroyed 64 houses and killed 10 people, and 73 people were reported to be missing. In this study, we focused on the contributions of six historical earthquakes to the collapse of rocks in the source area and the landslide amplification caused by the entrainment of pre-existing deposits on the landslide path. The historical earthquakes in this area contributed different levels of damage on the source rocks; particularly, the 1933 Diexi earthquake induced a few initial cracks in rocks at the crest of the steep hillslope. Under the combined time-effects of long-term weathering and gravity, the rock blocks in the source area were detached from the crest along the intersecting cracks and over-dip soft layer (namely, the sliding surface). The avalanche was amplified by the entrainment of colluvium on its path. The influence of the colluvium on the movement process of the Xinmo landslide was modelled by the GPU-based parallel discontinuous deformation analysis (DDA) method, and the effect of the impact and entrainment in the debris flow was rigorously analysed. The results indicate that the impact and entrainment can continuously increase the volume of debris flow, and the thickness of the colluvium was established to be in the range of 25.9–33.9 m based on the analysis of the deposit pattern and movement duration of the avalanche. There were numerous unstable rocks induced by historical earthquakes, at the crests of steep hill slopes in the earthquake-stricken area. With long-term damage caused by environmental factors and gravity, the failure of unstable rocks with high potential energy is likely to cause severe disasters. The excess pore water pressure likely to be present in the colluvium or deposits during impact and entrainment can considerably reduce the effective stress and degrade friction resistance along the runout path. As a result, the landslide can be intensively amplified owing to the effect of impact and entrainment under a lower friction resistance

Experimental study on bubble and droplet entrainment in vertical churn and annular flows and their relationship

Bubble and droplet entrainment plays an important role in the mass transfer between the liquid film and the gas core in churn and annular flow. In this work, bubble and droplet entrainments in vertical churn and annular flows are systematically studied in an air-water two-phase vertical flow experiment using a high-speed camera. The results show that there are three kinds of bubble entrainment and five kinds of droplet entrainment in churn and annular flows. The entrainment can be categorized as primary entrainment and secondary entrainment. The primary entrainment includes one kind of bubble entrainment, which is a large wave roll and overturn, and three kinds of droplet entrainment, which are shearing-off of waves containing bubbles, bag break-up of waves, and ligament break-up of waves. The secondary entrainment includes two kinds of bubble entrainment, which are synchronized bubble entrainment along with droplet entrainment and droplets from the gas core impacting the liquid film, and two kinds of droplet entrainment, which are droplets impacting the liquid film and the burst of bubbles. With the flow pattern transition from churn to annular flow, the dominant bubble entrainment changes from large wave roll and overturn to the synchronized bubble entrainment along with droplet entrainment accompanied by a decreasing intensity. The dominant droplet entrainment changes from shearing-off of waves containing bubbles to ligament break-up of waves with the intensity first decreasing and then increasing. There is a close relationship between bubble entrainment and droplet entrainment. The entrained bubbles reduce the strength of large-scale waves and facilitate the shearing-off of waves. A small number of droplets are entrained due to the burst of residual bubbles in the liquid film. Droplets that are generated by bag or ligament break-up of waves can directly impact the nearby liquid film, leading to synchronous bubble entrainment. A small number of bubbles can be entrapped by droplets from the gas core impacting the liquid film. The primary entrainment intensity is significantly greater than that of the secondary entrainment. With the transition from churn to annular flow, the change in the primary entrainment intensity determines the variety of the secondary entrainment intensity.

Development of a droplet entrainment rate model for a vertical adiabatic two-phase flow

The authors have assessed some well-known droplet entrainment and deposition rate models against a wide range of vertical annular-mist flow experiments. The results of assessment showed that the Hewitt model predicts quite well for low-pressure air-water experiments and the Hewitt and the Okawa models are reasonably good for high-pressure steam-water experiments. However, the models still have large errors and significant deviations. Especially, it is known that their entrainment rate models are inappropriate to the developing region of an annular-mist flow because they were developed using experimental data estimated with the equilibrium assumption that the entrainment rate is equal to the deposition rate. To solve these problems, a new entrainment rate model is proposed here.Key variables for droplet entrainment are deduced from the analysis of experimental data and then the basic form of the new model is suggested as an empirical correlation. For the least-square fitting of the new correlation, the experimental data set of entrainment rate versus flow condition were needed. However, the entrainment rate cannot be measured in experiments but should be estimated. In this study, the droplet entrainment rates of 349 experiments were realistically estimated by using the continuity equation of the liquid film in an annular-mist flow, where the measured axial change of the liquid film mass flux can be taken into account, that is, the equilibrium assumption is not always needed. The new model was fitted using the data set and, then, assessed against 470 annular-mist flow experiments. The results showed that the new entrainment rate model in combination with the Okawa’s deposition rate model is definitely better than the existing models in terms of both accuracy and precision.

Development of an adaptive bypass element for passive entrainment flow control in dry powder inhalers

The release of drug from dry powder inhalers is strongly dependent on the patient's inhalation profile. To maximise the effect of the treatment, it is necessary to optimise dry powder inhalers to achieve drug delivery that (A) is independent of the inhalation manoeuvre and (B) is targeted to the correct site in the lung. The purpose of this study is to develop a dry powder inhaler with an adaptive bypass element that achieves desired drug delivery behaviour. Computational and experimental methods are used. First, the effect of a generic variable bypass element on entrainment behaviour is modelled. This is done by modelling a dry powder inhaler as a network of flow. Second, the behaviour of a potential variable bypass element, a flap valve, is studied both computationally and experimentally. Third, the flow resistances are optimised to achieve consistent and desired entrainment behaviour for patients with very different inhalation manoeuvres. A simulated dry powder inhaler device design was found that achieves an approximately constant entrainment flow rate of 12 L/min when total flow rates larger than 20 L/min are applied. The developed dry powder inhaler is predicted to accurately deliver drug for patients with highly different inhalation manoeuvres.

Experimental Study of Air Entrainment Rates due to Inclined Liquid Jets

AbstractThe air entrainment rate due to inclined liquid jet plunging into a pool was investigated experimentally. Three types of fluids with varying physical properties in terms of viscosity and surface tension were utilized. For the impinging jet test section, nozzles with different inner diameters were selected. The inclination angles and liquid jet velocities at the nozzle outlet were varied and the entrained air rate was measured by the soap meniscus method. Taking the falling velocity of the liquid jet as a characteristic velocity, it was found that the air entrainment rate under the present experimental condition largely depended on the Weber number. From the obtained database, a new empirical model dependent on the Weber number and Laplace length scale is proposed which is capable of predicting the air entrainment flow rate at a mean absolute relative deviation of 21.7 %.