Lower Initial Velocity Research Articles

Simulation of flow field based on constrained fluid for electrolyte jet control

In previous studies, jets were controlled by adjusting parameters such as velocity, pressure, size and shape of nozzle, and temperature. In this study, a new method of controlling electrolyte jets was proposed by introducing a constrained fluid. To analyze the effects of the constrained fluid on the jet flow field, the initial jet velocity and velocity ratio of the flow field were examined. Simulation results showed that the proposed method can avoid stray deposition caused when the electrolyte is spread on the substrate surface. The electrolyte jet could be controlled by the constrained fluid, indicating the characteristic of cyclical fluctuation. The initial jet velocity and velocity ratio had different impacts on the flow field; the former mainly affected the velocity and pressure distribution. The relationship between the velocity of flow field and initial jet velocity was observed to be linear. However, the relationship between pressure of the flow field and initial jet velocity was quadratic. The velocity ratio had a significant impact on the state of electrolyte jet. At low initial jet velocities, the electrolyte jet was not continuous at different velocity ratios. When the initial jet velocity increased, cyclical fluctuations in the electrolyte jet weakened under high velocity ratios. The electrolyte jet diameter also decreased linearly as the velocity ratio increased. The velocity and pressure of the flow field were quadratically related to the velocity ratio. This indicates that the constrained fluid had a considerable impact on the flow field and state of the electrolyte jet. Moreover, the electrolyte jet could be controlled effectively by adjusting the initial jet velocity and velocity ratio to obtain a stable and small electrolyte jet diameter.

Effect of modified method based on recoil motion on interior ballistic performance of ultralight high-low pressure artillery

Light gun is of unique application prospect for its low body mass and projectile initial velocity. In order to solve the problem of calculation error caused by the large free recoil velocity of light artillery in traditional interior ballistic model, a more suitable internal ballistic model for ultralight artillery was proposed through the design and test of a light gun, the accuracy of the algorithm was verified. The model described the effect of the recoil motion of the gun on chamber pressure and projectile acceleration travel, which made the calculation result more accurate when the gun mass was lighter. Compared with the experimental data, the errors of the calculation results using the improved algorithm were 2.90 % and 0.33 % respectively, while that using the traditional algorithm were 20.38 % and 10.02 % respectively. At the same time, the extended calculation was carried out to analyze the impact of the buffer device on the interior ballistic performance. The result showed that the lighter the gun, the greater the improvement brought by the buffer device under the same mass and recoil resistance. When the mass of the projectile and the gun was both 3 kg, the resistance of 70 kN could increase the velocity of projectile by 16.2 %. Furthermore, the internal ballistic elements of the traditional algorithm and the improved algorithm were calculated respectively under the same working conditions. It was found that when the mass of the gun is 8.567 times the mass of the bullet, the error of the two algorithms was less than 1 %, and the error increased gradually with the mass ratio of gun and projectile decreasing.

Role of DNA-DNA sliding friction and nonequilibrium dynamics in viral genome ejection and packaging.

Many viruses eject their DNA via a nanochannel in the viral shell, driven by internal forces arising from the high-density genome packing. The speed of DNA exit is controlled by friction forces that limit the molecular mobility, but the nature of this friction is unknown. We introduce a method to probe the mobility of the tightly confined DNA by measuring DNA exit from phage phi29 capsids with optical tweezers. We measure extremely low initial exit velocity, a regime of exponentially increasing velocity, stochastic pausing that dominates the kinetics and large dynamic heterogeneity. Measurements with variable applied force provide evidence that the initial velocity is controlled by DNA-DNA sliding friction, consistent with a Frenkel-Kontorova model for nanoscale friction. We confirm several aspects of the ejection dynamics predicted by theoretical models. Features of the pausing suggest that it is connected to the phenomenon of 'clogging' in soft matter systems. Our results provide evidence that DNA-DNA friction and clogging control the DNA exit dynamics, but that this friction does not significantly affect DNA packaging.

Mass-stream trajectories with non-synchronously rotating donors

ABSTRACT Mass-transfer interactions in binary stars can lead to accretion disc formation, mass-loss from the system, and spin-up of the accretor. To determine the trajectory of the mass-transfer stream, and whether it directly impacts the accretor, or forms an accretion disc, requires numerical simulations. The mass-transfer stream is approximately ballistic, and analytic approximations based on such trajectories are used in many binary population synthesis codes as well as in detailed stellar evolution codes. We use binary population synthesis to explore the conditions under which mass transfer takes place. We then solve the reduced three-body equations to compute the trajectory of a particle in the stream for systems with varying system mass ratio, donor synchronicity, and initial stream velocity. Our results show that, on average, both more mass and more time are spent during mass transfer from a sub-synchronous donor than from a synchronous donor. Moreover, we find that at low initial stream velocity, the asynchronous rotation of the donor leads to self-accretion over a large range of mass ratios, especially for supersynchronous donors. The stream (self-)intersects in a narrow region of parameter space where it transitions between accreting on to the donor or the accretor. Increasing the initial stream velocity leads to larger areas of the parameter space where the stream accretes on to the accretor, but also more (self-)intersection. The radii of closest approach generally increase, but the range of specific angular momenta that these trajectories carry at the radius of closest approach gets broader. Our results are made publicly available.

Cavitation evolution and flow field characteristics of transient flows with multi-point water-column separations in undulating pipeline

Transient flows with water-flow-induced multi-point cavitation inside an undulating pipeline are investigated experimentally under conditions of fast valve closure for three typical test cases by using high-precision pressure transducers, high-speed photography and particle image velocimetry (PIV) techniques. For low initial velocity condition, the transient pressure at the monitoring point (PT1) downstream the valve quickly decreases to the cavitation pressure and fluctuates near it. The leading-edge velocity of the dispersed bubble-like cavitation group first increases with its occurrence and then accelerates and reaches an extreme value before decreasing to zero due to the viscous force and the decrease of water flow inertia, and finally the water flows upstream in reverse, i.e. the leading-edge accelerates and collapses behind the valve. At the same time, the transient pressure rises sharply to maximum 81.86 m due to the return of the compression wave and then decreases sharply to cavitation again because of the switch of the pressure waves. Then, the second cavitation intensity is weak and the corresponding pattern is fog-like, and finally decreases gradually with a cyclic period of about 0.38 s. The transient pressure at the monitoring point (PT2) at the peak of the undulating pipeline also maintains at the cavitation pressure corresponding to the big bubble occurrence moment in addition to its fluctuation near the cavitation pressure including first and second cavitation. Its maximum pressure is 72.16 m and the corresponding cavitation pattern is small and bigger bubbles at different cavitation stages. For middle initial velocity, the typical pattern of the corresponding cavitation is the large cavity with free surface at PT2, the corresponding higher maximum pressures are 102.36 m at PT1 and 84.81 m at PT2. The evolution process of the cavity including bubble mergence, morphological deformation, motion and collapse is analysed based on the broken wave and hydraulic dynamic theories. For high initial velocity, larger cavities occur both downstream the valve and at the peak point of undulating pipeline, and thus the corresponding higher maximum pressures due to larger cavity collapse are 140.56 m at PT1 and 127.98 m at PT2. In particular, complete water column separation with transparent large cavity occur downstream the valve.

Experimental investigation of various energy-absorbing layer materials and sodium alginate viscosities on the jet formation in laser-induced-forward-transfer (LIFT) bioprinting

Laser-induced-forward-transfer (LIFT) bioprinting technology has been viewed as a regenerative medicine technology because of its high printing quality and good cell viability. To stabilize the jet to achieve high-quality printing, an energy-absorbing layer (EAL) can be introduced. In this study, three materials (graphene, gelatin, and gold) were utilized as the EAL. The effect of each EAL on the jet generation process was investigated. Besides, the effect of graphene EAL thickness was addressed for various experimental conditions. The jet generation process using sodium alginate solutions with different concentrations (1 and 2 wt. %) was also discussed to investigate the effect of viscosity. The time sequence images of the formed jets utilizing three EALs showed that both graphene EAL and gelatin EAL can promote the formation of jet flow. For the gold EAL, no jet flow was observed. This study provides experimental verifications that the interaction between laser and EAL materials can result in different jets due to various dominant interaction mechanisms. For example, strong absorption in the infrared range for the graphene EAL, strong scattering loss for the gelatin EAL, and strong absorption in the ultraviolet range but weak absorption in the infrared for the gold EAL. We also observed the holes left on the EAL after the printing was completed. The thermal effect is dominant to create regular and round shape holes for the graphene EAL, but it changes to the mechanical effect for the gold EAL because of the existence of irregular and unorganized holes. In addition, we identified the existence of an input laser energy threshold value for a certain thickness graphene EAL. More laser energy is required to break down thicker graphene EALs, which will result in a higher initial jet velocity. Furthermore, we explored the effect of sodium alginate (SA) solution's viscosity on the generated jet. We found that a high-viscosity SA solution can result in a low initial jet velocity, a short jet, and small droplets on the receiving substrate. The findings from this study help determine the mechanisms of EAL–laser interaction with different EAL materials in the LIFT process. This work aims to facilitate the development of new EAL and bioink to achieve stable jet formation and high printing quality in future LIFT bioprinting.

Numerical investigation of the effect of reactive gas jets on the flame acceleration and DDT process

Jet obstacles can quickly induce the deflagration to detonation transition (DDT) process and reduce the thrust loss of an engine. However, there are few studies on the use of combustible premixed gases as the reactive jet obstacle. Based on the OpenFOAM platform, a detailed numerical investigation of the flame acceleration and DDT process is carried out with different initial jet velocities and the number of jet obstacles. The results show that, although the stronger flame generated by the reactive jet obstacles will reduce the virtual blockage ratio to a greater extent, the turbulence and combustion heat release effect generated by them still significantly promote the flame acceleration. In terms of flame acceleration, initially, increasing the initial jet velocity has no obvious effect on the flame acceleration. Later, turbulence and combustion heat release begin to dominate the flame acceleration and significantly promote the flame acceleration. Since the jets in this study adopt the same combustible premixed gases, increasing the number of reactive jet obstacles downstream does not improve the upstream flame acceleration. In DDT, increasing the initial velocity of the reactive jet can improve the detonation initiation, but the effect of the obstacle number on DDT is greatly affected by the initial jet velocity. Specifically, at a low initial jet velocity, more jet obstacles can significantly promote the detonation initiation, while at a high initial jet velocity, the enhanced turbulence caused by the jet is the dominant factor for the flame acceleration. Therefore, compared with the number of jet obstacles, the detonation initiation process of the premixed gases is more sensitive to the change in the initial jet velocity.

Active pole bending effect in pole support phase

ABSTRACT In pole vaulting, model analysis is one of the key methods to increase vaulting height. To date, the effects of athletes’ motions during ‘pole support phase’ have been measured and modelled to improve and set new world records. The motions were extracted based on the context of pole bending interaction and parameters to improve vaulting height were investigated. However, due to experimental, mechanical, and sensing restrictions, ranges and interactions of the parameters were poorly addressed. To investigate further, a parameter space must be globally explored. Here, we show parameter sensitivities and interactive effects between initial velocity, pole length, bending amplitude and switching time. From the simulation studies, we found that active pole bending enabled successful pole vaulting with lower initial velocity and longer poles. Vaulting height had a local maximum point at a specific initial velocity and positive bending could control conditions to deliver the local maximum height. Positive bending controls the rising-up speed of the pole and contributes to the verticalisation of the vaulting angle. Negative bending increases the vaulting speed and contributes to the robustness of the vaulting angle. Our results demonstrate how these parameters affect the vaulting performances and suggest how athletes should activate their bodies.

Theoretical and experimental study on the rheological properties of WIS grout and the dispersion and sealing mechanism

Recently a new grout material called water inflow sealing (WIS) was invented for sealing water inflow in tunneling and underground constructions. In this study, a special experimental method called intubated counter grouting (ICG) was proposed to investigate the influence of water dispersion on the rheological properties of the grout during the grouting process, and to testify the sealing performance of the grout, such as instant gelling ability (IGA) and anti-dispersion ability (ADA). In the experiment, dispersion was restricted in the downstream of the channel with a high turbulence intensity. The influences of ADA and IGA were therefore decoupled and evaluated separately. Experimental results revealed two distinctive sealing mechanisms of WIS. For a low initial velocity of water, WIS turned the shear flow of water into an overall movement of a plug by absorbing water into the particles. For a high initial velocity and the situation that the particles reached the outlet before sufficiently expanding, WIS modified the rheology of the water in the channel and reduced its velocity till the static state. The distinctive feature of WIS brings a reformation on the sealing mechanism and provides an effective way to control water inflow with high pressure and velocity.

Fluorescence-enabled evaluation of nasal tract deposition and coverage of pharmaceutical formulations in a silicone nasal cast using an innovative spray device

IntroductionThe characterisation of nasal formulations is a critical point. However, there are still no recommendations or guidelines in terms of standard approaches for evaluating the formulation's nasal deposition and/or coverage profile. This study optimises a method for quantifying silicone nasal cast deposition and coverage of liquid formulations using different nasal devices. ObjectivesThe present work investigates the nasal deposition and coverage patterns of innovative nasal spray nozzles producing slow velocity soft mists, using a nasal cavity replica and a fluorescent dye. MethodsThe study of the deposition pattern of a fluorescent liquid formulation in a transparent nasal cast was carried out in both the presence and absence of a simulated inhalation flow. The extent of the deposition pattern was investigated using ImageJ and fluorescence in the nasal cast, quantified by fluorometric analysis. The particle size distribution and initial droplet velocity were determined using a laser diffractometer and a high-speed camera with a frame rate of 1000 fps. ResultsA uniform intranasal coverage was obtained with droplets of a volume median particle size (Dv50) between 15 and 25 µm in airflow between 10 and 30 L/min. In these conditions, aerosol formulations can be uniformly deposited in the vestibule and turbinate cavity nasal regions, with less than 10 % passing beyond the nasopharyngeal region. ConclusionThe method applied allowed for the determination of the coverage of the nasal cast in different regions using images analysis and fluorometric analysis. Droplet velocity is a critical parameter in the deposition in the nasal cavity. With standard swirl nozzles, many droplets are found on the surface of the nasal vestibule. Soft mist nozzles produce smaller droplets at a much lower initial velocity (<1 m/s), resulting in a more uniform coverage.

Collision Chains among the Terrestrial Planets. III. Formation of the Moon

Abstract In the canonical model of Moon formation, a Mars-sized protoplanet “Theia” collides with proto-Earth at close to their mutual escape velocity v esc and a common impact angle ∼45°. The “graze-and-merge” collision strands a fraction of Theia’s mantle into orbit, while Earth accretes most of Theia and its momentum. Simulations show that this produces a hot, high angular momentum, silicate-dominated protolunar system, in substantial agreement with lunar geology, geochemistry, and dynamics. However, a Moon that derives mostly from Theia’s mantle, as angular momentum dictates, is challenged by the fact that O, Ti, Cr, radiogenic W, and other elements are indistinguishable in Earth and lunar rocks. Moreover, the model requires an improbably low initial velocity. Here we develop a scenario for Moon formation that begins with a somewhat faster collision, when proto-Theia impacts proto-Earth at ∼ 1.2v esc, also around ∼45°. Instead of merging, the bodies come into violent contact for a half hour and their major components escape, a “hit-and-run” collision. N-body evolutions show that the “runner” often returns ∼0.1–1 Myr later for a second giant impact, closer to v esc; this produces a postimpact disk of ∼2–3 lunar masses in smoothed particle hydrodynamics simulations, with angular momentum comparable to canonical scenarios. The disk ends up substantially inclined, in most cases, because the terminal collision is randomly oriented to the first. Moreover, proto-Earth contributions to the protolunar disk are enhanced by the compounded mixing and greater energy of a collision chain.

Segmented flow capillary microreactors for determination of kinetic rate constants of reactive zinc extraction system

Conventional techniques for kinetic rate constant acquisition are limited by diffusive transport, which can be well addressed by microfluidic techniques. Segmented flow capillary microreactors were used for the first time in this study to obtain kinetic rate constants for a reactive zinc extraction system. The influences of initial zinc ion concentration, total flow velocity, pH of the aqueous phase, and ionic strength on the kinetic rate constant determination were investigated. The control regimes were determined with the ratio of reaction to total resistance R, and the Hatta number. The kinetic rate constants obtained at low initial zinc concentrations, low acidity and high flow velocities exhibited minor deviations, indicating that the zinc sulfate/D2EHPA system could be under the kinetic control regime with the capillary microreactors, which also demonstrated the viability of the segmented flow approach. The kinetic rate constant data obtained with the segmented flow capillary microreactor from this work were much higher than those obtained from previous studies, which may be attributed to the fact that the microreactor minimizes the influence of diffusive transport. The purpose of this work was to provide reliable kinetic data of the reactive zinc extraction system and to verify the efficiency and superiority of the segmented flow microreactor approach over other techniques.

Experimental investigation on the water entry of a bulbous bow based on TR-PIV

The objective of this work is to experimentally investigate the dynamic evolution of flow structure and pressure distribution on the water entry of a bulbous bow. We focus on the water entry of an arbitrary bow based on the 18.5 station line of the KRISO container ship with a typical bulbous bow. Time-resolved particle image velocimetry (TR-PIV) is utilized to capture the transient flow information during the bow slamming. The instantaneous pressure is estimated by solving the incompressible Navier–Stokes equation using the Lagrangian approach. Our findings demonstrate the formation and migration of a saddle point, the generation of a strong shear layer near the concave zone at a low initial velocity, and the deformation of an air pocket at a high initial velocity. Furthermore, we offer insight into the reconstructed pressure field over the entire duration of the slamming, especially the dramatic slamming pressure during the secondary impact. It is interesting that the peak value locates on the critical point of the upward and downward flow caused by the pressing of the bow flare. In addition, the pressure close to the flare continuously keeps the global maximum.

Efficiency characterization of the V-shape asteroid family detection method

Following the break up of a parent body, the Yarkovsky effect causes asteroid family members to spread in orbital semimajor axis with a rate often inversely proportional to their diameter. This size dependent semimajor axis drift causes family members to form structures in the semimajor axis vs inverse diameter plane that have the shape of the letter V. The V-shape method consists in finding the borders of such V-shapes of unknown center and opening. Although successfully employed to find some very old families in the inner main asteroid belt, the V-shape searching method is very sensitive to many parameters. In this work, we first created and evolved a synthetic asteroid family over billions of years. Then, by adding uncertainties to semimajor axis and diameter of the evolved synthetic family components, we randomly generated additional 99 similar, but not perfectly V-shaped, family clones. We chose a fairly low initial velocity dispersion of 20 m/s for our family. Thus, we can more easily relate the spreading in semimajor axis with the family’s age (slope of the evolving V). A synthetic background with an initially randomly distributed components was also created and evolved for 100 Myr. Thus, by setting different levels of ratio of the synthetic family and background asteroids, we derived a detection efficiency map for the V-shape method and determined how sensitive the results can be based on signal-to-noise levels. We also determined optimal parameter values for the method’s efficiency. We found that, families older than ≈3 Gyr are likely undetectable, with a method efficiency of 50% or less, whereas younger families (0.5–2.5 Gyr) are more easily detected by the method, with an efficiency of ≳80%.

Constrained adaptive backstepping take-off control for a morphing hybrid aerial underwater vehicle

This paper studies the inclined take-off control for a kind of morphing hybrid aerial underwater vehicles. At the initial stage, the hybrid aerial underwater vehicle could not achieve trim condition for the low speed. Then the altitude could have an obvious decrease during the take-off process. This could lead the vehicle to crashing into water. To escape from the unbalanced stage, an adaptive backstepping controller is designed to track the target angle of attack and the thrust is opened to maximum within limitation. The radial basis function Neural Networks and parameter adaption are employed in order to deal with the time-varying nonlinear variable sweep dynamics of the vehicle. Bisides, the control input of the elevator deflection is easy to be saturated for the low initial velocity. This is taken into account in the design of a constraint adaptive backstepping controller to prevent aggressive adaptions and to provide reliable input commands. Simulations are presented to varify the proposed controller.

Formation of film cooling on the turbine blade back and pressure side in the case of using V-shaped dimples

Alongside the development of methods of intensifying convective heat transfer inside the blade, development of methods of local improvement of the efficiency of film cooling of the blades surface is still of immediate interest. The film is formed on the blade surface in conditions of high-camber shape and low initial velocity of the gas flow in the vicinity of the leading edge with its subsequent abrupt acceleration. The paper presents some data on the peculiarities of film formation on the back and pressure side of the blade in the vicinity of the leading edge. Experimental temperature distribution over the adiabatic wall was obtained with the use of a FLIR-E 64501 thermal imager. It was found that the conditions for the film formation on the blade back are more favorable than those on the pressure side. It manifests itself in the fact that optimal blowing parameters on the blade back are considerably lower than those on the pressure side. The use of V-shaped dimples located on the wall immediately behind the holes for blowing was suggested as a measure for local improvement of film cooling efficiency. The efficiencies of film cooling in the formation of a curtain, without the use and with the use of V-shaped dimples behind the holes for blowing were compared. Local improvement of efficiency and uniformity of film cooling distribution with the use of V-shaped dimples behind the holes for blowing was observed.

Improved pivot–slide model of the motion of a curling rock

The pivot–slide model (Shegelski and Lozowski) successfully predicts the slide and curl distances of a curling rock. However, in this model, there is no dependence of the curl distance on the initial velocity, because the ratio between the pivot to sliding times is constant. A refined model is presented, in which the ratio of the pivot to sliding times depends on the stone velocity via two parameters. Confidence limits for these parameters are deduced from experimental data, which show that the pivot–slide ratio depends on the stone velocity. However, precise values of these parameters could not be obtained with this study, as more precise experiment data are needed. The refined model allows one to qualitatively explain two characteristics of the stone trajectory observed in a curling game, namely the bigger final curl with lower initial velocity and the lower curl with the effect of sweeping the ice.

Exceptional Extended Field-of-view Observations by PROBA2/SWAP on 2017 April 1 and 3

Abstract On 2017 April 1 and 3, two large eruptions on the western solar limb, which were associated with M4.4- and M5.8-class flares, respectively, were observed with the Sun Watcher with Active Pixels and Image Processing (SWAP) Extreme Ultraviolet (EUV) solar telescope on board the Project for On Board Autonomy 2 (PROBA2) spacecraft. The large field-of-view (FOV) of SWAP, combined with an advantageous off-point, allows us to study the eruptions up to approximately 2 solar radii (Rs), where space-based coronagraph observations begin. These measurements provide us with some of the highest EUV observations of an eruption, giving crucial additional data points to track the early evolution of Coronal Mass Ejections. In SWAP observations, we track the evolution of off-limb erupting features as well as associated on-disk EUV waves, and the kinematics of both are calculated. The first eruption shows a clear deceleration throughout the lower corona into coronagraph observations, whereas the second eruption, which had a lower initial velocity, shows no obvious acceleration or deceleration profile. This paper presents a unique set of observations, allowing features observed in EUV to be traced to greater heights in the solar atmosphere, helping to bridge the gap to the FOV of white-light coronagraphs. Even with these favorable data sets, it remains a challenging task to associate features observed in EUV with those observed in white light, highlighting our urgent need for single-instrument observations of the combined lower and middle corona.

The eccentric behaviour of windy binary stars

Carbon-enhanced metal-poor stars, CH stars, barium stars, and extrinsic S stars, among other classes of chemically peculiar stars, are thought to be the products of the interaction of low- and intermediate-mass binaries, which occurred when the most evolved star was in the asymptotic giant branch (AGB) phase. Binary evolution models predict that because of the large sizes of AGB stars, if the initial orbital periods of such systems are shorter than a few thousand days, their orbits should have circularised due to tidal effects. However, observations of the progeny of AGB binary stars show that many of these objects have substantial eccentricities, up to e ≈ 0.9. In this work we explore the impact of wind mass transfer on the orbital parameters of AGB binary stars by performing numerical simulations in which the AGB wind is modelled using a hydrodynamical code and the dynamics of the stars is evolved using an N-body code. We find that in most models the effect of wind mass transfer contributes to the circularisation of the orbit, but on longer timescales than tidal circularisation if e ≲ 0.4. For relatively low initial wind velocities and pseudo-synchronisation of the donor star, we find a structure resembling wind Roche-lobe overflow as the stars approach periastron. In this case, the interaction between the gas and the star is stronger than when the initial wind velocity is high and the orbit shrinks while the eccentricity decreases. In one of our models wind interaction is found to pump the eccentricity of the orbit on a similar timescale as tidal circularisation. However, since the orbit of this model is shrinking tidal effects will become stronger during the evolution of the system. Although our study is based on a small sample of models, it offers some insight into the orbital evolution of eccentric binary stars interacting via winds. A larger grid of numerical models for different binary parameters is needed to test if a regime exists where hydrodynamical eccentricity pumping can effectively counteract tidal circularisation, and if this can explain the puzzling eccentricities of the descendants of AGB binaries.

Experimental study on entrained droplets in vertical two-phase churn and annular flows

In this paper, the air-water vertical two-phase pipe flow experiment was carried out at room temperature and pressure to study the characteristics of entrained droplets in churn and annular flows with the high-speed camera and phase Doppler anemometry. The results show that there are five kinds of droplet entrainment modes in churn and annular flows, namely shearing-off of waves, bag break-up of waves, ligament break-up of waves, droplet impacting liquid film, and burst of bubbles. With the flow pattern transition from churn flow to annular flow, the dominant droplet entrainment changes from shearing-off of waves to ligament break-up of waves. The gas superficial velocity plays a decisive role in determining the characteristics of entrained droplets, while the effect of the liquid superficial velocity can be neglected. In churn flow, the shearing-off of waves produces a small amount of entrained droplets with low sphericity and initial velocity. Large droplets tend to deposit, and small droplets undergo a certain degree of fragmentation and coalescence during the accelerated migrating in the gas core. In annular flow, the ligament break-up of waves produces a large amount of entrained droplets with high sphericity and initial velocity. Droplet size presents continuous distribution. The size of droplets increases slightly due to the coalescence of small droplets during the accelerated migrating in the gas core. Compared with churn flow, the momentum exchange between the gas phase and droplets is greater in annular flow, which leads to a significant increase in droplet momentum. The size distribution of the entrained droplets determines the momentum exchange between the gas core and the entrained droplets to a certain extent.