Collision dynamics of high-speed droplets upon layers of variable thickness

The collision dynamics between a droplet and a film has been studied with high-impact energy that can be grouped in a dimensionless Weber number, We, as normalized by surface energy. To accomplish this, we have developed a technique based on cutting of a high-speed jet, which can generate a single droplet with speed up to 23 m/s and We on the order of thousands. It was found that the boundaries indicating the occurrence of a central jet and that of a secondary droplet disintegrated from the jet decreased monotonically with increased dimensionless film thickness, H, and remained constant when the film thickness was larger than the crater depth. However, the transition designating multiple droplets that are originated from a central jet shows a non-monotonic trend with the variation of H, with a minimum We being at H ≈ 3, which is about the maximum crater depth, owing to a tuning behavior. The critical We for splashing that occurs at an early phase immediately after the impact is relatively sensitive to the film thickness only when H is between 1 and 2, which increases with reduced H. At large We (≳2,570 for high H), the ejected crown is closed to form a bubble and the transition boundary reveals a similar dependence on H as that for creation of a central jet.     

Wall jet similarity of impinging planar underexpanded jets

Velocity profiles and wall shear stress values in the wall jet region of planar underexpanded impinging jets are parameterized based on nozzle parameters (stand-off height, jet hydraulic diameter, and nozzle pressure ratio). Computational fluid dynamics is used to calculate the velocity fields of impinging jets with height-to-diameter ratios in the range of 15–30 and nozzle pressure ratio in the range of 1.2–3.0. The wall jet has an incomplete self-similar profile with a typical triple-layer structure as in traditional wall jets. The effects of compressibility are found to be insignificant for wall jets with Ma < 0.8. Wall jet analysis yielded power-law relationships with source dependent coefficients describing maximum velocity, friction velocity, and wall distances for maximum and half-maximum velocities. Source dependency is determined using the conjugate gradient method. These power-law relationships can be used for mapping wall shear stress as a function of nozzle parameters.     

Analytical and experimental investigations of the pulsed air–water jet

This paper describes a new way of generating pulsed air–water jet by entraining and mixing air into the cavity of a pulsed water jet nozzle. Based on the theory of hydro-acoustics and fluid dynamics, a theoretical model which describes the frequency characteristic of the pulsed air–water jet is outlined aimed at gaining a better understanding of this nozzle for generating pulses. The calculated result indicates that as the air hold-up increases, the jet oscillation frequency has an abrupt decrease firstly, and then reaches a minimum gradually at α (air hold-up)=0.5, finally it gets increased slightly. Furthermore, a vibration test was conducted to validate the present theoretical result. By this way, the jet oscillation frequency can be obtained by analyzing the vibration acceleration of the equal strength beam affected by the jet impinging. Thereby, it is found that the experimental result shows similar trend with the prediction of the present model. Also, the relationship between vibration acceleration and cavity length for the pulsed water jet follows a similar tendency in accord with the pulsed air–water jet, i.e. there exists a maximum for each curve and the maximum occurs at the ratio of L/d₁ (the ratio of cavity length and upstream nozzle diameter) =2.5 and 2.2, respectively. In addition, experimental results on specimens impinged by the pulsed water jet and pulsed air–water jet show that the erosion depth increases slightly with air addition within a certain range of cavity length. Further, this behavior is very close to the vibration test results. As for erosion volume, the air entrained into the cavity significantly affects the material removal rate.     

Effect of Droplet Size and Atomization on Spray Formation: A Priori Study Using Large-Eddy Simulation

The paper is mainly focused to the vast number of researchers who work within direct injection (DI) engine fuel spray simulations. The most common simulation framework today within the community is the Reynolds Averaged Navier Stokes (RANS) approach together with the Lagrangian Particle Tracking (LPT) method. In fact, this study is one of the first studies where high resolution LES/LPT diesel spray modeling is considered. The potential of LES to deepen the present day multidimensional LPT fuel spray simulations is discussed. Spray evolution is studied far from an injector by modeling a spray as a particle laden jet (PLJ). The effect of d on mixing in non-atomizing and atomizing sprays is thoroughly investigated at jet inlet Reynolds number Re?=?10⁴ and Mach number Ma?=?0.3. Based on and justified by rather recent and also quite old ideas, novel and compact views on droplet breakup in turbulent flows are pointed out from the literature. We use LES/LPT to illustrate that even in a low Weber number flow (We?<?13) the droplet breakup modeling may need considerable attention in contrast to what is typically assumed in the present-day breakup models. LES and LPT techniques are first applied to essentially confirm certain expected droplet size effects on spray shape in non-atomizing monodisperse sprays. In the simulations LES e.g. produces an expected turbulent dispersion pattern that depends on droplet diameter (d) without a droplet dispersion model in contrast to RANS. A new compact droplet breakup model is formulated and tested for droplets that break with a natural resonance time rate according to the Poisson process. As a result of the study: 1) the analysis gives a rigorous and enriching proof of currently existing views on droplet size effects on mixing, and 2) the presented a priori analysis points out the importance of modeling the resonance breakup even at a low We.     

Combined aerodynamic and electrostatic atomization of dielectric liquid jets

The electrical and atomization performance of a plane?Cplane charge injection atomizer using a dielectric liquid, and operating at pump pressures ranging from 15 to 35?bar corresponding to injection velocities of up to 50?m/s, is explored via low current electrical measurements, spray imaging and phase Doppler anemometry. The work is aimed at understanding the contribution of electrostatic charging relevant to typical higher pressure fuel injection systems such as those employed in the aeronautical, automotive and marine sectors. Results show that mean-specific charge increases with injection velocity significantly. The effect of electrostatic charge is advantageous at the 15?C35?bar range, and an arithmetic mean diameter D ₁₀ as low as 0.2d is achievable in the spray core and lower still in the periphery where d is the orifice diameter. Using the data available from this higher pressure system and from previous high Reynolds number systems (Shrimpton and Yule Exp Fluids 26:460?C469, 1999), the promotion of primary atomization has been analysed by examining the effect that charge has on liquid jet surface and liquid jet bulk instability. The results suggest that for the low charge density Q _v?~?2?C/m³ cases under consideration here, a significant increase in primary atomization is observed due to a combination of electrical and aerodynamic forces acting on the jet surface, attributed to the significantly higher jet Weber number (We _j) when compared to low injection pressure cases. Analysis of Sauter mean diameter results shows that for jets with elevated specific charge density of the order Q _v?~?6?C/m³, the jet creates droplets that a conventional turbulent jet would, but with a significantly lower power requirement. This suggests that ??turbulent?? primary atomization, the turbulence being induced by electrical forces, may be achieved under injection pressures that would produce laminar jets.     

Computations of breakup modes in laminar compound liquid jets in a coflowing fluid

We present a numerical investigation of breakup modes of an axisymmetric, laminar compound jet of immiscible fluids, which flows in a coflowing immiscible outer fluid. We use a front-tracking/finite difference method to track the unsteady evolution and breakup of the compound jet, which is governed by the Navier–Stokes equations for incompressible Newtonian fluids. Numerical results show that depending on parameters such as the Reynolds number Re (in the range of 5–30) and Weber Number We (in the range of 0.1–0.7), based on the inner jet radius and inner fluid properties, the compound jet can break up into drops in various modes: inner dripping–outer dripping (dripping), inner jetting–outer jetting (jetting), and mixed dripping–jetting. Decreasing Re or increasing We promotes the jetting mode. The transition from dripping to jetting is also strongly affected by the velocity ratios, U₂₁ (intermediate to inner velocities) and U₃₁ (outer to inner velocities). Increasing U₂₁ makes the inner jet thinner and stretches the outer jet and thus promotes jetting. In contrast, increasing U₃₁ thins the outer jet, and thus, when the inner jet is dripping, the outer jet can break up into drops in the mixed dripping–jetting mode. Continuously increasing U₃₁ results in thinning both inner and outer jets and thus produces small drops in the jetting mode. In addition, starting from dripping, a decrease in the interfacial tension ratio of the outer to inner interfaces results in the mixed dripping–jetting and jetting modes. These modes produce various types of drops: simple drops, and compound drops with a single inner drop (single-core compound drops) or a few inner drops (multi-core compound drops).     

Flow behaviour of negatively buoyant jets in immiscible ambient fluid

In this paper we investigate experimentally the injection of a negatively buoyant jet into a homogenous immiscible ambient fluid. Experiments are carried out by injecting a jet of dyed fresh water through a nozzle in the base of a cylindrical tank containing rapeseed oil. The fountain inlet flow rate and nozzle diameter were varied to cover a wide range of Richardson Ri (8 × 10⁻⁴ < Ri < 1.98), Reynolds Re (467 < Re < 5,928) and Weber We (2.40 < We < 308.56) numbers. Based on the Re, Ri and We values for the experiments, we have determined a regime map to define how these values may control the occurrence of the observed flow types. Whereas Ri plays a stronger role when determining the maximum penetration height, the effect of the Reynolds number is stronger predicting the flow behaviour for a specific nozzle diameter and injection velocity.     

Dependence of the self-oscillation period for a conical jet aerator cap on the jet width in the nozzle outlet

The penetration of free hollow thin-walled turbulent water jets into water is considered. These jets are generated in a conical jet aerator whose apex angle is 60°. The periods of steady regular self-oscillations appearing during the process of penetration are studied experimentally. A dependence of these periods on the annular nozzle gap width δ, 0.07 ≤ δ ≤ 0.12 cm, is analyzed for the jet discharge range 160 ≤ Q ≤ 550 cm³/s when the height H of the annular nozzle above the water surface belongs to the range 1 ≤ H ≤ 28 cm.     

On the fragmentation characteristics of melt jets quenched in water

Experiments were carried out to investigate the characteristics of jet breakup and debris formation after melt jets fall into a subcooled water pool, which may occur in industrial processes such as the interactions of molten corium with coolant during a severe accident of light water reactors. A high-speed visualization system developed previously at KTH was used to capture the jet fragmentation phenomenon. Molten metal (Woods metal or tin) and mixture of binary oxides (WO₃-Bi₂O₃ or WO₃-ZrO₂) were employed separately as melt materials to address different breakup mechanisms (e.g., hydrodynamic vs. thermodynamic fragmentation) and material effect. Moreover, the parameters related to melt and water conditions, including superheat, jet diameter and velocity of melt as well as subcooling of water, were scrutinized for their influences on jet fragmentation characteristics. The experimental data were acquired on the melt jet fragmentation patterns, breakup length, droplet size spectrum, droplet breakup and solidification as well as debris morphology, which can be useful for validation of the codes used for the prediction of debris formation phenomena.     

90°锥头弹丸不同速度下垂直入水冲击引起的空泡特性

用高速摄像拍摄了90°锥头弹丸低速入水的空泡形态演变过程,全面讨论了不同入水冲击速度下空泡的闭合方式及其演变过程,分析了空泡闭合时间、闭合点水深和弹头空泡长度随入水速度的变化规律以及不同水深位置空泡直径的变化规律;研究了水幕闭合和近液面空泡收缩上升所形成的射流现象及其相互耦合作用过程,探讨了空泡深闭合后其壁面波动规律。结果表明:随着入水速度的增加,空泡分别发生准静态闭合、浅闭合、深闭合和表面闭合,每种闭合方式对应的一个速度区间;弹头产生空泡的临界入水速度为0.657 m/s;不同水深位置的空泡直径呈现非线性变化;随着水深的增加空泡扩张初速增大,空泡最大直径减小,扩张段缩短,收缩段延长;同一时刻水深越大空泡扩张收缩的加速度也越高;水幕闭合后会产生向上和向下两股射流,向下射流速度较大时会对弹丸运动产生影响;近液面空泡收缩上升时会产生强度正比于空泡体积大小和闭合点水深的射流,并与上两股射流相互耦合形成一股更强的向上射流;空泡深闭合后长度缩短和产生的向下射流使弹丸受力发生改变,弹丸速度因受力产生的变化带动了流体质点速度的波动,进而导致空泡壁面发生波动,壁面波动遵循空泡截面独立扩张原理。     

Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation

Characteristic flow modes, flow evolution processes, jet spread width, turbulence properties, and dispersion characteristics of swirling double-concentric jets were studied experimentally. Jet pulsations were induced by means of acoustic excitation. Streak pictures of smoke flow patterns, illuminated by a laser-light sheet, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocity instabilities in the flow. Jet spread width was obtained through a binary edge identification technique. Tracer-gas concentrations were measured for information on jet dispersions. Two characteristic flow patterns were observed: (1) synchronized vortex rings appeared in the low excitation intensity regime (the excitation intensity less than one) and (2) synchronized puffing turbulent jets appeared in the high excitation intensity regime (the excitation intensity greater than one). In the high excitation intensity regime, the “suction back” phenomenon occurred and therefore induced in-tube mixing. The jet spread width and turbulent fluctuation intensity exhibited particularly large values in the high excitation intensity regime at the excitation Strouhal numbers smaller than 0.85. At the excitation Strouhal numbers >0.85, the high-frequency effect caused significant decay of jet breakup and dispersion—the jet spread width and fluctuation intensity decreased sharply and may, at very high Strouhal numbers, asymptotically approach values almost the same as the values associated with unexcited jets. Exciting the jets at the high excitation intensity regime, the effects of puffing motion and in-tube mixing caused breakup of the jet in the near field and therefore resulted in a small Lagrangian integral time and small length scales of fluctuating eddies. This effect, in turn, caused drastic dispersion of the central jet fluids. It is possible that the excited jets can attain 90 % more improvements than the unexcited jets. We provide a domain regarding excitation intensity and Strouhal number to facilitate identification of characteristic flow modes.     

Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.     

An experimental and numerical study into turbulent condensing steam jets in air

 Temperatures, velocities, and droplet sizes are measured in turbulent condensing steam jets produced by a facial sauna, for varying nozzle diameters and varying initial velocities (Re=3,600–9,200). The release of latent heat due to droplet condensation causes the temperature in the two-phase jet to be significantly higher than in a single-phase jet. At some distance from the nozzle, droplets reach a maximum size and start to evaporate again, which results in a change in sign of latent heat release. The distance of maximum size is determined from droplet size measurements. The experimental results are compared with semi-analytical expressions and with a fully coupled numerical model of the turbulent condensing steam jet. The increase in centreline temperature due to droplet condensation is successfully predicted. Received: 5 April 2000 / Accepted: 15 November 2000     

Droplet size and velocity characteristics of water-air impinging jet atomizer

A water-air impinging jets atomizer is investigated in this study, which consists of flow visualization using high speed photography and mean droplet size and velocity distribution measurements of the spray using Phase Doppler Anemometry (PDA). Topological structures and break up details of the generated spray in the far and near fields are presented with and without air jet and for an impinging angle of 90°. Spray angle increases with the water jet velocity, air flow rate and impinging angle. PDA results indicate that droplet size is smallest in the spray center, with minimum value of Sauter mean diameter (SMD) of 50 µm at the air flow rate of Q_m = 13.50 g/min. SMD of droplets increases towards the spray outer region gradually to about 120 µm. The mean droplet velocity component W along the air-jet axis is highest in the spray center and decreases gradually with increasing distance from the spray center. SMD normalized by the air nozzle diameter is found firstly to decrease with gas-to-liquid mass ratio (GLR) and air-to-liquid momentum ratio (ALMR) and then remain almost constant. Its increasing with aerodynamic Weber number indicates an exponential variation. The study sheds light on the performance of water-air impinging jets atomizers providing useful information for future CFD simulation works.     

UV-mediated coalescence and mixing of inkjet printed drops

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.     

The influence of gas phase velocity fluctuations on primary atomization and droplet deformation

The effects of grid-generated velocity fluctuations on the primary atomization and subsequent droplet deformation of a range of laminar liquid jets are examined using microscopic high-speed backlit imaging of the break-up zone and laser Doppler anemometry of the gas phase separately. This is done for fixed gas mean flow conditions in a miniature wind tunnel experiment utilizing a selection of fuels, turbulence-generating grids and two syringe sizes. The constant mean flow allows for an isolated study of velocity fluctuation effects on primary atomization in a close approximation to homogeneous decaying turbulence. The qualitative morphology of the primary break-up region is examined over a range of turbulence intensities, and spectral analysis is performed in order to ascertain the break-up frequency which, for a case of no grid, compares well with the existing literature. The addition of velocity fluctuations tends to randomize the break-up process. Slightly downstream of the break-up region, image processing is conducted in order to extract a number of metrics, which do not depend on droplet sphericity, and these include droplet aspect ratio and orientation, the latter quantity being somewhat unconventional in spray characterization. A turbulent Weber number $We^{\prime}$ which takes into account gas phase fluctuations is utilized to characterize the resulting droplet shapes, in addition to a mean Weber number <We _d>. Above a $We^{\prime}>0.05$ a clear positive relationship exists between the mean aspect ratio of droplets and the turbulent Weber number where $We^{\prime}$ is varied by altering all relevant variables including the velocity root mean square, the initial droplet diameter, the surface tension and the density.     

An experimental study of circular sand–water wall jets

Laboratory experiments were carried out to study the effects of sand particles on circular sand–water wall jets. Mean and turbulence characteristics of sand particles in the sand–water wall jets were measured for different sand concentrations c_o ranging from 0.5% to 2.5%. Effects of sand particle size on the centerline sand velocity of the jets were evaluated for sand size ranging from 0.21 mm to 0.54 mm. Interesting results with the range of measurements are presented in this paper. It was found that the centerline sand velocity of the wall jets with larger particle size were 15% higher than the jets with smaller particle size. Concentration profiles in the vertical direction showed a peak value at x/d = 5 (where x is the longitudinal distance from the nozzle and d is the nozzle diameter) and the sand concentration decreased linearly for x/d > 5. Experimental results showed that the turbulence level enhanced from the nozzle to x/d = 10. For sand–water wall jets with a higher concentration (c_o = 1.5–2.5%), the turbulence intensity became smaller than the corresponding single-phase wall jets by 34% due to turbulent modulation. A modified logarithmic formulation was introduced to model the longitudinal turbulent intensity at the centerline and along the axis of the jet.     

Time-averaged flow characteristics and mixing properties of double-concentric jets

We examined the flow behaviors and mixing characteristics of double-concentric jets using laser-assisted smoke flow visualization method to analyze typical flow patterns and binary boundary detection technique to investigate jet spread width. Time-averaged velocity vectors, streamline patterns, velocity distributions, turbulence properties, and vorticity contours were analyzed using Particle Image Velocimetry (PIV). Topological flow patterns were analyzed to interpret the vortical flow structures. Mixing properties were investigated using a tracer-gas concentration detection method. Four characteristic modes were observed: annular flow dominated mode, transition mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. The jets’ mixing properties were enhanced by two major phenomena: the merging of annular flow and central jet at the centerline and the large turbulence fluctuations produced in the flow field. The merging of the jets induced stagnation points on the central axis in the annular flow dominated mode, which caused reverse flow on the central axis and drastic turbulence fluctuations of the near field region. When the central jet penetrated the recirculation region in the other three modes, the stagnation points on the central axis and the reverse flow vanished. Therefore, the mixing behaviors were prominently enhanced in the annular flow dominated mode.     

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρ_l/ρ_v,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρ_l/ρ_v,clip = 43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.     

Compressible spreading rates of supersonic coaxial jets

The compressible spreading rates of two supersonic coaxial jets were studied experimentally. The center jet had a fully-expanded Mach number of 3, and the outer jet of M = 1.8. The geometries of the center jet were circular and rectangular with two configurations, both with a 3∶1 aspect-ratio. The convective Mach numbers were varied in the range between 0.25 < M _c< 2.25. This was accomplished by varying the density ratio between the center jet and the coaxial jet, as well as the velocity ratio. The experimental compressible spreading rate was determined using Schlieren photography and total-pressure measurements. The spreading rate of the center circular jet decreased with increasing convective Mach number until it reached a constant value of 0.2 to 0.3 of the incompressible spreading rate for M _c>1.4. The rectangular jets exhibited a similar drop, at the same range of M _c, but their spreading rate was higher relative to the circular jet in the entire convective Mach number range.