Secondary atomization of water and isooctane drops impinging on tilted heated surfaces

The present paper reports an experimental study aimed at characterizing the effects of heat transfer on the secondary atomization, which occurs during droplet impact on hot surfaces at conditions reproducing those occurring at fuel injection in internal combustion engines. The experiments consider single isooctane and water droplets impacting at different angles on a stainless steel surface with known roughness and encompass a range of Weber numbers from 240 to 600 and heat transfer regimes from the film-vaporization up to the Leidenfrost regime. The mechanisms of secondary breakup are inferred from the temporal evolution of the morphology of the impact imaged with a CCD camera, together with instantaneous measurements of droplet size and velocity. The combination of a technique for image processing with a phase Doppler instrument allows evaluating extended size distributions from 5.5 μm up to a few millimetres and to cover the full range of secondary droplet sizes observed at all heat transfer regimes and impaction angles. Temporal evolution of the size and velocity distributions are then determined. The experiments are reported at impact conditions at which disintegration does not occur at ambient temperature. So, any alteration observed in droplet impact behavior is thermally induced. The analysis is relevant for port fuel injection systems, where droplets injected to impact on the back surface of the valves, behave differently depending on fuel properties, particularly when the use of alcohols is considered, even as an additive to gasoline.     

Impaction of spray droplets on leaves: influence of formulation and leaf character on shatter,bounce and adhesion

This paper combines experimental data with simple mathematical models to investigate the influence of spray formulation type and leaf character (wettability) on shatter, bounce and adhesion of droplets impacting with cotton, rice and wheat leaves. Impaction criteria that allow for different angles of the leaf surface and the droplet impact trajectory are presented; their predictions are based on whether combinations of droplet size and velocity lie above or below bounce and shatter boundaries. In the experimental component, real leaves are used, with all their inherent natural variability. Further, commercial agricultural spray nozzles are employed, resulting in a range of droplet characteristics. Given this natural variability, there is broad agreement between the data and predictions. As predicted, the shatter of droplets was found to increase as droplet size and velocity increased, and the surface became harder to wet. Bouncing of droplets occurred most frequently on hard-to-wet surfaces with high-surface-tension mixtures. On the other hand, a number of small droplets with low impact velocity were observed to bounce when predicted to lie well within the adhering regime. We believe this discrepancy between the predictions and experimental data could be due to air layer effects that were not taken into account in the current bounce equations. Other discrepancies between experiment and theory are thought to be due to the current assumption of a dry impact surface, whereas, in practice, the leaf surfaces became increasingly covered with fluid throughout the spray test runs.     

Droplet production from disintegrating bubbles at water surfaces. Single vs. multiple bubbles

We experimentally determine the droplet production rate at a water surface where either single or multiple bubbles (bubbly flow) with similar mean diameters disintegrate and produce film and jet droplets. A detailed assessment of film drop production from bubbly flow is important, since most presently used correlations are based on single-bubble measurements. Moreover, jet drops––even though they contain a much larger fluid volume––are de-entrained into the water surface in most technical and geophysical applications. Detailed phase Doppler anemometry (PDA) measurements are performed in the vicinity of the water surface with long sampling times. For a considered mean diameter of approximately 3 mm, the size distribution of the non spherical bubbles is determined from photographic images. From single-bubble measurements we find, consistent with literature data, a narrow size distribution of the jet drops with a mean diameter of 477 μm. For bubbly flow, the maximum is shifted to somewhat smaller jet drop diameters (425 μm) and the production of film droplets increases significantly. We relate this increase to the coalescence of bubbles prior to their disintegration at the surface. Our results therefore show that for a fixed bubble size and gas flow rate the number of film drops entrained from a bubbly flow is underestimated, if the estimate is based on single-bubble data.     

Droplet imaging velocimeter and sizer: a two-dimensional technique to measure droplet size

This paper discusses the droplet imaging velocimeter and sizer (DIVAS), a two-dimensional technique to measure droplet size from very small droplets (a few microns) to very large drops (millimeters), as well as droplet velocity and concentration. DIVAS uses a particle imaging velocimetry (PIV) configuration; that is, pulsed lasers illuminate the droplets in the measurement plane and CCD cameras collect the off-axis scattered light, thus yielding a small measurement volume and correspondingly a high number density measurement capability. DIVAS is based on glare point imaging as described by van de Hulst and Wang (Appl Opt 30(33):4755–4763, 1991). By measuring the separation of cross-polarized glare points, DIVAS is capable of measuring droplet size down to a few microns with sub-pixel resolution. The measurement of large drops from the separation of glare points is straightforward. However, the paper discusses that large drops are often non-spherical and more advanced signal processing is required to retrieve their size. Both analytical studies and experiments are presented to demonstrate the feasibility of the technique.     

Fluctuating flow in a liquid layer and secondary spray created by an impacting spray

A spray impacting onto a wall produces a flow of secondary droplets. For relatively sparse spray these secondary droplets are produced by the splashing of the impacting drops and their interactions. For dense sprays, like Diesel injection sprays, these secondary droplets are created by the fluctuating liquid film created on the wall. In the present paper hydrodynamic models are presented for these two extreme cases. The velocities of the secondary droplets produced by the crown splash in a sparse spray are described theoretically. Next, the fluctuations in the motion of the liquid film created by a dense impacting spray are analyzed statistically. This motion yields the formation of finger-like jets, as observed in experiments of a Diesel spray impacting onto a rigid wall. The characteristic size and velocity of the film fluctuations are estimated. These two theoretical models are validated by comparison with the experimental data.     

Flow characteristics of spray impingement in PFI injection systems

The present paper addresses an experimental study of the dynamic exchanges between the impact of an intermittent spray and the liquid film formed over the target, based on detailed phase Doppler anemometry (PDA) measurements of droplet size, velocity and volume flux in the vicinity of the impact. The flow configuration is that of a pulsed injector spraying gasoline onto a flat disc to simulate the port fuel injection (PFI) of an internal combustion engine operating at cold-start conditions. The measurements evidence that the outcome of impact cannot be accurately predicted based on the characteristics of the free spray, but requires precise knowledge of the flow structure, induced by the target. The implications for spray–wall interaction modelling are then discussed based on the application of conservation equations to the mass, momentum and energy exchanged between the impinging droplets and the liquid film. The results show that the liquid film starts to form in the vicinity of the stagnation region at early stages of injection and a non-negligible proportion of droplets impinging at outer regions splash after interaction with the film. Film disruption is mainly driven by the intermittent axial momentum of impinging droplets, which enhances the vertical oscillations. The radial momentum imparted to the liquid film at the stagnation region is fed back onto secondary droplets emerging later during the injection cycle at outwards locations, where momentum of impacting droplets is much smaller. As a consequence, although the number of splashed droplets is enhanced by normal momentum, their size and ejection velocity depends more on the radial spread induced onto the liquid film and, hence, on the radial momentum at impact. The analysis further shows that existing spray–wall interaction models can be improved if the dynamic exchanges between the impacting spray and the liquid film are accounted.     

Breakup and atomization characteristics of mono-dispersed diesel droplets in a cross-flow air stream

This paper describes the microscopic and macroscopic breakup characteristics, as well as the velocity and size distributions, of mono-dispersed droplets in relation to the breakup regimes. For this experiment, a droplet generator equipped with a piezo stack produced mono-dispersed droplets. The droplet-breakup phenomenon due to the cross-flow was captured in microscopic and macroscopic views by using the following: a spark lamp, a Nd:YAG laser, a long distance microscope and a CCD camera as a function of the Weber number. Along with the analysis of the images, the droplet size and velocity distributions were measured in the near nozzle region by a phase Doppler particle analyzer system at bag, stretching and thinning, and catastrophic breakup regimes. The results of this study showed the size and velocity distributions of disintegrated droplets at the bag, stretching and thinning, and catastrophic breakup regimes. In the bag breakup regime, the droplets separated into small and large droplets during breakup. Alternatively, the droplets disintegrated at a shorter duration and formed a cloud, similar to a fuel spray injected through an injector, in the stretching and thinning and catastrophic breakup regimes.     

Numerical analysis of secondary droplets characteristics due to drop impacting on 3D cylinders considering dynamic contact angle

This study is investigating three-dimensional numerical simulation of a Newtonian droplet impact and break on two square cylinders based on dynamic contact angle of droplet at the spatial interface between two solid–fluid phases. The droplet impact details and morphology studied in the present work could provide ideas for the spray wall impingement modeling in the simulation of many industrial applications, such as spray painting and liquid cooling of surfaces. The droplet impact is investigated on two square cylinders in 9 different modes with different droplet diameters and physical conditions such as different positions of droplet. The volume of fluid (VOF) method was used with open-source software. The results have been compared and validated quantitatively and qualitatively with the experimental results. Results represent droplet diameter into cylinder dimension and velocity profiles are affected on number of broken droplets, break times and droplet deformation. Also, mean velocities of droplet after impact on two square cylinders at first break time were 0, 0.025, 0.12, 0.47, 0.11, 0.08, 0.2, 0.012, 0.19 m/s for cases 1–9, respectively. Moreover, in case 7 that droplet diameter into cylinder dimension was 2, the maximum number of break-up into secondary droplets was 10 drops that occurred for 4 times.

     

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75^°. The effect of water to oil flow rate ratio (Q_wat/Q_oil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Q_wat/Q_oil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Q_wat/Q_oil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.     

Single droplet impingement of urea water solution on a heated substrate

In the present work, impingement behaviour of an aqueous urea solution is investigated experimentally. The effects of droplet diameter, impact velocity and substrate temperature are evaluated by monitoring single droplet impingement with a high-speed camera. Results allow the formulation of four different interaction regimes and a regime map depending on hydrodynamic and thermal parameters is proposed. The regimes deposition, splash, boiling-induced breakup, rebound with breakup and the transition boundaries are discussed in detail. Results show that the solute significantly affects the outcome of droplet impingement promoting droplet disintegration by enhanced nucleation and bubble formation. Comparison with literature data reveal the strong dependency of droplet impact behavior on the Weber number as a combination of initial droplet diameter and impact velocity.     

用格子Boltzmann方法模拟液滴撞击固壁动力学行为

首次用格子Boltzmann方法中的伪势模型对液滴撞击固壁的动力学行为进行了数值模拟.详细研究了液滴在壁面上的流动状态以及各种因素对撞击过程的影响.通过数值模拟得到:壁面的可润湿性越小,液滴越容易发生反弹,液滴的回缩速度越快;液滴的撞击速度越大,所得到的相对直径越大,回缩速度越快;液滴的粘性越小,所得到的相对直径越大;液滴的表面张力越大,液滴越容易发生反弹现象.另外,液滴的最大相对直径与We数满足一定的线性关系,这些结果与前人的理论预测和实验结果完全吻合.     

The periodicity of droplets emanating from interconnected orifices

The periodicity of droplets emanating from a single and from two orifices with a common fluid reservoir between them was investigated. Experiments were conducted in which the effects of variations in mass flow rate, orifice diameter and common reservoir volume were determined. The results reported herein indicate that dripping from an orifice at relatively low mass flow rates is singly periodic and that the period between droplets is inversely proportional to the imposed mass flow rate. A simple model of the singly periodic droplet emission process is developed and supported by the experimental results. Period doubling initiates and continues to develop with further increases in the mass flow rate. It is marked by the introduction of additional, smaller diameter droplets that are interspersed temporally among the larger, primary droplets. The presence of a common fluid reservoir volume between two orifices of the same diameter is shown for the singly periodic regime not to alter the droplet emission rate of either orifice as compared to its single orifice counterpart. The volume of the reservoir, however, does affect the mass flow rate per orifice necessary for initial period doubling, with this mass flow rate being lower for a smaller reservoir volume.     

Impact of liquid drops on a rough surface comprising microgrooves

Impact of water drops on a stainless steel surface comprising rectangular shaped parallel grooves is studied experimentally. Geometric parameters of the surface groove structure such as groove depth, groove width and solid pillar width separating any two successive grooves were kept at 7.5, 136 and 66 μm, respectively. The study was confined to the impact of drops in inertia dominated flow regime with Weber number in the range 15–257. Experimental results of drop impact process obtained for the grooved surface were compared with those obtained for a smooth surface to elucidate the influence of surface grooves on the impact process. The grooves definitely influence both spreading and receding processes of impacting liquid drops. A more striking observation from this study is that the receding process of impacting liquid drops is dramatically changed by the groove structure for all droplet Weber number.     

Drag coefficients of spherical liquid droplets Part 2: Turbulent gaseous fields

The drag of non-evaporating, spherical, liquid droplets was measured in turbulent flow fields at parametric ranges relevant to spray combustion, characterized by the droplet Reynolds number, and the intensity and spatial scales of turbulence. The experimental apparatus comprised a wind-tunnel and a piezo-electric droplet generator. The procedure was to inject water droplets of uniform size co-currently and continuously with vertical turbulent air flows while droplet velocity was measured at different elevations using laser-Doppler velocimetry. Turbulence was characterized using hot-wire anemometry prior to droplet injection. Drag coefficients were calculated using these main measurements and the law of conservation of mechanical energy. Reynolds numbers were investigated in the range 10–100, in terms of the equivalent spherical diameter of a droplet, and the mean relative speed between the ambient gaseous field and the droplets. Weber numbers were much less than unity so droplets were effectively spherical. Relative intensities of turbulence were investigated in the range 20–65 percent, in terms of the mean relative speed. Spatial scales of turbulence were large in comparison to the droplets; the ratio between the spatial integral scale and the droplet diameter was in the range 11–38, and the Kolmogorov scale was comparable in size or smaller than the droplet diameter. Experimental data showed that the drag in turbulent fields under these conditions is not significantly different than that of solid spheres in a quiescent field at the same Reynolds number.The financial support of the Natural Sciences and Engineering Research Council of Canada and the Manufacturing Research Corporation of Ontario is gratefully acknowledged.     

Maximum spreading of electrically charged droplets impacting on dielectric substrates

In this work, the electric charging effect on the spreading of droplet impacting on dielectric substrates has been investigated. The charged water droplets were directed on the paraffin wax and the Teflon-coated plates. The impinging behavior was visualized and recorded using a CCD camera to identify the maximum extent of the flattened droplets. Droplet diameter and velocity approaching the wall were measured as well. The diameter of the electrically charged droplet at the maximum spread turned out to be larger compared to that of neutral droplet (at the maximum spread), and the difference becomes larger with increasing of the electric charge ratio (defined as the ratio of the actual electric charge to the Rayleigh limit). This phenomenon is considered to be due to reduction of effective interfacial tensions between the liquid and the gas and between the liquid and the solid by electric charging. Finally, an improved model was proposed to predict the maximum spreading ratio for electrically charged droplets by introducing correlations on the liquid–gas and the liquid–solid interfacial tensions.     

Evaluation of FFT-based cross-correlation algorithms for PIV in a periodic grooved channel

The design of a pneumatic droplet generator to produce small (~0.2 mm diameter) water droplets on demand is described. It consists of a cylindrical, liquid-filled chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Rapidly opening and closing the valve sends a pressure pulse to the liquid, ejecting a single droplet through the nozzle. Gas in the chamber escapes through a vent hole so that the pressure drops rapidly and more droplets do not emerge. We photographed droplets as they emerged from the nozzle, and recorded pressure fluctuations in the chamber. We determined the duration of the pressure pulse required to generate a single drop; longer pulses produced satellite drops. The length of the water jet when its tip detached and the diameter of the droplet that formed could be predicted using results from linear stability analysis. The peak pressure in the cavity could be increased by raising the supply pressure, increasing the width of the pressure pulse, or by reducing the size of the pressure relief vent.     

Performance of slotted pores in particle manufacture using rotating membrane emulsification

This paper addresses the use of different slotted pores in rotating membrane emulsification technology. Pores of square and rectangular shapes were studied to understand the effect of aspect ratio （1-3.5） and their orientation on oil droplet formation. Increasing the membrane rotation speed decreased the droplet size, and the oil droplets produced were more uniform using slotted pores as compared to circular geometry. At a given rotation speed, the droplet size was mainly determined by the pore size and the fluid velocity of oil through the pore （pore fluid velocity）. The ratio of droplet diameter to the equivalent diameter of the slotted pore increased with the pore fluid velocity. At a given pore fluid velocity and rotation speed, pore orientation significantly influences the droplet formation rate： horizontally disposed pores （with their longer side perpendicular to the membrane axis） generate droplets at double the rate of vertically disposed pores. This work indicates practical benefits in the use of slotted membranes over conventional methods.     

Performance of slotted pores in particle manufacture using rotating membrane emulsification

This paper addresses the use of different slotted pores in rotating membrane emulsification technology.Pores of square and rectangular shapes were studied to understand the effect of aspect ratio (1-3.5) and their orientation on oil droplet formation.Increasing the membrane rotation speed decreased the droplet size,and the oil droplets produced were more uniform using slotted pores as compared to circular geometry.At a given rotation speed,the droplet size was mainly determined by the pore size and the fluid velocity of oil through the pore (pore fluid velocity).The ratio of droplet diameter to the equivalent diameter of the slotted pore increased with the pore fluid velocity.At a given pore fluid velocity and rotation speed,pore orientation significantly influences the droplet formation rate: horizontally disposed pores (with their longer side perpendicular to the membrane axis) generate droplets at double the rate of vertically disposed pores.This work indicates practical benefits in the use of slotted membranes over conventional methods.     

The use of single-point measurements to characterize dynamic behavior in sprays

In an effort to derive dynamic information from a single-particle counting device such as the phase-Doppler interferometer (PDI), a one-dimensional motion model and a sequential sorting algorithm have been developed to calculate the distance and velocity difference between consecutive droplets in sprays, as well as the number of droplets in close proximity (grouplets) and the frequency of grouplet formation. By focusing on sprays that are primarily uni-directional, this model can be used to highlight dynamic behavior and provide information that may indicate tendencies for clustering, collision, coalescence or evaporation among the droplets – information that is not available when considering the histograms of droplet size and velocity normally produced by PDIs. The motion model and sorting algorithm are validated for a mono-sized droplet stream generated by piezo-electric excitation, and are then applied to two other sprays to determine dynamic behavior. For a pressurized-liquid nozzle emitting water, the expected trends of droplet deceleration and spatial dispersion are found moving away from the exit of the nozzle, and significant in velocity differences between closely-spaced droplet pairs are observed. No dominant frequencies are present in the grouping of the droplets. For a droplet-laden air jet excited at a known frequency, similar results are calculated for droplet spacing, grouping and velocity differences, and the driving frequency is also obtained from analysis of the sequentially-sorted PDI data. Although the models developed assume an idealized PDI system, real operating characteristics of PDI systems place limits on the accuracy of these analyses. Received: 10 October 1998/Accepted: 5 October 1999     

煤油液滴直径对两相旋转爆轰发动机流场的影响

为探究煤油液滴不同初始直径对气液两相旋转爆轰发动机流场的影响,假设初始注入的煤油液滴具有均匀直径,考虑雾化破碎、蒸发等过程,建立了非定常两相爆轰的Eulerian-Lagrangian模型,进行了液态煤油/高温空气爆轰的非预混二维数值模拟。结果表明：在初始液滴直径为1～70μm的工况范围,燃烧室内均形成了单个稳定传播的旋转爆轰波;全局当量比为1时,爆轰波前的空气区域大于液滴煤油的蒸气区域,导致波前燃料空气混合不均匀,波前均存在富油区和贫油区,两相速度差导致分离出的空气形成低温条带;当煤油液滴的初始直径较小时,波前的反应物混合过程主要受蒸发的影响,爆轰波可稳定传播;当直径减小至1μm时,煤油液滴在入口处即蒸发,旋转爆轰波表现为气相传播的特性,爆轰波结构平整;当煤油液滴的初始直径较大时,波前的反应物混合过程主要受液滴破碎的影响;对于相同的燃料质量流量,在不同初始煤油液滴直径工况下,煤油液滴最大的停留时间均占爆轰波传播时间尺度的80%以上;爆轰波前燃料预蒸发为气相的占比越高,爆轰波的传播速度越高;初始液滴直径为10～70μm的工况范围内,爆轰波的速度随初始直径的增大先升高后降低。