Spray Formation Mechanism Research Articles

Development of test rig for optical diagnostics of cryogenic spray

Spray cooling is the primary method for conducting chilldown and fill of cryogenic propellant tanks. In a typical spray injection process, a liquid sheet/jet that exits the spray nozzle undergoes primary breakup by the development of surface instabilities. Droplets and ligaments generated after the primary breakup undergo secondary breakup to create a dispersion of droplets, which extract heat on impact with the tank walls. In the existing literature, there is limited data on the primary breakup of cryogenic sprays and their detailed visualization. Moreover, an insight into the spray characteristics such as primary breakup length and cone angle is vital to the development of computational models. In this investigation, optical diagnostics of cryogenic spray breakup and measurements of cryogenic spray characteristics have been conducted. Liquid nitrogen is the selected cryogen for the present analysis. To capture the transient nature of spray breakup after the injection, a novel shadowgraph technique is developed to photographically freeze the spray motion. Cryogenic spray characteristics such as cone angle and primary breakup length are obtained from the shadowgraph. In addition, droplet velocity measurement using Particle Image Velocimetry is explored. The comprehensive experimental dataset obtained herein not only provides insights into the mechanism of spray formation for cryogenic fluids but also helps in designing of spray cooling system for tank chilldown.

Factors affecting water flux distribution of fire sprinklers and effect of water flux uniformity on fire suppression characteristics

The water flux distribution of the sprinkler spray is not uniform due to the spray formation mechanism impinging on the deflector. In this study, the effect of factors such as installation height, flow rate, and deflector design on the uniformity of the water flux was investigated by experiment. Then, the water flux distribution has been reproduced with simulation and compared with measured data. When the installation height was increased, there was no significant change in the water flux pattern, only radial direction mixing. The water flux distribution predicted by changing the installation height through simulation showed excellent reproducibility with an R2 value over 0.77. Increasing the flow rate maintained the water flux and its pattern near the sprinkler head, however increased the water flux away from the sprinkler head. The modified sprinkler deflector has a deep slot design that increases the water flux near the sprinkler head, which improves the water flux uniformity. Also, the large tines of the modified sprinkler deflector restored the flow after the frame arm, eliminating the effect of the frame arm.

Numerical and experimental investigation of flow phenomena in rotating step-holes for direct-spray-cooled electric motors

Despite their high efficiency, electric motors are thermally limited in some operating points by several types of losses. Whenever temperature–critical components threaten to overheat, the performance is reduced for component protection (derating). The use of a suitable cooling concept may reduce the derating. The design of efficient cooling concepts of electric motors in traction drives with increased power densities is challenging, caused by the fact that the heat releases in the components vary considerably with the operating point. One option to reduce the temperatures is to place the heat sinks close to heat sources. Therefore, direct spray cooling with nozzles located in the rotor shaft is often used for cooling the end windings. The dielectric fluid (e.g. oil) is introduced into the mainly air-filled interior of the electric motor. In the following study, the behavior of the jet in the rotating step-holes at different volumetric flow rates is examined. To carry out the investigation, a new test rig and a novel optically accessible electric motor were designed. In this specifically designed test environment, the shape of the jets of different operating points is investigated by direct high-speed visualization. The cinematography setup is made of a four-light-emitting diode system in combination with a high-speed camera. A combined approach of experiment and simulation is used to find basic mechanisms of spray formation produced by rotating step-holes. Depending on the volumetric flow rate and the rotational speed, the direction of the oil jet gets more curved in relation to the rotating nozzle after exiting the small bore. If the deflection is large, the jet impinges on the wall of the large bore before reaching the end of the nozzle. The jet formation at the exit of the step-hole is mainly driven by the divergent forces in the liquid caused by impingement and the counteracting Coriolis force. Depending on the volumetric flow rate with constant rotational speed, different cross-sectional shapes of the jet at the exit are observed. These characteristic shapes can be grouped as a round undisturbed jet, strands with a connecting lamella and a C-shaped cross-section.

Spray formation mechanism of diverging-tapered-hole GDI injector and its potentials for GDI engine applications

The gasoline-direct-injection injector adopting the diverging-tapered-hole nozzle has appeared in the market recently, which has shown the improved spray atomization and lower engine particulate emissions compared to conventional straight-hole nozzles. However, the near-nozzle spray formation mechanism associated with this atomization potential of the diverging-tapered-hole nozzle has not been thoroughly clarified due to the difficulties in the near-field flow observation. The current study compares the in- and near-nozzle flow characteristics of the straight-hole and diverging-tapered-hole GDI nozzles using X-ray phase-contrast and absorption imaging techniques to unravel engaged mechanisms and discuss the potentials of the diverging-tapered-hole nozzle for GDI engine applications. The analyses focus on the in- and near-nozzle flow structure, and flow dynamics and stability outside the nozzle. The results showed that the diverging-tapered-hole nozzle coupled with proper hole length generated the radially expanding crescent-moon-like flow structure in the hole that promoted the spray atomization and dispersion outside the nozzle. The expansion of the hole cross-sectional area in the diverging-tapered-hole nozzle was appeared to attenuate the mutual interactions of fuel flows in the hole from various origins that formed rather stable sprays. These flow characteristics are discussed to be promising for the compact and robust spray generation for GDI engine applications.

Visualization of spatial distribution of the droplet size and velocity in flash boiling spray with extended glare-point imaging technique

Flash boiling is an effective way to improve the atomization quality. Droplet diameter and velocity distribution of the flash boiling spray extremely impact the mixture formation of direct injection engines. We visualize and measure the spatial distribution of the droplet size and velocity of flash boiling spray with extended glare-point imaging technique in this paper. An improved correlation-based processing algorithm for droplet identification is proposed. The algorithm is capable of identifying the droplet positions and sizes as well as tracking and matching of the droplet displacement vectors. The employed template matching algorithm using serial particle mask and the centroid method provide higher accuracy for estimating the droplet displacement, and the cross-correlation matching algorithm and the two-frame particle tracking algorithm further ensure higher matching rates for the droplet pairs. The performance of the algorithm is well validated by simulations. Furthermore, the method is applied for the experimental study of flash boiling spray with extended glare-point imaging technique in combination with particle tracking velocimetry for measuring the particle size and velocity with higher accuracy, the results of which provides improved understanding of the flash boiling spray formation mechanism.

Characterization and pressure drop correlation for sprays under the effect of micro scale cavitation

In this study, spray formation and atomization, droplet evolutions, break-up, and corresponding cavitating flows at the outlet of a short micro-channel with an inner diameter of 152 μm were experimentally studied at different injection pressures with the use of a high speed visualization system. High speed visualization was performed at five different segments to cover a ∼27.5 mm distance beginning from the micro-channel outlet (Five segments, at distances of 0–5.5, 5.5–11, 11–16.5, 16.5–22 and 22–27.5 mm from the micro-channel outlet) to assess the spray formation mechanism. High speed visualization revealed that droplet evolution is initiated from the second segment at low upstream pressures (5–30 bars), whereas droplets are discretized from the liquid jet in the fourth and fifth segments at medium and high upstream pressures (40–100 bars). Bigger size droplets formed at the outlet up to an injection pressure of 30 bars, while cavitation effect of intensified cavitating flows became dominant beyond this injection pressure, leading to smaller droplet sizes and a more conical spray. Pressure drop was correlated together with Martinelli parameter for cavitating flows and a new correlation for two-phase pressure drop was developed. Moreover, in order to segment the discretized droplets at low upstream pressure (5–30 bars) from captured images and to perform an in-depth analysis on them, an active contour approach utilizing curve evolution and level set formulation was implemented. As shown by experimental results, droplets were successfully segmented at different low pressure levels. The droplet/bubble evolution can be exploited in biomedical and engineering applications, where destructive effects of bubbly cavitating flows are needed.

Spray formation in a quasiplanar gas-liquid mixing layer at moderate density ratios: A numerical closeup

Spray formation and atomization in a gas-liquid mixing layer is an important fundamental problem of multiphase flows. It is highly desirable to visualize the detailed atomization process and to analyze the instabilities and mechanisms involved, and massive numerical simulations are required, in addition to experiment. Rapid development of numerical methods and computer technology in the past decades now allows large-scale three-dimensional direct numerical simulations of atomization to be performed. Nevertheless, the fundamental question, whether all the physical scales involved in the primary breakup process are faithfully resolved, remains unclear. In the present study, we conduct direct numerical simulations of spray formation in a gas-liquid mixing layer with state-of-the-art computational resources (using up to 4 billion cells and 16384 cores), in order to obtain a high-fidelity numerical closeup of the detailed mechanisms of spray formation. We also aim to examine whether present computational resources are sufficient for a fully resolved direct numerical simulation of atomization.

Analysis of effervescent spray quality for oil-fired furnace application

The quality of liquid fuel atomization is a key factor in the formation of gaseous pollutants and particulate emissions. This paper investigates the quality of effervescent spray, which has established itself as a promising alternative to conventional spray formation mechanisms. The reported results were obtained using a dual-phase Doppler analyser. Droplet data were collected in multiple locations varying both axially and radially, in intermediate spray region. The results display spatial development of Sauter mean diameter (SMD) in effervescent sprays at different operating conditions, but they also show that SMD may be insufficient to characterize the spray, especially near to the dense spray region. This conclusion is based on the observed large variance of drop size and velocity and on their mutual correlation. It is concluded that for modelling purposes, joint distributions of drop size and velocity would be more appropriate to fully characterize the primary atomization in an effervescent spray.

High‐speed imagery captures new sea spray formation mechanism

When strong winds blow over ocean waves, small droplets of sea spray rise into the air, enhancing the exchange of heat, mass, and energy between the air and the sea. How effective sea spray is at mediating each of these dynamics depends on the rate at which droplets are created and the drop size distribution of the mist. Unfortunately, research has been limited by a dearth of observational evidence that could explain the details of sea spray generation, including understanding the drop size distribution or the effects of different wind speeds. Previous research with high‐speed cameras aiming to capture the moment of drop formation was limited by camera resolutions too low to see all but the largest drops.

MECHANISM OF PERFORATION BASED ON SPREADING PROPERTIES OF EMULSIFIED OILS

During an agricultural application, sprays are formed by atomization through a hydraulic nozzle. Fine spray droplets can cause off-target contamination with agrochemicals when they move by air from the application site. Dilute oil-in-water emulsions create coarser sprays than water, when atomized through a flat fan nozzle, and are therefore interesting for drift control purposes. However, different emulsions influence the spray droplet size to a different extent and this effect is not yet well understood. With an aim to develop products with drift reduction properties it is necessary to comprehend the mechanism of interactions between emulsion droplets and the continuous aqueous phase. In this study, we analyze systematically the effects of different physical properties of oils on the spray formation process and investigate how the type of the emulsified oil, the concentration of emulsifier, emulsion droplet size, the spreading properties of the emulsified oil, and its viscosity influence the spray droplet size. Based on the experimental data, we propose a spray formation mechanism of dilute oil-in-water emulsions. We argue that when the liquid sheet leaves the nozzle, some emulsion droplets might merge with the air/water interface of the liquid sheet, spread there and induce a subphase flow, which, reinforced by perturbations in a turbulent flow, will initiate spray atomization. In mixtures of a dilute emulsion and a water soluble surfactant, dynamic surface tension at the interface of the liquid sheet controls the spray formation process. Surfactants located at the interface form a kinetic barrier for the spreading emulsion droplets and thus delay the atomization onset.

Detailed Numerical Simulation toward Understanding the Liquid Spray Formation Mechanisms

Detailed numerical simulations of Diesel liquid jet sprays have been conducted to elucidate the liquid atomization mechanisms. The obtained flow field is very complicated, but the spray behavior can be understood physically. The jet head formation leads to vortex shedding and subsequent atomization from the head edge. The liquid core surface is also unstable due to gas boundary layer instability and atomization also occurs from here. The final process of droplet generation from a ligament is basically the same as the past research results, and can be explained by the capillary wave destabilization. Such findings can be used for droplet modeling for practical-scale simulations.

Mechanisms of spray formation and combustion from a multi-hole injector with E85 and gasoline

The spray formation and combustion characteristics of gasoline and E85 (85% ethanol, 15% gasoline) have been investigated using a multi-hole injector with asymmetric nozzle-hole arrangement. Experiments were carried out in a quiescent optical chamber using high-speed shadowgraphy (9 kHz) to characterise the spray sensitivity to both injector temperature and ambient pressure in the range of 20–120 °C and 0.5, 1.0 bar. Spray-tip penetrations and ‘umbrella’ spray cone angles were calculated for all conditions. Phase Doppler Anemometry was also used to measure droplet sizes in the core of one of the spray plumes, 25 mm below the injector tip. To study the effect of fuel properties on vaporisation and mixture preparation under realistic operating conditions, a separate set of experiments was carried out in a direct-injection spark-ignition optical engine. The engine was run at 1500 RPM under cold and fully warmed-up conditions (20 °C and 90 °C) at part load and full load (0.5 and 1.0 bar intake pressure). Floodlit laser Mie-scattering images of the sprays on two orthogonal planes corresponding to the swirl and tumble planes of in-cylinder flow motion were acquired to study the full injection event and post-injection mixing stage. These were used to make comparisons with the static chamber sprays and to quantify the liquid-to-vapour phase evaporation process for both fuels by calculating the projected ‘footprint’ of the sprays at different conditions. Analysis of the macroscopic structure and turbulent primary break-up properties of the sprays was undertaken in light of jet exit conditions described in terms of non-dimensional numbers. The effects on stoichiometric combustion were investigated by imaging the natural flame chemiluminescence through the engine’s piston crown (swirl plane) and by post-processing to derive flame growth rates and trajectories of flame motion.

On the mechanism of spray formation from liquid jets

A fundamental physical mechanism whereby sprays are formed from liquid jets is formulated. It is shown that a combination of axial disturbances cannot produce the necessary conditions for nonaxial evolution of drops. These conditions are satisfied by a nonaxial sequence of superimposed disturbances, propagating one on top of the other. The resulting model is used to describe the evolution of liquid jets into sprays. It is postulated that every consecutive superimposed disturbance, which is characterized by a self-instability parameter, travels tangent to the surface that supports its propagation. Model outputs show that starting from the first superimposed disturbance, highly complex profiles of the jet surface are generated. Fourier analysis of the derived superimposed disturbance functions is performed in conjunction with the basic building blocks of classic instability theory. This is achieved by assigning to each term a self-instability factor. The sum of these building blocks results in intricate profiles of the jet. In these profiles, multiplicity of radial position of the jet interface as a function of axial distance provides the necessary conditions for evolution of nonaxial drops. The model accuracy, which depends on the disturbance rank, is sufficient to disclose the mechanism that turns the jet into a spray. The observed jump in the level of error is commensurate with the sudden increase in flow complexity that follows an increase in the disturbance rank. Finally, model outputs are used to study the effect of instability parameters on the evolution patterns of the jet and the nonaxial discharge of drops.

An Analysis of Swirl Injector Spray Properties under High Injection Pressure Using the PIV Method

The flow velocity fields of swirl injector sprays for direct-injection gasoline engines were measured by using the PIV method. This method made it possible to measure not only transient and complicated spray droplet velocities but also the surrounding air velocity caused by spray formation and to analyze the mechanism of spray formation. In this study, two nozzles with different spray angles were used to analyze the effects of injection pressure and ambient pressure on spray properties and the results made clear the mechanism of spray formation. It was observed in particular that under a condition of high injection pressure (20 Mpa) with high ambient pressure, spray penetration after the injection period was longer than that obtained under low injection pressure (10 Mpa). This is attributed to both the higher spray velocity and the greater surrounding air velocity.

How adjuvants influence spray formation with different hydraulic nozzles

Formation of sprays by five hydraulic nozzles (standard flat fan, pre-orifice, hollow cone, low-pressure and evenspray nozzles) were investigated in order to establish whether the proven effects of adjuvants on sprays from a standard flat fan nozzle are reproduced by other hydraulic nozzles. Measurements were made of droplet size and velocities and spray volume distribution patterns with seven spray liquids (water alone or water plus one of six adjuvants). All spray liquids had similar spray formation mechanisms through each of the nozzles and similar effects on liquid sheet length and spray volume distribution patterns. However, changes in droplet size were not the same for all nozzles, with the hollow cone and pre-orifice nozzles showing substantial differences. The resulting problems in developing a model to predict the effects of adjuvants on droplet sizes and drift potential are discussed.

The effect of some adjuvants on sprays produced by agricultural flat fan nozzles

Six commercially-available agricultural spray adjuvants were mixed with water and sprayed through a medium quality flat fan nozzle to evaluate their effect on the spray formation process and droplet size distributions. Droplet size and velocity distributions were significantly changed. Ethokem, which reduced droplet sizes and LI-700, which increased them, were selected for more detailed study with medium and fine quality flat fan nozzles. The adjuvants significantly affected the variation of droplet size within the spray and the thickness of the spray fan. Photographs showed that the mechanism of spray formation was changed by the addition of LI-700 and that droplets containing Ethokem had air inclusions.

Liquid Jet Disintegration and Spray Characteristics in Subsonic Air Streams.

The present study focuses on the problem of a liquid jet injected transversely into a subsonic air stream. Droplet sizes and droplet velocities were measured, and empirical equations of mean droplet size and mean droplet velocity profile were deduced. On the basis of the observation, the spray formation mechanism is classified into two mechanisms : one due to the instability of the liquid jet surface and the other due to the instability of the liquid jet itself. The maximum size of the mean droplet size profile in the liquid injection direction decreases with increasing air velocity and decreasing momentum ratio of the liquid and the air. The location in the liquid injection direction where the mean droplet velocity becomes minimum is coincident with the location where the mean droplet size becomes maximum. There is a strong correlation between the droplet size and droplet velocity, and larger particles have smaller velocities.

Sauter mean diameter of a diesel spray injected into an environment of high pressure.

The new empirical equations to represent the Sauter mean diameter of a spray injected from a diesel nozzle were reported in this paper. In order to determine the new equations of the mean diameter, drop sizes of a diesel spray were analyzed by a laser diffraction technique. Liquids with different viscosities and different surface tensions were tested. The maximum injection pressure and the maximum ambient pressure were 90 MPa and 3 MPa, respectively. According to the tendencies of break-up length and drop sizes of a spray for an injection velocity and an ambient pressure, the spray formation mechanism could be divided into two categories as a function of the injection velocity and physical properties of the liquid. Finally, dimensionless analysis for each category of a spray could lead tot the empirical equations of the Sauter mean diameter.

Sprays formed by flashing liquid jets

AbstractLiquids forced from a high‐pressure zone into a low‐pressure zone often cross the equilibrium pressure for the liquid temperature and disintegrate into a spray by partial evolution of vapor. The ordinary aerosol dispenser is a common example of this operation, and flash boiling is another.This paper reports on a study of the sprays formed by such a process and of the mechanism of spray formation. Sprays from water and Freon‐11 jets were analyzed for drop sizes, drop velocities, and spray patterns. The breakup mechanism was analyzed and data presented to show some of the controlling factors.A critical superheat was found, above which the jet of liquid is shattered by rapid bubble growth within it. The bubble‐growth rate was correlated with the Weber number, and a critical value of the Weber number was found to be 12.5 for low‐viscosity liquids. The mean drop size was also correlated with Weber number and degree of superheat.The spray from rough orifices and sharp‐edged orifices was compared with sprays produced from cold liquids by other techniques and was found to be comparable in all respects except temperature.

Dispersion of Sprays in Solid-Injection Oil Engines

Abstract In investigating the problem of how to improve atomization and dispersion in the solid-injection oil engine, the knowledge of the mechanism of spray formation was found to be scant. In the researches covered by this paper, it was not attempted to develop a spray theory. A few experiments were made for the purpose of clearing up which physical forces do or do not participate in the atomization and distribution process. A large number of charts have been made representing sprays under various conditions, these seeming to point to the following general conclusions: Dispersion becomes more even when injection pressure is increased, when the oil viscosity is decreased, and when the air density is increased; the cone angle increases with increasing oil pressure, increasing air density, and decreasing viscosity of oil; a larger percentage of oil reaches a given distance in a spray having a slenderer spray cone; no definite influence of orifice dimensions was noticeable in using cylindrical orifices of the dimensions employed in the tests discussed.