A STUDY OF LIQUID METAL ATOMIZATION

LIQUID METAL ATOMIZATION USING CLOSE-COUPLED NOZZLES, 1 1

Copyright © 2005 by Begell House, Inc.

Atomization and Sprays, vol. 15, pp. xx–yy, 2005

1

A STUDY OF LIQUID METAL ATOMIZATIONUSING CLOSE-COUPLED NOZZLES, PART 1:GAS DYNAMIC BEHAVIOR

S. P. MatesMetallurgy Division, National Institute of Standards and Technology, Gaithersburg,Maryland, USA

G. S. SettlesGas Dynamics Lab, Department of Mechanical and Nuclear Engineering, Penn StateUniversity[[AU: ADD City, State, USA]]

Original Manuscript Submitted: 2/26/03; Final Draft Received: 8/8/03

Liquid metal atomization using close-coupled nozzles is an established technique for fabricating fine(< 100-µm) metal powders for a variety of industrial uses. Despite its widespread use, however, theinterrelationships among gas dynamics, nozzle geometry, processing parameters, and particle sizeremain ill-defined. As a result, efforts to reduce powder costs by improving particle size control andenergy efficiency remain hindered. This study examines and compares examples of a convergent anda converging-diverging (c-d) close-coupled nozzle on the basis of their gas dynamic behavior (Part 1)and their liquid metal atomization performance (Part 2). In Part 1, Schlieren photography and Machnumber and Pitot pressure measurements are used to characterize the gas dynamic behavior of thenozzles (without liquid metal present) operating at stagnation pressures between 2 and 5 MPa. InPart 2, their liquid metal atomization behavior is examined by high-speed Schlieren photography, andparticle size distributions are measured to compare their atomization performance. Results showed thatthe two nozzles performed similarly in gas flow and atomization tests over most of the range of Po

examined, despite their significantly different geometries. The active atomization zone appeared toextend far downstream, indicating that gas velocity decay by turbulent diffusion may play a limitingrole in atomization. This also suggests that the importance of the gas-to-liquid mass flux ratio has aphysical basis associated with a ratio of velocity decay length to breakup length scales. These observationshave potentially important implications for designing efficient liquid metal atomization processes forproducing low-cost metal powders.

INTRODUCTION

Liquid metal atomization using close-coupled nozzle technology is an established methodof fabricating fine metal powders for use in powder metallurgy (P/M) manufacturing aswell as a variety of other industrial uses [1–5]. A typical gas atomizer configuration issketched in Fig. 1a. In a close-coupled nozzle arrangement, molten metal is fed through acentral tube that is surrounded by a coaxial gas nozzle. The nozzle generates a high-velocitygas stream that disintegrates the slower melt stream into fine droplets, which then freezein-flight into solid powder particles. Liquid metal atomization is usually conducted inside

The authors wish to thank J. D. Miller and Lori Dodson-Dreibelbis of the Penn State Gas Dynamics Lab,and Dr. Ali Ünal and Dr. Dennis McLaughlin for their valuable input. This work was supported by the NationalScience Foundation (CTS-9221863).

[[AU: NOTE THAT EQUATION NUMBERS HAVE BEEN CORRECTED.PLEASE CHECK ALL REFS TO EQ. NOS.]]

S. P. MATES AND G. S. SETTLES2

A* critical throat area, m2

d diameter (nozzle, droplet), mL length, mm mass flow, kg/sM Mach numberP pressure, kPaq dynamic pressure, kPaT temperature, Kγ specific heat ratioµ viscosity, N s/m2

ρ density, kg/m3

σ surface tension, n/m; std. dev.

Subscripts

a ambiente exito stagnations sonic point

NOMENCLATURE

Fig. 1 Typical arrangement of a liquid metal atomizer (a) employing a close-coupled nozzle to produce fine metalpowder (based on [3]). (b) Typical atomized metal powder.


an enclosed chamber to both confine the powder particles and, in some cases, to limitparticle oxidation. The particles are typically spherical in shape (Fig. 1b) and range fromabout 1 to 250 µm. Metals atomized commercially range from tin and tin–lead alloys toaluminum, steel, and nickel-based alloys. Gases used usually include air, nitrogen, or argon.Industrial-scale atomizers can produce tens of kilograms of powder per minute, whileresearch-scale units generally produce significantly less. Gas-atomized powders are used asrocket propellant additives, chemical catalysts, feedstock for P/M consolidation and ther-mal spray coatings, and in many other capacities.

Gas atomization is a relatively costly method for producing metal powders. Contrib-uting to its high cost are inefficiencies arising from poor energy usage and lack of particlesize control. Improvements in these areas would have a significant impact on a wide rangeof metal powder industries. Unfortunately, efforts to optimize the liquid metal atomizationprocess have been hampered by an incomplete understanding of the underlying physics,particularly that involving supersonic gas flow. For example, the role of shock waves in theatomizing gas flow has often been debated but remains unclear. Further, no reliableguidelines exist for optimizing nozzle geometry to achieve efficient atomization. Forexample, it remains unclear whether the improved supersonic expansion provided byconverging-diverging (c-d) nozzles necessarily leads to better atomization performance(smaller particle sizes) compared to convergent nozzles. Finally, although it is known thatincreasing gas pressure generally reduces particle size, the physical role of gas pressureremains poorly understood. For example, increasing gas pressure increases both the fullyexpanded gas velocity as well as the gas mass flux, either of which may contribute toreducing particle size. Is one or the other the dominant effect, or do they play equal roles?These lingering questions limit efforts to optimize the liquid metal atomization process.

In many respects this process is similar to the more often studied twin-fluid fuelatomization processes [6]. Although liquid metal solidifies readily in the cold atomizinggas stream, the solidification step generally follows the atomization step [2], so thatatomization behavior is determined primarily by liquid instability rather than phasechange. However, the unique properties of liquid metals give rise to some importantdifferences. Because the high surface tension of liquid metals makes them difficult toatomize, supersonic gas velocities are usually employed. Further, because liquid metal caneasily freeze inside the nozzle and interrupt atomization, heat transfer is a critical issue. Thenozzle must not only thermally insulate the melt, it must be chemically inert toward themelt to prevent powder contamination and/or its own erosion. The use of supersonic gasvelocities and the importance of heat transfer and chemical reactivity in the nozzle makethis a unique twin-fluid atomization process.

Optimizing a liquid metal atomization process for increased powder affordabilitygenerally involves maximizing the yield of powder particles within some desired size range(usually narrow) while consuming the least possible amount of energy in terms of com-pressed gas. Typical uses for metal powders call for narrow or specific particle size rangesthat are usually some subset of the raw powder size distribution obtained from theatomizer. For example, metal injection molding (MIM) requires feedstock powders below20 µm, while in thermal spray applications the powder must typically be larger than 20 µmand less than about 50 µm. Thus only a fraction of the powder produced meets the sizerequirements of any one application. Alternative uses must be found for the remainingpowder, or else it must be remelted and reatomized, adding cost. The ability to “tune” anatomizer for particular sizes would therefore reduce the overall powder cost by minimizing


off-size material. Energy usage in liquid metal atomization is often put in terms of theamount of gas consumed per quantity of powder produced. Thus the gas-to-metal massflow ratio (G:M), in addition to being an important physical parameter, is a key indicatorof process economy. Typical values range between 2 and 10 for industrial atomizers. Lowervalues are associated with less costly production, while higher values usually result in finerpowder, creating a trade-off between particle size and powder cost. Another desirable goalis to increase the yield of the finest particles, typically below 10 mðm. Fine powder is moreexpensive because it represents a relatively small fraction of the overall yield. Improvingparticle size control and increasing fine powder yield on industrial scales are thus the mostcritical research goals.

Research on liquid metal atomization involving close-coupled nozzles goes backdecades. The earliest scientific work known to the authors in this field was that ofThompson [7], who concentrated on aluminum. Thompson investigated the effect of themajor process variables, such as gas pressure, metal flow rate, and metal temperature, onparticle size using various nozzle arrangements. More recently, interest in rapid solidifica-tion processing in the 1980s prompted an expanded interest in liquid metal atomization.Rapidly solidified metal droplets produced by atomization were found to possess glassy(noncrystalline) microstructures with unique and desirable properties [8]. Under the rapidsolidification research initiative, an array of sophisticated experimental tools was applied toexamine the atomization of a wide range of metals and alloys. Schlieren optical flowvisualization was applied to study the supersonic gas flow patterns produced by close-coupled nozzles when operating gas-only [9–12] and when atomizing water as a surrogateliquid [13]. High-speed cinematography [9, 10] and holography [14] were used tovisualize melt breakup phenomena. Particle size analysis by laser-diffraction, including in-situ measurements [15], have been employed to measure fine powder yield at variousprocessing conditions and with different nozzle designs. Computational fluid dynamicshas been applied to model gas-only flow [16, 17] as well as liquid metal atomization itself[18]. Both convergent [9–11] and converging-diverging (c-d) [12] close-coupled nozzleconcepts, employing either annular or discrete-jet arrangements, have been evaluated in anattempt to find optimum performers. The effects of gas species [12] and gas temperature[19] on particle size have also been studied. While these efforts have led to improvementsin atomizer performance and enhanced process understanding, substantial gaps in knowl-edge, particularly regarding gas dynamics topics such as the role of gas pressure, shockwaves, and nozzle expansion conditions, remain.

In addition to these gas dynamics issues, a further uncertainty involves the primarybreakup mechanism that has traditionally been associated with close-coupled liquid metalatomization. Primary breakup is believed to involve the formation of a thin melt sheet atthe nozzle tip [1, 9–12], shown schematically in Fig. 2a. Upward-flowing gas along thecentral axis, along with radial pressure gradients acting across the base of the liquid deliverytube, are believed to create the melt sheet by forcing the liquid metal out to the nozzle edgeand into the supersonic cross flow. It is generally believed that fine powder is created by thesubsequent disintegration of this melt sheet by a wave instability mechanism akin to thecase studied by Dombrowski and Johns [20]. Despite widespread adherence to this theory,however, many observations suggest that the melt sheet does not always form. While sheetformation is observed when the liquid metal flow rate is low [21], as the flow increases, itbecomes far less certain [21, 22]. In Thompson’s original experiments, sheet formation didnot occur at all. Instead, the liquid metal formed a miniature “fountain” at the nozzle tip,


with a short intact liquid core, shown schematically in Fig. 2b. Others have described asimilar “jetting” effect under some conditions [2].

As will be shown in Part 2 of this article, the conditions examined in the presentstudy do not result in a melt sheet. Sheet formation is therefore not the only possibleatomization mechanism at work in this process. In fact, industrial-scale atomizers, whichoperate with large melt flow rates, are significantly less likely to produce melt sheets thanthe research-scale units in which sheeting has been observed. Despite this possibility, fewattempts have been made to analyze atomization under non-sheet-forming conditions.Investigating this regime further will expand the overall understanding of this atomizationprocess and may suggest new ways to approach improving particle size control andimproving efficiency. For example, our initial observations of non-sheet-forming behaviorindicated that fine powder appears to be created mostly by the secondary breakup of largeglobules that are produced by a relatively coarse primary breakup stage. Further, it was notuncommon for secondary breakup to persist well downstream of the region where sheetbreakup was previously thought to occur [23]. Thus our observations indicate that, undernon-sheet-forming conditions, the important powder-forming breakup mechanisms ap-pear to be quite different.

In fact, even under conditions favoring melt sheet formation, one investigator sug-gested that secondary breakup is important and that atomization is not necessarily limitedto the nozzle tip region. Ünal, who investigated aluminum atomization, estimated that onlyabout one-quarter of the powder was created directly by the initial (primary) disintegrationof the sheet [12]. The rest, he hypothesized, was created by the secondary breakup of coarsedaughter droplets resulting from primary sheet breakup. Further, Ünal found that nozzlesproducing longer supersonic jets generally also produced finer powder [13]. With a longersupersonic jet, he argued, elevated gas velocities are maintained farther downstream,leading to more effective secondary breakup far from the nozzle tip. Despite these obser-

Fig. 2 Melt sheet [1, 9–12] and “fountain” [7] primary breakup models.


vations, most previous efforts to improve atomization have focused on enhancing sheetbreakup, while little attention has been paid to influencing secondary breakup.

Ünal’s observations not only show that secondary breakup may be important, theyalso suggest a novel connection between the gas dynamic behavior of supersonic nozzlesand atomization performance. That is, a relationship appears to exist between the super-sonic length of the gas jet produced by the nozzle and the outcome of secondary breakupevents occurring far downstream from the nozzle tip. Understanding this connection mayshed significant light on the fundamental behavior of the liquid metal atomization process,from which important insights on optimization may be gained. For example, among rocketnozzles it is known that supersonic jet length increases significantly as the nozzle stagnationpressure increases [24]. However, no prior study has examined the effect of stagnationpressure on supersonic jet length among liquid metal atomization nozzles, or on how thiseffect might influence atomization performance. Another implication is that converging-diverging nozzles ought to produce finer powder than convergent nozzles, since the formerproduce longer supersonic jets, at least for round (rather than annular) nozzle geometries[25–27]. Ünal reported that c-d nozzles were generally superior to convergent nozzlesamong basic configurations he studied [28]. However, others claim excellent performanceusing convergent nozzles, raising doubts about the universal benefit of c-d nozzles [2]. Anexamination of the effect of stagnation pressure on supersonic jet length behavior amongclose-coupled nozzles, including comparisons between successful examples of convergentand c-d designs, would therefore prove revealing.

In the present study, the gas dynamic behavior and atomization characteristics of aconvergent and a converging-diverging close-coupled nozzle are examined and compared.A laboratory-scale atomizer is constructed with the ability to atomize small quantities ofmolten tin. The two nozzles chosen for this study are based on successful designs describedin prior literature as having an ability to atomize liquid metal effectively without sufferingfrom excessive melt freeze-off problems. In Part 1 of this article we examine the gasdynamic behavior of these nozzles over a range of stagnation pressures. Schlieren opticalflow visualization and Mach number and Pitot pressure measurements are used to char-acterize the mean flow behavior of the two nozzles both in the neighborhood of the nozzletip and farther downstream. In Part 2 we examine the nozzles liquid metal atomizationbehavior under conditions that do not favor melt sheet formation. Breakup phenomenataking place in the compressible jet flow are observed using microsecond-exposure Schlierenphotography. Particle size analysis is then performed to quantify and compare the twonozzles under equivalent atomizing conditions and to examine how nozzle stagnationpressure affects particle size. The observations and data are synthesized into an overallphysical picture of the atomization process, including both near-field and far-field effects,to supplement the current understanding.

EXPERIMENTAL DESIGN

The two close-coupled nozzles chosen for this study are shown in Fig. 3. They include anannular convergent nozzle, based on a discrete-jet design described in [9], and a converg-ing-diverging nozzle based on [12]. The nozzle dimensions were chosen to best emulatethe optimum designs identified in the cited references while maintaining some dimen-sional uniformity (e.g., nozzle tip diameter) to facilitate comparisons. It must be noted that,


in addition to differing in gas channel design (e.g., convergent versus converging-diverg-ing), these nozzles differ in other characteristics such as nozzle tip protrusion and taperangle (Fig. 2), owing to their separate heritages. Previous studies of such nozzles haveindicated that atomization performance can be sensitive to small changes about theseoptimum configurations [10, 28]. However, we did not elect to investigate these variantshere and instead focus on the difference in their degree of exit flow expansion (convergentversus c-d design) when comparing their behavior.

The nominal sonic throat area of the c-d nozzle was somewhat larger than that of theconvergent nozzle (13 mm2 to 9 mm2), causing a difference in mass flux at any givenstagnation pressure. This difference was accounted for by normalizing the results by themeasured mass fluxes when appropriate. The isentropic “design” pressure ratio of the c-d nozzle is 35 based on its exit-to-throat area ratio of 4:1. In this study, the c-d nozzle isoften operated at “off-design” conditions in order to study the effect of stagnation pressureon its gas dynamic and atomization behavior. The term “design” is misleading in thepresent context, however, since it pertains specifically to the use of c-d nozzles in thrust-producing devices and not in liquid atomizers. It is well known that operating a c-d nozzleat its “design” pressure ratio produces maximum thrust, while operating under off-designconditions diminishes thrust [29]. However, the thrust performance of a nozzle dependsonly on the net pressure force acting on its walls. Thrust is unaffected by the gas velocityfield existing beyond the nozzle exit. However, the atomization performance of a c-d nozzledepends only on the characteristics of the external gas velocity field that drives the atomi-zation event. While the external velocity field is certainly influenced by off-“design”operation (shock waves are introduced, for example), it is not clear that atomizationperformance should necessarily suffer as a result. Thus, operating at “design” conditionsdoes not have the same implications for atomizers as it does for thrust-producing devices.Further, the present c-d nozzle will never behave like an ideal converging-diverging nozzleeven when operating at its design pressure ratio, since shock waves will be formed becauseof the conical diverging section and the presence of the central liquid delivery tube.

Fig. 3 Close-coupled metal atomization nozzles investigated. Linear dimensions are in mm.


The atomizer facility is shown in Fig. 4. Compressed air is supplied to the atomizerassembly from a 454-L reservoir that is maintained at 7 MPa. The stagnation pressure inthe atomizer assembly is controlled via a two-stage regulator, and on–off flow control isprovided by a ball valve located upstream of the regulator. The system allowed a maximumstagnation pressure of approximately 5.5 MPa. Exhaust air from the nozzle is contained bydirecting it into a 200-L enclosed plastic container through a 15-cm inlet duct protrudingthrough the container’s cover. Excess air exits the container through a side-mountedexhaust duct. When metal is atomized, the container is partially filled with water to scrubthe atomized particles from the exhaust air. Further details on powder handling arediscussed in Part 2. Air mass flow is measured with a Venturi flow meter selected tominimize stagnation pressure losses [30]. The expanded uncertainty of the air mass flowmeasurements was determined to be ±7%. This uncertainty is larger than the nominal ±1%usually assigned to flow meters of this type because of the small differential pressuresachieved across the meter during operation, which resulted in a relatively low signal-to-noise ratio. Pressure measurements for the air flow meter and in the nozzle stagnationchamber were made using diaphragm-type pressure transducers. Gas and liquid metaltemperatures were measured using K-type thermocouples. Voltage data from the pressuretransducers and thermocouples were acquired at 10 Hz and averaged over several secondsto obtain steady-state measurements.

The atomizer assembly, also shown in Fig. 4, is made from a short section of schedule80 pipe (76.2 mm i.d.) fitted with extra-heavy pipe flanges. The plenum is sealed bycapping the flanges with 9.25-mm-thick aluminum plates. Holes cut in each cap plateallow the nozzle centerbody to protrude through the plenum at each end to allow liquidmetal delivery to the nozzle tip. The liquid reservoir sits atop the centerbody and connectsto a quartz capillary tube that is inserted into the centerbody to insulate the molten metaland protect the centerbody from being slowly dissolved. The nozzle shape is formed by theshape of the centerbody tip and the shape of the orifice in the bottom cap plate throughwhich the centerbody tip protrudes (see Fig. 3). Three set screws located upstream of the

Fig. 4 Laboratory atomizer facility. Linear dimensions are in mm.


nozzle throat were used to carefully align the centerbody tip inside the bottom plate orificein order to obtain a symmetric gas flow pattern. During gas-only experiments, the liquiddelivery tube is sealed to prevent excess air from being aspirated through the orifice.

A single-pass lens-type Schlieren system [31] (Fig. 5) was used for visualizing thecompressible flow patterns generated by the two nozzles. Images were captured with aCCD video camera and recorded on S-VHS videotape, after which still frames weredigitized with a PC-based frame grabber and subsequently processed for clarity. Longexposures were obtained using a halogen light source, yielding an effective exposure timeof 33 ms. Microsecond-exposure images were obtained using a strobed xenon-arc lamplight source that was synchronized to the CCD camera field acquisition rate of 60 Hz.

Radial Pitot pressure surveys of the gas flow pattern in the neighborhood of thenozzle tip were conducted using a 0.9-mm-diameter steel Pitot tube mounted on a three-axis stepper-motor-driven traverse. To minimize gas consumption, the probe was sweptthrough the flow at constant speed while recording pressure data. Probe position wasmonitored using a variable-resistance position transducer. Axial Pitot pressure surveysconducted along the gas flow centerline were performed with a 1.5-mm-diameter steelPitot tube mounted on a manually driven traverse. In the axial surveys, the probe waspositioned at each measurement location before pressurizing the nozzle. Centerline Machnumber measurements were made optically using the Schlieren system and a 10° half-wedge. Mach numbers were computed from the angle of the oblique shock wave formedat the wedge tip using two-dimensional oblique shock theory [32]. The expanded uncer-tainty in the Mach number data was estimated to be ±0.2 Mach. Beyond the optical fieldof view, Mach numbers were estimated directly from the measured Pitot pressure levelsusing the Rayleigh-Pitot formula [33] and by assuming ambient static pressure (nominally100 kPa). The sonic point was taken to be the axial location where the pitot pressureequaled 1.89 times the ambient pressure.

RESULTS AND DISCUSSION

Nozzle Tip Region Flow Structure

Our initial investigation focused on the gas flow patterns produced by the convergentand c-d nozzles in the nozzle tip region, where primary breakup is thought to occur.

Fig. 5 Schlieren optical system.


Differences in the nozzle’s gas dynamic behavior are anticipated primarily because ofdifferences in their exit pressure ratios, although nozzle tip protrusion and taper angle alsocontribute. The exit pressure ratio is the ratio of the static pressure of the flow at the nozzleexit (Pe) to the surrounding ambient pressure (Pa). It determines the amount of expansion(or compression) the flow undergoes upon leaving the nozzle to adjust to the pressure ofthe surroundings. Table 1 compares the exit pressure ratios (Pe /Pa) of the two nozzles atlow, medium, and high values of Po/Pa. These values, being computed assuming idealisentropic one-dimensional flow, are only approximate. Table 1 shows that convergentnozzle’s exit pressure ratio is always well above unity, meaning its flow will expandsignificantly in response to the surrounding pressure in the exhaust environment (e.g., thelaboratory). Because of this, the convergent nozzle is said to be highly underexpanded. Bycontrast, the c-d nozzle goes from being slightly overexpanded to being slightly under-expanded over the range tested, meaning the flow it generates undergoes only mild pressureadjustments in response to the surrounding pressure. Differences in the amount of expan-sion give rise to differences in the strength and pattern of the shock waves that develop inthese flows. The higher exit pressure ratios achieved by the convergent nozzle mean thatstronger expansion waves will form, which, by wave reflection, will subsequently give riseto stronger shock waves compared to the c-d nozzle’s flow. One objective of the presentstudy is to determine whether the stronger shock waves expected in the convergent nozzle’sflow will affect its atomization performance relative to the c-d nozzle.

Figure 6 describes the typical flow pattern produced by the convergent nozzle in theneighborhood of the liquid delivery tube. Because this flow pattern is very similar to thatproduced by plug nozzles studied in connection with rocket propulsion applications [34],many of the terms used here to label flow features are borrowed from this earlier work.Dominating the flow is the strong internal shock wave, which is created by reflections ofthe strong expansion waves formed at the nozzle exit. The internal shock wave originatesclose to the nozzle exit, then curves inward toward the flow centerline, finally terminatingon an unseen boundary. This boundary is the embedded sonic surface surrounding aninternal subsonic flow region called the wake. The wake is created as the supersonic flowexiting the nozzle separates from the outer rim of the liquid delivery tube, leaving asubsonic region of circulating flow, sometimes called a separation bubble. Because thewake is surrounded by a sonic surface, shock waves cannot cut across it. Instead, as in thecase of the internal shock, they reflect off the wake boundary away from centerline as anexpansion fan. Upon reaching the outer subsonic boundary surrounding the jet, theexpansion fan reflects again as a compression fan, creating a new shock wave. Successivereflections of these waves off sonic surfaces give rise to the classic repeating shock cellstructure known as shock “diamonds.”

Table 1 Convergent and c-d Nozzle Exit PressureRatios Calculated Using a One-Dimensional Isentropic Approximation

Pe/Pa

Po/Pa Convergent nozzle c-d nozzle

14 7.29 0.3928 14.57 0.7948 25.50 1.38


Inside the wake, the mean flow consists of a circulating pattern, as depicted quali-tatively in Fig. 6. This circulation pattern is typical of supersonic base flows studied inrocket propulsion [35]. The end of the circulation zone is marked by a single stagnationpoint on the flow axis. Beyond this point the flow accelerates once again in the streamwisedirection, eventually reaching supersonic velocities. The faint horizontal stripes visible inthe Schlieren picture are caused by a streamwise vortex sheet present within in the annularsupersonic flow region surrounding the wake. Streamwise vortices are commonly observedin underexpanded supersonic jets emanating from round nozzles [36]. Pitot pressure dataplotted alongside the Schlieren image in Fig. 6 are used to further define the shape andextent of the wake region and to reveal the structure of the supersonic jet surrounding it.The subsonic region extends approximately two nozzle diameters from the tip of the liquiddelivery tube. After six nozzle diameters, the initial “doughnut” Pitot profile begins to giveway to a single-peaked profile. Natural turbulent diffusion processes and entrainment ofsurrounding air act to smooth and broaden the jet velocity profile as one proceeds down-stream. Although velocity information cannot be extracted from the Pitot pressure mea-surements because the static pressure distribution is unknown, the Pitot data neverthelessshow that velocities are generally much lower in the wake region compared to the super-sonic jet surrounding it. Therefore liquid breakup will be much more intense outside ofthe wake region than inside it.

In Fig. 7, a series of Schlieren pictures reveals how the convergent nozzle flow patternvaries with stagnation pressure. Of particular interest is that, when the overall pressure ratio(Po/Pa) exceeds about 52, the flow pattern changes dramatically and a Mach disk appearsas the internal shock crosses itself. In round nozzles, a Mach disk is generally present whenPe/Pa > 2 [37]. Since the exit pressure ratio of the convergent nozzle always exceeds 2(Table 1), one might have expected to see a Mach disk at all the pressures examined here.

Fig. 6 Schlieren image of the convergent nozzles’ near-region flow field (left) in the open-wake configuration(Po/Pa = 45), a flow field sketch (right), and Pitot pressure (Pt2) data (top).


However, because of the presence of the embedded sonic boundary surrounding the wake,the internal shock is prevented from crossing to form the Mach disk at pressure ratiosbelow 52. The transition occurs because, as Po increases, the point where the internal shockis intercepted by the wake boundary moves progressively farther downstream, as Fig. 7shows. Eventually this causes the wake to suddenly collapse to a smaller size, allowing theinternal shock to propagate freely through the jet and cross to form a Mach disk. Thecollapse of the wake and the subsequent Mach disk formation occurs very suddenly, at awell-defined pressure ratio. We label the flow pattern containing the Mach disk as being“closed wake,” while the previous pattern is referred to as “open wake.” The terms “closedwake” and “open wake” are borrowed from [34], but here they carry a slightly differentmeaning. In the present context they refer specifically to the presence or absence of theMach disk structure. In [34], “wake closure” refers to the point at which the wake regionloses pressure communication with the surroundings, which happens when the wake firstbecomes completely enclosed by supersonic flow. In our case, however, the wake is never“open” in this sense, since it is always completely enclosed by supersonic flow.

When the wake closes, the pressure acting at the delivery tube orifice drops dramati-cally. This is shown in Fig. 8, which plots the pressure measured in the liquid delivery tubeagainst overall pressure ratio. The pressure acting at the tip of the liquid delivery tube iscommonly referred to as the aspiration pressure, since it is usually below the ambientpressure and thereby helps aspirate liquid through the tube. At wake closure, the aspirationpressure of the convergent nozzle drops sharply. Similar aspiration pressure behaviorobserved by other investigators [9, 10] indicates that wake closure is common amongconvergent close-coupled nozzles used for atomizing liquid metal.

Understandably, there has been considerable discussion of the possible effects of wakeclosure on atomization performance. Some have argued that the presence of a Mach diskprobably enhances atomization due to the rapid pressure rise across the wave [10]. Othershave suggested that wake closure may affect melt flow stability, which can contribute to poorparticle size control [38]. In the present experiments, Schlieren photography revealed that theintroduction of molten metal into the flow causes the wake to reopen and the Mach disk to

Fig. 7 Schlieren images of the convergent nozzles’ near-region flow field at increasing values of Po/Pa (indicatedbelow each image). Wake closure occurs in the region 52 < Po/Pa < 55.


disappear [23]. It was therefore concluded that wake closure did not influence atomizationperformance in our experiments. Wake closure may have an impact, perhaps, if the condi-tions are such that transitions between closed-wake and open-wake structures occur ran-domly while the atomizer is in operation. In such a case, the resulting abrupt changes inaspiration pressure might then affect the stability of the metal flow rate and thereby influenceatomization results. Such an extremely unstable situation requires careful balance betweenmelt flow rate and stagnation pressure, however, making it an unlikely operating mode formost atomizers. For the present study, wake closure is not considered to have an importanteffect on the atomization behavior of the convergent nozzle.

The flow generated by the c-d nozzle in the near region is described in Fig. 9. Becauseits exit pressure ratio is always near unity, the c-d nozzle produces a much less prominentinternal shock wave compared to the convergent nozzle. As a result, wake closure does notoccur. As Fig. 8 shows, the aspiration pressure declines smoothly as Po/Pa increases and thereare no sudden changes. Further, the c-d nozzle’s flow lacks the prominent streamwise vorticesthat were present in the convergent nozzle’s highly underexpanded flow. The wake flowpattern is assumed to be similar to that of the convergent nozzle, one of low-velocitycirculation. However, c-d nozzle’s wake is somewhat broader, and it lacks the concave shapeof the convergent nozzle’s wake. These differences result from its straight-walled liquiddelivery tube design and the lack of a strong internal shock wave. As a result, the velocity andpressure distributions inside the c-d nozzle’s wake are different from the convergent nozzle’swake, possibly enough to alter primary breakup behavior. This will be investigated further inPart 2 of this article.

The foregoing results pointed out a number of similarities, and a number of differ-ences, in the gas dynamic behavior of the two nozzles in the near region. Both produce alarge wake consisting of a low-speed circulating flow surrounded by a high-velocity,supersonic annular jet. Differences in the exit pressure ratio as well as the delivery tubegeometry of the two nozzles lead to different shock wave patterns and wake zone charac-teristics. Because of its highly underexpanded exit flow, the convergent nozzle undergoes

Fig. 8 Variation in delivery tube pressure (Ptip) with Po/Pa.


wake closure, in which the wake suddenly collapses, the aspiration pressure drops dramati-cally, and a Mach disk appears. By contrast, the c-d nozzle undergoes no such wake closureevent, most likely because of its near-proper supersonic expansion, resulting in a weakerinternal shock wave. However, since the Mach disk is eliminated from the convergentnozzle’s flow when atomization begins, wake closure is not a factor in the present experi-ments. In most other respects, the experimental results indicated little reason to expect thereto be gross differences in the atomization behavior of the two nozzles in the near region.This will be examined in Part 2.

Far-Field Flow Behavior and Comparison

Schlieren images of the gas flow patterns produced by the two nozzles at up to 12diameters downstream of the nozzle tip are shown in Fig. 10. Images are taken whenoperating the nozzles at nominally low, medium, and high stagnation pressures. In eachcase, the annular jet structure near the nozzle exit gives way to a predominantly round jetstructure a few diameters downstream of the nozzle tip. The round jets exhibit the classicrepeating shock “diamond” patterns typical of imperfectly expanded jets produced byround nozzles [39]. Figure 10 also reveals the dramatic effect of Po on the length of thesupersonic region extending from the nozzle exit. This effect, which has generally beenoverlooked in previous work, is anticipated to have an important impact on secondaryatomization events that take place well downstream of the nozzle tip region.

Comparing the two nozzles, Fig. 10 indicates that, at Po /Pa ≈ 14, the c-d nozzleproduces a supersonic jet that is roughly twice the length of the jet produced by theconvergent nozzle. Because of the limited field of view of the schlieren optics, however,comparisons at higher stagnation pressures cannot be made. Instead, jet lengths are com-pared using a combination of centerline Pitot pressure measurements and wedge Machnumber measurements. These data, shown in Fig. 11, confirm that the c-d nozzle pro-

Fig. 9 Schlieren images of the c-d nozzles’ near-region flow field at increasing values of Po/Pa (left), flow field sketch(right, bottom), and Pitot pressure (Pt2) data (right, top) acquired at Po/Pa = 53.


duces a significantly longer supersonic jet than the convergent nozzle at Po/Pa ≈ 14.However, at the higher two stagnation pressures (Po/Pa ≈ 28, 48), the supersonic lengthsare comparable, although the c-d nozzle enjoys a slight advantage at Po/Pa ≈ 48. Some ofits advantage derives from its greater mass flux, as noted in the experimental section.Overall, the data indicate that, despite large differences in exit pressure ratio, the twonozzles produce roughly similar supersonic jet lengths.

This result appears to contradict literature data that suggest c-d nozzles producesignificantly longer supersonic jets than convergent nozzles under equivalent conditions[25–27]. One might argue that the present c-d nozzle fails to produce a longer supersonicjet because it never achieves shock-free flow. However, the oblique shocks formed in mildlyoff-design supersonic jets have been shown to have little influence on velocity decay orsupersonic length [39]. The surprise, therefore, may not be that the c-d nozzle producesa shorter supersonic jet than it should. Rather, it seems that the convergent nozzle performsbetter than expected. The literature suggests a possible explanation for this apparentlyunexpected result. The shortening of supersonic length due to underexpansion effectsappears to be related specifically to the presence of a Mach disk in the jet. As alreadymentioned, oblique shock cells are known to have little effect on jet velocity decay. Further,experimental data indicate that supersonic lengths produced by convergent and c-d nozzlesbegin to diverge at jet Mach numbers above about 1.5 [40], which is close to the pointwhere the Mach disk forms in convergent nozzles (e.g., when Pe /Pa > 2) [37]. The presentconvergent nozzle does not produce a Mach disk until Po/Pa > 52, just beyond the pressure

Fig. 10 Far-field flow patterns produced by the convergent and c-d nozzles at various values of Po/Pa (as indicated).


range under investigation. Thus its surprisingly good performance may be due to the factthat it avoids producing a Mach disk despite being highly underexpanded because of thepresence of the embedded wake zone. In fact, subsequent supersonic jet length measure-ments made on either side of wake closure show that supersonic length is indeed reducedas soon as the Mach disk appears [41].

Figure 12 shows that the present close-coupled nozzles produce significantly shortersupersonic jets compared to ideal round supersonic nozzles [24] under equivalent condi-tions. The points in Fig. 12 are computed by normalizing the supersonic length measure-

Fig. 11 Centerline Mach numbers from wedge measurements (circles) and Pitot pressure measurements (squaresand triangles). Squares are calculated from the Rayleigh-Pitot formula [32], and triangles are calculated assumingisentropic subsonic flow (Po/Pa values in parentheses).


ments from Fig. 10 by the exit diameter of a hypothetical perfectly expanded c-d nozzleoperating with the same overall pressure ratio (Po/Pa) and critical throat area (A *). Thisnormalization procedure (details of which appear in [42]) eliminates mass flow from thecomparison, highlighting the effect of nozzle geometry on supersonic length. Gas massflow data needed to normalize the supersonic length data are shown in Fig. 13. The datashow a power-law dependence on Po rather than the expected linear dependence. Webelieve the nonlinear behavior of the mass flow data is caused by small expansion of thesonic throats of the nozzles due to pressure-induced stresses, which cause the bottomplate to flex slightly with increasing Po. Discharge coefficients computed from themeasured mass flow rates using the nominal critical throat diameters are also plotted inFig. 13. The wide variation in Cd with pressure ratio is uncharacteristic of critical nozzlesand is also thought to be due to throat expansion effects. Mass flow curves obtained afterrepeated nozzle installations agreed to within the measurement uncertainties, however,indicating that the installation procedure produced consistent unstressed sonic throatdimensions and that the nozzle did not deform plastically when stressed.

The mass-flow normalized supersonic length data in Fig. 12 are fitted reasonablywell by the following empirical equation (following [24]):

Fig. 12 Comparison of normalized supersonic length measurements with an empirical correlation for perfectlyexpanded round supersonic jets [24].


2(5 0.8)L

M Ad= + ⋅ (1)

An attenuation coefficient (A ) has been included to account for nozzle geometry effects.Equation (1) is plotted to fit the present data in Fig. 12 using A = 0.65. Thus thesupersonic jets produced by these close-coupled nozzles are approximately 35% shorterthan equivalent perfectly expanded supersonic jets produced by round (rather than annu-lar) nozzles. The attenuation is most likely caused by the annular geometry of the close-coupled nozzles, which leads to a larger jet surface area compared to round nozzles, leadingto more rapid turbulent diffusion of the jet’s momentum. The Schlieren photographs ofFig. 10 confirm that by the time the present jets acquire a full velocity profile (a few

Fig. 13 Air mass flow rates measured by Venturi flow meter and power-law regressions (top) and dischargecoefficient estimates (bottom) for convergent and c-d nozzles.


diameters downstream of the wake region), they are 25–50% thicker than their perfectlyexpanded round equivalents. The jet produced by the c-d nozzle at Po /Pa ≈ 14 fails to followthis pattern, however, as it apparently does not suffer from significantly accelerated diffu-sion (Fig. 12). At present, this behavior is not well understood. However, it indicates thatthe attenuation of these jets is more complex than the simple model presented here.

In addition to supersonic length, an additional flow quantity of importance toatomization behavior is the dynamic pressure, q. Dynamic pressure in part determines theWeber number applicable to both primary and secondary breakup. It can be calculatedfrom the centerline Mach number data if the local static pressure (P ) of the flow is knownusing the following [43]:

212

q PM�= (2)

Thus, for a given static pressure, higher Mach numbers mean higher dynamic pressuresand larger corresponding Weber numbers. However, in the present case the static pressuredistribution is unknown and varies due to the presence of shock cells. An approximationof the average dynamic pressure along the centerline of these jets can be made by assumingthe static pressure is everywhere equal to the ambient pressure. Essentially, this averagesout the effect of the shock cells, across which the static pressure rises and falls about theambient pressure. Smooth polynomial fits are drawn through the centerline Mach numberdata obtained with the convergent nozzle in Fig. 14 to approximate how the centerlinedynamic pressure distributions change with increases in Po. For the convergent nozzle,there is a significant increase in the peak centerline Mach number when Po /Pa increases

Fig. 14 Effect of pressure ratio on the average centerline Mach number distributions produced by the convergentnozzle.


from 14 to 28, but there is little increase beyond this pressure. Thus, while the maximumdynamic pressure rises initially with Po, it begins to level off with further increases.However, due to the large increase in supersonic length aided by the increasing mass flux,the dynamic pressure remains at a high level over a significantly longer distance. Therefore,increasing Po/Pa from 28 to 48 on the convergent nozzle primarily extends the high regionof high dynamic pressures rather than increases the maximum dynamic pressure available.Thus Po has a more dramatic effect on the extent of the aerodynamic forces available toatomize liquid metal than on their peak magnitude. Thus atomization enhancementachieved by increasing Po may have much to do with its effect on secondary breakupoccurring downstream of the nozzle tip, rather than on primary breakup.

SUMMARY

This study examined and compared the gas dynamic characteristics of convergent andconverging-diverging close-coupled nozzles used for atomizing liquid metals to producefine metal powders. Schlieren photography and Pitot pressure surveys were used to char-acterize the mean gas flow fields generated at different nozzle stagnation pressures in thenozzle tip region as well as farther downstream. Each flow field contained an embeddedregion of separated flow (wake) at the base of the liquid metal delivery tube surrounded byan annular region of supersonic flow. This low-velocity circulating flow zone typicallyextended two to four nozzle tip diameters downstream. Beyond the wake, the annularsupersonic flow region gradually gives way to a supersonic jet with a full (single-peaked)velocity profile, similar to an imperfectly expanded supersonic jet generated by a roundnozzle. The convergent nozzle flow is highly underexpanded over the entire test range. Itsnear-region flow pattern is dominated by a strong internal shock wave. The interactionbetween this strong shock wave and the wake region causes an abrupt change in flowstructure, called wake closure, at a pressure ratio between 52 and 55. Wake closure involvesthe sudden appearance of a Mach disk structure and a precipitous drop in the pressureacting at the liquid delivery tube orifice. The c-d nozzle, by contrast, because it operateswith a near-unity exit pressure, does not produce a strong internal shock wave and suffersno wake-closure event.

Increasing the nozzle pressure ration from 14 to 55 produced a two- to fourfold increasein supersonic jet length and more moderate increases in the maximum dynamic pressuregenerated along the flow centerline. Both effects are believed to play a role in enhancingatomization (reducing mean powder particle size) as the stagnation pressure increases, particu-larly through their influence on secondary breakup. The convergent and c-d nozzles producedsimilar supersonic jet lengths and dynamic pressures at equivalent values of Po, indicating thatnozzle geometry had less influence on the external velocity fields generated by these nozzles thandid Po. The c-d nozzle did produce a significantly longer supersonic jet than the convergentnozzle at the low end of the stagnation pressure range (Po/Pa ≈ 14), however. The overallsimilarity in the gas dynamic behavior of these nozzle suggests that they also ought to performsimilarly in atomizing liquid metal at medium and high pressure ratios, although not at Po/Pa

≈ 14, where the c-d nozzle generates a longer supersonic jet. In Part 2 of this article, theatomization behavior of these nozzles is examined and compared at low, medium, and highstagnation pressures to test this assumption and to examine the effect of increasing supersonicjet length with Po on particle size.


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A STUDY OF LIQUID METAL ATOMIZATION