thermoacoustic streaming and ultrasonic processing of low melting melts e.h. trinh

8/9/2019 Thermoacoustic Streaming and Ultrasonic Processing of Low Melting Melts E.H. Trinh

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ABSTRACT

Ultrasonic levitation allows the processing of low

melting materials both in 1 G as well as in microgravity.The free suspension of the melts also facilitatesundcrcooling, permitting the measurement of the physicalproperties of the metastable liquids. A convenient methodto melt a levitated sample involves its spot heatingthrough a focused radiant source, the heat input to thesample is controlled by the material emittance as well asthe external convective flows. Because of high intensitysound fields required for levitation, thermoacousticstreaming will significantly increase the heat transfer fromthe sample to the environment, and it will thereforedecrease the heating efficiency. Experimental measurementinvolving flow visualization and power input monitoringhave allowed the quantitative assessment of thisenhancement in heat transfer at ultrasonic frequencies andfor millimeter-size samples. A decrease of temperature byup to 150 C for a sample initially at 550 C without thesound has been measured. Other results involving normal1 G and low gravity flow visualization and materialprocessing arc presented.

THERMOACOUSTIC STREAMING AND ULTRASONIC PROCESSING OFLOW MELTING MELTS

E.H. TrinhJet Propulsion Laboratory

California Institute of Technology

Pasadena, CA

INTRODUCTION

A low gravity environment is ideally suited forexperimental studies involving the melting andsolidification of materials in the absence of a container.The drastic reduction in the effects of the Earthgravitational acceleration allows the use of much lower

levels in the electromagnetic 12, electrostatic 3, oracoustic

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fields used to remotely position the sarnplc ofinterest. Depending upon the material properties, thecomposition of the processing medium, and the levitationtechnique, the reduction of gravity could allow the studyofphenomena not ordinarily observed, or it could lead to

more accurate experimental measurements. For exampthe decrease in the magnitude of the electromagnetic firequired for positioning a molten metal droplet in l

gravity could result in the enhanced supercooling of tliquid sample. The reduction of the acoustic intensity usfor sample levitation in a gaseous host medium almeans a reduced level of acoustically induced convectflow around the levitated sample. The effects of acoussheaming asswiated with ultrasonic levitation is tsubject of this experimental investigation,

The specific results to be reported below have beobtained from a flow visualization study of streamiflow fields in a single-axis ultrasonic hwitator operating25 kHz and in 1 G. A quantitative assessment of tenhanced convective heat transfer from a locally heatsample has been obtained for moderately high samptemperature, and the results arc summarized. Finalsome results of the levitation processing of low-meltimaterials in 1 G using ultrasonic techniques arc describe

1. THE EXPERIMENTAL TECHNIQUE

A standard single-axis ultrasonic levitator (Trinh, 198is used to levitate the liquid and solid samples toprocessed in a gaseous environment and at the focuseither a Nd-Yag laser or a Xenon arc lamp. Figurprovides a schematic representation of the apparatus, ultrasonic standing wave with an integer number of hwavclcngths (typically 2 or 3) is established betweendriver and reflector, and a sample can be levitated at aone of the pressure nodes (acoustic velocity antinodcs).radial dependence of the acoustic pressure distribution, dto natural beam divergence as well as due to the curvatof the rcfkxtor, generates a lateral restoring force usedcenter the sample along the symmetry axis of levitator. In addition to the second-order radiation presswhich is at the origin of the levitation force, a seco


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order steady-state convective flow, or acoustic streamingfield, is also generated, These flows are the ultrasonicequivalent of the streaming flow fields investigated byGopinath and Mills 5 (1993) in the same context of theapplication ofacouslic levitation. The visualization ofthese flows is carried out through light scattering bysmoke particles under a laser sheet illumination. The

resulting streamlines are imaged by a video camera andrccordcd on tape,The temperature of the heated samples is determined

through embedded thermocouples when they aremechanically suspended, and by the use of an infraredimaging camera when they are levitated,

FIGURE 1SCHEMATIC DESCRIPTION OF EXPERIMENTALAPPARATUS. THE SAMPLE (E) IS LEVITATED IN THEAXISYMMETRIC STANDING WAVE BETWEEN THEULTRASONIC DRIVER (C) AND THE REFLECTOR (D). THELIGHT SHEET (B) IS USED TO VISUALIZE THE FLOWFIELD THROUGH SCATTERING FROM SMOKEPARTICLES. A XENON ARC LAMP (A) BEAM ISFOCUSED ON THE SAMPLE THROUGH A LENS (F).

Il. FLOW VISUALIZATION OFTHERMOACOUSTIC STREAMINGIncense smoke was used as a tracer for these flow

visualization studies. Previous investigations underisothermal conditions have revealed that the axialsymmetry of the levitator was extended to the acousticstreaming flow field, This symmetry would notnecessarily be preserved, however, in the case of a locally

heated levitated or mechanically held sample because spot heating is generally not along the axis of symmetWith this caveat in mind, we have carried out flovisualization using sheet lighting parallel to the levitaaxis of symmetry which is vertically oriented parallelthe gravity axis. The heated sample was simulated bcylindrical mechanically held thermistor (1,9 mm diame

and 6.35 mm long). The temperature of the thermiswas varied by changing its drive voltage; its resistanwas monitored by measuring the current input.experiments were carried out at an ambient rootemperature of about 23 C.

The ultrasonic frcxjuency was 25 kHz and the maximuSound Pressure Level or SPL was 155 dB. The SPLdefined as a logarithmic relative pressure measuremwith a fixed reference ~ef = 0.0002 I .LBar expressed in dB

Figure 2 is a photograph of a video frame showing flow pattern around the unheated cylindrical thermiswith ultrasound at 145 dB. The sound driver and t

refktor can be seen at the bottom and top of the pictutogether with the primary and secondary eddies. Aenclosure has been placed around the levitation regionorder to slow smoke dissipation, This enclosure does nsignificantly contribute to the primary standing wavethe region of interest around the sample. The primary of eddies would be present in the chamber even withthe thermistor, while the secondary eddies are attachedthe thermistor and are caused by its presence. The primaeddy flow direction is counter-clockwise on the right sand clockwise on the left side of the picture; the directiof flow in the secondary eddies is directly opposiTheory predicts a symmetrical set of vortices above abelow the cylinder for streaming around a cylindricashaped body. In practice, the superposition of the primstreaming flows caused by the enclosure reinforces tupper set of eddies, but opposes the lower vortices.

Figure 3 shows a still video frame for a thermisheated to 150 C in an ultrasonic standing wave with tSPL at 140 dB. In addition to the secondary vorticattached to the upper part of the thermistor, a lower setvortices reappear on the lower side of the thermistor. Treappearance of the lower vortices is probably duenatural convective flows coupled to the temperatugradient caused by the hot sample immersed in a coogas environment.

Figure 4 shows the streaming flow field immediate

around the thermistor for a temperature of 450 C and tSPL at 145 dB. The upper tidies are no longer prominebut the lower eddies are sharper and detached fromsample. These lower eddies are also split into two setscounter-rotating vortices. The outer component of tpair periodically stretches outward, sheds, and reforcloser to the inner component, The frequency of vort


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shedding is controlled by the tuning of the ultrasonicstanding wave. For a given sample temperature, a steadyflow pattern can be established and maintained if carefultuning of the ultrasound is implemented. The flow pattcmaway from the sample is also significantly modificxl bythe addition of heat transfer. The influence of naturalconvection manifests itself through the shift in the

location of the attached secondary vortkxs: free convectioninduces a flow opposite to the original primary streamingflow of the isothermal chamber and reinforces the lowerset of secondary vortices while washing out the upperpair. In a low gravity environment the morphology of theflow would be strictly dictated by the primary streamingestablished by the ultrasonic wave and the walls of theenclosure.

FIGURE 2FLOW PATTERN AROUND THE UNHEATED CYLINDRICALTHERMISTOR WITH AN ULIRASONIC WAVE AT 145 dB.

FIGURE 3FLOW PATTERN AROUND THE THERMISTOR HEATEDTO 150 C WITH SOUND AT 145 dB.

FIGURE 4FLOW PATfERN WITH THERMISTOR AT 450 C ANDSOUND AT 145 dB


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Acoustic drive voltage (mV)

FIGURE 6

THERMISTOR TEMPERATURE AS A FUNCTION OF ACOUSTIC DRIVE FOR DIFFERENT lNITtAL TEMPERATURESIV. DISCUSSION

Previous experimental studies of thermoacoustAT(C) h(W/m2K) Nu=hd/K Nu/(RJ005 streaming have also included flow visualization and htransfer coefficient measurement 7*8. MOSL of these rcsuwere obtained for a horizontally directed fraveling sou

57 63 4.13 1.99 wave normal to the axis of the cylinder and for a ratio132 125.6

the acoustic half-wavelength to the cylinder diame7.02 3.46 which is at least equal to 6 (L/2d is equal to 3 in t

307 1S6 6.70 3.74 present work). The ratio of the acoustic particdisplacement to the diameter of the cylinder a/d was equ

397 165.4 6.41 3.75 to 0.016 in the previous work while it ranges from 0.0to 0.028 in the current investigation, In additionprevious flow visualization results were obtained w

TABLE I flowing smoke streams while the present ones have beRESULTS OF THE CALCULATION OF THE AVERAGE based on initially stationary smoke particles. The previoHEAT TRANSFER COEFFICIENT AND THE AVERAGE studies could not therefore reveal the superpositionNUSSELT NUMBER FOR FOUR DIFFERENT isothermal streaming with the thermoacoustic componeTEMPERATURES. These differences explain why the current measurement

the average heat transfer coefficient for forced convectcmoling yield results which area factor of 5 larger than . values obtained in the earlier studies.


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The current flow visualization results suggest thatnatural convection plays a significant role in thedetermination of the morphology of the flow field: theresulting flow configuration is a superposition of theisothermal streaming field, natural Convcxlive flow, andthe streaming flow associated with the sample itself, The

principal controlling parameters arc the temperaturedifference AT and the acoustic parlicle displacementamplitude relative to the sample diameter a/d. Formoderate temperature differences ( AT c 500 C ), twodistinct flow field regimes appear to dominate near thesample: asymmetric paucm of four eddies both above andbelow the sample at lower sound intensity (SPL c 145dB) and an asymmetric distribution where two sets ofcounter-rotating tidies are found at the lower half of thesample. Vortex shedding can happen, and it is probablyrelated to the stability of the standing ultrasonic waveunder thermal fluctuations. Careful tuning of the acousticwave always appears to stabilize any large scaleoscillation of the flow field. One must keep in mind that

the quasi-isothermal streaming flow field characteristic ofthe chamber provides a background which must be takeninto account in the detailed analysis of the overall fluidflow distribution. At very high sound levels ( SPL >150dB) the smoke particle visualizxitiontcchniquc is no longereffective because of the high velocities attained, and noclear structure can be resolved, It seems, however, that thelower vortex pattcm attached to the sample still remains,even in the midst of what would appear to be turbuIcntflow.

v. SAMPLE Processing EXPERIMENTS

Ultrasonic levitation melting, undcrcooling, andsolidification in a gaseous environment of low meltingmetals, and organic or inorganic materials have beencarried out in ground-based laboratories in isothermallyheated chambers

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, Spot heating oflevitatcxt samples ismore difficull, however, because of the local thermaldisturbance affecting an essentially rather well tunedsystem, The non-trivial task of maintaining levitationunder the inftuencc ofthcrmo-acoustic streaming has beeninvestigated in 1 G, and it can be accomplished for asample-environment temperature difference of about 500 Cfor low density materials. This implies that themicrogravity implementation of such a capability has

been verified. The extension to higher temperatureprocessing could bc accomplished through thecombination of a high temperature isothermal facilitycoupled to a spot heating capability.

The spot heating of materials for containcrlessprocessing is advantageous in terms of high heating and

ceding rates, but it presents the disadvantage of requirthe implementation of reliable remote temperatmeasurement capabilities. In addition to standard sinand multi-color pyrometers, we have chosen to impleminfra-red imaging cameras for high temperature Earth-badevelopmental studies as well as for experimentsmaterial processing.

Figure 7 reproduces video displays of an infra-imaging camera recorded during the levitation melting ospherical PO] ymcr sample in 1 G, The sample startsoften and increases in volume due to boiling of volacomponents. It ultimately distorts and solidifies intnon spherical sample.

Because of the thermal input from the spot heating, ultrasonic standing wave undergoes oscillations whcause sample translational vibrations. These positioinstabilities (also accompanied by rotational motionturn initiates thermal fluctuations because the sample a time-dependent motion into and out of the heating be

or focal region. Earth-based processing is more sensitto this instability process because of the high soupressure lCVCIS required for levitation. The practisolution to this problem involves real-time, fast responultrasound retuning as well as slow increases in the hinpuL A closed-loop feedbacksystcm coupling the soutuning and intensity to the sample motion and heat inis required for Earth-based controlled processing. Tsample behavior in microgravity should remqualitatively similar cxccpt for slower positiofluctuations duc to the smaller magnitude restoring foThe same active control capabilities for the levitator heat input, however, will still be required.

Figure 8 are photographs of the IRimagcr output forEarth-based levitation heating of a low density shuttlesample (3 mm in diameter). The series of photograshows the typical fluctuations in position of the sampThe maximum temperature recorded is approximately to 550 C, A substantial thermal gradient exists on surface of tic sample as it is heated from onc directiona 2 mm diameter gaussian beam from a Nd-YAG Claser.

Figure 9 shows the various stages of melting solidification of an O-Terphcnyl sample in low gravduring the parabolic flight of the NASA KC-135 airplaAbout 15 seconds are avaitablc at an acceleration leveabout 0.05 G for positioning, melting, and solidifying

sample. The resulting oblate shape is caused by the higintensity sound required for levitation under 1.8 G durthe high G part of the parabolic flight.


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W. SUMMARYThese preliminary experiments have provided str

evidence for the significant role played by thcrmo-acoustreaming in the levitation processing of materials. Fvisualization has shown that the resulting convectiveis a combination of forced isothermal streamcharacteristic of the chamber, near sample boundstreaming, and finally natural buoyancy flows. reduction of the gravitational component tsignificantly alters the flow morphology by virtueliminating the natural buoyancy component andreducing the maximum acoustic field intensity requiredsample positioning.

Results of the measurements carried out in this walso suggest that the enhancement of heat transfer quiincreases for sound pressure levels up to 150 dB,

levels out at the very high SPL. Since the lower enthe sound intensity scale is likely to be implementelow gravity, the effect of thermo-acoustic streamshould remain an important factor in micrograprocessing of materials in gases. Thermo-acoustreaming affwts not only the efficiency of spot heatinfreely suspended samples, but it also significantly degrtheir positional and rotational stability within ultrasonic field. An appropriately designed feedbsystcm tying the sound tuning and intensity andinput controls to sample positional information wilrequired for both ground-based and micrograprocessing.

Earth-based processing of low-melting polymeric metallic materials has provided early cvidcncc offeasibility of spot heating for moderate samenvironment temperature difference. An cxpcrimcntaldetermined upper limit of about 500 C for the samenvironment temperature difference has been obtainedground-based processing of low temperature samples. environment temperature can of course by as highrequired as long as isothermal conditions arcmaintaand the positioning (levitation) system remains effecti

FIGURE 7

LEVITATION PROCESSING OF A SPHERICALPOLYMERIC SAMPLE IN AN ULTRASONIC FACILITYWITH FOCUSED XENON ARC LAMP HEATING


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FIGURE 8

IR IMAGER OUTPUT DURING THE SPOT HEATING OF A LEVITATED SHUITLE TILE SAMPLE IN 1 Go


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thermoacoustic streaming and ultrasonic processing of low melting melts e.h. trinh

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