Airborne particle focusing in heated chamber using thermal emission?

  1. taylaron

    taylaron 384
    Gold Member

    Greetings PFers,

    I'm working on a project where micron size metal or plastic particles are carried inside an argon or nitrogen stream at ~1 atm. The particles travel through at 1/4'' ID nylon tube before they enter a chamber where they then travel through a pyrex tube. I have four 4'', 500W halogen light bulbs around the tube perimeter that are meant to heat up the airborne particles to their melting point.

    My problem is the particles enter the heater chamber with a stream diameter of 1/4''. I need to focus those particles down to under 0.02 in. Due to the hot nature of the chamber, stray particles will inevitably adhere to the glass tube and eventually clog the system. I'm trying to find an aerodynamic solution to my problem. OR, find a way to use the halogen heaters as an electron sprayer and have high voltage plates to focus the now charged particles into a small beam diameter.

    Is there any way I can get the electrons to pass through the glass walls of the halogen bulbs and impact the particle stream?

    Image notes:
    yellow objects are the halogen bulbs
    Pink object is the pyrex tube with focusing rings
    blue objects are the insulating caps
    grey object on the bottom is the final focusing chamber where the air is removed from around the particle stream.
    grey object on the top is a push to connect fitting
    Black objcts are stainless steel rods holding the assembly together

    Attached Files:

  2. jcsd
  3. Baluncore

    Baluncore 2,441
    Science Advisor

    The focussing of a particle stream using electrostatic or magnetic fields will be sensitive to variation of the mass of the particles and variation of their charge.
    If the nozzle that extrudes the particles in the gas stream then by connecting that nozzle to a HV DC supply would generate charged particles. That is powder coating or spray painting technology.
  4. taylaron

    taylaron 384
    Gold Member

    Thanks Baluncore,
    There will be mass variations of the particulates and obviously their charge.
    What i'm describing is indeed similar to powder coating, however, the particles in the chamber need to be heated to their melting point before leaving the chamber. I'm trying to find a way to focus and heat those particles without physical contact. I know tungsten when heated sufficiently will spray electrons from thermal emission (more so with special coatings...), but i'm concerned the glass will block the electrons. It would be very difficult to remove the glass chamber and just have the tungsten element because of the unavoidable partial oxygen environment. I need the process to be under argon because I don't want the metal particles to oxidize once they melt.

    Acoustic Focusing*
    Since the original post, i've discovered the principle of acoustic focusing of aerosol particles. I know it's a bit out of the scope of this thread, but I was hoping we could discuss the possibility of using this as a focusing technique as well.
    With acoustic focusing, I could have an acoustic wave guide couple with the top of the heater chamber to focus the particles before they enter, then another wave guide to do the fine-focusing after they're heated. This way, I'm not messing with thermal emission, burning my tungsten, and ionizing particles.

    My question would be wither or not I can use transverse acoustic waves to focus the particles as they travel the length of the tube, not just in a small area as is typically done using longitudinal waves. If I place the acoustic wave guide to the side of the tube, transverse waves should be generated perpendicular to the longitudinal wave's propagation. These transverse waves would be coaxial to the heating chamber tube, which would continuously focus the particles as they travel through the chamber. Am I right?

    transverse wavguide:

    Last edited: Dec 25, 2013
  5. Baluncore

    Baluncore 2,441
    Science Advisor

    I have no immediate answer to your question.

    Regarding the heat source as a source of charge, I doubt very much that you will get a significant charge through the glass tube.

    An ellipse has two foci. If your light source is at one focus and the particle stream is at the second focus then you will have very efficient heat transfer. With four axial light sources in four partial elliptical cylinders you can move the heat sources some way from the particle stream and improve the efficiency. You will need forced air cooling through the light chambers, that is, outside the inner pyrex tube. Because the light is convergent on the particle stream it will pass through a wide area of the pyrex tube wall. A laminar inert gas flow on the inside surface of the pyrex tube should help to keep it clean.

    I cannot see a better heat source than your quartz halogen light tubes, apart from the addition of the partial elliptical mirrors. The pyrex tube is needed to isolate the violent cooling air from the controlled particle stream. I believe all those are now “givens”.

    So the focussing challenge can now be considered a fluid dynamics problem inside the pyrex tube.

    Having a number of smaller particle jets converging about the central axis would result in a tighter helical flow along the axis of the cylinder. I believe that would be more stable and hold the stream together better.

    I now have an 8 hour drive ahead, acoustic focussing of a helical stream will give me something to think about.
  6. AlephZero

    AlephZero 7,298
    Science Advisor
    Homework Helper

    I'm not very convinced about the helical flow idea. Since the particles are denser that the gas, this is basically a centrifuge, so the particles will tend to get "de-focussed" towards the outside rather than focussed in the center.

    Another idea would be to heat the gas sufficiently before injecting the particles into it somehow, e.g. mix an annular flow of hot gas around the central gas stream with the particles.
  7. taylaron

    taylaron 384
    Gold Member

    I had envisioned a parabolic or elliptical mirror on the outside of the four halogen bulbs to reflect the outward rays.
    I'm not sure how hot the halogen bulbs can get. If the quartz glass and ceramic insulators cannot withstand the high temperatures, I will need to add forced air (assuming the mirrors don't melt...). If I do that, ceramic fiber insulation on the outside will not be needed.

    The multiple particle streams necessary for a helical flow would not be difficult in my application, but the particle density in the individual streams will not be constant. However, I'm confused as to how a helical stream of airborne particles can exist in a laminar flow due to their swirling nature.

    Aleph Zero-
    Are you suggesting we use air streams to focus the particles before they enter the chamber? If so, it would eliminate the laminar flow and the original stream diameter would not be restored through the heating chamber. Is this what you're thinking? I'd imagine not since the air doesn't need to be heated prior to injection.

    Regarding the particle streams, I need to control the stream contents to 0.1% tolerance. I'm thinking of either using:

    1. micro auger system. trouble is with the pulsing nature of an auger system. I need constant flow. Also the most complex and probably expensive and difficult to maintain. However, probably the most accurate.

    2. ultrasonic vibrator on a conveyor tray. Modulating the transducer to adjust the flow of particles down the tray slope and into the air stream. Probably the easiest to maintain, but particle injection consistency is my primary concern.

    3. air-particle spray system. Perhaps the simplest solution, but ensuring a consistent amount of particles at the tip of the air nozzle needs to be addressed. (Think of a cheap abrasive partilce pray booth) Perhaps a tall hopper of particles above a spray nozzle with a vibrator on the hopper. Varying the airflow at the tip could adjust the amount of particles being injected.

    4. Grinding up the particles using a high speed burr and having the shavings blown into an air stream. Perhaps the least accurate, highest maintenance, low particle consistency and wide variety of particle diameters. Also very expensive with replacing burrs.

  8. Bobbywhy

    Bobbywhy 1,908
    Gold Member

    Hi Tay! Long time no see here! Welcome back to Physics Forums!

    Your latest project is fascinating. Since the particles may be metallic or non-conducting, I would eliminate electrostatic focusing from your options. I’ve been looking at how others focus streams of particles in flow cytometers and in mass spectrometers. The same principles that apply to liquid streams with particles may well apply to gaseous streams with aerosol particles. Here are some examples:

    If you use acoustic focusing it may require four piezo-electric ceramic transducers (Barium Titanate, for instance) surrounding the transport tube. Since all those high intensity lamps may overheat the transducers, some shielding for them may be required.

    Aerodynamic focusing uses only nozzle and throat geometries to focus the aerosol stream. The first six references below describe these. The remaining four are examples of acoustic focusing.

    1. J . Fluid Mech. (1988). Vol1. 95, p p . 1-21
    Aerodynamic focusing of particles in a carrier gas
    This article covers aerosol focusing and high-resolution aero dynamical focusing.,d.b2I

    J.E. Brockmann, R. C. Dykhuizen, R. Cote, T. Roemer

    3. NASA Tech Briefs:
    “Nozzles for Focusing Aerosol Particles”, 01 December 2010
    These nozzles aerodynamically focus aerosol particles into a small-diameter jet.

    4. Akhatov’s research projects
    “Collimated Aerosol Beam and its Application to Direct-Write Technology”

    5. Army Research Laboratory
    Nozzles for Focusing Aerosol Particles

    6. Aerosol Science and Technology
    Volume 39, Issue 3, 2005
    Particle–Focusing Characteristics of Matched Aerodynamic Lenses

    7. Anal Chem. 2007 Nov 15;79(22):8740-6. Epub 2007 Oct 9.
    Analytical performance of an ultrasonic particle focusing flow cytometer.
    Goddard GR, Sanders CK, Martin JC, Kaduchak G, Graves SW.
    8. Ultrasonic Particle-Concentration for Sheathless Focusing of Particles for Analysis
    in a Flow Cytometer
    Gregory Goddard, John C. Martin, Steven W. Graves, and Gregory Kaduchak

    9. Real-time detection method and system for identifying individual aerosol particles
    US 7260483 B2

    10. This patent, although it describes particle manipulation in an aqueous solution, may be applicable to an aerosol of particles.
    “Method for non-contact particle manipulation and control of particle spacing along an axis”
    United States Patent 8528406
    One or more of the embodiments of the present invention provide for a method of non-contact particle manipulation and control of particle spacing along an axis which includes axial and radial acoustic standing wave fields. Particles are suspended in an aqueous solution, and this solution then flows into the cylindrical flow channel. While the solution flows through the flow channel, the outer structure of the flow channel is vibrated at a resonant frequency, causing a radial acoustic standing wave field to form inside the flow channel in the solution. These radial acoustic standing waves focus the particles suspended in the solution to the center axis of the cylindrical flow channel.
    At the same time, a transducer is used to create an axial acoustic standing wave field in the flow channel parallel to the axis of the flow channel. This drives the particles, which are already being focused to the center axis of the flow channel, to nodes or anti-nodes of the axial standing wave at half-wavelength intervals, depending on whether the particles are more or less dense and more or less compressible than the surrounding fluid.”

    There are more interesting existing focusing mechanisms that may apply to your project. You may check “the Journal of the Acoustical Society of America” and the “Review of Scientific Instruments”. The trouble with these excellent sources is that they charge $$money just to read the article! Maybe your Engineering Design and manufacturing company will spring for them! Or, your college library subscribes to them.

    Cheers, Bobbywhy
  9. taylaron

    taylaron 384
    Gold Member

    Thank you for your detailed post Bobby, I really appreciate it.

    I've been working on the acoustic focusing of the particles in the carrier gas. I calculated the frequency needed to generate a standing wave perpendicular to the 1/2'' diameter heater chamber tube.
    The speed of sound in Nitrogen is 353 m/s. With a 1/2'' glass tube ID (0.0127m)
    353m/s = 0.0127m *F Freqency = 27,795 Hz

    Most tweeters have a frequency response curve that can handle 20KHz, but most slowly drop off after that. So, a decent tweeter should be able to produce ~28KHz
    Like the Radioshack Super Tweeter: which has a decent response up to 40KHz.

    Based on the knowledge that particles subjected to a standing wave will congregate to the least turbulent area- the nodes, one can design a device to focus the particles coaxially.
    A speaker mounted perpendicular to the tube axis generating a standing wave will obviously create oscillating high and low pressure areas (except the center node...). Those oscillating high and low pressure areas will cause the air in the tube to be pushed and pulled parallel to the tube axis to respond to the pressure changes. The resulting wave generated in the tube is a transverse acoustic wave. This wave will cause particles on the perimeter to congregate to the center of the tube which is essentially one long node. The result is a straight Constant* stream of particles.

    I wonder if the acoustic waves will still drive the particles to the center of the tube even if the tube isn't straight. Sounds like a waveguide to me.

    The acoustic focusing described in this document:
    would indeed work in my situation, but you're right, maintaining a working temperature for the piezoelectric elements would be a challenge. Amplitude might also be a concern. Ultrasonic transducers aren't that expensive and i'd only need 4 (Two at the top, two at the bottom, each 90 degrees apart). The glass will get very hot which will make adhering the ultrasonic transducer to the glass will be difficult. Not many adhesives can maintain such high temperatures.

    My concern with using small orifices and laminar flows described in this paper:
    is that clogging will occurr due to the molten particles.

    The use of air streams and puffs of air to focus particles is far too sophisticated for my application. i'd like to find a simpler solution

    Attached Files:

  10. Bobbywhy

    Bobbywhy 1,908
    Gold Member

    You are correct, if the Piezoelectric (PZ) transducers are subject to high heat then the adhesive holding them on the tube must not deteriorate, nor should its acoustic impedance change significantly. Once more I remind you that the intense heat may degrade the acoustic focusing because the dynamic characteristics PZ material itself changes with temperature.

    Good comprehensive reviews of General Ultrasonic theory and applications are found below in references 1. and 2. References 3. 4. and 5. describe the behavior of common PZ materials under temperature variations.



    3. History and Challenges of Barium Titanate: Part I
    M. M. Vijatović, J. D. Bobić, B. D. Stojanović

    4. History and Challenges of Barium Titanate: Part II
    M. M. Vijatović, J. D. Bobić, B. D. Stojanović

    5. Barium titanate and phase changes
    The temperature at which the spontaneous polarisation disappears is called the Curie temperature, TC.

    Cheers, Bobbywhy
  11. taylaron

    taylaron 384
    Gold Member

    Acoustic Focusing

    Thanks for the references Bobby, but i'm going to try and avoid PZ elements because of the technical difficulties in my application. We've concluded electrostatic, magnetic, aerodynamic and now PZ aren't the best options, so i'd like to focus on acoustic waves (longitudinal or transverse)

    Because nobody seems to know wither or not longitudinal waves indeed give rise to transverse waves which would coaxially focus the particles, it seems an experiment is in order. Unfortunately that will take a while to setup.

    For an acoustic waveguide that generates a standing wave, i'd need a rectangular or circular (I've chosen circular) tube with a diameter just under 1/2 the wavelength (making that just under 1/4'') and a waveguide length some integer multiple of the wavelength. The waveguide material needs to be stiff, but can be flexible.
    Can I use a 1/4'' ID nylon tubing for the waveguide? As long as it doesn't get kinked, wouldn't it work?

    I'll model what i'm thinking here and post a picture...
  12. taylaron

    taylaron 384
    Gold Member

    I've attached what i've got in mind for acoustic focusing. It separates the powder injector and focusing source from the heater chamber. The tubing is 1/4'' nylon.

    Any obvious problems?


    Attached Files:

  13. Baluncore

    Baluncore 2,441
    Science Advisor

    It is important that your reflectors are elliptical. If they are parabolic then opposite light sources will heat each other, but not the particle stream.

    With partial elliptical reflectors each light source will have a halogen tube at one focus, and the particle stream at the other. The lights will indirectly heat each other, but only after being intensely concentrated on the particle stream.
  14. Bobbywhy

    Bobbywhy 1,908
    Gold Member

    The attached cartoon sketch shows a “tweeter” speaker mounted in a tapered waveguide that looks a lot like the Bromer Sound Transverse Waveguide. Its purpose is to act as a speaker cabinet to produce bass frequencies. In this invention a bass speaker is mounted inside and directed inward near the closed (smaller) end of a truncated radial waveguide. “The waveguide then mimics a portion of the surface of a large, pulsating sphere and a portion of the air surrounding it.” (from Patent No. US 8,066,095 B1) Thus bass frequencies are radiated outward from the open (large) opening.

    In the drawing you’ve shown, however, the large end is closed and the small end is open where it enters the stream tube; just the reverse of the Bromer invention. You propose using a tweeter as a sound source of ~38kHz. Even though this structure is intended for bass frequencies, the ultrasonic frequency may indeed exit the small end (open) waveguide and enter the gas/particle stream. But there is no reason to believe these acoustic waves will induce longitudinal (axial) standing waves in the stream column (tube). Even if they did form an axial acoustic standing wave field it would tend to cause “bunching” of the particles into groups at the wave node sites.

    The literature I’ve been able to uncover that describes “acoustic focusing” of particulate matter in flowing streams all use transducers attached to the outside wall of the transport tube. When the outer structure of the flow channel is vibrated at a resonant frequency it causes a radial acoustic standing wave field to form inside the cylindrical resonator. These radial acoustic standing waves focus the particles suspended in the stream towards the central axis of the cylindrical flow channel.

    Here’s another example:
    "Acoustic particle focusing device
    Our technology enables focusing of gas-suspended particles without a reduction of pressure. Focusing is achieved by means of a pressure field which is generated inside an X-shaped channel (Fig. 1). The channel is comprised of four resonating hyperbolic walls. Each wall emits a sound wave towards the center of the channel. These quadrupole waves interact to create a pressure field rising towards the channel's walls (Fig. 2). As gas flows through the channel, particles are drawn towards the axis by viscous forces."

    Cheers, Bobbywhy
  15. taylaron

    taylaron 384
    Gold Member

    I only expect a standing wave to be generated in the tweeter waveguide. With my current configuration illustrated in my last drawing, it would indeed be impossible to generate a standing wave parallel to the air flow without a cap at the bottom with a small orifice for the particle stream. I might end up putting a cap on the bottom to increase the amplitude of the (assumed) transverse waves.

    If i'm right, the particles will focus in a single stream (not along nodes). If I'm wrong, then the particles will bunch into nodes. It'll take me a while to get the money to fund the experiment though...
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