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Author Topic: Watch the burn: slo-mo potato gun  (Read 3834 times)

goatroper

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Watch the burn: slo-mo potato gun
« on: June 24, 2016, 02:32:29 pm »

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    JesseL

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    Re: Watch the burn: slo-mo potato gun
    « Reply #1 on: June 24, 2016, 04:33:19 pm »
    I expect that the central ignition point probably yields higher velocities since it produces two flame fronts instead of one. That should make the burn rate nearly twice as high, raising pressure more quickly and accelerating the potato faster.

    I'm a bit surprised that aspect wasn't addressed.
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    louie the lumberjack

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    Re: Watch the burn: slo-mo potato gun
    « Reply #2 on: June 24, 2016, 05:50:22 pm »
    Quote from: JesseL on June 24, 2016, 04:33:19 pm
    I expect that the central ignition point probably yields higher velocities since it produces two flame fronts instead of one. That should make the burn rate nearly twice as high, raising pressure more quickly and accelerating the potato faster.

    I'm a bit surprised that aspect wasn't addressed.

    In fairness, not that many people would consider the applicability of the combined gas law when running around with a b____in potato gun.

    louie the lumberjack
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    coelacanth

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    Re: Watch the burn: slo-mo potato gun
    « Reply #3 on: June 25, 2016, 12:36:31 am »
    Yeah, there is that but again in all fairness they also wouldn't make it out of clear polycarbonate and film it at night with a 20,000 fps camera while wearing a "...science is about to happen..." T-shirt either.   :hmm
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    Deer Hunter

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    Re: Watch the burn: slo-mo potato gun
    « Reply #4 on: June 25, 2016, 12:53:01 am »
    Quote from: JesseL on June 24, 2016, 04:33:19 pm
    I expect that the central ignition point probably yields higher velocities since it produces two flame fronts instead of one. That should make the burn rate nearly twice as high, raising pressure more quickly and accelerating the potato faster.

    I'm a bit surprised that aspect wasn't addressed.
    I need to watch the video tomorrow when I'm in front of a computer, but the rate law may be first order with respect to initial concentration and the pressure change is largely effected by a molar change in the gas composition.
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    Plebian

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    Re: Watch the burn: slo-mo potato gun
    « Reply #5 on: June 27, 2016, 04:10:58 pm »
    Quote from: JesseL on June 24, 2016, 04:33:19 pm
    I expect that the central ignition point probably yields higher velocities since it produces two flame fronts instead of one. That should make the burn rate nearly twice as high, raising pressure more quickly and accelerating the potato faster.

    I'm a bit surprised that aspect wasn't addressed.

    I have never seen anyone do velocity testing on tator guns. It would be neat to play with variables like location of ignition, location of fuel introduction, air fuel mixing etc etc.

    I could likely geek out for a few hours watching a few folks that really know their combustion and interior ballistics make the nerdiest/most efficient spud gun.
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    JesseL

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    Re: Watch the burn: slo-mo potato gun
    « Reply #6 on: June 27, 2016, 05:08:26 pm »
     :hmm
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    booksmart

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    Re: Watch the burn: slo-mo potato gun
    « Reply #7 on: June 27, 2016, 10:12:32 pm »
    Yeah, I was wondering about two combustion points, equidistant from each other and the ends of the chamber....
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    Deer Hunter

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    Re: Watch the burn: slo-mo potato gun
    « Reply #8 on: June 28, 2016, 12:16:53 am »
    Quote from: Plebian on June 27, 2016, 04:10:58 pm
    I have never seen anyone do velocity testing on tator guns. It would be neat to play with variables like location of ignition, location of fuel introduction, air fuel mixing etc etc.

    I could likely geek out for a few hours watching a few folks that really know their combustion and interior ballistics make the nerdiest/most efficient spud gun.

    And to really get a decent understanding, well I hope you've brushed up on your partial differential equations, heat and mass transfer, thermo, etc.

    A good start for combustion reaction rates would be here.

    http://webserver.dmt.upm.es/~isidoro/bk3/c15/Combustion%20kinetics.pdf

    And if you need some established rate constants for combustion reactions,

    http://www.nist.gov/data/nsrds/NSRDS-NBS67.pdf

    I could probably take a look at my reaction design textbook and come up with some sort of graph to model the reactions...
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    JesseL

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    Re: Watch the burn: slo-mo potato gun
    « Reply #9 on: June 28, 2016, 12:44:01 am »
    A little less science, more engineering!  :P

    I've got some propane injectors laying around at work. Making up a timer for precise injection quantities shouldn't be hard. Nichrome wire heaters in an insulated combustion chamber, and maybe a brushless fan inside to promote turbulence just prior to ignition.

    Use MAPP gas or maybe even straight acetlyene and we could get a full up detonation. Assuming it holds together, it would be awesome.
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    Deer Hunter

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    Re: Watch the burn: slo-mo potato gun
    « Reply #10 on: June 28, 2016, 10:58:08 am »
    Quote from: JesseL on June 28, 2016, 12:44:01 am
    A little less science, more engineering!  :P

    I've got some propane injectors laying around at work. Making up a timer for precise injection quantities shouldn't be hard. Nichrome wire heaters in an insulated combustion chamber, and maybe a brushless fan inside to promote turbulence just prior to ignition.

    Use MAPP gas or maybe even straight acetlyene and we could get a full up detonation. Assuming it holds together, it would be awesome.

    That's going to be the biggest variable for reaction rate in my opinion. If you want numerous data points, I suggest a run with and without the fan being on.
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    coelacanth

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    Re: Watch the burn: slo-mo potato gun
    « Reply #11 on: June 28, 2016, 11:30:43 am »
    Why not uncomplicate it and just use a venturi tube like a carbeurator.  For that matter you could probably get some pretty good energy going with a liquid fuel like alcohol instead of propane.   Also, it would be hard to get accurate results with a projectile as variable as a potato.  Maybe something like golf balls would be better.   :shrug
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    JesseL

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    Re: Watch the burn: slo-mo potato gun
    « Reply #12 on: June 28, 2016, 02:07:37 pm »
    That's basically what a spray bottle does, but it wouldn't keep the fuel-air mixture moving very well at the time it's ignited.

    As far as fuels go, the ease of perfectly mixing a gaseous fuel like propane probably trumps the marginal extra energy content of alcohol or gasoline. Something like nitromethane though, that might be worthwhile.
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    coelacanth

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    Re: Watch the burn: slo-mo potato gun
    « Reply #13 on: June 29, 2016, 12:26:44 am »
    See?  Now you're talking my language  .  .  .   :cool   
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