A commonly accepted temperature when wood starts to pyrolyze to form burnable gases is 250C. If you light those gases with a flame they will burn. But if you heat wood up in a closed box with no flame to start it, it may require a temperature of 450C before it spontaneously bursts into flame. Since temperature is raised by thermal exposure over a period of time faster than dissipation- then, with proper factors determined, it can be easily calculated the maximum thermal exposure time necessary to heat brush (and dry grass) to ignition temperature and design a solution that is well below this at all times.
Dave is correct- a glo-plug does not put out a lot of energy per second so it is easy to "swamp" it with fluid or airflow or thermal mass. Turning off the glo-plug will result in very rapid cool-down. It would probably require direct contact with an active glo-plug element by dry grass (or similar) to get an ignition. This is a very safe way to go, IMHO. If you go with a powder solution, you will need a non-flammable powder, preferably also on that does not clump in humidity nor stick to itself or other things. It's thermal breakdown by-products should not be toxic (as it passes through the engine flame) and it ideally should be safe to breathe the dust in aerosol form in low concentrations and limited exposure time. A tiny CO2 cartridge might be considered as one possibility for an easy propellant/dispersal energy source- control a valve from the cpu and pass through a venturi of some type for mixing. On Thu, Oct 7, 2010 at 11:13 PM, Dave Camarillo <dave.camari...@gmail.com>wrote: > I suspect that if a glo-plug technique were used, there's some maximum > volume of fluid you could spray over the glow plug, after which the fluid > would simply cool the glo plug instead of burning the fluid. I suspect this > would boil down to an energy conversion problem based watts into the plug > and energy required to combust the fluid... But it would have a calculatable > upper bound (and the existence of the upper bound would key to know and > sufficiently avoid in such a design)... > > One idea I was pondering, not sure if it's a good idea, perhaps using some > kind of fine white dust or powder, perhaps powder from drywall filler or > baking flour and a mechanical worm-drive feed mechanism to drive the > material out at a specific volume per second... The ultrasonic atomizer > would probably work for this too... I'm not sure how well this would work, > but there's no hot-things involved.... > > My gut feeling says that if an thermo/electronic approach was chosen, > perhaps with glo plug(s) or nichorme wire or other elements, sufficient > safety could be put in place to avoid fire on the ground. One can design > some simple circuitry that will get you 99.9+ percent reliability, and if > you really cared, you could assume worst case scenario, and know the max > thermal output of the glo plug and design the frame such that it can > dissipate the heat fast enough to keep surface temperature down below X > threshold, where X is consider a safe threshold for not starting fires... > > > > On Thu, Oct 7, 2010 at 8:39 PM, Doug Ausmus <daus...@gmail.com> wrote: > >> As a licensed pyro, I agree- do NOT use a pyro technique for the smoke. >> Yes, Potassium Nitrate and melted sugar ignite... in fact it is a very >> effective rocket fuel! And it burns WAY too fast for this application and is >> quite hot, but it does make smoke <g>. Smoke compounds have things that make >> them burn very slow (I have a very slow burning, non-flaming formula for >> creating huge volumes of smoke), and there are easy methods to ignite them >> 100% with an e-match using some special ignition formulas, but that's all >> beside the point- I highly suggest that you do not use pyro to generate the >> smoke for visual air flow in this application. >> >> Smoke fluid (such as castor oil), using a glow plug is a good idea, but >> drowning it may not give you the results you actually need- in addition >> there is a significant lag between energy and smoke, so controlling it per >> velocity (although a good idea) may not perform quickly enough to give you >> the smoke you need when you need it. >> >> Another thought occurs to me, however, what about a non-smoke solution? >> There are very benign foggers (not actually smoke) for theatrical use that >> make non-toxic 'smoke'. Three questions that would need to be answered: >> >> 1) can there be a miniature light version? >> 2) if so, can it produce enough smoke for the velocities that will be >> experienced? >> 3) Can it be energy efficient? >> >> >> Last idea: >> Will a miniature Ultrasonic atomizer work sufficiently well enough (with >> some fluid to simulate a smoke or fog) to provide the "smoke" volumes needed >> for the velocities experienced? >> If so, same questions as above. >> >> Doug >> >> On Thu, Oct 7, 2010 at 5:21 PM, Keith Packard <kei...@keithp.com> wrote: >> >>> On Thu, 7 Oct 2010 15:38:16 -0700, Dave Camarillo < >>> dave.camari...@gmail.com> wrote: >>> >>> > It seems like it should be OK to light stuff on fire and/or make smoke >>> > electrically anytime after the motor has started up... I hope that >>> wouldn't >>> > be frowned upon? >>> >>> It has been; the concern is in having something burning after the rocket >>> lands. >>> >>> The desert is dry, and the rocket often lands far from roads which make >>> fire fighting difficult. And, yes, rockets have landed and caused >>> significant fires. >>> >>> -- >>> keith.pack...@intel.com >>> >>> _______________________________________________ >>> psas-avionics mailing list >>> psas-avionics@lists.psas.pdx.edu >>> http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics >>> >>> >> >
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