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Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: D_Hall » Sun May 04, 2008 5:47 pm

spudfarm wrote:no..
the projectile was a marble "1.6cm" and i don't think that has much friction at all. th "oops" was made when i thought a marble was 0.5" ans then i realised that .5" is 1.27cm

How closely matched are your marbles and your barrel? Projectile blow-by gets more significant as pressures go up and bores go down... and I fully conceed that blow-by is not something I've looked at closely in HGDT (IE, I haven't confirmed that it's doing what it's supposed to be doing).
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Unread postAuthor: SpudFarm » Sun May 04, 2008 5:51 pm

it has a bit of gap around it.. can't tell how much but like 0.5mm all in all
mabe a little more since this was a small marble..

when i test my golf ball hybrid i am going to check all of this and put it up against the program 99%accurate and may take some pics of the ball in the barrel so you can run it on HGDT since you understand this fully.
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Unread postAuthor: drex » Sun May 04, 2008 7:33 pm

i was wandering, could yous this program to model a regular combustion cannon by putting the mix at 1X, if so would it be accurate?
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Unread postAuthor: Ragnarok » Sun May 04, 2008 7:38 pm

drex wrote:i was wandering, could yous this program to model a regular combustion cannon by putting the mix at 1X

Yes, that's part of the idea behind it.

if so would it be accurate?

Probably reasonably, but exactly how accurate is something of an unknown - but with D_Hall's latest update, it seems to be getting very close to Latke's data, so things are looking good.
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Unread postAuthor: D_Hall » Sun May 04, 2008 8:03 pm

drex wrote:i was wandering, could yous this program to model a regular combustion cannon by putting the mix at 1X, if so would it be accurate?

Yes and no.

You CAN model a regular combustion cannon, but you don't do it by putting the mix at 1X. You do it by putting the disc failure at 0 psi (HGDT will then determine the mix appropriate to the altitude you live at...which may or may not be 1X).
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Unread postAuthor: JDP12 » Sun May 04, 2008 8:09 pm

cool thanks D_hall, i was wondering the same thing myself
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Unread postAuthor: drex » Sun May 04, 2008 8:14 pm

D_Hall wrote:
drex wrote:i was wandering, could yous this program to model a regular combustion cannon by putting the mix at 1X, if so would it be accurate?

Yes and no.

You CAN model a regular combustion cannon, but you don't do it by putting the mix at 1X. You do it by putting the disc failure at 0 psi (HGDT will then determine the mix appropriate to the altitude you live at...which may or may not be 1X).


how do i tell hgdt what altitude i live at?

EDIT: never mind i figured it out
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Unread postAuthor: jimmy101 » Wed May 07, 2008 1:48 pm

D_Hall wrote:A question for the masses....

Not that it's horribly surprising, but it appears that chamber fan performance may very well have a significant effect on system performance. Currently, HGDT simply has a "yes/no" option for chamber fans (turbulence induction). I've decided that this is insufficient.

The question then becomes precisely what IS sufficient?

Should I have a pulldown box that allows you to select from none, low, med, high, extreme?

Or should I have a text box that allows you to manually input a burn rate delta-V?

The first would be nice because it's simple for the user and includes a few options. But it does open the door for "low is too low, med is too high" scenarios.

The second allows one to really dial in on a gun, but it requires a bit more smarts from the user.

A third option would be a whole 'nuther input section dedicated to the chamber fan....


Thoughts?


I think I would go with a more qualitative (to the user) approach;
None: No fan (duh), gases assumed to be queiscent.
Low: Single fan several inches from the spark gap. For example, fan 1" from breech and single spark at the center of a 12" long chamber.
High: Fan within a couple inches of the spark.

Not sure how to deal with the fan at the breech but several spark gaps along the chamber. First gap is a "high", subsequent gaps are more like "lows".

Not sure how to deal with fan diameter ~ chamber diameter versus fan diameter ~1/2, 1/3... chamber diameter.

A more detailed and/or quantitative treatment is just going to get into a bunch of parameters (fan diameter, air velocity, distance from spark, fan alignment with spark, fan alignment with chamber axis, ...) that few users will have accurate numbers for.
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Unread postAuthor: jimmy101 » Wed May 07, 2008 1:58 pm

D_Hall

Can you give a bit of info on what the "Insulation" options represent?

None: High heat transfer? Perhaps what you would get with a metal chamber -- coefficient of thermal conductivity 50~400 J/m/k/s, high density material.

Insulated: Lower heat transfer? Perhaps what you would get with a plastic chamber -- coefficient of thermal conductivity ~0.2 J/m/k/s, low density material.

Adiabatic: No heat transfer from gases to gun.
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Unread postAuthor: D_Hall » Thu May 08, 2008 6:26 pm

jimmy101 wrote:D_Hall

Can you give a bit of info on what the "Insulation" options represent?

None: High heat transfer? Perhaps what you would get with a metal chamber -- coefficient of thermal conductivity 50~400 J/m/k/s, high density material.

Insulated: Lower heat transfer? Perhaps what you would get with a plastic chamber -- coefficient of thermal conductivity ~0.2 J/m/k/s, low density material.

Adiabatic: No heat transfer from gases to gun.


None was actually made to corralate to the data in your webpage for the technical spud gun.

Adiabatic is obvious.

Insulated is half-way between the two. Not sure why anyone would use it unless they truly had done a good job of insulating their chamber (who does that?), but since it took mere seconds to code I threw it in there.
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Unread postAuthor: jimmy101 » Fri May 09, 2008 11:59 am

How much difference, in terms of heat loss, is there between metal and a plastic gun? The Coefficient of Thermal Conductivities are hugely different;
PVC 0.2 W/m/K
iron/steel 20~80 W/m/K
copper 400 W/m/K

Just wondering. Is the heat loss much greater in a metal gun than a plastic one? Is the greater heat loss significant? Is a PVC gun basically insulated?
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Unread postAuthor: D_Hall » Fri May 09, 2008 2:53 pm

I looked into that early on... Remember it's not just thermal conductivity, but also specific heat. In any event, I don't remember the numbers but I decided that it probably wasn't worth worrying about the differences at this time. There may come a time when I do, but until such time as the basic model can be well verified I'm not gonna lose much sleep over it. I mean, if my choice combustion model makes it impossible for me to ever predict something to closer than (say) 10%, does it really make sense to worry about tweaking something that only makes a 1% difference?
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Unread postAuthor: jimmy101 » Fri May 09, 2008 3:48 pm

I agree, not worth diddlin around with something that'll get lost in the noise.

Still, the difference in thermal conductivity is huge, the difference in heat capacity is almost nothing.
<table border="1"><tr><td>Material</td><td>Coefficient of Thermal Conductivity, K (W/m/K)</td><td>Density (g/cc)</td><td>Specific Heat Capacity (J/g/K)</td><td>Volumetric Heat Capacity (J/cc/K)</td></tr><tr><td>Silica aerogel</td><td>0.02</td><td>0.144</td><td>0</td><td>.</td></tr><tr><td>Air</td><td>0.024</td><td>0.0013</td><td>1</td><td>0.0013</td></tr><tr><td>PVC (cell core)</td><td>0.062</td><td>0.6</td><td>.</td><td>.</td></tr><tr><td>PVC (solid)</td><td>0.18</td><td>1.4</td><td>0.9</td><td>1.3</td></tr><tr><td>Glass</td><td>1</td><td>2.4 - 2.8</td><td>0.5 - 0.84</td><td>~1.7</td></tr><tr><td>Stainless Steel</td><td>16</td><td>.</td><td>.</td><td>.</td></tr><tr><td>Hi-carbon Steel</td><td>35</td><td>7.7 - 8</td><td>0.49</td><td>~3.9</td></tr><tr><td>Cast Iron</td><td>80</td><td>7.15</td><td>0.46</td><td>3.3</td></tr><tr><td>Aluminum</td><td>237</td><td>2.7</td><td>0.87</td><td>2.3</td></tr><tr><td>Copper</td><td>400</td><td>8.96</td><td>0.38</td><td>3.4</td></tr></table>
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