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| D_Hall |
 Posted: 05/04/2008 17:47 PM Post subject: |
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Donating Member
Joined: 07 Feb 2008
Posts: 463
1102.12 Spud Bux
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| 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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| SpudFarm |
| Posted: 05/04/2008 17:51 PM Post subject: |
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Hybrid Guy
Joined: 04 Nov 2006
Posts: 1499
230.81 Spud Bux
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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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| drex |
| Posted: 05/04/2008 19:33 PM Post subject: |
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Sergeant
Joined: 21 Apr 2008
Posts: 86
118.16 Spud Bux
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| 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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| Ragnarok |
| Posted: 05/04/2008 19:38 PM Post subject: |
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I am Copperman
Joined: 19 Dec 2006
Posts: 1720
5278.55 Spud Bux
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| 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.
| Quote: | | 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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| D_Hall |
| Posted: 05/04/2008 20:03 PM Post subject: |
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Donating Member
Joined: 07 Feb 2008
Posts: 463
1102.12 Spud Bux
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| 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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| ilovetoblowthingsup |
| Posted: 05/04/2008 20:09 PM Post subject: |
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Brigadier General
Joined: 30 Mar 2007
Posts: 822
1491.90 Spud Bux
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| cool thanks D_hall, i was wondering the same thing myself |
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| drex |
| Posted: 05/04/2008 20:14 PM Post subject: |
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Sergeant
Joined: 21 Apr 2008
Posts: 86
118.16 Spud Bux
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| 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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| jimmy101 |
| Posted: 05/07/2008 13:48 PM Post subject: |
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Major General
Joined: 28 Mar 2007
Posts: 1426
6048.19 Spud Bux
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| 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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| jimmy101 |
| Posted: 05/07/2008 13:58 PM Post subject: |
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Major General
Joined: 28 Mar 2007
Posts: 1426
6048.19 Spud Bux
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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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| D_Hall |
| Posted: 05/08/2008 18:26 PM Post subject: |
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Donating Member
Joined: 07 Feb 2008
Posts: 463
1102.12 Spud Bux
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| 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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| jimmy101 |
| Posted: 05/09/2008 11:59 AM Post subject: |
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Major General
Joined: 28 Mar 2007
Posts: 1426
6048.19 Spud Bux
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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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| D_Hall |
| Posted: 05/09/2008 14:53 PM Post subject: |
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Donating Member
Joined: 07 Feb 2008
Posts: 463
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| 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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| jimmy101 |
| Posted: 05/09/2008 15:48 PM Post subject: |
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Major General
Joined: 28 Mar 2007
Posts: 1426
6048.19 Spud Bux
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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.
| Material | Coefficient of Thermal Conductivity, K (W/m/K) | Density (g/cc) | Specific Heat Capacity (J/g/K) | Volumetric Heat Capacity (J/cc/K) | | Silica aerogel | 0.02 | 0.144 | 0 | . | | Air | 0.024 | 0.0013 | 1 | 0.0013 | | PVC (cell core) | 0.062 | 0.6 | . | . | | PVC (solid) | 0.18 | 1.4 | 0.9 | 1.3 | | Glass | 1 | 2.4 - 2.8 | 0.5 - 0.84 | ~1.7 | | Stainless Steel | 16 | . | . | . | | Hi-carbon Steel | 35 | 7.7 - 8 | 0.49 | ~3.9 | | Cast Iron | 80 | 7.15 | 0.46 | 3.3 | | Aluminum | 237 | 2.7 | 0.87 | 2.3 | | Copper | 400 | 8.96 | 0.38 | 3.4 |
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