Long joke -> discussion

by **boilingleadbath** » Tue Mar 13, 2007 6:48 pm

Well, I recently made a long, complicated post on spudfiles (intended as a joke; yes, it did have a punchline)...
It's here because I thought you guys might aprecciate both the joke and the concepts behind it; I welcome serious discussion of it.

Here's the origional post. (interpret in the context of a thread asking "which do you think is cooler: pneumatics or combustions?"
**********************************************************

I felt like making a useless post:

Well, you lazy person, go use the GGDT. You'll find that the gasses, on exit, are significantly below the freezing point of water - to the tune of -50 to -100*F

We then note that any degree of research will show that combustions derive their pressure from temperature.
A bit more research will lead you to write this formula:
(VpP0T0) / (V0Pf) = Tp

Where V0 is the initial volume, Vp is the volume at the optimum (perfect) ratio, P0 is the initial particle count (in the case of propane-air, 26), Pf is the post-reaction particle count at (27), T0 is the initial temperature in absolute units, and Tp is the temperature at the optimum ratio.

This is based on the assumption that the pressure at the optimum ratio is equal to the starting pressure. This is not true for hybrids - but you should be able to figure out how to account for that factor.

We substitute the values like so:
(2.25L*26p*293K)/(1L*27p) = Tp
...and solve to find that Tp equals 635K.

Now, we note that this calculation ignores any evaporation of potato juices, but the optimum ratio for the latke tests using the sabot slugs suggest that this, at least up to the optimum ratio, is at most minor effect.
Also, you might note that this is the temperature when the pressure equals zero PSIG, and that the gasses might further cool off due to their momentum or such. I will not dispute that these effects might exist, but I doubt their seriousness.

So, I feel reasonably secure in saying that the exhaust gasses of a combustion spudgun are significantly hotter, on average, than the exhaust gasses of a pneumatic launcher, or, as you put it, that pneumatics are cooler.

(that was fun)

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(get the joke? See, it's a pun: he ment "which do you find more exciting?", but I answeared "which one is colder?". The long technical discussion of something which you expect to be a matter of taste is also somewhat amusing.)

by **Darrel** » Tue Mar 13, 2007 9:54 pm

by **flyingspuds2007** » Tue Mar 13, 2007 10:08 pm

I mess with my friends heads like that every day, always analyzing things and sometimes answering the occasional rhetorical question.

by **sgort87** » Wed Mar 14, 2007 12:26 am

Sounds like you wasted your time BLB. You know as well as I do that when something on that intelligence level is posted on SpudFiles, no one reads it. I'm pretty sure they stop reading when you say "GGDT".

Funny, mostly in the "I can't believe you spent all that time with that" sense, but still pretty funny.

**Sponsored**

by DK » Wed Mar 14, 2007 1:35 am

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Posted by BLB: 
P0 is the initial particle count (in the case of propane-air, 26), Pf is the post-reaction particle count at (27)<hr height="1" noshade id="quote"></blockquote id="quote">

What's this about particle count of propane to air? I really doubt such a ratio exists, unless it was intended as part of the joke...?

by **draculon** » Wed Mar 14, 2007 3:48 am

I didn't get it... :evil:

by **draculon** » Wed Mar 14, 2007 4:48 am

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by DK
[br]<blockquote id="quote">quote:<hr height="1" noshade id="quote">Posted by BLB: 
P0 is the initial particle count (in the case of propane-air, 26), Pf is the post-reaction particle count at (27)<hr height="1" noshade id="quote"></blockquote id="quote">

What's this about particle count of propane to air? I really doubt such a ratio exists, unless it was intended as part of the joke...?
<hr height="1" noshade id="quote"></blockquote id="quote">

It's basically the fuel to air ratio.

EDIT: Oops, sorry about the double post.

by **boilingleadbath** » Wed Mar 14, 2007 2:17 pm

Eh, not really. I couldn't think of a good single-leter variable, so I went with P for 'partical' - but it represents the mole ratio of the products to the reactants.

As I said, I think the analysis is valid.

And as to the number 26/27:
C3H8 + 5O2 -> 3CO2 + 4H2O
...but we have some non-reactive species (mostly nitrogen):
C3H8 + 5O2 + 20N2 -> 3CO2 + 4H2O 20N2
We find that there are 26 molecules on the reactants side ("P0") and 27 on the products side ("Pf")

And yes, it would probably be (mostly) a waste of time if:
1) I already knew that the exaust gasses where at 635K (ok, I figure that's within 30K, so that's an excessive number of digits)
2) I hadn't posted it here for serious discussion.

by DK » Wed Mar 14, 2007 9:52 pm

Alright, i never thought of the reaction in that manner before. Thanks for the clarification.

by **BewareOfDog** » Wed Mar 14, 2007 10:08 pm