The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 01-05-2001, 06:27 PM
Ad Noctum Ad Noctum is offline
Guest
 
Join Date: Jun 2000
as I was gazing at the fireplace, with the doors open, when a spark jumped out at me, tried to bite me, and instead burned my arm. (yes, I had just added wood to the fire) and I thought; "Hey, that hurt". and the question hit me, what is the temperature where fire exists, and solid no longer does. now, there's got to be a SET tempurature, isn't there? I mean, a set temperature where a dry piece of wood, or watever will burn. what is the minimum? and is there a maximum tempurature where fire will exist?-you know, before it turns to plasma?

__________________
Ad Noctum, Resident Pyro and moron.
__________________
Silver Fire--- Ad Noctum is, in all seriousness, a poster I wouldn't mind seeing around for a while. awww... Thanks babe
so I've got that goin' for me... Which is nice.
Someone could shoot you for being a Squacist Squonky or a Squigoted Squacker.-- Thanks to TurboDog
Reply With Quote
Advertisements  
  #2  
Old 01-05-2001, 07:36 PM
hightechburrito hightechburrito is offline
Guest
 
Join Date: Nov 1999
Paper will catch on fire at 451 degrees fahrenhiet. Krpp in mind that the flame is probably much hotter after it starts burning.
Reply With Quote
  #3  
Old 01-05-2001, 10:25 PM
Gunslinger Gunslinger is offline
Guest
 
Join Date: Jun 2000
Pure alcohol burns at 70 degree F. There's no set temperature; it depends on the ignition temperature of what you're burning.
__________________
and then they made me their chief.
Reply With Quote
  #4  
Old 01-05-2001, 11:12 PM
Una Persson Una Persson is offline
Straight Dope Science Advisory Board
 
Join Date: Mar 2000
Location: On the dance floor.
Posts: 14,922
I could tell you an awful lot about flames, but I'm not sure if I understand your question properly. Are you asking for the highest temperature at which there are no solids, just flame? Or what is the lowest temperature that one can have a flame at? I'm not certain.

hightechburrito - yes, the peak flame temperature is always much hotter than the ignition temperature. Consider this - paper may ignite at 451 F or so as per the novel, but the flame temperature from a newspaper fire I have measured with my Fluke thermocouple meter to be as high as 1000 or so F, and up to 1300 F when blown with a bellows. I've also measured a wood fire in a fireplace that was 1900 F when blown with a small bellows.
__________________
SDMB records held:
* Most title changes
* Longest Ignore list
Reply With Quote
  #5  
Old 01-05-2001, 11:47 PM
Badtz Maru Badtz Maru is offline
BANNED
 
Join Date: Aug 2000
Location: Dallas, Texas
Posts: 5,665
If pure alcohol burned at 70 F, then a bottle of Everclear would burst into flames as soon as it was exposed to oxygen. I think it has to be hotter than that to ignite.

Flash paper seems to have a pretty low ignition point, but I don't know what it is.
Reply With Quote
  #6  
Old 01-06-2001, 12:30 AM
Homer Homer is offline
Guest
 
Join Date: Sep 1999
Everclear is far from pure.

--Tim
Reply With Quote
  #7  
Old 01-06-2001, 06:19 AM
zigaretten zigaretten is offline
Member
 
Join Date: Oct 2000
Location: Oakland
Posts: 506
Ad Noctum - I believe that the answer to your question is NO, there is no minimum temperature at which material will burn. I think you have the wrong picture in your head. The solid does not turn into "flame" and cease to exist, it just chemically reacts with oxygen in the air and turns into different materials. For instance, wood is mostly cellulose which is mostly carbon and hydrogen. When it burns it it reacts with oxygen and forms carbon dioxide (carbon and oxygen) and water (hydrogen and oxygen)(The water is in the form of steam as long as the fire is burning at over 100C). So all of the "matter" is still "matter" (though you started with a solid and a gas and ended with two gases).

The "flame" (and the heat) comes from the fact that the new compounds which are formed are lower in energy than were the initial compounds. The excess energy has to go somewhere; so it is given off as heat and light. The light is the flame. (What actually happens is that the gases which result from the reaction are in very high energy states with electrons in high energy orbits. These electrons quickly fall back into lower energy orbits and emit light. This is why different materials can give off different color flames.)

The temperature of the flame depends a lot on how quickly the reaction takes place. If you use a bellows to provide more oxygen then the reaction will speed up and you'll get more heat and light (hotter fire/bigger flame).

It takes a certain amount of energy to get the reaction to start; thats you with the match. But once it is started it supplies its own energy to keep itself going. And that will be true whether you are in the desert or at the north pole.

When you burn different materials you get different reactions. Some require a lot of energy to get them started and some require very little. Getting wood started is a lot harder that getting gasoline started. Also, different reactions give off different amounts of energy. Some give off almost no energy at all. In fact, there is a smooth transition to reactions which actually need an input of energy, but since these would not give off heat and flame, I won't bore you with them.

I'm afraid this may not completely make sense, but my point is that once you get a fire started it will keep itself burning no matter how cold it is outside. And as for the minimum temperature to get one started, well think about nitroglycerine. (Explosives are really just things that burn really, really fast) It can go off with no application of heat at all.

About alcohol; 70F is the FLASHPOINT of 70% ethanol. The flashpoint is the temperature at which the liquid gives off enough fumes so that the fumes will burn. The liquid never burns. If liquids burst into flames at their flashpoints we would all be in a lot of trouble because gasoline has a flashpoint of negative 40F.

Sorry about the length of this and I'm afraid I don't know too much about plasmas except, of course, it wouldn't be the "fire" that turns into plasma, but I guess that the hot gases created by the burning could become plasma.
Reply With Quote
  #8  
Old 01-06-2001, 07:36 AM
kanicbird kanicbird is offline
Guest
 
Join Date: May 1999
can you have a flame temp of something low enough that you could put and leave you hand in it and not get hurt?
Reply With Quote
  #9  
Old 01-06-2001, 09:57 AM
zigaretten zigaretten is offline
Member
 
Join Date: Oct 2000
Location: Oakland
Posts: 506
I think I may have answered too quickly before. At first I was just going to tell k2dave: Yes, but at such low temperatures you wouldn't be able to see the Flame. But now I'm thinking that if you can't see it, then it isn't really a flame, is it?

Everything that I said in my previous answer would still be correct if we could all walk around with those military infrared goggles all the time because then we could see the flames from all those low temperature reactions. But as we can't then I don't think it is proper to refer to those as fires.

So - BETTER ANSWER (I hope): bodies will first radiate in the visible range at 400C (barely visible). For reference, candles generally burn at 1700C.

I guess it all depends on exactly how you define fire and flame.
Reply With Quote
  #10  
Old 01-06-2001, 10:22 AM
Una Persson Una Persson is offline
Straight Dope Science Advisory Board
 
Join Date: Mar 2000
Location: On the dance floor.
Posts: 14,922
Quote:
Originally posted by zigaretten
(Explosives are really just things that burn really, really fast)
Your post shows that you know a good deal about this - but this line isn't exactly true, or were you merely oversimplifying for clarity? For example, if we want to compare detonation and deflagration, we can look at this example:

Try to visualize a one-dimensional stationary combustion wave of a flammable gas and air mixture in a tube. We will assume this is a premixed flame, which is defined as one where the reactants are mixed perfectly before the chemical reaction. (as opposed to a diffusion flame, where the reactants diffuse into each other during the chemical reaction). Wood burning generates a diffusion flame, as opposed to a torch or gas burner which generates a premixed flame.

Code:
___________________________________________________________
                             |
    (Unburned gas)           |F      (Burned Gas Products)
                             |l
    U1------->               |a    U2--------->
                             |m
                             |e
     p1, T1, P1              |   p2, T2, P2
_____________________________|______________________________
Where U is the velocity of the gas, p is the density, T is the temperature, and P is the pressure, and C is the local sonic velocity. By looking at these variables, we can express relations between the two sides of the flame front to say whether what we have is either:

* deflagration - where the combustion wave propagates at subsonic speed (controlled burning, in other words). These are found in Region III of the Hugonoit curve (and yes, technically, Region IV, except it is very hard to get anything into region IV since it is very hard to get combustion products to depart from the combustion wave at supersonic speeds), or

* detonation - where the combustion wave propagates at supersonic speed (an uncontrolled burning). These are found in Regions I and Regions II of the Hugonoit curve, with said regions being seperated by the Upper Chapman-Jouget point, which divide them into Strong and Weak detonation points.

Deflagration and detonation are divided by Region V of the Hugonoit curve, which is an imaginary region since the Rayleigh-line expression implies that U1 (see below) is imaginary, and thus it is a physically impossible region.

But I digress.

For these cases, we have the following relationships:
Code:
Property    Detonation             Deflagration
------------------------------------------------------
U1/C        5-10 (supersonic)      0.0001 - 0.03 (subsonic)
U2/U1       0.4-0.7 (deceleration) 4-6 (acceleration)
P2/P2       13-55 (compression)    0.98 (explansion)
T2/T1       8-21 (heating)         4-16 (heating)
p2/p1       1.7-2.6                0.06-0.25
Which just tells us in a stable flame, we have a subsonic flame front, acceleration of hot exhaust gases away from the flame, a slight expansion of the exhaust gases due to lower pressure, a large heat addition (duh!), and a decrease in density. As opposed to detonation, where the supersonic flame front causes a large spike in pressure and density, casuing the "knock" one hears in car engines.
__________________
SDMB records held:
* Most title changes
* Longest Ignore list
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:20 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.