Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Flame speed question

I teach part time HVAC for a local vocational school in the evenings and a question arose as we were going over combustion principals.
If there is such a thing, I'm looking for a real simple definition of flame speed.
For example; Natural gas has a burning speed of 1 foot per/sec and propane is aprox. 2.8 ft per/sec. Under what conditions? The question from one of my students was, " if nat. gas is 1 ft per/sec how can you have an explosion? Isn't that awful slow?" He commented that " ..you're telling me that if I was standing 20 feet away from a 20 foot long tube with Nat. gas in it, it would take 20 seconds for the flame to reach me?"
I thought,..good question. I'm not sure how to answer it in laymen's terms. I was guessing it all depended on the air mixture or lack of it. Does this refer to a LEL mixture in a controlled chamber? How do they measure this speed and what is the mixture of gas vs. air? Is pressure applied to control velocity?
Could you help me out with a little more detail.

Thanks and have a great day!

Jeff Boshea
Service Manager / Technical Support
Auer Steel Twin Cities
763-971-2910 x238 Fax: 763-971-2920
jeff.boshea@auersteel.com

Comments

  • Fred Harwood
    Fred Harwood Member Posts: 261
    Flame speed

    Look up explosion or detonation. For example, a line of black powder in the open will burn. In a confined space, such as a closed cardboard tube, black powder detonates, or violently decomposes by propagation of a shock wave through the powder at near the speed of sound, roughly 1,000 feet per second. Interesting ordinary stuff has explosive potential, and processors must handle such things as flour, mustard, cornstarch, and even asprin carefully. For example, see Mark's Standard Handbook for ME, Table 23, Explosive Characteristics of Various Dusts.

    Today you might not find much on the Internet about such things, but try Google and detonation or explosive.
  • jim sokolovic
    jim sokolovic Member Posts: 439
    Ignition velocity vs. air/fuel mixture...

    This site has some good info and a graph:

    http://www.technocarb.com/natgasproperties.htm

    I hope this works, this is the first time I've posted a site like this here.
  • jim sokolovic
    jim sokolovic Member Posts: 439
    Sorry, I can't find that article now...

    when I try it - I had it from 3 years ago.

    It gave a range of 25:1 to 7:1 air/fuel ratio for natural gas ignition. At the bottom part of this range, flame velocity is low - but increases rapidly with richer mixtures, reaching it's peak around stociometric - then it falls off again. Heating the mixture increases the speed, also. Hope this at least helps.
  • Dale
    Dale Member Posts: 1,317
    Speed

    Natural gas has a very very steep curve for flame speed. This is why the smallest primary air change makes so much difference in flame appearance. It is not a bell shaped curve. As to the speed it is slow if view with gas on one side and air on the other. If you can borrow a lexan clear model gas house from the local utility or instrument vendor. You can watch the gas flame go at about a foot per second until it reaches a point where it runs in all directions and has the poof. The explosion is the air being pushed in a confined area in front of the gas. The same amount of gas igniting outdoors causes no explosion since the flame front isn't confined.
  • Mitch_4
    Mitch_4 Member Posts: 955
    I look at it this way

    Combustion is a CONTROLLED process. If natural gas could be laid out like a fuse, it would (Under controlled conditions..like a burner) burn at a rate of 1 foot per second.

    An explosion, is UNcontrolled, where it will expand as fast as it is possible to consume the fuel in as short of a span of time as it can. You will not have 20 seconds because it is not controlled. In fact you will have substantially less. a lot less....:-0

    In a burner, the rate that the fuel/air mix exits the burner ports is designed to be approximately 1'/sec. this results in a stable, controllable flame, neither flashing back into the burner (when less than 1'/sec), or lifting off the burner(greater than 1'/sec).

    Your students can see the difference in a blocked burner that results in delayed ignition (also known as "audible indication of successful light off"). When it lights...that will be an explosion, it will then settle into a controlled condion when the burner operates normally.

    Hopeit helps.

    Mitch

  • jim sokolovic
    jim sokolovic Member Posts: 439
    Maybe now I can show you...

    the article, as an attachment here (I hope):

    The Technocarb company had removed it from their site, but were nice enough to send it to me - look under the last section "Combustion".
  • J. Boshea
    J. Boshea Member Posts: 3


    Thank you all for your input and so quick! The internet can be a useful tool even with all it's faults.
    Thanks again.
  • S Ebels
    S Ebels Member Posts: 2,322
    \"Audible indication\"..............

    I like that. A calm way of describing what can be a very disconcerting event.
  • bob_44
    bob_44 Member Posts: 112
    Speed

    Flame speed for Nat. gas. bob
  • bob_44
    bob_44 Member Posts: 112
    Sorry guys

    i don't know why that speed graph is so big. bob
  • Constantin
    Constantin Member Posts: 3,796
    Hi Bob!

    Is this better? I just rotated the file, increased the contrast, downscaled it, and converted it to a gif. How does it look to you?
  • J. Boshea
    J. Boshea Member Posts: 3
    Chart

    I just emailed the image to myself and resized and printed. It's fine. Thanks
  • Bob Harper
    Bob Harper Member Posts: 1,081
    other factors

    affecting flame speed include the temp. of the fuel and primary air, burner temp., size/shape of burner or combustion chamber and oxygen % and pressure at the flame front. The charts so often quoted show the curves indicating stoichiometric mixtures at the peak. They don't take these other factors into consideration. If you burn a quantity of NG in the open, it burns rather slowly. Put it into a confined space where the pressure spikes and the walls of the container heat up and you have a bomb. As previously noted, the difference btw deflagration vs. detonation is when the flame speed exceeds the speed of sound. Whoomf! vs. Boom!

    Take a normal stoichiometric mixture and add in an oxidizer. It speeds up. Now starve it for air, keeping the air/fuel ratio constant and it will slow.

    The charts are also referring to lamninar flames, which are faster than turbulent flames.
    HTH,
  • bob_44
    bob_44 Member Posts: 112
    Show-off

    Constantain, we ain't all cumpyuter literate like you.
    That was my first scan and it took two hours on the phone with Canon to get that! Thanks, bob
  • Constantin
    Constantin Member Posts: 3,796
    Bob,

    I don't mean to be a show-off and I apologize if I offended you. The above picture was a simple conversion for me to do with my graphics program, I do it all the time before posting stuff here. But believe me, I was just trying to help.

    Given my recent performance with a Weil-Mc LGB steamer, there is ample improvement opportunity in my life. :-)

    Anyway, check out Irfanview. I used to use it all the time on my Windoze machine. It's a great graphics program, and best of all it's free.
  • Mitch_4
    Mitch_4 Member Posts: 955
    feel free to uuse it

    does sound nicer that the door blew off (saw that once on a teledyne pool heater.. drove itseld into a 1 x 6 fence board like a thrown hatchet. Glad I wasn't in front or it...what a mess.
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,525
    Constantin...

    I thought I read some dry humor in Bob's post. You could take it as a compliment. ;~) Keep on educating us about the evil computer.

    Yours, Larry
  • Constantin
    Constantin Member Posts: 3,796
    But Larry...

    ... a computer is only evil if it's running Windows... :-P

    ...OK back to work...
  • bob_44
    bob_44 Member Posts: 112
    Larry's right

    Constantain, I don't think that Infran thing will run on my Mac. How do you open files with the suffex CFM I see a lot of them on the Wall. I would think CFM would be on a forced air site. I'm envious of your computer skills not pissed. bob
  • Constantin
    Constantin Member Posts: 3,796
    A Mac user!

    Hello fellow rebel! If you use a Mac, I recommend GraphicConverter from Lemkesoft. It's what I use every day for pics here. It's quick and not too feature ridden to become overbearing. Plus, it understands almost every image format out there.

    I had the same issue as you with .CFM images from this website. As best as I can tell, it's an issue between the server and your browser, where a CFM application at Inivision.com mishandles your browser request. Hence, the pictures are classified as "CFM" files, which are usually used in web development, not imaging.

    The slow way to fix the problem is to manually rename the file from xyz.cfm to xyz.jpg. That will force preview or graphicconverter to open the file for you as a JPG. Even if you get the extension wrong, the image program usually recognizes the error and corrects it on the fly.

    I'll try to put a auto-renamer together for you in the coming days... Cheers!
  • Mitch_4
    Mitch_4 Member Posts: 955
    shouldn't that say...

    "...acomputer is only evil if its running Windows...:-XP?

    note the "X"

    hehehe
This discussion has been closed.