Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Biot number

  1. Apr 21, 2014 #1

    Maylis

    User Avatar
    Gold Member

    Hello

    I confused about the equation for the biot number

    Bi = hLc/k

    For a slab, sphere, and cylinder, what are the characteristic lengths?
     
  2. jcsd
  3. Apr 21, 2014 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    According to this article:

    http://en.wikipedia.org/wiki/Biot_number

    the characteristic length Lc is usually the volume of the body divided by its surface area, or

    Lc = Vbody/Asurface
     
  4. Apr 21, 2014 #3

    Maylis

    User Avatar
    Gold Member

    I think that is for biot numbers less than 0.1
     
  5. Apr 22, 2014 #4

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    If you read the quoted article, it discusses what the implications are for Biot Nos. < 0.1 and Biot Nos. > 0.1 of an object in terms of heat transfer. It does not, however, indicate that Lc is modified depending on the value of the Biot No., as you could never tell if you are calculating the correct Biot No. with such a definition.

    If the Wiki definition is not satisfactory, try this one:

    http://www.tufts.edu/as/tampl/en43/lecture_notes/ch4.html

    or this one:

    http://ocw.mit.edu/courses/chemical...ng-spring-2007/lecture-notes/biot_numbers.pdf
     
  6. Apr 22, 2014 #5

    Maylis

    User Avatar
    Gold Member

    Yes, the tufts article is where I am getting the 0.1 figure from. I'm just confused because our professor told us something contradictory, basically the the characteristic length is one half the diameter
     
  7. Apr 22, 2014 #6

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    "Characteristic lengths" in fluid dynamics are not an exact science. In the Tufts article, the behavior of the system doesn't suddenly change to something completely different when the non-dimensional parameter changes from 0.0999 to 0.1001. The important thing is the physical interpretation i.e. the "resistance" to heat flow across the surface, compared with the "resistance" inside the body, and what that means for the way the temperature varies with time. At one extreme, the surface temperature stays almost constant. At the other extreme, the internal temperatures stay almost uniform.
     
  8. Apr 22, 2014 #7
    See how McAdams defines the Biot number for various shapes. Also, check out how Bird et al do it.

    Chet
     
  9. Apr 23, 2014 #8

    Maylis

    User Avatar
    Gold Member

    It turns out that we use the radius because the charts given to us use that as the characteristic length
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Biot number
  1. Biot-Savart Law (Replies: 0)

  2. Reynold's Number (Replies: 14)

  3. Reynolds Number (Replies: 2)

  4. Stanton Number? (Replies: 1)

Loading...