Convection

[karan87]

I have a hard time understanding characteristic length in convection. I am trying to figure out the natural and forced convection inside a tube that contains couple of heating elements. Air is being heated up in this tube. Using Gr/Re^2 can tell me whether forced or natural convection dominates. Both grashof number and reynolds number have characteristic length in their formulas. For reynolds number, the characteristic length should be
Diameter(tube) - Diameter (elements) * n(number of elements).

For grashof number, I am confused on whether I need to include diameter of the tube. My understaning is that since air would flow over the elements, I only need to consider the elements. Can anybody shed some light on this. Thanks

 

[racookpe1978]

Unfortunately, I'm not good enough to understand your description of the problem geometry. Lengths vs diameters maybe incorrectly described?

Please submit a sketch (axial and sectional) of your tube.

 

[vpl]

Agree with racookpe1978 - a sketch would be helpful. I spent some time looking through one of my heat transfer books -- my initial reaction was that the characteristic length may change dependant upon the geometry.

My gut reaction is that this may not be simple depending on how your "couple of heating elements" take up space inside your tube.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[ione]

For a horizontal cylindrical cavity with internal natural convection the characteristic length is the cylinder diameter. For a vertical cylindrical cavity with internal natural convection the characteristic length is the cylinder height.
Anyway and as indicated above a sketch showing your real situation would put us in condition to give you a better advice.