How to determine characteristic length?

[goggles31]

There are characteristic lengths for Reynold's, Grashof, Nusselt, and Biot number but the method of obtaining them is not given in my notes. I would like to know how to do so for a plane wall, cylinder and sphere. Thank you.

 

[256bits]

There are characteristic lengths for Reynold's, Grashof, Nusselt, and Biot number but the method of obtaining them is not given in my notes. I would like to know how to do so for a plane wall, cylinder and sphere. Thank you.

Have you tried a search, and see to where that leads.
https://en.wikipedia.org/wiki/Reynolds_number
https://en.wikipedia.org/wiki/Biot_number
https://en.wikipedia.org/wiki/Nusselt_number
etc. as a starter.

A pipe, for example, would have a Reynold's characteristic length of its diameter.

 

[pigna]

The biot number characteristic length is volume/(eternl surface).
For convention problems the characteristic lengths are in according to the correlation u are using. Normaly They are the Most intuitive ones. I suggest you to look for the incropera textbook. U can find the pdf version easily searching on Google.