How to determine characteristic length?

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Characteristic lengths for Reynolds, Grashof, Nusselt, and Biot numbers vary based on the geometry of the object, such as a plane wall, cylinder, or sphere. For instance, the characteristic length for Reynolds number in a pipe is its diameter, while for the Biot number, it is calculated as volume divided by external surface area. The choice of characteristic length often aligns with the correlation being used in specific problems. Resources like Wikipedia and the Incropera textbook provide foundational information on these concepts. Understanding these lengths is crucial for accurate fluid dynamics and heat transfer calculations.
goggles31
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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.
 
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goggles31 said:
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.
 
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.
 
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