Characteristic length in Rayleigh number
 

Hi, I'm new in Fluent and Heat transfer. I'm trying to simulate a 2D natural convection from a horizontal cylinder in an enclosed vertical rectangle cavity. The cylinder's diameter is 40mm and the cavity's dimensions is 760mm in height and 260mm in width. My questions are

• How do I set and calculate the Rayleigh number? What is the characteristic length? Will it be the diameter of cylinder or the length or height of the cavity? If I want to increased or decreased the Ra number, what value can I play around with?
  
• How do we calculate the bulk temperature, to be input in reference value? I have tried to use mass-weighted average temperature of the fluid as the input, but the Nusselt number is lower than the value that i found in literature. Someone suggested to find the bulk temperature outside the the boundary layer, but I'm not sure how to do that.

Please help me in this matter. I never use CFD software before. Thank you!

Help pls..

http://adf.ly/1dLnuNThis can help

Hi Mohawk, thank you for your reply. I been to that page before. But it doesn't mention what to use for the characteristic length etc. plus the geometry file cant be open.