[CZKuba961]

Hi,
I´m doing a natural convection model for my passive cooling device. I was wondering how to calculate proper thickness of tehrmal boundary layer when I do not have any velocity value. I know the air will flow with increasing temperature and the calculation with Nusselt number etc. but all these calculations are done with velocity input. Does anybody know how to calculate it? Thank you.

[FMDenaro]

Originally Posted by CZKuba961

Hi,
I´m doing a natural convection model for my passive cooling device. I was wondering how to calculate proper thickness of tehrmal boundary layer when I do not have any velocity value. I know the air will flow with increasing temperature and the calculation with Nusselt number etc. but all these calculations are done with velocity input. Does anybody know how to calculate it? Thank you.

There are a lot of references in literature for the natural convection problem. The characteristic velocity is evaluated from the assumption that the non dimensional number describing the flow problem is O(1).
Just as an example see
https://www.thermopedia.com/content/786/