calculate thermal cresistance of heatsink

Hi every body
i have a question about how can calculate thermal resistance of heatsinks under natural convection air and forced air (both of them) if the equation independent of temperature and depends to physical dimension of hestsink.
thanks

I would primarly rely on heatsink manufacturer data.

In addition you find empirical heat transfer formulas for various surface forms in technical thermodynamics text books, used e.g. to design heat exchangers. As thermodynamic properties of fluids, e.g. air are temperature dependent to some extent, heat sink thermal resistance will be, too. Other parameters like exact flow pattern will probably matter more.

For answers to your many questions find a good book on heat sink design.
You can calculate many things but in real life, a good test is best.

In my projects I estimated the power to be lost in an enclosure, and put a light bulb as a heating resistor (car lamp bulbs are suitable). Then I put a thermometer inside, an switched the power on.
During 3-4 hours I recorder temperature readings. If the top temperature exceeded 50 deg.C (most components are specified for it_), I had to design an improved cooling like a fan or a heat sink with fins protruding outside. And another run to confirm the design.
I never erred by this procedure.

Have a look to the thread:

https://www.edaboard.com/thread313437.html#post1340180

thanks a lot for their answer.
but i have know how calculate thermal resistance in terms of physical characterize and independent of temperature or power dissipated or without testing, when the length of heat sink is changed as shown in many company e.g. Fischer electronic. it plot thermal resistance Vs. length of heat sink . the method shown in thread "Heat sink thermal resistance calculation" used forced convection not natural convection. for natural convection there is not velocity. ???
thanks

Heat transfer of vertical plates by natural convection is e.g. described by Churchill and Chu formula http://en.wikipedia.org/wiki/Heat_tr...vertical_plane

As already mentioned, this are empirical formulas but the compliance with measurements is usually good.

thanks FvM
It was helpful. and about thermal resistance equation and Rayleigh number. what are them??