Calculating Grashof's Number for Natural Convection

[lazypast]

HI

when dealing with natural convection, we use grashof's number. suppose we have a heated plate of temperature Tp. and heat being transferred to surrounding air by convection, air of temp Ta.

So film temp is Tf = 0.5(Tp + Ta)

If grashofs numbers, $$Gr = \frac { \beta gL^3 \delta T}{\nu^2}$$

im wondering what $$\delta T$$ and what $$\beta$$ we use.

thanks

 

[Mapes]

Does http://books.google.com/books?id=nr...r&ei=YivmSc7SI4fmkATB8O2bAQ&client=firefox-a" help?

 

[lazypast]

Does http://books.google.com/books?id=nr...r&ei=YivmSc7SI4fmkATB8O2bAQ&client=firefox-a" help?

it seems a good book, I've tried various temperatures and can just think that the answer is wrong

thanks anyway