for deciding internal or external ask yourself 2 questions:
1. do i care about the flow as it leaves and goes into the environment
2. do i know how to characterize the boundary conditions at the openings
if the answer to 1 is no and the answer to 2 is yes (i probably need to rephrase them so both are yes or no), then you can go internal.
to enable natural convection, just enable gravity
for fans, no problem
if you go internal and they lie on the pheriphery of the model, just use external inlet or outlet fans
if they are completely inside, use an internal fan and make sure you have the appropriate settings for internal cavities
if you go external, everything will be an internal fan.
if you haven't done so already, there is an electronics enclosure tutorial that will probably help you get started. but also you should go through the tutorials from the begining to end because each builds on the other
thank you for your answer.
I have another one question, maybe you think it is silly question..
If I do the internal analysis with time dependent setting; the reason is because I want to observe temperature increment due to time increment (physical time is the real time, am I right?)...is it reasonable?
Internal vs external choice is irrespective of steady or transient. So your internal transient is a reasonable combo.