"my question is what should be direction of the air flow, inside out or vice versa."

1. If you are planning to direct the flow on the heatsink then the direction is already decided. BUT please ensure that the rear of the fan is facing a open space / louvers OUTSIDE the cabinet. Idea is to suck in cool air and blow it on the heatsink. And dont forget a path out of the cabinet for the hot air to be blown off - without this path any fan will not solve your problem.

2. Other indirect method is to have a "artificial draft" with proper routing louvers : Assume you have the heat sink in the middle of the cabinet ; then you provide a filtered opening in the front of the cabinet and directly in line with it, mount a cooling fan in the rear in such a way that it sucks from the INSIDE and throws the air OUTSIDE. In this case the fan needs to be more powerfull as it has to suck past the filter .

3. And when you mount heatsinks just make sure the fins are vertical so that a natural draft is maintained - from bottom to top. Hot air rises blah blah..

( And incidentally if you are somewhere near Delhi, where the ambient itself is at 40 C +, nothing short of a chiller will work ;-)


Raghu