It depends on how much heat you need to dissipate.  Often, it is desirable to be conduction cooled from inside the box to the outside of the box, mainly to keep condensing, and other bad air away from electronics.  Then either air-cooled exterior of the box, or conduction cooled to the frame.   Really high-power electronics may need to be liquid cooled.  There are other exotic techniques that can be found for special cases.

Dear Sir,

Thank you for your answer. The heat needed to dissipate is aproximately 5 kW. We are planning to desisgn liquid cooling system:

1) Cold plate devices and Polyalphaolefin (PAO) as a coolant. 

2) TEC with liquid cooled micro channel heat sink.

I am sending the configuration of the both system, personally. Please do provide your valuable suggestion whether any improvement can be done and which will be more efficient one? We also need to consider the light weight components. if you have any literature related to this, please let me know.

Regards,

Liquid cooling systems are used for extracting heat from small size devices that dissipates high amount of power and transferring that heat to the ambient, somewhere else using a rather large radiator. As you may know, compared to conventional forced-air cooling systems it has shorter MTBF and occupies more space.

 In some cases liquid cooling can be replaced by introducing a well designed copper heat spreader between the device that dissipates power and the aluminum heat sink.

 5kW of power for cooling means at least 5kW of average RF power which is unusually high for airborne radars! If your transmitter is SSPA, there is good chance you can design it in a way that can be cooled using conventional forced-air method.

Considering the compact ergonomics of airborne radar system, liquid cooling is must for Tx average  power of 5 KW or more. As a coolant proportionate mixure of Ethylene Glycol and Water can be used. As a heat exchanger, compact FIN type plate and associated pump system is required for efficient management of heat dissipation.